Global Perspective of Hydrothermal Liquefaction of Algae: a Review of the Process, Kinetics, and Economics Analysis

Hydrothermal liquefaction (HTL) is a green technology for biocrude production at high temperatures (200–500?°C) and high pressure (5–30?MPa). There are important gaps in HTL reaction optimization, process design, and the effect of operating parameters. To facilitate overcoming these research gaps in future studies, this review summarizes the scientific and engineering applications of HTL. The objective of this study is to assess the production of biocrude from algae using HTL and compare it with wood and waste biomasses as a potential feedstock. The influence of effective parameters on the optimum HTL biocrude yield was investigated. Moreover, kinetic, economic, and exergy analyses have been considered in HTL studies. The result showed that the highest biocrude yield was attained at 50 wt%, 30 wt%, and 20 wt% using microalgae, wood, and macroalgae at an optimum temperature range of 300–350?°C for less than 60?min. The kinetic models were successful at all reaction temperatures and times for biocrude yield prediction. Moreover, the minimum fuel selling price varied between $1.70 and $22/GGE. The exergy studies indicated that the overall exergy efficiency was in the range of 20–96%. However, direct HTL has some drawbacks such as severe operation conditions and biocrude production with high nitrogen and oxygen contents. Several processes were considered to address these problems. The two-stage, microwave, catalytic cracking, additives, and hydro-treatment upgrading process improve the biocrude properties, while the supercritical fluids and emulsification upgrading process helped dissolve insoluble materials. Furthermore, pretreatment processes such as bead milling, ultrasonic, and microwave were suggested to promote biomass cell wall disruption.