Biomass Pyrolysis Pathways for Renewable Energy and Sustainable Resource Recovery: A Critical Review of Processes, Parameters, and Product Valorization

Abstract

The increasing demand for renewable energy has intensified research on lignocellulosic biomass pyrolysis as a versatile route for sustainable energy and resource recovery. This study provides a comparative overview of main pyrolysis regimes (slow, intermediate, fast, and flash), emphasizing operational parameters, typical product yields, and technological readiness levels (TRLs). Reactor configurations, including fixed-bed, fluidized-bed, rotary kiln, auger, and microwave-assisted systems, are analyzed in terms of design, advantages, limitations, and TRL status. Key process parameters, such as temperature, heating rate, vapor residence time, reaction atmosphere, and catalyst type, critically influence the yields and properties of biochar, bio-oil, and syngas. Increased temperatures and fast heating rates favor liquid and gas production, whereas lower temperatures and longer residence times enhance biochar yield and carbon content. CO₂ and H₂O atmospheres modify product distribution, with CO₂ increasing gas formation and biochar surface area and steam enhancing bio-oil yield at the expense of solid carbon. Catalytic pyrolysis improves selectivity toward target products, though trade-offs exist between char and oil yields depending on feedstock and catalyst choice. These insights underscore the interdependent effects of process parameters and reactor design, highlighting opportunities for optimizing pyrolysis pathways for energy recovery, material valorization, and sustainable bioeconomy applications.

1. Introduction

The continuous rise in fossil fuel prices, concerns over their eventual depletion, and the escalating impacts of pollution and global climate change have collectively driven the global shift toward alternative, sustainable energy sources. Since the energy crisis of the 1970s, interest in biomass as a renewable and sustainable fuel resource has grown significantly, aiming to diversify domestic energy portfolios, accelerate renewable technology deployment, and reduce the environmental footprint of conventional energy. Biomass holds considerable potential as an energy source, with applications and benefits spanning both industrialized and developing regions. Its decentralized nature and wide availability contribute to increased energy security and rural development, particularly in regions with limited access to conventional energy infrastructure.

Biomass constitutes a heterogeneous and renewable source of organic matter, encompassing wood and forestry residues, agricultural crops and their residual by-products, municipal solid waste, and food waste. Moreover, it includes animal waste, by-products from wood and agri-food industries, post-consumer wood, and aquatic biomass such as plants and algae [1,2]. This broad diversity of feedstock offers extensive opportunities for energy recovery and chemical valorization, positioning biomass as a sustainable alternative to conventional fossil fuels [3]. Seasonal and geographical variability of these feedstocks requires flexible and adaptable conversion technologies.

The strategic role of biomass as a bioenergy source is closely linked to its sustained and diversified availability. Globally, bioenergy accounts for about three-quarters of renewable energy use, with more than half stemming from traditional biomass consumption. According to the International Renewable Energy Agency [4], bioenergy power capacity reached 151 GW by 2024, representing roughly 4.4% of total renewable energy capacity. Bioenergy contributed approximately 9% to the global final energy consumption in 2019, making it a significant component of the global energy mix. In the European Union (EU), bioenergy generation reached around 12,762 GWh in 2022, accounting for 18% of the EU’s renewable electricity production. These trends highlight the sustained growth of bioenergy driven by technological progress, policy support, and increasing environmental awareness, reinforcing biomass’s vital role in the ongoing energy transition. As illustrated in Figure 1, the EU is projected to maintain and even expand its supply of sustainable biomass through 2050, with significant contributions arising from agricultural residues, forest biomass, and organic waste streams. Agricultural and forestry biomass are the primary contributors to the overall biomass potential, serving as key drivers of resource availability for energy production, while biowaste contributes only marginally to the total volume. The quantity of biomass available for bioenergy is expected to range between 520 and 860 million dry tons (equivalent to 208–344 Mtoe) by 2030 and between 539 and 915 million dry tons (215–366 Mtoe) by 2050, depending on policy developments, sustainability criteria, and technological advancements [5]. These projections underscore the urgent need to develop efficient thermochemical technologies to harness biomass potential within a circular, decarbonized economy, where research and innovation improve conversion efficiencies and reduce environmental impacts.

This growing availability reinforces the need to explore and optimize the multiple energy services that biomass can provide across different sectors and scales.

Biomass acts as a versatile energy carrier utilized across various applications, including electricity generation, residential and district heating, transportation fuels, and the provision of process heat in industrial operations [6]. In the current transition towards a circular and decarbonized economy, recovering energy from biomass through thermal and thermochemical processes plays a critical role in diversifying energy sources and reducing the volume of biomass residues disposed of in landfills. This approach is strongly supported by the European legislative framework, which establishes requirements and limits for the sustainable use of biomass for energy purposes. Moreover, integrating biomass into existing energy systems contributes to the reduction of fossil fuel dependency and supports the development of hybrid energy infrastructures that combine renewables.

Key regulatory instruments include the Renewable Energy Directive (EU) 2018/2001 (RED II) [7], which promotes the use of energy from renewable sources, as well as Directive (EU) 2023/2413 [8], which amends RED II under the framework of RED III and reinforces the role of biomass in achieving renewable energy targets. The Industrial Emissions Directive (IED) also plays a crucial role by regulating combustion and co-incineration facilities processing biomass. Additionally, various directives and regulations support national schemes for renewable energy production. These regulatory frameworks align with broader EU climate and energy policy goals, including the European Green Deal and the “Fit for 55” package, which set binding targets to reduce greenhouse gas (GHG) emissions by at least 55% by 2030 [9]. In its 2020 Energy Strategy, the European Commission urged Member States to expand the integration of renewable energy sources into their national systems [10].

Meanwhile, the European Council outlined the overarching long-term objectives and guiding principles for this transition. The strategy sets a target for the EU and other industrialized nations to cut GHG emissions by 80–95% by 2050. Biomass is recognized as a key renewable energy source contributing to these targets and supporting the EU’s ambition to achieve climate neutrality by 2050, particularly in hard-to-decarbonize sectors such as heating and industrial energy use [11]. In some Member States, legislation allows the inclusion of a small proportion of biomass (e.g., up to 5%) in installations primarily powered by fossil fuels without altering the energy classification of the facility. However, this limit is not uniformly mandated by a specific European directive, and its implementation varies according to national legislation and installation type. The harmonization of these national approaches remains an open challenge, with implications for both market integration and regulatory coherence.

In recent years, the EU has intensified efforts to transition towards a low-carbon energy system. In 2022, renewable energy sources represented approximately 18% of the EU’s total energy supply, increasing to 24.5% in 2023. Biomass accounted for nearly 60% of this renewable share, making it the most significant renewable energy source and the fourth-largest overall energy source in the EU, after oil, natural gas, and nuclear power. This upward trend reflects the progressive implementation of legislative instruments such as Directive (EU) 2018/2001 [7] and its amendment, Directive (EU) 2023/2413 [8], which aim to accelerate the deployment of renewables and support the EU’s 2030 and 2050 decarbonization targets [12]. Such statistics highlight the central role of biomass in Europe’s renewable energy portfolio and emphasize the importance of further investments in advanced biomass conversion technologies.

This review provides a comprehensive and structured overview of biomass pyrolysis, focusing on the operational characteristics, key process parameters, reactor types, and resulting products. It summarizes the applications and valorization pathways of the three primary pyrolysis products (biochar, bio-oil, and syngas) within the frameworks of sustainable resource management and circular economy. The technological maturity of various pyrolysis reactors is also critically discussed to assess their readiness for practical implementation. The objective of this study is to systematically analyze and correlate reactor configurations and operating parameters with product quality and valorization routes, thereby offering a clearer understanding of biomass pyrolysis as a promising thermochemical pathway for renewable energy production and material recovery. Our study aims to provide a more integrative perspective compared to existing reviews, highlighting recent technical developments and identifying challenges and opportunities for future research and industrial adoption. By doing so, it contributes to advancing sustainable bioenergy solutions and supports ongoing efforts to decarbonize the energy sector through efficient waste-to-energy conversion technologies.

In this context, the present review specifically focuses on biomass pyrolysis due to its unique capability to convert diverse feedstocks into valuable products such as bio-oil, biochar, and syngas, with a high degree of operational flexibility. Pyrolysis offers advantages over other thermochemical processes, including gasification and combustion, in terms of product diversity and adaptability to varying feedstock types and moisture contents. This emphasis complements our recent comprehensive review on biomass gasification [11], providing a more complete perspective on thermochemical biomass conversion technologies and their role in sustainable energy transition.

2. Biomass Conversion Pathways and Thermochemical Processes

To effectively harness biomass potential, a variety of conversion processes are employed to break down its complex organic matrix into value-added products. In addition to biochemical and physicochemical methods, thermochemical methods are extensively explored for specific feedstocks or applications [11,13]. As illustrated in Figure 2, biomass conversion pathways can be broadly categorized based on transformation mechanisms and the nature of the final products. This classification highlights the interplay between biochemical, physicochemical, and thermochemical methods and the types of energy carriers or chemical intermediates they generate. Such categorization facilitates process selection based on feedstock characteristics, desired outputs, and environmental considerations.

Thermochemical treatment of biomass involves high-temperature processes that convert solid organic matter into valuable energy carriers and intermediates such as syngas, bio-oil, biochar, and heat, along with by-products including tars and ash [11]. Key thermochemical conversion technologies include pyrolysis, gasification, combustion, and co-combustion, each characterized by specific operational parameters, energy efficiencies, and potential for integration within industrial and energy systems [14].

Recent advances in thermochemical biomass conversion have introduced novel methods such as hydrothermal gasification, which effectively processes wet biomass under moderate temperatures and high pressures to produce clean syngas with high efficiency. Additionally, emerging pyrolysis innovations include catalytic approaches aimed at enhancing selectivity and yield of target bio-oils, as well as co-hydropyrolysis techniques that combine hydrogen and pyrolysis processes to improve product quality and reduce oxygen content. However, despite these promising advances, challenges related to process scalability, catalyst stability, and feedstock variability continue to limit widespread industrial adoption. These advanced methods complement traditional thermochemical routes by expanding feedstock flexibility and enabling more sustainable and economically viable bioenergy production.

The selection of a suitable thermochemical pathway depends on multiple factors, including moisture content, feedstock composition, and desired energy form or chemical product. Moreover, high-pressure liquefaction and processes adapted to wet biomass, such as direct liquefaction, supercritical water extraction, and hydrothermal processing, have demonstrated promising conversion efficiencies [15]. These wet-processing techniques offer advantages for feedstocks with high water content that are otherwise unsuitable for conventional dry thermochemical routes.

Figure 2. Classification of biomass conversion technologies by output and end use (adapted from [16], under the terms of the Creative Commons Attribution License (CC BY 4.0)).

Figure 2. Classification of biomass conversion technologies by output and end use (adapted from [16], under the terms of the Creative Commons Attribution License (CC BY 4.0)).

Emerging approaches like supercritical torrefaction and catalytic decomposition are attracting growing research interest due to their potential to improve product yields, selectivity, and environmental performance. Nevertheless, further research is required to fully understand their long-term environmental impacts and techno-economic feasibility. Energy production through biomass thermochemical conversion is considered CO₂-neutral, since the emitted biogenic CO₂ was previously absorbed from the atmosphere by plants during photosynthesis [17]. These conversion routes not only enable energy recovery from non-recyclable biomass fractions and reduce residual volumes but also contribute to the stabilization of hazardous substances and lower GHG emissions compared to landfilling or traditional combustion [18,19]. Additionally, integrating catalytic systems and advanced reactor designs can further lower reaction temperatures, improve energy efficiency, and reduce pollutant formation. Furthermore, many of these processes can be customized to produce alternative fuels or chemical precursors, reinforcing their strategic role in sustainable biomass valorization. This flexibility supports the development of integrated biorefineries that co-produce energy, fuels, and high-value chemicals.

Given this versatility, it is critical to address the current gaps in understanding the complex interactions between process parameters and feedstock properties to optimize pyrolysis performance at commercial scale. The subsequent sections not only describe the process factors and specific products that define the versatility and value of biomass pyrolysis but also offer a critical analysis of how these parameters influence product distribution, identify current knowledge gaps, and highlight strategies for ensuring process scalability.

3. Classification and Operational Characteristics of Biomass Pyrolysis Processes

3.1. General Aspects

Biomass pyrolysis, originally developed for charcoal (biochar) production, has evolved into a sophisticated and dynamic research field focused on energy and chemical recovery from lignocellulosic feedstocks. In recent years, it has attracted growing interest due to its flexibility in converting a wide variety of biomass types into valuable solid, liquid, and gaseous products. Moreover, pyrolysis offers an environmentally sustainable alternative to conventional waste management practices such as incineration and uncontrolled landfilling. Its growing relevance is driven by the dual goals of renewable energy generation and organic waste valorization.

Also known as devolatilization, pyrolysis is a thermochemical decomposition process that takes place in the absence or near-absence of oxygen. It typically operates within the temperature range of 300 to 700 °C, although higher temperatures (up to 1100 °C) can be employed under specific conditions. The process requires an external heat source to drive the endothermic reactions responsible for breaking down complex organic macromolecules into smaller chemical constituents [20,21]. This thermal cracking leads to the formation of intermediate radicals and volatile compounds that subsequently recombine into bio-oil and gases, while a carbon-rich solid residue remains.

The operational characteristics of pyrolysis, such as final temperature, heating rate, residence time, and process atmosphere (e.g., nitrogen, argon, carbon dioxide, or steam), determine the specific regime and configuration of the conversion process. In cases where inert or no carrier gases are used, the process is often referred to as carbonization or devolatilization [22]. The fine-tuning of these parameters allows for targeted product distributions, making pyrolysis highly adaptable to diverse valorization strategies.

The pyrolysis process yields three primary product streams: a solid fraction (biochar), a liquid fraction (bio-oil or tar), and a gaseous fraction (syngas), which consists mainly of CO, H₂, CH₄, and light hydrocarbons. Each of these products has potential applications in energy recovery or as industrial feedstocks, depending on their quality and chemical composition [11]. Their composition varies considerably with feedstock type and operating conditions, necessitating tailored process control for consistent product quality.

Among thermochemical routes, pyrolysis is particularly favored for bio-oil production due to its fast reaction kinetics, relatively simple reactor design, and high bio-oil yield, typically in the range of 50–70% [22]. However, crude bio-oil produced by conventional pyrolysis is characterized by high oxygen content, which negatively affects its calorific value, chemical stability, and suitability for direct use as transportation fuel. To overcome these limitations, two main upgrading strategies are commonly employed. The first involves post-processing through hydrotreatment, which effectively reduces oxygen content and enhances the energy density and thermal stability of the oil. The second strategy is catalytic pyrolysis, in which catalysts are introduced during the thermochemical conversion step to improve product selectivity and reduce oxygenated compounds in situ [23]. These upgrading approaches are critical to expanding the industrial applicability of pyrolysis-derived bio-oils and integrating them into existing energy infrastructures.

In contrast to incineration, pyrolysis takes place under oxygen-limited conditions and typically at lower temperatures, which substantially reduces the formation of harmful pollutants such as dioxins, furans, and nitrogen oxides (NO_x). This makes pyrolysis a safer and more environmentally benign option for biomass valorization [22]. In some pyrolysis configurations, especially at temperatures of 1000–1100 °C, the process begins to resemble gasification, favoring the production of combustible gases suitable for combined heat and power (CHP) or synthetic fuel synthesis. Such high-temperature regimes are explored particularly for optimizing syngas quality and reducing tar formation. Nevertheless, the precise control required at these high temperatures to prevent undesirable side reactions and catalyst degradation remains a key technical challenge.

While most classifications of pyrolysis focus on slow, intermediate, fast, and flash regimes, a more holistic approach to biomass thermal conversion includes pretreatment technologies such as torrefaction and hydrothermal carbonization. Although distinct from pyrolysis, these processes play a crucial role in improving the physicochemical properties of feedstocks, thereby improving the efficiency and quality of the pyrolysis products. Pretreatment steps can increase reactivity, facilitate feedstock handling, and stabilize biomass for storage and transport.

Torrefaction is a mild thermal pretreatment that enhances the fuel properties of solid biomass by partially removing moisture and volatile matter. It is conducted at temperatures between 200 and 300 °C in an inert or low-oxygen environment, leading to partial thermal degradation, predominantly of hemicellulose [24]. Torrefied biomass exhibits improved energy density, hydrophobicity, and grindability, along with reduced moisture and oxygen content [25]. These enhancements increase its chemical and thermal stability, making it better suited for subsequent thermochemical conversion processes, such as pyrolysis, gasification, and combustion. Torrefied biomass contributes to higher syngas yields, lower tar formation, and more stable reactor operation, thereby supporting more efficient energy recovery and reduced pollutant emissions. In addition, torrefied biomass can be pelletized for use in co-firing applications in existing coal-fired power plants. As a result, torrefaction has gained considerable interest as a commercially viable pretreatment technology. However, its economic feasibility at large scale depends strongly on regional biomass availability and integration into existing fuel supply chains.

Hydrothermal carbonization (HTC), on the other hand, is a wet thermochemical process designed for high-moisture biomass. It typically operates at 180–250 °C (up to 300 °C) and pressures of 2–4 MPa [26]. HTC converts wet biomass into a carbon-rich solid product known as hydrochar, which is structurally and energetically like lignite coal. Hydrochar usually requires post-drying before use but possesses higher energy content than conventional biochar and can serve both as fuel and soil amendment [27]. A significant advantage of HTC lies in its ability to process biomass with high moisture content, such as food waste, sewage sludge, or aquatic biomass, without energy-intensive drying. In terms of gas production, HTC yields a syngas with a higher CO₂ content and a lower CO concentration compared to torrefaction. However, HTC can outperform torrefaction in overall syngas yield and energy content [28]. While torrefaction primarily reduces moisture and volatile components, HTC densifies the biomass structure and significantly enhances its energy density. Hydrochar produced by HTC also shows promise for carbon sequestration and heavy metal adsorption, owing to its porous structure and surface chemistry. Although still considered an emerging technology, HTC holds strong potential for industrial application, with ongoing research focused on scaling up and optimizing the process for wider deployment. Yet, challenges remain regarding the handling of process water and potential emissions of organic contaminants.

While several thermochemical conversion methods, including hydrothermal carbonization, offer specific advantages depending on feedstock characteristics and product goals, pyrolysis remains one of the most flexible and widely studied approaches. To optimize pyrolysis technologies and understand their operational scope, it is essential to examine the defining characteristics of the main pyrolysis processes, such as temperature regimes, heating rates, and product distributions. The following subsections provide a detailed analysis of these parameters, clarifying their influence on product quality and yield.

3.2. Characteristics of the Main Pyrolysis Processes

Depending on key process parameters such as final temperature, heating rate, and residence time, pyrolysis can be classified into several types. The most widely applied are slow, intermediate, fast, and flash pyrolysis. In contrast, alternative methods such as microwave-assisted, catalytic, vacuum, or hydropyrolysis remain niche or emerging technologies with limited industrial adoption. Consequently, the following sections will focus on these four main conventional pyrolysis routes.

These four categories encompass a wide operational spectrum, offering flexibility in tuning product distributions toward biochar, bio-oil, or syngas, depending on energy needs and application goals.

Before discussing each type individually, it is useful to note that the technological readiness levels (TRLs) of pyrolysis processes vary significantly depending on reactor design, operational scale, and intended products. In the European context, slow pyrolysis is generally the most mature, with several commercial plants in operation and a TRL of 8–9. Fast pyrolysis, aimed at bio-oil production, is at TRL 6–8, with multiple pilot and demonstration-scale facilities. Flash and catalytic pyrolysis remain at lower TRL levels (3–5), primarily under laboratory investigation or early pilot studies [29,30,31,32]. These differences in maturity reflect the historical focus of each process: biochar for slow pyrolysis, liquid fuels for fast pyrolysis, and high-efficiency gas production or selective compound generation for flash or catalytic pathways.

This disparity underscores the importance of aligning process development with market and regulatory incentives for valorizing specific pyrolysis products. Moreover, TRL variations impact investment attractiveness and dictate the scale-up potential of each technology in real-world applications.

3.2.1. Slow Pyrolysis

Slow pyrolysis, also referred to as carbonization or conventional pyrolysis, is a thermochemical process primarily optimized to produce biochar. Historically, slow pyrolysis, often regarded as the traditional form of pyrolysis, has been utilized industrially since the early 20th century. In its early applications, biomass, particularly wood, was subjected to prolonged thermal decomposition, typically lasting up to 24 h, to yield a range of products such as methanol, ethanol, acetic acid, and solid carbonaceous residues [31]. However, conventional approaches have often suffered from low energy efficiency and high pollutant emissions. To address these limitations, recent technological advancements have focused on improving process control and sustainability, aiming to reduce environmental impacts and improve overall performance. Modern systems integrate temperature monitoring, feedstock pretreatment, and emission control technologies to ensure higher efficiency, lower tar formation, and better-quality products.

