Fuels doi: 10.3390/fuels7020041

Authors: Yang Li Seyed Bahram Nourani Najafi P. V. Aravind Anatoli Mokhov

Dry reforming of methane (DRM) is an attractive route for H2 production and simultaneous CO2 utilization, but its practical implementation is limited by catalyst deactivation. This study experimentally investigates the catalytic performance of Ni/Al2O3 and Gd-doped ceria-promoted Ni/GDC–Al2O3 catalysts for DRM in a fixed-bed quartz reactor over 400–800 °C at gas residence times of 0.1 s and 0.4 s. Increasing temperature and residence time enhanced CH4 and CO2 conversion as well as H2 and CO yields for both catalysts. The GDC-promoted catalyst exhibited markedly improved activity, achieving conversions and product yields at 0.1 s comparable to those of Ni/Al2O3 at 0.4 s and reaching complete CH4 conversion at about 650 °C, approximately 100 °C lower than the Ni/Al2O3 catalyst. Long-term testing at 650 °C showed stable catalytic behavior of the Ni/GDC–Al2O3 catalyst, while operational observations qualitatively suggested the absence of significant carbon deposition, consistent with equilibrium calculations.

Fuels doi: 10.3390/fuels7020040

Authors: Maria Laura Mastellone

Plastics pyrolysis is increasingly pursued as a pathway for producing circular hydrocarbon feedstocks for petrochemical integration. However, non-integrated reactor configurations often exhibit limited heat-transfer control, significant char-handling requirements, and variable product distributions. This work presents a system-level interpretation of the MLM-R™ process, an integrated pyrolysis–combustion loop in which a circulating solid heat carrier enables continuous thermal supply through internal oxidation of carbonaceous residues. Material Flow Analysis (MFA) was applied to reconcile mass, elemental carbon, and chemical energy distributions across the defined process boundary. For the representative case study (1000 kg polyolefin basis), ~81% of feed carbon and ~83% of feed chemical energy (HHV basis) were recovered in the condensed liquid product, while ~7% of feed carbon was internally combusted to sustain autothermal operation. Simulated distillation analysis indicates that removal—aimed at further reprocessing—of a ~15 wt% C34+ heavy fraction from the pyrolysis vapor stream enables compliance with refinery-relevant boiling range targets (≥95% below 480 °C). The MFA results, supported by the physicochemical interpretation, suggest that integrated control of solids circulation and heat transfer contributes to product selectivity and process scalability in circular feedstock production.

Fuels doi: 10.3390/fuels7020039

Authors: Ruguo Dong Yongli Liu Lixin Li

The permeability of coal seams plays a critical role in the efficiency of coalbed methane extraction and gas disaster prevention. Traditional permeability models often overlook the anisotropic and dynamic evolution characteristics of coal under varying stress and gas adsorption conditions. This paper proposes a novel permeability evolution model that integrates the effects of effective stress variation and gas sorption-induced deformation on coal permeability. Starting from the concept of face porosity and utilizing a representative voxel approach, the model incorporates the anisotropy of mechanical parameters and adsorption expansion strain to derive the evolution of permeability in three dimensions. The model is validated against experimental permeability data from two distinct coal samples (Sulcis and Sydney), demonstrating its ability to accurately capture permeability changes under different boundary conditions. Furthermore, the concept of “internal expansion strain coefficient” is introduced to quantify the impact of adsorption-induced matrix deformation on permeability. The model provides a theoretical foundation for predicting gas flow behavior in coal seams under complex in-situ conditions and offers significant insights into the optimization of gas extraction strategies.

Fuels doi: 10.3390/fuels7020038

Authors: Anca-Iuliana Dumitru Sibel Osman Grigore Cican Bartosz Ciupek Łukasz Brodzik

This study evaluates the impact of long-term storage on aviation fuel blends composed of Jet A and camelina-derived biodiesel. The physicochemical properties of the pure biodiesel were assessed according to EN 14214 and ASTM D6751 standards, while the resulting Jet A–biodiesel blends were evaluated against ASTM D1655 aviation fuel specifications. Particular attention was given to the evolution of density during storage as an indicator of fuel stability. The results show that camelina methyl esters exhibit generally satisfactory physicochemical characteristics; however, the iodine value remains a critical limitation. The measured value of approximately 155 significantly exceeds the maximum limit of 120 established by European standards, reflecting the high degree of unsaturation of the feedstock. Long-term monitoring of the blends revealed a clear relationship between biodiesel concentration and the rate of fuel degradation. Increasing the biodiesel fraction led to more pronounced variations in density during storage, indicating reduced stability of the fuel system. Consequently, instability risks increase proportionally with the biodiesel-to-Jet A ratio, highlighting the need for appropriate storage strategies and technological optimization when considering higher concentrations of camelina-derived biodiesel in aviation fuel blends.

Fuels doi: 10.3390/fuels7020037

Authors: Hassan Niazi Kamran Taghizad-Tavana Ali Esmaeel Nezhad Afshin Canani Mehrdad Tarafdar Hagh Pouya Paidar

Green hydrogen is increasingly discussed as an energy carrier that can link electricity, gas, heat, and transport sectors. However, many existing reviews address this topic from separate viewpoints, such as hydrogen production technologies, Artificial Intelligence (AI) applications, or system integration, with less attention to how policy and market conditions affect deployment. This review brings these related aspects together in one structured discussion. The paper first reviews the hydrogen supply chain, including production, storage, transport, and utilization. It then discusses an integrated multi-energy architecture in which hydrogen interacts with electricity, natural gas, heat, and cooling networks. Policy instruments in five major economies, including the European Union, the United States, China, Japan, and India, are compared. The review also summarizes the main barriers to large-scale deployment, including high production costs, limited infrastructure, technological challenges, regulatory uncertainty, and supply-chain constraints. In addition, the current market structure and selected large-scale hydrogen projects planned in the United States are reviewed. The paper also examines the role of artificial intelligence in green hydrogen systems. AI applications are grouped into four main stages of the hydrogen value chain: forecasting renewable energy generation, improving electrolyzer design and operation, optimizing storage and distribution, and supporting system-level techno-economic assessment. Recent Machine Learning (ML) studies are compared based on their methods and their contributions to operation and planning. Overall, this review highlights the role of AI in enabling green hydrogen integration within multi-energy systems.

Fuels doi: 10.3390/fuels7020036

Authors: Jasim Al Shehihi Nitin Raut

Two-Stage Anaerobic Digesters (TSADs) have emerged as an effective strategy for improving the stability and efficiency of biogas production from high-strength substrates such as food waste. The separation of acidogenic and methanogenic phases enables better environmental control for distinct microbial communities, thereby enhancing methane yield and reducing process instability. This study investigates the dynamics of microbial populations of acidogens, acetogens, and methanogens in a TSAD using an extended Anaerobic Digester Model No. 1 framework incorporating stage-specific microbial growth kinetics. Simulation scenarios were performed across a range of operational parameters, including OLR (1–8 kg VS/m3 day), pH (5.0–8.0), temperature (35 °C and 45 °C), and HRT (10–30 days). The results demonstrate that balanced microbial population dynamics and syntrophic interactions strongly influence methane production and overall digester performance. Optimal methane yields were achieved within an OLR range of 3.5–4.5 kg VS/m3 day under mesophilic conditions. Elevated loading rates led to VFA accumulation and pH decline, resulting in the inhibition of methanogenic populations and reduced methane output. Preliminary parametric analysis suggests that the acetoclastic methanogen growth rate and ammonia inhibition constants are influential parameters affecting system performance. The findings highlight the importance of integrating microbial population dynamics into AD models to enhance predictive accuracy and support the development of intelligent control strategies for sustainable waste-to-energy systems.

Fuels doi: 10.3390/fuels7020035

Authors: Rui Liu Kang Liu Meiming He Mingqi Sun Yuxuan Yang Wanfen Pu

This study investigates improving the flowability of heavy crude oil using non-ionic surfactants that modify interfacial properties, thereby enhancing emulsification and dispersion. A mixture of Span 85 (HLB = 1.8) and Tween 20 (HLB = 16.7) was selected to meet the affinity requirements of both oil and water phases. Experiments were conducted on five different densities of heavy crude oil, evaluating viscosity reduction, emulsion droplet size distribution, and interfacial tension. Notably, this work presents the first systematic examination of interactions between various heavy crude oil densities and mixed emulsifiers. Results show that aligning the HLB value of the mixed emulsifier with that of the heavy crude oil enhances electrostatic repulsion between droplets, reducing droplet size and optimizing surfactant arrangement at the interface. The optimal HLB value for viscosity reduction was determined to be 8.0, at which a viscosity reduction rate of over 89% was achieved for high-density heavy crude oil. A quantitative relationship between emulsion droplet size and viscosity reduction rate was also established, leading to improved emulsion stability and significant viscosity reduction. These findings provide a theoretical framework for applying non-ionic mixed surfactants to enhance heavy crude oil flowability, and deliver experimental data to support field applications in petroleum engineering.

Fuels doi: 10.3390/fuels7020034

Authors: Constantinos Katrakylidis Dimitrios Dimitriadis

This study investigates whether countries converge toward common long-run paths in renewable energy consumption and examines the implications for global fuel transition dynamics. Using a balanced panel of 108 countries over the period 1990–2022, we implement an integrated econometric framework that combines stochastic convergence tests, β- and σ-convergence analysis, the Phillips–Sul club convergence methodology, ordered logit modelling, and heterogeneous panel causality tests. The results reject global stochastic convergence, indicating that countries do not share a common transition trajectory. However, evidence of β- and σ-convergence suggests the presence of partial and bounded catch-up dynamics. The Phillips–Sul approach identifies four distinct convergence regimes, implying multiple steady-state equilibria in global energy systems. Structural analysis shows that income and governance quality increase the probability of belonging to higher-renewable-energy regimes, while carbon intensity constrains upward transitions. Regime-specific causality results further reveal that the drivers of renewable energy dynamics differ across structural contexts. Overall, the findings demonstrate that global energy transitions are characterized by persistent heterogeneity and regime-dependent adjustment processes rather than uniform convergence. This study contributes by integrating convergence analysis with structural modelling and regime-based interpretation, offering a more comprehensive framework for understanding differentiated decarbonization pathways. The results carry important policy implications, highlighting that effective energy transition strategies must be tailored to regime-specific conditions rather than relying on uniform policy approaches.

Fuels doi: 10.3390/fuels7020033

Authors: Xiaofei Liu Xin Chen Zhengwei Zhang Huayu Wen Dongbo Chen Haoyu Yin Haozhe Jin Chao Wang Lite Zhang

Hydrotreatment is vital for producing high-quality liquid fuels in petroleum refining and its air coolers are critical components prone to severe corrosion under high-temperature and high-pressure conditions. Ammonium salts from NH3-HCl and NH3-H2S reactions, particularly ammonium chloride precipitated during cooling, readily deposit on tube surfaces. Strong temperature gradients and complex flow conditions may severely affect air cooler inlets and front sections. To enhance the refining process reliability, an experimental setup was established to investigate the water injection removal of ammonium chloride particle deposits in air cooler tube bundles. Results show that water injection effectively removes ammonium chloride particles. Particle size has a minor influence, whereas inlet velocity, temperature, and water injection rate significantly affect removal efficiency. Increasing inlet velocity from 2 to 5 m/s, temperature from 80 to 110 °C, and water injection rate all enhance removal efficiency. Furthermore, differences between two-row tubes were also observed: the second-row tube exhibits a higher removal ratio due to liquid film formation, which increases Reynolds number and shear force, thereby enhancing dissolution. These findings provide experimental support for optimizing water injection strategies to mitigate corrosion, improving hydrotreatment unit reliability and safety, ensuring the continuous operation of the petroleum and fuel processing industry.

Fuels doi: 10.3390/fuels7020032

Authors: Wang Yubin Tang Lingjun Jiang Luxin Li Zhengbing Chen Lei

The blending of naphtha and light fuel oil with gasoline and diesel during pipeline batch transportation poses risks of quality non-compliance. This study experimentally investigates the impact of these contaminants on key quality indicators to establish quantitative blending thresholds. Based on a volume-gradient experimental design (0–50 vol% for naphtha, 0–15 vol% for light fuel oil), diesel blends were tested for pour point, flash point, and 95% recovery temperature, while gasoline blends were tested for final boiling point, all in accordance with Chinese national standards. Results demonstrate that light fuel oil contamination in gasoline causes a linear increase in the final boiling point (y = 18.77x + 185.09, R2 = 0.91), exceeding the 205 °C limit at concentrations above 1.2 vol%. Naphtha contamination in diesel leads to a sharp linear decline in flash point (y = −13.20x + 101.83, R2 = 0.84), falling below the 60 °C threshold at concentrations above 3.0 vol%. Diesel pour point increases linearly with light fuel oil concentration (y = 0.39x − 26.41, R2 = 0.88) but remains within specification up to 15 vol%. These quantitative thresholds, derived from statistically significant regression models, provide a scientific basis for optimizing cut-point strategies and mitigating safety risks in product oil pipeline operations.

Fuels doi: 10.3390/fuels7020031

Authors: José Miguel Mahía-Prados Ignacio Arias-Fernández Manuel Romero Gómez Sandrina Pereira

The transition towards alternative marine fuels introduces new safety challenges related to onboard storage, distribution, and fuel management, due to the markedly different physical and chemical properties of methane, methanol, ammonia, and hydrogen. While numerous studies address the risks of individual fuels, there is a lack of structured and comparable risk-assessment methodologies to support early-stage fuel selection and preliminary system design under a unified framework. This study introduces the Methodology to Alternative-fuels Hazardous Identification, a hybrid framework that integrates HAZOP-based deviation analysis with HAZID-style risk classification to enable a consistent qualitative–quantitative comparison of alternative marine fuel systems. The methodology is applied to representative storage and distribution architectures for methane, methanol, ammonia, compressed hydrogen, and liquefied hydrogen, allowing the identification of dominant risk drivers and system-level vulnerabilities across fuel options. The results reveal distinct fuel-specific risk profiles. Methane and methanol are mainly associated with moderate risks linked to operational temperature deviations and system controllability. Ammonia exhibits the most severe risk profile due to the high consequences of toxic releases, particularly under pressure-related failures. Compressed hydrogen is dominated by high-risk scenarios driven by extreme storage pressures, while liquefied hydrogen presents a mixed profile governed by the interaction between cryogenic temperature control and pressure regulation. By providing a comparative and scalable risk-assessment framework, the Methodology to Alternative-fuels Hazardous Identification (MAHI) supports informed decision-making in early design phases and complements existing regulatory safety analyses, contributing to a safer energy transition in maritime transport.

Fuels doi: 10.3390/fuels7020030

Authors: Kangkai Xu Peng Wang Shuai Wang Shuyi Xie Bohong Wang

Ensuring the integrity of weld seams in pipeline components is critical for the safe and reliable transportation of oil and natural gas. This paper presents a systematic failure investigation of a cracked weld in a reducer located at a natural gas transmission station in Western China, aiming to identify the failure mechanism and assess its implications for pipeline safety management. A comprehensive analysis was conducted using macroscopic examination, chemical composition analysis, mechanical property testing, metallographic observation, and microscopic fracture characterization. The results reveal that the heat-affected zone (HAZ) exhibited abnormally high hardness (up to 588 HV0.1), indicating insufficient toughness that made it susceptible to cracking. The base metal showed a high carbon equivalent (CEV), placing it in the “difficult-to-weld” category and increasing its sensitivity to improper welding thermal cycles. On-site investigation further identified significant deficiencies in welding process control, including inadequate preheating, improper interpass temperature management, and insufficient post-weld heat treatment (PWHT). These deficiencies allowed welding residual stresses to persist and failed to mitigate the hardened HAZ microstructure. The combination of poor material weldability and inadequate on-site welding practices ultimately led to brittle fracture under service conditions. This failure highlights a critical vulnerability in pipeline transportation infrastructure and underscores the necessity of strict adherence to qualified welding procedures for high-carbon-equivalent steels. The findings provide practical guidance for enhancing welding quality control and ensuring the long-term operational safety of natural gas pipeline systems.

Fuels doi: 10.3390/fuels7020029

Authors: Jiong Wang Zihan Wang Gang Liu Lei Chen

Ammonia, as a hydrogen carrier and clean fuel, has an increasingly urgent demand for large-scale transportation. Utilizing the existing refined oil pipeline network for sequential transportation of ammonia and refined oil is an economically and efficiently feasible solution. However, the unique micro-solubility characteristics of ammonia and refined oil can cause significant differences in the mixing mechanism of the two substances during sequential transportation in the pipeline compared to traditional oil products. This study conducts transient flow numerical simulation and mechanism research on the mixing problem during the sequential transportation process of ammonia and refined oil under the influence of micro-solubility transfer. Using the ANSYS Fluent platform and combining it with the dynamic mesh technology, a sequential transportation pipeline model was constructed. In the VOF multiphase flow model framework, the Fick diffusion and convective transfer theories were coupled. Through the development of user-defined functions, a transfer model was established to describe the ammonia dissolution process in refined oil during sequential transportation. This model characterizes the axial transfer process of the two-phase flow and the dissolution transfer in the pipeline. Then, the correctness and accuracy of the transfer model were verified, proving that the model has reliable simulation capabilities. To evaluate the comprehensive influence of various engineering factors on the mixing law, this study selected seven key parameters. It then designed and simulated multiple sets of comparative conditions. The influence of each parameter on the development of the mixing section was analyzed, and a sensitivity analysis was conducted. Subsequently, using the growth rate of the mixing length (dL/dt) as the dependent variable to represent the dynamic development of the mixing process, and using the above seven parameters as independent variables, a semi-empirical fitting formula was established. This formula can comprehensively reflect the coupling effect of multiple factors. The results show that the model has good generalization ability and extrapolation robustness. It provides a prediction model and theoretical tool with certain engineering practical value. This can be used for predicting the amount of mixing and optimizing operating parameters in actual pipeline sequential transportation systems.

