Progress in Hydrothermal Liquefaction of Algal Biomass and Hydrothermal Upgrading of the Subsequent Crude Bio-Oil: A Mini Review

Abstract

Algae biomass has recently attracted the attention of the green energy industry as a raw material for biofuels production. Their high-water content has led to the choice of hydrothermal liquefaction as a suitable way to convert them into biooil. From algae species to bio-oil as fuel, many steps are required, including the selection of algae species and process parameters (including catalysts), the liquefaction process, product separation, recovery of crude bio-oil, and subsequent upgrading (if the goal is to use the bio-oil as transportation fuel). This review gives some biochemical, elemental, and inorganic compositions of algae. The reaction mechanism of hydrothermal liquefaction is briefly described, with an emphasis on the influence of process parameters on the yield and quality of the crude bio-oil. The use of organic solvents as reaction media or for recovery of crude bio-oil from product mixtures is discussed. The research work on the catalytic hydrothermal upgrading of algae liquefied crude bio-oil in recent years is reviewed, and some conclusions are put forward. According to recent reports, there is a section devoted to the techno-economic analysis of the hydrothermal liquefaction process. Finally, the challenges that future research and new development strategies may face are proposed.

Biofuel production from Macroalgae: present scenario and future scope

“…Furthermore, the processing of algae may generate significant food products such as alginate and agar [ 85 ]. Protein extraction from macroalgae biomass is widely studied; however, the additional market is dominated by algal goods, which have proved to be an economically viable alternative to other marine protein resources [ 86 ]. Chemicals such as levulinic acid, 2,5-furandi-carboxylic acid, succinic acid, and lactic acid have been proposed as possible biorefinery products, however, the feasibility of the process varies depending on the chemicals generated.…”

Catalytic hydrothermal liquefaction of Scenedesmus sp. biomass integrated with dark-fermentation: biocrude and low-carbon fuel production in a biorefinery approach

“…7,30,31,41 At the subcritical state, the hetero atoms bound to the carbon in DAB may dissociate to form reactive fragments, which further adjoin to produce hydrocarbons. 30,31,42 The higher yields of biocrude in H-HTL than N-HTL indicated the role of H 2 as a promoter for the hydrogenation (Fig. 7).…”

“…41 At the same time, high temperatures may increase collision frequency and high kinetic energy, which accelerates the cracking, converting long-chain alkanes into short-chain alkanes and CH 4 . 23,42 Increasing the temperature may also result in the formation of undesirable products, sudden pressure surge, coking, and deactivation of the catalysts. 24,40 The carbon and nitrogen recovery into HTL products is significantly influenced by reaction temperature.…”

Key Targets for Improving Algal Biofuel Production

“…Three major steps have been identified: depolymerisation, decomposition, and recombination [104]. Macromolecules depolymerise into their constituent of building blocks (monomers or oligomers) and decompose via multi-pathway routes (e.g., cellulose degrades to glucose and undergoes dehydration to produce anhydro-sugars and the amino acid tyrosine degrades to generate aromatic hydrocarbons).…”

“…Hydrothermal liquefaction (HTL) is carried out in water at temperatures in the range of 280-370 °C and high pressures (10-25 MPa) (Figure 7) generating bio-oil as a main product along with the gaseous, aqueous, and solid phase by-products [103]. Three major steps have been identified: depolymerisation, decomposition, and recombination [104]. Macromolecules depolymerise into their constituent of building blocks (monomers or oligomers) and decompose via multi-pathway routes (e.g., cellulose degrades to glucose and undergoes dehydration to produce anhydro-sugars and the amino acid tyrosine degrades to generate aromatic hydrocarbons).…”