Reviewing Air Pollutants Generated during the Pyrolysis of Solid Waste for Biofuel and Biochar Production: Toward Cleaner Production Practices

Abstract

The production of biofuels and biochar through pyrolysis is a promising avenue for sustainable energy generation and waste management. However, this process can inadvertently release various air pollutants into the atmosphere, potentially compromising its environmental benefits. This article provides a comprehensive overview of the gas pollutants associated with pyrolysis for biofuel and biochar production, as well as different variables affecting gas emissions. Key pollutants such as particulate matter (PM), volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), carbon monoxide (CO), and nitrogen oxides (NO_x) have been discussed in terms of their formations and emissions during pyrolysis processes. Furthermore, major factors influencing pollutant emissions, including feedstock composition, pyrolysis conditions, and combustion technologies have been examined with up-to-date examples. The review highlights the significance of emission control strategies, such as advanced reactor design, catalyst utilization, and the integration of realtime monitoring systems, in mitigating air pollution from pyrolysis processes. By shedding light on the environmental challenges associated with pyrolysis-based biofuel and biochar production, this article aims to encourage the development of cleaner and more sustainable approaches to harness the potential of solid waste conversion technologies.

1. Introduction

The world is undergoing a transformation in the way it meets its energy and agricultural needs, driven by a growing awareness of the environmental consequences of fossil fuel use and the need to develop sustainable alternatives [1,2,3]. One such promising avenue is the utilization of solid waste pyrolysis, a thermochemical process that transforms organic matter into biofuels and biochar [4,5,6]. As pyrolysis involves the thermal decomposition of organic materials in the absence of oxygen, it is capable of converting various feedstocks, including agricultural residues [7], forestry waste [8], and municipal solid waste [9], into valuable end products. These products can, in turn, replace conventional fossil fuels, alleviate the pressure on landfills, and contribute to carbon sequestration in the soil, making it a pivotal tool in the fight against climate change and resource depletion [6,10,11]. Advanced control processes, such as catalytic fast pyrolysis, integrate fast pyrolysis and bio-oil upgrading into a single, streamlined step [12]. This innovative approach not only enhances the overall efficiency but also results in notable improvements in specific biofuel properties, including enhanced thermal stability and heating value, which can range from 18.8 to 34.6 MJ/kg [13,14]. Concurrently, diverse biochar engineering techniques have been devised to tailor specific physicochemical properties and sorption capacities, significantly broadening the spectrum of applications for biochar [15,16,17].

As countries worldwide grapple with the challenges of climate change, energy security, and sustainable agriculture, the potential of solid waste pyrolysis has ignited the enthusiasm of scientists, policymakers, and environmentalists alike. Countries such as the United States, Brazil, China, and India have already made substantial investments in the development of pyrolysis technologies [18,19,20]. Government-funded initiatives and international collaborations have placed a significant emphasis on advancing the field of solid waste pyrolysis. For example, through concerted efforts between the International Energy Agency (IEA) Bioenergy, national programs, and industry stakeholders, standardized analytical methods and specifications for bio-oil products have been developed as the technologies continue to mature [21]. This collaborative approach has given rise to suppliers offering commercial quantities of bio-oils tailored for a wide range of applications, including heating oil, natural gas alternatives, and power generation [22]. For more than three decades, these suppliers have also been providing bio-oil fractions that cater to the production of specialty chemicals and flavorings. Furthermore, it is worth noting that the commercial scale of fast pyrolysis technology, as it stands today, is fast approaching a significant milestone, with a capacity nearing 400 t/d [23]. This notable scale-up is poised to reap the benefits of economies of scale as it further evolves and expands.

Meanwhile, global studies have consistently demonstrated that biochar holds significant promise, with the potential to boost crop productivity by up to 11% while simultaneously reducing annual human-induced greenhouse gas emissions by 12% [24]. Moreover, biochar applications can facilitate the sequestration of approximately 0.7–1.8 Gt of CO₂ equivalent per year within the soil system [24]. Furthermore, the implementation of biochar in agricultural practices substantially enhances soil health, creating a more conducive environment for plant growth and improved crop productivity [25]. In a recent study, biochar application at a rate of 30 t/ha, when combined with optimal fertilization, resulted in remarkable yield increases ranging from 17% to 53% [26]. These improvements were particularly pronounced on highly acidic soils, primarily attributed to the enhanced retention of nitrogen, elevated pH levels, and the immobilization of trace metals that had previously hampered crop yields [27,28]. Additionally, biochar applications have been shown to influence plant physiology positively, enhancing the resilience of plant systems against both biotic and abiotic stressors [24,29]. The advancement of biochar engineering techniques has further broadened its applications, extending to groundwater and soil remediation, as well as water and wastewater treatment and reuse [15,30,31].

However, as the world enthusiastically embraces solid waste pyrolysis as a sustainable technology, it faces a significant challenge in the form of air pollution generated during the pyrolysis process [32,33]. The exhaust gases produced during pyrolysis can contain a range of harmful air pollutants, including particulate matter (PM), volatile organic compounds (VOCs), carbon monoxide (CO), nitrogen oxides (NO_x), etc. These pollutants not only have immediate implications for air quality but also contribute to long-term environmental and health issues [34]. Research studies and reports from various regions, including Europe, Asia, and North America, have highlighted the urgent need to address the air pollution concerns associated with solid waste pyrolysis [35]. The impact of these emissions on local air quality and their potential for global climate change necessitates thorough investigation and effective mitigation strategies.

The growing awareness of air pollution concerns stemming from solid waste pyrolysis has spurred governments and international organizations to establish regulatory frameworks and emission standards. As an example, the United States Environmental Protection Agency (USEPA) has initiated an advance notice of proposed rulemaking (ANPRM) with the aim of assisting in the potential development of regulations for pyrolysis and gasification units [36]. These units play a pivotal role in converting solid or semisolid feedstocks, which encompass materials such as municipal solid waste, industrial waste (e.g., plastics and tires), agricultural and animal waste, and organic contaminants in soils and oily sludges, into valuable products like energy, fuels, and chemical commodities. The ANPRM serves as a platform for a diverse array of stakeholders, including potentially affected facilities, small businesses, and state, local, and tribal governments, to partake in the data and information-gathering process [36]. Their input regarding the intricacies of pyrolysis and gasification units and processes is invaluable. Based on the data and information acquired through this ANPRM, the agency will assess the most effective approaches to regulate pyrolysis and gasification units. Simultaneously, environmental agencies and organizations worldwide have set forth stringent guidelines aimed at curbing the release of harmful air pollutants during the pyrolysis process [37,38,39]. These regulations are designed to strike a balance between harnessing the benefits of biofuel and biochar production through pyrolysis and the imperative of safeguarding human health and the environment.

This comprehensive review article seeks to shed light on the critical aspects of air pollutants during pyrolysis for biofuel and biochar production. It delves into the sources and composition of these pollutants, their environmental and health impacts, and the current state of regulatory expectations and compliance in various regions. Furthermore, this review explores the strategies and technologies employed to minimize emissions during the pyrolysis process, paving the way for more sustainable and ecofriendlier biofuel and biochar production. In doing so, this article provides a comprehensive overview of the challenges and opportunities presented by solid waste pyrolysis, offering insights into its role in the transition towards a cleaner and more sustainable energy and agriculture sector.

2. Factors Affecting the Types of Air Pollutants during Pyrolysis

Due to its thermal degradation characteristics, the pyrolysis of solid waste, including biomass, stands out as a leading technology for both waste volume reduction and the creation of various forms of fuel [18]. While pyrolysis effectively decreases the emission of pollutants, it also gives rise to the production of diverse types of pollutants [37]. The types and levels of pollutants generated during pyrolysis are influenced by a variety of factors, which can vary depending on the feedstock, pyrolysis process conditions, and equipment used [40].

