Hydrothermal Liquefaction
Hydrothermal liquefaction (HTL) is a thermochemical depolymerisation process in an enclosed reactor to convert wet biomass into biocrude oil and chemicals at moderate temperature (200–400°C) and high pressure (10–25 MPa) [1]. This process is synonym of hydrous pyrolysis but compared to pyrolysis, HTL is carried at lower temperatures and heating rates. In other words, hydrothermal liquefaction of biomass is the thermochemical conversion of biomass into liquid fuels by processing in a hot, pressurized water environment for sufficient time to break down the solid bio polymeric structure to mainly liquid components [2]. Water serves as an important reactant. As the reaction condition approaches to the critical point of water, several properties of water are drastically changed and able to bring about fast, homogeneous, and efficient reactions. The product yield and physiochemical properties of an HTL are primarily affected by the types of feedstock, processing conditions (primarily reaction temperature and time), and existence of a catalyst [1].
Biomass Feedstock
The feedstock used for HTL is classified into two types: dry/lignocellulose biomass and wet/algal biomass.
Dry Feedstock
The tangled biomass properties of dry biomass give it a complex and rigid crystallinity structure which makes it tougher to process. Despite, the intrinsic source of hydrogen and carbon, the processing of the lignocellulose biomass is limited due to the substantial amount of the oxygen. Woody biomass/lignocellulose is composed of cellulose, hemicellulose and lignin. Cellulose is a long chain polysaccharide with high molecular weight and high degree of polymerisation due to the monomer glucose molecules being stiffly bonded by strong intra – and inter-molecular interactions. Hemicellulose is a branched structure hetero polymer with amorphous structure consisting of pentose and hexose as a polymer and is weaker in comparison to cellulose. Lignin possess similar morphological characteristic of amorphous form as hemicellulose, less solubility similar to cellulose and behaviour of hydrophobic nature [2].