Fuels doi: 10.3390/fuels7020028

Authors: Vidhi Saini Anubhav Pratap Singh Anand Chauhan Ankit Agrawal Vinay Kumar

The growing demand for energy and emerging environmental concerns are making it necessary to look for more sustainable alternatives. To address the limitations of first-generation biofuels and reduce dependence on fossil fuels, this study focuses on second-generation bioethanol sourced from non-edible pomegranate waste. This study develops and analyses a supply chain optimization model for the sustainable production of biofuel from pomegranate waste and solves it using a genetic algorithm. The framework assesses key supply chain elements, including collection centres for pomegranate waste, processing plants, bio-refineries for conversion and distribution centres for final bioethanol. The primary objective of the optimization is to reduce the total cost of the biofuel production system and to maximize positive environmental impact through waste valorization. A numerical example validates the framework, and a sensitivity analysis further evaluates the economic viability of the supply chain under fluctuating market conditions, such as variations in the purchasing cost of waste, the production cost of bioethanol and the opening cost of plants. Biofuel production supports the Sustainable Development Goals (SDG-12 and -13) by transforming waste into renewable energy. This study aims to address gaps in biofuel research by focusing on the underutilized area of pomegranate-based biofuel through an integrated supply chain optimization framework. The findings offer practical values for researchers working on renewable energy solutions, policymakers and business leaders.

Fuels doi: 10.3390/fuels7020027

Authors: Mariane Fe A. Abesamis Alec Paolo V. Dy Pico Rosanne May E. Marilag Javinel P. Servano Queenee Mosera M. Ibrahim Cymae O. Oguis Alexander Jr. Q. Bello Kenth Michael U. Uy Joevin Mar B. Tumongha Rodel D. Guerrero Ralf Ruffel M. Abarca Alexander O. Mosqueda

In the Philippines’ agricultural setup, pre-harvest cacao (Theobroma cacao) fruits are wrapped with low-density polyethylene (LDPE) for moisture retention and damage protection. Responding to the growing concern for its waste volume and scarcity of treatment, this research explores the co-hydrothermal carbonization (co-HTC) of cacao shells (CS) and LDPE as a method to convert agricultural waste with plastic into hydrochar for potential energy applications. Thus, observations on the thermal, physicochemical, and morphological changes from feedstocks to hydrochar are carried out. Optimal conditions of 200 °C for 60 min resulted in hydrochar with 21.11 MJ/kg and appreciable thermal properties. SEM micrographs show that hydrochar had increased surface area, a good fuel characteristic, and surface flaking on oversized LDPE film, suggesting relative LDPE degradation. EDX analysis reveals C, K, Ca, and Zn metals that affect chemical pathways. FTIR analysis further supports chemical synergy by preservation of functional groups innate from both parent materials. Kinetic and thermal evolutions are also investigated to reveal the influence of pretreatment on the stability of cacao shell-dominated hydrochar and the effectivity of biomass integration to facilitate relatively easier cracking of LDPE. The findings support co-HTC as a viable technology to enhance the circular economy by valorizing LDPE and cacao shells while promoting energy recovery and solid fuel production.

Fuels doi: 10.3390/fuels7020026

Authors: Bárbara Lôpo de Lima Antonio José Vinha Zanuncio Fernando Colen Talita Baldin Edy Eime Pereira Baraúna Marina Donária Chaves Arantes Alfredo Napoli Amelia Guimarães Carvalho Lorena dos Santos Almeida Silva Eliane Favalessa Shoraia Germani Winter Felipe Gomes da Silva Maria Auxiliadora Drumond

Bioenergy production from agro-industrial waste has the potential to contribute to climate change mitigation. In Brazil, the pequi (Caryocar brasiliense Camb.) production chain makes an economic, environmental, and social contribution. However, the collection and processing of the fruit produce large amounts of waste, such as the peel, whose improper disposal leads to significant environmental impacts. This study evaluated how moisture and carbonization temperature influence the energy properties of charcoal briquettes made from pequi peel waste. Carbonization was performed at two final temperatures (360 °C/480 °C) with a heating rate of 1.5 °C min−1 and residence times of 4 h and 5 h 20 min, respectively. Carbonization yields were calculated based on dry mass. Briquettes were produced from pequi peel at moisture contents of 5%, 7.5%, and 10% (wet basis). After carbonization, the charcoal briquette samples were characterized by proximate analysis, higher heating value (HHV), bulk density, energy density, and mechanical durability. Carbonization temperature exerted a more pronounced effect on the properties of the carbonized briquettes than the initial moisture content. Carbonization at 480 °C increased the fixed carbon content (76.38%, 74.25%, and 75.10% for treatments 1, 2, and 3) and the HHV (25.10–25.31 MJ kg−1), while reducing the gravimetric yield (32.84–33.25%). The influence of moisture content was more evident in carbonizations carried out at 360 °C, indicating a temperature-dependent interaction. The use of pequi peel for solid biofuel production promotes the valorization of agro-industrial residues and supports strategies aimed at the circular bioeconomy and the decarbonization of the energy matrix.

Fuels doi: 10.3390/fuels7020025

Authors: Xu Zheng Bing Liang Weiji Sun Zhuang Li Zipeng Wei Yan Li

To investigate the synergistic effect of hydraulic fracturing and hot water injection on enhancing methane extraction from low-permeability coalbeds and elucidate the underlying thermal-hydraulic coupling mechanism, methane desorption experiments were conducted in coal samples with varying fracture networks using a self-developed multi-field coupling experimental system. Tests were performed under different injection pressures and temperatures to analyze coal temperature evolution and methane desorption-seepage characteristics. The results demonstrate that hydraulic fracturing significantly improves pore structure and connectivity, thereby optimizing methane desorption behavior. The methane migration in the samples is influenced by water injection, exhibiting an initial promotion followed by inhibition. The combined fracturing-thermal injection approach effectively reduces the dynamic viscosity of water, mitigates the water lock effect, and enhances the desorption capacity. The hydraulic fracturing and the hot water injection complement each other, achieving synergistic production enhancement. The optimal injection pressure and water temperature can be selected according to specific reservoir conditions to balance the production increase and cost efficiency. This laboratory-scale study provides theoretical support for optimizing hydraulic measures and thermal injection techniques in coalbed methane extraction, revealing complementary synergies between these two methods and offering new insights into multi-field coupling enhancement mechanisms with practical application guidelines.

Fuels doi: 10.3390/fuels7020024

Authors: Ali Raza Marva Hadia Zunaira Tu Zehra Sajjad Miran Muhammad Khurram Ghulam Murtaza

Fossil fuel depletion has increased interest in renewable alternatives such as biodiesel derived from non-edible plant oils. Droplet evaporation is a key process influencing fuel–air mixing and combustion efficiency in diesel engines. In this study, the evaporation characteristics of diesel and two non-edible biofuels, Jatropha and Castor, are investigated using computational fluid dynamics (CFD) under high-temperature and high-pressure conditions representative of engine environments. The numerical model incorporates the conservation equations of mass, momentum, and energy, together with the k–ε turbulence model and a discrete phase model to simulate droplet heating, motion, and mass transfer during evaporation. A comparative CFD analysis is performed to examine how fuel properties, ambient temperature, and droplet size affect the evaporation behaviour of diesel, Jatropha, and Castor droplets under identical engine-like conditions. The evolution of droplet diameter, temperature, velocity, and lifetime is analysed, and the applicability of the classical D2-law is evaluated under different operating conditions. The results indicate that biofuel droplets generally evaporate faster than diesel droplets at lower temperatures, while evaporation trends become similar at higher temperatures. These findings provide insight into the evaporation behaviour of Jatropha and Castor fuels and their potential application in diesel engines.

Fuels doi: 10.3390/fuels7020023

Authors: Mario Morales-Máximo Ramiro Gudiño-Macedo José Guadalupe Rutiaga-Quiñones Juan Carlos Coral-Huacuz Luis Fernando Pintor-Ibarra Luis Bernardo López-Sosa Víctor Manuel Ruíz-García

The energy utilization of residual woody biomass is a relevant strategy for the decentralized energy transition and local waste management in rural areas. The objective of this study was to characterize (physically, chemically, and energetically) five types of residual biomass: pine branches, huinumo (this material refers to the long, thin pine needles that, after drying and falling, form a layer on the forest floor), cherry branches and leaves, and grass waste generated in the community of San Francisco Pichátaro, Michoacán, Mexico, in order to evaluate its viability for the production of densified solid biofuels. A comprehensive analysis was conducted, including moisture content, higher heating value, proximate characterization, structural chemical analysis (using the Van Soest method), elemental CHONS analysis, ash microanalysis (by ICP-OES), and a multicriteria analysis with normalized energy and compositional indicators. The results showed that huinumo and cherry leaves were the most outstanding biomasses, presenting the highest heating values (20.7 MJ/kg) and low moisture and ash contents. Pine branches obtained the most balanced results, characterized by their equilibrium in fixed carbon and lignin, as well as their low potassium content. The multicriteria analysis showed that there is no absolute optimal biomass; however, it indicates that pine branches and huinumo are the most robust feedstocks for the production of briquettes or pellets. The results confirm the significant technical and environmental potential of local lignocellulosic residues for the production of solid biofuels and for contributing to sustainable energy solutions at the local scale.

Fuels doi: 10.3390/fuels7020022

Authors: Maria Căldărar Mădălin Dombrovschi Tiberius-Florian Frigioescu Gabriel-Petre Badea Laurentiu Ceatra Răzvan Roman

The increasing need to reduce reliance on fossil-derived aviation fuels and mitigate environmental impacts has intensified research into renewable alternatives for aviation energy systems. The growing interest in ethanol-based fuels is primarily driven by their simple oxygen-rich molecular structure and advantageous physicochemical characteristics, yet experimental studies examining their application in hybrid power architectures, including micro-turboprop engine-based power sources, are still limited. This study presents an experimental investigation of ethanol–Jet A1 fuel blends used in a micro-turboprop engine operating as a power generation unit for unmanned aerial vehicle applications. Ethanol was blended with Jet A1 at volumetric fractions of 10%, 20% and 30% and the engine was tested under multiple operating regimes corresponding to different electrical power outputs. Exhaust gas temperature, electrical power output and gaseous emissions (CO and NOx) were measured for each operating condition. The results indicate that low ethanol fractions (E10) provide performance comparable to neat kerosene, while higher ethanol fractions lead to a reduction in exhaust gas temperature at low-power regimes due to the lower heating value and high latent heat of vaporization of ethanol. Emission measurements showed a decrease in NOx emissions with increasing ethanol content, associated with lower combustion temperatures, while CO emissions increased at low-power regimes due to incomplete combustion under lean conditions. Additionally, combustion instability was observed during rapid transitions from maximum to idle regime operation for higher ethanol blends, attributed to transient ultra-lean mixtures, evaporative cooling, and reduced reaction rates. The results demonstrate that ethanol–kerosene blends can be used in micro-turboprop systems at low blend ratios without major performance penalties, but transient operating conditions impose stability limits that must be considered in practical UAV power system applications.

Fuels doi: 10.3390/fuels7020021

Authors: Jacob Mdigo Arthur Santos Gerald Duggan Prajay Vora Kira Shonkwiler Daniel Zimmerle

This work presents a mechanistic modeling approach for simulating methane emissions from triethylene glycol (TEG) dehydrators used in oil & gas (O&G) operations. The model was developed as a modular component of the Mechanistic Air Emissions Simulator (MAES) tool, incorporating species-specific absorption and emission dynamics through two-level, second-order polynomial regression (PR) models trained on ProMax simulation data: (1) species-level regression models that track the transfer rates of individual gas species within the dehydrator unit streams, and (2) outlet flow stream regression models that predict the fraction of inlet gas distributed among the outlet streams of the dehydrator unit. These behaviors were characterized over a range of glycol circulation ratios, wet gas pressures, and temperatures. The model was validated using root mean square error (RMSE) analysis. The species-level PR achieved low root mean square error (RMSE) values (<0.03) for light hydrocarbon species across all dehydrator components, ranging from 0.0009 for methane to 0.029 for normal pentane. Similarly, the outlet-level PR yielded RMSE values below 0.002 for the dry gas fraction, 0.001 for the flash tank fraction, and 0.002 for the still vent fraction, demonstrating strong agreement between predicted and reference ProMax values. When deployed at field facilities, the model significantly improved MAES-simulated dehydrator emissions, revealing that gas-assisted glycol pump emissions are the dominant contributors to both dehydrator-level and site-level methane emissions under uncontrolled conditions. Further analysis of the 154 dehydrator units reported by operators under the AMI 2024 project showed that 54 units (31%) used gas-driven glycol pumps, of which 6 units (11%) operated with uncontrolled flash tanks, and 22 units (40.7%) were identified as potentially oversized. Of the six dehydrator units with uncontrolled gas-assisted pumps, pump emissions accounted for 90.25% of total dehydrator emissions and 63.10% of total site-level emissions. These findings highlight substantial opportunities for emissions mitigation through equipment upgrades.

Fuels doi: 10.3390/fuels7020020

Authors: Zhengyuan Zhang Shixuan Lu Liming Dai Na Jia

Vibration-assisted water flooding (VA-WF) can improve sweep efficiency. However, unclear macro-scale mechanisms limit its wider adoption in heavy oil reservoirs. This study combines previous sandpack experiments with two-dimensional Volume-of-Fluid (VOF) simulations to show how vibrations reshape permeability fields and, in turn, pressure and production behaviour. Heavy oil sandpacks were water-flooded under conditions of no vibration and 2 Hz and 5 Hz axial excitation. Measured injection pressure histories and oil production were used to calibrate a VOF model in which absolute permeability follows a log-normal distribution with directional anisotropy. Only when axial and radial permeabilities were assigned a negative local correlation did the model reproduce key observations: secondary pressure spikes, irregular viscous-fingering morphologies, delayed production drops, and variability in cumulative recovery. Parameter sweeps quantify the sensitivity of VA-WF performance to the variance and correlation of the permeability field, and multiple runs estimate the variability in outcomes introduced by stochastic heterogeneity. This study proposes a transferable workflow—comprising sample testing, parameter inference, and probabilistic simulation—to screen excitation conditions and forecast VA-WF performance prior to field implementation, enabling operators to optimize vibration frequency based on reservoir-specific permeability characteristics and to anticipate production variability under uncertainty. These results highlight the dominant factors affecting swept volume and oil recovery, supporting data-driven decision making in VA-WF projects.

Fuels doi: 10.3390/fuels7020019

Authors: Cezary Behrendt Włodzimierz Kamiński Oleh Klyus

The International Maritime Organization’s (IMO) greenhouse gas (GHG) strategy aims for a 40% reduction in carbon intensity by 2030 and a 70% reduction by 2050, relative to 2008 levels. Attainment of these objectives necessitates an integrated strategy encompassing technological advancements, operational optimization, and the adoption of innovative practices to curtail fuel consumption and enhance vessel performance. The Ship Energy Efficiency Management Plan (SEEMP), mandated by MEPC 62 in 2011, establishes a systematic framework for the continual enhancement of energy efficiency. SEEMP is intrinsically associated with reductions in fuel consumption, enabling maritime organizations to systematically monitor and control energy performance via the Energy Efficiency Operational Indicator (EEOI). This metric enables operators to assess operational energy performance and implement measures such as optimized voyage planning and fuel-saving technologies. However, the effectiveness of SEEMP varies widely across companies and vessel types, often due to limited crew awareness. To enhance daily implementation, it is essential to improve crew training and streamline SEEMP documentation. Simplifying SEEMP structures within ship management companies can further facilitate usability and compliance. By focusing on these areas, the maritime industry can better align with IMO’s GHG reduction targets and promote more sustainable operations and fuel-saving technologies.

Fuels doi: 10.3390/fuels7010018

Authors: Li Shen Felix Leach

Ammonia, a zero carbon energy vector, is under consideration for decarbonising marine and energy storage applications due to its high mass-based energy density compared to many alternatives. In addition, there is widespread existing supply and transportation infrastructure due to ammonia’s use as a fertiliser. When injected in its liquid form, however, ammonia behaves quite differently to traditional fuels due to its high saturation pressure and enthalpy of vaporisation, amongst other things. This means that fundamental data on ammonia sprays need to be collected in order to understand ammonia spray behaviour and calibrate models of ammonia sprays needed for design in the virtual world. Previous work on ammonia sprays has mostly focused on spray morphology at a macroscopic level (such as liquid penetration length). However, there are fewer studies of ammonia sprays at a microscopic level. In this study, liquid ammonia was injected into a constant-volume chamber using a direct injector at two injection pressures (100 bar and 150 bar) and a range of ambient pressures from 3–13 bar. This range of ambient conditions spans regimes from flash-boiling to non-flash-boiling, thereby enabling systematic investigation of the transition between these regimes. A laser diffraction technique was used for measuring the droplet sizes of the spray at different locations (in a cylindrical volume with a diameter of 10 mm) within the spray plume at 10 kHz, and the nominal droplet sizes were quantified by the Sauter Mean Diameter (SMD). These SMD values provided, at a microscopic level, an insight of the atomisation of the spray as it left the nozzle and penetrated into an environment with different densities. It was found that the tested injector leads to a breakup dominant spray behaviour with liquid ammonia and hence the SMD values decrease as ambient pressure increases. In addition, the droplets are generally smaller at the outer edge of the spray plume compared to the inner part and both the injection pressure and injection duration have a strong effect on the droplet sizes.

