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•In-situ pyrolysis regulation and post-modification of biochar were summarized.•The relationship between physicochemical properties and applications of biochar was analyzed.•The ...future research requirements for biochar preparation and applications were proposed.
Biochar is a carbon-rich product obtained from the thermo-chemical conversion of biomass. Studying the evolution properties of biochar by in-situ modification or post-modification is of great significance for improving the utilisation value of lignocellulosic biomass. In this paper, the production methods of biochar are reviewed. The effects of the biomass feedstock characteristics, production processes, reaction conditions (temperature, heating rate, etc.) as well as in-situ activation, heteroatomic doping, and functional group modification on the physical and chemical properties of biochar are compared. Based on its unique physicochemical properties, recent research advances with respect to the use of biochar in pollutant adsorbents, catalysts, and energy storage are reviewed. The relationship between biochar structure and its application are also revealed. It is suggested that a more effective control of biochar structure and its corresponding properties should be further investigated to develop a variety of biochar for targeted applications.
•A review of supercritical water gasification of real biomass rather than model compounds.•The relationship between hydrogen yield and biomass type is elucidated.•The impact of all process variables, ...including the use of catalysts are quantified.•A technoeconomic study to show how the process could become viable through scale up.
Supercritical water gasification (SCWG) is a combined thermal decomposition and hydrolysis process for converting wet biomass feedstock with high water content potentially (80 wt%) to syngas. The process bypasses the need for an energy intensive pre-drying step and also needs relatively shorter residence times (of the order of seconds to minutes) when compared to conventional gasification. The main target of SCWG is to obtain syngas rich in hydrogen whilst minimising char formation. In recent years, SCWG studies have advanced from using model compounds (e.g. glucose and cellulose) towards the use of real biomass and its waste (e.g. sugarcane trash). The use of biomass as a feedstock creates real opportunities for the technology since it is available in some form, regardless of location. This review discusses the findings from SCWG studies that have used real biomass as a feedstock. The effects of reaction temperature, pressure, residence time and feedstock concentration to the hydrogen yields are presented. The relationship between the main components in biomass (cellulose, hemicellulose and lignin) and hydrogen yields are also discussed. Homogeneous and heterogeneous catalysts have been used to enhance SCWG with real biomass feedstock and the benefits of these approaches are also considered. The economic benefits of running the catalytic SCWG at 400 °C compared to non-catalytic operation at 600 °C is evaluated. Reactor configuration and process conditions vary across the literature, and various authors describe the associated challenges (char formation and plugging, corrosion) as well as promising solutions to tackle these key challenges.
Composting is a biodegradation and transformation process that converts lignocellulosic biomass into value-added products, such as humic substances (HSs). However, the recalcitrant nature of ...lignocellulose hinders the utilization of cellulose and hemicellulose, decreasing the bioconversion efficiency of lignocellulose. Pretreatment is an essential step to disrupt the structure of lignocellulosic biomass. Many pretreatment methods for composting may cause microbial inactivation and death. Thus, the pretreatment methods suitable for composting can promote the degradation and transformation of lignocellulosic biomass. Therefore, this review summarizes the pretreatment methods suitable for composting. Microbial consortium pretreatment, Fenton pretreatment and surfactant-assisted pretreatment for composting may improve the bioconversion process. Microbial consortium pretreatment is a cost-effective pretreatment method to enhance HSs yields during composting. On the other hand, the efficiency of enzyme production during composting is very important for the degradation of lignocellulose, whose action mechanism is unknown. Therefore, this review describes the mechanism of action of lignocellulase, the predominant microbes producing lignocellulase and their related genes. Finally, optimizing pretreatment conditions and increasing enzymatic hydrolysis to improve the quality of composts by controlling suitable microenvironmental factors and core target microbial activities as a research focus in the bioconversion of lignocellulose during composting in the future.
•The pretreatment destroys the structure of refractory lignocellulose.•Combined pretreatment and composting have a synergistic effect on HSs formation.•The key roles of lignocellulase during composting have been confirmed.•Directional control HSs formation has been proposed.