2.1. Feedstock Composition

The nature of the feedstock significantly influences the types of pollutants generated during pyrolysis. Different feedstocks vary in terms of their organic composition, moisture levels, and impurities, leading to variations in emissions [40,41]. Forestry waste, agricultural residues, waste tires, and plastics each possess distinct chemical compositions, resulting in diverse pollutant profiles during pyrolysis [42].

Research has demonstrated that the pyrolysis of furan binders may result in the emission of significant quantities of gaseous pollutants, including cresols, benzene, toluene, and m,p,o-xylenes [43,44]. In the case of waste tires undergoing pyrolysis, significant environmental concerns arise due to the emergence of polycyclic aromatic hydrocarbons (PAHs), sulfur-based compounds, and nitrogen-based pollutants [45]. These compounds primarily originate from the decomposition of the tire’s original constituents during pyrolysis and are exacerbated by the formation of high molecular-weight cyclic compounds (known as cycloadducts) through Diels-Alder reactions, a chemical reaction between a conjugated diene and a substituted alkene [46]. PAHs encompass a broad spectrum of over a hundred chemically related compounds, each characterized by distinct structures and associated toxicity, making them significant environmental pollutants [47]. Their detrimental impact on the environment is attributed to their ability to induce toxicity through various mechanisms, affecting the biota and potentially leading to carcinogenic and mutagenic consequences upon exposure [47].

In addition to PAHs, nitrogen-based compounds (NO_x) must be carefully monitored, as they can contribute to atmospheric concerns such as acid rain formation and ozone depletion [48]. Understanding the reaction pathways that can mitigate the presence of these pollutants during waste tire decomposition is crucial. Moreover, the nitrogen content in the original materials (fuel-N) significantly contributes to the formation of nitrogen-based pollutants within the resulting products, which can manifest in various pyrolysis products. The major constituents of these nitrogen-based compounds, including hydrogen cyanide (HCN), ammonia (NH₃), and isocyanic acid, are prevalent within the pyrolysis oil derived from waste tires [49].

Sulfur presents a formidable challenge in the proper disposal of waste tires, primarily arising from the vulcanization process employed in rubber manufacturing [50]. This process aims to enhance the rubber’s toughness, heat resistance, and overall stability [51]. Sulfur compounds in waste tires form cross-linked structures within the long-chain polymers of vulcanized rubber, in addition to serving as antioxidants [51]. Consequently, sulfur compounds resist degradation, further contributing to the complexity of waste tire pyrolysis [50].

Notably, preprocessing steps, such as drying and size reduction of feedstock, can influence the moisture content, chemical composition, physical characteristics, structural components, and potential reactivity of the feedstock, thus affecting the pyrolysis process and emissions [52]. A recent study delved into the influence of feedstock drying on the emissions of PAHs associated with particulate matter (PM) production in the co-generation of biochar and bioenergy [52]. The study involved the generation of raw pyrolysis volatile compound mixtures from rice husk at temperatures ranging from 400 °C to 800 °C, utilizing a laboratory-scale continuous pyrolysis-combustion system, followed by combustion at 850 °C. The findings revealed a notable contrast when employing as-received (AR) rice husk in comparison to dried rice husk [52]. Specifically, the utilization of AR rice husk resulted in significantly higher energy-based PM₁₀ yields, with a 1.2-fold increase at 400 °C and a 1.6-fold increase at 800 °C, in comparison to dried rice husk. This increase primarily pertained to the PM_2.1–10 size fraction. Additionally, the concentration of PM-bound PAHs for AR rice husk was 2.1 and 2.8 times higher at 400 °C and 800 °C, respectively, compared to dried rice husk [52]. Consequently, this led to a notable rise in the energy-based yield of PAHs across the entire range of volatile production temperatures for AR rice husk. Nevertheless, it is noteworthy that the majority of the PM-bound PAH species generated from AR rice husk comprised 2- and 3-ring PAHs, namely naphthalene, acenaphthylene, and acenaphthene, which generally possess relatively low toxicity [47]. Moreover, the concentration of 4-, 5-, and 6-ring PAHs was typically lower in the case of AR rice husk, resulting in the benzo(a)pyrene-equivalent toxicity of the PM derived from AR rice husk being lower than that of the dried counterpart [52].

2.2. Pyrolysis Temperature

The pyrolysis process occurs over a range of temperatures, and the temperature at which it is conducted can significantly impact the types of products and pollutants produced [42]. Lower temperatures may favor the formation of biochar and bio-oil, while higher temperatures can lead to the generation of syngas [27]. Temperature also influences the composition of tars, gases, and other byproducts [42].

In a recent study, analytical pyrolysis was conducted to assess the emission of major hazardous air pollutants (HAPs) during the pyrolysis of bituminous coal and a furan binder [43]. For bituminous coal, the predominant HAP emissions included cresols, benzene, toluene, phenol, and naphthalene, whereas, for the furan binder, m,p,o-xylenes were the notable emissions [43]. It was determined that xylene emissions were primarily attributed to xylenesulfonic acid, the acidic catalyst present in the furan binder [43]. Notably, for both casting materials, the majority of emissions occurred within the temperature range of 350–700 °C [43].

The reaction temperature stands as a crucial parameter with a significant influence on the emission of NO_x precursors. Elevated temperatures contribute to improved yields of NO_x precursors. Notably, it has been documented that lower temperatures promote the generation of NH₃, while higher temperatures tend to favor the formation of HCN. It is evident that, during the pyrolysis of most biomass, NH₃ is more abundantly generated at lower temperatures, typically below 500 °C (Figure 1). This phenomenon can be attributed to the deamination reaction of unstable amine compounds or the direct depolymerization of proteins. As the temperature approaches approximately 500 °C, the secondary cleavage of intermediates (e.g., heterocyclic-N) and the decomposition of amines contribute to the formation of HCN. Furthermore, it is noteworthy that the NH₃ yield curve exhibits either a plateau or experiences an increase within the temperature range of 600−900 °C, whereas HCN exhibits more substantial releases (Figure 1). This is primarily due to the continued decomposition of nitrile-N and heterocyclic-N·NH₃ at higher temperatures. The generation of NH₃ at high temperatures originates from two distinct mechanisms: the first involves the hydrogenation reaction between HCN, coke, and H radicals, while the second entails the cleavage of heterocyclics in tar. Throughout the heating process, the sensitivity of the NO_x precursor formation pathway to temperature determines which N-pollutants dominate during each stage. The transformation of N in biomass with rising pyrolysis temperature is summarized in Figure 1.

2.3. Residence Time

The amount of time the feedstock spends in the pyrolysis reactor, also known as residence time, affects the extent of thermal decomposition and the resulting pollutants [53]. Longer residence times may lead to more mass being volatilized and relatively higher emissions [27]. A recent study revealed that a residence time within the range of 10 to 100 min had a marginal impact on both rapeseed stem biochar yield and biogas emissions. This suggests that a residence time of less than this range should be favored for minimizing emissions [53]. In the same study, a statistical analysis of the literature data indicates that there is no significant correlation between residence time and biochar yield (p > 0.1), implying that biogas emissions would also be minimally affected [53]. It is worth mentioning that in certain cases, residence time demonstrated a significant negative relationship with yield within a lower residence time range. For example, the biochar yield from Saccharina japonica exhibited a reduction from 86.6% to 59.1% as the pyrolysis residence time extended from 1 to 5 min at 380 °C [54]. This change corresponded to a notable increase in both biogas and bio-oil generation.

In addition, with the extension of residence time, a greater proportion of heavy metals in some feedstock (e.g., biosolids) was either volatilized or transferred into biogas. However, as reported in a recent study, a significant quantity remained sequestered within the biochars, contributing to heightened total concentrations of heavy metals [55]. This retention was validated through extraction experiments, affirming the immobilization of heavy metals within the biochars as they transitioned from active chemical forms to more stable states [55]. The outcomes of the risk assessment underscored that longer residence times mitigated the potential environmental risks associated with heavy metals within the biochars.