Fuels doi: 10.3390/fuels7010017

Authors: Paul Sebastian Selvaraj Aswin Kuttykattil Parameswari Ettiyagounder Ilakiya Tamilselvan Kalaiselvi Periyasamy Sadish Oumabady Poornima Ramesh Kavitha Ramadass Thava Palanisami

An exorbitant amount of organic fractions of the municipal solid waste, i.e., fruit and vegetable waste (FVW), generated from farm to fork are being treated through anaerobic digestion (AD). Anaerobic digestion (AD) of FVW only achieves <60% methane potential due to methanogen loss and indirect electron transfer. Hence, the technology necessitates further improvements in performance to maximise the methane gas yield by stabilising the methanogens using a potential additive. Magnetic biochar is a budding and promising additive in anaerobic digestion that amplifies biomethanation performance. This study focuses on the role of magnetic biochar in enhancing the viability of the AD system in biogas production from organic waste fractions. Herein, the magnetic biochar was produced using a FeCl3-impregnated walnut shell and then characterized. The derived magnetite was identified as the major crystalline phase in biochar with the presence of several oxygenated functional groups. The specific surface area, pore volume, and pore diameter were found to be 360.99 m2 g−1, 0.089 cm3 g−1, and 0.98 nm, respectively. The SEM and TEM images illustrated a good dispersion of the material, with size ranging between 18.2 and 46.6 nm, thus indicating the porous nature of the magnetic biochar. The incorporation of magnetic biochar in the CN ratio modified the AD system with enhanced methane production and the highest volume (1523.4 mL) reported in treatment, with a CN ratio of 25:1 and 0.5% magnetic biochar. The resulted gas yield is 35% more than the control (1125 ML) with reduced lag phase (4 vs. 12 days). It concludes that walnut shell MBC uniquely combines DIET conduits and biofilm support and enhances methane production from FVW. However, 16S rRNA confirmations of syntrophs, continuous reactor validation, and magnetic biochar recovery and reuse potential studies are essential for further scaleup.

Fuels doi: 10.3390/fuels7010016

Authors: Wenqiang Liu Dong Li Yong Huo Zhengguang Zhao Zhanwen Fu Yibo Tian

Production profiling is essential for optimizing production strategies in oil and gas wells. Conventional production logging tools provide only discrete, time-limited measurements and face operational challenges in long or complex horizontal wells. Distributed fiber-optic sensing (DTS/DAS) enables continuous, full-wellbore monitoring but each sensing modality has limitations when used alone: DTS interpretation is influenced by wellbore disturbances and thermal hysteresis, while DAS acoustic energy does not always correspond to actual inflow zones. This study proposes a joint interpretation method integrating DTS-based temperature inversion with DAS frequency-band energy and apparent velocity analysis. DTS data are processed using a coupled wellbore–formation heat-transfer model to obtain segmental flow rates, while DAS data are analyzed using short-time Fourier transform, cross-correlation, and Hough transform to extract positive and negative apparent velocities indicating fluid migration directions. Field results show that high-production intervals at 4126–4486 m correlate with positive apparent velocities, whereas medium-/low-production and shut-in stages exhibit persistent negative velocities linked to backflow and reinjection. The combined interpretation effectively distinguishes reservoir inflow from wellbore flow by jointly constraining thermal response and flow direction, thereby reducing uncertainties associated with single-method analysis.

Fuels doi: 10.3390/fuels7010015

Authors: José Apolonio Venegas-Venegas Deb Raj Aryal René Pinto-Ruiz Francisco Guevara-Hernández Mariela Beatriz Reyes-Sosa Alberto Pérez-Fernández José Alfredo Castellanos-Suárez

Biogas production from animal manure has huge potential in mitigating greenhouse gas emissions and replacing the higher environmental footprint energy sources. This study aimed to assess the technical functionality, environmental benefits, and economic advantages of low-cost biodigesters suitable for rural areas, which can produce biogas from animal manure. Four low-cost polyethylene tubular biodigesters with a concrete retaining wall with capacities ranging from 4 to 14 m3 were installed in small dairy production units in Chiapas, Mexico. Four profitability indicators were calculated. The IPCC’s methodology was used to calculate emissions from biogas and firewood burning, and the emission reduction from manure management. These biodigesters generate between 526 and 1993 m3 of biogas year−1 and represent a savings of USD 197–744 year−1 in energy costs. The four profitability indicators were favorable. Moreover, these biodigesters reduce 70–73% of greenhouse gas (GHG) emissions through manure management, that is, between 1.5 and 5.1 t CO2e year−1, and 1.3–5.1 t CO2e year−1 from firewood displacement. These findings provide critical insights into the potential of sustainable and low-cost biodigesters that can be implemented effectively in small-scale dairy farms in rural areas in many parts of the world.

Fuels doi: 10.3390/fuels7010014

Authors: Zhen Wang Zunmin Li Lili Ma Wenli Ma Xiaolong Wang Zhiqiang Zhao Xusheng Zhang Guohe Jiang

The global effort to mitigate hazardous particulate matter (PM) emissions from diesel engines relies significantly on advances in separations technologies. The diesel particulate filter (DPF) is a critical component designed to trap soot and ash from diesel engine exhaust, ensuring cleaner emissions and compliance with environmental regulations. In the current paper, a gas-particle two-phase flow model in the microchannels of a DPF is developed. A novel statistical approach based on probability sampling is proposed aimed at generating a particle ensemble that adheres to the real-world soot particle size distribution (PSD). The Eulerian-Lagrangian approach is employed to model the soot-laden gas flow, where the gas phase flow field is solved in the Eulerian framework, while the particle phase motion is tracked in the Lagrangian framework. The results demonstrate that the through-wall velocity plays a predominant role in the overall deposition behavior of the mixed-sized particle group. Increasing upstream velocity shifts initial particle deposition positions further from the channel inlet and enhances mass accumulation at the channel’s terminal section. Reduced filtration wall permeability promotes the uniformity of soot deposition along the channel. A permeability of 5 × 10−13 m2 is identified as the critical threshold, below which the soot deposition distribution approaches near-complete uniformity.

Fuels doi: 10.3390/fuels7010013

Authors: Ricardo Situmeang Jana Mazancová Hynek Roubík

Biogas is increasingly recognized as a strategic component of Indonesia’s clean energy transition; however, household-level adoption remains limited, even in livestock-dense regions. This study provides one of the first empirical assessments in Indonesia that integrates socioeconomic, behavioral, and institutional determinants of household biogas adoption within a unified analytical framework. Focusing on dairy-farming households in West Java Province, we examine why adoption remains low despite significant manure-based energy potential. Guided by the hypothesis that institutional support and household perceptions exert stronger influence on adoption than resource availability alone, we apply a binary logistic regression model to data from 201 households (101 adopters and 100 non-adopters). The analysis incorporates structural variables (income, livestock ownership, and electricity access) together with perceptual and experiential factors (fuel-cost pressure, perceived time savings, and participation in training). Contrary to conventional expectations, higher education is negatively associated with adoption, reflecting Indonesia’s LPG price distortions and aspirational energy preferences. In contrast, fuel-cost pressure, livestock ownership, perceived time savings, and training participation significantly increase adoption likelihood. These findings underscore that effective biogas dissemination requires not only physical resources but also strengthened institutional support, improved technical capacity, and targeted awareness-building interventions.

Fuels doi: 10.3390/fuels7010012

Authors: Darzhan Aitbekova Murzabek Baikenov Assanali Ainabayev Nazerke Balpanova Sairagul Tyanakh Zaure Absat Nazym Rakhimzhanova Yelena Kochegina

In this work, the kinetics of the redistribution of oils, resins, and asphaltenes in high-viscosity oil from the Karazhanbas field (Republic of Kazakhstan) were investigated. This was achieved with an ultrasonic treatment (22 kHz, 50 W) in the presence of a zeolite catalyst (1.0 wt%). The parameters of the technological process were established as a temperature range from 30 to 70 °C and an exposure time of 3 to 11 min. This allowed us to increase the oil content by 14.8% and decrease the concentration of resins by 12.2% and asphaltenes by 2.6%. Conversion schemes (“oils ↔ resins” and “resins ↔ asphaltenes”) were developed, which made it possible to determine the main direction of the reaction processes. The most rapid process is the conversion of resins to oils (k2 = 0.1148–0.1860 min−1). The process of the cracking of asphaltenes with the formation of resins (k4 = 0.1023–0.1413 min−1) ranks second in rates. Condensation reactions, including the transition of oils to resins (k1 = 0.0175–0.0252 min−1) and resins to asphaltenes (k3 = 0.0139–0.0194 min−1), occur significantly more slowly. The calculated activation energies (7.0–10.4 kJ/mol) show that the cavitation treatment of high-viscosity oil in the presence of a catalyst effectuates the processing of heavy oil with minimal energy consumption. A group composition analysis of the light and middle oil fractions demonstrated an increase in paraffinic, naphthenic, benzenic, and olefinic hydrocarbons, with a simultaneous decrease in naphthalenes and heteroatomic compounds. The results obtained confirm the effectiveness of ultrasonic–catalytic treatment for the structural cracking of high-viscosity oil and the formation of lighter hydrocarbon fractions.

Fuels doi: 10.3390/fuels7010011

Authors: Qiang Wu Zhaofeng Wang Liguo Wang Shujun Ma Yongxin Sun Shijie Li Boyu Lin

In the process of using the freezing method to uncover coal from stone gates, the thermal evolution profiles of the coal body during the freezing process tend to be complex due to the presence of gas and moisture. To investigate the temperature response of coal containing gas under different freezing temperature conditions, a self-developed low-temperature freezing test system for coal containing water and gas was used to conduct freezing and cooling tests at different freezing temperatures (−5 °C to −30 °C). The temperature changes at various measuring points inside the coal over time were monitored in real time, and the temperature distribution, cooling law, and strain evolution process of the coal in the axial and radial directions were analyzed. The experimental results show that the cooling process of the center point of the coal can be divided into four stages: rapid cooling, extremely slow temperature drop, relatively slow cooling, and stable constant temperature. The time required to reach the stable constant temperature stage is inversely proportional to the freezing temperature, and corresponding prediction formulas have been established based on this. The standardized coal briquettes exhibit a gradient distribution characteristic of gradually increasing temperature from outside to inside in both axial and radial directions, with the radial temperature distribution being well matched by an exponential decay model. The strain of coal is affected by both thermal shrinkage and ice-induced expansion. The occurrence time of frost heave is positively correlated with freezing temperature, while the strain of frost heave is negatively correlated with freezing temperature. The axial frost heave effect is significantly stronger than the radial effect, but the radial frost heave occurs slightly earlier than the axial effect. This study reveals the thermal-mechanical coupling response mechanism of gas-containing coal during the low-temperature freezing process, and the research results can provide theoretical support for parameter optimization and engineering application of low-temperature freezing anti-outburst technology.

Fuels doi: 10.3390/fuels7010010

Authors: Xinting Fan Liang Chen Senyuan Zheng Qiongqiong He Ruize Gao Haiting Zhang Yutao Li

As an important type of power and domestic coal, long-flame coal plays a significant role in China’s energy structure. In this study, long-flame coal from Dongsheng, Inner Mongolia (DS) and its vitrinite-enriched fraction (DSV) prepared by organic solvent flotation separation method were selected as research objects. Simultaneous thermal analyzer (TGA), thermogravimetry-gas chromatography-mass spectrometry (TG-GC/MS), and Gray-King assay of coal were mainly employed to investigate their pyrolysis characteristics and differences in pyrolysis products. Results indicate that at the same final pyrolysis temperature, the CO2 content in the pyrolysis gas of DS is higher than that of DSV, while CO, CH4, and CmHn follow the order of DSV > DS. At 400−600 °C, pyrolysis tar mainly comprises monocyclic aromatic hydrocarbons (MAHs), polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons, phenols and other oxygen heteroatom-containing organics (OHCs). Except for aliphatic hydrocarbons and OHCs, the contents of other components reach their maximum values at 500 °C, with peak area intensities of 3.192 × 108, 5.841 × 108, and 8.562 × 108, respectively. In summary, when compared with DS, DSV exhibits more pronounced volatile release and higher reactivity.

Fuels doi: 10.3390/fuels7010009

Authors: Amr Abbass

A Cantera-based combustion-kinetics framework that maps the operating space of hydrogen compression ignition (H2-CI) engines and establishes a structured charter to guide experiments. Beginning with a diesel (n-dodecane) baseline at an intake temperature of 300 K, the model is virtually converted to neat hydrogen and evaluated across intake temperatures of 400–600 K, compression ratios (CR) of 20–28, and exhaust gas recirculation (EGR) levels of 0–15%. Hydrogen demonstrates stable operation across a broad equivalence ratio window (ϕ = 0.45–2.1), achieving power outputs of 16–22 kW and higher efficiencies with substantially lower fuel mass than diesel. The optimal operating region is identified at an approximately 400 K intake temperature, CR = 24–28, and EGR between 5% and 10%, where power remains high (20–18 kW), efficiency increases (above 50%), and NOx emissions are markedly reduced (from 332 ppm at zero EGR to 48 ppm at 5% EGR and 10 ppm at 10% EGR), with only modest hydrogen slip (0.07–0.11). The kinetics-based framework thus provides a systematic and validated roadmap for experimental calibration, research, and development of compression ignition engines working on pure hydrogen.

Fuels doi: 10.3390/fuels7010008

Authors: Sabrina Anderson Beker Beni Jequicene Mussengue Chaúque Marcela Marmitt Guilherme Brittes Benitez Frederick J. Passman Fatima Menezes Bento

Microbial contamination of aviation fuels is a persistent operational and safety challenge, compromising fuel quality and accelerating material degradation. The global transition toward sustainable aviation fuels (SAF) amplifies the need to reassess microbial risks across both conventional and alternative fuel systems. Here, we present the first systematic review and meta-analysis to synthesize evidence on microbial prevalence in jet fuel environments and to evaluate implications for SAF deployment. Of 2837 records screened, 37 studies fulfilled the inclusion criteria. Microorganisms were detected in up to 87% of jet fuel systems worldwide (95% CI: 76–100%); however, this pooled estimate was associated with substantial heterogeneity (I2 = 96%) and should therefore be interpreted with caution as reflecting an overall trend rather than a precise global value. Taxonomic analysis identified consistently reported bacterial genera (Actinomycetes, Halomonas, Mycobacterium, Nocardioides, Rhodococcus, Stenotrophomonas) and fungal genera (Aspergillus, Alternaria, Amorphotheca, Byssochlamys, Candida, Fusarium, Saccharomyces, Schizosaccharomyces, Talaromyces, Trichocomaceae). Deteriorative organisms dominated (bacteria 57%; fungi 75%) relative to non-deteriorative taxa (12% and 32%, respectively). These findings highlight microbial spoilage as a pervasive and underrecognized threat to fuel integrity. Importantly, they suggest that risks currently documented in conventional systems are likely to extend to SAF, reinforcing the urgent need for proactive monitoring frameworks and bio-contamination mitigation strategies to ensure aviation fuel reliability.

Fuels doi: 10.3390/fuels7010007

Authors: Jintao Wu Lei Zhang Zhennan Gao Chenxu Yang Linna Sun

The water flooding characteristic curve is a crucial tool in reservoir dynamic analysis, commonly employed to estimate water-driven geological reserves and recoverable reserves. However, due to approximations in theoretical derivations—such as equating average water saturation with outlet saturation or assuming that water cut approaches unity—most conventional curves achieve high accuracy only during the high water-cut stage (>80%). This study eliminates systematic errors and enhances calculation accuracy by establishing an improved water flooding curve equation. Firstly, a theoretical analysis of the error in a WOR (water–oil ratio)-type water flooding characteristic curve is performed. The results demonstrate that as water cut increases, calculated dynamic geological and recoverable reserves gradually rise, approaching actual values only when the water cut exceeds 90%. Secondly, a new type of water flooding characteristic curve is derived by using the Buckley–Leverett water drive oil theory and the Welge equation to modify the saturation approximation. Comparative analysis via reservoir numerical simulation demonstrates that the proposed curve significantly enhances prediction accuracy across all water-cut stages above 50%, outperforming conventional curves. After the water cut reaches 50%, the calculation error of dynamic geological reserves is less than 10%, and the calculation error of recoverable reserves is less than 5%. Field application shows that the new water flooding characteristic curve maintains a stable linear shape under certain development conditions. After the adjustment of development conditions, it jumps to form a new stable straight-line segment, which is conducive to the rapid and accurate evaluation of the adjustment effect.

Fuels doi: 10.3390/fuels7010006

Authors: Mosammat Mustari Khanaum Shafiqur Rahman Md. Saidul Borhan

Laboratory-based research on microbial fuel cells (MFCs) is often costly and limited to a small number of variables, making optimization challenging. However, machine learning (ML) offers a promising solution by enabling efficient multivariate principal component analysis (PCA) and multivariable optimization. These techniques can provide significant insights and optimization opportunities. The goal of this study is to propose an ML-based approach to explore the relationships between bioelectricity generation (in terms of voltage, power density (PD), current density (CD), and coulombic efficiency (CE)) and two key variables, chemical oxygen demand (COD) and pH, as well as to recommend their optimal combinations. Specifically, the objectives are to (1) integrate a laboratory-based MFC study with multivariate data analyses; (2) apply PCA to reduce data complexity by focusing on the principal components that account for the greatest variance, thus improving interpretability; and (3) identify the optimal combinations of COD and pH for maximizing bioelectricity generation. The PCA results demonstrated that COD positively influenced the generated voltage while having an inverse effect on CE. Additionally, both PD and CD increased with higher pH values. The optimal combination of COD and pH improved CD, PD, and CE; however, their optimal combination for generated voltage differed, with higher COD leading to higher voltage. The optimal predicted voltage, CD, PD, and CE of the study were 795.71 (mV), 1451.80 (mA/m2), 57.46 (mW/m2), and 4.85%, respectively. By integrating ML approaches, this study contributed to the optimization of bioelectricity generation from wastewater and offered valuable insights for researchers working in this field.

