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Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Applied Chemistry".

Deadline for manuscript submissions: closed (31 December 2023) | Viewed by 22170

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Dr. Zhuozhi Wang

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School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China
Interests: biomass upgradation; solid waste; thermal conversion; co-utilization; carbon-neutral fuel
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Boxiong Shen

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School of Chemical Engineering & Technology, Hebei University of Technology, Tianjin 300130, China
Interests: solid waste; NOx reduction; petroleum engineering; catalyst characterization; SCR
Special Issues, Collections and Topics in MDPI journals

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Dear Colleagues,

The extensive utilization of fossil fuels in energy generation and industrial production is creating serious environmental problems, especially regarding the massive emission of greenhouse gas (GHG), predominantly CO₂. Therefore, the substitution of carbon-neutral fuels (agricultural by-products, microalgae, forestry waste, etc.) for fossil fuels seems to be an appropriate disposal method for reducing the emission of GHG in energy generation and the chemical industry. The appropriate utilization of carbon-neutral fuels would not only abate environmental pollution problems and GHG emissions, but promote the conversion of waste into resources. To realize the aim of carbon neutralization for energy generation and the chemical industry, various methods have been proposed to achieve the high efficiency and clean utilization of carbon-neutral fuels, including catalytic/non-catalytic pyrolysis, co-combustion, gasification, hydrothermal carbonization, and lignocellulose biomass compost. In these typical reaction processes, the conditions and procedures play vital roles in affecting the generation characteristics and chemical performances of the gaseous, liquid and solid products. Consequently, investigations on novel approaches for carbon-neutral fuel conversion and utilization are welcome to be submitted to this Special Issue.

Dr. Zhuozhi Wang
Prof. Dr. Boxiong Shen
Guest Editors

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Keywords

carbon-neutral fuel
greenhouse gas emission
biomass thermal conversion
high-efficiency and clean utilization technology
chemical performance of product derived from biomass based fuels

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Renewable Energy, Fuels and Chemicals from Biomass in Molecules (10 articles)

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15 pages, 6312 KiB

Open AccessArticle

Effect of Equivalence Ratio on Pollutant Formation in CH₄O/H₂/NH₃ Blend Combustion

by Jingyun Sun, Qianqian Liu, Mingyan Gu and Yang Wang

Molecules 2024, 29(1), 176; https://doi.org/10.3390/molecules29010176 - 28 Dec 2023

Viewed by 1276

Abstract

This paper investigates the effect of equivalence ratio on pollutant formation characteristics of CH₄O/H₂/NH₃ ternary fuel combustion and analyzes the pollutant formation mechanisms of CO, CO₂, and NO_X at the molecular level. It was found that lowering the equivalence ratio accelerates the decomposition of CH₄O, H₂, and NH₃ in general. The fastest rate of consumption of each fuel was found at φ = 0.33, while the rates of CH₄O and NH₃ decomposition were similar for the φ = 0.66 and φ = 0.4. CO shows an inverted U-shaped trend with time, and peaks at φ = 0.5. The rate and amount of CO₂ formation are inversely proportional to the equivalence ratio. The effect of equivalence ratio on CO₂ is obvious when φ > 0.5. NO₂ is the main component of NO_X. When φ < 0.66, NO_X shows a continuous increasing trend, while when φ ≥ 0.66, NO_X shows an increasing and then stabilizing trend. Reaction path analysis showed that intermediates such as CH₃ and CH₄ were added to the CH₄O to CH₂O conversion stage as the equivalence ratio decreased with φ ≥ 0.5. New pathways, CH₄O→CH₃→CH₂O and CH₄O→CH₃→CH₄→CH₂O, were added. At φ ≤ 0.5, new intermediates CHO₂ and CH₂O₂ were added to the CH₂O to CO₂ conversion stage, and new pathways are added: CH₂O→CO→CHO₂→CO₂, CH₂O→CO→CO₂, CH₂O→CHO→CO→CHO₂→CO₂, and CH₂O→CH₂O₂→CO₂. The reduction in the number of radical reactions required for the conversion of NH₃ to NO from five to two directly contributes to the large amount of NO_X formation. Equivalent ratios from 1 to 0.33 corresponded to 12%, 21.4%, 34%, 46.95%, and 48.86% of NO₂ remaining, respectively. This is due to the fact that as the equivalence ratio decreases, more O₂ collides to form OH and some of the O₂ is directly involved in the reaction forming NO₂. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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21 pages, 6033 KiB

