Algal biorefinery to value-added products by using combined processes based on thermochemical conversion: A review

Thermochemical processes, including gasification, liquefaction, and pyrolysis, are promising technologies for algal conversion. Gasification is effective to convert algal biomass into fuel gases while liquefaction and pyrolysis are favorable for the production of bio-oil with low molecular weight and biocrude with high energy density, respectively. To understand the role of algal components (proteins, lipids, and carbohydrates) on thermochemical conversion processes, this paper reviews the properties of biofuels from the thermochemical conversion of algal components and their model compounds. The characteristic fingerprints of algal components differ from one another. Consequently, the thermochemical conversion of the total algal biomass results in heterogeneity of the biofuels. The unfavorable nitrogenous compound production also leads to resource and energy losses, which are the critical bottleneck of algal biorefinery. As such, this review tackles some combined processes. The combination of the hydrothermal liquefaction of algal biomass and the hydrothermal gasification of an aqueous fraction shows potential for applications that improve fuel gas production. Lipid extraction combined with thermochemical residue conversion contributes to an increase in total oil yield. Protein extraction combined with thermochemical residue conversion decreases the risk of nitrogenous compound contamination in bio-oil and increases the recovery of value-added protein-derived products. Protein and lipid extraction before thermochemical conversion should be further explored to maximize the exploitation of multiple value-added products from algal biomass. [Display omitted] •Thermochemical conversion of algal components and model compounds is reviewed.•Challenges of thermochemical processes for algal biorefinery are discussed.•Some combined processes for optimizing algal biorefinery are reviewed.•Protein and lipid extraction before thermocehmical conversion is suggested.