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| Fig. 1: Hydrothermal Liquefaction Process. (Image source: F.Mehnaz after Kirtana. [12]) |
Municipal Solid Waste (MSW) comprises a diverse range of materials, including waste plastics, biodegradable organic materials such as food scraps and spoiled vegetables, garden waste, inert substances like metals and sand, and other items like jute, fabric rags, and wrappers. Additionally, MSW contains a significant amount of moisture. Many developing countries face significant challenges in managing Municipal Solid Waste (MSW). [1] Bio-inorganic materials like plastics and metals in municipal solid waste (MSW) take a long time to degrade and can contaminate groundwater, especially in the rainy season. [1] Incineration and landfilling pose significant risks of air, water, and land pollution. [2] Hydrothermal liquefaction is a thermochemical process that utilizes water as a reaction medium at high pressure and temperature. It converts biomass and waste materials into biofuels and recovers water and nutrients from the feedstock. This method overcomes several technical challenges associated with existing technologies like anaerobic digestion, incineration, gasification, and pyrolysis, offering a promising solution to the interconnected issues of water, energy, and climate change. [3] Fig. 1 shows a hydrothermal liquefaction process.
HTL is a thermochemical reaction process that occurs at low temperatures, typically between 250°C and 400°C , and under high pressures ranging from 5 to 20 MPa. During this process, organic solid waste is converted into three types of products in aqueous solvents: biofuel, gas, and solid residue. [4] The hydrothermal liquefaction (HTL) process consists of three key reactions: depolymerization, decomposition, and recombination. [5,6]
Initially, the depolymerization reaction takes place, during which lipids, proteins, and carbohydrates are converted into fatty acids, amino acids, and monosaccharides like pentose and hexose. This stage of depolymerization, which serves as a hydrothermal pretreatment method, has been the focus of considerable research. The typical temperature range for this reaction is between 150°C and 250°C , depending on the specific characteristics of the substrates involved. [4]
The next stage is decomposition, where the products of depolymerization are further broken down into unstable and reactive elements. This phase mainly involves the removal of water, carbon dioxide, oxygen, and hydrogen through processes such as dehydration, decarboxylation, and dehydrogenation, as well as bond cleavage that occurs at high temperatures. The temperature for decomposition usually falls between 180°C and 340°C , which varies based on the functional groups involved. During the hydrothermal liquefaction (HTL) process, carbohydrates decompose first, followed by proteins, and finally, lipids. [4] A recombination reaction refers to the rearrangement of various active fragments produced by decomposition reactions at temperatures exceeding 300°C , leading to the formation of bio-crude compounds. The bio-crude primarily originates from the large molecular substances generated during the recombination reaction. Organic molecular groups such as aromatics, ketones, esters, amides, and amines are formed during the recombination reaction process involving long-chain fatty acids. Additionally, complex alcohol molecules are produced through the cyclization and hydration reactions of olefins. [4] The yield of bio-crude, gas phase, aqueous phase, and solid residue from hydrothermal liquefaction (HTL) depends on factors such as raw materials, residence time, temperature, pressure, and catalyst type. Each component's composition is also affected by these conditions. [7,8]
The current Hydrothermal Liquefaction (HTL) technology depends on electricity from fossil fuels to operate and manage heat losses, which hinders commercialization. Optimal reaction parameters for HTL are influenced by the feedstock's composition, and variability in feedstock complicates the development of a standardized optimization process. Additionally, the quality of the biocrude produced is affected by the feedstock and the varying concentrations of contaminated PFAS or microplastics at different locations, impacting the design of HTL operating conditions to maximize removal efficiency. The quality of biocrude produced from biomass waste differs from that of conventional petroleum crude oil due to its inferior properties, such as a low heating value and high nitrogen content. Despite these differences, biocrude is carbon neutral regarding greenhouse gas emissions when used as fuel. Moreover, the varying compositions of microplastic (MP) contaminants can lead to inconsistent quality and low production yields. [9]
Wet (or high-moisture) biomass waste is commonly managed through landfilling and conventional composting; however, hydrothermal liquefaction (HTL) offers a more innovative solution. HTL effectively transforms this waste into high-value products, including energy-rich biocrude oil, carbon-rich materials, and nutrient-rich aqueous phases. This process not only enhances waste management but also creates valuable resources. [10] Furthermore, biocrude oil derived from sustainable liquid fuels is crucial to addressing the significant challenges of energy and climate due to its carbon neutrality. [11] These contributions make HTL an essential technology for fostering a sustainable and carbon-circular economy. [9]
© Fareen Mehnaz. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. Mahesh et al., "Hydrothermal Liquefaction of Municipal Solid Wastes For High Quality Bio-Crude Production Using Glycerol as Co-Solvent," Bioresour. Technol. 339, 125537 (2021).
[2] M. Sharholy et al., "Municipal Solid Waste Management in Indian Cities - A Review," Waste Manag. 28, 459 (2008).
[3] R. Kumar, "A Review on the Modelling of Hydrothermal Liquefaction of Biomass and Waste Feedstocks," Energy Nexus 5, 100042 (2022).
[4] T. Yang, "Hydrothermal Liquefaction of Organic Solid Waste to Produce Biofuel," in Thermochemical Conversion of Biomass Feedstock and Solid Waste into Biofuels, ed. by Y. Hu et al. (Woodhead Publishing, 2025).
[5] I. A. Basar et al., "A Review on Key Design and Operational Parameters to Pptimize and Develop Hydrothermal Liquefaction of Biomass For Biorefinery Applications," Green Chem. 23, 1404 (2021).
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[8] B. Zhang et al., "Hydrothermal Liquefaction of Fresh Lemon-Peel: Parameter Optimisation and Product Chemistry," Renew. Energy 143, 512 (2019).
[9] H. D. Pham et al., "Hydrothermal Liquefaction: A Promising Technology For Renewable Energy and Environmental Clean-Up Applications," Biomass Bioenergy 201 108151 (2025)..
[10] D. V. Cabrera et al., "Enhancing Energy Recovery of Wastewater Treatment Plants Through Hydrothermal Liquefaction," Environ. Sci. Water Res. Technol. 9, 474 (2023).
[11] J. Hoffmann et al., "Conceptual Design of an Integrated Hydrothermal Liquefaction and Biogas Plant For Sustainable Bioenergy Production," Bioresour. Technol. 129, 402 (2013).
[12] K. Kirtania, "Thermochemical Conversion Processes For Waste Biorefinery," in Waste Biorefinery, ed. by T. Bhaskar et al. (Elsevier, 2018).