2.4. Heating Rate

The rate at which the temperature is increased during the pyrolysis process, known as the heating rate, can influence the types of gases and tars produced [56]. This rate directly impacts the speed of chemical reactions occurring within the process. A faster heating rate accelerates the breakdown of organic materials, intensifying chemical reactions and generating larger volumes of volatiles and gases as feedstock decomposes more rapidly [53]. This swift decomposition encourages the rapid vaporization of volatile compounds present in the feedstock, including methane (CH₄), hydrogen (H₂), carbon monoxide (CO), and diverse organic compounds, consequently elevating gas emissions [33,53].

Furthermore, the heating rate shapes the temperature distribution within the pyrolysis reactor. Rapid heating may induce uneven temperature distribution or spikes in specific reactor areas, influencing the types and volumes of gases produced [53]. Consequently, varying heating rates can alter the yield distribution of pyrolysis products. Higher heating rates tend to favor the production of gases and bio-oils over biochar, resulting in the production of lighter hydrocarbons and gases, whereas slower heating rates may enhance biochar formation while reducing gas release, leading to the formation of heavier hydrocarbons [57,58,59]. Adjusting the heating rate thus crucially influences the output composition and quantities of pyrolysis products [33], marking it as an important parameter in process optimization. For instance, in a study utilizing rapeseed straw for biochar production, elevating heat rates beyond 50 °C/min led to reduced biochar yield and increased bio-oil or biogas production [53,60]. This shift was attributed to heightened organic breakdown and the release of carbon-rich vapor, phenomena amplified at higher heating rates [53].

2.5. Reactor Design

Pyrolysis reactors come in various designs and configurations, each with its distinct characteristics impacting the pyrolysis process and the generation of air pollutants [61,62]. Three primary types—fixed bed, fluidized bed, and rotary kiln—stand out in their configurations and their influence on temperature, pressure, and residence time within the reactor, consequently shaping the types and quantities of pollutants emitted [63].

Fixed-bed reactors consist of a stationary bed of feedstock (Figure 2a) [64]. The uniform arrangement allows for precise control over residence time and temperature profiles. However, limited heat and mass transfer might occur within the fixed bed, potentially leading to localized temperature variations and incomplete pyrolysis [65]. This setup can result in the production of tar and char, contributing to emissions of PAHs and VOCs [66].

Characterized by a bed of particles suspended and agitated by a flow of gas, fluidized bed reactors facilitate excellent heat and mass transfer (Figure 2b) [67]. The increased contact between the feedstock and the heating medium enhances the pyrolysis efficiency [62]. Nevertheless, due to the higher temperatures and rapid reactions in the fluidized bed, they may lead to increased NO_x emissions from nitrogen-containing compounds present in the feedstock [68].

Rotary-kiln reactors utilize a rotating cylindrical vessel to heat and process the feedstock (Figure 2c) [69]. The continuous movement ensures a relatively uniform temperature throughout the material [70]. However, fluctuations in heating and cooling zones along the reactor length may lead to diverse pyrolysis reactions [69]. This variation can produce a range of pollutants, including tar, soot, and gases such as CO, CO₂, and hydrocarbons, due to varying degrees of thermal decomposition and residence times [71,72,73].

The design and configuration of each reactor influence the temperature distribution, residence time, and contact between the feedstock and heating medium [63]. Variations in these factors affect the pyrolysis reactions, leading to the formation of different pollutants. Factors like incomplete combustion, inadequate heat transfer, and variations in temperature zones within the reactor contribute to the release of pollutants such as PAHs, VOCs, NO_x, and carbonaceous particles [63]. Therefore, optimizing reactor design and operational parameters is crucial in minimizing pollutant emissions during pyrolysis for biofuel and biochar production.

2.6. Type of Pyrolysis

Fast pyrolysis is a rapid thermochemical process that involves swiftly heating the feedstock in the absence of oxygen [74]. Operating at temperatures typically ranging from 400 to 600 °C and with residence times on the order of seconds, fast pyrolysis is known for its expeditious and efficient production of renewable fuels and chemicals [12]. This process is particularly well suited for large-scale applications where higher throughput is a priority [75]. Fast pyrolysis tends to yield higher concentrations of volatile gases, including hydrocarbons, phenols, and various organic compounds [76]. In a recent study focusing on biochar production through fast pyrolysis, conducted at a high heating rate of 100 °C per second until reaching close to 800 °C, gas yields from 14 different plant feedstocks varied between 18% and 28% for live plant species, and from 16% to 25% for dead plant species [77]. The gases generated from these diverse feedstocks exhibited varying compositions. Concentrations of CO ranged from 53.4% to 63.0% for live plant species and 55.4% to 60.5% for dead plant species. Notably, a considerable amount of phenol was detected during the biochar production process with all 14 different feedstocks, accounting for nearly 27% of all the hydrocarbons generated [77].

Slow pyrolysis is characterized by a relatively low heating rate and operates at temperatures typically ranging between 300 and 500 °C [78]. Distinguished from faster pyrolysis processes, slow pyrolysis involves a more gradual and controlled heating of the material. One of its notable outputs is the substantial production of biochar [79]. Despite its slower kinetics compared to other pyrolysis methods, slow pyrolysis also generates bio-oil and combustible gases [6]. The extended residence time of the material in the pyrolysis reactor during slow pyrolysis allows for more thorough thermal decomposition, resulting in a broader spectrum of end products [6]. In a recent study, slow pyrolysis conducted at temperatures ranging from 25 to 550 °C reported BTX (benzene, toluene, xylene) compound emissions reaching as high as 0.33 mg C/min. The predominant hydrocarbons in this study were lighter hydrocarbons (C2–C4), constituting up to 80% of the total hydrocarbons generated [80].

Flash pyrolysis is an expeditious thermal decomposition process involving the rapid heating of biomass or other organic materials to elevated temperatures, typically within the range of 500 to 1000 °C, through a brief and intense heat pulse [81]. The term “flash” pertains to the remarkably brief residence time of the material within the high-temperature zone, typically measured in milliseconds [81]. In the flash pyrolysis process, biomass experiences an abrupt and intense heat source, inducing swift thermal decomposition. The abbreviated residence time curtails complete material breakdown, resulting in the generation of bio-oil, gas, and biochar. The intense heating conditions sometimes facilitate a more pronounced secondary cracking of pyrolysis vapors, leading to the increased formation of CO, CH₄, C₂H₂, C₂H₄, and C₂H₆ [82].

Limited research has explored gas emissions in emerging pyrolysis technologies, including microwave pyrolysis, solar pyrolysis, and plasma pyrolysis. Given their distinct heating mechanisms, comparing these innovative pyrolysis processes with conventional methods presents challenges. There is an urgent need for new protocols and guidelines to regulate these evolving pyrolysis processes.

2.7. Gas Atmosphere

Pyrolysis, conducted within diverse gas atmospheres, profoundly impacts the composition of pyrolysis products and resultant air pollutants [83]. Several atmospheres—such as inert gases, air, or steam—alter the pyrolysis process and consequent pollutant generation. The choice of pyrolysis atmosphere influences the balance between various reactions occurring during the process [84].

Employing inert gases such as nitrogen (N₂) or helium (He) prevents oxidation reactions during pyrolysis [85]. This atmosphere inhibits the combustion of feedstock, maintaining a reducing environment that impacts product distribution [85]. Inert gas pyrolysis often leads to the production of biochar and volatile compounds. However, incomplete combustion and limited oxygen may cause the formation of carbon monoxide (CO) and potentially harmful hydrocarbons [86].

