A Review of Hydrothermal Liquefaction of Biomass for Biofuels Production with a Special Focus on the Effect of Process Parameters, Co-Solvents, and Extraction Solvents

Abstract

Hydrothermal liquefaction is one of the common thermochemical conversion methods adapted to convert high-water content biomass feedstocks to biofuels and many other valuable industrial chemicals. The hydrothermal process is broadly classified into carbonization, liquefaction, and gasification with hydrothermal liquefaction conducted in the intermediate temperature range of 250–374 °C and pressure of 4–25 MPa. Due to the ease of adaptability, there has been considerable research into the process on using various types of biomass feedstocks. Over the years, various solvents and co-solvents have been used as mediums of conversion, to promote easy decomposition of the lignocellulosic components in biomass. The product separation process, to obtain the final products, typically involves multiple extraction and evaporation steps, which greatly depend on the type of extractive solvents and process parameters. In general, the main aim of the hydrothermal process is to produce a primary product, such as bio-oil, biochar, gases, or industrial chemicals, such as adhesives, benzene, toluene, and xylene. All of the secondary products become part of the side streams. The optimum process parameters are obtained to improve the yield and quality of the primary products. A great deal of the process depends on understanding the underlined reaction chemistry during the process. Therefore, this article reviews the major works conducted in the field of hydrothermal liquefaction in order to understand the mechanism of lignocellulosic conversion, describing the concept of a batch and a continuous process with the most recent state-of-art technologies in the field. Further, the article provides detailed insight into the effects of various process parameters, co-solvents, and extraction solvents, and their effects on the products’ yield and quality. It also provides information about possible applications of products obtained through liquefaction. Lastly, it addresses gaps in research and provides suggestions for future studies.

A review on the interplay between bioeconomy and soil organic carbon stocks maintenance.

“…Catalysts can influence the HTL processes to improve the bio-oil yield and properties, decrease the hydrochar yield [58], reduce operation times, hindering secondary reactions, and reduce pressure and temperature dependency of the process [63]. The effect on the process is influenced by the type of catalyst, process conditions and biomass used [59].…”

A review on the interplay between bioeconomy and soil organic carbon stocks maintenance.

“…HTL allows to input a feedstock with only 5-20% DM [62] and is carried out at low temperatures around 250-374°C, pressures of 4-25MPa, and retention times around 20-30 minutes [63], [64].…”

The crop residue conundrum: maintaining long-term soil organic carbon stocks while reinforcing the bioeconomy, compatible endeavors?

“…f The use of catalysts, specially K 2 CO 3 accelerates the water gas shift reaction in low temperatures HTL processes, which yields higher rates of bio-oil (targeted product) than hydrochar. The use of catalysts tends to be more common (Mathanker et al, 2021), therefore, the Cc and Cr values stem from such process conditions. g Some simulation units involve the use of manure as organic fertilizer.…”