Renewable Fuels and Chemical Recycling of Plastics via Hydrothermal Liquefaction

ConspectusHydrothermal liquefaction (HTL) converts a wide range of biomass feedstocks into a renewable bio-oil via reactions in hot, compressed water. Other products that form partition into the gas, aqueous, or solid phases. The reactions taking place during HTL include hydrolysis, decarboxylation, condensations, additions, deamination, and dehydration. Bio-oil production from HTL has been demonstrated for many renewable materials including microalgae, macroalgae, sludge from water treatment, food waste, agricultural residues, bacteria, yeast, and a wide variety of lignocellulosic materials. HTL of whole biomass is an energy densification process as it generally recovers about 70–80% of chemical energy in an oil product that is just 20–50 wt % of the mass of the original feedstock. The oil is typically rich in oxygen (10 -20 wt %) and also in nitrogen (about 5 wt %), if the biomass contains protein. The oil also contains metals such as iron. Therefore, an upgrading and/or refining process would be required to convert the crude bio-oil to a finished fuel. Each biochemical component in biomass (e.g., polysaccharides, protein, lipids, lignin) contributes different proportions of its initial mass to biocrude. Lipids provide the highest biocrude yields whereas polysaccharides and lignin provide the lowest. Kinetics models have been developed that incorporate different reactivities for the different biochemical components. These models, which are effective in correlating and predicting the yields of biocrude and other product fractions from HTL, can be used in technoeconomic analysis and life-cycle assessments to advance commercial adoption of the technology.HTL is also effective in producing an oil product from decomposition of many different plastics. Thus, it can be used for chemical recycling or valorization of post-consumer waste plastics. Polyolefins and polycarbonates can give oil yields exceeding 90 wt %. Polyethylene terephthalate (PET) gives very low oil yields. The major product from PET hydrolysis, terephthalic acid, is a solid at room temperature and not soluble in the organic solvents typically used to recover oil.Hydrothermal decomposition of isolated biochemical components of biomass and individual synthetic polymers provides insights into the governing chemical reaction pathways. Results from such experiments also enable development of component additivity models that can predict oil yields and other outcomes from HTL of biomass, plastics, and their mixtures.Research needs in this field include more detailed, molecular-level kinetics models, experimental work on HTL and hydrolysis of the many synthetic plastics that have not yet been adequately explored, and experiments and modeling for mixtures of different plastics and mixtures of plastics with biomass.