Open AccessArticle

Reactive Molecular Dynamics Study of Pollutant Formation Mechanism in Hydrogen/Ammonia/Methanol Ternary Carbon-Neutral Fuel Blend Combustion

by Jingyun Sun, Qianqian Liu, Yang Wang, Mingyan Gu and Xiangyong Huang

Molecules 2023, 28(24), 8140; https://doi.org/10.3390/molecules28248140 - 17 Dec 2023

Viewed by 1368

Abstract

Hydrogen, ammonia, and methanol are typical carbon-neutral fuels. Combustion characteristics and pollutant formation problems can be significantly improved by their blending. In this paper, reactive molecular dynamics were used to investigate the pollutant formation characteristics of hydrogen/ammonia/methanol blended fuel combustion and to analyze the mechanisms of CO, CO₂, and NO_X formation at different temperatures and blending ratios. It was found that heating can significantly increase blending and combustion efficiency, leading to more active oxidizing groups and thus inhibiting N₂ production. Blended combustion pollutant formation was affected by coupling effects. NH₃ depressed the rate of CO production when CH₄O was greater than 30%, but the amount of CO and CO₂ was mainly determined by CH₄O. This is because CH₄O provides more OH, H, and carbon atoms for CO and CO₂ to collide efficiently. CH₄O facilitates the combustion of NH₃ by simplifying the reaction pathway, making it easier to form NO_X. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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15 pages, 3072 KiB

Open AccessArticle

Gas-Pressurized Torrefaction of Lignocellulosic Solid Wastes: Deoxygenation and Aromatization Mechanisms of Cellulose

by Liu Shi, Yiming Sun, Xian Li, Shuo Li, Bing Peng, Zhenzhong Hu, Hongyun Hu, Guangqian Luo and Hong Yao

Molecules 2023, 28(22), 7671; https://doi.org/10.3390/molecules28227671 - 20 Nov 2023

Cited by 2 | Viewed by 1171

Abstract

A novel gas-pressurized (GP) torrefaction method at 250 °C has recently been developed that realizes the deep decomposition of cellulose in lignocellulosic solid wastes (LSW) to as high as 90% through deoxygenation and aromatization reactions. However, the deoxygenation and aromatization mechanisms are currently unclear. In this work, these mechanisms were studied through a developed molecular structure calculation method and the GP torrefaction of pure cellulose. The results demonstrate that GP torrefaction at 250 °C causes 47 wt.% of mass loss and 72 wt.% of O removal for cellulose, while traditional torrefaction at atmospheric pressure has almost no impact on cellulose decomposition. The GP-torrefied cellulose is determined to be composed of an aromatic furans nucleus with branch aliphatic C through conventional characterization. A molecular structure calculation method and its principles were developed for further investigation of molecular-level mechanisms. It was found 2-ring furans aromatic compound intermediate is formed by intra- and inter-molecular dehydroxylation reactions of amorphous cellulose, and the removal of O-containing function groups is mainly through the production of H₂O. The three-ring furans aromatic compound intermediate and GP-torrefied cellulose are further formed through the polymerization reaction, which enhances the removal of ketones and aldehydes function groups in intermediate torrefied cellulose and form gaseous CO and O-containing organic molecules. A deoxygenation and aromatization mechanism model was developed based on the above investigation. This work provides theoretical guidance for the optimization of the gas-pressurized torrefaction method and a study method for the determination of molecular-level structure and the mechanism investigation of the thermal conversion processes of LSW. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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18 pages, 14782 KiB

Open AccessArticle

Electrochemical Performance of Nitrogen Self-Doping Carbon Materials Prepared by Pyrolysis and Activation of Defatted Microalgae

by Xin Wang, Lu Zuo, Yi Wang, Mengmeng Zhen, Lianfei Xu, Wenwen Kong and Boxiong Shen