Fuels doi: 10.3390/fuels7010005

Authors: Ozan Öztürk Melih Yıldız

The aviation industry, responsible for approximately 2.5–3.5% of global greenhouse gas emissions, faces increasing pressure to adopt sustainable energy solutions. Hydrogen, with its high gravimetric energy density and zero carbon emissions during use, has emerged as a promising alternative fuel to support aviation decarbonization. However, its large-scale implementation remains hindered by cryogenic storage requirements, safety risks, infrastructure adaptation, and economic constraints. This study aims to identify and evaluate the primary technical and operational risks associated with hydrogen utilization in aviation through a comprehensive Monte Carlo Simulation-based risk assessment. The analysis specifically focuses on four key domains—hydrogen leakage, cryogenic storage, explosion hazards, and infrastructure challenges—while excluding economic and lifecycle aspects to maintain a technical scope only. A 10,000-iteration simulation was conducted to quantify the probability and impact of each risk factor. Results indicate that hydrogen leakage and explosion hazards represent the most critical risks, with mean risk scores exceeding 20 on a 25-point scale, whereas investment costs and technical expertise were ranked as comparatively low-level risks. Based on these findings, strategic mitigation measures—including real-time leak detection systems, composite cryotank technologies, and standardized safety protocols—are proposed to enhance system reliability and support the safe integration of hydrogen-powered aviation. This study contributes to a data-driven understanding of hydrogen-related risks and provides a technological roadmap for advancing carbon-neutral air transport.

Fuels doi: 10.3390/fuels7010004

Authors: Marcella Frauscher Bettina Ronai Nicole Dörr Alexandra Rögner

Succinimide additives play an important role in combating engine deposits and are therefore commonly blended in fuels. As many of the methods currently used to quantify them in fuel rely on time-consuming techniques and the use of expensive laboratory equipment, a more practical approach was explored. For this purpose, an existing method for aqueous samples involving a colour reaction with Rose Bengal dye and spectrophotometric detection in the UV/Vis range was modified for usage in the nonpolar fuel matrix and tested for applicability. The result was an accessible method for determining the succinimide additive content of diesel fuel—including biodiesel—that is easy to implement in the laboratory routine.

Fuels doi: 10.3390/fuels7010003

Authors: Ayaaz Yasin Saaras Pakanati Kishan Bellur

Due to its high energy density, liquid Hydrogen is an essential fuel for both terrestrial energy systems and space propulsion. However, uncontrolled evaporation poses a challenge for cryogenic storage and transport technologies. Accurate modeling of evaporation remains difficult due to the multiscale menisci formed by the wetting liquid phase. Thin liquid films form near the walls of containers, ranging from millimeters to nanometers in thickness. Heat conduction through the solid walls enables high evaporation rates in this region. Discrepancies in the reported values of the accommodation coefficients (necessary inputs to models) further complicate evaporation calculations. In this study, we present a novel multiscale model for CFD simulations of evaporating Hydrogen menisci. Film profiles below 10 μm are computed by a subgrid model using a lubrication-type thin film equation. The microscale model is combined with a macroscale model above 10 μm. Evaporation rates are computed using a kinetic phase change model combined with in situ calculations of the accommodation coefficient using transition state theory. The submodels are implemented in Ansys FluentTM using User-Defined Functions (UDFs), and a method to establish two-way coupling is detailed. The modeling results are in good agreement with cryo-neutron experiments and show improvement over prior models. The model, including UDFs, is made available through a public repository.

Fuels doi: 10.3390/fuels7010002

Authors: Kaye Papa Jeffrey Lavarias Melba Denson Danila Paragas Mari Rowena Tanquilut Arly Morico

Pili nutshell (PS), an abundant agro-industrial byproduct in the Bicol Region, Philippines, demonstrates substantial potential as a solid biofuel and bioenergy feedstock. Proximate and ultimate analyses revealed high volatile matter (72.00 ± 0.20 wt%), low ash content (4.33 ± 0.76 wt%), and a higher heating value of 20.60 MJ/kg, indicating strong suitability as a solid fuel for thermochemical conversion and biofuel production. Thermogravimetric analysis (TGA) was conducted from 30 °C to 900 °C at heating rates of 10, 15, and 20 °C/min under nitrogen to examine its thermal decomposition behavior. The process followed three stages: initial moisture loss, active devolatilization, and lignin-rich char formation. The resulting kinetic and thermodynamic parameters are directly relevant for designing fast pyrolysis processes aimed at liquid biofuel production and optimizing downstream fuel utilization of the derived bio-oil and char. Kinetic analysis using the Coats–Redfern method identified third-order reaction (CR03) and diffusion-controlled (DM6) models as best-fitting, with activation energies ranging from 64.03–96.21 kJ/mol (CR03) and 66.98–104.72 kJ/mol (DM6). Corresponding thermodynamic parameters—ΔH (58.67–90.95 kJ/mol), ΔG (201.51–231.46 kJ/mol), and ΔS (−174.57 to −255.08 kJ/mol·K)—indicated an endothermic, non-spontaneous, entropy-reducing reaction pathway. Model-free methods confirmed a highly reactive zone at α = 0.3–0.6, with consistent Ea values (~130–190 kJ/mol). These findings affirm the viability of PS for fast pyrolysis, offering data-driven insights for optimizing advanced fuel and bioenergy systems in line with circular economy objectives.

Fuels doi: 10.3390/fuels7010001

Authors: Sijiang Wu Xiao Luo Wei Li Peng Ren Dongyu Wang Baobin Gao Alhaji Safiwu

Methane gas (CH4) leakage and gas extraction efficiency in drillholes present persistent challenges in coal mine gas management. To address these issues, a novel gas leakage detection device and a precision secondary grouting and thickening system were developed and field-tested at the Li YaZhuang Coal Mine, China. The system enables accurate identification of leakage zones and provides adjustable sealing length and depth, withstanding grouting pressures up to 2.0 MPa to achieve the full-section sealing of drillholes. Field application on 23 drillholes demonstrated a significant improvement in gas extraction performance. The average methane concentration and pure gas flow rate increased by more than 2-fold (2.61 and 3.05, respectively) compared with the pregrouting values, indicating substantial increases in gas extraction stability and duration. This study validates the effectiveness and practicality of the proposed secondary grouting technology for restoring failed drillholes, mitigating gas leakage, and improving methane recovery. The results provide a technical reference for advancing gas control strategies in high-gas coal seams.

Fuels doi: 10.3390/fuels6040093

Authors: Jing Zhang Sai Zhang Yueli Feng Jianxin Liu Hao Bai Ziliang Li Erdong Yao Fujian Zhou

To address the challenges of strong heterogeneity and poor crude oil mobility in tight conglomerate reservoirs of the Mahu Oilfield, this study systematically evaluated the effects of different surfactants on wettability alteration, spontaneous imbibition, and relative permeability through high-temperature/high-pressure spontaneous imbibition experiments, online Nuclear Magnetic Resonance (NMR) monitoring, and relative permeability measurements. Core samples from the Jinlong and Madong areas (porosity: 5.98–17.55%; permeability: 0.005–0.148 mD) were characterized alongside X-Ray Diffraction (XRD) data (clay mineral content: 22–35.7%) to compare the performance of anionic, cationic, nonionic, and biosurfactants. The results indicated that the nonionic surfactant AEO-2 (Fatty Alcohol Polyoxyethylene Ether) (0.2% concentration) at 80 °C exhibited optimal performance, achieving the following results: 1. a reduction in wettability contact angles by 80–90° (transitioning from oil-wet to water-wet); 2. a decrease in interfacial tension to 0.64 mN/m; 3. an imbibition recovery rate of 40.14%—5 to 10 percentage points higher than conventional fracturing fluids. NMR data revealed that nanopores (<50 nm) contributed 75.36% of the total recovery, serving as the primary channels for oil mobilization. Relative permeability tests confirmed that AEO-2 reduced residual oil saturation by 6.21–6.38%, significantly improving fluid flow in highly heterogeneous reservoirs. Mechanistic analysis highlighted that the synergy between wettability reversal and interfacial tension reduction was the key driver of recovery enhancement. This study provides a theoretical foundation and practical solutions for the efficient development of tight conglomerate reservoirs.

Fuels doi: 10.3390/fuels6040092

Authors: Sagar Kansara Kourosh Rezanejad Mohammad Jahanbakht Diogo M. F. Santos

The urgent need to address climate change has driven the exploration of sustainable energy solutions, with wave energy and green hydrogen emerging as prominent alternatives to traditional fossil fuels. This study examines the potential synergy between wave energy and hydrogen production, with a focus on the economic viability of integrating these technologies. Through a detailed analysis of the levelised cost of electricity (LCOE) and the levelised cost of hydrogen (LCOH), this paper examines how coastal regions in Portugal and across Western Europe can harness wave energy to produce green hydrogen, a crucial component in the global energy transition. The techno-economic assessment accounts for capital and operational costs, energy efficiency, and lifetime performance to determine how design and location affect economic feasibility. Preliminary analysis indicates that regions with significant wave power potential present opportunities for competitive LCOE values, with some coastal areas achieving LCOE figures as low as 0.10 €/kWh. Additionally, the LCOH analysis reveals that among various storage methods, compressed gas hydrogen at 350 bar stands out as the most cost-effective option. This research highlights the transformative potential of wave energy-driven hydrogen production as a crucial solution for decarbonising the maritime sector. Future technological advancements and cost efficiencies are poised to overcome current economic barriers and accelerate the transition to a sustainable, low-carbon energy landscape.

Fuels doi: 10.3390/fuels6040091

Authors: Andreea Cristina Mangra Florin Gabriel Florean Cristian Carlanescu

The global concern regarding the reduction of carbon emissions has led to the development of hydrogen as a clean, carbon-free fuel for combustion systems. The present work combines Particle Image Velocimetry flow field measurements and Reynolds-Averaged Navier–Stokes numerical simulations to investigate the reactive flow downstream of a newly developed flame holder as part of a hydrogen-fueled afterburner system. The obtained numerical results are in reasonable agreement, for a RANS simulation, with the PIV measured data. According to the results presented in this article, it can be seen that ignition occurs, the flame is attached to the flame holder, and vortices develop downstream of the flame holder. These vortices facilitate the mixing of hydrogen with the flue gas coming from the gas generator. The recirculation zone generated by the flame holder in the flow measures approximately 100 mm, with the peak negative velocity reaching around 10 m/s. Downstream of the recirculation zone, the far-field free stream velocity on the centerline reaches 20 m/s. Outside the recirculation region, in the radial direction, the free stream is accelerated to an experimentally measured value of approximately 40 m/s, at 20 mm downstream from the flame holder, and 35 m/s at 100 mm downstream of the flame holder. The information gathered thus far will aid further investigation of the presented hydrogen-fueled afterburner system.

Fuels doi: 10.3390/fuels6040090

Authors: Miriam Martínez-González Miguel Angel Ramos-López Ana L. Villagómez-Aranda José Alberto Rodríguez-Morales Juan Campos-Guillén Karla Elizabeth Mariscal-Ureta Aldo Amaro-Reyes Juan Antonio Valencia-Hernández Diana Saenz de la O Carlos Eduardo Zavala-Gómez

The current rise in global energy demand and environmental degradation has highlighted the need to use renewable energy as an alternative to fossil fuels. Ricinus communis L. (castor bean oil) has emerged as a promising source for biofuels production due to high oil content (45–55%), ability to grow on marginal soils, and resistance to adverse conditions. This review analyzes 93 relevant studies from 2019 to 2025, selected by the PRISMA method (Preferred Reporting Items for Systematic reviews and Meta-Analyses) from databases such as Google Scholar and Web of Science. There were identified that agronomic techniques such as optimized plant spacing, balanced fertilization, and elicitation can significantly increase productivity. Among the production methods used, heterogeneous catalysis (96.8%) and enzymatic processes (90%) stand up for their sustainability and efficiency. However, the main limitation remains the high viscosity of castor biodiesel (14–18 mm2/s at 40 °C), which exceeds international quality standards. Even so, castor biodiesel offers excellent lubricity (reduces injection wear by 20%), has standard oxidative stability, and has a relatively low cetane number (38–42), which poses challenges for ignition quality. Improvement strategies such as blending, enzymatic modification, and additive incorporation have shown potential to mitigate these limitations. The review also addresses environmental benefits, regulatory challenges, and market opportunities where the castor biodiesel offers competitive advantages. Enhancing research and innovation, supported by targeted public policies and technical standards, is essential to overcome current barriers and enable the commercial adoption of castor biodiesel as part of a more sustainable and diversified energy future.

Fuels doi: 10.3390/fuels6040089

Authors: Zhibing Shen Ruiyuan Tang Shengrong Liang Juntao Zhang Leyuan Li Shangli Zhang

The conversion of polycyclic aromatic hydrocarbons (PAHs) to monocyclic aromatic hydrocarbons holds significant importance in the petrochemical and coal chemical industries, as it enables the production of high-value-added chemicals. In this study, we investigated the methane-assisted hydroconversion of PAHs to monocyclic aromatic hydrocarbons with methyl side chains over Zn-based catalysts from Hβ zeolites treated with citric acid (CA) at different concentrations. The CA-modified Hβ catalysts were characterized using X-ray diffraction (XRD), N2 adsorption–desorption, pyridine–Fourier transform infrared spectroscopy (Py-FTIR), and ammonia temperature-programmed desorption (NH3-TPD). The results show that low CA concentrations facilitate the removal of amorphous aluminum from the zeolite framework, thereby increasing the specific surface area, pore volume, and pore diameter of the Zn/Hβ catalyst, as well as improving its Lewis/Brønsted (L/B) acid ratio. In contrast, excessive CA treatment causes the undesirable removal of framework aluminum and leads to structural collapse in the mesoporous regions formed at the interfaces between certain crystal aggregates. This, in turn, has a negative impact on the catalyst’s specific surface area, pore volume, pore size distribution, total acidity, and L/B ratio. Experimental data further indicate that the optimal Zn/Hβ catalyst, prepared using Hβ treated with 0.08 M CA, achieves a naphthalene conversion rate of up to 99% and a benzene–toluene–xylene (BTX) selectivity of 60% in the liquid product over a 10 h reaction period. These findings confirm that CA treatment not only enhances the catalytic activity of Zn/Hβ but also significantly improves its operational stability. This work provides new insights into the rational design of catalysts for the efficient conversion of PAHs to monocyclic aromatic hydrocarbons and the utilization of methane resources.

Fuels doi: 10.3390/fuels6040088

Authors: Muhammad Ammar Babatunde Oyeleke Oyewale Ahmed Elseragy Ibrahim M. Albayati Aliyu M. Aliyu

Hydrogen is emerging as a key energy carrier in the transition to a low-carbon economy. This study reviews blue and green hydrogen, analysing their production technologies, environmental impacts, economic viability and global deployment trends. Blue hydrogen, derived from natural gas, coal or biomass with carbon capture, utilisation and storage, offers a transitional pathway by reducing emissions relative to unabated fossil routes, but its benefits depend on high CO2 capture efficiencies and strict methane leakage control. Green hydrogen, produced via renewable-powered electrolysis and advanced thermochemical, photochemical and photoelectrochemical methods, represents the most sustainable long-term solution, though it is currently limited by cost and scale. This comparative assessment shows that green hydrogen’s production emissions, in the range of 0.67 kgCO-eq/kgH to 1.74 kgCO2-eq/kgH2, are substantially lower than those of blue hydrogen, in the range of 1.21 kgCO2-eq/kgH2 to 4.56 kgCO2-eq/kgH2, reinforcing its alignment with climate neutrality goals. Global production remains below 1% from low-emission sources, yet momentum is growing, with renewable-rich regions investing in large-scale electrolysers. A long short-term memory forecast suggests that blue hydrogen will dominate in the short term, but green hydrogen will surpass it around 2042. Together, both pathways are essential, blue hydrogen as a bridging option and green hydrogen as the foundation of a sustainable hydrogen economy.

Fuels doi: 10.3390/fuels6040087

Authors: Livhuwani Modau Charles Muzenda Tebogo Mashola Touhami Mokrani Rudzani Sigwadi Fulufhelo Nemavhola

SiO2 is a versatile inorganic substance with a wide spectrum of applications in areas such as fuel cells. In this study, pristine (p-SiO2) and sulfonated silica (s-SiO2) particles were synthesised using the sol–gel and Stober methods. Furthermore, this study investigated the impact of calcination time and surface changes on the morphology, and hence functionality, of silica particles synthesised as potential fuel cell membrane additives. Tetraethyl orthosilicate (TEOS) was used as a silica precursor dissolved in water, with sulfuric acid serving as the sulfonation agent. Parametric data on particle morphology, such as particle size, porosity, total surface area, and agglomeration, were measured and evaluated using BET, Fourier-transform infrared (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The amorphous nature of silica particles was confirmed by XRD analysis. The BET outcome data acquired for the synthesised silica particles were a surface area ranging from 271 to 487 m2/g, a pore diameter of 12.10–21.02 nm, and a total pore volume of 0.76–1.58 cm3/g. These data give crucial characteristics for designing appropriate silica nanofillers for hybrid fuel cell membranes. As a result, the data gathered can be used to make future decisions about silica synthesis methods for various specific applications, such as fuel cell applications.

Fuels doi: 10.3390/fuels6040086

Authors: Abdulwahab Saad Kehinde Temitope Alao Idris Temitope Bello Fawziyah Oyefunke Olarinoye Abdulhammed K. Hamzat

Dry reforming of methane (DRM) is a promising method for turning two major greenhouse gases, CO2 and CH4, into syngas (H2 + CO). This syngas has the right H2/CO ratio for making valuable chemicals and liquid fuels. However, there are significant challenges that make it tough to implement commercially. One big issue is that the process requires a lot of energy because it is highly endothermic, needing temperatures over 700 °C. This high heat can quickly deactivate the catalyst due to carbon build-up (coking) and the thermal sintering of metal nanoparticles. Researchers increasingly recognize mechanochemistry—a non-thermal, solid-state technique employing mechanical force to drive chemical transformations—as a sustainable, solvent-free strategy to address these DRM challenges. This mini-review critically assesses the dual role of mechanochemistry in advancing DRM. First, we examine its established role in creating advanced catalysts at lower temperatures. Here, mechanochemical methods help produce well-dispersed nanoparticles, enhance strong interactions between metal and support, and develop bimetallic alloys that resist coke formation and show great stability. Second, we delve into the exciting possibility of using mechanochemistry to directly engage in the DRM reaction at near-ambient temperatures, which marks a major shift from traditional thermocatalysis. Lastly, we discuss the key challenges ahead, like scalability and understanding the mechanisms involved, while also outlining future directions for research to fully harness mechanochemistry for converting greenhouse gases sustainably.

