Thermochemical Conversion of Microalgae: Challenges and Prospective of HTL Pathway for Algae Biorefinery

Abstract Hydro Thermal Liquefaction (HTL) emerges as a promising method for converting wet biomass into liquid fuels. However, additional processing of the resulting HTL biocrude is imperative. Elevated levels of oxygen and nitrogen in HTL-produced biocrude necessitate deoxygenation and denitrogenation before it can be effectively used as a transport fuel. Managing the by-product aqueous stream is crucial for the success of an algal biorefinery employing HTL. Consequently, maximizing HTL efficiency and optimizing the utilization of biocrude and co-products, especially aqueous by-products, are current research priorities in biorefinery studies. To boost HTL efficiency, the focus is on using only carbon and hydrogen for hydrocarbon liquid fuels, as the presence of oxygen and nitrogen is undesirable for oil applications. Oxygen lacks inherent heating value, and nitrogen, if combusted, contributes to environmental pollution. Hence, HTL involves concurrent deoxygenation and denitrogenation during biocrude formation. The primary role of HTL lies in sustainable energy and chemicals production, aligning with a commitment to environmental preservation. Biocrude, characterized by high oxygen and nitrogen contents, along with elevated molecular weight and viscosity, typically undergoes upgrading processes like solvent extraction/distillation, hydrogenation/hydrodeoxygenation, catalytic cracking, esterification, and hybrid techniques. These processes lead to the partial removal of oxygen as CO2 or H2O and the conversion of nitrogen into ammonium. The study explores potential routes for the thermochemical conversion of microalgae, distinguishing between dry processes (pyrolysis and gasification) and wet processes (near-critical water hydrothermal liquefaction and hydrothermal gasification). The work identifies key engineering advantages and challenges, focusing on biofuel production for transportation. The future perspectives for each route are presented.