Comparative techno-economic analysis of algal biofuel production via hydrothermal liquefaction: One stage versus two stages

Hydrothermal liquefaction is a promising process for conversion of algae to bio-oil that is especially suitable for the high moisture content of algal feedstock. A two-stage sequential hydrothermal liquefaction (SEQHTL) alternative was developed to facilitate production of co-products in addition to bio-oil at reduced temperatures and pressures compared with conventional one-stage direct hydrothermal liquefaction (DHTL). In this study, Aspen simulation and comprehensive techno-economic analysis were conducted for both SEQHTL and DHTL processes to assess their performance when used to convert the same algal strain (Chlorella sorokiniana) to bio-oil intermediates. The technical and economic evaluation also included the subsequent upgrading of the bio-crude to biofuels via hydrotreatment. The minimum fuel selling price for SEQHTL and DHTL was $1.61/L, and $2.10/L, respectively. The milder operating conditions of SEQHTL process resulted in both reduced capital and operating cost. The total installed cost of the facilities for hydrothermal processing 1215 metric tons/day of algae was $89 million for SEQHTL and $112 million for DHTL. A higher energy returned on energy invested was realized by SEQHTL (6.73) owing to its greater amount of fuel blendstock produced and less energy required for production in comparison with DHTL (5.31). The sensitivity analysis showed that improving the yield and quality of both the bio-oil and co-products, as well as increasing the feed concentration, may lead to a much lower production cost. This study provided new insights of hydrothermal liquefaction process design with highlighting the potential of recycling of nutrient streams and fractionation algal biomass at milder operating conditions.