Recent development of hydrothermal liquefaction for algal biorefinery

Hydrothermal liquefaction (HTL) is considered as one of the most promising methods for converting algal biomass to bio-oil and other value-added chemicals. The conventional HTL process, however, is limited by its incapability of recovering high-value bioactive compounds that are desirable for enhancing overall process feasibility. The two-stage sequential hydrothermal liquefaction (SEQHTL) process was proposed as an alternative to overcome this limitation. SEQHTL operates at reduced temperature and pressure to facilitate production of coproducts in addition to bio-oil. This article offers a comprehensive review of the SEQHTL process in comparison with conventional HTL. Main topics include: operation principles and targeting final products of HTL, reaction mechanisms of algal biomass in the HTL process, recent publications on algae-related HTL studies, features of the SEQHTL process, advantages of the SEQHTL process for production of high quality bio-oil as well as extraction of prospective high-value co-products that may be harvested from microalgae biomass, cost advantage of the SEQHTL process, challenges identified with techno-economic and life-cycle assessments, and suggested future HTL research and development in comparison with other conversion technologies. All these aspects collectively provide an overview of the SEQHTL technology.