Fuels doi: 10.3390/fuels6040085

Authors: Shreyas Mysore Guruprasad Yajing Chen Ann-Katrin Müller Gabriel Sultan Agnetha Flore

As global decarbonization accelerates, the environmental and economic viability of hydrogen production largely depends on the evolving electricity supply mix. This study focused on alkaline water electrolysis (AWE) to identify the key factors affecting the competitiveness of green hydrogen. In this study, the temporal dynamics of grid transformation in Germany and the EU over a 20-year period (2025–2045) were addressed by developing a multi-objective optimization framework that integrates environmental impact analysis with machine-learning surrogate models to evaluate trade-offs between the carbon footprint and energy cost per kilogram of hydrogen. Grid-mix scenarios were generated via constrained Latin Hypercube Sampling under policy constraints, including coal phase-out and ≥80% renewables, screened for Pareto optimality, and clustered into distinct archetypes. The results indicated that cost-effective, low-carbon hydrogen production can be achieved through balanced portfolios that emphasize hydropower, biomass, and solar energy. Scenarios that minimize energy costs alone tend to breach environmental targets, whereas ultra-low-emission paths incur steep energy cost penalties. A representative scenario for 2034 (GWP = 24.57 kg CO2-eq/kg H2; Energy Cost = 9.47 €/kg H2) demonstrated a realistic synergy between policy constraints, cost, and environmental impact.

Fuels doi: 10.3390/fuels6040084

Authors: Habib Ouadi Olusegun Stanley Tomomewo Yahia Zakaria Benkhira Gizem Yildirim Aimen Laalam Abderrahmane Mellak

The development of unconventional resources continues to be propelled by innovations that enhance economic efficiency and maximize reservoir contact within operational constraints. Among the most recent of these is the U-shaped, or “horseshoe,” well design, which connects two parallel horizontal laterals with a 180° turn, effectively doubling the reservoir exposure from a single wellhead. This paper provides a comprehensive literature review of the current state of knowledge on U-shaped well technology. It surveys the operational drivers for their adoption, the critical drilling, and completion technologies that have enabled its successful implementation and discusses key design considerations for a successful operation. Field data from major North American shale basins, including the Permian, Eagle Ford, Bakken, and Haynesville, demonstrate substantial economic benefits, such as capital cost savings of approximately 20–25% compared to traditional methods. Lifecycle assessments indicate notable environmental advantages, including a 29.3% reduction in carbon emissions, a 15.8% reduction in water use, and a 50% decrease in land disturbance. Despite these clear benefits, gaps remain regarding long-term performance validation, stimulation of curved sections, and fracture modeling accuracy. Addressing these gaps is essential to fully realize the potential of U-shaped wells as a sustainable and economically attractive approach in the evolving landscape of unconventional energy development.

Fuels doi: 10.3390/fuels6040083

Authors: Peixuan Sun Ruiyuan Tang Lixia He Zhibing Shen Lingying Wang Yuanyu Tian Juntao Zhang

Amid growing concerns over fossil fuel depletion and environmental degradation, developing alternative energy sources is imperative. While MoS2-based catalysts are known for their syngas conversion activity, their selectivity toward alcohols remains limited. This study addresses this gap by developing Cu-promoted MoS2 catalysts to enhance alcohol synthesis. The results indicated that the introduction of copper significantly modulates the catalytic performance of MoS2. We demonstrate that incorporating Cu significantly modulates the catalytic properties of MoS2. The optimized catalyst with 9 wt% Cu loading exhibited a CO conversion of 17.9% and a markedly improved total alcohol selectivity of 46.4%, with a space-time yield of 67.6 mg·g−1·h−1. Although Cu addition slightly reduced CO conversion, it markedly improved alcohol selectivity by facilitating active site dispersion, suppressing Fischer-Tropsch side reactions, and stabilizing heteroatomic active phases. Finally, a catalytic mechanism for the synthesis of low-carbon alcohols from syngas on MoS2-based catalysts was proposed based on the catalyst analysis and reaction results.

Fuels doi: 10.3390/fuels6040082

Authors: Krzysztof Kogut Piotr Narloch Katarzyna Kapusta Ewa Zięba

The article presents a comparison between the pressure conditions of a real low-pressure gas network and the results of hydraulic calculations obtained using various simulation programs and empirical equations. The calculations were performed using specialized gas network analysis software: STANET (ver 10.0.26), SimNet SSGas 7, and SONET. Additionally, the simulation results were compared with calculations based on the empirical Darcy–Weisbach and Renouard equations. In the first part of the analysis, two calculation models were compared. In one model, the geodetic elevation of individual network nodes was included (elevation-aware model), while in the second, calculations were performed without considering node elevation (flat model). For low-pressure gas networks, accounting for elevation is critical due to the presence of the pressure recovery phenomenon, which does not occur in medium- and high-pressure networks. Furthermore, considering the growing need to increase the share of renewable energy, the study also examined the network’s operating conditions when using natural gas–hydrogen mixtures. The following hydrogen concentrations were considered: 2.5%, 5.0%, 10.0%, 20.0%, and 50.0%. The results confirm the importance of incorporating elevation data in the modeling of low-pressure gas networks. This is supported by the small differences between calculated results and actual pressure measurements taken from the operating network. Moreover, increasing the hydrogen content in the mixture intensifies the pressure recovery effect. The hydraulic results obtained using different computational tools were consistent and showed only minor discrepancies.

Fuels doi: 10.3390/fuels6040081

Authors: Nataliya Korol Viktor Yankovych

Biomass briquettes are increasingly used as renewable solid fuels, yet their durability under humid storage remains a key limitation. This study evaluated the mechanical performance and water resistance of briquettes made from fine (0–1 mm) and coarse (0–3 mm) charcoal fractions using molasses as a primary binder, polyvinyl alcohol (PVA, 3–7%) as a synthetic binder, and liquid soap (1–9%) as a surfactant additive. Compressive strength was measured in the dry state, after four days of water immersion, and after re-drying, while water absorption was monitored over immersion times from 15 min to 4 days. Fine-fraction briquettes showed higher strength and lower water uptake than coarse fractions, with optimal PVA contents of 6–7% providing maximum dry and post-drying strength. Moderate soap addition (2–3%) improved binder dispersion and early wet strength, whereas higher levels (>5%) reduced durability. Water absorption kinetics indicated that particle size controlled early swelling, while binder composition influenced the rate but not the final saturation. The best performance in humid storage was achieved by 0–1 mm + 4% PVA and 0–1 mm + 5% PVA + 3% soap formulations. These results support the formulation of eco-friendly binder systems that balance strength, moisture resistance, and cost for large-scale biomass briquette production.

Fuels doi: 10.3390/fuels6040080

Authors: Kaizhi Li Yanqi Chen Zhaofeng Wang Liguo Wang Demin Chen Shujun Ma Shijie Li

Coal mining has entered the stage of deep mining, and the prevention and control of gas disasters are facing significant challenges. Coal seam water injection, as an effective means of preventing and controlling gas disasters, has dual effects of pressure relief, permeability enhancement, and displacement sodium dodecyl benzene sulfonate (SDBS), as an anionic surfactant, can reduce surface tension to a certain extent in its aqueous solution and is therefore commonly used in coal seam water injection technology. In order to clarify the effect of SDBS on the water absorption capacity of coal and whether it will affect the gas adsorption capacity of coal, imbibition tests were conducted on dried coal samples in different concentrations of SDBS solutions, as well as gas adsorption tests on dried coal samples after imbibition was completed. Research shows that the key concentration range of SDBS for practical application is 0.050–0.075 wt%. When the concentration of SDBS solution is lower than 0.050 wt%, as the concentration of SDBS solution increases, the spontaneous imbibition capacity of coal increases significantly, and the adsorption capacity of coal to gas decreases significantly. When the concentration of SDBS solution is higher than 0.075 wt%, the spontaneous imbibition water capacity and gas adsorption capacity of coal hardly change significantly with the increase in solution concentration. Considering the effects of SDBS on coal water absorption and gas adsorption capacity, as well as environmental protection factors, it is recommended to use SDBS as a surfactant with a solution concentration of 0.050 wt%.

Fuels doi: 10.3390/fuels6040079

Authors: Alexandru-Constantin Bozonc Vlad-Cristian Sandu Alexia-Maria Buzila Ana-Maria Cormos

A multiscale 3D CFD model of CO2 methanation over Ni/Al2O3 was developed in COMSOL Multiphysics 6.3 for a lab-scale isothermal fixed-bed Sabatier reactor and validated against published data. The multiscale approach integrated bulk convection–diffusion, fluid flow, and pressure distribution with intraparticle diffusion–reaction phenomena coupled with Langmuir–Hinshelwood–Hougen–Watson-based kinetics, thus solving mass-transfer limitations without empirical effectiveness factors. Model validation was carried out by (i) kinetics, (ii) reactor performance, and (iii) hydrodynamics. Simulation results showed strong diffusion-dominated species transport at the bed entrance that lessened downstream as partial pressures decreased and products accumulated, resulting in a diffusion-relieved regime near the outlet. Sensitivity studies identified 320–350 °C and up to 10 bar as favorable conditions for high CH4 yield. Additionally, slightly H2-rich feed accelerated approach to equilibrium, while lower flow rates achieved near-complete conversion within the first half of the reactor bed. Simulations were carried out in COMSOL Multiphysics 6.3 on a dual Intel Xeon Platinum 8168 (48 cores at 2.7 GHz) workstation with 512 GB RAM to solve a 12-million-element mesh. The developed framework identifies a practical operating window and quantifies the conversion–throughput trade-off with flow rate, guiding operating condition selection and providing a basis for process intensification and lab-to-pilot scale-up of CO2 methanation.

Fuels doi: 10.3390/fuels6040078

Authors: Ahmet Fatih Kaya Marco Puglia Nicolò Morselli Giulio Allesina Simone Pedrazzi

In this study, the impact of the electric motor size and the hybridization ratio of a Fuel Cell Electric Bus on its vehicle performance (i.e., gradeability and acceleration) and fuel consumption was investigated using the ADVISOR software. The investigation first involved a parametric analysis with different electric motor and fuel cell sizes for the dynamic performance metrics, specifically the 0–60 km/h vehicle acceleration and the maximum gradeability (%) at a constant speed of 20 km/h. The results revealed that the acceleration is most sensitive to fuel cell power. Regarding gradeability, a more complex relationship was observed: when the electric motor power was below 215 kW, gradeability remained consistently low regardless of the fuel cell size. However, for motors exceeding 215 kW, fuel cell power then became a significant influencing factor on the vehicle’s climbing capability. Subsequently, the analysis focused on the effect of the hybridization ratio, which represents the power balance between the fuel cell and the energy storage system, varied between 0 and 0.8. Results showed that increasing the hybridization ratio decreases gradeability and acceleration performance and increases total energy consumption. This trade-off is quantitatively illustrated by the results over the Central Business District (CBD) driving cycle. For instance, the pure battery-electric configuration (a hybridization ratio of 0), featuring a 296 kW battery system, recorded a gradeability of 12.4% and an acceleration time of 16.3 s, while consuming 28,916 kJ. At an intermediate hybridization ratio of 0.4 (composed of a 118.4 kW fuel cell and a 177.6 kW battery), performance remained high with a gradeability of 12.2% and an acceleration of 17.3 s, but the energy consumption increased to 43,128 kJ. Finally, in the fuel-cell-dominant configuration with a hybridization ratio of approximately 0.8 (a 236.8 kW fuel cell and a 59.2 kW battery), gradeability dropped to 8.4%, acceleration time deteriorated to 38.9 s, and total energy consumption increased further to 52,678 kJ over the CBD driving cycle.

Fuels doi: 10.3390/fuels6040077

Authors: Pariya Zendehdel Amir Karimian Torghabeh Hossein Jowkar Nuno Pimentel

This study presents an integrated petrophysical workflow for the comprehensive characterization of the Upper Dalan and Kangan carbonate gas reservoirs in the Shanul Field, southwest Iran. By combining advanced cross-plot techniques (including M-N, MID, and RHOma-Uma plots) with probabilistic porosity modeling calibrated to core data, this work achieves a higher-resolution discrimination of lithology and more robust estimation of fluid properties compared to conventional single-log approaches. The results reveal significant heterogeneity within both formations but demonstrate the superior reservoir quality of the Upper Dalan, particularly within the UD2 subzone, and in the Ka-2a subzone of the Kangan. The improved workflow enables more accurate zonation and identification of high-quality, productive intervals, supporting optimized field development strategies. These findings provide methodological advances for challenging and heterogeneous carbonate systems, offering a reference framework for similar reservoirs in the Zagros Basin and beyond.

Fuels doi: 10.3390/fuels6040076

Authors: Jun Hu Hongyang Yu Yanan Wang

The chemical looping steam reforming of methane (CLSR) employing Fe-containing oxygen carriers can produce syngas and hydrogen simultaneously. However, Fe-based oxygen carriers exhibit low CH4 activation ability and cyclic stability. In this work, oxygen carriers with fixed Fe content and different Fe/Ni ratios were synthesized by the sol–gel method to investigate the effects of Ni-Fe-Al interactions on CLSR performance. Ni-Fe-Al interactions promote the growth of the spinel structure and regulate both the catalytic sites and the available lattice oxygen, resulting in the CH4 conversion and CO selectivity being maintained at 96–98% and above 98% for the most promising oxygen carrier, with an Fe2O3 content of 20 wt% and Fe/Ni molar ratio of 10. The surface, phase, and particle size were kept the same over 90 cycles, leading to high stability. During the CLSR cycles, conversion from Fe3+ to Fe2+/Fe0 occurs, along with transformation between Ni2+ in NiAl2O4 and Ni0. Overall, the results demonstrate the feasibility of using spinel containing earth-abundant elements in CLSR and the importance of cooperation between oxygen release and CH4 activation.

Fuels doi: 10.3390/fuels6040075

Authors: Mohammad Al-Ghnemi Erdal Ozkan Hossein Kazemi

Emissions of carbon dioxide (CO2) resulting from steam-driven enhanced oil recovery (EOR) operations present an environmental challenge as well as an opportunity to further enhance oil recovery. Using numerical simulations with realistic input data from field and laboratory measurements, we demonstrate a prudent approach to reduce CO2 emissions by capturing CO2 from steam generators of a steam-driven enhanced oil recovery (EOR) project and injecting it in a nearby oil field to improve oil recovery in this neighboring field. The proposed use of CO2 as a water-alternating-CO2 (WAG-CO2) EOR project in a small, 144-acre, sector of a target limestone reservoir would yield 42% incremental EOR oil while sequestering CO2 with a net utilization ratio (NUR) of 3100 standard cubic feet CO2 per stock tank barrel (SCF/STB) of EOR oil in a single five-spot pattern consisting of a central producer and four surrounding injectors. This EOR application sequesters 135,000, 165,000, and 213,000 metric tons of CO2 in five, ten, and twenty years in the single five spot pattern (i.e., our sector target), respectively. As a related matter, the CO2 emissions from nearby steam oil recovery project consisting of ten 58-ton steam/hr boilers amounts to 119,000 metric tons of CO2 per year with an estimated social cost of USD 440 million over 20 years. Upscaling the results from the single five-spot pattern to a four-pattern field scale increases the sequestered amount of CO2 by a factor of 4 without recycling and to 11 with recycling produced CO2 from the EOR project. Furthermore, the numerical model indicates that initiating CO2 injection earlier at higher residual oil saturations improves EOR efficiency while somewhat decreases sequestration per incremental EOR barrel. The most significant conclusion is that the proposed venture is an economically viable EOR idea in addition to being an effective sequestration project. Other sources of CO2 emissions in oil fields and nearby refineries or power generators may also be considered for similar projects.

Fuels doi: 10.3390/fuels6040074

Authors: Autumn Elniski Biljana M. Bujanovic

Pelletizing enhances competitiveness of lignocellulosic biomass (LCB) as a fuel by increasing its bulk and energy density. However, LCB pellets are prone to degradation from moisture, have high ash, and pose safety risks due to carbon monoxide (CO) emissions during storage. Hot water extraction (HWE), a mild hydrothermal treatment particularly effective for angiosperms, removes most hemicelluloses (xylans), reduces ash, and increases lignin content in remaining HWE-LCB. Based on the current understanding of CO formation, these changes suggested that HWE could reduce CO emissions. In this study, we evaluated the effects of HWE on pellets made from shrub willow, miscanthus, and wheat straw. A statistical analysis was conducted on ash, energy content, bulk density, durability, pellet length and density, moisture absorption, and CO emissions. All HWE-LCB pellets demonstrated significant increases in energy content (up to 3.54%) and reductions in moisture absorption (up to 23.84%). Although not all effects reached statistical significance, HWE generally had positive effects on ash content, bulk density, durability, and average pellet length and density. Contrary to expectations, HWE-LCB pellets emitted significantly more CO under both ambient and isothermal temperature conditions (up to 4.25 times overall increase), although still less than commercial hardwood/softwood blend pellets (<200 ppm in HWE-LCB vs. >300 ppm).

