Hydrothermal liquefaction (HTL) is a thermal depolymerization process used to convert wet biomass, and other macromolecules, into crude-like oil under moderate temperature and high pressure.
[1] The crude-like oil has high energy density with a lower heating value of 33.8-36.9 MJ/kg and 5-20 wt% oxygen and renewable chemicals.
[1] Carbon and hydrogen of an organic material, such as biomass, peat or low-ranked coals (lignite) are thermo-chemically converted into hydrophobic compounds with low viscosity and high solubility.
[5] Simple heating without water, anhydrous pyrolysis has long been considered to take place naturally during the catagenesis of kerogens to fossil fuels.
[8] Other organizations that have previously demonstrated HTL of biomass include Hochschule für Angewandte Wissenschaften Hamburg, Germany, SCF Technologies in Copenhagen, Denmark, EPA’s Water Engineering Research Laboratory, Cincinnati, Ohio, USA, and Changing World Technology Inc. (CWT), Philadelphia, Pennsylvania, USA.
[9] Construction has begun in Teesside, UK, for a catalytic hydrothermal liquefaction plant that aims to process 80,000 tonnes per year of mixed plastic waste by 2022.
[10] In hydrothermal liquefaction processes, long carbon chain molecules in biomass are thermally cracked and oxygen is removed in the form of H2O (dehydration) and CO2 (decarboxylation).
[1][6][8][14] The composition of cellulose, hemicellulose, protein, and lignin in the feedstock influence the yield and quality of the oil from the process.
Barbero-López et al.,[16] tested in the University of Eastern Finland the use of spent mushroom substrate and tomato plant residues as feedstock for hydrothermal liquefaction.
A commercialized process[17] using hydrous pyrolysis (see the article Thermal depolymerization) used by Changing World Technologies, Inc. (CWT) and its subsidiary Renewable Environmental Solutions, LLC (RES) to convert turkey offal.
Pressure (along with temperature) determines the super- or subcritical state of solvents as well as overall reaction kinetics and the energy inputs required to yield the desirable HTL products (oil, gas, chemicals, char etc.).
[1] Recently, Steeper Energy announced that the carbon intensity (CI) of its Hydrofaction™ oil is 15 CO2eq/MJ according to GHGenius model (version 4.03a), while diesel fuel is 93.55 CO2eq/MJ.
[1] Instead the heteroatoms, including nitrogen, sulfur, and chlorine, are converted into harmless byproducts such as N2 and inorganic acids that can be neutralized with bases.
[23] HTL oil has good potential to yield bio-oil with "drop-in" properties that can be directly distributed in existing petroleum infrastructure.