This method typically operates at relatively low temperatures, generally in the range of 300–500 °C [33], although higher temperatures of up to 900 °C may also be employed [34]. The process is characterized by a slow heating rate, typically between 0.1 and 1 °C/s [35], and extended biomass residence times within the reactor, which can vary from several minutes to 12 h, depending on the particle size and the specific process goals [34]. The particle size of biomass used in slow pyrolysis typically ranges from 0.075 to 19 mm. These operating conditions promote secondary reactions and allow thorough decomposition of hemicellulose and cellulose, while favoring char formation from lignin-rich fractions.

Slow pyrolysis can accommodate various types of biomasses, including those with high moisture content, making it a versatile option for diverse feedstock characteristics [36]. Due to the prolonged retention of both solids and volatiles within the system, this process favors the formation of carbon-rich solid residues (biochar), while yielding comparatively lower amounts of liquid and gaseous products. The biochar yield from slow pyrolysis generally falls within the range of 30–40 wt% [35,37], with 35–45% bio-oil and 25–40% gaseous products [38].

Biochar, produced through slow pyrolysis and characterized by a high fixed carbon content, has attracted increasing attention due to its versatility and multifunctional applications, including soil improvement, carbon sequestration, activated carbon production, and metallurgical uses. Its high chemical stability enables long-term carbon storage, thereby contributing to negative emission technologies and climate change mitigation strategies. When applied to agricultural soils, biochar can improve water retention, reduce nutrient leaching, and decrease reliance on synthetic fertilizers, which collectively enhance soil health, promote sustainable agricultural practices, and increase the resilience of farming systems to environmental stresses.

A wide range of reactor configurations have been developed for slow pyrolysis, with the most common including cylindrical fixed-bed systems, batch reactors, rotary kilns, and packed-bed designs. The choice of reactor depends on feedstock characteristics, desired throughput, heat transfer efficiency, and intended product valorization strategy. Batch systems offer flexibility and simplicity for heterogeneous feedstocks, while rotary kilns provide better temperature uniformity and scalability.

Figure 3 presents a schematic representation of a typical batch-type reactor system used in slow pyrolysis applications. The configuration includes three fundamental components: a feed unit, where the biomass is often preheated and partially liquefied; the pyrolysis chamber, operating at 300–700 °C; and a downstream product recovery section for collecting condensed vapors and solid residues. Inert gas purging, usually with nitrogen, ensures an oxygen-free environment, while the condensation and filtration stages are designed to prevent contamination and enable efficient separation of tar, bio-oil, and gases.

Despite these advances, the energy efficiency and economic feasibility of slow pyrolysis systems remain highly dependent on heat integration strategies and product valorization routes, which must be optimized for each application context. This underlines the importance of system integration and process intensification to enhance competitiveness.

Slow pyrolysis is highly versatile regarding feedstock, as it can process biomass with a wide range of moisture contents and particle sizes. This includes woody biomass, agricultural residues, and organic waste streams. The ability to handle higher moisture content materials reduces preprocessing costs and expands the availability of feedstock sources, making slow pyrolysis adaptable to diverse biomass supply chains.

The main product, biochar, is widely applied as a soil amendment to improve water retention, nutrient availability, and microbial activity in agricultural soils. Additionally, biochar contributes to carbon sequestration strategies due to its recalcitrant carbon content. Other applications include activated carbon production, metallurgical uses, and as a renewable solid fuel [11]. Slow pyrolysis systems can be integrated into rural and decentralized settings, supporting local circular economy initiatives by valorizing agricultural and forestry residues.

Slow pyrolysis promotes carbon-negative outcomes by stabilizing biomass carbon in biochar and reducing GHG emissions associated with open burning or landfilling of residues. Furthermore, the process reduces pollutant emissions through controlled thermal decomposition and modern emission control technologies. Optimized heat integration enhances overall energy efficiency, lowering the environmental footprint. However, longer processing times and lower throughput can impact economic viability if not carefully managed.

Advantages of slow pyrolysis include the production of high-quality biochar with superior carbon stability, the ability to process moist and heterogeneous biomass, and relative technological simplicity. However, limitations involve longer processing times, lower yields of bio-oil and syngas compared to faster pyrolysis methods, and economic feasibility that is highly sensitive to heat integration and product valorization strategies. Additionally, reactor design complexity varies depending on scale and feedstock diversity, which influences capital and operational costs.

3.2.2. Intermediate Pyrolysis

Intermediate pyrolysis operates at temperatures between 500 and 700 °C, with heating rates ranging from 0.4 to 10 °C/s and biomass residence times of 0.5–40 s [39]. This method enables a balanced distribution of solid, liquid, and gaseous products, making it particularly attractive for integrated biorefinery applications aimed at efficiently valorizing all product fractions. Such integrated valorization is crucial in the context of sustainable waste management and circular economy models, especially in regions generating large volumes of agricultural or agro-industrial residues.

The intermediate regime bridges the gap between biochar-oriented and bio-oil-oriented processes, offering operational flexibility with moderate complexity and investment costs.

Typically, intermediate pyrolysis yields 40–50 wt% of liquid products, along with non-condensable gases and solid char, resulting in product distributions that lie between those of slow and fast pyrolysis [40]. The bio-oil generated is notable for its low viscosity and reduced tar content, enhancing its suitability for direct thermal use. Concurrently, the dry and brittle biochar obtained is well-suited for use as a biofertilizer or as feedstock for subsequent gasification [41].

The porous structure and mineral content of biochar from intermediate pyrolysis can enhance soil health and microbial activity when applied as an amendment, particularly in degraded or nutrient-poor soils. Its potential use in water filtration systems and as a renewable adsorbent material is also being explored.

This process can also yield a bio-oil with improved phase stability and lower water content compared to fast pyrolysis, depending on reactor conditions and feedstock. In terms of gas composition, the intermediate regime can provide a syngas richer in CH₄ and H₂ compared to slow pyrolysis, which is advantageous for downstream combustion or reforming processes.

A key advantage of intermediate pyrolysis is its high feedstock flexibility. Intermediate pyrolysis can efficiently process a broad range of biomass types, including woody residues, straws, grasses, and other agricultural and food and beverage industry by-products [42], in various physical forms (coarse, shredded, chopped, or ground), without requiring extensive size reduction [34]. This versatility significantly enhances operational adaptability, making the process particularly effective for heterogeneous biomass streams. Notably, olive pomace, grape marc, wheat straw, and brewers’ spent grain are among the most suitable feedstocks for this process due to their balanced lignocellulosic composition and widespread availability in agro-industrial regions.

The minimized preprocessing requirements lower energy demand, thereby improving the overall energy balance of the system, a critical sustainability metric when evaluating lifecycle GHG emissions. Moreover, this advantage facilitates decentralized deployment in rural or agro-industrial settings where raw biomass is abundant, but infrastructure is limited. As a result, intermediate pyrolysis systems are increasingly considered for local energy hubs or farm-scale bio-refineries that aim to transform waste into energy, fertilizers, and fuel while reducing transport-related emissions.

Multiple reactor configurations are employed for intermediate pyrolysis, including screw-based systems, fluidized bed reactors, and rotary kilns. Among these, screw-based systems are especially advantageous because of their compact design, ease of control, and continuous-feed operation, which facilitate scalability. Fluidized bed reactors provide superior heat transfer and more uniform temperature distribution but tend to be more complex to operate and maintain.

Figure 4 illustrates the schematic layout of a continuous-feed retort system used in intermediate pyrolysis. The system consists of four primary modules: a feed unit with an agitator and screw conveyor for steady biomass delivery; a cylindrical pyrolysis reactor with integrated, temperature-controlled heating elements; a cooling system employing ethylene glycol for effective vapor condensation; and a control unit equipped with a PID controller and thermocouples for precise thermal regulation across the process. Such modular systems are increasingly used in pilot and demonstration-scale plants aimed at evaluating feedstock variability, product optimization, and techno-economic viability.

Despite its flexibility, challenges such as reactor fouling, heat integration, and the need for effective product upgrading technologies must be addressed to improve scalability and long-term operational sustainability. It is important to note that fouling and tar buildup can significantly affect operational continuity and maintenance costs, which are often underestimated in pilot studies.

Intermediate pyrolysis excels in processing a diverse range of biomass feedstocks, including agricultural residues, woody materials, and agro-industrial by-products. Its tolerance to variable particle sizes and moisture content reduces preprocessing requirements, facilitating the use of heterogeneous feedstock streams and enhancing operational adaptability. This flexibility supports decentralized and rural bio-refinery deployments where feedstock variability is common.

This pyrolysis regime is well-suited for integrated biorefinery approaches, enabling balanced recovery of biochar, bio-oil, and syngas for multiple uses. Biochar serves as a soil amendment or biofertilizer, improving soil health and supporting sustainable agriculture. Bio-oil can be used directly for heat generation or as a precursor for upgraded fuels, while syngas contributes to combined heat and power (CHP) systems. These multiple product streams enhance economic viability and sustainability.

Intermediate pyrolysis contributes to waste valorization and circular economy objectives by converting biomass residues into valuable products and reducing open burning. Improved energy balances due to moderate heating rates and reduced preprocessing lower GHG emissions. The biochar produced aids carbon sequestration and soil restoration, while utilization of syngas and bio-oil reduces reliance on fossil fuels. Effective heat integration and emission control further enhance environmental benefits.

Advantages of intermediate pyrolysis include operational flexibility, balanced product yields, and suitability for decentralized applications with lower preprocessing costs. However, challenges involve managing reactor fouling, ensuring efficient heat integration, and upgrading bio-oil for broader use. Capital investment and operational complexity lie between slow and fast pyrolysis, making intermediate pyrolysis a promising compromise for scalable and sustainable biomass conversion.

Further research into catalyst development and process intensification could enhance bio-oil quality and process economics, making intermediate pyrolysis more competitive in commercial settings.

3.2.3. Fast Pyrolysis

Fast pyrolysis is considered an advanced thermochemical conversion technology specifically designed to maximize bio-oil yield through the rapid thermal decomposition of biomass under tightly controlled process conditions. Typically, the process is carried out at temperatures ranging from 400 to 600 °C [44], although broader operational windows from 300 °C up to 1400 °C have also been reported [34].

To achieve high bio-oil yields and minimize secondary reactions, biomass must be heated rapidly, often reaching 500 °C, with solid residence times limited to 0.5–10 s and vapor residence times restricted to 1–5 s, commonly under 2 s [44]. These short contact times help preserve condensable volatiles and prevent over-cracking. Under such optimized conditions, bio-oil yields can reach 50–75 wt% (dry basis), while char and non-condensable gases are formed in comparatively lower amounts [45]. Similar product distributions have been confirmed in other studies, reporting 60–75% bio-oil, 15–25% char, and 10–20% syngas [46], or 12–25% char and 13–25% syngas [47]. These results are attributed to precise control of the thermal cracking window, which ensures rapid heating without extended exposure to high temperatures [34].

A critical requirement for optimal process performance is achieving high heating rates, typically exceeding 10 °C/s and reaching up to 200 °C/s [48]. To enable efficient and uniform heat transfer, feedstock is pre-treated by reducing moisture content below 12 wt% and milling to small particle sizes (1–2 mm). Maintaining these stringent thermal conditions often necessitates electrically heated systems equipped with temperature regulation via thermocouples and feedback control.

Consequently, a variety of reactor designs have been developed for fast pyrolysis, including bubbling and circulating fluidized beds, auger screw reactors, and rotating cone reactors, as well as alternative systems such as ablative and vacuum pyrolizers. Among these, fluidized bed reactors are the most commonly used at pilot and demonstration scales due to their excellent heat transfer, uniform temperature distribution, and strong scalability potential [49]. Rotating cones and ablative reactors, although less common, provide high heating rates and are suitable for feedstocks with higher particle size or moisture content.

The reactor constitutes the core of any fast pyrolysis system, ensuring optimal heat and mass transfer for efficient thermochemical conversion. A typical process configuration integrates several auxiliary units, including a biomass preconditioning module for particle size and moisture adjustment; the pyrolysis reactor, where vapors, char, and gases are generated; a solid–vapor separation unit such as a cyclone; a condensation and collection module for bio-oil recovery; and a thermal integration system designed for heat recovery and improved process efficiency [50]. Figure 5 depicts this typical process configuration.

The bio-oil obtained from fast pyrolysis is a dark, flammable liquid with a heating value of approximately 18 MJ/kg [51]. However, it typically contains 10–15 wt% water and a high concentration of oxygenated compounds, which contribute to its low pH (around 2), chemical instability, and poor miscibility with conventional fossil fuels. Additionally, its low cetane number (10–25) limits its direct application as a diesel engine fuel without prior upgrading [52]. Several upgrading strategies have been explored to improve the physicochemical properties of fast pyrolysis bio-oil, including catalytic hydrotreatment, emulsification, and fractional condensation. These methods aim to reduce water and oxygen content, enhance stability, and increase calorific value, thereby extending its potential for transportation fuel applications [51,52].

Biochar produced through fast pyrolysis generally exhibits a higher oxygen-to-carbon (O/C) atomic ratio compared to biochar from slow pyrolysis. Since lower O/C ratios are associated with greater chemical recalcitrance and structural durability, biochar from slow pyrolysis tends to possess superior structural and functional properties [53]. Nonetheless, biochar from fast pyrolysis still retains potential for soil amendment, for carbon sequestration, and as a precursor for activated carbon, particularly when subjected to post-treatment processes such as physical or chemical activation.

Fast pyrolysis bio-oil serves as a renewable liquid fuel intermediate, suitable for co-processing in existing refineries or as a standalone source for heat and power generation. Its use in agriculture includes the application of biochar as a soil amendment to improve fertility, water retention, and carbon sequestration. The process also fits well in circular economy models, enabling valorization of residues that would otherwise be discarded or landfilled.

Fast pyrolysis has thus become a cornerstone technology within thermochemical biorefinery concepts, providing a high-yield pathway for liquid biofuel production while enabling the economic valorization of by-products through parallel recovery and upgrading strategies.

Fast pyrolysis is compatible with a variety of biomass feedstocks, including woody residues, agricultural wastes (such as straws and husks), energy crops, and certain types of food and industrial by-products. However, optimal yields are typically obtained with feedstocks that have low moisture content (<12%) and small particle size, requiring preprocessing such as drying and milling. This feedstock versatility supports integration within diverse biomass supply chains.

When integrated with efficient heat recovery and emission control, fast pyrolysis can offer lower GHG emissions compared to fossil fuels. The biochar captures stable carbon in soils, contributing to negative carbon emissions over time. Moreover, converting agricultural residues reduces open burning practices, mitigating air pollution and improving local air quality. Life cycle assessments generally indicate favorable energy balances and carbon footprints, though results depend on feedstock source and process design.

Advantages of fast pyrolysis include high bio-oil yields, rapid processing times, and adaptability to various biomass types. The technology is scalable from pilot to commercial scale, particularly with fluidized bed reactors. However, challenges remain in bio-oil stability, storage, and upgrading costs. Feedstock preparation and moisture control add to operational complexity. Capital investment for reactors and auxiliary systems can be substantial, requiring techno-economic assessment for project viability.

In view of the above, fast pyrolysis is an advanced and efficient technology for bio-oil production; however, its large-scale implementation depends on optimizing preprocessing costs and developing effective upgrading methods. Integrating advanced process control systems alongside ongoing research in catalysts and post-treatment techniques is essential to enhance the sustainability and economic viability of technology.

3.2.4. Flash Pyrolysis

Flash pyrolysis represents the most advanced form of pyrolysis, engineered to maximize yields of condensable vapors and syngas while minimizing the formation of solid residues. This process typically employs electrically powered heating systems to sustain high reactor temperatures, with precise thermal regulation achieved through thermocouples and closed-loop control mechanisms.

Operating at very high temperatures between 600 and 1400 °C, flash pyrolysis is characterized by extremely rapid heating rates ranging from approximately 1000 to 21,000 °C/s [54]. Reaction times are exceptionally short, often below 0.5 s and up to 2 s [44], facilitating the rapid thermal decomposition of biomass. Such brief exposure leads to swift breakdown of organic macromolecules like lignin and cellulose, releasing volatile compounds that are promptly extracted from the high-temperature zone and rapidly cooled to inhibit secondary reactions. Solid residence times for biomass particles sized between 0.05 and 2 mm range from 0.016 to 0.34 min. Achieving these performance metrics requires finely pulverized biomass to ensure efficient and uniform heat transfer [34].

Flash pyrolysis of biomass typically yields a combination of gaseous products, bio-oil, and solid biochar. The distribution of these products by weight is generally around 75% gases, 13% liquid bio-oil, and 12% solid residue, depending on the feedstock and operational parameters [55]. Under optimized conditions, flash pyrolysis can yield bio-oil amounts as high as 75–80 wt% [45], making it particularly attractive for liquid fuel production. Nevertheless, due to the extremely high temperatures involved, the chemical composition of the bio-oil can vary significantly, often exhibiting a high oxygen content (up to 35–50 wt%), low stability, and high acidity. The resulting bio-oil is often chemically unstable and corrosive. Additionally, trace contaminants such as heavy metals and nitrogenous compounds may persist, necessitating further purification and upgrading, commonly via catalytic hydrogenation [56]. These additional steps increase the complexity and cost of downstream processing. Recent advances in catalytic vapor upgrading and in-situ reactive condensation have shown promise in mitigating some of these limitations by partially deoxygenating and stabilizing the bio-oil directly within the pyrolysis system.

Various reactor designs and operational concepts have been investigated, including ablative motion-based systems (e.g., rotating or cyclonic reactors), screw-based conveyance (auger reactors), fluidized particle beds, and entrained flow configurations. Among these, auger screw reactors have demonstrated effectiveness in conducting flash pyrolysis of biomass. Fluidized bed systems, however, provide superior heat transfer and scalability, particularly when combined with inert gas circulation and staged condensation setups to maximize product recovery and reduce fouling.

A typical fast pyrolysis installation comprises several key units: a solid feedstock introduction mechanism; an inert gas injection system; the central pyrolysis reactor; a fine particulate capture unit for vapor streams; a condensation and recovery module for liquid products; and an analytical setup to monitor gaseous product composition, as illustrated in Figure 6. Additionally, safety systems such as gas flaring units and pressure relief mechanisms are critical due to the high-temperature, high-pressure nature of the process, especially when scaling from laboratory to industrial level.

Flash pyrolysis requires finely ground biomass with very low moisture content to achieve rapid and uniform heating. Suitable feedstocks include agricultural residues, wood powders, and specially prepared energy crops. The need for very small particle size (<2 mm) and low moisture (<10%) somewhat limits the variety of raw materials compared to other pyrolysis types, but ongoing research aims to expand feedstock tolerance.

Due to its high gas and vapor yields, flash pyrolysis is particularly attractive for producing syngas and volatile compounds for chemical synthesis, fuels, and energy generation. The relatively low liquid bio-oil yield is compensated by high syngas output, useful for integrated biorefinery processes. Biochar, although produced in lower quantities, still finds applications in soil amendment and carbon sequestration. Its integration in industrial settings with advanced gas cleaning and heat recovery systems enhances overall process efficiency and sustainability.

Flash pyrolysis offers the potential to significantly reduce solid waste and convert biomass into valuable energy carriers with lower emissions than traditional combustion. The process can mitigate open burning of agricultural residues, decreasing local air pollution. However, the high operational temperatures and need for finely milled feedstock result in higher energy input, which must be balanced by efficient heat integration and use of renewable electricity sources to maintain overall environmental benefits.

Advantages of flash pyrolysis include ultra-rapid processing times, high syngas yields, and the ability to produce a range of valuable chemicals and fuels. The advanced reactor designs enable precise control over product distribution. However, limitations involve the complexity and cost of reactor systems capable of withstanding extreme thermal stress, extensive feedstock preparation, and challenges related to bio-oil stability and purification. Scale-up from laboratory to industrial scale remains technically demanding but promising.

Table 1. Comparative overview of torrefaction and pyrolysis types based on key process parameters [authors’ own elaboration].

Technique	Residence Time	Temperature (°C)	Heating Rate	Primary Products
Torrefaction (mild pyrolysis)	15–60 min	200–300	10–50 °C/min (mild)	Mainly solid (torrefied biomass); minor volatiles and syngas
Slow pyrolysis (conventional)	5–12 h	300–500 (up to 900)	0.1–1 °C/s (very low)	30–40% biochar, 35–45% bio-oil, 25–40% syngas
Intermediate pyrolysis	0.5–40 s	500–700	0.4–10 °C/s (medium)	40–50% bio-oil, 25–35% biochar, 20–30% syngas
Fast pyrolysis	0.5–2 s (<2 s)	400–600 (optimal)	10–200 °C/s	60–75% bio-oil, 12–25% char, 10–20% syngas
Flash pyrolysis	<0.5 s	600–1400	1000–21,000 °C/s (very high)	75–80% bio-oil, 10–20% char, 10–20% syngas

summarizes the main types of pyrolysis, their typical operating parameters, and the approximate distribution of the resulting products.