Although pyrolysis is supposed to be conducted in oxygen-limited conditions, the presence of air is common in practice [87]. The introduction of oxygen allows partial oxidation of the feedstock, leading to increased combustion and potentially reducing the formation of biochar [12,41]. However, the presence of oxygen can also escalate the generation of harmful pollutants like NO_x due to the reaction between nitrogen in the feedstock and oxygen at elevated temperatures [88]. Additionally, the higher oxygen content may lead to increased emissions of CO₂ [89].

Introducing steam in pyrolysis reactors facilitates steam reforming reactions, altering product distribution [90]. Steam can enhance the decomposition of volatile compounds, promoting hydrogen-rich gas production [91]. However, high-temperature steam can also facilitate the release of VOCs and potentially contribute to the formation of NO_x [88,92,93]. According to a recent study, an abundance of water vapor in the combustion atmosphere would hinder the liberation of fuel-N during the devolatilization phase and encourage the creation of specific reducing gases. Aside from the expected dilution impact, introducing steam could also abbreviate the timeframe for both homogeneous and heterogeneous NO_x reduction by the reducing gases and char. It is theorized that the surge in NO_x emissions might be attributed to the formation of OH radicals at elevated temperatures and high O₂ content [94].

The variation in pyrolysis gas atmospheres significantly impacts the pathways of pyrolysis reactions, affecting the distribution of pyrolysis products and subsequently influencing the emission of diverse air pollutants such as CO, CO₂, NO_x, VOCs, and other organic compounds [83,84,85]. Therefore, as shown in

Table 1. Gaseous contaminant emissions under various pyrolysis conditions with different types of feedstock.

Feedstock	Pyro. Temp. (°C)	Residence Time (min)	Heating Rate (°C/min)	Atm. Gas	Reactor Type	Gaseous Contaminant	Ref.
Sewage sludge	500–800	20	-	-	Rotary kiln	PAHs 0.22–421 µg/m3	[95]
Xylan, lignin, cellulose	800	2.6 s	-	N2 100 mL/min	Fixed bed	PAHs 11.9–48.8 µg/g	[76]
Wood pellets, e-waste	850	6–15	12–25	N2 500 mL/min	Fixed bed	CO 227.6 mg/g CO2 107.7 mg/g VOCs 91.9 mg/g	[96]
Coal, pine sawdust	300–1000	60	3	N2 100 mL/min	Fixed bed	CO 1.9–7.2% CO2 2.3–21.1% CH4 3.4–19.2%	[97]
Municipal solid waste	600–800	6	-	N2 200 mL/min	Fixed bed	CO 5.8 mol/kg CH4 3.2 mol/kg	[83]
Wheat straw	350–650	-	20	N2 100 mL/min	Fixed bed	CO 19–39% CO2 15–64%	[98]
Municipal sludge	300–700	60	10	N2 1 L/min	Fixed bed	H2S ~900 mg/m3 SO2 ~220 mg/m3 NO ~140 mg/m3 CO ~26,000 mg/m3	[99]
Municipal sludge	300–700	60	10	CO2 1 L/min	Fixed bed	H2S ~620 mg/m3 SO2 ~280 mg/m3 NO ~120 mg/m3 CO ~24,000 mg/m3	[99]
Furfural residues	500	60	10	N2 60 mL/min	Fixed bed	CO 34.66–62.29% CO2 12.17–48.26%	[100]
Rice husk	400–800	-	-	N2 250 mL/min	Screw reactor	CO 20–25% CO2 8–27% CH4 2–6% PM 21.5%	[52]
Dairy manure	200–500	60	-	-	Fixed bed	PM 12.5 ± 2.7 mg/g	[42]
Conifer chip	300–500	120	-	-	Fixed bed	CO 28,000 ppm VOCs 634 ppm	[101]

, choosing the appropriate gas atmosphere in pyrolysis reactors is critical to tailor product yields and minimize the generation of harmful pollutants.

2.8. Catalysts

The incorporation of catalysts in the pyrolysis process introduces a versatile avenue for tailoring product composition. While catalysts can enhance targeted product yields, various types of catalysts exhibit distinct functionalities, influencing the pyrolysis reactions and subsequent pollutant formation [79,102,103].

Zeolites and metal catalysts are commonly employed to enhance specific product yields during pyrolysis [104]. Zeolites act as solid acidic catalysts, promoting dehydration and cracking reactions [104]. Metal catalysts, such as nickel or iron, facilitate reforming reactions, particularly in the presence of steam, leading to an increased production of hydrogen-rich gases [105,106]. However, the use of metal catalysts may also contribute to the generation of NO_x due to their role in nitrogen-containing compound reactions. According to a previous study, the emission of NO_x was suppressed under catalytic conditions at elevated combustion temperatures, whereas it was encouraged at lower temperatures [107]. The impact of catalysts on NO_x emission levels indicates how significantly the catalytic effect influences both char-N oxidation and the NO-char reaction in the combustion process of nitrogen-containing char.

Basic or alkaline catalysts, such as alkali metals or alkaline earth metals, can influence the pyrolysis process by promoting deoxygenation reactions and reducing the oxygen content in the products [108]. While they enhance the yield of bio-oils and decrease oxygenated compounds, they may also contribute to the formation of carbonates and potentially emit CO₂ during the pyrolysis process [108]. Earlier research indicated that alkali metals have the potential to stimulate the depolymerization and fragmentation of feedstock constituents. For example, Banks et al. explored the influence of potassium on the rapid pyrolysis of beech wood within a bubbling fluidized-bed reactor, revealing that the presence of potassium resulted in a decrease in bio-oil yield, accompanied by an increase in both noncondensable gases and char yield [109]. Compared to alkali metals, alkaline earth metals exhibit stronger Lewis acid characteristics, which enhances the formation of dehydration products [108]. In a study by Case et al., pine underwent pretreatment using different calcium compounds to generate bio-oil in a fluidized bed pyrolysis reactor. Calcium formate served as the pretreatment compound due to its role as a hydrogen donor upon heating. At temperatures exceeding 450 °C, pyrolysis of calcium formate leads to the breakdown of the formate salt, resulting in the formation of solid calcium carbonate alongside gaseous CO and H₂ [110].

3. Environmental and Health Impacts of Air Pollutants

During pyrolysis for biochar production, several air pollutants can be emitted, depending on various factors as described above. Some of the major air pollutants typically emitted during pyrolysis include particulate matter, VOCs, PAHs, NO_x, CO, and sulfur compounds.

3.1. Particulate Matter (PM)

Pyrolysis stands as a promising technique for biochar production, yet it generates particulate matter (PM) emissions comprising carbonaceous particles, ash, and solid residues [42]. These diverse particles, varying in size and composition, wield significant implications for both air quality and human health [111]. Recognizing the environmental and health impacts of PM emissions from pyrolysis is pivotal in implementing effective mitigation strategies and fostering sustainable biochar production practices.

The PM emissions from pyrolysis constitute a complex amalgamation of carbonaceous particles, ash, and solid residues derived from the feedstock [52]. The composition and size distribution of PM exhibit variability influenced by distinct feedstock properties, pyrolysis conditions, and reactor designs [42]. For instance, studies on different feedstocks such as wood, agricultural residues, or waste materials showcase the diverse composition and emission levels of PM, highlighting the nuanced impact of operational parameters on PM characteristics [42].

These emissions significantly contribute to air pollution, influencing atmospheric visibility and contributing to smog formation [112]. PM can undergo long-range transport, affecting regional air quality and ecosystems [111]. For example, research demonstrates the role of PM in altering precipitation patterns, influencing cloud formation, and impacting environmental health and ecological systems [112,113]. Instances of PM’s impact on atmospheric processes, especially in regions affected by biomass burning or industrial pyrolysis, underline its far-reaching consequences on ecosystems and human health [113,114].

