Superstructure optimization of hydrothermal liquefaction for microalgae biorefinery considering environmental impacts and economics

Microalgae biorefinery superstructure connections from algae to muti-products are addressed. The superstructure connections affect eCO2 emissions, product variety, and profits. Through the superstructure optimization algorithm for the illustrated four scenarios within the same lipid content of 25 % and 200 kg weight of microalgae, (i) Scenario-1 for maximizing the net profit, the best net profit of 32.54 USD is achieved due to producing the green electricity and biodiesel, (ii) Scenario-2 for minimizing total eCO2 emissions, the lowest total eCO2 emissions with 78.28 kg eCO2 is guaranteed if the power generation from biogas is assumed to fully cover load demands of the process units but the corresponding net profit is down to 10.84 USD, (iii) Scenario-3 for balancing net profit and eCO2 emissions, the net profit is returned to 17.65 USD since the acetone/butanol/ethanol by the ABE fermentation and biooil/green diesel are added, (iv) Scenario-4 for maximizing biooil production, the total revenue is higher than Scenarios 1–3 by 16∼164 % but the corresponding eCO2 emissions is also higher than Scenarios 1–3 by 65∼160 %. Although the superstructure optimization problem induces the trade-off results, it is validated that the combination of ABE fermentation and HTL is superior to the combination of transesterification and anaerobic digestion.