While the main pyrolysis regimes differ in temperature, heating rate, and product yield, further distinctions can be made based on the use of catalysts and reactor configurations.

Critical considerations for flash pyrolysis include the substantial energy requirements driven by its extremely high heating rates and temperatures, necessitating effective heat integration and, ideally, the use of renewable energy sources to maintain environmental sustainability. The prerequisite for finely milled, low-moisture feedstock increases preprocessing costs and may constrain feedstock flexibility, impacting overall economic viability. Moreover, the inherent chemical instability and high oxygen content of the bio-oil demand complex and costly upgrading steps to achieve fuel-grade quality. Advanced reactor materials and designs are essential to withstand the severe thermal stresses, further adding to capital and operational expenditures. Continued research into integrated catalytic upgrading and vapor-phase stabilization remains crucial to enhance process efficiency, product quality, and commercial scalability.

3.3. Catalytic Pyrolysis

Beyond the classical thermal pyrolysis classifications discussed previously, pyrolysis processes can also be differentiated based on the use of catalysts during the reaction. This distinction leads to two primary categories: thermal pyrolysis and catalytic (thermo-catalytic) pyrolysis, which differ significantly in their reaction mechanisms and resulting product distributions.

Thermal pyrolysis relies solely on the heat-driven decomposition of biomass, typically producing a product distribution dominated by gaseous compounds that can constitute up to 93% of the total recovered products. In contrast, catalytic pyrolysis employs catalysts such as zeolites (e.g., ZSM-5), cobalt–molybdenum oxide supported on alumina (CoMo/Al₂O₃), or zinc oxide (ZnO) to facilitate the breakdown of polymeric biomass components at moderate temperatures ranging from 350 to 600 °C [57,58,59]. The presence of these catalysts significantly enhances liquid product yields while simultaneously reducing residual carbon and ash contents. Consequently, catalytic pyrolysis improves both overall product quality and process efficiency, making it a promising approach for the tailored valorization of biomass.

In recent years, catalytic pyrolysis has attracted considerable attention for its ability to tailor product composition and enhance bio-oil quality. For example, zeolite catalysts such as ZSM-5 have been widely studied for their capacity to promote deoxygenation reactions, thereby increasing aromatic hydrocarbon yields and reducing oxygenated compounds in bio-oil [57]. Similarly, metal oxide catalysts such as CoMo/Al₂O₃ have demonstrated effectiveness in cracking heavy molecular weight compounds, thereby improving liquid fuel characteristics [60]. These catalytic systems allow pyrolysis not only to generate higher-value liquid fuels but also to reduce the formation of undesirable char and gaseous by-products, thereby improving process efficiency and environmental performance. Furthermore, catalytic pyrolysis contributes to reducing the oxygen content in bio-oil to below 15 wt%, increases its heating value to 30–36 MJ/kg, and improves its miscibility with conventional hydrocarbon fuels [58,60]. Such enhancements are essential for downstream applications, especially in transportation and industrial sectors requiring higher fuel quality.

Catalytic pyrolysis can be performed using two main experimental configurations: in situ and ex situ. In the in situ approach, biomass and catalyst are mixed and fed simultaneously into the reactor, allowing simultaneous decomposition and catalytic upgrading. This close contact enhances reaction rates but limits the optimization of temperature conditions and may lead to catalyst deactivation due to coke formation [61,62]. Conversely, ex situ pyrolysis separates the pyrolysis and catalytic upgrading steps, with biomass first pyrolyzed to produce vapors that then pass through a catalytic bed. This configuration allows independent control of pyrolysis and catalytic reaction temperatures, improving catalyst efficiency and product selectivity. However, it requires additional equipment and can increase operational costs [62,63].

Hybrid configurations combining both in situ and ex situ approaches have also been investigated to balance the advantages and mitigate the limitations of each method. These systems aim to optimize vapor residence time, heat distribution, and catalyst exposure, ultimately improving the quality and yield of upgraded bio-oils [63].

Despite these advances, significant challenges related to catalyst deactivation, regeneration, and associated costs remain key barriers to commercial implementation.

Catalytic pyrolysis can process a broad range of lignocellulosic biomass feedstocks, including woody materials, agricultural residues, and industrial by-products. However, feedstock pretreatment to reduce moisture and control particle size is generally required to optimize catalyst performance and to prevent rapid deactivation. The compatibility of various biomass types with different catalyst formulations enables tailored product outcomes.

This process is particularly promising for producing upgraded bio-oils with improved fuel properties suitable for transportation and industrial uses. Enhanced deoxygenation and reduced acidity broaden the applicability of bio-oil as a renewable drop-in fuel. Additionally, catalytic pyrolysis facilitates the production of valuable aromatic hydrocarbons and specialty chemicals, supporting integrated biorefinery concepts. Its adaptability to both in situ and ex situ configurations allows flexibility in scale and operational design.

By increasing liquid fuel yield and quality, catalytic pyrolysis can reduce the environmental footprint of biomass conversion processes through more efficient resource utilization and lower formation of char and gaseous by-products. However, challenges remain related to catalyst manufacturing, regeneration, disposal, and sensitivity to feedstock impurities, which can lead to catalyst deactivation, particularly due to coke formation.

Developing durable and regenerable catalysts, alongside using renewable energy for process heating, is critical to improving the overall sustainability of catalytic pyrolysis. While catalytic pyrolysis offers advantages such as improved bio-oil quality with lower oxygen content, higher energy density, and enhanced chemical stability, facilitating downstream upgrading and utilization, these benefits are counterbalanced by catalyst costs, process complexity (especially in ex situ configurations), and the need to optimize catalyst lifespan. Research efforts continue to focus on this balance to enable scalable commercial applications.

Nonetheless, several critical challenges remain before widespread commercial adoption can be realized. These include catalyst deactivation caused by coke formation and feedstock impurities, as well as the environmental impacts associated with catalyst lifecycle, including sourcing, regeneration, and disposal. Further research is necessary to optimize catalyst formulations tailored to diverse biomass types and to improve process integration for both in situ and ex situ configurations, reducing operational complexity and energy consumption.

Environmental sustainability considerations must extend beyond process emissions to encompass the full catalyst lifecycle. Moreover, scaling laboratory findings to industrial levels require robust process control strategies and advanced reactor designs that maintain catalyst performance over prolonged operation. Future efforts should also focus on integrating renewable energy sources for process heating and exploring hybrid catalytic systems that synergistically combine the advantages of multiple catalytic approaches. Addressing these gaps will be crucial to unlocking the full potential of catalytic pyrolysis as a versatile, scalable, and sustainable technology within the evolving bioeconomy.

Despite the significant advancements in catalytic pyrolysis, several critical challenges must be addressed before its widespread commercial implementation can be realized. Catalyst deactivation, primarily caused by coke deposition and impurities present in diverse biomass feedstocks, remains a major obstacle impacting process stability and longevity. Furthermore, environmental sustainability considerations extend beyond operational emissions to include the entire catalyst lifecycle, encompassing raw material sourcing, regeneration, and disposal impacts. Effective scale-up from laboratory to industrial scales necessitates robust process control strategies and advanced reactor designs capable of maintaining catalyst performance over prolonged operational periods. Future research should focus on optimizing catalyst formulations tailored to varying biomass characteristics and improving integration of in situ and ex situ catalytic configurations to reduce complexity and energy consumption. Additionally, incorporating renewable energy sources for process heating and exploring hybrid catalytic systems that leverage synergistic effects hold promise for enhancing both economic viability and environmental sustainability. Addressing these technological and environmental challenges is essential to unlocking the full potential of catalytic pyrolysis as a versatile, scalable, and sustainable approach within the evolving bioeconomy.

3.4. Pyrolysis Stages

The thermal decomposition of biomass encompasses a complex network of simultaneous and consecutive chemical reactions, wherein the polymeric structures of cellulose, hemicellulose, and lignin are broken down into smaller molecules. The overall pyrolysis reaction can be summarized as follows [20,22]:

$$C_n{H}_m{O}_P\left(biomass/polymer\right)\to {\sum }_{liquid}{C}_x{H}_y{O}_z+{\sum }_{gas}{C}_a{H}_b{O}_c+H_{2}O+C\left(char\right)$$

(1)

This global reaction equation reflects the non-stoichiometric nature of biomass decomposition, as it depends on feedstock composition and process conditions. It is important to emphasize that the variability in feedstock composition leads to a wide range of product distributions, which requires adaptive process control.

The physicochemical characteristics of biomass feedstock, such as moisture content, particle size, ash composition, and lignin-to-cellulose ratio, play a crucial role in determining reaction pathways and product yields throughout the pyrolysis stages. Hence, thorough feedstock characterization and pretreatment are critical for process optimization and reproducibility.

Pyrolysis conversion can be understood as a complex process involving the superposition of three main reaction pathways, namely char formation, depolymerization, and fragmentation, alongside a series of secondary reactions [64]. The complexity of biomass pyrolysis arises from the interplay between primary thermal degradation, fragmentation, and subsequent secondary reactions, as schematically represented in Figure 7. Secondary reactions often contribute significantly to tar and char formation, negatively impacting bio-oil yield and quality if not properly controlled.

These transformations affect both the composition and yield of the resulting products. The rate and extent of these reactions are influenced by parameters such as heating rate, residence time, reactor configuration, and biomass physicochemical properties (e.g., ash content, lignin-to-cellulose ratio).

• Primary pyrolysis

Primary pyrolysis typically occurs within the temperature range of 250–500 °C and represents the initial stage of thermal decomposition of the main organic macromolecules found in lignocellulosic biomass, namely, cellulose, hemicellulose, and lignin [19]. During this stage, elevated temperatures in an oxygen-free environment induce the breakdown of these biopolymers, generating condensable vapors (which condense into bio-oil or tar), non-condensable gases (syngas), and a solid carbonaceous residue (biochar) [65]. Basically, the decomposition of these components occurs over distinct temperature intervals, as illustrated in Figure 8.

The differentiation in thermal stability among cellulose, hemicellulose, and lignin offers opportunities to manipulate process parameters for selective product formation.

Below 200 °C, moisture is removed from the biomass through drying. The feedstock should have a moisture content below 10% for optimal fast pyrolysis performance. Between 200 and 280 °C, hemicellulose begins to decompose, primarily producing syngas and initiating the formation of bio-oil. Between 240 and 350 °C, cellulose undergoes significant depolymerization, yielding anhydro-sugars (such as levoglucosan) and furan derivatives and contributing to both bio-oil and biochar formation. Between 280 and 500 °C, lignin, the most thermally resistant component, decomposes more gradually, producing phenolic compounds and contributing significantly to char formation due to its aromatic structure. Moisture content above recommended levels can lead to excessive energy consumption and reduced product yields.

Understanding the distinct thermal decomposition intervals for hemicellulose, cellulose, and lignin allows for targeted control strategies in reactor design and operation. For example, fast heating rates favor volatile release and bio-oil formation during primary pyrolysis, while slower heating and extended residence times enhance char yield through secondary pyrolysis. These operational choices influence process scalability and integration into biorefinery schemes.

The decomposition behavior is also influenced by lignin subtype (guaiacyl, syringyl, p-hydroxyphenyl units), which affects the reactivity and product spectrum.

These thermally induced reactions also involve fragmentation mechanisms that release small molecules such as CO, CO₂, CH₄, and light hydrocarbons. The specific distribution and yield of products are strongly influenced by the nature of the biomass, heating rate, particle size, and reactor type. Optimizing these parameters is crucial for directing the process toward a desired product fraction, whether targeting energy-rich gases, chemical intermediates, or carbon-rich solids. Process control at this stage is critical to minimize undesirable secondary reactions and improve product selectivity.

This staged decomposition mechanism is essential for understanding the complex product profile of pyrolysis and optimizing the process according to the desired output, whether targeting bio-oil, syngas, or biochar.

The staged nature of pyrolysis reactions offers opportunities for optimizing energy efficiency and minimizing pollutant formation by adjusting reaction conditions to favor desired products. For instance, limiting secondary cracking can preserve bio-oil quality and reduce tar formation, thereby decreasing downstream upgrading demands. Moreover, the ability to sequester carbon in biochar during secondary pyrolysis contributes to climate mitigation strategies.

• Secondary pyrolysis

Secondary pyrolysis involves further transformation of the initially formed volatile products and the solid biochar residue. These subsequent reactions include thermal cracking, polymerization, dehydration, condensation, and carbonization, which significantly affect the final yield and chemical composition of pyrolysis products [20]. Moreover, during secondary pyrolysis, some volatile intermediates can undergo repolymerization (Figure 9), contributing to the formation of more complex aromatic structures within biochar or heavy tar fractions. This process, along with cracking, dehydration, and condensation, influences the overall quality and stability of the pyrolysis products [67]. Increased residence time and temperature during secondary pyrolysis generally result in lower bio-oil yields and higher char and tar production.

Secondary reactions typically occur at higher residence times and temperatures above 500 °C and are responsible for reducing bio-oil quality by increasing the content of heavier, more oxygenated compounds.

Controlling the balance between primary and secondary pyrolysis stages is crucial for tailoring product distribution and quality, but it demands precise thermal management and advanced reactor technologies. This balance involves a complex interplay among char formation, depolymerization of biopolymers, fragmentation into smaller molecules, and secondary reactions, all of which govern the composition and yield of solid, liquid, and gaseous products. Achieving this balance requires advanced sensor integration and dynamic process control, especially for scalable industrial reactors.

While optimizing these processes offers flexibility, it also introduces challenges such as trade-offs between yield and product quality, increased process complexity, and difficulties in scaling laboratory findings to industrial applications. Continuous advancements in sensor technology and process modeling are essential to address these challenges and enhance reactor design for improved energy and material efficiency. Further research into catalyst integration during pyrolysis may mitigate some secondary reaction drawbacks by promoting selective pathways.

4. Types of Pyrolysis Reactors

The performance and efficiency of any pyrolysis process are strongly influenced by the design and operation of the reactor. Selecting an appropriate reactor configuration is critical for achieving optimal yields, particularly of bio-oil, and requires careful consideration of factors such as heating rate, vapor residence time, temperature control, and system pressure.

Reactor selection also depends on the characteristics of the biomass feedstock, its moisture content, and particle size, as well as on the scale of operation and intended downstream processing.

Over the years, a wide variety of reactor types have been developed to accommodate different feedstocks, operating conditions, and product targets. These include both conventional and advanced systems, each with specific strengths and limitations depending on the pyrolysis mode (slow, intermediate, fast, flash, etc.) and desired product distribution.

The following subsections provide an overview of the most commonly used pyrolysis reactors, highlighting their operating principles, advantages, and challenges.

4.1. Fixed-Bed Reactors

Fixed-bed reactors are widely employed in small-scale biomass pyrolysis due to their structural simplicity, low operational costs, and reliable performance, especially when processing feedstocks with consistent particle size and minimal fines content [66,68]. The standard setup typically consists of a vertical reactor column paired with a gas treatment system, which includes vapor condensation and particulate removal units such as cyclone separators, wet scrubbers, and dry filters (Figure 10). The reactor is usually constructed from stainless steel, firebricks, or concrete, and integrates essential components like a biomass feeding unit, ash collection zone, and gas outlet. Some configurations incorporate solid catalysts directly into the reactor bed, enabling catalytic pyrolysis [66]. During operation, biomass descends through a stationary bed in a counter-current flow setup, meeting an upward-moving stream of carrier gas (e.g., nitrogen), which aids in transporting volatile pyrolysis vapors. These systems generally operate at moderate heating rates, with prolonged solid residence times, high carbon retention, and low superficial gas velocities, making them suitable for decentralized energy generation [68].

However, the relatively low heating rates and extended residence times inherent to fixed-bed designs often favor char production over bio-oil yield, which should be considered when selecting reactor type for specific applications.

The bed material in fixed-bed reactors commonly includes inert materials such as sand, quartz, or ceramic particles, which help to distribute heat evenly and prevent channeling of gases through the biomass. In some designs, refractory materials are used to enhance thermal stability and insulation. These inert bed materials also facilitate heat transfer to the biomass particles and support uniform temperature profiles within the reactor [22]. Careful selection and maintenance of bed materials are critical to prevent agglomeration and ensure consistent thermal performance.

Fixed-bed reactors are particularly effective for feedstocks with uniform particle size and low moisture content, such as wood chips, nut shells, and agricultural residues that have undergone pre-processing. However, their limited capability to handle fine particles or heterogeneous biomass can restrict feedstock flexibility compared to fluidized or screw reactors. This limitation reduces operational versatility, particularly in contexts where feedstock quality varies or preprocessing infrastructure is limited.

Furthermore, fixed-bed reactors typically allow easy integration of temperature control zones, which can optimize the pyrolysis process by maintaining desired reaction temperatures and minimizing hotspots. Their relatively low gas velocity ensures a high solid residence time, favoring char formation but potentially limiting bio-oil yields. Therefore, careful control of operating parameters is essential depending on the targeted product distribution. The trade-off between char and liquid product yields necessitates precise temperature and flow management to tailor output to specific end-uses.

From a sustainability perspective, fixed-bed reactors offer advantages due to their simplicity, enabling low-energy operation and minimal auxiliary systems, thereby reducing lifecycle environmental impacts. Moreover, their capacity to produce high-quality biochar supports carbon sequestration and soil amendment applications, aligning well with circular economy principles. However, lower throughput and scale-up challenges limit their use primarily to decentralized or small-scale installations. Scaling fixed-bed systems to larger capacities is often constrained by heat transfer limitations and increased pressure drop, which can reduce process efficiency.

In terms of technology readiness, fixed-bed reactors are well-established and widely used in small-scale applications, corresponding to a TRL of 8–9 [29,31,32]. Their proven reliability makes them a favorable choice for early-stage or rural biomass valorization projects, although innovation is needed to overcome feedstock and scale constraints.

4.2. Fluidized Bed Reactors

Fluidized bed reactors (FBRs) are among the most widely used technologies for biomass pyrolysis, owing to their excellent heat and mass transfer capabilities. These systems suspend solid biomass particles in a fluid-like state by means of an upward flow of pressurized gas, ensuring uniform temperature distribution and efficient mixing throughout the reactor.

The main types of fluidized bed reactors are [66] bubbling fluidized bed (BFB), suitable for small to medium-scale operations due to its simplicity and low cost; circulating fluidized bed (CFB), which operates at higher gas velocities, allowing continuous operation with effective solid recirculation, thus increasing throughput and control; dual fluidized bed (DFB), which separates pyrolysis and combustion zones into different chambers, improving temperature control and optimizing the yield of bio-oil and syngas.

BFB reactors typically process biomass particles smaller than 2–3 mm and are known for their ability to produce high-quality bio-oil, with liquid yields reaching approximately 70–75% of the dry biomass input. In these systems, biochar is rapidly removed from the bed, avoiding accumulation [68].

As illustrated in Figure 11, the fluidized bed reactor utilizes a high-velocity stream of inert gas (e.g., nitrogen or helium) to suspend finely ground biomass particles. The bed material, often composed of inert substances like sand, silica, or alumina, behaves as a fluid under gas flow, promoting intense mixing and thermal uniformity [69,70]. A distributor plate at the reactor’s base ensures even gas distribution and prevents channeling or dead zones, thus supporting consistent reaction conditions and high product selectivity [71].

Common bed materials used in FBRs include sand, alumina, and olivine, selected for their thermal stability, density, and inertness. Olivine, particularly, has gained attention due to its catalytic properties that can improve bio-oil quality by cracking heavier compounds and reducing tar formation [22].

Fluidized bed reactors excel in processing a broad range of biomass feedstocks, including agricultural residues, forestry waste, and energy crops, if particle size and moisture content are appropriately controlled. Their high heat and mass transfer rates facilitate rapid and uniform pyrolysis, resulting in enhanced yields and product consistency.

These reactors are commonly operated in continuous mode, allowing for steady biomass feeding and the simultaneous extraction of pyrolysis vapors and solid residues. However, maintaining stable operation requires precise control of fluidization parameters. In particular, the flow rate of the carrier gas must be carefully tuned to sustain stable fluidization, as deviations can lead to poor suspension or excessive particle entrainment [72]. These requirements contribute to increased operational complexity and maintenance demands, which can elevate costs and constrain the suitability of FBRs for decentralized or small-scale applications.