•Deep eutectic solvent studies on biomass pretreatment have been reviewed.•Basics of DES fractionation of lignocellulosic biomass have been summarized.•Mechanisms of using DES for biomass ...fractionation have been discussed.•Prospects and challenges for future works have been outlined.
Biomass recalcitrance hinders efficient utilization of lignocellulosic biomass, making pretreatment process a crucial step for successful biorefinery process. Pretreatment processes have been developed for processing biomass, while technical obstacles including intensive energy requirement, high operational cost, equipment corrosions resulted from currently applied techniques promote the development of new pretreatment process for biomass. The deep eutectic solvent (DES) has been recognized as a promising solvent for biomass pretreatment, although the DES application toward biomass is still in its nascent stage. This review summarized the current researches using DES for biomass pretreatment, focusing particularly on lignin extraction and saccharification enhancement of lignocellulosic biomass. The mechanisms for biomass fractionation using DES as agents are introduced. Prospect and challenge were outlined.
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•This study reviews thermochemical conversion processes in lignocellulosic biomass.•Lignocellulosic biomass is a promising renewable resource.•Thermochemical conversion processes ...convert biomass into products at a faster rate.•Experimental works can be reduced by artificial intelligence and machine learning.
Lignocellulosic biomass is one of the most promising renewable resources and can replace fossil fuels via various biorefinery processes. Through this study, we addressed and analyzed recent advances in the thermochemical conversion of various lignocellulosic biomasses. We summarized the operation conditions and results related to each thermochemical conversion processes such as pyrolysis (torrefaction), hydrothermal treatment, gasification and combustion. This review indicates that using thermochemical conversion processes in biorefineries is techno-economically feasible, easy, and effective compared with biological processes. The challenges experienced in thermochemical conversion processes are also presented in this study for better understanding the future of thermochemical conversion processes for biorefinery. With the aid of artificial intelligence and machine learning, we can reduce time-consumption and experimental work for bio-oil production and syngas production processes.
The pretreatment of lignocellulosic biomass enhances the conversion efficiency to produce biofuels and value-added chemicals, which have the potential to replace fossil fuels. Compared to ...physicochemical and other pretreatment techniques, the hydrothermal methods are considered eco-friendly and cost-effective. This paper reviews the strengths, weaknesses, opportunities and threats of steam explosion and subcritical water hydrolysis as the two promising hydrothermal technologies for the pretreatment of lignocellulosic biomass. Although the principle of the steam explosion in depolymerizing the lignin and exposing the cellulose fibers for bioconversion to liquid fuels is well known, its underlying mechanism for solid biofuel production is less identified. Therefore, this review provides an insight into different operating conditions of steam explosion and subcritical water hydrolysis for a wide variety of feedstocks. The mechanisms of subcritical water hydrolysis including dehydration, decarboxylation and carbonization of waste biomass are comprehensively described. Finally, the role of microwave heating in the hydrothermal pretreatment of biomass is elucidated.
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•Hydrothermal pretreatments such as steam explosion and subcritical water are elaborated.•Properties, mechanism, pros and cons of these hydrothermal pretreatments are explained.•Microwave-assisted pretreatments are more advantageous than conventional techniques.•These pretreatments are economical and efficient to depolymerize lignin and hemicellulose.•Biomass degradation products e.g., furfurals and phenolics can inhibit fermentation process.
Hydrothermal carbonization (HTC) as a promising thermochemical process can convert organic solid wastes (e.g., biomass, plastics) into valuable products (i.e., hydrochar) at relatively low ...temperatures (180–250 °C) and saturated pressures (2–10 MPa). Hydrothermal conversion generally occurs via dehydration, polymerization and finally carbonization reactions. The carbon materials derived from hydrochar have high potential in various applications such as solid fuel, supercapacitor, fuel cell, and sorbent. Although the energy densification of hydrochar was increased at higher temperatures, most of the benefit was achieved at modest temperatures. Chemical structures of hydrochars include crosslinks of aromatic polymer, surface porosity, organic functional groups and ultimate components. All of these characteristics can be changed significantly by HTC, influencing the reactivity and fuel properties of hydrochars. The reaction pathways including negative and positive effects during (co)-HTC of biomass and plastic wastes are thoroughly concluded. In particular, the co-HTC of chlorinated plastic (e.g., PVC) and biomass can enhance the dechlorination and inorganics removal from hydrochar.
