Economic analysis of a second-generation ethanol and electricity biorefinery using superstructural optimization

This paper investigates the economic feasibility of expanding a first-generation sugarcane distillery into a biorefinery by producing second-generation ethanol and increasing electricity production. Both products compete for using the same biomass by-products, sugarcane bagasse and straw, as raw-material. Two pathways for electricity generation were considered: a Rankine Cycle and a Biomass Integrated Gasification Combined Cycle. Also, the feasibility of producing biogas from anaerobic digestion of vinasse was assessed. Superstructural optimization was employed to evaluate the optimal fraction of bagasse that should be addressed for either ethanol or electricity product and to define the most economic route for generating electricity. The objective function was based on the benefit-cost ratio (BCR) criterion for investment. A Monte-Carlo method was used to quantify the level of uncertainty associated with the preliminary cost estimations and varying selling prices of products. In all scenarios, the optimal conditions were the hydrolysis of all available bagasse and the combustion of sugarcane straw, lignin, and biogas in a Rankine Cycle. With actual values, the expansion would cost US$ 535.2 million. Even in more favourable scenarios, the expansion of the distillery was considered economically unattractive with a BCR of 0.6209 due to high investment and operating costs.