Molecules 2023, 28(21), 7280; https://doi.org/10.3390/molecules28217280 - 26 Oct 2023

Cited by 1 | Viewed by 1459

Abstract

Pyrolysis and activation processes are important pathways to utilize residues after lipid extraction from microalgae in a high-value way. The obtained microalgae-based nitrogen-doped activated carbon has excellent electrochemical performance. It has the advantage of nitrogen self-doping using high elemental nitrogen in microalgae. In this study, two kinds of microalgae, Nanochloropsis and Chlorella, were used as feedstock for lipid extraction. The microalgae residue was firstly pyrolyzed at 500 °C to obtain biochar. Then, nitrogen-doped activated carbons were synthesized at an activation temperature of 700–900 °C with different ratios of biochar and KOH (1:1, 1:2, and 1:4). The obtained carbon materials presented rich nitrogen functional groups, including quaternary-N, pyridine-N-oxide, pyrrolic-N, and pyridinic-N. The nitrogen content of microalgae-based activated carbon material was up to 2.62%. The obtained materials had a specific surface area of up to 3186 m²/g and a pore volume in the range of 0.78–1.54 cm³/g. The microporous pore sizes of these materials were distributed at around 0.4 nm. Through electrochemical testing such as cyclic voltammetry and galvanostatic charge–discharge of materials, the materials exhibited good reversibility and high charge–discharge efficiency. The sample, sourced from microalgae Chlorella residue at activation conditions of 700 °C and biochar/KOH = 1:4, exhibited excellent endurance of 94.1% over 5000 cycles at 2 A/g. Its high specific capacitance was 432 F/g at 1 A/g. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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17 pages, 6870 KiB

Open AccessArticle

Enhanced Assembling of N-and-K-Riched Macroalgae as Carbon Adsorbent for CO₂ Capture with Ni(NO₃)₂/KOH as Co-Catalysts

by Huijuan Ying, Ganning Zeng, Yaohong He, Yanjun Hou and Ning Ai

Molecules 2023, 28(17), 6242; https://doi.org/10.3390/molecules28176242 - 25 Aug 2023

Cited by 1 | Viewed by 1066

Abstract

Porous-activated carbons have drawn great attention due to their important role in CO₂ capture. Ni(NO₃)₂/KOH, as co-catalysts under different temperatures, were studied to obtain porous graphitized carbon from Sargassum horneri feedstock. The results indicated that the properties of the porous graphitized carbon generated at 850 °C were greatly enhanced, showing a large specific surface area of 1486.38 cm³·g⁻¹ with narrowly distributed micropores (~0.67 nm) and abundant functional groups, which endowed high CO₂ uptake; moreover, the high CO₂ uptake was mainly attributed to the synergistic effect of Ni(NO₃)₂ and KOH, both in chemical modification and pore formation. The fitted values of the four kinetic models showed that the double exponential model provided the best description of carbon adsorption, indicating both physical and chemical adsorption. It is worth noting that carbon could be reused four times in the adsorption/desorption procedure in this research with good stability. This work focuses on the high-value-added comprehensive utilization of macroalgae, which not only is important for high-performance adsorbent preparation but also has positive benefits for the development and utilization of macroalgae resources. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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18 pages, 2087 KiB

Open AccessArticle

Effects of Torrefaction Pretreatment on the Structural Features and Combustion Characteristics of Biomass-Based Fuel

by Xu Yang, Yaying Zhao, Lei Zhang, Zhuozhi Wang, Zhong Zhao, Wenkun Zhu, Jiao Ma and Boxiong Shen

Molecules 2023, 28(12), 4732; https://doi.org/10.3390/molecules28124732 - 13 Jun 2023

