Hydrothermal liquefaction of biomass

Biomass is one of the most abundant sources of renewable energy, and will be an important part of a more sustainable future energy system. In addition to direct combustion, there is growing attention on conversion of biomass into liquid energy carriers. These conversion methods are divided into biochemical/biotechnical methods and thermochemical methods, such as direct combustion, pyrolysis, gasification, liquefaction, etc. This chapter focuses on hydrothermal liquefaction, where high pressures and intermediate temperatures together with the presence of water are used to convert biomass into liquid biofuels, with the aim of describing the current status and development challenges of the technology. During the hydrothermal liquefaction process, the biomass macromolecules are first hydrolyzed and/or degraded into smaller molecules. Many of the produced molecules are unstable and reactive and can recombine into larger ones. During this process, a substantial part of the oxygen in the biomass is removed by dehydration or decarboxylation. The chemical properties of the product are mostly dependent of the biomass substrate composition. Biomass consists of various components such as carbohydrates, lignin, protein, and fat, and each of them produce distinct groups of compounds when processed individually. When processed together in different ratios, they will most likely cross-influence each other and thus the composition of the product. Processing conditions including temperature, pressure, residence time, catalyst, and type of solvent are important for the bio-oil yield and product quality.