Moreover, the physical characteristics of the biomass, particularly particle size, are critical for achieving uniform heating and minimizing thermal gradients. Despite their advantages, FBRs also face technical challenges, including reactor wall erosion from continuous particle impact; the need for complex gas-solid separation systems to recover entrained particles; sophisticated instrumentation for reliable operation. Such challenges necessitate advanced materials and process control strategies, potentially limiting the deployment of FBRs in resource-constrained environments.

The operational complexity and maintenance requirements of fluidized bed reactors may increase costs and limit their suitability for small-scale or decentralized contexts. Nonetheless, their scalability and flexibility make them well-suited for integration into modern biorefineries focused on valorizing diverse biomass streams and maximizing biofuel production efficiency. Advanced fluidized bed designs sometimes incorporate external cyclones or electrostatic precipitators to enhance particle separation efficiency, as well as heat recovery systems to improve overall energy efficiency.

From a sustainability perspective, fluidized bed reactors offer high energy efficiency and potential for integration with heat recovery and emissions control systems. Their ability to convert heterogeneous biomass feedstocks supports circular economy goals by utilizing agricultural and forestry residues that might otherwise be discarded or burned. However, the energy demand for maintaining fluidization, along with the necessary feedstock preprocessing such as drying and milling, can impact the overall process economics and carbon footprint. Optimization of these factors through process integration and renewable energy use is vital to enhance the sustainability of FBR-based pyrolysis.

Fluidized-bed reactors, particularly bubbling and circulating types, have reached a TRL of 6–8, with successful demonstrations at pilot and industrial scales for fast pyrolysis [29,31,32].

4.3. Entrained Flow Reactors

The entrained flow reactor, also known as a drop tube furnace, is a highly advanced pyrolysis system widely used for decades in converting coal and biomass into energy fuels. It consists of an externally heated vertical tube through which hot laminar gas flows, thermally decomposing finely pulverized biomass feedstock, typically with particle sizes around 100 µm [73]. The reactor is generally operated isothermally, with the gas temperature controlled by the reactor wall, ensuring uniform heating rates, temperature, and residence times throughout the reactor tube.

In biomass pyrolysis applications, dried feedstock is introduced through a feeder system that precisely controls the particle feed rate. A primary, preheated transport gas conveys the biomass particles into the reactor, with particle velocity regulated by the gas flow rate. To prevent premature contact between biomass particles and hot gases, a secondary gas stream (usually nitrogen) is injected through the hopper and screw feeder. Additionally, a pressure regulation unit is incorporated to maintain optimal operating conditions [74].

The reactor tube is usually constructed from high-temperature resistant materials such as quartz, alumina, or silicon carbide, which provide excellent thermal stability and chemical resistance at temperatures exceeding 1000 °C. The choice of reactor lining material is critical for minimizing corrosion, erosion, and thermal stress during continuous operation. Entrained flow reactors require sophisticated feeding systems capable of precisely metering ultra-fine biomass particles and maintaining steady flow to ensure uniform pyrolysis conditions [22].

Heat transfer in entrained flow reactors is primarily by conduction through the reactor walls and convective heat transfer from the hot gas flow, enabling extremely rapid heating rates. Rapid heating combined with short residence times helps minimize secondary reactions such as tar cracking, improving product selectivity toward desired syngas or bio-oil fractions.

Due to the very fine particle size requirement, feedstock preparation often involves milling and drying steps, which can increase operational complexity and costs. Moreover, ash management is critical because high temperature can lead to slagging, fouling, or reactor plugging if the ash melting characteristics are not compatible with the reactor design. To mitigate this, careful selection of feedstock and sometimes additives or bed materials is needed.

Entrained flow reactors (Figure 12) operate at elevated temperatures, often exceeding 1000 °C and reaching up to 1500 °C, achieving extremely rapid heating rates on the order of 10,000 °C/s [75]. Combined with short residence times of approximately 5 s [76] and finely pulverized feedstock, these conditions enable maximum conversion efficiency.

Based on ash handling, the reactors can be classified as slagging or non-slagging: slagging reactors melt ash into liquid slag that exits the system, while non-slagging types are better suited for biomass with low ash content. The design of entrained flow reactors allows uniform temperature distribution and efficient heat transfer, optimizing the conversion of solid biomass into gaseous and liquid products. These features enable processing of diverse biomass types at high pressures and temperatures [77], making the technology suitable for both laboratory-scale pyrolysis studies and scalable industrial applications.

Entrained flow reactors demonstrate significant potential for coupling with downstream gas cleaning and catalytic upgrading units due to the high quality and consistent composition of pyrolysis vapors produced. The extremely short residence times and high temperatures facilitate production of syngas rich in H₂ and CO, which can be further valorized in Fischer–Tropsch synthesis or chemical feedstock production.

However, the intensive feedstock preparation requirements and elevated thermal management demands result in high energy consumption, potentially reducing the net energy efficiency and economic feasibility of entrained flow pyrolysis. This limits its applicability primarily to industrial facilities with access to high-value coproduct markets or integrated biorefinery operations. These factors currently restrict the widespread deployment of entrained flow pyrolysis systems to specialized industrial or research settings with sufficient capital and technical expertise.

Entrained-flow reactors are mostly used for gasification but have been adapted for pyrolysis in advanced systems. Their technology readiness level typically ranges between TRL 6 and 7, with some pilot- and demonstration-scale units available [29,31,32]. Future research and development efforts should focus on improving feedstock preprocessing technologies, optimizing ash management strategies, and enhancing reactor materials to extend operational lifespan and reduce maintenance costs, thus enabling broader commercial viability.

4.4. Rotary Kiln Reactors

The rotary kiln reactors have long been employed in incineration and are pivotal in biomass pyrolysis due to their adaptability and efficiency in processing a wide range of feedstocks, including wood, plastics, and waste tires [78]. The main types include both internally and externally heated designs, which use various heating methods such as steam, gas, or electric systems, each adapted to specific pyrolysis needs [79]. Typically operating in the 320–500 °C range, these reactors ensure short residence times for solids, while allowing adjustable residence durations for gases and pyrolysis vapors.

Enhanced heat transfer is achieved through an internally heated screw mechanism, purged with nitrogen, and the use of metal spheres that improve thermal conduction. The rotary kiln is usually slightly inclined, enabling uniform axial temperature distribution and supporting continuous feedstock flow, features that make it ideal for industrial-scale applications [80].

The design tolerates a wide variety of biomass particle sizes and shapes, minimizing the need for pre-treatment or size reduction [81]. Operational control is relatively straightforward, and recent innovations have aimed to improve throughput while reducing energy demands. Furthermore, the reactor’s gentle mixing promotes char flowability and reduces oxidation, enhancing its suitability not only for standalone pyrolysis but also as a pretreatment stage in multi-step gasification.

Common materials for kiln shells and internal components include high-grade stainless steel or refractory-lined steel to withstand abrasion and corrosive pyrolysis gases. The use of metal spheres or ceramic balls as bed material improves heat transfer by increasing contact surface and mixing efficiency. Temperature sensors and rotational speed controls allow precise process optimization, while seals at kiln ends prevent air ingress, maintaining an inert atmosphere inside [22].

Rotary kiln reactors provide robust performance for heterogeneous feedstocks, including those with higher moisture content and irregular particle size distributions, thereby reducing the need for preprocessing and associated energy consumption. This flexibility makes rotary kilns especially suitable for decentralized applications where feedstock variability is high and preprocessing infrastructure is limited. Their mechanical design supports continuous operation with relatively low maintenance, making them well-suited for decentralized and industrial biochar production. The inherent capability to control residence time of solids independently from gas phase allows better tuning of product distributions, enabling the optimization of yields for biochar, bio-oil, or syngas, depending on operational goals.

Despite these advantages, rotary kilns generally have slower heating rates compared to fluidized or entrained flow reactors, which may limit bio-oil yield and quality in certain applications. This slower heating can lead to increased secondary reactions, potentially affecting product stability and requiring further downstream upgrading for liquid fractions. However, their durability and operational flexibility compensate for these constraints in many industrial scenarios.

As a result, rotary kilns are widely acknowledged for their role in efficient char production and the thermal conversion of particulate biomass. Figure 13 illustrates a typical rotary kiln setup for biomass pyrolysis.

Rotary kiln reactors are generally used for slow and intermediate pyrolysis processes and operate at a TRL of 7–8, depending on the design and targeted end-products [29,31,32]. Nonetheless, challenges related to energy efficiency and emissions control remain areas for ongoing research, especially in optimizing kiln insulation and integrating heat recovery systems to improve overall process sustainability.

4.5. Ablative Reactors

Ablative pyrolysis reactors are generally categorized into two principal designs: the ablative vortex reactor and the ablative rotating disk reactor. In both configurations, biomass undergoes thermal decomposition through direct contact with a heated reactor surface, without the involvement of a carrier or fluidizing gas [68]. Heat is transferred via a molten or softened biomass layer formed on the hot reactor wall, and pyrolysis is initiated by mechanically pressing the biomass against this surface. The motion induced by the reactor, typically rotational or vortex flow, facilitates rapid thermal degradation, leading to the release of pyrolysis vapors that are subsequently condensed into bio-oil [83].

One of the key advantages of ablative reactors is their ability to handle relatively large biomass particles (up to 20 mm, including biomass pellets), thereby reducing the need for extensive grinding and pretreatment [84]. However, the mechanical complexity of these reactors poses engineering and maintenance challenges. Heat transfer in ablative systems is limited to the contact surface area, which inherently constrains scalability. Unlike volumetric heating systems, the scale-up of ablative reactors tends to be linear and less efficient, which restricts their industrial feasibility [66]. Despite their limitations, ablative pyrolysis reactors (Figure 14) provide high heating rates, efficient thermal contact, and minimal dependence on inert gases. These features make them particularly suitable for specialized or decentralized applications where limited feedstock preprocessing and process simplicity are key priorities.

Materials used for the heated surfaces in ablative reactors are typically high thermal conductivity metals such as copper or aluminum alloys, sometimes coated with refractory or anti-corrosive layers to withstand high temperatures and abrasive biomass feedstocks. The mechanical pressing mechanism requires robust design to ensure continuous and uniform contact, while minimizing wear and maintenance intervals [22].

Additional operational benefits include reduced energy consumption due to direct heat transfer and elimination of carrier gas usage, potentially lowering GHG emissions associated with inert gas generation and handling. However, the abrasive nature of biomass feedstock causes mechanical wear that demands frequent maintenance, increasing downtime and operational costs. Feedstock heterogeneity can affect surface heat transfer efficiency, requiring consistent biomass quality or adaptive control strategies to maintain stable reactor performance.

Precise control of rotational speed and pressure applied to the biomass is essential, as these parameters directly influence residence time and heat transfer efficiency. Cooling systems may be integrated to prevent overheating of mechanical components, and effective sealing is critical to avoid air ingress that could cause unwanted oxidation. Given the high mechanical stresses, ablative reactors also require specialized equipment to detect wear and ensure operational safety, adding further to system complexity and maintenance requirements.

Due to their high heat transfer efficiency but complex design, ablative pyrolysis systems remain mostly at TRL 4–5, with limited large-scale applications [29,31,32]. Future research efforts should focus on improving material durability, reducing maintenance frequency, and developing adaptive control strategies to enhance the scalability and reliability of ablative reactors for broader commercial adoption.

4.6. Auger Reactors

Among the various reactor configurations employed in biomass pyrolysis, auger reactors are mechanically driven system that enable continuous operation without the need of using fluidizing gases. Their compact design and relatively straightforward operational requirements make them particularly suitable for decentralized or small-scale applications. They can process a variety of lignocellulosic feedstocks, including wood chips, nutshells, agricultural residues, and pelletized or ground biomass, provided that particle size and moisture content are adequately controlled.

An auger-based conveying mechanism typically consists of a rotating helical screw enclosed within a sealed casing, designed to transport granular materials efficiently. In addition to feeding biomass into the reactor and extracting the resulting solid residues, a well-designed auger can significantly enhance particle mixing and facilitate efficient heat exchange between solid heat carriers and biomass particles [86]. Notably, auger reactors offer improved axial dispersion, promoting a more uniform thermal residence time for the feedstock along the length of the reactor [87].

The auger pyrolysis reactor illustrated in Figure 15 features a horizontally oriented cylindrical chamber containing the rotating screw (auger), which is powered by an external motor. Biomass feedstock is introduced at one end of the reactor and transported along the heated reactor walls by the auger, enabling uniform thermal decomposition under controlled conditions, typically around 400 °C [87]. During this process, volatile compounds are released and extracted through vapor outlets positioned in the upper section of the reactor, while the solid residue, comprising primarily char and ash, is discharged from the opposite end [88,89].

This configuration is particularly suited for continuous processing and allows for modular integration into decentralized biomass conversion systems in rural or agro-industrial settings.

Materials commonly used for the reactor shell and auger include stainless steel or other corrosion-resistant alloys that withstand high temperatures and erosive biomass particles. The reactor walls are often lined or jacketed with heating elements or use indirect heating methods to maintain precise temperature control. Additionally, some designs incorporate solid heat carriers such as sand or ceramic beads to improve heat transfer efficiency [22].

Challenges associated with auger reactors include mechanical wear of the screw caused by abrasive biomass particles, which requires the use of durable materials or coatings. Furthermore, vapor residence time control is critical, as prolonged exposure to high temperatures can lead to secondary cracking, reducing liquid yields and altering product composition. Advanced designs may integrate rapid vapor removal systems or optimized reactor geometry to mitigate these effects.

Despite the auger reactor’s mechanical simplicity and operational convenience, several technical limitations remain. These include relatively long vapor residence times, which may favor secondary cracking reactions, ultimately decreasing bio-oil yield and quality while promoting increased char formation [90]. Furthermore, scaling up auger reactors is hindered by suboptimal heat transfer characteristics, particularly in larger units, where maintaining uniform temperature profiles across the reactor becomes progressively more difficult. In addition, variations in feedstock properties such as ash content, bulk density, and grindability can significantly affect the reactor’s performance and product distribution, highlighting the need for pre-treatment or feedstock standardization in some cases. Moreover, maintenance considerations related to wear and potential clogging must be factored into operational planning to ensure reactor longevity and minimize downtime.

Due to their compact configuration and suitability for decentralized biomass conversion, auger pyrolysis reactors have reached technological readiness levels typically between TRL 6 and 8, depending on system design and feedstock compatibility [29,31,32]. Their moderate capital and operational costs, combined with the ability to handle distributed biomass resources, make them attractive for on-site energy generation, soil amendment (via biochar), and decentralized biorefining applications. Ongoing research aims to optimize reactor geometry and heating strategies to enhance heat transfer uniformity and reduce secondary cracking, thereby improving bio-oil yield and quality at larger scales.

4.7. Rotating Cone Reactors

Among the advanced pyrolysis reactor designs, the rotating cone reactor (Figure 16) stands out due to its compact structure and enhanced heat transfer performance. This system utilizes centrifugal forces to create a swirling flow of solid particles, significantly improving both heat and mass transfer rates. Biomass feedstock, typically ground to a fine particle size, is introduced in the conical chamber that rotates at high speed, driven by an electric motor [89]. External heaters apply thermal energy to the cone walls, and as the cone spins, the biomass is rapidly transported along the heated surface, ensuring short vapor residence times and rapid thermal decomposition [67]. The pyrolysis vapors, including condensable volatiles and permanent gases, exit the reactor through the top section, while the solid char and ash fractions are simultaneously expelled.

Materials used for the rotating cone and reactor housing are typically stainless steel or other heat-resistant alloys, often coated to minimize fouling and corrosion due to biomass ash and tars. The reactor’s rotational speed and temperature control systems are critical for maintaining optimal heat transfer and preventing biomass agglomeration on the hot surfaces. Advanced designs may include automated cleaning systems or vibrations to reduce wall buildup [22].

In terms of feedstock adaptability, rotating cone reactors are generally limited to finely ground (often <1 mm particle size to ensure smooth flow) and dry lignocellulosic biomass, which may restrict their applicability when dealing with wet, fibrous, or highly heterogeneous raw materials. Moreover, the precise control over residence time and temperature makes this configuration well-suited for high-value bio-oil production, especially in research or pilot contexts where rapid analysis and optimization are required. One notable advantage of this system is the absence of a need for an inert heat carrier, such as sand, thereby simplifying the process and reducing operational costs [91]. However, the need for extremely fine and homogeneous feedstock may increase preprocessing energy and costs, which should be considered in lifecycle assessments.

Despite its benefits, the rotating cone reactor also faces key limitations. These include restricted feed capacity, primarily due to the necessity of fine and homogeneous biomass particles, and difficulties in thermal scale-up, as maintaining uniform temperature profiles across the rotating cone surface becomes increasingly complex in larger systems. Additionally, particle agglomeration and wall fouling may occur, especially under suboptimal moisture or ash content, impacting both product quality and mechanical operation. These challenges currently restrict their widespread adoption in industrial-scale continuous pyrolysis processes. Operational challenges such as wear of moving parts and the need for frequent maintenance or cleaning cycles may also impact reactor availability and overall economics.

Nevertheless, due to their modular structure, rotating cone reactors are relatively easy to integrate into compact pilot plants or mobile units, making them particularly attractive for decentralized biomass valorization schemes.

Rotating cone reactors, primarily used for fast pyrolysis of finely ground biomass, have achieved TRLs in the range of 5 to 7, with several pilot-scale systems developed and tested, though commercial deployment remains limited [29,31,32].

4.8. Vacuum Reactors

Vacuum pyrolysis reactors operate under slow pyrolysis conditions, characterized by low heating rates and extended residence times, which favor the formation of liquid and solid products [66]. This technique enables the thermal decomposition of organic materials at lower operating temperatures, thus reducing the overall energy input required for the process. The process supports the use of larger biomass particle sizes, thereby reducing the extent of feedstock pretreatment required.

Moreover, vacuum pyrolysis can accommodate a diverse range of lignocellulosic feedstocks, including forestry residues, agricultural waste, and industrial biomass, without extensive pre-processing. This flexibility enhances its potential for valorizing locally available and low-cost raw materials.

In the configuration illustrated in Figure 17, biomass is fed into a heated vacuum chamber via a metal conveyor belt that incorporates periodic mechanical agitation, facilitating improved heat transfer and uniform contact between particles [92]. Thermal energy is supplied through an external burner, while inductive heating using molten salts as heat carriers may also be applied to allow precise temperature control and enhance overall thermal efficiency.

Operating under reduced pressure, vacuum pyrolysis suppresses oxidation reactions and limits secondary cracking, which can improve the quality of the resulting bio-oil. The bio-oil obtained via vacuum pyrolysis typically contains polycyclic macromolecular compounds and has a yield of 35–50 wt%, with a relatively high heating value ranging from 22.4 to 40 MJ/kg [93]. The solid fraction, primarily biochar, retains significant carbon content and can be further utilized as a soil amendment or solid fuel.

These characteristics make vacuum pyrolysis particularly suitable for producing high-quality bio-oil from thermally sensitive feedstocks, while also generating biochar with enhanced carbon sequestration potential.

The vacuum environment also reduces the partial pressure of volatile compounds, facilitating their removal at lower temperatures and thus preventing thermal degradation of sensitive molecules. Reactor construction requires robust sealing systems and vacuum pumps capable of maintaining stable, low-pressure conditions throughout operation. The feeding and discharge systems must be carefully designed to preserve vacuum integrity, often involving lock hoppers or rotary valves. Additionally, materials used must resist vacuum-induced mechanical stresses and thermal cycling. However, the complexity and cost of maintaining vacuum integrity and specialized feed/discharge equipment increase operational challenges and limit economic viability, especially at larger scales.

Nevertheless, the technical complexity of the system, including the need for specialized feeding and discharge mechanisms that maintain vacuum integrity, leads to elevated capital and maintenance costs. These factors currently hinder the economic feasibility of vacuum pyrolysis in large-scale applications, where simpler and more scalable reactor designs are generally preferred. As a result, its practical applicability remains limited compared to other pyrolysis configurations, especially in decentralized or resource-constrained settings.

Vacuum pyrolysis reactors typically operate at TRLs between 4 and 6, being mostly confined to laboratory and pilot-scale applications focused on maximizing liquid product recovery under reduced pressure conditions [29,31,32]. Despite these limitations, continued research into material compatibility, heat transfer enhancement, and vacuum system efficiency may enable broader adoption of this method for niche applications demanding high-value liquid fractions or thermally sensitive biomass processing. Future developments might also explore hybrid systems combining vacuum pyrolysis with other reactor types to optimize product yields and process economics.