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•Hydrothermal carbonization of biomass and plastic wastes was reviewed.•Hydrochar with high energy density can be used as a coal-alternative solid fuel.•Hydrochar can be chemically activated into porous carbons for energy storage.•HTC is mostly influenced by the feedstock types and processing conditions.•Co-HTC of plastics and biomass can improve the quality of hydrochar.
Increasing fossil fuel consumption and global warming has been driving the worldwide revolution towards renewable energy. Biomass is abundant and low-cost resource whereas it requires environmentally ...friendly and cost-effective conversion technique. Pyrolysis of biomass into valuable bio-oil has attracted much attention in the past decades due to its feasibility and huge commercial outlook. However, the complex chemical compositions and high water content in bio-oil greatly hinder the large-scale application and commercialization. Therefore, catalytic pyrolysis of biomass for selective production of specific chemicals will stand out as a unique pathway. This review aims to improve the understanding for the process by illustrating the chemistry of non-catalytic and catalytic pyrolysis of biomass at the temperatures ranging from 400 to 650 °C. The focus is to introduce recent progress about producing value-added hydrocarbons, phenols, anhydrosugars, and nitrogen-containing compounds from catalytic pyrolysis of biomass over zeolites, metal oxides, etc. via different reaction pathways including cracking, Diels-Alder/aromatization, ketonization/aldol condensation, and ammoniation. The potential challenges and future directions for this technique are discussed in deep.
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•Properties of lignocellulosic biomass and its pyrolysis chemistry are extensively reviewed.•Catalytic pyrolysis of lignocellulosic biomass for selective production of valuable chemicals is outlined.•Different catalytic reforming pathways are summarized.•Future research directions and technological challenges are proposed.
Lignocellulosic biomass (LCB) is the most abundantly available bioresource amounting to about a global yield of up to 1. 3 billion tons per year. The hydrolysis of LCB results in the release of ...various reducing sugars which are highly valued in the production of biofuels such as bioethanol and biogas, various organic acids, phenols, and aldehydes. The majority of LCB is composed of biological polymers such as cellulose, hemicellulose, and lignin, which are strongly associated with each other by covalent and hydrogen bonds thus forming a highly recalcitrant structure. The presence of lignin renders the bio-polymeric structure highly resistant to solubilization thereby inhibiting the hydrolysis of cellulose and hemicellulose which presents a significant challenge for the isolation of the respective bio-polymeric components. This has led to extensive research in the development of various pretreatment techniques utilizing various physical, chemical, physicochemical, and biological approaches which are specifically tailored toward the source biomaterial and its application. The objective of this review is to discuss the various pretreatment strategies currently in use and provide an overview of their utilization for the isolation of high-value bio-polymeric components. The article further discusses the advantages and disadvantages of the various pretreatment methodologies as well as addresses the role of various key factors that are likely to have a significant impact on the pretreatment and digestibility of LCB.
Bioethanol is one of the most promising and eco-friendly alternatives to fossil fuels, which is produced from renewable sources. Although almost all the current fuel ethanol is generated from edible ...sources (sugars and starch), lignocellulosic biomass (LCB) has drawn much attention in recent times. However, the conversion efficiency as well as ethanol yield of the biomass differs greatly with respect to the source and nature of LCB, primarily due to the variation in lignocellulosic content. Two major polysaccharides in LCB, namely, cellulose and hemicellulose firmly link to lignin and form a complex lignocellulosic network, which is highly robust and recalcitrant to depolymerization. For this reason, generation of ethanol from LCB requires a complicated conversion process that has made it commercially non-competitive. As attempts to exploit LCBs into commercial ethanol production, recent research efforts have been devoted to the techno-economic improvements of the overall conversion process, in addition to screen out promising feedstocks. This review paper presents an overview on the diversity of biomass, technological approaches and microbial contribution to the conversion of LCB into ethanol.
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