Biogas Production from Anaerobic Digestion of Vinasse in Sugarcane Biorefinery: A Techno-economic and Environmental Analysis

Abstract

Anaerobic digestion (AD) of effluents in sugarcane biorefineries may improve the energy balance in the bioethanol production process and the environmental suitability of wastes disposal. This study analyses the inclusion of AD of vinasse (stillage) into different configurations of sugarcane biorefineries, using technical, economic and environmental metrics. The three base-case scenarios are a first generation (1G) ethanol plant, an integrated first and second generation (1G2G) ethanol plant with alcoholic fermentation of the hemicellulose fraction (C5), and an 1G2G ethanol plant with biodigestion of the C5 fraction. These three base scenarios are compared with the similar ones including the biodigestion of vinasse. The implementation of vinasse biodigestion had positive impact on the productivity and sustainability of sugarcane biorefineries. The use of biogas from vinasse for a 1G ethanol plant increased in 9.20% the surplus of electric energy yielded to the grid. This scenario had a positive net present value, NPV: + 11.5 × 106 USD, assuming minimum acceptable rate of return, MARR = 11%. For the 1G2G + C5 fermentation scenario the NPV was + 4.63 × 106 USD. Assuming NPV = 0, the IRR for 1G becomes 19.7% and for 1G2G + C5, 13.6%, an IRR higher than the MARR. The inclusion of AD of vinasse when the C5 fraction is biodigested was not an economically feasible option. In all scenarios, vinasse biodigestion reduced environmental impacts, and the 1G2G processes exhibit better results than the consolidated 1G for almost all environmental impact categories. The AD of vinasse can be considered an environmental-friendly and economical-feasible technology to improve the process in sugarcane biorefineries.