Hydrothermal carbonization and liquefaction for sustainable production of hydrochar and aromatics
Introduction
Developing sustainable production of fuels, materials, and valuable chemicals from renewable feedstock have received considerable attention due to the rapid transition from fossil fuel [1]. Biomass is a renewable organic material including wood, wood processing waste, yard and garden waste, energy crop, algae, and municipal solid waste, etc. [2]. Lignocellulosic biomass, such as woody waste and agricultural residues, is the most abundant resource with an estimated annual global production of approximately 170 billion tons, and the utilization of these renewable resources ensures a more sustainable society [3]. In terms of environmental protection, using lignocellulosic biomass could effectively abate greenhouse gases (GHGs) emissions, contributing to the mitigation of climate change by carbon sequestration [4].
Lignocellulosic biomass mainly consists of cellulose, hemicellulose, lignin, and extracts [5]. Most of the frontier biofuel production strategies focused on the conversion of hemicellulose and cellulose [6,7]. These precursors are the promising sources for producing lignocellulose-derived sugars and valuable chemicals such as bioethanol, hydroxymethylfurfural (HMF), furfural, and levulinic acid [[8], [9], [10], [11], [12]]. Nowadays, large amounts of lignocellulosic biomass are processed in the pulp and paper industry which generates around 150–180 million tons of technical lignin as industrial byproduct every year [13]. Lignin is one of the most abundant sources of sustainable aromatics on the planet, but it is generally underutilized in these cellulosic projects and mostly burned as fuel due to the challenges associated with its recalcitrance and inherent heterogeneity [14,15]. The variability of biomass composition would result in products with different physicochemical properties and energy values. The societal drive for a sustainable future highlights the recognition of lignin depolymerization into aromatics (e.g., aromatic monomers, dimers, and low-molecular-weight oligomers), which could improve the overall economic feasibility and sustainability metrics of biorefinery [[16], [17], [18]]. To achieve sustainable production, not only the biomass feedstock but also the process conditions should be carefully designed and tuned to obtain the desirable products.
Lignocellulosic biomass, though abundant in reserve for bioenergy production, has its own drawbacks due to high moisture, bulk volume, and low heating value [19,20]. Hydrothermal processing is an effective and advanced technology that can directly convert carbon-rich et high moisture content feedstock without prior drying step. This process is endothermic and usually carried out under subcritical or supercritical water conditions, where the feedstock is fractionated into valuable constituents through hydrolysis, depolymerization, and condensation [21]. Water is the greenest solvent, and it can simultaneously act as a reactant and a suitable medium for acid-catalyzed reactions to break down the biomass into small fractions by the high concentration of H+ ions generated from subcritical water [21,22]. Hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL), and hydrothermal gasification (HTG) are involved in the processes for the selective production of carbonized solids (i.e., hydrochar), liquid bio-oils, and fuel gases depending on the operating temperatures [23,24]. HTC is conducted at temperatures between 180 °C and 250 °C and produces hydrochar with properties similar to brown coal [25,26]. The cellulose and hemicellulose fractions of lignocellulosic biomass can be largely degraded into sugar monomers and derivatives during the HTC process, whereas the lignin component is mildly modified because of its recalcitrance. With increasing reaction temperatures, the lignin-rich solid residues could be completely converted into liquid biofuel during the HTL process at 250–370 °C and be gasified during the HTG process (370–750 °C) [[27], [28], [29]]. Thus, these routes could realize various design of products with pre-defined properties and whole-biomass valorization by an integrated process.
By far, HTC and HTL have been particularly popular for the processing of biomass into value-added products at low to intermediate temperatures, e.g., sustainable carbon-rich materials produced by HTC process, high selective production of aromatic monomers from HTL of lignin-rich feedstock, and novel catalytic approaches for HTL of algae towards biofuel production [[30], [31], [32], [33]]. The number of publications related to hydrothermal processes between 2011 and 2020 (according to Web of Science™) is shown in Fig. 1a. In particular, the interest in the production of hydrochar and aromatics has significantly increased because hydrochar can be a source of low-cost fuel/energy replacing fossil fuels, and thereby lignin-rich feedstock offers significant valorization potential beyond its heating value for lignocellulose refining (Fig. 1b). Hydrothermal processing of lignocellulose-/lignin-rich feedstock presents an economically attractive and environmentally friendly approach for sustainable biorefinery. However, several fundamental and technical concerns related to the variability in biomass, operating parameters, and efficient catalysis need to be addressed in the hydrothermal processing of biomass into desired products and commercialization of biofuels and bioproducts [34].
An overview of hydrothermal processing and its fundamentals as well as crucial issues of hydrothermal conversion have been discussed above and reviewed in previous studies [23,24,31]. With regard to different hydrothermal routes, it is even more important to critically review the advantages and challenges faced in the selective production of target products (e.g., hydrochar and aromatics), because the required physical/chemical processing and conversion might differ considerably when treating various types of biomass feedstock. HTC process is the main subject of biorefinery research for producing solid hydrochar with different structures and functional properties [35,36]. The key factors including reaction temperature, time, pressure, loading ratio, and use of catalyst can directly alter the reaction pathways and modify hydrochar for different applications [20]. Hydrochar with a high energy density can be applied for energy storage while it can also serve as a sustainable carbon material for environmental remediation and catalysis [[37], [38], [39]]. HTL of lignin-rich feedstock can produce aromatic products with properties similar to petroleum-derived counterparts. The influences of operating parameters on the selective conversion of lignin have been widely studied to provide practical guidance, metrics, and methods [40,41]. Recent studies have highlighted that the selective production of targeted aromatics would be determined by the inherent properties of lignin-rich feedstock [[42], [43], [44]]. Traditionally, most studies focus on the optimization of reaction parameters to tune the yields and qualities of biofuel and bioproducts. Nevertheless, the critical impacts of feedstock properties are often overlooked, which should be considered at the earliest stage to achieve high-efficiency conversion. In-depth understanding with advanced analysis of feedstock variability is essential for developing better production methods and understanding complex reaction mechanisms involved in the hydrothermal processes. In this case, extensive efforts are still required to identify the correlations and interactions among feedstock properties and targeted products.
