Endogenous bioethanol production by solid-state prefermentation for enhanced crude bio-oil recovery through integrated hydrothermal liquefaction of seaweeds

The present study evaluated the action of endogenous bioethanol produced during solid-state fermentation of Ulva spp. on subsequent hydrothermal liquefaction (HTL). HTL of raw biomass (RB) was compared with HTL of the fermented biomass after bioethanol separation (F-Aqua) and HTL of the whole fermentation broth containing the bioethanol (F-Eth). Optimization of fermentation conditions increased the ethanol yield efficiency (Yeff) and specific ethanol yield (SEY) from 23.55% and 0.120 g g−1 sugar consumed to 43.97% and 0.224 g g−1 sugar consumed, respectively. TGA and FTIR analysis confirmed noticeable changes in the thermal profile and functional groups of the fermented residue with increased volatiles and lower ash content. Among different HTL treatments, F-Eth showed the highest bio-oil yield of 24.96% at 250 °C, which was 37.7% and 39.0% higher than that of RB and F-Aqua, respectively. In addition, hydrocarbons and esters in the bio-oil represented the dominant compounds in F-Aqua and F-Eth, altogether 28.28% and 68.58%, respectively, compared to 17.96% in RB. Economic analysis for a proposed 100 ton year−1 plant confirmed that HTL of the whole fermentation broth containing endogenous bioethanol enhanced the gross energy output to 8.152 GJ ton−1, which represented 4.2-time, 51.1%, and 23.3% higher than individual bioethanol production, individual bio-oil production, and sequential bioethanol/bio-oil recovery, with the highest net annual profit of 29553 US$ compared to 23391 US$ for the sequential production.