Effects of the use of acetone as co-solvent on the financial viability of bio-crude production by hydrothermal liquefaction of CO2 captured by microalgae

Abstract

Hydrothermal Liquefaction (HTL) is a promising technology to produce biocrude from microalgal biomass that has captured gaseous CO₂. However, several problems of this technology must still be solved to make it economically and technically feasible. One of the main problems in the financial viability of the HTL process is the low yield obtained when only water is used as a solvent, with results close to or lower than 30 wt%. This, in turn, increases the production cost to over 120 USD/BBL. In recent years, some authors have focused their efforts on increasing biocrude production through the extensive use of organic solvents, without considering the effects on economic viability. The present work evaluated the financial effect of using acetone as an organic co-solvent, finding that high acetone contents increased operating costs of the process, mainly due to losses in its handling and recovery. On the contrary, very low acetone contents had very little effect on the biocrude yield. It was possible to establish that concentrations close to 5 wt% of acetone, mixed with water, resulted in a yield of about 60 wt% and a production cost of 50 USD/BBL of biocrude, with an energetic densification of 30.25 MJ/kg and 8.60^∘ API, classifying it as heavy crude, which makes it necessary to include refining processes for heteroatom removal. Chemical characterization of biocrude revealed a high content of nitrogenous compounds (23.6 wt%) and oxygenated compounds (10.7 wt%), which have to be removed for the subsequent production of commercial liquid fuels. It is concluded that the use of a water-acetone mixture allows for obtaining positive operating profits, which helps to the high capital costs involved in this type of technology, making it more financially comparable with the conventional petroleum industry.