The health risks associated with exposure to PM emissions from pyrolysis are substantial. Fine and ultrafine particles possess the ability to penetrate the respiratory system deeply, leading to respiratory diseases, cardiovascular issues, and heightened mortality rates [111]. Numerous studies establish the direct correlation between PM exposure and various adverse health outcomes [111,112,115], emphasizing the urgent need to mitigate PM emissions from pyrolysis operations.

Efficient mitigation strategies demand technological advancements, enhanced reactor designs, and stringent emission control measures [115]. Research initiatives focusing on refining pyrolysis technologies, optimizing operational parameters, and implementing robust particle capture and filtration systems are pivotal. Developing cleaner pyrolysis methodologies and efficient filtration systems, as evidenced by studies exploring advanced filtration materials or modified reactor designs [75,78,116], illustrates the potential for reducing PM emissions and associated environmental and health impacts.

3.2. Volatile Organic Compounds (VOCs)

Volatile organic compounds (VOCs) released during pyrolysis constitute a diverse array of organic chemicals that readily vaporize at ambient temperatures [117]. Benzene, toluene, xylene, and other VOCs contribute significantly to air pollution and entail potential health hazards [118].

VOCs discharged during pyrolysis actively partake in air pollution by interacting with nitrogen oxides and sunlight, culminating in the formation of ground-level ozone and secondary organic aerosols [119]. These compounds hold a critical role in fostering smog and instigating photochemical smog incidents [119]. Extensive research in urban and industrial locales affected by pyrolysis activities substantiates the correlation between VOC emissions and escalated concentrations of ground-level ozone, highlighting their contribution to regional air pollution [120,121,122].

Furthermore, VOCs are capable of long-range dispersion, contributing to the genesis of atmospheric particulate matter [117]. Research instances accentuate the involvement of VOCs in the creation of secondary organic aerosols, impacting atmospheric visibility and influencing climate dynamics [119]. Investigations were conducted in regions affected by biomass burning to elucidate how VOC emissions influence aerosol formation, thereby shaping atmospheric processes and impacting environmental health [123,124].

Exposure to VOCs emanating from pyrolysis poses substantial health risks to individuals through inhalation or skin contact, leading to adverse health outcomes [118]. Epidemiological studies consistently associate VOC exposure with respiratory issues, exacerbating asthma, bronchitis, and other pulmonary ailments [125]. For instance, research in proximity to pyrolysis facilities underscores elevated respiratory symptoms and heightened hospital admissions among exposed populations [126].

Moreover, several VOCs, such as benzene, ethylene oxide, and formaldehyde, are classified as carcinogens or suspected carcinogens by regulatory agencies [127]. A plethora of studies establish the nexus between occupational or chronic exposure to specific VOCs and heightened cancer risks [128,129,130]. This underscores the pressing need for stringent regulatory interventions and control measures to mitigate these health risks associated with VOC emissions. Continued research aimed at elucidating VOC composition, exposure levels, and their variable impacts across diverse pyrolysis settings is imperative in safeguarding environmental quality and human health.

3.3. Polycyclic Aromatic Hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) are a cluster of organic compounds synthesized through incomplete combustion or pyrolysis of organic materials [131]. The US Environmental Protection Agency (EPA) has identified 16 PAHs as high-priority substances due to their potential toxicity in both humans and other organisms, as well as their widespread presence and persistence in the environment. Notably recognized for their carcinogenic nature, these PAHs pose significant environmental and health hazards when released during biochar production via pyrolysis [52].

PAHs discharged during biochar production via pyrolysis contribute substantially to environmental pollution. These compounds traverse the environment, depositing in various matrices [132]. Research demonstrates the persistence of PAHs in soil and sediment, posing risks to terrestrial and aquatic ecosystems [133]. Investigations conducted in pyrolysis-affected regions highlight correlations between PAH emissions and adverse impacts on soil microbial communities and aquatic organisms [134,135]. Furthermore, PAHs exhibit a propensity for bioaccumulation in food chains, posing threats to higher trophic-level organisms [136]. Studies exploring PAH accumulation in plants and aquatic species underscore their ability to traverse the food web, potentially disrupting ecosystem dynamics and imperiling biodiversity [137,138].

Human exposure to PAHs released during biochar production via pyrolysis presents substantial health risks [138]. Inhalation or dermal contact with PAHs leads to adverse health outcomes [139]. Epidemiological studies consistently associate PAH exposure with respiratory issues and cardiovascular diseases. Proximity to pyrolysis sites correlates with heightened incidences of respiratory symptoms and cardiovascular ailments among exposed populations [140,141,142,143]. Moreover, specific PAH compounds like benzo[a]pyrene (BaP) are identified as potent carcinogens by regulatory agencies [144]. Extensive research establishes a direct association between chronic PAH exposure and heightened cancer risks [145,146], underscoring the imperative for stringent regulatory measures and effective control strategies to mitigate the health hazards linked to PAH emissions.

3.4. Nitrogen Oxides (NO_x)

Nitrogen oxide (NO_x) gases, composed of nitric oxide (NO) and nitrogen dioxide (NO₂), are produced during pyrolysis processes due to high-temperature reactions involving nitrogen-containing compounds [88]. The N content of the feedstock is a primary determinant of NO_x formation. During pyrolysis, the breakdown of N-containing compounds releases N as molecular N and other reactive N species [33]. Studies have shown that rapid heating rates and short residence times may favor NO_x formation [147,148,149]. At the same time, surface reactions involving N-containing compounds on the char surface, especially in the presence of reactive C radicals, can also contribute to NO_x emissions [88].

NO_x emissions contribute notably to environmental degradation, inducing smog formation that hampers air quality and visibility [150]. Additionally, these gases undergo atmospheric reactions, generating nitrogen-based aerosols that contribute to the formation of acid rain, harming soil, water bodies, and vegetation [151,152]. Studies in pyrolysis-affected regions demonstrate the direct link between NO_x emissions and detrimental effects on ecosystems and biodiversity [33,88,153].

Furthermore, NO_x gases play a role in the creation of secondary organic aerosols, exacerbating air quality issues and influencing atmospheric processes [154]. Investigations into NO_x-related atmospheric transformations highlight their significance in atmospheric chemistry, impacting environmental health and ecological systems [150]. Anthropogenic NO_x and VOCs are widely acknowledged as precursors that contribute to the formation of near-ground O₃. Analyzing the interconnected patterns among the time series of O₃ and its precursors alongside their temporal changes, nonlinear techniques like the empirical kinetic modeling approach (EKMA) and air quality models (AQM) offer insights into how O₃ reacts to its precursors across various time scales [155].

Exposure to NO_x emissions from pyrolysis poses significant health risks to human populations [150]. Inhalation of these gases can trigger respiratory issues, worsen asthma, and cause irritation in the respiratory tract [156]. Consistent epidemiological studies link NO_x exposure to increased occurrences of respiratory ailments and exacerbation of existing conditions [157]. Additionally, NO_x exposure can heighten susceptibility to respiratory infections, lung diseases, and potentially cancer. It also contributes to the formation of the brownish haze observed in overcrowded regions and plays a role in the occurrence of acid rain [158].

3.5. Carbon Monoxide (CO) and Carbon Dioxide (CO₂)

Carbon monoxide (CO) and carbon dioxide (CO₂) are commonly produced as byproducts of incomplete combustion or pyrolysis processes [152]. While CO is a colorless, odorless gas posing severe health risks when inhaled due to its interference with the bloodstream’s oxygen-carrying capacity [159], CO₂ functions as a greenhouse gas, significantly influencing climate change [160].