Cited by 6 | Viewed by 1746

Abstract

Wheat straw, a typical agricultural solid waste, was employed to clarify the effects of torrefaction on the structural features and combustion reactivity of biomass. Two typical torrefaction temperatures (543 K and 573 K), four atmospheres (argon, 6 vol.% O₂, dry flue gas and raw flue gas) were selected. The elemental distribution, compositional variation, surface physicochemical structure and combustion reactivity of each sample were identified using elemental analysis, XPS, N₂ adsorption, TGA and FOW methods. Oxidative torrefaction tended to optimize the fuel quality of biomass effectively, and the enhancement of torrefaction severity improved the fuel quality of wheat straw. The O₂, CO₂ and H₂O in flue gas could synergistically enhance the desorption of hydrophilic structures during oxidative torrefaction process, especially at high temperatures. Meanwhile, the variations in microstructure of wheat straw promoted the conversion of N-A into edge nitrogen structures (N-5 and N-6), especially N-5, which is a precursor of HCN. Additionally, mild surface oxidation tended to promote the generation of some new oxygen-containing functionalities with high reactivity on the surface of wheat straw particles after undergoing oxidative torrefaction pretreatment. Due to the removal of hemicellulose and cellulose from wheat straw particles and the generation of new functional groups on the particle surfaces, the ignition temperature of each torrefied sample expressed an increasing tendency, while the Ea clearly decreased. According to the results obtained from this research, it could be concluded that torrefaction conducted in a raw flue gas atmosphere at 573 K would improve the fuel quality and reactivity of wheat straw most significantly. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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13 pages, 2084 KiB

Open AccessArticle

The Role of the Mannich Reaction in Nitrogen Migration during the Co-Hydrothermal Carbonization of Bovine Serum Albumin and Lignin with Various Forms of Acid–Alcohol Assistance

by Qiang Zhang, Kai Mu, Bo Zhao and Linlin Yi

Molecules 2023, 28(11), 4408; https://doi.org/10.3390/molecules28114408 - 29 May 2023

Cited by 1 | Viewed by 1580

Abstract

Co-hydrothermal carbonization (co-HTC) of N-rich and lignocellulosic biomass is a potential way to produce hydrochar with high yield and quality, but the nitrogen will also enrich in a solid product. In this study, a novel co-HTC with acid–alcohol assistance is proposed, and the model compounds bovine serum albumin (BSA) and lignin were used to investigate the role of the acid–alcohol-enhanced Mannich reaction in nitrogen migration. The results showed that the acid–alcohol mixture could inhibit nitrogen enrichment in solids and the order of the denitrification rate was acetic acid > oxalic acid > citric acid. Acetic acid promoted solid-N hydrolysis to NH₄⁺ while oxalic acid preferred to convert it to oil-N. More tertiary amines and phenols were generated with oxalic acid–ethanol addition and then formed quaternary-N and N-containing aromatic compounds through the Mannich reaction. In the citric acid–ethanol–water solution, NH₄⁺ and amino acids were captured to form diazoxide derivatives in oil and pyrroles in solids through both nucleophilic substitution and the Mannich reaction. The results are able to guide biomass hydrochar production with the targeted regulation of nitrogen content and species. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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22 pages, 5654 KiB

Open AccessArticle

Optimization of Briquette Fuels by Co-Torrefaction of Residual Biomass and Plastic Waste Using Response Surface Methodology

by Shuai Guo, Lidong Liu, Deng Zhao, Chenchen Zhao, Xingcan Li and Guangyu Li

Molecules 2023, 28(6), 2568; https://doi.org/10.3390/molecules28062568 - 11 Mar 2023

Cited by 4 | Viewed by 2388

Abstract

Combining biomass, a clean and renewable energy source, with waste plastic, which serves as a good auxiliary fuel, can produce high-quality clean fuel. The performance of biomass-derived fuel can be improved by torrefaction. This study optimized the co-torrefaction of fungus bran and polypropylene (PP) waste plastic to obtain clean solid biofuel with high calorific value and low ash content (AC) using response surface methodology. Two sets of mixed biochars were investigated using a multiobjective optimization method: mass yield–higher heating value–ash content (MY-HHV-AC) and energy yield–ash content (EY-AC). PP increased the heat value, decreased AC, and acted as a binder. The optimal operating conditions regarding reaction temperature, reaction time, and PP blending ratio were 230.68 °C, 30 min, and 20%, respectively, for the MY-HHV-AC set and 220 °C, 30 min, 20%, respectively, for the EY-AC set. The MY-HHV-AC set had properties close to those of peat and lignite. Furthermore, compared with that of the pure biochar, the AC of the two sets decreased by 15.71% and 14.88%, respectively, indicating that the prepared mixed biochars served as ideal biofuels. Finally, a circular economy framework for biobriquette fuel was proposed and prospects for preparing pellets provided. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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16 pages, 11027 KiB

