What Is Hydrothermal Liquefaction?

By Owais AliReviewed by Lexie CornerMar 10 2025

Global plastic waste continues to rise, largely due to low recycling rates. Only 10-15 % of plastic waste is recycled each year, while the rest is incinerated, landfilled, or discarded.

Traditional recycling methods struggle to process mixed and contaminated plastics, making them inefficient for large-scale waste management. Hydrothermal liquefaction (HTL) has been developed as a scalable alternative for plastic recycling.

But what is HTL, how does it work, and could it be the key to reducing plastic waste?

Image Credit: Mykolastock/Shutterstock.com

Overview of Hydrothermal Liquefaction Process

Hydrothermal liquefaction (HTL) is a thermochemical process that uses high temperatures and pressures near the critical point of water (approximately 374 °C) to decompose complex organic materials, with water acting as a solvent and catalyst.

This process is commonly applied to wet biomass, breaking it down into smaller energy-dense products such as biofuels and chemicals.¹

Stages of HTL Process

The HTL process involves a series of chemical transformations that convert waste into high-quality hydrocarbons, making it suitable for circular recycling.

1. Hydrolysis:

Hydrolysis initiates the breakdown of complex organic compounds by dissociating chemical bonds via water molecules. At subcritical temperatures, water exhibits increased ionization, generating H₃O⁺ and OH⁻ ions, accelerating the decomposition of macromolecules into smaller constituents.

This stage prepares the feedstock for subsequent chemical transformations.

2. Depolymerization:

Following hydrolysis, as the temperature exceeds 210 °C, the dissociation of hydrogen bonds within polymer chains facilitates the degradation of cellulose, hemicellulose, and lignin.

This process converts these biopolymers into oligosaccharides, monosaccharides, and other low-molecular-weight intermediates, further participating in subsequent thermochemical reactions.

3. Dehydration:

Removing water from reaction intermediates results in highly reactive compounds, such as furfural, hydroxymethylfurfural, and acetic acid. Simultaneously, sugar compounds, like glucose and xylose, undergo isomerization, cyclization, and condensation, interacting with phenols, ketones, and acids to produce low-molecular-weight products.

4. Repolymerization and Condensation:

Reactive intermediates recombine through repolymerization and condensation, forming more stable compounds. The resulting bio-oils and solid char vary in composition depending on temperature, pressure, and residence time.

5. Supercritical Phase:

At temperatures near the water's supercritical point (374 °C), phase boundaries disappear, significantly enhancing mass transfer. In this state, water functions as a highly efficient solvent, accelerating nucleophilic substitution and elimination reactions while facilitating the breakdown of aromatic and heterocyclic compounds through free radical mechanisms.

Meanwhile, nitrogen-containing proteins hydrolyze, releasing amino acids that undergo decarboxylation and deamination, leading to the formation of ammonia and organic acids.

6. Maillard Reactions and Final Product Formation:

In the final stage, Maillard reactions occur as reducing sugars react with amino acids, forming melanoidin-like polymers and polycyclic compounds.

These compounds decompose into monocyclic products such as pyrroles, pyrazines, indoles, and aromatic amines, which undergo further thermal degradation, rearrangement, and polymerization reactions to produce bio-oil and gaseous products.^2,3

Key Process Parameters

The HTL process is influenced by several key parameters, which significantly impact its efficiency and the quality of the resulting products.

1. The optimal bio-oil yield occurs between 300 and 350 °C, as lower temperatures result in incomplete biomass breakdown, while higher temperatures promote gasification and char formation.
2. Maintaining high pressure ensures water remains in the liquid phase, improving solvent density and biomass penetration, though its effect diminishes under supercritical conditions.
3. Residence time influences product composition, with shorter durations favoring bio-oil production, whereas prolonged exposure promotes secondary reactions that generate gas and char.
4. Feedstock characteristics and particle size also play a key role, as smaller particles enhance heat transfer and reaction efficiency, though excessive grinding increases processing costs.
5. Additionally, pH affects the solubility of biomass components, catalyst activity, and bio-oil stability, with higher pH levels (approximately 8.1) improving product yields and reducing char formation.
6. Enhancing catalytic activity further optimizes hydrolysis efficiency, facilitating the breakdown of complex molecules and improving overall process performance.^2,3

HTL vs. Traditional Methods

Advantages of HTL

Versatile Feedstock Processing

HTL processes mixed and contaminated waste streams, including lignocellulosic materials, aquatic biomass, and organic waste, without requiring extensive pre-treatment.

In contrast, anaerobic digestion techniques struggle with complex feedstocks and demand precise microbial conditions, limiting their ability to handle diverse or contaminated waste.⁴

Low Emissions and Carbon Sequestration

When integrated with carbon capture and storage (CCS), HTL can achieve negative carbon emissions by permanently sequestering CO₂.

Unlike traditional incineration, which releases large amounts of greenhouse gases and pollutants, HTL operates in a closed system, minimizing emissions such as ammonia, nitrogen oxides (NOₓ), and sulfur oxides (SOₓ).⁵

Reduced Energy Consumption

Pyrolysis requires extensive drying, whereas HTL efficiently processes wet biomass under moderate temperatures and high pressures, reducing thermal energy consumption by at least 35 %. Using water as a reaction medium, HTL eliminates energy losses associated with evaporative drying, making it a more cost-effective and sustainable alternative.²

Production of Valuable Byproducts

HTL produces biofuels, biochar, and nutrient-rich aqueous solutions that can be repurposed for soil enhancement and fertilizer recovery. In contrast, gasification primarily yields syngas with limited byproduct utilization, making HTL a more resource-efficient process.⁶

Disadvantages of HTL

High Capital and Maintenance Costs

HTL requires specialized reactors capable of withstanding high temperatures and pressures, leading to elevated capital and maintenance costs. In contrast, anaerobic digestion techniques operate under lower temperature and pressure conditions, making it a more cost-effective option with simpler reactor designs.

