Techno-economic analysis of alternative aqueous phase treatment methods for microalgae hydrothermal liquefaction and biocrude upgrading system

An economic analysis was conducted for an algae hydrothermal liquefaction (HTL) and biocrude upgrading system with alternative aqueous phase treatment methods. Using experimental data, three different methods were modeled and compared: 1) direct recycle of HTL aqueous phase to the algae farm, 2) catalytic hydrothermal gasification (CHG), and 3) anaerobic digestion (AD). Direct recycle consumes the most natural gas because unlike AD and CHG, no methane is generated from the aqueous phase organics conversion. However, direct recycle has the lowest cost compared to other two cases because it only requires the addition of pumps and buffer tankage related to the aqueous phase recycle. The CHG and AD cases require significant capital and operating costs for aqueous phase treatment, which increase the minimum fuel selling price (MFSP) over the direct recycle case by 11% and 2.9%, respectively. The CHG and AD respectively increase the conversion cost only (excludes the cost to grow, harvest, and dewater algae) by 75% and 21%. Sensitivity analysis suggests that the key factors affecting the total cost of fuel production are the feedstock cost, the biocrude yield, the plant scale, and the method of aqueous phase treatment.