Hydrothermal liquefaction of microalgal biomass for biocrude production

Abstract

Biofuels are one of renewable energies that are actively developed in order to solve global climate change and fossil fuel depletion. Among the biofuels, microalgal biofuel is considered promising due to high productivity and lipid contents. However, conventional conversion method involving lipid extraction suffers low extraction efficiency from wet microalgal biomass. Hydrothermal liquefaction (HTL) is a promising method of converting wet microalgal biomass into biocrude. Especially, the potential HTL of microalgae to produce biocrude regardless of the types of microalgae (biomass-agnostic) has been highly noted. However, the quality of biocrude and the Energy Return On Investment (EROI) of HTL have not been investigated in detail, and only the high yield of biocrude tends to be emphasized. Here we report that the yield as well as the quality of biocrude is directly dependent on the lipid contents of microalgae, and the importance of high lipid content in HTL is revealed using statistical analysis and Energy Return On Investment (EROI) calculation. HTL was conducted with Nannochloropsis oceanica (30.2% lipid content) and Golenkinia sp. (17.3% lipid content), and various operation conditions were tested. The quality of HTL was assessed in terms of asphaltene/non-asphaltene contents, heteroatom (O, N, and S) concentrations, and the effective hydrogen-to-carbon ratios $(H/C_{eff})$ of the biocrudes. N. oceanica with high lipid content produced biocrude more suitable for catalytic upgrading to transportation fuels than the biocrude from Golenkinia sp. with low lipid content. It is shown that carbohydrate and protein in microalgae mainly contributes to formation of asphaltene, which is not suitable for catalytic upgrading due to excess amount of heteroatoms and polyaromatic structure. Based on analysis of components in aqueous phase and biocrude, a general reaction pathway diagram was suggested. Multivariate statistical analysis (redundancy analysis) with the data gathered from the literature confirmed that lipid content has strong positive correlations with biocrude yield as well as quality, represented by H/Ceff. As the result of EROI calculation, low temperature $(200^\circ C)$ is beneficial for HTL of high-lipid microalgae such as N. oceanica, while high temperature $(300^\circ C)$ is suitable for low-lipid microalgae such as Golenkinia sp. The result also shows that production of transportation fuel is only feasible with high-lipid microalgae. These results strongly support that the choice of microalgae with high lipid content is still a key factor that must be considered for biocrude production as much as it is regarded important for biodiesel production using microalgal biomass.