Research progress on characteristics and utilization of products from hydrothermal liquefaction of biomass

Abstract: In recent years, hydrothermal liquefaction (HTL) has attracted great attention because it has obvious advantages, such as various substrate types, total conversion of organic components from biomass (fat, carbohydrates and proteins), etc., compared with other biomass conversion technologies. In addition, HTL do not need to dry the raw materials, even biomass with high moisture content (more than 70%) can be used to produce biocrude oil via HTL. This paper reviews the latest progress in the HTL study of biomass, describes the separation process during HTL, and concentrates on the characteristics and utilization of HTL products (Biocrude oil, aqueous phase, solid residues and gases). Biocrude production of several model components from biomass was investigated, the results showed that biocrude yield of lipid was the highest (above 80%), followed by protein (20%-30%), and the yield of carbohydrate is the lowest (less than 10%). The aqueous phase is the main by-product of biomass HTL. There was 20%-50% of the organic matter in the substrate transferred to the aqueous phase. Different from the biocrude oil and aqueous phase, the organic components in the solid residues are much lower, the solid residue mainly contained inorganic components (ash content > 50%). Furthermore, we summarized the key elements migration in HTL products. In the process of HTL, 62%-98% of Ca, Mg, Al, Fe, Cu, Pb, Cd and Zn in the raw materials are transferred to the solid residues. Biocrude oil can be used as fuels, or can be used to extract high value-added products, the aqueous phase can be utilized for microalgae cultivation, methane production through anaerobic digestion or hydrogen production via microbial electrolysis cells, the solid residue is able to be used as biochar after further treatment, and gas phase can be used as gas fertilizer in the greenhouse. At last, the research direction in HTL is prospected. At present, a lot of researches on HTL of biomass were performed, but still some problems need to be further explored, mainly in the following aspects: 1) Biocrude oil components are complex, GC-MS of the biocrude oil can only obtain the information of low boiling point compounds, lack of understanding for high boiling point macromolecular compounds in biocrude oil, FTICR-MS or other technologies can be carried out to get a comprehensive understanding of the compounds. 2) The conversion mechanism of biomass HTL needs to be studied. At present, only some characterization of the product is carried out to speculate its reaction path, while the reaction intermediates are not monitored, which can be deeply analyzed by some in-situ on-line monitoring methods. Biomass HTL can improve the quality of biocrude oil by catalytic hydrogenation, and then the biocrude oil can be utilized by distillation. Although the above bottlenecks need to be explored and studied by researchers, the production of biocrude oil by HTL is expected to solve the problem of energy shortage in China because of its remarkable advantages, especially as a liquid fuel, so HTL technology has great potential in the renewable fuel production. This paper can provide references for future HTL study of biomass and the downstream utilization of HTL products.