High Energy Consumption

The high energy demand for heating biomass in HTL significantly contributes to operational expenses, particularly in reactor and heat exchanger components. Gasification, while also energy-intensive, benefits from a more established heat recovery infrastructure, reducing overall energy consumption compared to HTL.⁷

Complex Feed Pressurization

Pressurization of HTL feedstock requires complex pumping systems prone to frequent fluctuations and mechanical failures. Meanwhile, traditional hydrocracking, which also requires high pressures, benefits from well-established pump designs that mitigate such operational inefficiencies.

Severe Corrosion Issues

Corrosion remains a major issue in HTL due to the high-temperature, high-pressure aqueous environment, which accelerates material degradation. In contrast, Fischer-Tropsch synthesis, while operating at high temperatures, does not expose reactor materials to the same level of aqueous corrosion, improving equipment longevity.^3,8

HTL in Plastic Waste Recycling

Plastic pollution is escalating, with global plastic waste production expected to exceed one billion tonnes annually by 2060. Traditional mechanical recycling struggles with mixed and low-quality plastics, leading to 40 % of plastic waste being landfilled and 25 % incinerated.

While pyrolysis offers an alternative, it has limitations, including up to 20 % char formation and diverting output to fuel rather than new plastics. HTL presents a more effective solution by converting diverse plastic waste into high-quality hydrocarbons suitable for true circular recycling.

Mura Technology, a UK-based plastic recycling company, recently developed a commercial-scale HTL process that employs supercritical water at over 400 °C and 220 atmospheres of pressure to break down polymers. This method enhances heat transfer, stabilizes reactive carbon radicals, minimizes char formation, and improves conversion efficiency compared to pyrolysis.

The resulting hydrocarbons are fractionated into naphtha, gas oils, and heavy residual oil, which can be refined into new plastics and chemicals.

The process converts plastic waste in approximately 30 minutes—significantly faster than pyrolysis—while efficiently removing contaminants, enabling the treatment of lower-quality feedstock.

Mura will deploy this technology in its UK facility, the world's first commercial-scale HTL plant, set to launch later this year with a capacity to process 23,000 tons of plastic annually. This facility marks a critical step toward large-scale plastic circularity, reducing reliance on virgin plastics and diverting substantial waste from landfills and incineration.⁹

Future Prospects in Plastic Recycling

Chemical recycling remains a niche industry, with an estimated 17 million tons of plastic waste expected to be processed annually by 2034. HTL offers a scalable solution, converting diverse plastic waste into high-quality hydrocarbons for circular recycling.

Mura has already announced plans for additional plants in Germany, the United States, Singapore, Japan, and South Korea, signaling the potential for widespread deployment.⁹ As the technology expands, it could help close the loop on plastic waste, complement mechanical recycling, and reshape the global recycling industry.

To learn more about advancements in plastic recycling, please visit:

• New Method for Near-Complete PMMA Plastic Breakdown
• Why Oxo-Biodegradable Plastics are Essential for Addressing Plastic Waste
• Role of Nutritional Amino Acids and Water in Plastic Decomposition by Mealworms
• What are the Main Polymer Recycling Methods?
• An Interview with Greenpeace: The Harmful Reality of Plastic Recycling

References and Further Reading

1. Feuerbach, S., Toor, S. S., Costa, P. A., Paradela, F., Marques, P. A., Castello, D. (2023). Hydrothermal Co-Liquefaction of Food and Plastic Waste for Biocrude Production. Energies. https://doi.org/10.3390/en17092098
2. Ranjbar, S., Malcata, F. X. (2022). Hydrothermal Liquefaction: How the Holistic Approach by Nature Will Help Solve the Environmental Conundrum. Molecules. https://doi.org/10.3390/molecules28248127
3. Liu, M., Zeng, Y. (2022). Key Processing Factors in Hydrothermal Liquefaction and Their Impacts on Corrosion of Reactor Alloys. Sustainability. https://doi.org/10.3390/su15129317
4. Subramanya, SM., Savage, PE. (2025). Hydrothermal Liquefaction for Processing Municipal Solid Waste without Separation. [Online] PennState. Available at: https://drawdown.psu.edu/sites/default/files/posters/study-hydrothermal-liquefaction-process-utilizing-municipal-solid-waste-without-separation.pdf
5. Lozano, E., Pedersen, T., Rosendahl, L. (2020). Integration of hydrothermal liquefaction and carbon capture and storage for the production of advanced liquid biofuels with negative CO2 emissions. Applied Energy. https://doi.org/10.1016/j.apenergy.2020.115753
6. CORDIS – EU. (2021). Hydrothermal liquefaction: Enhanced performance and feedstock flexibility for efficient biofuel production. [Online] CORDIS. Available at: https://cordis.europa.eu/project/id/764734
7. Ghavami, N., Özdenkçi, K., Salierno, G. et al.(2023). Analysis of operational issues in hydrothermal liquefaction and supercritical water gasification processes: a review. Biomass Conv. Bioref. https://doi.org/10.1007/s13399-021-02176-4
8. Ghadge, R., Nagwani, N., Saxena, N., Dasgupta, S., Sapre, A. (2022). Design and scale-up challenges in hydrothermal liquefaction process for biocrude production and its upgradation. Energy Conversion and Management: X. https://doi.org/10.1016/j.ecmx.2022.100223
9. Peplow, M. (2025). Can this revolutionary plastics-recycling plant help solve the pollution crisis? [Online] Nature. Available at: https://doi.org/10.1038/d41586-025-00293-y

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