4.9. Plasma Reactors

Plasma pyrolysis is an advanced thermochemical process that utilizes extremely high temperatures generated by plasma arcs to convert biomass into valuable gases and solids. The reactor configuration typically comprises a cylindrical quartz tube fitted with copper electrodes at each end [94]. Biomass feedstock is introduced axially near the center of the reactor via a variable-speed screw feeder, allowing accurate control of the feeding rate and ensuring uniform exposure to the plasma zone.

Inside the reactor (Figure 18), plasma energy is generated by applying a high-voltage electric current across the electrodes, which ionizes the working gas to form a highly energetic plasma field. Inert gases, such as argon or nitrogen, are introduced into the system to purge atmospheric oxygen and maintain an oxygen-free environment, a condition that is critical both to sustain plasma formation and to prevent unwanted combustion reactions [95].

The extreme temperatures in the plasma zone, often exceeding 3000 °C, ensure instantaneous pyrolysis and gasification of the biomass, which significantly limits tar formation, a common drawback in conventional slow pyrolysis methods [66]. The high energy density also enables effective cracking of heavy hydrocarbons and promotes the formation of clean syngas, rich in H₂ and CO.

The intense thermal environment allows for the complete conversion of complex or hazardous organic feedstocks, including sewage sludge, plastic-rich waste, or biomedical residues, which are often unsuitable for conventional reactors. This makes plasma reactors particularly suited for decentralized or mobile waste-to-energy units, where robustness and environmental safety are prioritized over energy efficiency. Despite their high electricity demands and complex operational requirements, plasma pyrolysis systems effectively treat heterogeneous, wet, or hazardous feedstocks, making them valuable for specialized waste-to-energy applications and the destruction of toxic organic compounds. Due to minimal solid residues and high gas yields, plasma reactors also offer potential for near-zero-emission processing, supporting circular economy objectives in specific industrial contexts.

Key technical considerations include the need for durable electrode materials resistant to high temperatures and plasma erosion, as well as robust cooling systems to prevent overheating [22]. Reactor design must ensure stable plasma arc formation and efficient gas flow to maximize conversion efficiency. The high power consumption and capital costs restrict plasma pyrolysis mainly to niche or pilot applications, although advances in electrode materials and cooling technologies may help improve operational reliability and reduce maintenance intervals. The near-complete destruction of organics and the production of low-ash, non-leachable residues provide significant environmental benefits, especially when treating persistent pollutants. Furthermore, coupling with renewable power sources (e.g., solar or wind) could mitigate energy concerns and improve overall sustainability of these systems.

Plasma pyrolysis reactors are currently at low technological readiness levels (TRL 3–5), with ongoing research efforts focused on niche applications such as hazardous waste treatment and high-value material recovery [29,31,32]. Future developments may explore hybrid plasma-pyrolysis systems or integration with catalytic upgrading to enhance product selectivity and energy efficiency.

4.10. Microwave Reactors

Microwave pyrolysis represents a cutting-edge advancement in thermochemical biomass conversion, offering significant improvements in efficiency, selectivity, and scalability compared to conventional heating methods. Unlike traditional systems that rely on external heating followed by thermal transfer through conduction, convection, or radiation, microwave pyrolysis employs volumetric heating, a process in which electromagnetic energy interacts directly with the biomass. This occurs via the excitation of polar molecules and free ions within the feedstock, resulting in rapid and uniform internal heating [96].

This volumetric heating mechanism significantly reduces temperature gradients inside biomass particles, facilitating a more homogeneous thermal decomposition. Moreover, microwave heating allows for more rapid heating rates and shorter start-up times, enhancing operational responsiveness. The process takes place in a sealed microwave chamber connected to an electrical power source. Biomass is introduced into the reactor, while an inert carrier gas such as nitrogen or argon is continuously supplied to displace oxygen and maintain an oxygen-free environment, thereby preventing undesired combustion reactions and preserving process selectivity.

One of the primary advantages of microwave pyrolysis is its high energy efficiency, as thermal losses are minimized compared to conventional systems where heat must be transferred indirectly to the feedstock [97]. Furthermore, this technology allows for precise control over heating rates and target temperatures, which enhances the reproducibility and tunability of product yields. Shorter residence times for volatiles also favor the production of gaseous products and minimize secondary reactions that lead to tar formation [98]. Additionally, the rapid heating achieved in microwave systems can improve product selectivity by limiting the formation of unwanted by-products.

A unique feature of microwave reactors is the formation of localized micro-plasma regions, or “hot spots”, where temperatures can momentarily exceed the average bulk temperature. These regions can enhance cracking reactions and potentially stimulate in situ catalytic activity, promoting hydrogen production and increasing gas-phase yields [99]. These micro-plasmas can also facilitate radical-driven reactions, contributing to the formation of novel chemical species.

To further optimize the process and improve selectivity, microwave-assisted catalytic pyrolysis has been widely studied. Catalysts such as zeolites (notably ZSM-5 and HZSM-5), metal oxides (e.g., Al₂O₃, CaO, MgO, Fe₂O₃, TiO₂), and carbon-based materials (e.g., activated carbon, graphite) are commonly employed. These materials enhance the breakdown of heavy organics and favor the formation of target compounds, including syngas and light aromatics [100,101,102]. The interaction between microwave fields and catalyst materials can also influence reaction mechanisms, potentially enabling more efficient conversion pathways.

Microwave pyrolysis has shown strong potential for chemical recovery, particularly in the selective production of high-value compounds such as phenolics, furans, and levoglucosan. This makes the technology especially attractive for biorefinery applications and circular economy models. Recent technological advancements have enabled scaling microwave reactors from laboratory to pilot and industrial applications, with increasing interest in integrating them into waste valorization and bioenergy production systems. However, challenges such as limited microwave penetration depth and non-uniform field distribution remain significant barriers to large-scale deployment. The limited penetration depth of microwaves can cause non-uniform heating, especially in larger reactor volumes, leading to hot spots or cold zones that affect product quality and yield consistency.

Materials used in microwave reactors are typically composed of microwave-transparent ceramics or quartz for the reaction chamber, ensuring efficient penetration of microwave radiation [22]. The design must consider uniform microwave field distribution to avoid uneven heating and localized overheating. Power modulation and sensor feedback loops are critical for process control. The integration of catalyst beds inside the microwave cavity requires careful optimization to prevent catalyst sintering and ensure effective microwave absorption.

A schematic representation of a typical microwave-assisted pyrolysis reactor used for biomass conversion is provided in Figure 19.

Further research into advanced microwave applicator designs, such as multimode cavities or continuous flow reactors with optimized waveguides, could improve uniformity and scalability. Additionally, combining microwave heating with other energy inputs (e.g., conventional or inductive heating) may overcome some penetration limitations. Microwave-assisted pyrolysis is an emerging technology still at a low TRL (3–5), primarily under laboratory research or early pilot testing [29,31,32]. Continued development and demonstration of microwave pyrolysis at larger scales will be essential to assess operational reliability, energy efficiency, and economic viability.

4.11. Solar-Driven Reactors

Solar-driven pyrolysis reactors offer a sustainable and environmentally friendly alternative for the thermochemical conversion of biomass, using concentrated solar radiation as the sole external heat source. Unlike conventional pyrolysis systems, where part of the feedstock is combusted to generate process heat, solar reactors eliminate the need for fossil-based or internal combustion-derived energy inputs, thus improving both energy efficiency and product purity [104].

A standard solar pyrolysis system integrates a quartz or ceramic tubular reactor, often enclosed in opaque, heat-resistant external walls, into a solar concentrator setup, such as a parabolic dish or heliostat field, which directs and focuses solar radiation onto the reactor surface (Figure 20). The concentrated solar flux can raise the internal reactor temperature above 700 °C, enabling pyrolysis in the fast or intermediate regime [105]. This high-temperature environment facilitates rapid biomass decomposition and conversion into valuable products such as biochar, bio-oil, and syngas.

Solar-driven pyrolysis is compatible with a wide range of lignocellulosic biomass feedstocks, including agricultural residues, forest waste, and organic industrial by-products, provided they are sufficiently dry and of relatively uniform particle size.

The external, non-combustive heating method used in solar reactors significantly reduces pollutant emissions, particularly NO_x, sulfur compounds, and particulate matter, while also preventing dilution of pyrolysis vapors with flue gases [66]. The resulting products are of higher purity, and the process becomes more environmentally benign. However, the dependency on direct solar irradiation limits the applicability of these reactors to regions with high solar availability and require sophisticated solar tracking systems and thermal storage to ensure consistent performance. The intermittency of solar energy inherently challenges continuous operation; thus, integrating efficient thermal energy storage and hybrid heating approaches is crucial to overcoming these limitations and achieving industrial relevance.

In terms of operational advantages, solar-driven systems exhibit a rapid thermal response, characterized by faster startup and shutdown times compared to traditional slow pyrolysis reactors [106]. Moreover, when integrated with thermal energy storage units, such as molten salt baths or phase-change materials, these reactors gain additional operational stability and continuity, even during fluctuating solar irradiance conditions [107].

Figure 20. Schematic diagram of a solar-driven reactor setup for biomass pyrolysis (adapted from [108], under the terms of the Creative Commons Attribution License (CC BY 4.0)).

Figure 20. Schematic diagram of a solar-driven reactor setup for biomass pyrolysis (adapted from [108], under the terms of the Creative Commons Attribution License (CC BY 4.0)).

Despite their high energy efficiency and emission-free operation, solar pyrolysis systems face challenges related to scalability, system complexity, and capital costs associated with solar concentrators and specialized reactor materials. Their practical applicability is currently limited to pilot-scale setups or laboratory environments, and the requirement for real-time solar flux control adds further technical barriers to widespread implementation.

Recent innovations also explore hybrid solar configurations, which combine solar input with auxiliary heating technologies such as microwave or resistive electric heating. These hybrids enable fine-tuned control of reaction kinetics, improve heating uniformity, and enhance overall system efficiency and flexibility [109,110].

Materials used in solar-driven reactors must withstand intense thermal cycling and concentrated solar flux; thus, high-purity quartz or advanced ceramics are preferred for reactor tubes due to their high thermal shock resistance and transparency to solar radiation. The concentrator optics require precise alignment and cleaning to maintain efficiency. Control systems must dynamically adjust auxiliary heating and feed rates to compensate for solar intermittency. Materials and components must also address long-term durability under cyclic thermal stresses to reduce maintenance costs and downtime in operational settings. Additionally, the large physical footprint and land requirements for solar concentrator fields may constrain site selection.

By maximizing biomass utilization, minimizing emissions, and eliminating fossil-derived process heat, solar pyrolysis reactors contribute significantly to the development of renewable, low-carbon technologies for biomass valorization and support the global transition to circular and sustainable energy systems. When deployed strategically in decentralized settings, especially in remote or off-grid areas with abundant solar resources, these reactors could support local energy generation and waste management with minimal environmental impact.

Solar-driven pyrolysis reactors are at early technological readiness levels (TRL 2–4), as they remain largely experimental and are primarily explored in academic research for sustainable and carbon-neutral biomass valorization [29,31,32]. Further research and pilot demonstrations are needed to validate system integration, optimize thermal storage solutions, and lower capital costs for viable commercial adoption.

Table 2. Strengths and limitations of different pyrolysis reactors [authors’ own elaboration].

Reactor Type	Strengths	Limitations	TRL
Fixed-bed	Simple and robust design. Low capital and operational costs. High char yield. Biomass size flexibility. The thermal bed material must be resistant to high temperatures and provide good thermal conductivity, for example sand or alumina.	Low heat transfer efficiency. Long residence time. Poor scalability. Difficult char removal. Uneven temperature profiles. Batch operation limits productivity.	8–9
Bubbling fluidized bed	Simple design and operation. Excellent heat and mass transfer. Uniform temperature distribution. Suitable for continuous operation. Commercially (large scale) proven for bio-oil production.	Requires small, uniform particles. Risk of bed agglomeration. Char removal is difficult. High gas flow rates are needed. Erosion of internals may occur over time. Fluidized bed materials must resist erosion caused by particles and high temperatures.	6–8
Circulating fluidized bed	Mature technology. Better heat control than bubbling bed. Suitable for higher throughput, large particle size. Enhanced mixing of solids and gases.	More complex design. Char particles may be too fine. Not suitable for large production. Higher operational complexity. Higher maintenance requirements.	6–8
Rotary kiln	Can handle a wide range of biomass types and sizes. Suitable for continuous operation. Good temperature control. Robust and scalable design. Tolerant to feedstock impurities.	Lower heat transfer efficiency. Requires longer residence time. Potential for incomplete pyrolysis. Difficult gas sealing. Larger footprint. Slow heating rates.	7–8
Ablative	No inert gas is required. Can process larger biomass particles. Moderate temperature control. High heating rate. Requires materials with high resistance to mechanical wear and high temperatures, often special alloys or composite materials.	High mechanical complexity. Requires high pressure contact. Costly reactor design. Limited scalability due to mechanical constraints.	4–5
Auger (screw)	Compact design. Continuous operation. No need for inert gas. Process temperature is low. Good for medium-scale use.	Limited scale-up potential. Wear of moving parts in hot zone. Poor heat transfer at large scale. Feedstock must be dry and pretreated.	6–8
Rotating cone	No need for carrier gas. Fast heat transfer. Efficient contact between biomass and heat carrier.	Requires finely ground feedstock. Complex mechanics. Challenging to scale up. Sensitive to particle size variability.	5–7
Vacuum	Cleaner bio-oil (less oxygenated, it does not contain biochar). Allows larger biomass particles. Operates at lower temperatures. Carrier gas is not required.	Slow pyrolysis rates. Poor heat transfer. Long residence time. Water consumption is high. Higher moisture in products. Bulky equipment. High operational cost.	4–6
Plasma	Extremely high temperature. High reaction rates. Energy density is high. Effective for hazardous waste destruction.	Very high energy consumption. High operating costs. Fine biomass particles are required. Complex safety and shielding systems needed.	3–5
Microwave	Rapid and volumetric heating. Uniform temperature distribution. Compact and modular design. It allows for volumetric internal heating, which reduces temperature gradients and optimizes the process.	High power demand. Limited industrial experience. Expensive components. Difficult to control heating homogeneity at large scale.	3–5
Solar-driven	Renewable heat source. Low emissions. Suitable for remote areas.	Weather-dependent. High initial cost. Lower controllability of process conditions. Requires thermal storage or hybridization for continuous use.	2–4

, compiled by the authors based on information synthesized from relevant sources in the literature, such as [29,31,66,111], provides a comparative overview of the most used pyrolysis reactor types, namely fixed-bed, fluidized-bed, rotating cone, ablative, auger (screw), vacuum, microwave, plasma, and solar reactors, highlighting their main advantages, disadvantages, and technology readiness levels (TRLs).

To ensure efficient pyrolysis, two key parameters must be carefully addressed during reactor design: achieving effective heat transfer to the biomass feedstock and maintaining a short vapor residence time. One of the main challenges lies in delivering thermal energy efficiently not only to the bulk of biomass particles but also to the reactor vessel itself.

In conventional pyrolysis processes, heat is generated externally and transferred to the biomass via convection, conduction, and radiation. However, the efficiency of this heat transfer is limited by surface temperature constraints and the thermal properties of the material. As heat moves inward from the particle surface, an internal temperature gradient forms, which can result in uneven heating and affect the quality and yield of the pyrolysis products [112].

Most reactor designs implement one or a combination of heat transfer mechanisms, as illustrated in Figure 21.

Direct heat transfer can be accomplished using a solid heat carrier (such as sand, metal, or ceramic beads) or a hot gas stream, which often also fluidizes the biomass particles. These heat carriers or gases are typically heated by combusting the solid residue from the pyrolysis process, commonly known as biochar. Materials used for heat carriers must combine high thermal conductivity with resistance to abrasion and chemical inertness to prevent contamination of pyrolysis products. Another effective heating method involves partial combustion inside the reactor itself by controlled air injection, enabling the maintenance of required temperatures without relying on external heating sources. However, this approach may complicate process control, increase local temperature gradients, and promote secondary cracking if air distribution is not uniform. Additionally, the oxidation of part of the biomass inevitably reduces liquid yields, which may be detrimental in applications prioritizing bio-oil production.

In contrast, indirect heat transfer occurs via the reactor walls or embedded heat exchangers, such as tubes or plates. These systems may be powered by various external heat sources, including hot gases, thermal oils, or electric heaters. Indirect heating allows better control over reaction temperature but may limit heating rates and uniformity compared to direct contact methods. The lower heating rates in indirect systems can favor char formation, while the need for large heat-transfer surfaces increases reactor size and capital cost. In industrial-scale units, maintaining uniform temperature profiles across large heat exchange areas remains a key engineering challenge. Furthermore, research gaps persist in optimizing wall and exchanger geometries for rapid heating of heterogeneous feedstocks and in developing coatings or materials that minimize fouling from condensable vapors.

The choice between direct and indirect heating, and the design thereof, depends on the reactor type, desired product distribution, feedstock characteristics, and operational scale, each presenting distinct trade-offs in terms of energy efficiency, product quality, and process complexity.

5. Key Factors Influencing Pyrolysis Performance

The efficiency, product distribution, and selectivity of biomass pyrolysis are significantly influenced by a range of interrelated process parameters. These include reactor configuration, the composition of the reaction atmosphere, final pyrolysis temperature, and vapor and solid residence times, as well as biomass-specific factors such as chemical composition, particle size, moisture content, and the presence of inorganic impurities. Other operational factors, such as heating rate and pressure, also play a pivotal role in determining the reaction pathways and product characteristics [23,64]. A comprehensive understanding of the influence of each parameter on the thermochemical transformation of lignocellulosic biomass is essential for optimizing the production yields and quality of biochar, bio-oil, and syngas.

5.1. Reactor Configuration

The design and configuration of the pyrolysis reactor are critical factors that influence heat transfer efficiency, residence time control, and, ultimately, product distribution and quality. Various reactor types are employed in biomass pyrolysis, including fixed-bed, fluidized-bed, rotary kiln and drum, auger (or screw), ablative, and microwave reactors [113].

Fixed-bed reactors are valued for their structural simplicity and are typically used in laboratory or small-scale applications; however, they suffer from limited heat transfer efficiency and scale-up challenges. In fixed-bed systems, the thermal gradient across the bed can lead to incomplete pyrolysis and non-uniform product composition. Additionally, the lack of solid mixing reduces heat penetration into the biomass core, especially for large particles. These limitations critically restrict the feasibility of fixed-bed reactors in industrial-scale applications, particularly where high throughput and uniform product quality are demanded. Moreover, the tendency for hotspot formation raises concerns over localized degradation, impacting bio-oil quality.

In contrast, fluidized-bed reactors are the most widely adopted design for commercial bio-oil production due to their superior heat and mass transfer characteristics. These systems ensure uniform temperature distribution and rapid heating of finely divided biomass particles, thereby enhancing reaction kinetics and yield [114]. Moreover, the intense solid mixing in the fluidized bed allows for better temperature control, minimizes local overheating, and supports catalyst integration in catalytic pyrolysis setups. However, fine particle entrainment in the gas stream requires efficient downstream separation. This entrainment not only complicates downstream processing but can also lead to operational inefficiencies and increased maintenance, particularly in long-term continuous operation. Furthermore, fluidized-bed reactors demand high energy input for fluidization, which may affect overall process sustainability.

Rotary kiln reactors, often used in continuous processing, offer improved control over residence time and feed handling flexibility, although they may exhibit lower heat transfer efficiency compared to fluidized systems [11]. Nevertheless, they are highly versatile for co-processing mixed waste and biomass and can operate under varied atmospheric conditions (inert, reducing, or oxidizing). The rotational motion also helps mitigate reactor fouling and channeling. Despite these advantages, the slower heating rates inherent to rotary kilns can negatively impact bio-oil yield and quality, which restricts their applicability in processes targeting liquid fuel maximization. Moreover, the mechanical complexity of kiln rotation and sealing can increase capital and operational costs.

Auger or screw reactors utilize mechanical conveyance to transport biomass through a heated zone, offering consistent feedstock flow and moderate temperature control, making them suitable for decentralized, medium-scale operations. Their compact design makes them ideal for mobile or modular pyrolysis units. However, mechanical wear of the screw and potential for clogging with sticky feedstocks are key limitations. Additionally, the relatively long vapor residence times associated with auger reactors can favor secondary cracking reactions, which lower liquid yields and alter product composition, thus demanding careful optimization of operational parameters.

Microwave reactors, though still under research and development, offer rapid volumetric heating and selective interaction with polar components in biomass, leading to unique product distributions and potential energy savings. However, challenges related to scale-up, equipment cost, and non-uniform heating remain significant barriers to commercialization. The inherent non-uniformity of microwave energy distribution and difficulties in achieving consistent temperature profiles at scale limit their current industrial viability, despite promising laboratory-scale results.