Open AccessArticle

Experimental and DFT Research on the Effects of O₂/CO₂ and O₂/H₂O Pretreatments on the Combustion Characteristics of Char

by Lei Zhang, Jie Xu, Rui Sun, Zhuozhi Wang, Xingyi Wang, Mengfan Yuan and Jiangquan Wu

Molecules 2023, 28(4), 1638; https://doi.org/10.3390/molecules28041638 - 8 Feb 2023

Cited by 2 | Viewed by 1486

Abstract

The use of a coal-based energy structure generates a large amount of CO₂ and NOx. The numerous emissions from these agents result in acid rain, photochemical smog, and haze. This environmental problem is considered one of the greatest challenges facing humankind in this century. Preheating combustion technology is considered an essential method for lowering the emissions of CO₂ and NO. In this research, the char prepared from O₂/CO₂ and O₂/H₂O atmospheres was employed to reveal the effects of the addition of an oxidizing agent on the combustion characteristics of char. The structural features and combustion characteristics of preheated chars were determined by Raman, temperature-programmed desorption (TPD), and non-isothermal, thermo-gravimetric (TGA) experiments. According to the experimental results, the addition of oxidizing agents promoted the generation of smaller aromatic ring structures and oxygen-containing functional groups. The improvement in the surface physicochemical properties enhanced the reactivity of char and lowered its combustion activation energy. Furthermore, the combustion mechanisms of the char prepared from the O₂/CO₂ and O₂/H₂O atmospheres were investigated using the density functional theory (DFT). The simulation results illustrated that the combustion essence of char could be attributed to the migration of active atoms, the fracture of the benzene ring structure, and the reorganization of new systems. The addition of oxidizing agents weakened the conjugated components of the aromatic ring systems, promoting the successive decomposition of CO and NO. The results of this study can provide a theoretical basis for regulating the reaction atmosphere in the preheating process and promoting the development of clean combustion for high-rank coals. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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17 pages, 12903 KiB

Open AccessArticle

Fuel Characteristics and Removal of AAEMs in Hydrochars Derived from Sewage Sludge and Corn Straw

by Shuai Guo, Weinan Xiao, Zhaoyuan Liu, Deng Zhao, Kaixin Chen, Chenchen Zhao, Xingcan Li and Guangyu Li

Molecules 2023, 28(2), 781; https://doi.org/10.3390/molecules28020781 - 12 Jan 2023

Cited by 2 | Viewed by 1369

Abstract

Co-hydrothermal carbonization (Co-HTC) of sewage sludge (SS) and corn straw (CS) for fuel preparation is a waste treatment method that reduces the pre-treatment cost of solid waste and biomass fuel. Based on the response surface methodology (RSM), a test was designed to prepare SS and CS hydrochars using a hydrothermal high-pressure reactor. The test examined the higher heating value (HHV) and the concentrations of alkali metals and alkaline earth metals (AAEMs) and Cl. The HHV of SS-hydrochar decreased with an increase in reaction temperature, but that of CS-hydrochar increased. The yield of CS-hydrochar was at 26.74–61.26%, substantially lower than that of SS-hydrochar. Co-hydrochar has the advantages of HHV and an acceptable yield. The HHV of co-hydrochar was 9215.51–12,083.2 kJ/kg, representing an increase of 12.6–47.6% over single component hydrochar, while the yield of co-hydrochar was 41.46–72.81%. In addition, the stabilities of AAEM and Cl in the co-hydrochar were Mg > Ca > K > Na > Cl. SS and CS had a synergistic effect on dechlorination efficiency (DE), which had a negative effect on the removal efficiency (RE) of Ca and Na. The optimal hydrocharization conditions were a temperature of approximately 246.14 °C, a residence time of approximately 90 min, and a mixing ratio of SS–CS of approximately 57.18%. The results offer a way to utilize SS and CS by Co-HTC and convert them into low-chlorine and low-alkali fuel, thus pushing the improvement of this promising waste-to-energy technology. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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18 pages, 3918 KiB