Fuels doi: 10.3390/fuels6030073

Authors: Paul C. Ani Hasan J. Al-Abedi Joseph D. Smith Zeyad Zeitoun

This study investigates the influence of feedstock blending on the structural and thermal properties of biochar produced via downdraft gasification at 850 °C. Biochars from 100% oak, a 1:1 oak-–pine blend, and a ternary blend of 50% oak, 30% pine, and 20% RDF were analyzed using SEM, BET, TGA, XRD, Raman spectroscopy, and CHN elemental analysis. The oak biochar exhibited the highest surface area (107.7 m2/g) and fixed-carbon content (79.94%), while the RDF-based biochar showed a 99.2% decrease in surface area (0.86 m2/g) and a 19.7% reduction in fixed carbon. These findings underscore RDF’s detrimental impact on porosity and stability, despite its waste valorization potential, suggesting its limited use in applications requiring high adsorption or structural integrity. Further studies should optimize RDF preprocessing and blending ratios to balance sustainability with functional performance.

Fuels doi: 10.3390/fuels6030072

Authors: Hikaru Kaneko Yusuke Ozaki Jun Takezaki Hiroyuki Daimon

This study addresses volatile fatty acid (VFA) accumulation, a key issue limiting methane fermentation under high organic loading rate (OLR) conditions. Batch experiments were conducted with GAC (0–10%) under various OLRs (1:0.5–1:10) to investigate its effect on biogas yield, methane purity, and microbial interactions. Higher GAC levels (7.5% and 10%) significantly enhanced biogas production (750–800 mL/g VS) and methane concentration (–70%) while shortening stabilization time. A continuous system with 10% GAC showed suppressed VFA accumulation, stable pH (7.0–8.1), and improved organic matter degradation. This work quantitatively evaluates the link between GAC dosage, DIET induction, and microbial community shifts under high OLR. These findings highlight GAC as an operationally simple and potentially cost-beneficial strategy for stabilizing methane fermentation, particularly in decentralized or small-scale applications.

Fuels doi: 10.3390/fuels6030071

Authors: Paul C. Ani Hasan J. Al-Abedi Joseph D. Smith Zeyad Zeitoun

This study investigates how the inclusion of refuse-derived fuel (RDF) alters the surface chemistry and electrostatic behavior of oak-based biochar. Biochars were produced using downdraft gasification at 850 °C from 100% oak (HW) and a ternary blend comprising 50% oak, 30% pine, and 20% RDF (HW/SW/RDF). Characterization using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), zeta potential, pH, and electrophoretic mobility was conducted to assess surface functionality and colloidal behavior. The RDF-containing biochar exhibited a 43.3% increase in surface nitrogen content (from 0.24% to 0.90%) and a 6.6% rise in calcium content (from 2.07% to 2.27%) alongside the introduction of chlorine (0.20%) and elevated silicon levels (0.69%) compared to RDF-free counterparts. A concurrent reduction in oxygen-containing functional groups was observed, as O1s decreased from 15.75% in HW to 13.37% in HW/SW/RDF. Electrokinetic measurements revealed a notable decrease in zeta potential magnitude from −31.5 mV in HW to −24.2 mV in HW/SW/RDF, indicating diminished surface charge and colloidal stability. Moreover, the pH declined from 10.25 to 7.76, suggesting a loss of alkalinity and buffering capacity. These compositional and electrostatic shifts demonstrate that RDF inclusion significantly modifies the surface reactivity of biochar, influencing its performance in catalysis, ion exchange, and nutrient retention. The findings underscore the need for tailored post-treatment strategies to enhance the functionality of RDF-modified biochars in environmental applications.

Fuels doi: 10.3390/fuels6030070

Authors: K. Sudha Madhuri Syed Altaf Hussain Rohit Kumar Upendra Rajak Tikendra Nath Verma

Without changing any of its constituents, tyre pyrolysis oil energy (TPOE) has frequently been subjected to Diesel-RK (D-RK) analyses in diesel engines in an effort to serve as a substitute for diesel fuel. Environmentally beneficial TPOE features, such as biodegradability, renewability, and ease and safety of handling, are highly sought after. In addition to its beneficial aspects, TOPE also has drawbacks. The BTE and SFC of performance metrics, as well as the smoke and NOx of emission parameters of alternative fuel, do not meet the emission limits specified by regulatory authorities. Nano-additions have been shown to be effective for boosting fuel quality for improved performance and production characteristics. In this study, TPOE–diesel blends are blended with ceramic oxide (CeO2 of 50 and 100 ppm) nanoparticles and subjected to a performance and production investigation of engine working physiognomies in diesel engines. For the blend TPOE10CDF80 + D, the numerical results show a positive outcome of a 1.0% rise in BTE, a 2.0% decrease in SFC, a 17.7% decrease in smoke emission, and an 18.2% increase in NOx emission as compared to diesel fuel (CDF).

Fuels doi: 10.3390/fuels6030069

Authors: Đurđica Kovačić

This study provides a comprehensive review of the application of pulsed electric field (PEF) technology as a pretreatment method for enhancing biogas production from various organic substrates. A comparative bibliometric analysis was conducted using four databases, Web of Science Core Collection, Scopus, Dimensions, and Google Scholar, to evaluate research activity, interdisciplinarity, and geographic distribution of PEF-related literature. The results show that, although biomass pretreatment research has grown considerably over the past two decades, the number of studies focused specifically on PEF remains extremely low, accounting for less than 0.5% in each database. A detailed overview of 66 studies further confirms PEF’s potential to improve methane yield through substrate disintegration and microbial community enhancement, yet highlights the need for standardization and scalability. Optimization studies reveal promising outcomes, particularly for sludge and algal substrates, though most were limited to laboratory scale. Two full-scale studies demonstrated economic feasibility, yet long-term stability, energy balance, and integration into existing anaerobic digestion systems remain underexplored. The analysis of author countries and institutions shows that research is concentrated in China, Sweden, and France. Overall, this review identifies major research gaps and outlines future directions aimed at including a more diverse range of substrates, improving comparability, and validating PEF in real-scale biogas production systems.

Fuels doi: 10.3390/fuels6030068

Authors: Myo Myo Khaing Chuck Hookham Janaka Ruwanpura Shunde Yin

This paper compares national hydrogen (H2) infrastructure plans in Canada, the United States (the USA), Singapore, and Sri Lanka, four countries with varying geographic and economic outlooks but shared targets for reaching net-zero emissions by 2050. It examines how each country approaches hydrogen production, pipeline infrastructure, policy incentives, and international collaboration. Canada focuses on large-scale hydrogen production utilizing natural resources and retrofitted natural gas pipelines supplemented by carbon capture technology. The USA promotes regional hydrogen hubs with federal investment and intersectoral collaboration. Singapore suggests an innovation-based, import-dominant strategy featuring hydrogen-compatible infrastructure in a land-constrained region. Sri Lanka maintains an import-facilitated, pilot-scale model facilitated by donor funding and foreign collaboration. This study identifies common challenges such as hydrogen embrittlement, leakages, and infrastructure scalability, as well as fundamental differences based on local conditions. Based on these findings, strategic frameworks are proposed, including scalability, adaptability, partnership, policy architecture, digitalization, and equity. The findings highlight the importance of localized hydrogen solutions, supported by strong international cooperation and international partnerships.

Fuels doi: 10.3390/fuels6030067

Authors: Sai Manoj Rayapureddy Jonas Matijošius

To achieve the goal of climate neutrality set by the European Union, it is important to find an efficient strategy to simultaneously lower nitrogen oxide, carbon monoxide, and particle emissions. When a portion of exhaust gas is reintroduced back into the combustion chamber, it reduces the combustion temperature. This reduces NOX emissions but has a negative impact on CO and particle emissions due to the lower concentration of O2. Reducing the combustion temperature can also indirectly influence particle formation. By including an oxygen-rich alternative fuel, CO emissions are reduced by 28% and 33% at 60 and 90 Nm, respectively. To further reduce particle emissions, which have significant health risks, acoustic waves are introduced to achieve better filtration through conventional DPFs that filter particles with larger diameters. With 21 kHz of acoustic frequency and 0% EGR, a 6% increase in large particles is observed. With moderate rise in the recirculation percentage, a higher combined efficiency of EGR and acoustic waves is observed. With 21 kHz acoustic frequency and 10% EGR, a 73% increase in larger particles is observed at lower loads and a 32% increase at higher loads is observed. Simultaneous emission reduction can be achieved by combining the benefits of using oxygen-rich fuel, acoustics, and EGR at a moderate rate.

Fuels doi: 10.3390/fuels6030066

Authors: Emilio Seijo-Bestilleiro Ignacio Arias-Fernández Diego Carro-López Manuel Naveiro

Crude oil accounts for approximately 40% of global energy consumption, and the refining sector is a major contributor to greenhouse gas (GHG) emissions, particularly through the production of hard-to-abate fuels such as aviation fuel and fuel oil. This study disaggregates the refinery into its key process units to identify decarbonization opportunities along the entire production chain. Units are categorized into combustion-based processes—including crude and vacuum distillation, hydrogen production, coking, and fluid catalytic cracking—and non-combustion processes, which exhibit lower emission intensities. The analysis reveals that GHG emissions can be reduced by up to 60% with currently available technologies, without requiring major structural changes. Electrification, residual heat recovery, renewable hydrogen for desulfurization, and process optimization through digital twins are identified as priority measures, many of which are also economically viable in the short term. However, achieving full decarbonization and alignment with net-zero targets will require the deployment of carbon capture technologies. These results highlight the significant potential for emission reduction in refineries and reinforce their strategic role in enabling the transition toward low-carbon fuels.

Fuels doi: 10.3390/fuels6030065

Authors: Guy Trudon Muya Ali Fellah Sun Yaquan Yasmina Boukhchana Samuel Molima Matthieu Kanyama Amsini Sadiki

Since the hydrogen-production process is not yet fully efficient, this paper proposes a poly-generation system that is driven by a geothermal energy source and utilizes a combined Kalina/organic Rankine cycle coupled with an electrolyzer unit to produce, simultaneously, power and green hydrogen in an efficient way. A comprehensive thermodynamic analysis and an exergetic evaluation are carried out to assess the effect of key system parameters (geothermal temperature, high pressure, ammonia–water concentration ratio, and terminal thermal difference) on the performance of concurrent production of power and green hydrogen. Thereby, two configurations are investigated with/without the separation of turbines. The optimal ammonia mass fraction of the basic solution in KC is identified, which leads to an overall optimal system performance in terms of exergy efficiency and green hydrogen production rate. In both configurations, the optimal evaluation is made possible by conducting a genetic algorithm optimization. The simulation results without/with the separation of turbines demonstrate the potential of the suggested cycle combination and emphasize its effectiveness and efficiency. Exemplary, for the case without the separation of turbines, it turns out that the combination of ammonia–water and MD2M provides the best performance with net power of 1470 kW, energy efficiency of 0.1184, and exergy efficiency of 0.1258 while producing a significant green hydrogen amount of 620.17 kg/day. Finally, an economic study allows to determine the total investment and payback time of $3,342,000 and 5.37 years, respectively. The levelized cost of hydrogen (LCOH) for the proposed system is estimated at 3.007 USD/kg H2, aligning well with values reported in the literature.

Fuels doi: 10.3390/fuels6030064

Authors: Loukia P. Chrysikou Vasiliki Dagonikou Stella Bezergianni

This study conducts a life cycle assessment (LCA) of a microalgae-based biorefinery producing biofuels, using a well-to-tank approach. Microalgae were cultivated using greenhouse wastewater, while the extracted lipids were converted to biofuels via catalytic hydrotreatment. Experimental data supported an Aspen Plus model to generate inventory data for the LCA. The assessment incorporated multiple environmental metrics, including global warming potential (GWP), net energy ratio (NER) etc., under variant energy sources scenarios. Results show a low GWP (0.86 kg CO2-eq/MJ) and a NER (3.7), indicating favorable environmental performance, while the downstream processes were identified as the most energy-intensive. Sensitivity analysis highlighted the critical role of energy sources, with renewable energy improving the sustainability compared to fossil-based inputs. Overall, the results support the viability of wastewater-grown microalgae for sustainable biofuel production, particularly when integrated with low-impact energy sources.

Fuels doi: 10.3390/fuels6030063

Authors: Lihle Mdleleni Sithenkosi Mlala Tobeka Naki Edson L. Meyer Mojeed A. Agoro Nicholas Rono

Fuel cells have become a fundamental technology in the development of clean energy systems, playing a vital role in the global shift toward a low-carbon future. With the growing need for sustainable hydrogen production, perfluoro sulfonic acid (PFSA) ionomer membranes play a critical role in optimizing green hydrogen technologies and fuel cells. This study aims to investigate the effects of different environmental and solvent treatments on the chemical and physical properties of Nafion N−115 membranes to evaluate their suitability for both hydrogen production in proton exchange membrane (PEM) electrolyzers and hydrogen utilization in fuel cells, supporting integrated applications in the local and global green hydrogen economy. To achieve this, Nafion N−115 membranes were partially dissolved in various solvent mixtures, including ethanol/isopropanol (EI), isopropanol/water (IW), dimethylformamide/N-methyl-2-pyrrolidone (DN), and ethanol/methanol/isopropanol (EMI), evaluated under water immersion and thermal stress, and characterized for chemical stability, mechanical strength, water uptake, and proton conductivity using advanced electrochemical and spectroscopic techniques. The results demonstrated that the EMI-treated membrane showed the highest proton conductivity and maintained its structural integrity, making it the most promising for hydrogen electrolysis applications. Conversely, the DN-treated membrane exhibited reduced stability and lower conductivity due to solvent-induced degradation. This study highlights the potential of EMI as an optimal solvent mixture for enhancing PFSA membranes performance in green hydrogen production, contributing to the advancement of sustainable energy solutions.

Fuels doi: 10.3390/fuels6030061

Authors: Ian K. Feole Bruce C. Folkedahl

Plugs of lignite coal from multiple formations were subjected to a series of tests to determine the amount of rare-earth elements (REEs) to be extracted from coal in an in situ mining operation. These tests were used to determine if extraction of REEs and other critical minerals in an in situ environment would be possible for future attempts as an alternative to extraction mining. The tests involved subjecting whole lignite coal plugs from the Twin Butte coal seams in North Dakota to flow-through tests of water, and concentrations of 1.0 M ammonium nitrate, 1.0 M and 1.5 M sulfuric acid, and 1.0 M and 1.5 M hydrochloric acid (HCl) solvents at different concentrations and combinations. The flow-through testing was conducted by alternating the solvent and water flow-through to simulate an in situ mining scenario. The samples were analyzed for their concentrations of REEs (lanthanum [La], cerium [Ce], praseodymium [Pr], neodymium [Nd], samarium [Sm], europium [Eu], gadolinium [Gd], terbium [Tb], dysprosium [Dy], holmium [Ho], erbium [Er], thulium [Tm], ytterbium [Yb], lutetium [Lu], yttrium [Y], and scandium [Sc], as well as germanium [Ge] and cobalt [Co], manganese [Mn], nickel [Ni], and barium [Ba]). Results from the testing showed that REEs were extracted in concentrations that were on average higher using sulfuric acid (8.9%) than with HCl (5.8%), which had a higher recovery than ammonium nitrate. Tests were performed over a standard time interval for comparison between solvents, while a second set of testing was done to determine recovery rates of REEs and critical minerals under certain static and constant flow-through times to determine extraction in relation to time. Critical minerals had a higher recovery rate than the REEs across all tests, with a slightly higher recovery of light REEs over heavy REEs.

Fuels doi: 10.3390/fuels6030062

Authors: Robert Wilson Calvin Kwesi Gafrey

Samples of refined petroleum fuels from the three major oil-marketing companies (GOIL Company Limited, Total Energies Ghana Limited and Shell Vivo Ghana Limited) in Ghana have been analysed for elemental concentrations using an X-ray fluorescence facility at the National Nuclear Research Institute, Ghana Atomic Energy Commission. The samples were acquired from seven different fuel service stations where customers directly purchase refined petroleum fuels such as diesel, petrol and kerosene. The X-ray fluorescence method was considered for the study because sample preparation does not require the addition of reagents, and the fluorescence measurements involve a direct electron transition effect. The fluorescence study was carried out to estimate the concentrations of sulphur and other contaminants in the major refined petroleum fuel products patronised in Ghana. The average sulphur concentration in the samples of diesel products were 17.543, 25.805 and 26.813 ppm in DFS, DE and DXP samples compared to 22.258, 22.623 and 15.748 ppm in petrol samples of PE, PXP and VP. Also, the sulphur concentration of sample KE, kerosene products, is 33.250 ppm. Among the diesel samples, DE and DXP recorded the highest but most comparable average concentration of elemental contaminants, and DFS the least, while PXP recorded the least among the petrol samples. The study estimated the concentrations of four heavy metal elements that are toxic to biological life (Hg, Pb, Cr and Mn) to be less than 10.0 ppm, except Cr. The study concluded that most of the elemental contaminants of heavy metals in the samples were relatively less than ultra-low levels. Therefore, exhaust emissions may have little impact on the environment. Also, the content of the ash-producing metal elements in each sample of the seven refined fuel products is between 10.0 and 50.0 ppm. Since the concentration of sulphur and a few other elemental contaminants could not meet the internationally accepted standard (<10.0 ppm), the imported refined fuel products used in Ghana may be considered relatively good but not environmentally safe.