Each reactor type requires tailored optimization of operational parameters to achieve the desired balance among product yield, energy efficiency, and output quality. Key design considerations include reactor insulation, heating method (direct vs. indirect), feedstock properties (particle size, moisture, bulk density), and product recovery systems (condensers, cyclones, filters). Thus, the choice of reactor configuration plays a central role in the design and performance of biomass pyrolysis systems [22]. A critical evaluation of these factors must weigh trade-offs among operational complexity, scalability, and economic viability to guide reactor selection for specific biomass valorization goals.

5.2. Reaction Atmosphere

The composition of the reaction atmosphere plays a pivotal role in directing pyrolysis pathways and influencing product selectivity. Pyrolysis is typically carried out in an oxygen-free environment using inert carrier gases such as nitrogen (N₂), helium (He), or argon (Ar) to prevent combustion and maintain a controlled thermal environment. Although chemically inert, these gases can subtly affect reaction kinetics and the distribution of volatile products through their influence on heat and mass transfer dynamics. In particular, the choice of carrier gas impacts the residence time of volatiles, affecting secondary cracking and recombination reactions. However, the extent to which inert gases influence reaction pathways remains underexplored in many studies, potentially overlooking subtle but significant impacts on product yields and composition, which calls for more detailed mechanistic investigations.

From a practical standpoint, the choice of atmosphere must also balance availability, cost, and safety, especially when scaling the process to industrial levels. Nitrogen remains the most common carrier due to its low cost and widespread accessibility.

A detailed investigation by [115] explored the effects of different reaction atmosphere, including H₂O, N₂, H₂, CO₂, and CO, on the fluidized-bed fast pyrolysis of biomass. Through advanced computational fluid dynamics (CFD) simulations combined with a comprehensive chemical kinetic mechanism comprising 134 species and 4169 reactions, the study examined how each gas influences bio-oil yield, vapor-phase product distribution, turbulence intensity, and residence time. The findings underscore the critical role of gas-phase composition in steering primary decomposition mechanisms and optimizing the yield and quality of pyrolysis products. While such modeling efforts provide valuable insights, experimental validation under diverse feedstock and scale conditions is necessary to confirm these predictions and evaluate practical feasibility.

Additionally, recent experiments reviewed by [37] demonstrated that replacing inert nitrogen with CO₂ during slow pyrolysis significantly enhances biochar yield, increases carbon content and aromaticity, and reduces O/C and H/C ratios. This adjustment leads to superior biochar structural and agro-environmental properties compared to inert atmospheres. CO₂’s mild oxidative character enables partial gasification reactions, enhancing fixed carbon formation in the char matrix. Such results are particularly relevant for biochar production in carbon sequestration or soil amendment applications, where elevated aromaticity and stability are desired. Moreover, CO₂ valorization in this context could contribute to circular carbon utilization strategies. Nevertheless, the mildly reactive nature of CO₂ introduces process complexity, including potential reactor material corrosion and higher energy requirements due to endothermic reactions, which must be weighed against the benefits.

For instance, steam-assisted pyrolysis enhances the formation of oxygenated compounds such as aldehydes, carboxylic acids, and ketones by promoting biomass depolymerization and reduces secondary cracking reactions, thereby improving the liquid phase complexity [116,117]. Steam also acts as a hydrogen donor in certain thermal ranges, influencing tar reforming and improving the liquid phase complexity. Hydrogen-rich atmospheres, often used in catalytic pyrolysis or hydropyrolysis, facilitate deoxygenation and hydrogenation processes, thereby improving bio-oil stability and calorific value. Consistent with these mechanistic insights, Özbay et al. [118] confirmed that steam introduction can increase pyrolysis oil yield, albeit reducing biocarbon yield due to suppression of secondary cracking, favoring retention of condensable volatiles in the liquid phase. Furthermore, Önal et al. [119] reported that steam-assisted pyrolysis can enhance the specific surface area of the residual biocarbon, potentially improving its suitability for adsorption-related applications. Despite these advantages, the use of steam or hydrogen requires a robust reactor design to manage increased corrosion risk and maintain safety, as well as more sophisticated control of process conditions, adding to capital and operational expenses.

Methane-enriched environments have been shown to increase the yield of condensable liquids by 10–15%, primarily by suppressing coke formation and enhancing in situ vapor-phase reforming. This effect is particularly relevant when CH₄ is co-fed into high-temperature reactors alongside catalysts such as Ni or zeolites, enabling simultaneous pyrolysis and reforming. However, the use of reactive or flammable gases such as H₂ or CH₄ entails higher safety risks, more complex gas handling infrastructure, and increased operational costs, which must be considered when designing pilot- or industrial-scale units. These operational challenges may limit their widespread adoption despite the clear improvements in product yields and qualities.

Carbon dioxide (CO₂), though not fully inert, acts as a mild gasifying agent that can inhibit the formation of undesirable polycyclic aromatic hydrocarbons (PAHs) such as benzene and naphthalene. Overall, steam and CO₂ atmospheres tend to promote higher selectivity toward liquid products, while inert gases like N₂ and He are preferred for maintaining stable operating conditions without significantly altering the pyrolysis chemistry [117,120]. However, CO₂ and H₂O may increase the complexity of reactor operation due to endothermic side reactions and variable heat loads, requiring robust temperature control systems. The impact of CO₂ on pyrolysis products is feedstock- and temperature-dependent. Manyà et al. [121] found that slow pyrolysis under a CO₂ atmosphere did not significantly alter overall product distribution or the stability of biocarbon, suggesting that CO₂ can enable the simultaneous production of both biocarbon and pyrolysis gas. In contrast, Pilon and Lavoie [122] reported a reduction in biocarbon yield at 500 °C when N₂ was replaced by CO₂, highlighting that gas environment effects are sensitive to operating conditions. Lee et al. [123] further demonstrated that CO₂ enhanced gas production and reduced tar formation during peat pyrolysis at temperatures above 440 °C, while also increasing the surface area of the biocarbon, indicating a role for CO₂ in pore development. Together, these studies underscore the dual function of CO₂ as both a pore-forming agent and a modifier of product distribution, with benefits or drawbacks that vary according to the pyrolysis objectives. Therefore, optimizing the reaction atmosphere involves trade-offs among process selectivity, energy input, product quality, environmental impact, and economic feasibility. This delicate balance highlights the need for integrated techno-economic and life cycle assessments to guide atmosphere selection in commercial pyrolysis processes.

Thus, the reaction atmosphere plays a crucial role in determining the thermal behavior of biomass, influencing product distribution, composition, and quality.

Table 3. Effects of reaction atmosphere on biomass pyrolysis behavior and product yields [authors’ own elaboration].

Atmosphere	Effect on Pyrolysis	Impact on Product Yields and Properties
N2 (nitrogen)	Inert, widely used to prevent oxidation and maintain thermal stability	Serves as a baseline atmosphere; enables comparability across studies. It does not actively alter product composition.
He (helium)	Inert, high thermal conductivity	May slightly enhance heat transfer, leading to marginal increases in volatile products release. However, the economic and safety costs of using helium limit its practical application in large-scale pyrolysis.
CO2 (carbon dioxide)	Mildly reactive; acts as a gasifying agent	Increases biochar yield, carbon content, and aromaticity; reduces O/C and H/C ratios; can suppress PAHs formation.
H2 (hydrogen)	Reactive; promotes hydrogenation and deoxygenation	Enhances bio-oil quality by increasing stability and calorific value; reduces oxygenated compounds. But the flammability and storage risks require stringent safety measures and infrastructure investments.
H2O (steam)	Promotes biomass depolymerization and secondary vapor reactions	Increases yield of oxygenated compounds (e.g., aldehydes, ketones); suppresses coke formation; improves biochar porosity.
CH4 (methane)	Acts as a hydrogen donor and reforming agent	Boosts liquid product yield by 10–15%; suppresses coke; aids in in situ reforming.
CO (carbon monoxide)	Mild reducing agent; can participate in gas-phase reactions	Alters gas composition; limited direct effect on pyrolysis yield but influences secondary reactions.

summarizes the most common atmospheres used during pyrolysis and their specific effects on the process and resulting products.

5.3. Catalysts

Catalysts play a pivotal role in enhancing the efficiency and selectivity of biomass pyrolysis reactions. Their incorporation can significantly influence the quality and yield of bio-oil by promoting the cleavage of complex macromolecules and directing the formation of target compounds. Catalysts also lower the overall activation energy, allowing operation at milder temperatures and improving reaction kinetics [124].

Catalytic pyrolysis is typically carried out in fluidized-bed and fixed-bed reactors, where catalysts may be employed in solid or vapor-phase form. Commonly used catalytic agents in biomass pyrolysis and gasification include alkali and alkaline earth metals such as Na, K, Mg, and Ca, as well as transition metals like Fe, Co, Ni, Cu, and Ti [125]. In addition, naturally occurring minerals and engineered materials, such as dolomite, olivine, aluminum oxide, calcium oxide, and alkali metal salts, facilitate cracking and reforming reactions, improving gas yields while reducing tar formation [126].

Selecting the appropriate catalyst is essential for optimizing liquid yields while minimizing undesirable by-products such as tar and coke. Some catalysts effectively promote cracking reactions that break down heavy molecules into lighter and more valuable hydrocarbons, thereby improving the composition of the resulting bio-oil [127]. This catalytic transformation also reduces viscosity, acidity, and corrosiveness, ultimately enhancing its chemical stability and compatibility with existing petroleum-based fuels.

Experimental studies highlight the nuanced and feedstock-dependent effects of specific catalysts and their combinations. Aho et al. [128] demonstrated that the use of a zeolite-based catalyst at 450 °C increased biocarbon yield compared to noncatalytic pyrolysis, primarily due to catalyst coking, which enhances carbon retention. Wan et al. [129] evaluated multiple catalysts (Na₂HPO₄, H₃BO₃, MgCl₂, Al₂O₃, KaC, K₂Cr₂O₇) in microwave-assisted pyrolysis of corn stover and found that only Al₂O₃ increased biocarbon yield, whereas several others (H₃BO₃, MgCl₂, Al₂O₃, KaC, K₂Cr₂O₇) mainly enhanced pyrolysis oil production at the expense of solid and gaseous fractions, likely by acting as microwave absorbers and accelerating heating. Mohamed et al. [100] further showed that individual and mixed catalysts, including bentonite, K₃PO₄, and clinoptilolite, can alter microwave heating rates and improve catalytic performance, with mixtures often producing synergistic effects. These findings underscore that catalyst selection requires balancing multiple trade-offs, including char versus liquid yields, heating efficiency, and feedstock-specific behavior, highlighting the complex role of catalysts in pyrolysis.

In practical terms, catalytic pyrolysis enables more efficient upgrading of a wide range of biomass feedstocks, including lignocellulosic agricultural waste, forestry residues, and algae, into higher-quality fuel intermediates. Its ability to reduce oxygenated compounds improves downstream processing and storage stability, making it especially suitable for bio-refineries and distributed energy systems.

Furthermore, tailored catalysts can selectively favor the production of high-value chemicals, including furans and phenolics, which serve as essential building blocks in the chemical and pharmaceutical industries [16]. Catalysts may also inhibit coke deposition by facilitating secondary reactions such as the gasification of carbonaceous residues or by accelerating the decomposition of coke precursors. Similarly, tar formation is suppressed through catalytic tar-cracking mechanisms, which not only improve the quality of the liquid product but also help maintain operational stability in downstream processing units.

Despite these benefits, significant challenges persist that limit broader commercial adoption. Catalyst deactivation remains a major issue, with fouling and poisoning caused by biomass impurities such as alkali metals and silica leading to reduced catalytic activity and lifespan. Regeneration processes, while available, add operational complexity and cost. Moreover, many catalytic materials involve rare or expensive metals, raising concerns about economic scalability and supply chain sustainability. Reactor design must balance catalyst exposure and residence time to maximize contact efficiency without inducing excessive pressure drops or thermal degradation, while achieving uniform catalyst distribution and minimizing hot spots are technical challenges that require sophisticated reactor engineering, often increasing capital expenditure.

From a sustainability standpoint, many catalytic systems offer the added benefit of renewability and reusability, contributing to the economic viability and circular nature of the biomass pyrolysis process. However, lifecycle assessments are required to fully evaluate environmental impacts related to catalyst production, use, and disposal, particularly considering energy inputs and waste streams from regeneration or replacement. Future research should prioritize developing low-cost, earth-abundant, and highly durable catalysts to enhance the overall sustainability of catalytic pyrolysis.

5.4. Temperature

Temperature is the primary factor driving biomass pyrolysis, directly influencing the decomposition mechanisms and the distribution of final products. Pyrolysis reactions can initiate at temperatures as low as 200 °C. Hemicellulose typically decomposes between 200 and 305 °C, generating primarily acids and CO₂. Cellulose undergoes thermal degradation in the range of 300–400 °C, producing compounds such as levoglucosan and various volatile organic species. Lignin decomposes more gradually over a broad temperature span, from approximately 160 °C to 900 °C, contributing significantly to biochar formation and the generation of aromatic compounds; the main lignin breakdown occurs predominantly between 350 and 500 °C [130].

At relatively low pyrolysis temperatures (350–450 °C), the process favors higher yields of solid char and lower conversion efficiency to volatile products, making this regime suitable for biochar production applications. As temperature increases, biochar yield decreases, while the generation of syngas components intensifies [131]. Fast pyrolysis is commonly performed within the 480–520 °C temperature window, which optimizes bio-oil production by ensuring rapid devolatilization of biomass and minimizing secondary cracking of vapors. Above 600 °C, during flash pyrolysis conditions, thermal cracking reactions become dominant, significantly increasing syngas (CO, H₂, CH₄) formation at the expense of liquid bio-oil yield [132].

This temperature-dependent selectivity enables tailored product recovery for specific applications, such as maximizing biochar for soil remediation or enhancing syngas output for fuel synthesis, thereby increasing the practical applicability of the process.

Furthermore, elevated temperatures induce compositional changes in the products: biochar tends to have increased carbon content and improved aromaticity, while bio-oil exhibits reduced oxygen content, improving its fuel quality. Temperature also strongly affects the physicochemical properties of biochar, including elemental composition, pore structure, surface area, and the distribution of surface functional groups, which are critical for applications in soil amendment, adsorption, or catalysis [133]. However, operating at high temperatures introduces several drawbacks. Increased thermal stress requires reactors to be constructed from more advanced and often costly materials, and the higher energy input can reduce overall process efficiency and economic viability. Additionally, elevated temperatures may exacerbate secondary cracking reactions, potentially leading to increased formation of undesired gaseous by-products and reducing liquid yields, which challenges process optimization. Balancing temperature to optimize product distribution while minimizing energy consumption remains a key area for process improvement and techno-economic analysis. Figure 22 illustrates the combined effects of pyrolysis temperature and vapor residence time on the relative yields of biochar, bio-oil, and syngas products.

The trends illustrated in Figure 22 are consistent with findings from multiple experimental studies, emphasizing the influence of temperature on biochar physicochemical characteristics and the trade-offs in product distribution.

The relationship between fixed carbon content in biochar and the severity of thermal treatment during pyrolysis is well-documented. Altamer et al. [134] reported that pyrolysis of wild Brassica juncea L. seeds within the temperature range of 350–600 °C, at a constant heating rate of 10 °C/min, resulted in a progressive decline in biocarbon yield. The yield of pyrolysis oil increased with temperature up to approximately 475 °C, beyond which it exhibited a decreasing trend. Concurrently, the proportion of pyrolysis gas generated increased over the investigated temperature range. The authors further noted that the applied heating rate exerts a significant influence on the overall product distribution during pyrolysis. For example, Crombie et al. [135] reported that when the pyrolysis temperature increased from 200 °C to 700 °C for rapeseed biomass and from 300 °C to 750 °C for wheat straw, green waste, dried algae, and pinewood chips, a substantial increase in fixed carbon was observed, primarily due to the progressive release of volatile matter during heating. Tomczyk et al. [136] reported a similar pattern, showing that carbon content in biochar can rise dramatically from 62.2% to 92.4% as pyrolysis temperature increases. They further suggested that higher degrees of polymerization at elevated temperatures produce a more condensed and thermally stable carbon structure. Likewise, Tag et al. [137] observed that orange pomace biochar became more carbonaceous (from 56.8% to 68.1%) when the pyrolysis temperature was raised from 300 °C to 600 °C, reinforcing the trend identified in [136]. These findings collectively confirm the inverse relationship between volatile matter content and fixed carbon. As volatile compounds are released more extensively at higher temperatures, micropores are generated, which in turn enlarge the specific surface area of the resulting biochar. Sigmund et al. [138] demonstrated this effect quantitatively, reporting a 300% increase in surface area and pore volume when the pyrolysis temperature was raised from 105 °C to 300 °C. Since the specific surface area is a critical property that governs the sorption potential of biochar for pollutants, such structural changes have direct implications for its environmental applications. Taken together, these studies suggest that increasing pyrolysis temperature not only enhances fixed carbon content but also improves the physicochemical quality of biochar by producing a more porous and reactive surface. However, the magnitude of these effects can vary depending on the biomass feedstock and heating profile, indicating that optimal conditions for maximizing both carbon content and functional properties must be determined on a case-by-case basis.

5.5. Residence Time

The residence time of both solid biomass and pyrolysis vapors is a crucial parameter in controlling secondary reactions and ultimately determining product yields and quality. Short vapor residence times, typically less than 2 s, are essential in fast pyrolysis processes to minimize repolymerization and thermal cracking of volatile compounds, thereby maximizing the yield of condensable bio-oil. When vapor residence time extends beyond 5 to 10 s, especially at elevated temperatures, a significant portion of the primary volatiles undergoes secondary reactions, converting into non-condensable gases such as CO, CO₂, and CH₄, as well as additional char formation, which leads to decreased bio-oil yields [22].

Moreover, prolonged vapor residence in the reactor may also alter the composition of the bio-oil, increase its water content, and reduce its energy density, thus impacting its suitability for downstream applications. Hence, precise control of vapor residence time is critical to ensure product quality and maximize liquid yields, especially in continuous or scaled-up pyrolysis systems.

However, achieving and maintaining such short vapor residence times at commercial scales is challenging due to constraints in reactor design and vapor handling systems. In practice, longer vapor residence times often result from suboptimal vapor extraction or insufficient carrier gas flow rates, leading to greater secondary cracking and product degradation. Thus, vapor residence time control often represents a key bottleneck in scaling fast pyrolysis technologies.

Regarding the solid phase, biomass residence times vary widely depending on particle size, reactor type, and heating method. In slow pyrolysis, solid residence times may range from several minutes to hours, allowing for more complete carbonization and enhanced fixed-carbon content in the resulting biochar. In contrast, fast pyrolysis maintains very short solid residence times, usually under 5 s, to favor liquid production. Solid residence time is also affected by the reactor geometry and the presence of internal or mixing mechanisms. For instance, in fluidized bed systems, enhanced mixing promotes more uniform heating and can reduce the variability in residence time distribution across particles. This uniformity is crucial for reproducibility and consistent product quality in industrial processes.

Extended solid residence times at moderate temperatures (below 600–700 °C) promote increased carbonization, resulting in biochar with higher fixed-carbon content and altered physicochemical properties [139]. At lower pyrolysis temperatures, around 300 °C, prolonged residence time can cause a gradual decrease in biochar yield but tends to increase its pH and iodine adsorption capacity, indicating changes in surface chemistry beneficial for certain applications. However, at higher temperatures near 600 °C, residence time has a minimal effect on biochar yield and pH but may reduce its iodine adsorption capacity, likely due to changes in surface functional groups or porosity [140].

While extended solid residence times enhance certain desirable biochar properties, they may also result in reduced process throughput and increased energy consumption, potentially limiting economic feasibility in industrial contexts. Balancing residence time to optimize biochar quality without compromising process efficiency remains a key challenge. Moreover, solid residence time control can be less precise than vapor residence time management, especially in fixed-bed or rotary kiln reactors, where particle flow and mixing are less uniform.

Therefore, optimizing residence time in relation to target products (biochar, bio-oil, or syngas) is essential for tailoring the pyrolysis process to specific application goals. Advanced reactor designs increasingly incorporate adjustable residence time controls to fine-tune product distribution dynamically during operation. The development of real-time monitoring and control systems for residence time, possibly coupled with advanced modeling techniques, could improve process adaptability and product consistency, particularly in flexible biorefinery operations where feedstock variability is high.

