A review on the hydrothermal processing of microalgal biomass to bio-oil - Knowledge gaps and recent advances

Abstract

This state of the art review had presented hydrothermal processing techniques such as hydrothermal gasification, hydrothermal carbonization, and hydrothermal liquefaction. Further, pros and cons of each hydrothermal technique were compared concerning the yield and quality of bio-crude, other products yield, nutrients of an aqueous phase, energy consumption and overall economics. Based on the numerous research and review articles survey, this critical review article had ascertained hydrothermal liquefaction as an ideal technique for microalgal bio-crude production and thus, recent research works undertaken on the hydrothermal liquefaction of different microalgal species has been summarized and discussed. This article has evaluated and described the impact of various factors such as reaction temperature, residence time, pressure, catalyst dose, feedstock selection and its load on the hydrothermal liquefaction efficiency. A separate section dealing with upgrading of bio-oil delivers options to produce maximal bio-crude with higher heating value, low nitrogen, and oxygen heteroatoms. The key contribution of this review is information and scope itemized in this manuscript on hydrothermal liquefaction process can be useful in propelling the prospect research to standardize an intact and efficient hydrothermal liquefaction system for microalgae.