During pyrolysis operations, especially those involving biomass or fossil fuel feedstocks, significant emissions of CO can occur [161]. For instance, recent studies on biomass pyrolysis have highlighted the considerable release of CO due to incomplete combustion or inefficient pyrolysis conditions [12,161,162]. Similarly, when waste materials are burned or subjected to incomplete combustion during pyrolysis processes, elevated levels of CO₂ emissions are observed [12,73,152], impacting both local air quality and global climate dynamics.

Robust control measures and the adoption of cleaner pyrolysis technologies are imperative to mitigate CO and CO₂ emissions, addressing their detrimental impacts on both local air quality and global climate stability. For instance, ongoing research focuses on optimizing pyrolysis conditions to minimize CO emissions [163] and developing carbon capture and storage techniques to mitigate CO₂ release [164], aiming to promote environmentally sustainable pyrolysis practices.

3.6. Sulfur Compounds

Sulfur compounds present in specific feedstocks have the potential to emit gases like hydrogen sulfide (H₂S) during pyrolysis, significantly contributing to air pollution while often carrying a distinct and unpleasant odor [165,166].

In recent research, investigations into biomass pyrolysis involving sulfur-rich feedstocks have highlighted the notable generation of hydrogen sulfide (H₂S) during the process [165]. Additionally, studies focused on waste materials, such as tires or rubber-based compounds, have demonstrated the liberation of sulfur compounds, notably H₂S, when subjected to pyrolysis conditions [166]. These findings emphasize the diverse sources of sulfur emissions in pyrolysis and their consequential impact on air quality.

The emissions of sulfur compounds like H₂S not only contribute to air pollution but also possess an identifiable odor that serves as a marker for environmental contamination. The odoriferous nature of these emissions signals their potential adverse effects on both environmental and human health. Prolonged exposure levels below 10 ppm have long been linked to odor aversion, along with ocular, nasal, respiratory, and neurological effects. Remarkably, exposure to even lower levels, under 0.03 ppm (30 ppb), has shown an increased incidence of neurological effects, while concentrations below 0.001 ppm (1 ppb) of H₂S have been linked to ocular, nasal, and respiratory effects [167].

Efforts to curtail the release of sulfur-containing gases during pyrolysis are paramount to prevent air quality degradation. Ongoing research is focusing on refining techniques to handle sulfur-rich feedstocks more efficiently and innovating pyrolysis methodologies aimed at minimizing the liberation of sulfur compounds [168]. These advancements aim to mitigate the negative implications of sulfur emissions on air quality and their associated risks to human health, aligning with sustainable pyrolysis practices.

4. Current State of Regulatory Expectations and Compliance in Various Regions

In the global context of waste management, diverse regulatory frameworks govern the disposal and treatment of solid waste. The current state of regulatory expectations and compliance in various regions regarding emissions from pyrolysis processes reflects an evolving landscape focused on mitigating environmental and health impacts. This section explores the regulatory landscapes of key regions, including the United States, the European Union, and the Asia–Pacific Region, shedding light on the distinct approaches and policies influencing waste management practices.

4.1. United States

In the United States, the regulatory landscape regarding emissions from industrial processes, including pyrolysis, is governed by a mix of federal and state-level regulations aimed at safeguarding air quality. The Environmental Protection Agency (EPA) plays a pivotal role in establishing and enforcing guidelines to limit air pollutants.

The EPA’s regulations encompass a wide array of air quality standards applicable to various industries, including those employing pyrolysis for biochar or biofuel production. The Clean Air Act, administered by the EPA, sets national ambient air quality standards (NAAQS) for specific pollutants like particulate matter (PM), nitrogen oxides (NO_x), sulfur dioxide (SO₂), and volatile organic compounds (VOCs), which may be emitted during pyrolysis [169]. By setting these standards, the Clean Air Act provides a regulatory foundation for monitoring and controlling air pollution, ultimately contributing to the broader mission of environmental protection. Compliance with NAAQS requires industries to adhere to emission limits and implement control technologies [170].

States often adopt and enforce regulations that align with federal standards while addressing localized concerns. States may impose additional regulations or more stringent standards based on specific air quality challenges within their jurisdictions. For instance, California’s stringent air quality standards often exceed federal regulations, influencing industries operating within the state [171]. In addition, California has implemented robust climate change mitigation measures aimed at improving air quality while also reaping public health benefits. These initiatives encompass advanced clean car standards, the promotion of renewable energy sources, a sustainable community strategy to curb suburban sprawl, the establishment of a low-carbon fuel standard, and initiatives focused on enhancing energy efficiency. Among these measures is a market-based mechanism known as the cap-and-trade program, permitting capped facilities to trade greenhouse gas (GHG) emissions allowances issued by the state. The “cap” sets a ceiling on total GHG emissions from covered sources, progressively decreasing over time to effectively reduce overall emissions [172].

Compliance with EPA regulations involves implementing emission control technologies and practices to mitigate pollutants stemming from pyrolysis [173]. Continuous monitoring of emissions, regular reporting to regulatory authorities, and strict adherence to emission standards are essential components of compliance measures. Technologies such as tar scrubbers [174], catalytic converters [175], and advanced filtration systems [176] are deployed to reduce and control emissions of pollutants like PM and VOCs. For instance, the utilization of a catalytic converter containing palladium and platinum catalysts resulted in an approximately 20% reduction in PAH emissions [177]. In recent years, studies have emphasized the importance of compliance with EPA regulations in industries using pyrolysis [178,179]. Research efforts have demonstrated the efficacy of various emission control technologies in reducing pollutants emitted during pyrolysis processes [180]. For instance, studies on biomass pyrolysis have highlighted the effectiveness of advanced gas cleaning systems in lowering emissions of PM and VOCs, aligning with EPA emission standards [181,182].

4.2. European Union

The European Union (EU) has a robust regulatory framework in place to address emissions from industrial processes, including pyrolysis, ensuring stringent environmental standards.

The European Environment Agency (EEA) plays a crucial role in monitoring and reporting on environmental issues across the EU. It supports the development and implementation of policies aimed at reducing emissions and improving air quality [183].

The Industrial Emissions Directive (IED) sets comprehensive standards for controlling emissions from industrial activities, encompassing pyrolysis operations. It establishes emission limit values (ELVs) for various pollutants, including particulate matter, nitrogen oxides, sulfur dioxide, and volatile organic compounds, which may arise during pyrolysis [184,185]. Compliance with ELVs is mandatory for industries and requires the adoption of best available techniques (BAT) to minimize emissions [186].

Recent studies conducted in the EU have focused on the implementation of BATs in pyrolysis facilities to comply with emission standards. For example, research has evaluated the effectiveness of advanced scrubbing and filtering technologies in reducing PM and other pollutant emissions from biomass pyrolysis [187]. Data from compliance reports highlight the successful implementation of these techniques [188], showcasing reduced emissions and improved air quality outcomes in areas with pyrolysis operations.

The Integrated Pollution Prevention and Control (IPPC) Directive is another regulatory instrument within the EU that aims to prevent or reduce emissions from various industrial activities to achieve a high level of environmental protection. Pyrolysis facilities fall under this directive and must adhere to stringent emission standards and permit requirements [189]. Additionally, the EU develops BAT reference documents (BREFs) that provide guidance on best practices and techniques to minimize emissions from specific industrial sectors, including pyrolysis. These documents outline efficient methods and technologies to control and reduce pollutant discharges [190].

The IED and IPPC Directive are closely related and often used interchangeably. The IED aims to regulate and control industrial emissions, ensuring a high level of environmental protection. Whereas, the IPPC, initially introduced in 1996 and later replaced by the IED in 2010, focused on preventing and controlling pollution from various industrial activities. The IED builds upon the principles of the IPPC Directive but expands its scope and strengthens the regulatory framework. It addresses a broader range of industrial activities and introduces more stringent requirements for environmental performance. The goal of both directives is to achieve a high level of protection for the environment as a whole by controlling emissions and promoting the use of best available techniques.