Fuels doi: 10.3390/fuels6030060

Authors: Yira Victoria Hurtado Ghazal Azadi Eduardo Lins de Barros Neto Jean-Michel Lavoie

This work focuses on the fabrication, characterization, and performance of a structured iron catalyst to produce hydrocarbons by the Fischer–Tropsch synthesis (FTS). The structured catalyst enhances the heat and mass transfer and provides a larger surface area and lower pressure drop. Iron-based structured catalysts indicate more activity in lower H2/CO ratios and improve carbon conversion as compared to other metals. These catalysts were manufactured using the sponge replication method (powder metallurgy). The performance of the structured iron catalyst was assessed in a fixed-bed reactor under industrially relevant conditions (250 °C and 20 bar). The feed gas was a synthetic syngas with a H2/CO ratio of 1.2, simulating a bio-syngas derived from lignocellulosic biomass gasification. Notably, the best result was reached under these conditions, obtaining a CO conversion of 84.8% and a CH4 selectivity of 10.4%, where the catalyst exhibited a superior catalytic activity and selectivity toward desired hydrocarbon products, including light olefins and long-chain paraffins. The resulting structured catalyst reached a one-pass CO conversion of 84.8% with a 10.4% selectivity to CH4 compared to a traditionally produced catalyst, for which the conversion was 18% and the selectivity was 19%, respectively. The results indicate that the developed structured iron catalyst holds considerable potential for efficient and sustainable hydrocarbon production, mainly C10–C20 (diesel-range hydrocarbons), via Fischer–Tropsch synthesis. The catalyst’s excellent performance and improved stability and selectivity offer promising prospects for its application in commercial-scale hydrocarbon synthesis processes.

Fuels doi: 10.3390/fuels6030059

Authors: Mehrdad Kiani Ali Akbar Abbasian Arani Ehsan Houshfar Mehdi Ashjaee Pouriya H. Niknam

The utilization of ammonia as a fuel for gas turbines involves practical challenges due to its low reactivity, narrow flammability limits, and slow laminar flame propagation. One of the potential solutions to enhance the combustion reactivity of ammonia is co-firing with syngas. This paper presents an experimental and numerical investigation of the laminar burning velocity (LBV) of ammonia/syngas/air mixtures under elevated pressures (up to 10 bar) and temperatures (up to 473 K). Experiments were conducted in a constant-volume combustion chamber with a total volume of 11 L equipped with a dual-electrode capacitive discharge ignition system. A systematic sensitivity analysis was conducted to experimentally evaluate the system performance under various syngas compositions and equivalence ratios from 0.7 to 1.6 and ultimately identify the factors with the most impact on the system. As a complement to the experiments, a detailed numerical simulation was carried out integrating available kinetic mechanisms—chemical reaction sets and their rates—to support advancements in the understanding and optimization of ammonia/syngas co-firing dynamics. The sensitivity analysis results reveal that LBV is significantly enhanced by increasing the hydrogen content (>50%). Furthermore, the LBV of the gas mixture is found to increase with the use of a rich flame, higher mole fractions of syngas, and higher initial temperatures. The results indicate that higher pressure reduces LBV by 40% but at the same time enhances the adiabatic flame temperature (by 100 K) due to an equilibrium shift. The analysis was also extended to quantify the impact of syngas mole fractions and elevated initial temperatures. The kinetics of the reactions are analyzed through the reaction pathways, and the results reveal how the preferred pathways vary under lean and rich flame conditions. These findings provide valid quantitative design data for optimizing the combustion kinetics of ammonia/syngas blends, offering valuable design data for ammonia-based combustion systems in industrial gas turbines and power generation applications, reducing NOₓ emissions by up to 30%, and guiding future research directions toward kinetic models and emission control strategies.

Fuels doi: 10.3390/fuels6030058

Authors: Joshua M. Hollingshead Makayla L. L. Ianuzzi Jeffrey D. Moore Grant A. Risha

Experimental research was conducted to characterize the ignition delay time and combustion performance of non-toxic reactants as a possible replacement for highly toxic fuels, such as hydrazine. The liquid fuel and oxidizer were N,N,N′,N′-tetramethylethylenediamine (TMEDA) and white fuming nitric acid (WFNA), respectively. The hypergolic ignition delay of the reactants was determined using 100% TMEDA with either >90% or >99.5% WFNA that was distilled, titrated, and droplet-tested in a laboratory setting while controlling the parameters that affect the quality of the yielded product. It was observed that >90% WFNA had three times longer average ignition delay than >99.5% WFNA with both mixtures producing ignition delay times less than 20 ms. Based upon the demonstrated hypergolic droplet test results, a fluid delivery feed system and hypergolic heavyweight bipropellant rocket engine were designed and fabricated to characterize the combustion efficiency of these non-toxic reactants. The rocket injector and characteristic length differed while operating under similar flow conditions to evaluate combustion efficiency. Results demonstrated similar engine performance between both cases of WFNA with improvements of over 30% in combustion efficiency with increased characteristic length. Tests using 100% TMEDA/>90% WFNA achieved a combustion efficiency of 88%.

Fuels doi: 10.3390/fuels6030057

Authors: Ahissan Innocent Adou Laura Brelle Pedro Marote Muriel Sylvestre Gerardo Cebriàn-Torrejòn Nadiège Nomede-Martyr

Vegetable oils are an important alternative to the massive use of fuels and lubricants from non-renewable energy sources. In this study, the physicochemical properties of coconut oil and waste cooking oil are investigated for biofuels and biolubricant applications. A transesterification of both oils was reached, and the transesterified oils were characterized by infrared analysis and gas chromatography. The lubricant performances of these oils have been evaluated using a ball-on-plane tribometer under an ambient atmosphere. Different formulations were developed using graphite particles as solid additive. Each initial and modified oil has been investigated as a base oil and as a liquid additive lubricant. The best friction reduction findings have been obtained for both initial oils as liquid additives, highlighting the key role of triglycerides in influencing tribological performances.

Fuels doi: 10.3390/fuels6030056

Authors: Shixuan Lu Zhengyuan Zhang Liming Dai Na Jia

Vibration-stimulated waterflooding (VS-WF) is a promising enhanced oil recovery (EOR) method, especially for reservoirs with high-viscosity or emulsified oil. This study explores the effect of low-frequency vibration (2 Hz and 5 Hz) on oil mobilization under constant pressure and flow rate, using both crude and emulsified oil samples. Vibration significantly improves recovery by inducing stick-slip flow, lowering the threshold pressure, and enhancing oil phase permeability while suppressing the water phase flow. Crude oil recovery increased by up to 24% under optimal vibration conditions, while emulsified oil showed smaller gains due to higher viscosity. Intermittent vibration achieved similar recovery rates to continuous vibration, but with reduced energy use. Statistical analysis revealed a strong correlation between pressure fluctuations and oil production in vibration-assisted tests, but no such relationship in non-vibration cases. These results provide insight into the mechanisms behind vibration-enhanced recovery, supported by analysis of pressure and flow rate responses during waterflooding.

Fuels doi: 10.3390/fuels6030055

Authors: Victor Alejandro Serrano-Echeverry Carlos Alberto Guerrero-Fajardo Karol Tatiana Castro-Tibabisco

Biobutanol is becoming more relevant as a promising alternative biofuel, primarily due to its advantageous characteristics. These include a higher energy content and density compared to traditional biofuels, as well as its ability to mix effectively with gasoline, further enhancing its viability as a potential replacement. A viable strategy for attaining carbon neutrality, reducing reliance on fossil fuels, and utilizing sustainable and renewable resources is the use of biomass to produce biobutanol. Lignocellulosic materials have gained widespread recognition as highly suitable feedstocks for the synthesis of butanol, together with various value-added byproducts. The successful generation of biobutanol hinges on three crucial factors: effective feedstock pretreatment, the choice of fermentation techniques, and the subsequent enhancement of the produced butanol. While biobutanol holds promise as an alternative biofuel, it is important to acknowledge certain drawbacks associated with its production and utilization. One significant limitation is the relatively high cost of production compared to other biofuels; additionally, the current reliance on lignocellulosic feedstocks necessitates significant advancements in pretreatment and bioconversion technologies to enhance overall process efficiency. Furthermore, the limited availability of biobutanol-compatible infrastructure, such as distribution and storage systems, poses a barrier to its widespread adoption. Addressing these drawbacks is crucial for maximizing the potential benefits of biobutanol as a sustainable fuel source. This document presents an extensive review encompassing the historical development of biobutanol production and explores emerging trends in the field.

Fuels doi: 10.3390/fuels6030054

Authors: Grigore Cican Alexandru Mitrache

This study investigates the energy performance of paraffin-based hybrid fuels enhanced with potassium nitrate (KNO3), ammonium nitrate (NH4NO3), aluminum (Al), titanium (Ti), and stearic acid additives. The fuels were evaluated using thermochemical calculations via ProPEP3 Version 1.0.3.0 software, revealing significant improvements in specific impulse (Isp) and combustion temperature. While formulations with nitrates and aluminum exhibited noticeable increases in combustion efficiency and thermal output, titanium-containing mixtures provided moderate improvements. Stearic acid improved fuel processability and provided a stable burning profile without significant energy penalties. These findings demonstrate that suitable combinations of additives can substantially improve the energy output of paraffin-based hybrid fuels, making them more viable for aerospace propulsion applications.

Fuels doi: 10.3390/fuels6030053

Authors: Nayeli Gutiérrez-Casiano Joaquín Estrada-García Karla Díaz-Castellanos José Vicente-Martínez César Antonio Ortiz-Sánchez Eduardo Hernández-Aguilar

Alternative energies have become relevant in global strategies to address climate change, and third-generation biodiesel derived from the generation of lipids from microalgae represents a viable option. This process can also be coupled with wastewater treatment to remove organic matter. To determine the effects of two catalyst levels (1 and 1.5% KOH) and two molar ratios of alcohol (methanol) with oil (1:6 and 1:9) on the conversion of lipids into FAMEs and the quality of the biodiesel produced, this work suggests a method for the ultrasonication-based extraction of lipids from C. vulgaris. It also employs an experimental 22 design and three replicates. It was found that with a molar ratio of 1:9 and a 1% catalyst, the highest yield of 98.48 ± 1.13% was achieved. The FAME profile was similar to the profiles obtained in cultures with bold basal medium or INETI. The quality of the biodiesel met ASTM standards, achieving refractive indices of 1.435–1.478. The flash point (FP) was 165 ± 18 °C, and the acid number was 0.31 ± 0.17 mg KOH/g. The viscosity ranged from 4.33 to 4.87 mm2/s. However, the rheological behavior was correlated with the Ostwald–de Waele model with pseudoplastic behavior.

Fuels doi: 10.3390/fuels6030052

Authors: Adnan Aftab Silvia J. Salgar-Chaparro Quan Xie Ali Saeedi Mohammad Sarmadivaleh

The global energy sector is aiming to substantially reduce CO2 emissions to meet the UN climate goals. Among the proposed strategies, underground storage solutions such as radioactive disposal, CO2, NH3, and underground H2 storage (UHS) have emerged as promising options for mitigating anthropogenic emissions. These approaches require rigorous research and development (R&D), often involving laboratory-scale experiments to establish their feasibility before being scaled up to pilot plant operations. Microorganisms, which are ubiquitous in laboratory environments, can significantly influence geochemical reactions under variable experimental conditions of porous media and a salt cavern. We have selected a consortium composed of Bacillus sp., Enterobacter sp., and Cronobacter sp. bacteria, which are typically present in the laboratory environment. These microorganisms can contaminate the rock sample and develop experimental artifacts in UHS experiments. Hence, it is pivotal to sterilize the rock prior to conduct experimental research related to effects of microorganisms in the porous media and the salt cavern for the investigation of UHS. This study investigated the efficacy of various disinfection and sterilization methods, including ultraviolet irradiation, autoclaving, oven heating, ethanol treatments, and gamma irradiation, in removing the microorganisms from silica sand. Additionally, the consideration of their effects on mineral properties are reviewed. A total of 567 vials, each filled with 9 mL of acid-producing bacteria (APB) media were used to test killing efficacy of the cleaning methods. We conducted serial dilutions up to 10−8 and repeated them three times to determine whether any deviation occurred. Our findings revealed that gamma irradiation and autoclaving were the most effective techniques for eradicating microbial contaminants, achieving sterilization without significantly altering the mineral characteristics. These findings underscore the necessity of robust cleaning protocols in hydrogeochemical research to ensure reliable, reproducible data, particularly in future studies where microbial contamination could induce artifacts in laboratory research.

Fuels doi: 10.3390/fuels6030051

Authors: Paul C. Ani Hayder Alhameedi Hasan J. Al-Abedi Haider Al-Rubaye Zeyad Zeitoun Ugochukwu Ewuzie Joseph D. Smith

This study presents a comprehensive characterization of oak biochar produced via downdraft gasification at 850 °C. The research employs a wide range of advanced analytical techniques to examine the biochar’s physical, chemical, and structural properties. Scanning electron microscopy (SEM) revealed a mesoporous structure, while Brunauer–Emmett–Teller (BET) analysis showed a surface area of 88.97 m2/g. Thermogravimetric analysis (TGA) demonstrated high thermal stability and carbon content (78.7%). X-ray photoelectron spectroscopy (XPS) and ultimate analysis confirmed the high degree of carbonization, with low O/C (0.178) and H/C (0.368) ratios indicating high aromaticity. Fourier transform infrared spectroscopy (FTIR) identified functional groups suggesting potential for CO2 adsorption. The biochar exhibited a negative zeta potential (−31.5 mV), indicating colloidal stability and potential for soil amendment applications. X-ray diffraction (XRD) and Raman spectroscopy provided insights into the biochar’s crystalline structure and graphitization degree. These findings highlight the oak biochar’s suitability for diverse applications, including soil improvement, carbon sequestration, and environmental remediation. By filling knowledge gaps in oak-specific biochar research, this study underscores the benefits of optimized downdraft gasification and sets a foundation for future advancements in sustainable biochar applications.

Fuels doi: 10.3390/fuels6030050

Authors: Niall J. English

In bulk liquid or on solid surfaces, nanobubbles (NBs) are gaseous domains at the nanoscale. They stand out due to their extended (meta)stability and great potential for use in practical settings. However, due to the high energy cost of bubble generation, maintenance issues, membrane bio-fouling, and the small actual population of NBs, significant advancements in nanobubble engineering through traditional mechanical generation approaches have been impeded thus far. With the introduction of the electric field approach to NB creation, which is based on electrostrictive NB generation from an incoming population of “electro-fragmented” meso-to micro bubbles (i.e., with bubble size broken down by the applied electric field), when properly engineered with a convective-flow turbulence profile, there have been noticeable improvements in solid-state operation and energy efficiency, even allowing for solar-powered deployment. Here, these innovative methods were applied to a selection of upstream and downstream activities in the oil–water–fuels nexus: advancing core flood tests, oil–water separation, boosting the performance of produced-water treatment, and improving the thermodynamic cycle efficiency and carbon footprint of internal combustion engines. It was found that the application of electric field NBs results in a superior performance in these disparate operations from a variety of perspectives; for instance, ~20 and 7% drops in surface tension for CO2- and air-NBs, respectively, a ~45% increase in core-flood yield for CO2-NBs and 55% for oil–water separation efficiency for air-NBs, a rough doubling of magnesium- and calcium-carbonate formation in produced-water treatment via CO2-NB addition, and air-NBs boosting diesel combustion efficiency by ~16%. This augurs well for NBs being a potent agent for sustainability in the oil and fuels sector (whether up-, mid-, or downstream), not least in terms of energy efficiency and environmental sustainability.

Fuels doi: 10.3390/fuels6030049

Authors: Tamer M. M. Abdellatief Ahmad Mustafa Mohamed Koraiem M. Handawy Muhammad Bakr Abdelghany Xiongbo Duan

This research work seeks to introduce eco-friendly, low-carbon, and high-octane biofuel gasoline production using a synergistic approach. Four types of high-octane gasoline, including SynergyFuel-92, SynergyFuel-95, SynergyFuel-98, and SynergyFuel-100, were generated, emphasizing the deliberate combination of petroleum-derived gasoline fractions using reformate, isomerate, and delayed coking (DC) naphtha with octane-boosting compounds—bio-methanol and bio-ethanol. A set of tests have been performed to examine the effects of antiknock properties, density, oxidation stability, distillation range characteristics, hydrocarbon composition, vapor pressure, and the volatility index on gasoline blends. The experimental results indicated that the gasoline blends made from biofuel (SynergyFuel-92, -95, -98, and 100) showed adherence to important fuel quality criteria in the USA, Europe, and China. These blends had good characteristics, such as low quantities of benzene and sulfur, regulated levels of olefins and aromatics, and good distillation qualities. By fulfilling these strict regulations, Synergy Fuel is positioned as a competitive and eco-friendly substitute for traditional gasoline. The results reported that SynergyFuel-100 demonstrated the strongest hot-fuel-handling qualities and resistance to vapor lock among all the mentioned Synergy Fuels. Finally, the emergence of eco-friendly, low-carbon, and high-octane biofuel gasoline production with synergistic benefits is a big step in the direction of sustainable transportation.

Fuels doi: 10.3390/fuels6020048

Authors: Yukun Feng Yifan Wu Pengzhou Du Yang Ma Zhaoyi Zhuang

To investigate the combustion characteristics of moxa under a nitrogen atmosphere, this study employed an integrated approach combining experimental and theoretical analysis. Twelve moxa floss samples with different leaf-to-floss ratios, geographical origins, and storage durations were selected for thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) of their carbonized products in nitrogen environment. Through TG-DTG analysis, the thermal degradation patterns of the twelve moxa floss samples under nitrogen atmosphere were systematically examined to elucidate their pyrolysis behaviors, with particular emphasis on the influence of pyrolysis temperature and leaf-to-floss ratio on combustion characteristics. The pyrolysis process occurred in three distinct stages, with the most significant mass loss (120–430 °C) observed in the second stage. Higher leaf–fiber ratios and longer storage years were found to promote more complete pyrolysis. Kinetic analysis was performed to fit thermogravimetric data, establishing a reaction kinetic model for moxa pyrolysis. Results indicated that samples with higher leaf–fiber ratios required greater activation energy, while storage duration showed negligible impact. Notably, Nanyang moxa demanded higher pyrolysis energy than Qichun moxa. FTIR analysis identified the primary components of carbonized products as water, ester compounds, flavonoids, and cellulose. These findings suggest that moxa carbonization products retain chemical reactivity, demonstrating potential applications in adsorption and catalysis processes.