It was well established that extended vapor residence times tend to promote biocarbon formation, whereas shorter residence times favor the recovery of condensable vapors and permanent gases. As noted by Kan et al. [45], reducing vapor residence time facilitates the rapid removal of organic vapors from the reaction zone, thereby suppressing secondary reactions such as cracking and polymerization, which would otherwise alter product composition. Conversely, Pahnila et al. [90] mention that prolonged vapor residence times increase the likelihood of secondary cracking, leading to the conversion of condensable volatiles into solid carbonaceous residues and light gases, ultimately resulting in higher biocarbon yields. A similar dynamic applies to biomass particle or feedstock residence time within the reactor. Experimental evidence reported by Solar et al. [141] for woody biomass waste and by Liu et al. [142] for peanut shells, demonstrated that increasing the residence time of the solid feedstock in continuous pyrolysis systems resulted in lower overall biocarbon yields. This effect is attributed to the progressive thermal decomposition of the initially formed biocarbon, which undergoes further devolatilization and gasification under prolonged exposure to elevated temperatures. The resulting shift in product distribution favors pyrolysis gas formation at the expense of the solid fraction. Collectively, these findings highlight a nonlinear relationship between residence time and biochar yield, in which excessively short durations may limit carbonization due to incomplete conversion, while excessively long durations can trigger secondary degradation of the solid phase. The optimal residence time is therefore feedstock-specific and dependent on reactor configuration, making it a critical variable for tailoring product quality and process efficiency in pyrolysis systems.

5.6. Chemical Composition of Biomass

The chemical composition of biomass is a fundamental factor influencing the yield and characteristics of pyrolysis products, due to the distinct thermal stability and decomposition pathways of its major constituents. Lignocellulosic biomass primarily consists of three structural polymers, cellulose (25–50 wt%), hemicellulose (15–40 wt%), and lignin (10–40 wt%), along with varying amounts of extractives (up to 15 wt%) and a small fraction of inorganic minerals [22]. These biopolymers differ significantly in molecular structure and thermal degradation behavior, which directly impacts the distribution and composition of pyrolysis products.

The ratio between these components determines both the volatility of the feedstock and the reactivity of pyrolysis intermediates, influencing the energy required for thermal conversion.

For example, hardwoods typically contain about 40–45% cellulose, 25–30% hemicellulose, and 20–25% lignin, whereas agricultural residues often have lower lignin content and higher hemicellulose proportions.

Table 4. Chemical composition of some lignocellulosic agricultural residues (percentages may vary depending on the variety, growing conditions, and maturity of the plants at harvest) [authors’ own elaboration, based on data provided in [22,44,50,66,126,143].

Biomass	Composition (% Dry Basis)
	Lignin (%)	Hemicellulose (%)	Cellulose (%)
Agricultural crop residues
Corn cobs	12–13	31–38	39–41
Corn stalks	42	46	3
Corn harvest residues	42	22	20
Corn straw	49	28	7.5
Barley straw	15–25	20–30	35–40
Oat straw	13–25	24–35	35–45
Rice straw	10–30	10–20	30–50
Rice husks	36	12	26
Millet husk	14	33.3	26.9
Sorghum straw	30–42	19–24	34–49
Wheat straw	11–26	20–45	32–45
Sunflower stalks	34 ± 0.6	20.8 ± 0.8	29.7 ± 0.6
Sunflower shell	17	34.6	48.4
Cotton stalks	41.6 ± 0.5	23.6 ± 0.4	23.3 ± 0.7
Cotton seed hairs	0	5–20	80–95
Jute fiber	21–26	18–21	45–53
Grass	47	36	12
Pasture grass	42	28	31
Dedicated energy crops
Giant reed	41.5 ± 2.6	20.5 ± 0.6	18.4 ± 1.4
Miscanthus	10–20	20–30	40–50
Water hyacinth	36.84 ± 0.8	27.7 ± 0.2	10.7 ± 0.4
Energetic willow	20–25	20–30	40–45
Energy poplar	18–25	20–30	40–50
Switchgrass	10–20	25–30	35–45
Energy sorghum	10–20	20–30	30–40
Bamboo	21–31	15–26	26–43
Wood residues
Softwood	28	24	41
Hardwood	20	35	39
Leaves	0	80–85	15–20
Sawdust	31.5 ± 1.3	26.1 ± 2.1	24.9 ± 1.7
Willow sawdust	35.6 ± 0.9	21.5 ± 0.9	28.7 ± 0.2
Pine wood	38.2 ± 0.3	24.1 ± 0.7	34.4 ± 0.3
Eucalyptus	52.07 ± 2.6	24.51 ± 1.1	25.2 ± 1.1
Birch	40.1 ± 0.6	17.5 ± 0.1	24.2 ± 0.1
Poplar	46 ± 0.1	16.7 ± 0.1	26.6 ± 0.3
Fruit and vegetable waste
Banana peels	10–20	10–20	30–50
Banana waste	14	14.8	13.2
Citrus peels	5–15	10–20	15–25
Citrus pulp	2–6	5–8	13–24
Apple and pear peels	5–10	10–20	20–30
Apple and pear residues (from juice production)	5–10	10–20	20–30
Grape marc	32–51	6–24	11–36
Disrobed bunches	34.9	26.64	31.1
Oil palm bunches, without fruit	38.5 ± 1.9	26.1 ± 1.1	11.6 ± 1.6
Sugarcane bagasse	46.1 ± 0.7	20.1 ± 0.6	20.3 ± 0.6
Cabbage leaves	2	3–5	3–11
Cauliflower, broccoli	5–10	10–15	20–30
Carrot, celery, beetroot stems and leaves	20–40	10–20	30–50
Walnut shells, almonds, hazelnuts	30–50	10–15	30–40
Bean waste	4–22	18–21	26–37
Tomato marc	15–30	14–27	25–32
Tea waste	40	19.9	30.20
Animal husbandry residues
Cattle manure	6–14	12–24	14–32
Pig manure	1–5	14–24	9–15
Poultry manure	4–8	18–22	14–24

, complied by the authors based on data from the literature [22,44,50,66,126,143], summarizes the chemical composition of various lignocellulosic biomass types, highlighting their diverse cellulose, hemicellulose, and lignin contents.

The wide variability in biomass composition not only challenges the standardization of pyrolysis processes but also necessitates flexible reactor and process designs that can accommodate such heterogeneity without compromising product yield or quality.

Its lignocellulosic composition influences biomass behavior during pyrolysis and determines its suitability for different process types [64]. Agricultural residues, generally rich in lignin, cellulose, and hemicellulose, are well-suited for fast or flash pyrolysis, which benefits from their high volatile content and rapid thermal decomposition [144]. Food waste may contain lignocellulosic material but is more compositionally variable due to the presence of seeds and other organic fractions, often favoring intermediate or slow pyrolysis depending on moisture and consistency [145,146]. Manure, characterized by low lignocellulosic content and high moisture and nitrogen levels, contains proteins, lipids, soluble organics, and residual lignocellulose, making it more appropriate for slow pyrolysis, where longer residence time facilitates comprehensive breakdown of complex organics [147,148].

Additionally, high nitrogen content in manure may lead to the formation of nitrogenous compounds such as pyrroles or amines during pyrolysis, which can affect the quality of the resulting bio-oil and require further upgrading. This nitrogen content can also impact emissions and catalyst deactivation in downstream processing.

The presence of such nitrogenous species poses environmental and operational challenges that must be addressed through tailored upgrading technologies or catalyst design to ensure sustainable biofuel production. Moreover, it highlights the need for careful feedstock characterization prior to pyrolysis.

Cellulose-rich biomass typically favors bio-oil production through the release of levoglucosan and other oxygenated volatiles. In contrast, lignin-rich feedstocks tend to generate higher biochar yields and more aromatic compounds, contributing to increased carbon retention in the solid phase. Hemicellulose decomposes more readily and often produces acidic condensates that can lower the energy content of bio-oil. Additionally, extractives and inorganic ash components influence secondary reactions, either catalyzing or inhibiting tar formation and affecting biochar properties.

Alkali and alkaline earth metals (AAEMs) present in ash, such as K, Na, Ca, and Mg, can catalyze char formation and reduce bio-oil yields by promoting cracking and repolymerization reactions. Understanding the AAEM content is therefore critical when selecting feedstocks or designing pre-treatment steps. From a process optimization perspective, pretreatment strategies (e.g., washing or torrefaction) that reduce AAEM content can significantly improve liquid yields and bio-oil quality, but they add complexity and cost to the overall process. This trade-off must be carefully evaluated, especially for large-scale or decentralized pyrolysis units.

Thermogravimetric analysis, as illustrated in Figure 23, reveals distinct degradation profiles for cellulose, hemicellulose, and lignin, each characterized by different mass loss rates over specific temperature ranges. Understanding these degradation patterns is essential for optimizing feedstock selection and pyrolysis conditions to achieve targeted product distributions, balancing volatile yields and solid residues. Such thermal analyses not only guide the selection of optimal heating rates and peak temperatures but also help predict reaction kinetics, activation energy, and thermal stability thresholds for various biomass types.

5.7. Moisture Content of Biomass

The moisture content of biomass is a critical parameter in pyrolysis processes, significantly affecting both energy efficiency and operational safety. High moisture levels (above 30% w/w) greatly reduce thermal efficiency, as a substantial portion of the supplied energy is consumed in vaporizing water rather than promoting pyrolytic decomposition reactions [68,149]. This leads to a pronounced decrease in gaseous product yields and overall conversion efficiency.

Excess moisture can also lower the effective temperature within the reactor, resulting in incomplete pyrolysis and increased char and tar formation due to slower heating rates. Moreover, rapid evaporation of moisture at elevated temperatures (typically above 400 °C) can generate pressure surges or fluctuations in continuous reactors, posing risks of mechanical damage or interruptions in biomass feeding. High moisture content also dilutes the concentration of organic compounds in the condensable fraction, resulting in lower energy density and diminished quality of the produced bio-oil [44].

The presence of moisture may also catalyze secondary reactions such as hydrolysis or steam reforming, which can alter the product distribution, often increasing the content of light gases like CO and H₂, but reducing liquid yields. Conversely, lowering biomass moisture content to below 15–20% typically enhances bio-oil yield and promotes increased production of hydrogen (H₂), carbon monoxide (CO), and light hydrocarbons in the syngas fraction. Pre-drying biomass to moisture levels under 10% significantly improves process stability, energy recovery, and product quality, but at the expense of additional energy consumption during the drying pretreatment stage [11,20].

While drying improves pyrolysis performance, the energy input and environmental impact of drying methods must be critically evaluated, especially for large-scale applications where drying energy demand can significantly offset the net energy gain from biofuel production. Hence, integrating energy-efficient drying technologies and heat recovery is essential for overall process sustainability.

However, overly aggressive drying may cause increased brittleness of biomass particles, leading to dust generation and handling challenges, which need to be managed in industrial settings.

To optimize moisture reduction while minimizing energy consumption, various drying methods can be employed, such as solar drying, convective hot-air drying, and low-temperature rotary drum drying. Solar drying is cost-effective and sustainable but weather-dependent, whereas convective drying offers faster and more controlled moisture removal. Integrating waste heat recovery from the pyrolysis process for biomass drying can further enhance overall energy efficiency and reduce operational costs. Additionally, optimizing particle size and feedstock handling can improve drying uniformity, preventing hotspots and moisture pockets that could impair pyrolysis performance.

Advanced drying techniques, such as microwave-assisted drying or superheated steam drying, have also shown promise in reducing drying times and improving moisture uniformity in biomass feedstocks. However, the adoption of advanced drying technologies involves higher capital costs and operational complexity, which may limit their deployment in decentralized or low-resource settings. Careful techno-economic analyses are therefore necessary to determine the feasibility of such methods relative to conventional drying, depending on the scale and intended application.

5.8. Particle Size of Biomass

Particle size of biomass significantly influences heat transfer rates, residence time, and the uniformity of thermal decomposition [126]. Biomass particles in the range of 0.15–0.5 mm promote increased gas production, particularly with elevated hydrogen concentrations, while simultaneously reducing char and tar yields. In contrast, larger particles (1–5 mm) tend to develop significant internal temperature gradients during pyrolysis, where the core remains cooler than the surface. This uneven heating results in incomplete devolatilization, producing higher char and tar yields alongside a reduced gas output [150].

The thermal conductivity and diffusivity of biomass materials further affect the severity of temperature gradients within larger particles, impacting the rate and extent of pyrolysis reactions.

Smaller particles (under 2 mm) are preferred in fast and flash pyrolysis processes, where rapid heating is essential to achieve high devolatilization rates. Particles smaller than 1 mm can reach the reaction temperature in less than 1 s, enhancing bio-oil yield and minimizing thermal gradients within the particles [151]. However, if particles are too fine (e.g., <200 µm), they risk entrainment in fluidized bed reactors, complicating separation processes and increasing biochar loss. Larger particles (>5 mm) experience slower heat transfer, leading to incomplete pyrolysis and increased biochar yield due to prolonged heating of the particle core [35].

In fluidized beds, excessive fine biomass particle can also lead to agglomeration and defluidization issues, affecting reactor operability. Maintaining uniform particle size distribution not only improves heat transfer and reaction kinetics but also enhances reactor stability and predictability of product distribution, which is critical for process scale-up and industrial application.

Particle size distribution control is therefore a key factor in process reproducibility and scale-up economics, influencing reactor design and operational parameters.

The particle shape and surface area also influence heat transfer and devolatilization rates. More irregular or porous particles have higher surface-to-volume ratios, facilitating faster heat penetration and volatile release. Biomass pretreatment methods such as milling, grinding, or sieving are employed to control particle size and shape, optimizing the feedstock for specific pyrolysis conditions. Nevertheless, these preprocessing steps introduce trade-offs between improved reaction efficiency and increased capital and operational costs, as well as potential degradation of biomass due to heat generated during mechanical size reduction. These factors must be carefully evaluated to optimize overall process economics and sustainability.

Advanced pretreatment techniques, including torrefaction and pelletization, may also influence effective particle size and bulk density, further impacting pyrolysis behavior and reactor feeding systems. However, while torrefaction can improve grindability and energy density, it also alters the chemical composition of biomass, potentially affecting pyrolysis product distribution and requiring adjusted operational parameters. Pelletization can enhance feeding consistency but may introduce additional energy consumption and binder-related complexities.

Liu et al. [142] observed that reducing biomass particle size resulted in lower biocarbon yields during pyrolysis at 600 °C. They attributed this trend to the reduced internal temperature gradients in smaller particles, which promote rapid devolatilization and consequently increase the formation of pyrolysis oil and gases at the expense of the solid fraction. In contrast, larger particles maintain a greater internal temperature gradient, favoring carbonization and leading to higher biocarbon yields. Similarly, Yu et al. [152] reported that larger particle sizes consistently produced more biocarbon over a wide temperature range (300–900 °C), with the effect being most pronounced below 400 °C. At elevated temperatures, the influence of particle size diminished, suggesting that intense thermal conditions override the diffusional and heat-transfer limitations that otherwise differentiate small and large particles. These studies suggest that particle size effects are both temperature-dependent and linked to intra-particle heat and mass transfer dynamics. While larger particles tend to favor char formation at moderate temperatures due to slower heat penetration and prolonged devolatilization, this advantage diminishes as pyrolysis approaches high-temperature regimes where reaction rates are dominated by thermal cracking and gasification processes. Optimizing particle size therefore requires balancing heat-transfer limitations against the desired product distribution for specific pyrolysis applications.

5.9. Impurities in Biomass

Biomass impurities, particularly mineral and metallic components, significantly influence both process performance and product characteristics.

Alkali metals (e.g., K, Na), alkaline earth metals (e.g., Ca, Mg), alkali metal salt catalysts (KCl, K₂SO₄, K₂CO₃, NaCl, Na₂SO₄, and Na₂CO₃), and anions such as chlorides, sulfates, and phosphates can act either as catalysts or inhibitors, depending on their concentration and chemical form [149,153,154]. For instance, potassium and sodium promote cracking reactions and biochar formation at temperatures as low as 400–500 °C, which reduces the liquid fraction while increasing gas yields. Calcium and magnesium may catalyze decarboxylation and demethoxylation reactions, thus influencing the acidity and oxygen content of the resulting bio-oil.

The speciation and mobility of these minerals during pyrolysis can also impact the catalytic behavior, potentially forming volatile inorganic compounds that affect downstream condensation and corrosion.

Moreover, transition metals such as iron, copper, or zinc, whether naturally present in the biomass or introduced through contamination, can lead to fouling and corrosion in the reactor and downstream equipment. Inorganic ash content above 5–6% can substantially reduce the stability and heating value of both biochar and bio-oil [155]. Consequently, ash-rich biomass types (e.g., straw, bark, or manure) tend to produce higher char yields and lower amounts of condensable volatiles compared to clean, woody biomass [156].

High ash and impurity levels therefore present a significant operational challenge, as they not only affect product quality but also exacerbate equipment wear and fouling, increasing downtime and maintenance costs. Additionally, high ash content can cause slagging and agglomeration within the reactor, reducing operational efficiency and increasing maintenance frequency. These physical phenomena can also alter heat transfer dynamics, leading to localized hot spots and non-uniform pyrolysis, which complicates scale-up efforts and reactor design optimization. The presence of chlorine in biomass is particularly problematic due to its role in promoting corrosion and formation of harmful emissions such as dioxins and furans during pyrolysis and downstream processing. Addressing chlorine content necessitates either feedstock pre-treatment, such as washing or blending with low-chlorine materials, or implementing advanced emission control technologies, which increase process complexity and costs. Such requirements can limit the economic viability of certain biomass types in pyrolysis applications, especially at industrial scale.

6. Pyrolysis Products: Biochar, Syngas, and Bio-Oil

The high temperatures applied during pyrolysis trigger the thermal decomposition of organic matter in biomass or waste feedstocks, causing substantial molecular rearrangements. Under such conditions, organic components become thermally unstable and undergo a series of complex chemical reactions, primarily governed by the final temperature and the specific thermal regime. Based on these operational parameters, three primary product categories typically emerge:

• Solid residue (biochar): Produced at lower temperatures, typically below 450 °C, and under slow heating conditions, biochar is a porous, carbon-rich material with multiple potential applications, including soil enhancement, carbon sequestration, and as a precursor for activated carbon production.
• Bio-oil (pyrolysis oil): Formed under intermediate temperatures and relatively high heating rates, this product is a complex mixture of oxygenated organic compounds with high water and acidity levels, which limit its direct use as fuel. However, it holds promise as a platform chemical or energy carrier after appropriate upgrading and stabilization.
• Syngas (pyrolytic gas): Generated at elevated temperatures, typically in the range of 400–700 °C, under an oxygen-free atmosphere, pyrolytic gas primarily consists of hydrogen (H₂), carbon monoxide (CO), methane (CH₄), and light hydrocarbons and differs in composition from conventional syngas produced via high-temperature gasification (>800 °C).

This thermal conversion pathway enables the valorization of organic waste by transforming it into energy carriers and valuable feedstocks for various industrial uses. In general, lower process temperatures tend to favor biochar and bio-oil formation, whereas higher temperatures lead to more extensive thermal cracking and increased gas yields. The final distribution and quality of pyrolysis products are strongly affected by multiple factors, including feedstock characteristics (chemical composition, particle size), the nature and flow rate of the carrier gas, heating rate, residence time, and the reactor configuration.

6.1. Biochar as a Pyrolysis Product

Biochar, also referred to as pyrolytic charcoal, is a solid carbonaceous material produced by the thermal decomposition of lignocellulosic biomass in an oxygen-deprived environment. It consists predominantly of carbon (typically 65–90%), along with hydrogen and various inorganic components. Its internal structure features a disordered arrangement of stacked graphene layers interspersed with amorphous aromatic regions, which confer distinctive physicochemical properties.

Biochar yields vary significantly, generally ranging from 10% to 40%, depending on the biomass type and pyrolysis conditions. The highest yields are typically achieved under slow pyrolysis at temperatures below 500 °C [157]. For example, 1 kg of lignocellulosic biomass can yield approximately 0.36–0.45 kg of biochar [22]. Higher heating rates (105–500 °C/s), shorter vapor residence times, and finely ground feedstocks result in finer biochar particles, while slow pyrolysis of coarser biomass (e.g., wood chips) produces coarser, more stable chars [158]. Agricultural residues and manure often generate finer biochar structures.

The particle size and feedstock moisture content also play important roles in determining biochar texture and porosity, impacting its application potential.

At lower pyrolysis temperatures (450–500 °C), limited devolatilization results in higher biochar yields. As the temperature increases to moderate levels (550–650 °C), the yield typically drops to around 8–10%, decreasing further beyond 650 °C due to enhanced carbon conversion into volatile gases [159].

The physical and chemical properties of biochar, such as ash and mineral content, degree of aromaticity, surface morphology, pH, and surface functional groups, are strongly influenced by the pyrolysis temperature and the nature of the biomass feedstock. In general, higher pyrolysis temperatures result in increased aromaticity and alkalinity. Plant-derived biochars tend to exhibit higher pH and alkalinity compared to those produced from manure. The hydrological behavior of biochar, including water retention, infiltration, and wettability, is also affected by several factors such as the feedstock origin, production method, particle size, and surface characteristics [150,160,161,162].