4.3. Asia–Pacific Region

The Asia–Pacific region has been increasingly proactive in addressing air quality concerns through evolving regulatory measures, especially concerning industrial emissions like those from pyrolysis processes.

China has been focusing on stringent emission standards to curb air pollution. For instance, the country has implemented the “Air Pollution Prevention and Control Action Plan”, setting specific targets for reducing air pollutants [191]. Regulations include emission standards for various industrial processes, emphasizing the reduction of PM, SO₂, NO_x, and VOCs. As of August 2015, China had issued a comprehensive set of 1890 environmental protection standards, with the environmental quality standard serving as the primary component of the environmental standard system. This is complemented by supplementary standards for pollutant emissions and pollutant monitoring [192]. Continuous monitoring and stringent compliance measures are central to these initiatives [193]. A sustained national commitment to biochar production, utilizing about 33% of the sustainably available crop residues in China, holds the potential to cut China’s GHG emissions significantly by approximately 54.27 Mt CO₂-eq annually [18]. Assessing the future impact on GHG emissions under a “moderate” scenario that involves processing 73% of crop residues into biochar and biofuels using carbon-negative technologies from 2020 to 2030, followed by coordinated deployment with bioenergy with carbon capture and storage (BECCS) post-2030, could yield a cumulative reduction in GHG emissions of up to 8620 Mt CO₂-eq by 2050 [18].

India, too, has been strengthening its regulatory framework to address air quality issues. The country has introduced the National Clean Air Programme (NCAP), aiming to reduce air pollution levels in various cities [194]. The program emphasizes the reduction of PM, SO₂, NO_x, and other pollutants. Emission standards for industrial processes, including pyrolysis, are being revised and implemented to align with cleaner technologies and strict emission limits [194]. As an illustration, the Indian National Ambient Air Quality Standards stipulate that the annual average PM_2.5 emissions from the power, industry, and transportation sectors must be brought below 40 µg/m³ nationwide. This stringent benchmark is comparable to the 35 µg/m³ in the United States but considerably higher than China’s 150 µg/m³ [195]. Addressing this challenge necessitates a coordinated approach involving urban, rural, and interstate responses.

Recent data from air quality monitoring stations in major cities across the Asia–Pacific region indicate the severity of air pollution caused by industrial emissions. Studies on emissions from pyrolysis and related processes in this region have highlighted the need for stricter regulations to control pollutants [81,196]. Regulatory measures across the Asia–Pacific region aim to mitigate the adverse impacts of industrial emissions, including those from pyrolysis processes, on air quality and public health. The focus is on enforcing stringent emission standards, promoting cleaner technologies, and fostering compliance to combat air pollution effectively.

4.4. Global Trend

Globally, there is a noticeable shift towards comprehensive regulatory frameworks that address emissions from pyrolysis and analogous processes, reflecting an increased emphasis on environmental sustainability and pollution control.

Various regions are investing in research and development to advance technologies that minimize emissions during pyrolysis [70,78,164]. For example, the integration of catalytic converters, advanced filtration systems, and gas-cleaning technologies in pyrolysis plants has shown significant promise in reducing pollutant emissions [163,166,175,176]. These advancements highlight the trend toward adopting cleaner and more efficient production methods.

Countries and regions worldwide are progressively setting stringent emission standards for pyrolysis operations. These standards encompass limits for PM, SO₂, NO_x, VOCs, and other pollutants generated during pyrolysis. Compliance with these standards is being enforced through continuous monitoring, reporting, and inspection measures. Notably, considerable disparities persist in the regulations pertaining to pollutants, encompassing variations in the criteria for setting limits, average timeframes, and concentration thresholds among these standards [197]. A comparative study conducted to assess these standards revealed that Mongolia, Russia, and many Central and Eastern European countries uphold comparatively stringent norms. In contrast, Central and Southeast Asian countries tend to have relatively lenient standards. Additionally, there significant diversity in the stringency of standards across South Asia, West Asia, and the Middle East regions [197].

Global initiatives are also encouraging sustainable practices in pyrolysis and related industries. These practices include the utilization of biomass and waste materials as feedstocks [64,79], promoting circular economy principles to reduce waste [3,185], and emphasizing the importance of reducing environmental footprints [89,198].

As shown in Figure 3, there is a growing trend toward international collaboration and knowledge sharing among countries and regions to develop harmonized standards and best practices for emissions control in pyrolysis [199]. This collaboration fosters the exchange of information, technological innovations, and strategies aimed at achieving global environmental goals. Between 2017 and 2022, China emerged as the dominant contributor to pyrolysis research, accounting for 56% of the published articles in this field. The USA and India followed, contributing 12% and 5.4%, respectively [199]. China also led in collaborative articles, with 968 publications during the same period, representing around 35% of the total output. Pakistan (84%), Germany (83%), and the United Kingdom (82%) had the highest proportions of collaborative articles. This collaborative trend has persisted consistently.

The evolving global trend is directed toward establishing comprehensive regulations that not only set strict emission standards but also prioritize the adoption of cleaner technologies and sustainable practices. This shift signifies a concerted effort to reduce the environmental impact of industrial processes like pyrolysis on a global scale, ensuring a more sustainable future.

5. Strategies and Technologies Employed to Minimize Emissions during Pyrolysis

The emissions of these pollutants can vary based on the feedstock characteristics, pyrolysis conditions (such as temperature, heating rate, and residence time), reactor design, and the gas atmosphere employed during the process. Proper control measures and optimization of pyrolysis conditions can help minimize the generation and release of these pollutants into the atmosphere, reducing their environmental impact. Strategies and technologies are integral to the ongoing efforts to mitigate emissions during the pyrolysis process, fostering cleaner production methods and ensuring sustainable practices in biochar production and similar industrial processes.

5.1. Improved Reactor Designs

Fluidized-bed reactors suspend feedstock particles in an upward-flowing stream of gas, facilitating excellent heat transfer and uniform temperature distribution. Research on fluidized-bed reactors, like the work done by Pienihäkkinen et al. (2021), has shown significant promise in achieving high biochar yields with minimized emissions [74]. The dynamic nature of fluidized beds allows better control over reaction conditions, ensuring efficient pyrolysis with reduced environmental impact. In their design, the issues related to bed agglomeration in the BFB unit were resolved by introducing a high-speed rotating mixer within the reactor to disrupt the agglomerates [74].

Fixed-bed reactors are known for their simplicity and reliability. They enable precise control over residence time and temperature distribution, critical factors influencing pyrolysis reactions [64]. Recent studies, such as those conducted by Al-Salem (2019), have focused on optimizing fixed-bed reactor designs to achieve higher biochar quality and quantity while limiting emissions of VOCs and other pollutants [200]. It was reported that The gaseous product contained over 70% of C₂ to C₄ hydrocarbons, primarily due to the increased activity of the carbon-carbon (C-C) chain scission reaction [200].

Cylindrical rotating kilns facilitate continuous feedstock movement, ensuring uniform heat distribution and residence time [69]. Innovative rotary kiln designs, as explored by Hu et al. (2022), have demonstrated improved energy efficiency and reduced emissions by precisely controlling the pyrolysis process parameters [201].

Research from Sekar et al. (2022) has emphasized the critical role of reactor design in achieving optimal pyrolysis performance [63]. Their findings underscore how specific design modifications in these reactors can significantly influence temperature gradients, residence times, and ultimately, emission profiles during the pyrolysis process [201,202].

Furthermore, computational fluid dynamics (CFD) simulations, such as those conducted by Yang et al. (2021), aid in modeling and optimizing reactor designs [203]. These simulations provide valuable insights into fluid flow dynamics, temperature distribution, and residence time, guiding the development of more efficient and environmentally friendly pyrolysis systems [204].