Fuels doi: 10.3390/fuels6020047

Authors: Murzabek Baikenov Dariya Izbastenova Yue Zhang Xintai Su Nazerke Balpanova Almas Tusipkhan Zeinep Akanova Amirbek Moldabayev Balzhan Tulebaeva Gulzhan Taurbaeva

This paper presents the results of a study of the catalytic hydrogenation of phenanthrene using catalysts based on chrysotile modified with nickel and titanium (chrysotile/NiTi), as well as coal shale. Complex characterization of catalysts in terms of acid, texture and morphological properties was carried out. Pre-reduction in the catalysts has been found to increase the yield of partially and fully hydrogenated products, including tetrahydronaphthalene, trans-decalin and dihydrophenanthrene. Particular attention is paid to the role of coal shale as a donor source of hydrogen in thermolysis conditions. The results of hydrogenation revealed complex mechanisms of phenanthrene transformations, including partial saturation of aromatic rings, desulfurization and the formation of alkyl-substituted compounds. The obtained data emphasize the prospects of using the studied catalysts in the processes of processing heavy and solid hydrocarbon raw materials, which opens up opportunities for creating new technologies for the production of liquid fuel.

Fuels doi: 10.3390/fuels6020046

Authors: Prantik Roy Chowdhury Adam C. Gladen

This study experimentally investigates various flow field designs for a direct ethanol-based proton exchange membrane (PEM) fuel cell operated at a temperature above the vaporization temperature of water. It expands the designs of flow fields investigated for high-temperature (HT) direct ethanol fuel cells by comparing four designs. It investigates the performance of these designs at various ethanol concentrations and flow rates. A series of polarization, constant current, and impedance spectroscopy experiments were carried out at different combinations of operating conditions. The result shows that all flow fields provide poorer performance at a high ethanol concentration (6 M), regardless of ethanol inlet flow rates. At a low concentration (3 M), the 2-channel spiral flow field exhibits higher cell power output (12–18% higher) with less mass transport loss and charge transfer resistance compared to other flow fields, although it has some voltage instability. As such, it is identified as a promising design, particularly for higher-power applications. The 4-channel serpentine, dual-triangle sandwich, and hybrid flow fields offer similar cell power output (max power: ~23 mW/cm2) and cell potentials. However, the cell potential instability and mass transport losses are higher in the hybrid flow field compared to the other two designs. Thus, it is not as promising a design for ethanol-based HT-PEM fuel cells. Since the dual-triangle has similar performance to the 4-channel serpentine, it could be an alternative to the serpentine for ethanol-based HT-PEM fuel cells.

Fuels doi: 10.3390/fuels6020045

Authors: Carolina Ardila-Suarez Jean-Paul Lacoursière Gervais Soucy Bruna Rego de Vasconcelos

Countries worldwide are focused on the objective of zero emissions by 2050. However, the accelerated implementation of clean technologies has had some drawbacks, remarkably those related to safety issues. Liquefied petroleum gas (LPG) emerges as a transition fuel in this context, considering the following two aspects. First, LPG is a fuel that has environmental advantages compared to other fossil fuels, so the extension of coverage as a replacement fuel is a key factor. Second, LPG has a well-developed storage and transportation infrastructure that can be used, sometimes without modifications, for clean fuels, helping their implementation. Therefore, the safety analysis and the study of the consequences related to the hazards of LPG is a current subject that contributes, through all the tools reviewed in this article, to not only reduce the risks of this fuel but also to connect with the safety issues of clean fuels. This review article provides a comprehensive overview through consequence modeling tools, highlighting computational fluid dynamics (CFD) and machine learning to pave the way for the full implementation of clean fuels that will power the future of humanity.

Fuels doi: 10.3390/fuels6020044

Authors: Nicholas O’Connell Dominik Stümpfl Rudolf Höß Raphael Lechner

Dimethyl carbonate (DMC) and polyoxymethylene dimethyl ethers (PODE also known as OME) are possible diesel additives that can be produced sustainably using green methanol. DMC can be produced from CO2 and methanol, while PODE can be produced from methanol and formaldehyde. In this study both DMC and PODE were investigated as drop-in diesel fuel additives regarding material compatibility, injection behavior, as well as particle and exhaust emissions. Both DMC and PODE are known to be incompatible with certain materials used as seals in the fuel injection system. Therefore, the material compatibility of both neat DMC and PODE as well as blends with B0 was investigated, with both PFTE and FFKM showing good compatibility. The hydraulic injection behavior of DMC–diesel and PODE–diesel blends was investigated experimentally, showing the need for compensating injection quantities for DMC and PODE blends to match neat diesel power output due to their lower calorific values. Energetic compensation can be achieved by higher injection pressures or longer injection durations. Engine tests have been conducted with both DMC–diesel and PODE–diesel blends, demonstrating the potential to mitigate the particle–NOX trade-off.

Fuels doi: 10.3390/fuels6020042

Authors: Elvin Hajiyev Marshall Watson Hossein Emadi Bassel Eissa Athar Hussain Abdul Rehman Baig Abdulrahman Shahin

Carbon Capture and Storage (CCS) technology presents a practical solution for reducing industrial carbon dioxide (CO2) emissions through underground anthropogenic CO2 storage in depleted hydrocarbon reservoirs. The long-term storage efficiency faces several CO2 leakage challenges that need to be addressed in the planning phase of the CCS project. Thus, effective risk assessment (RA) methodologies are crucial for ensuring safety, regulatory compliance, and public acceptance of CCS projects. This review examines RA parts and their corresponding technical and non-technical challenges. The analysis critically compares over 20 qualitative, semi-quantitative, quantitative, and hybrid RA techniques employed throughout GCS operations. Available quantitative RA tools do not deliver dependable results because they require technical data that become available late in the CCS project development process. Qualitative approaches work well for the initial screening of storage sites with limited data available, yet quantitative methods enable quantification of CO2 leakage. For the first time, a comparative analysis of two integrated assessment tools is presented in this paper. The techniques achieve success based on high-quality data and analysis of existing technical and non-technical challenges which this paper examines. The comparative analysis outlines the limitations and advantages of every methodology studied and emphasizes the need for integrated hybrid frameworks to boost decision-making in the RA process. Future research should focus on creating or improving existing hybrid frameworks for late-stage RA while utilizing qualitative frameworks in the initial site screening stage to advance GSC’s safe and effective implementation.

Fuels doi: 10.3390/fuels6020043

Authors: Sotir Sotirov Evdokia Sotirova Rosen Dinkov Dicho Stratiev Ivelina Shiskova Iliyan Kolev Georgi Argirov Georgi Georgiev Vesselina Bureva Krassimir Atanassov Radoslava Nikolova Anife Veli Svetoslav Nenov Denis Dichev Stratiev Svetlin Vasilev

The production of very-low-sulfur residual fuel oil is a great challenge for modern petroleum refining because of the instability issues caused by blending incompatible relatively high-sulfur residual oils and ultra-low-sulfur light distillates. Another obstacle in the production of very-low-sulfur residual fuel oil using hydroprocessing technology is the contradiction of hydrodesulfurization with hydrodemetallization, as well as the hydrodeasphaltization functions of the catalytic system used. Therefore, the production of very-low-sulfur residual fuel oil by employing hydroprocessing could be achieved by finding an appropriate residual oil to be hydroprocessed and optimal operating conditions and by controlling catalyst system condition management. In the current study, data on the characteristics of 120 samples of heavy fuel oils produced regularly over a period of 10 years from a high-complexity refinery utilizing H–oil vacuum residue hydrocrackers in its processing scheme, the crude oils refined during their production, the recipes of the heavy fuel oils, and the level of H–oil vacuum residue conversion have been analyzed by using intercriteria and regression analyses. Artificial neural network models were developed to predict the characteristics of hydrocracked vacuum residues, the main component for the production of heavy fuel oil. It was found that stable very-low-sulfur residual fuel oil can be manufactured from crude oils whose sulfur content is no higher than 0.9 wt.% by using ebullated bed hydrocracking technology. The diluents used to reduce residue viscosity were highly aromatic FCC gas oils, and the hydrodemetallization rate was higher than 93%.

Fuels doi: 10.3390/fuels6020041

Authors: Seong-Yeun Yoo Thi. Thu-Trang Ho Ahmad Nadeem Seong-Su Kim Kangil Choe Jai-Young Lee

Chicken manure (CM) is a nutrient-rich but environmentally problematic biomass that requires sustainable management. This study applied a three-step process consisting of hydrothermal activation (ZnCl2 or H3PO4), catalytic hydrothermal carbonization (HCl or FeCl3), and low-temperature pyrolysis (250 °C) to develop an energy-efficient method for producing biochar. The resulting biochars were systematically analyzed for their physicochemical properties, heavy metal content, and carbon sequestration potential, and compared with conventional pyrolysis-based biochars. Among the tested samples, the biochar produced via H3PO4 activation and HCl-catalyzed HTC [P-HTC(HCl)] exhibited the most favorable characteristics, including the highest carbon content (59.5 wt.%) and the lowest H/C ratio (0.65). As a result, it achieved the highest total potential carbon (TPC, 158.8 gcarbon/kgbiochar) and CO2 reduction potential (CRP, 465.9 gCO2-eq/kgbiochar), attributed to the strong dehydration and decarboxylation reactions and effective inorganic removal induced by Brønsted acid action. In contrast, conventional pyrolysis biochars showed significantly higher concentrations of heavy metals—up to 633 mg/kg of Cu and 2331 mg/kg of Zn—due to thermal concentration effects, whereas P-HTC(HCl) biochar presented a more balanced and environmentally acceptable heavy metal profile. In conclusion, the proposed low-temperature hydrothermal-assisted process demonstrates great potential for producing high-performance biochar from chicken manure with enhanced environmental safety and carbon storage efficiency.

Fuels doi: 10.3390/fuels6020040

Authors: Penka Zlateva Angel Terziev Kalin Krumov Mariana Murzova Nevena Mileva

The present study analyzes the combustion process of mixed biomass pellets in a domestic boiler. For the purposes of the research, experimental measurements of flue gases are combined with numerical simulations based on computational fluid dynamics (CFD). Special attention is given to the impact of the ratio between primary and secondary air on the combustion process, emission characteristics, and thermal balance. The results show that an air distribution ratio of 60/40 (primary/secondary) leads to more complete combustion, reducing emissions of carbon monoxide (CO) and nitrogen oxides (NOx), while also improving the efficiency of the boiler. The analysis of the numerical modeling results shows that CO emissions decrease by 12% and NOx emissions by 27%. The calculated model is validated using experimental data on flue gas temperature, oxygen (O2) and carbon dioxide (CO2) concentrations, and combustion efficiency, and a high degree of correspondence between theoretical and actual measurements is established. The simulations reveal the dynamics of the temperature field, the movement of flue gases, and the role of turbulence in the combustion chamber. Optimization of the air distribution is proven to improve the combustion process and reduce the harmful emissions generated. The obtained results highlight the potential of mixed biomass pellets as a sustainable alternative to conventional fuels, provided that combustion parameters are precisely regulated. They can serve as a foundation for the enhancement of biomass-based heating systems in order to achieve higher efficiency and environmental sustainability. A market research study is also conducted, revealing that mixed pellets are preferred due to their high calorific value, low cost, and low ash content.

Fuels doi: 10.3390/fuels6020039

Authors: Cecilia Pistolesi Alberto Giaconia Claudia Bassano Marcello De Falco

This study evaluates the economic feasibility of flexible, renewable ammonia production in Italy through a comprehensive sensitivity analysis of the levelized cost of ammonia (LCOA). Ammonia is produced through Haber–Bosch synthesis from green hydrogen and nitrogen coming from alkaline electrolysis and cryogenic air separation, respectively. The analysis examines the impact of key parameters such as renewable source peak power, Haber–Bosch reactor flexibility, energy mix, electrochemical and hydrogen storage, on the final production cost. The location considered for the PV and wind power output is Southern Italy. The results show that a wind-driven system with minimal battery storage and a flexibility factor (ratio between the minimum operating capacity and the nominal capacity of the plant) of 20% offers the most cost-effective solution, but production is scaled down to 64 tpd. With the 2030 cost structure, battery storage offers better integration with wind systems and flexible operation, even at low levels of turndown. For different combinations of process design choices and flexibility, the optimal LCOA for a green ammonia production is approximately 0.59 USD/kgNH3 in 2050. This cost of production could be competitive with grey ammonia, provided that a carbon emission allowance of USD 0.12/kgCO2 is applied.

Fuels doi: 10.3390/fuels6020038

Authors: Hasan J. Al-Abedi Joseph D. Smith Haider Al-Rubaye Paul C. Ani Caleb Moellenhoff Tyler McLeland Katarina Zagorac

This research delves into the co-pyrolysis of refuse-derived fuel (RDF) and oil shale (OS), utilizing a 50% weight ratio for each component. The study employs a fixed-bed reactor, augmented by electrical kiln heating, to conduct the co-pyrolysis process. A significant aspect of this research is the use of Aspen Plus software for process simulation, with the simulated results undergoing validation through experimental data. A commendable correlation was observed between the experimental outcomes and the model predictions, underscoring the reliability of the simulation approach. The investigation reveals distinct product yields from the pyrolysis of 100% RDF and 100% OS. Specifically, the pyrolysis of pure RDF yielded 45.26% gas, 20.67% oil, and 34.07% char by weight. In contrast, the pyrolysis of pure OS resulted in 14.51% gas, 8.32% liquid, and a significant 77.61% char by weight. The co-pyrolysis of RDF and OS in a 50% blend altered the product distribution to 31.98% gas, 12.58% liquid, and 55.09% char by weight. Furthermore, the Aspen Plus simulation model aligned closely with these findings, predicting yields of 31.40% gas, 11.9% oil, and 56.6% char by weight for the RDF-OS blend. This study not only elucidates the co-pyrolysis behavior of RDF and OS but also contributes valuable insights into the potential of these materials to address the pressing issue of plastic waste management and energy resource utilization. The findings underscore the efficacy of RDF and OS co-pyrolysis as a viable strategy for enhancing the value extraction from waste and underutilized energy resources, presenting a promising avenue for environmental and energy sustainability.

Fuels doi: 10.3390/fuels6020037

Authors: José Miguel Mahía-Prados Ignacio Arias-Fernández Manuel Romero Gómez Sandrina Pereira

The main motivation for this paper was the lack of studies and comparative analyses on the new generation of alternative fuels in the marine sector, such as methane, methanol, ammonia and hydrogen. Firstly, a review of international legislation and the status of these new fuels was carried out, highlighting the current situation and the different existing alternatives for reducing greenhouse gas (GHG) emissions. In addition, the status and evolution of the current order book for ships since the beginning of this decade were used for this analysis. Secondly, each fuel and its impact on the geometry and operation of the engine were evaluated in a theoretical engine called MW-1. Lastly, an economic analysis of the current situation of each fuel and its availability in the sector was carried out in order to select, using the indicated methodology, the most viable fuel at present to replace traditional fuels with a view to the decarbonization set for 2050.

Fuels doi: 10.3390/fuels6020036

Authors: Evdokia Sotirova Svetlin Vasilev Dicho Stratiev Ivelina Shishkova Sotir Sotirov Radoslava Nikolova Anife Veli Veselina Bureva Krassimir Atanassov Vanya Georgieva Denis Stratiev Svetoslav Nenov

All modern process simulators rely on thermodynamic methods to estimate physical properties and calculate phase equilibria. The critical properties of individual components or pseudo-components, which represent undefined mixtures, play a crucial role in these calculations. However, the chemical compositions and characteristics of whole crude oils, petroleum fractions, and fuels, which are very complex mixtures of individual hydrocarbons, can vary significantly depending on the specific crude oil and the processing involved. For instance, straight-run petroleum fractions differ from those obtained through cracking processes due to differences in unsaturated hydrocarbon content. Consequently, effective methods for predicting critical temperature and pressure must account for a wide range of compositional scenarios. To address this challenge, we utilized a database of 176 individual hydrocarbons to evaluate the existing correlations for critical temperature and pressure calculations. Intercriteria analysis was performed to evaluate the relations between the different variables to be used for critical temperature and pressure predictions. Additionally, we proposed new correlations and ANN models for these properties and assessed their performance. Our study aims to provide robust predictive models that can accurately estimate critical properties across diverse petroleum fractions and compositions.

Fuels doi: 10.3390/fuels6020035

Authors: Dicho Stratiev

The residue conversion processes, coking, visbreaking, and fluid catalytic cracking (FCC), have demonstrated that feedstock quality is the single factor that most affects process performance. While, for the FCC, it is known that the heavy oil conversion at a maximum gasoline yield point can vary between 50 and 85 wt. %, for the vacuum residue hydrocracking, no reports have appeared yet to reveal the dependence of conversion on the quality of vacuum residue being hydrocracked. In order to search for such a dependence, eight vacuum residues derived from medium, heavy, and extra heavy crude oils have been hydrocracked in a laboratory unit at different reaction temperatures. The current study has witnessed that the vacuum residue hydrocracking obeys the same rule as that of the other residue conversion processes, confirming that the feedstock quality has a great influence on the process performance. A conversion variation between 45 and 85 wt. % can be observed when the sediment content in the hydrocracked atmospheric residue is within the acceptable limit, guaranteeing the planned cycle length. An intercriteria analysis was performed, and it revealed that the vacuum residue conversion has negative consonances with the contents of nitrogen and metals. Correlations were developed which predict the conversion at constant operating conditions within the uncertainty of conversion measurement of 1.7 wt. % and correlation coefficient of 0.964. The conversion at constant hydrocracked atmospheric residue (HCAR) sediment content was predicted with a correlation coefficient of 0.985. The correlations developed in this work disclosed that the higher the contents of metals, nitrogen, and asphaltenes, and the lower the content of sulfur, the lower the conversion in the hydrocracking process is. It was also shown that vacuum residues, which have the same reactivity (the same conversion at identical operating conditions), can indicate significant difference in the conversion at the same HCAR sediment content due to their diverse propensity to form sediments in the process of hydrocracking.