Bioethanol fermentation as alternative valorization route of agricultural digestate according to a biorefinery approach

Abstract

This study investigates the feasibility of producing bioethanol from solid digestate after a mechanical fractionation (i.e. centrifugal milling), in order to improve the energy recovery from agricultural wastes and the sustainability of anaerobic digestion plants. A bioethanol yield of 37 g kg-1 TS was evaluated for the solid digestate fraction. Mass and energetic balances were performed and compared between two scenarios: (A) one-stage bioethanol fermentation and (B) two-stage anaerobic digestion-bioethanol fermentation, in order to evaluate the feasibility and the advantages of the two-stage process. Results revealed that, compared to the one-stage process, the dual anaerobic digestion-bioethanol process permitted: (i) to diversify biofuels production; (ii) to provide the thermal energy sufficient for drying digestate (13,351 kWhth day-1), for the subsequent milling step; (iii) to reduce the electric energy requirement for the milling step (from 23,880 to 3580 kWhel day-1); (iv) to produce extra electrical energy of 8483 kWhel day-1; (v) to improve the reduction of waste streams generated (from 13% to 54% of organic matter removal).