5.2. Gas Cleaning and Filtration Systems

Electrostatic precipitators use electrostatic forces to remove particulate matter and pollutants from the gas stream [205]. They have been widely applied in pyrolysis setups to capture fine particles effectively [206]. For instance, Zhang et al. (2023) successfully integrated electrostatic precipitators into their pyrolysis facility, resulting in an impressive reduction in fine particle emissions of over 94%, as observed through long-term monitoring [207].

Cyclone separators leverage centrifugal force to separate particulates from the gas stream [208]. Recent advancements in cyclone separator design, such as those studied by Duan et al. (2020), have shown increased efficiency in capturing smaller particles generated during pyrolysis [209]. The findings demonstrate that the new cyclone design expedites the discharge of purified gas through the vortex finder. Additionally, it prolongs the movement distance and residence time of fine particles, eliminates short-circuit flows and the vertical vortex under the vortex finder, prevents interference between the upflow and downflow in the cylinder, and expands the quasi-free vortex area in the cone [209].

Baghouse filters use fabric bags to capture particles and pollutants from the gas stream. Shewartz et al. (2020) introduced innovative baghouse filters in their pyrolysis plant, showcasing an impressive reduction in emissions [37]. The combustion of pyrolysis products complied with EPA emissions standards for CO at 10.6 ppm, NO_x at 16.8 ppm, and SO₂ at 2.3 ppm [37]. Studies indicated a consistent removal efficiency of over 95% for fine particulates, significantly improving air quality standards [210,211].

The advancements in gas cleaning and filtration systems underscore the commitment of the industry and research community to mitigate emissions effectively, ensuring environmentally friendly pyrolysis practices. Efforts to optimize and innovate these systems are crucial steps toward achieving cleaner emissions in pyrolysis operations.

5.3. Catalytic Converters

Catalytic converters play a pivotal role in reducing harmful emissions in pyrolysis processes. They are designed to accelerate the conversion of harmful compounds into less hazardous substances through catalytic reactions [73].

Jacob et al. (2022) implemented a novel catalytic converter in their pyrolysis setup, resulting in a significant reduction in NO_x emissions [212]. The findings demonstrated a reduction in NO_x concentrations by 55–60% compared to traditional systems [212]. The catalytic converter facilitated the conversion of NO_x into nitrogen and oxygen through selective catalytic reduction, thereby minimizing their release into the atmosphere [213].

Furthermore, recent research focuses on developing novel catalyst formulations tailored for pyrolysis conditions [214]. Preliminary results indicate that these advanced catalysts exhibit superior performance in reducing emissions of specific pollutants like PAHs and sulfur compounds [215,216], showcasing a potential breakthrough in emission control strategies for pyrolysis systems.

5.4. Controlled Pyrolysis Conditions

Controlled pyrolysis conditions represent a fundamental strategy in emission reduction during pyrolysis processes [217]. Optimization of key parameters such as temperature, heating rate, and residence time enables a more efficient process while mitigating harmful emissions [7,53,55].

Biochars usually exhibit reduced VOC levels as the pyrolysis temperature increases [218]. High-VOC biochar, releasing vapors and leachates, completely hindered the germination of cress seeds, indicating potential environmental risks [219].

It has been reported that postheating the softwood pellet biochar at 200 °C led to a 95% reduction in VOC concentrations [220]. Mixing high- and low-VOC biochars has shown promise in mitigating VOC toxicity [220]. Furthermore, pretreating VOC-rich biochar (e.g., through storage and rinsing) presents an alternative strategy to alleviate their phytotoxic effects [219]. Nevertheless, the most effective approach to prevent VOC contamination in biochar remains conducting pyrolysis in controlled conditions within well-designed pyrolysis units.

5.5. Renewable Energy Integration

Renewable energy integration into pyrolysis processes presents a significant opportunity to mitigate environmental impact and reduce reliance on nonrenewable resources. The incorporation of solar, wind, or other renewable energy sources offers promising avenues for cleaner and more sustainable pyrolysis operations.

Li et al. (2023) implemented a solar pyrolysis setup where solar thermal energy was used to pyrolyze feedstock [8]. This approach significantly reduced the energy demand from conventional sources, thereby diminishing the carbon footprint of the process [8]. Hou et al. (2022) revealed in their comparative study that the solar-powered process not only reduced overall energy consumption and carbon emissions but also yielded biochar with equivalent efficacy in enriching and stabilizing heavy metals in Chinese medicine residues when compared to traditional heating methods [221].

In a study by Li et al. (2019), they introduced the concept of a negative emission hybrid renewable energy system and explored its design for a self-sufficient rural island striving for net-zero emissions [222]. For an island covering 22.05 km² and housing 10,881 individuals, the optimal system presented a substantial negative emission capacity, potentially sequestering 2795 kg of CO₂-equivalent daily, along with an estimated daily profit of 455 US dollars [222].

5.6. Realtime Monitoring and Control Systems

Realtime monitoring and control systems integrated into pyrolysis operations are integral for ensuring compliance with emission standards and optimizing processes [223]. These systems, equipped with advanced sensors and analyzers, provide immediate insights into emission levels, enabling swift corrective actions and aiding in regulatory compliance [223].

Gas sensors are capable of detecting the compositions of various gases and vapors generated during pyrolysis. Utilizing the sensing data from these gas products allows for the adjustment and optimization of operating parameters within the pyrolysis and upgrading processes, thereby enhancing pyrolysis efficiency and the quality of synthetic energy products [224]. Many available gas sensors exhibit robustness, withstanding temperatures up to 1000 °C, offering advantages such as good selectivity and long-term stability [223]. These sensors prove highly useful for monitoring and optimizing pyrolysis and upgrading processes.

Analytical feedback control plays a pivotal role in refining pyrolysis efficiency and curbing emissions. Aboughaly et al. (2020) recently introduced a closed-loop control system tailored for pyrolysis reactors [225]. This intelligent feedback mechanism utilized realtime feedback signals from three online gas analyzers—CO, CO₂, and NO_x [225]. The controller’s output signal governed the radio frequency thermal plasma torch current, ensuring immediate temperature regulation. By merging realtime analytical data with automated control, this system could swiftly identify and adjust deviations in crucial process parameters like temperature and heating rates, thus optimizing pyrolysis conditions and minimizing emissions [225]. This innovative setup was able to ensure precise temperature profiles for both pyrolysis and gasification, elevate end-product yield, and effectively eliminate undesired byproducts like tar and char [225].

6. Conclusions

The process of pyrolysis, while effective in reducing emissions, generates a diverse range of pollutants influenced by various factors. Feedstock composition, such as forestry waste or plastics, yields different pollutant profiles. For instance, waste tires produce concerns due to the emergence of PAHs, sulfur-based, and nitrogen-based pollutants during pyrolysis. Moreover, factors like pyrolysis temperature, residence time, heating rate, reactor design, gas atmosphere, and catalysts all significantly impact the types and quantities of pollutants generated. These variables can influence emissions of PM, NO_x, VOCs, CO, CO₂, and other hazardous compounds, necessitating careful monitoring and process optimization to minimize environmental risks while producing biofuels and biochar.

The discourse on emissions from pyrolysis processes reflects a dynamic landscape underscored by both challenges and innovative strategies. From examining the diverse pollutants generated to delving into regional regulatory expectations and compliance measures in the United States, European Union, Asia–Pacific, and global trends, it is evident that a concerted global effort is underway to mitigate environmental impacts.

Strategies including improved reactor designs, advanced gas-cleaning systems, catalytic converters, biochar activation, and controlled pyrolysis conditions represent substantial strides in emission reduction. The integration of renewable energy sources and realtime monitoring systems further highlights the commitment to sustainable practices in pyrolysis. With ongoing research and innovation, the synergistic approach between regulatory frameworks, technological advancements, and sustainable practices holds promise in fostering environmentally responsible pyrolysis processes for a greener future.