Slow pyrolysis typically yields biochar with high structural stability, developed porosity, and elevated cation exchange capacity, making it well-suited for soil amendment and long-term nutrient retention. In contrast, biochar from fast pyrolysis exhibits greater surface area and microporosity, enhancing its effectiveness in pollutant adsorption, in water treatment, and as a catalyst support. Flash pyrolysis generates highly microporous biochar with potential applications in gas storage, in odor control, and as a lightweight additive in composite materials [163,164].

Table 5. Properties of biochar obtained from lignocellulosic biomass [authors’ own elaboration].

Property	Description	Typical Range/Observations
Feedstock type	Type of lignocellulosic biomass used (e.g., wood, straw, husks)	Strongly influences final properties of biochar
Pyrolysis temperature	Temperature at which pyrolysis is conducted	300–700 °C
Carbon content (C)	Fixed carbon concentration	50–90 wt%
Hydrogen content (H)	Indicates degree of aromaticity and stability	0.5–4 wt%
Oxygen content (O)	Affects surface functional groups and polarity	5–30 wt%
H/C ratio	Indicator of aromaticity and thermal maturity	<0.6 for stable biochars
O/C ratio	Related to surface polarity and degradation potential	<0.4 for highly stable biochar
Ash content	Inorganic residue after combustion	1–30 wt%, depending on biomass type
pH	Reflects alkalinity or acidity of biochar	6–11
BET 1 porosity	Specific surface area (m2/g)	10–500 m2/g (increases with temperature)
Pore structure	Micropores and mesopores; relevant for adsorption	Highly porous, especially at higher temperatures
Cation exchange capacity (CEC)	Affects nutrient retention and soil interaction	10–200 cmol (+)/kg depending on activation
Electrical conductivity (EC)	Salinity and ion availability for soil application	Variable; higher in biochars from nutrient-rich feedstocks
Heavy metals	May be present depending on feedstock and processing	Should be monitored for environmental applications
Functional groups	Oxygen-containing groups (e.g., -COOH, -OH)	Affect reactivity and sorption behavior

¹ BET porosity refers to the specific surface area of biochar as measured by the Brunauer–Emmett–Teller (BET) gas adsorption technique. This parameter reflects the internal porosity and adsorption capacity of the material, which are essential for applications such as soil amendment and pollutant removal. Higher BET surface area values indicate a more porous and chemically active structure. Biochar produced through pyrolysis, particularly at lower temperatures (350–550 °C), typically shows higher BET surface areas (200–400 m²/g) due to the preservation of microporous structures. In contrast, biochar derived from gasification at higher temperatures (800–1000 °C) often develops larger pores but may exhibit lower BET values (20–100 m²/g), as the carbon becomes more aromatized and less reactive [11].

presents some of the main properties of biochar obtained from the pyrolysis of lignocellulosic biomass.

A synthesis of the main biochar valorization pathways across different sectors and their associated benefits is provided in

Table 6. Applications and benefits of pyrolysis biochar [authors’ own elaboration].

Domain	Application	Benefits
Environment	Carbon sequestration	Long-term carbon storage in stable form (CCS)—reduces atmospheric CO2
	GHG emission reduction	Alters microbial soil processes and composting; reduces NOx and CH4 emissions
	Water filtration	Porous structure absorbs heavy metals, nutrients, pharmaceuticals—improves water quality
	Air purification	Adsorbs VOCs, NH3, H2S, and fine particles—improves indoor/outdoor air quality
	Soil remediation	Adsorbs heavy metals and organic pollutants—prevents leaching into groundwater
	Waste management	Valorization of organic waste (e.g., sludge, food waste)—reduces landfill burden
Agriculture	Soil fertility enhancement	Improves nutrient retention, cation exchange, and microbial diversity—increases crop yields
	Nutrient management	Reduces nutrient leaching and fertilizer loss—increases efficiency of fertilization
	Pest control	Alters soil conditions unfavorable to pests—reduces the need for pesticides
	Feed additive	Improves digestion in livestock—reduces methane in manure, enhances animal health
	Seed coating / germination aid	Enhances seedling growth and drought resistance—improves germination rates
	Animal bedding	Controls odor, moisture, and pathogens—improves hygiene and reduces emissions
Industry	Material improvement	Additives for plastics, rubber, concrete; catalyst support—enhances material properties
	Bioenergy enhancement	Boosts anaerobic digestion when co-added—increases CH4 yield and system stability
	Battery and supercapacitor materials	High surface area carbon for energy storage—used in electrodes
	Construction	Additive in asphalt or insulation materials—improves thermal and mechanical properties
	Packaging and textiles	Sustainable filler or additive—improves biodegradability and moisture control
Climate adaptation	Drought resilience	Improves soil water holding capacity—increases tolerance to drought conditions
	Erosion control	Enhances soil aggregation and stability—reduces wind/water erosion
Circular economy	Resource recovery	Recycles nutrients and carbon—supports sustainable waste-to-resource pathways
	Sustainable agriculture	Integrates into low-input farming systems—promotes regenerative practices

. A more detailed exploration of these applications has been recently published by [11].

Efficient utilization of biochar requires a detailed understanding of its chemical composition, physical characteristics, and the way these interact with the application environment (soil, water, air). This knowledge is essential for optimizing its performance in specific use cases, such as soil conditioning, pollutant removal, or energy storage. Moreover, tailoring biochar properties through controlled pyrolysis and post-treatment processes can significantly enhance its efficiency, ensuring compatibility with diverse environmental and industrial applications.

6.2. Bio-Oil as a Pyrolysis Product

During the pyrolysis process, vapor-phase products are rapidly cooled and condensed using dedicated systems, resulting in two distinct liquid fractions: bio-oil (pyrolytic oil) and an aqueous phase. These are separated from the non-condensable gases that remain in the system. The composition and yield of these fractions depend heavily on feedstock type, pyrolysis conditions, and condensation techniques, influencing their suitability for downstream applications such as fuel production, chemical recovery, or wastewater treatment.

Bio-oil is a viscous liquid ranging from light to dark brown in color and consists of a highly complex and heterogeneous mixture of organic compounds. These include hydrocarbons, benzene derivatives, aromatic oligomers, and a large fraction of oxygenated chemicals [51]. Chemically, bio-oil can be broadly divided into three major groups: carbonyl compounds (such as acids, ketones, and aldehydes), sugar-derived compounds (including furans and hydro-sugars), and lignin-derived compounds (notably phenols and aromatic oligomers). In addition, bio-oil typically contains 15–35 wt.% water, along with hundreds of other constituents such as phenols, ethers, alcohols, and esters. This complexity gives bio-oil a highly variable composition and properties (

Table 7. Physical properties and characteristics of pyrolysis bio-oil [22].

Property	Typical Value/Description	Observation
Appearance	Dark reddish-brown, free-flowing organic liquid	Due to micro-carbon and complex organics
Odor	Sharp, smokey	Resulting from volatile acids, aldehydes, and phenolic compounds
Density	1.2 kg/L (vs. 0.85 kg/L for fossil fuels)	Elevated due to high water and oxygen content
Viscosity	40–100 cP	Strongly influenced by feedstock, moisture content, and aging
Heating value	26.7 MJ/kg	Lower than fossil fuels due to high oxygen and moisture content
Aging behavior	Viscosity increases, phase separation, gum formation	Caused by polymerization and condensation reactions during storage
Polarity	Polar	Miscible with polar solvents only

) [143,164,165,166,167].

Bio-oil exhibits limited chemical stability, tending to undergo aging through polymerization and condensation reactions, which cause an increase in viscosity, phase separation, and gum formation during storage. These processes impact its shelf life and complicate long-term storage and transportation.

Thermally, bio-oil’s lower heating value and high acidity present challenges for direct combustion, as they can lead to corrosion and increased emissions in conventional boilers or engines. However, when used appropriately, it can serve as a renewable fuel in heat and power generation systems.

Despite its chemical instability, bio-oil is a versatile pyrolysis product with significant potential in both energy and materials applications. Due to its renewable origin and fuel-like properties, it is considered a sustainable alternative to petroleum-based fuel oil and can serve as both a bioenergy source and a platform to produce bio-based chemicals in industrial and agricultural sectors. However, to improve its stability, energy density, and compatibility with existing fuel infrastructure, bio-oil typically requires upgrading processes, such as catalytic hydrotreating, emulsification, or fractional distillation [168,169,170,171,172]. Advanced upgrading techniques including hydrodeoxygenation, catalytic cracking, and solvent addition are actively researched to reduce oxygen content and improve fuel properties.

The bio-oil composition can also vary significantly depending on the pyrolysis regime employed: slow pyrolysis tends to produce heavier and more viscous bio-oil fractions with lower volatility, whereas fast and flash pyrolysis generate lighter bio-oils with higher volatile content and different chemical profiles, influencing their downstream processing and applications.

Table 8. Applications and benefits of pyrolysis bio-oil [authors’ own elaboration].

Application Domain	Description	Challenges
Liquid fuel for combustion	Used as renewable fuel in boilers, furnaces, and turbines for heat and power generation	High water content (15–30%) and high acidity reduces the calorific value and storage stability
Transport fuel (after upgrading)	Upgraded via catalytic cracking or hydroprocessing to produce transportation fuels (e.g., green diesel, jet fuel)	Requires high-pressure processing and costly catalysts. Not yet fully commercial
Chemical production	Source of valuable chemicals such as phenols (resins, plastics), acetic acid, hydroxyacetaldehyde, furfural, and levoglucosan	Separation and purification of target compounds are technically complex
Soil amendment/bio-stimulant	Certain bio-oil fractions can stimulate microbial activity or act as soil conditioners	May require dilution and detoxification. Limited field validation
Fertilizer additive/slow release	Nitrogen-rich bio-oil fractions can be used to enhance fertilizer formulations	Toxicity concerns may arise depending on feedstock and pyrolysis temperature
Pharmaceutical/cosmetic precursors	Contains bioactive compounds that can serve as raw material in pharma and personal care industries	Requires strict refinement and safety testing; research still ongoing
Binder/adhesive formulation	Bio-oil can replace phenol in phenol–formaldehyde resins used in wood panels or adhesives	Requires viscosity and reactivity adjustments for industrial applicability
Carbon fiber precursor (experimental)	Under evaluation as a renewable precursor for carbon fiber production, due to its aromatic content	Early-stage research. Requires significant molecular tailoring
Antimicrobial agent (experimental)	Certain fractions exhibit antimicrobial properties and are being tested in food packaging or agricultural applications	Needs further toxicological and environmental impact assessments

outlines the main application domains, along with the respective benefits and limitations.

Given the broad application spectrum and complex composition of bio-oil, ongoing research efforts are focused on improving its stability, refining upgrading pathways, and enabling integration into existing energy and chemical value chains. In particular, the development of tailored catalysts and process intensification for cost-effective bio-oil upgrading is a critical area to overcome current limitations related to stability, oxygen content, and corrosivity, ultimately supporting its commercial viability.

6.3. Syngas as a Pyrolysis Product

Syngas (also referred to as pyrolysis gas or synthesis gas) is a complex mixture of gases produced through the thermal decomposition of biomass or other organic materials in the absence of oxygen. Its primary constituents include carbon monoxide (CO), hydrogen (H₂), methane (CH₄), ethane (C₂H₆), and other light hydrocarbons.

High pyrolysis temperatures, the use of catalysts, and extended residence times in the reactor are key parameters that favor the production of hydrogen-rich syngas from lignocellulosic biomass [151]. The thermal degradation of carbonyl and carboxyl functional groups leads to the formation of CO₂, while the cleavage of C–O–C and C=O bonds promote CO generation. Similarly, the breaking of C–H bonds and aromatic structures contributes to H₂ formation [173]. At lower pyrolysis temperatures, CO and CO₂ are typically the dominant gaseous products, whereas lignin depolymerization at elevated temperatures results in increased CH₄ generation.

Additionally, secondary reforming reactions such as steam reforming and water–gas shift reactions can further alter syngas composition, increasing hydrogen content under suitable conditions.

Emerging reactor designs and process intensification techniques are increasingly employed to optimize syngas yield and composition, improving hydrogen selectivity while minimizing tar formation, a common operational challenge.

Overall, pyrolysis gas exhibits a calorific value ranging from 10 to 20 MJ/Nm³. Due to this relatively high energy content and flexible composition, syngas can be used in a broad range of applications across the energy and chemical industries. For industrial uses, the removal of contaminants such as sulfur and chlorine is essential to ensure syngas quality and prevent corrosion or catalyst poisoning in downstream processes [174,175,176,177]. Advanced gas cleaning and conditioning systems, including scrubbers, filters, and catalytic converters, are crucial to meeting stringent purity requirements for high-value syngas applications.

Table 9. Properties of syngas obtained from lignocellulosic biomass [authors’ own elaboration].

Property	Description	Typical Range/Observations
Major components	Main gaseous products of pyrolysis	CO, H2, CH4, CO2, light hydrocarbons (C2–C4)
Heating value	Lower heating value of the syngas mixture	4–20 MJ/Nm3, depending on composition
CO content	Indicates fuel quality and reactivity	10–45 vol%
H2 content	Desired for combustion or synthesis applications	10–35 vol%
CH4 content	Contributes to energy content	2–15 vol%
CO2 content	Non-combustible; affects the calorific value	10–40 vol%
Tar content	Condensable organics that can cause operational issues	0.1–100 g/Nm3; higher in slow pyrolysis
Particulate matter	Fine solid residues carried in the gas stream	Requires removal before utilization
Sulfur compounds (e.g., H2S)	Present in small amounts; may require scrubbing	<0.1 vol% (feedstock dependent)
Nitrogen content	Typically low; may include NH3, HCN	<0.5 vol%, depending on biomass nitrogen content
Gas yield	Volume of syngas produced per unit of biomass	1–4 Nm3/kg dry biomass (varies with temperature and feedstock)
Gas temperature	Temperature of syngas at reactor outlet	300–800 °C (needs cooling for downstream use)
Gas temperature	Temperature of syngas at reactor outlet	300–800 °C (needs cooling for downstream use)
pH (of condensate)	Indicates acidity of condensable syngas fraction (used for environmental concerns)	Typically, acidic (pH 2–5)

presents a synthesis of the main properties of biochar obtained from the pyrolysis of lignocellulosic biomass.

Table 10. Applications and benefits pf pyrolysis syngas [authors’ own elaboration].

Application	Benefits
Hydrogen production	Source of clean energy for fuel cells, ammonia synthesis, and oil refining
Fischer–Tropsch fuel synthesis	Enables production of synthetic diesel, kerosene, and jet fuel
Electricity generation	Can be used in internal combustion engines, gas turbines, steam turbines, or fuel cells to efficiently generate power
Heat generation	Can be combusted in conventional boilers and combined heat and power (CHP) systems to produce thermal energy for industrial or residential use
Methanol production	Used as industrial solvent and precursor to formaldehyde and acetic acid
Ammonia synthesis	Key feedstock for fertilizers and various chemical processes
Basic chemicals production	Feedstock for alcohol, acids, plastics, and other petrochemical products
Methanation (synthetic natural gas)	Converts CO + H2 into CH4 for energy or chemical use
Amine and nitrogen compounds	Used in agriculture (pesticides, fertilizers), water treatment, and polymers
Substitute for solid fossil fuels	Cleaner combustion, lower emissions compared to coal or petroleum coke
Carbon monoxide recovery	Used in metallurgy and chemical reduction processes

highlights the main utilization pathways of syngas, along with the associated benefits.

Due to its versatility and high energy content, pyrolysis-derived syngas represents a strategic resource in the transition toward a sustainable energy system [178,179], with diverse applications across industry, transportation, and chemical sectors. A more detailed overview of syngas applications has been recently published by [11].

Recent advances in catalytic materials and gas cleaning technologies are focused on reducing tar and contaminant levels, thereby enhancing syngas reliability and expanding its use in fuel cells and synthesis processes, which are crucial for sustainable energy transitions.

Ongoing research on catalytic upgrading and improved gas cleaning tailored for pyrolysis-derived syngas aims to increase its quality and widen its industrial applications, thereby improving overall process economics.

7. Conclusions and Perspectives

This review provides a novel integrative perspective on biomass pyrolysis by systematically correlating reactor configurations, operational parameters, and product valorization strategies within the frameworks of sustainable resource management and circular economy. Unlike many previous studies focusing on isolated aspects, our study synthesizes recent technological advances and critically assesses the technological readiness levels (TRLs) of diverse pyrolysis systems, offering actionable insights for research prioritization and industrial scaling.

Biomass pyrolysis represents a versatile and scalable thermochemical route for the valorization of a wide range of organic residues into valuable products such as bio-oil, syngas, and biochar. This study highlights the diversity of reactor configurations available for pyrolysis processes, ranging from traditional fixed-bed systems to advanced microwave-assisted technologies, each with specific advantages and limitations depending on application scale, feedstock properties, and targeted product yields.

Fixed-bed reactors remain suitable for small-scale operations due to their simplicity and low cost, while fluidized-bed reactors offer superior heat transfer and are commonly applied in fast pyrolysis. Rotary kiln and auger reactors support continuous processing and are adaptable to various types of biomass. Emerging microwave-assisted reactors introduce volumetric heating and catalytic enhancement potential, showing promising results in terms of energy efficiency and gas-phase product yields, although operational complexity and cost remain challenges to be addressed.

An important consideration for practical deployment of pyrolysis technologies is their current TRL, which varies notably across different reactor designs and pyrolysis types. In the European context, slow pyrolysis technologies have reached a high TRL (8–9), supported by multiple commercial-scale operations, reflecting their maturity and market readiness. Fast pyrolysis, especially focused on bio-oil production, is at an intermediate stage (TRL 6–8), with numerous pilot and demonstration plants validating process feasibility. Conversely, emerging pyrolysis approaches such as flash and catalytic pyrolysis currently hold lower TRLs (3–5), largely confined to laboratory or early pilot-scale investigations. Similarly, reactor technologies exhibit a broad TRL spectrum: established systems like fixed-bed and fluidized-bed reactors demonstrate higher maturity levels, while innovative designs including microwave-assisted, plasma, and solar-driven reactors are still in earlier developmental phases. Recognizing these readiness levels is critical for directing research priorities, investment, and policy frameworks towards accelerating commercialization and scaling-up of pyrolysis for bioenergy and bioproduct applications.

Compared to other thermochemical technologies such as gasification and combustion, biomass pyrolysis offers distinct advantages in terms of product flexibility, adaptability to various feedstocks, and scalability. Market trends and recent forecasts predict significant growth in pyrolysis adoption driven by innovations in catalytic processes, reactor design, and integration with upgrading technologies. These developments position pyrolysis as a key enabling technology in the global shift towards renewable energy and low-carbon material recovery.

Product distribution in biomass pyrolysis is influenced by several process parameters, including temperature, heating rate, vapor residence time, and the presence of catalysts. The integration of suitable catalysts, particularly zeolites and metal oxides, has proven effective in improving selectivity toward high-value compounds and reducing unwanted by-products.

Looking ahead, widespread implementation of pyrolysis-based systems in the energy sector requires further efforts in multiple directions. These include optimizing reactor designs for enhanced thermal efficiency and process stability; developing robust control systems for continuous operation; improving the quality and standardization of pyrolysis products; and integrating pyrolysis within broader circular bioeconomy frameworks. Additionally, advancements in microwave reactor scalability, catalyst regeneration, and coupling pyrolysis with upgrading technologies for syngas or bio-oil will play a critical role in facilitating industrial adoption. Further research on catalyst longevity, process intensification, and energy integration will also be pivotal for economic competitiveness.

To fully unlock the potential of biomass pyrolysis, future research should emphasize cross-disciplinary collaboration, advanced process intensification, and robust lifecycle assessments. Policy frameworks and investment initiatives must support pilot-scale demonstrations and commercial deployment to facilitate the transition from promising laboratory-scale innovations to economically viable industrial solutions. The expansion of market infrastructure and regulatory frameworks for pyrolysis-derived products, such as bio-oil and biochar, is also crucial to enhance commercialization prospects. Establishing quality standards and certification schemes will increase stakeholder confidence and promote wider adoption. Integrating pyrolysis within multiproduct biorefinery systems and circular supply chains is essential to maximize resource efficiency and economic viability. Such hybrid approaches can valorize diverse biomass streams while generating energy, fuels, and specialty chemicals in a synergistic manner.

Moreover, future perspectives should incorporate comprehensive environmental impact assessments, including carbon footprint analysis and potential contributions to climate change mitigation. Biomass pyrolysis, by enabling carbon sequestration via biochar application and reducing reliance on fossil fuels, holds promise as a sustainable technology aligned with global decarbonization goals.