Subcritical Water Hydrolysis of Microalgal Biomass for Protein and Pyrolytic Bio-oil Recovery

Microalgae are a promising source of protein and biofuels. This study involved the extraction of soluble proteins from raw microalgae using subcritical water hydrolysis followed by pyrolysis of the resulting spent microalgal biomass for bio-oil production. The extraction process produced solubilized protein in amounts up to 10 wt% of the dry biomass. The effects of hydrolysis temperature (150–220 °C), process time (90–180 min), and initial pH (2–12) on the chemical compositions and reactivity of the spent biomass as biofuel intermediates were investigated. It was found that when the temperature and time increased, the protein and carbohydrate fractions of the spent biomass were reduced, while their lipid fraction increased. A low initial pH led to lower protein content in the spent biomass. Compared with the raw microalgae, the spent biomass gave a higher yield of pyrolytic bio-oil that contained much less of the N-containing compounds and higher amounts of long-chain fatty acids (C16) and C14–C20 long-chain hydrocarbons. In addition, enhanced energy recovery and a reduction in the energy consumption of the pyrolysis process were the other benefits acquired from the protein extraction. Therefore, subcritical water hydrolysis was considered to be an effective process to recover solubilized proteins, enhance the properties of the spent biomass, improve the energy balance of the subsequent pyrolysis process, and raise the quality of the bio-oil.