Open AccessArticle

Design and Performance of an Adsorption Bed with Activated Carbons for Biogas Purification

by Giulia Molino, Marta Gandiglio, Sonia Fiorilli, Andrea Lanzini, Davide Drago and Davide Papurello

Molecules 2022, 27(22), 7882; https://doi.org/10.3390/molecules27227882 - 15 Nov 2022

Cited by 5 | Viewed by 1882

Abstract

Organic waste can be efficiently converted into energy using highly efficient energy systems, such as SOFCs coupled to the anaerobic digestion process. SOFC systems fed by biogenous fuels, such as biogas or syngas, suffer long-term stability due to trace compound impacts. It follows that, a mandatory gas cleaning section is needed to remove these pollutants at lower concentrations. This work investigates the adsorption mechanism for micro-contaminant removal through experimental results achieved using solid sorbents. Samples of different sorbent materials were analyzed in the laboratory to determine their performances in terms of sulfur (mainly hydrogen sulfide) and siloxanes (mainly D4-Octamethylcyclotetrasiloxane) adsorption capacities. The analysis shows that the chemical composition of the samples influences the adsorption of H₂S (i.e., presence of calcium, iron, copper), while the effect of their textural properties mainly influences the adsorption of siloxane compounds, such as D4. A quantitative analysis was performed considering the influence of gas velocity on adsorption capacity. By increasing the biogas velocity (+45% and +89%), there was an indirect correlation with the H₂S adsorption capacity (−27% and −44%). This identified an aspect related to the residence time required to be able to remove and retain the trace compound. The results obtained and summarized were used to develop a strategy for the removal of trace compounds in large-scale plants, e.g., for water purification. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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24 pages, 3155 KiB

Open AccessReview

Emerging Strategies for Enhancing Propionate Conversion in Anaerobic Digestion: A Review

by Lan Mu, Yifan Wang, Fenglian Xu, Jinhe Li, Junyu Tao, Yunan Sun, Yingjin Song, Zhaodan Duan, Siyi Li and Guanyi Chen

Molecules 2023, 28(9), 3883; https://doi.org/10.3390/molecules28093883 - 4 May 2023

Cited by 9 | Viewed by 3753

Abstract

Anaerobic digestion (AD) is a triple-benefit biotechnology for organic waste treatment, renewable production, and carbon emission reduction. In the process of anaerobic digestion, pH, temperature, organic load, ammonia nitrogen, VFAs, and other factors affect fermentation efficiency and stability. The balance between the generation and consumption of volatile fatty acids (VFAs) in the anaerobic digestion process is the key to stable AD operation. However, the accumulation of VFAs frequently occurs, especially propionate, because its oxidation has the highest Gibbs free energy when compared to other VFAs. In order to solve this problem, some strategies, including buffering addition, suspension of feeding, decreased organic loading rate, and so on, have been proposed. Emerging methods, such as bioaugmentation, supplementary trace elements, the addition of electronic receptors, conductive materials, and the degasification of dissolved hydrogen, have been recently researched, presenting promising results. But the efficacy of these methods still requires further studies and tests regarding full-scale application. The main objective of this paper is to provide a comprehensive review of the mechanisms of propionate generation, the metabolic pathways and the influencing factors during the AD process, and the recent literature regarding the experimental research related to the efficacy of various strategies for enhancing propionate biodegradation. In addition, the issues that must be addressed in the future and the focus of future research are identified, and the potential directions for future development are predicted. Full article

(This article belongs to the Special Issue Technologies for Carbon-Neutral Fuels with High Efficiency and Clean Utilization)

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