Ethanol from Sugarcane Lignocellulosic Residues - Opportunities for Process Improvement and Production Cost Reduction

Sammanfattning: Bioethanol from sugarcane is a sustainable alternative to fossil fuels, and the increasing demand for fuel ethanol has prompted studies on the use of the lignocellulosic residues of sugarcane, namely bagasse and leaves, as new feedstock. This thesis describes various process designs and the economic feasibility of producing second generation (2G) ethanol from bagasse and leaves via the enzymatic route in an integrated sugarcane biorefinery, where first-generation (1G) ethanol is produced from sugarcane sugar. Techno-economic analysis have been performed for the Brazilian context to evaluate the influence of several process designs and the main production factors on the 2G ethanol process, in terms of energy efficiency, 2G ethanol production cost(2G MESP) and profitability.
The study of process design focused on ways to integrate the 1G and 2G ethanol processes, and on configurations to hydrolyse and ferment bagasse. The existing 1G ethanol process and the proposed 2G ethanol process were combined in a single plant by integration of thermal and material streams. The resulting synergies could improve the use of feedstock and reduce the 2G ethanol production cost. Simultaneous saccharification and fermentation (SSF)and time-separated hydrolysis and fermentation (tSHF) were the configurations investigated experimentally for the production of 2G ethanol from bagasse. In an attempt to increase the ethanol concentration before distillation, the fermented liquid of tSHF was also recirculated back to tSHF. The tSHF configurations showed a lower 2G MESP than SSF.
Process options were also investigated considering the pentose use and the addition of leaves to the 1G+2G process. Pentoses can either be fermented to ethanol or anaerobically digested to produce electricity from biogas combustion, and in the former case the highest potential reduction in 2G MESP could be achieved. The addition of leaves could improve the overall profitability of the 1G+2G process. Residence time and water-insoluble solids (WIS) loading in hydrolysis were the main process conditions considered together with costing factors, such as enzyme, sugarcane and leaves costs. The selling price of electricity and ethanol were found to have relevant impacts on the profitability of the 1G+2G ethanol process. Among the numerous operating conditions studied for the 2G ethanol process, the cases showing the best trade-off between technical and economic feasibility were also tested experimentally on laboratory scale obtaining promising results. In fact, it was possible to achieve high concentrations of 2G ethanol (47 g/L) in short time (60 hours), overcoming the mixing problems by
feeding repeatedly the pretreated bagasse up to 20% WIS.