We review the HTC of lignocellulose and HTL of lignin-rich feedstock for tailoring the selective production of multifunctional hydrochar and aromatics with the aim to provide a holistic and critical view on how the biomass variability, operational parameters, and catalyst fundamentally affect its valorization. The key contributions of this work are to: (a) present the important inherent properties of lignocellulosic biomass and physiochemical properties of their derived hydrochar/aromatics; (b) evaluate the critical impacts of feedstock characteristics, operating parameters, and use of catalysts on the yield and selective production of target products; (c) illustrate the reaction mechanisms during the HTC and HTL processes when using different lignocellulose-/lignin-rich feedstock with the optimized reaction parameters; (d) identify the current limitations related to the scale-up of hydrothermal processes and provide guidelines for sustainable biorefinery towards resource-efficient utilization in the future.
Section snippets
Lignocellulosic biomass and hydrothermal process
This section briefly summarizes the typical characteristics of lignocellulosic biomass and key factors involved in HTC and HTL processes for the production of hydrochar and aromatics.
Biomass feedstock
Feedstock plays a significant role in the product yield, surface functional groups, texture, structure, chemical composition, and surface morphology of hydrochar. The effects of feedstock on producing multifunctional hydrochar are reviewed in Table 3. The high content of lignin typically leads to an increase in the yield and thermal stability of hydrochar [70,71]. Hydrochar derived from carbohydrate-rich feedstock has abundant hydroxyl groups and enrichment of aromatic carbons [71]. Previous
Hydrothermal liquefaction
For the HTL process of lignin-rich feedstock, selective production of aromatics depends on the lignin source, specific reaction parameters, and catalysts used. The key factors involved in lignin pretreatment are the biomass source and the pretreatment conditions that influence the initial structure of lignin and govern its subsequent conversion. The lignin structure and catalytic systems are the vital factors in the HTL process (Fig. 6). We first discuss the lignin feedstock and its importance
Techno-economic assessment
Recent studies have conducted techno-economic assessment (TEA) for the production of biofuel and bio-products from lignocellulosic biomass [[162], [163]]. For the HTC process of yard waste, process models were developed for two different HTC plant configurations that included flash separators (case A) and heat exchangers (case B). In terms of costs, the price for case A was $3.3 per GJ more than that in case B, but case A was preferable in terms of energy production [164]. For case B,
Challenges and perspectives
The hydrothermal processing of lignocellulosic biomass into fuels and value-added products has received renewed emphasis thanks to recent advances and better understanding of the complex characteristics of feedstock variability, reaction mechanisms and pathways, and properties of target products and byproducts. Although considerable efforts have been made in fundamental research and technology development at a laboratory scale, inherent challenges associated with biomass heterogeneity still
Conclusions
Hydrothermal treatment of biomass has been extensively studied as a biorefinery technology for producing solid hydrochar and liquid bio-oil with high chemical functionality. In this critical review, we articulate the key factors (e.g., feedstock variability, reaction temperature, time, and catalyst) that determine the nature and properties of solid and liquid biofuel in the HTC and HTL processes. In particular, we critically review the influences of feedstock compositions and chemical
Credit author statement
Yang Cao: Methodology, Data curation, Investigation, Writing - Original draft preparation. Mingjing He: Methodology, Investigation, Writing - Review & Editing. Shanta Dutta: Methodology, Validation, Writing - Review & Editing. Gang Luo: Methodology, Validation, Writing - Review & Editing. Shicheng Zhang: Conceptualization, Methodology, Supervision, Writing - Review & Editing. Daniel C.W. Tsang: Conceptualization, Methodology, Supervision, Validation, Resources, Project administration, Funding
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors appreciate the financial support from the Hong Kong Environment and Conservation Fund (ECF Project 101/2020) and Hong Kong Research Grants Council (PolyU 15222020).
References (170)
- et al.
Pretreatment of lignocelluloses for enhanced biogas production: a review on influencing mechanisms and the importance of microbial diversity
Renew Sustain Energy Rev
(2021) - et al.
Advancement in valorization technologies to improve utilization of bio-based waste in bioeconomy context
Renew Sustain Energy Rev
(2020) - et al.
Valorization of biomass to hydroxymethylfurfural, levulinic acid, and fatty acid methyl ester by heterogeneous catalysts
Chem Eng J
(2017) - et al.
Kraft lignin biorefinery: a perspective
Bioresour Technol
(2018) - et al.
Hydrothermal carbonization of renewable waste biomass for solid biofuel production: a discussion on process mechanism, the influence of process parameters, environmental performance and fuel properties of hydrochar
Renew Sustain Energy Rev
(2020) - et al.
Recent development of hydrothermal liquefaction for algal biorefinery
Renew Sustain Energy Rev
(2020) - et al.
A review of hydrothermal biomass processing
Renew Sustain Energy Rev
(2014) - et al.
Effects of temperature, time and acidity of hydrothermal carbonization on the hydrochar properties and nitrogen recovery from corn stover
Biomass Bioenergy
(2019) - et al.
A comprehensive investigation of hydrothermal carbonization Energy potential of hydrochar derived from Virginia mallow
Renew Energy
(2020) - et al.
Catalytic hydrothermal liquefaction of lignin for production of aromatic hydrocarbon over metal supported mesoporous catalyst
Bioresour Technol
(2021)