Integrated energy production and reduction of the environmental impact at alcohol distillery plants

Abstract

In Brazil cane is being been grown at large scale to produce alcohol as an automotive fuel. Alcohol is the sole product, but there is generation of a large quantity of gaseous (CO2), liquid (vinasse) and solid (bagasse) by-products, which currently have very little or even negative value. By using steam turbines fuelled with bagasse combustion, electric power can be generated at a rate of 1 MWh per m3 of produced alcohol. Anaerobic digestion can be applied to vinasse to produce enough biogas for 0.5 MWh per m3 of alcohol, bringing total electric power production from subproducts to 1.5 MWh per m3 of alcohol. These operations are presently implemented at some distilleries at full scale. It has been shown at bench scale that by applying anaerobic digestion also to bagasse and burning the non-biodegradable residual, the power output can be increased to 2.25 MWh per m3 of alcohol, but the economic feasibility of this option depends on the maximum loading rate of the bagasse digester and the energy price. At the current alcohol production level of 13 × 106 m3/year, the power generation potential is 2.2 GW, which represents 4% of the power demand in Brazil. The digested waste water contains about 70% of the nutrient demand of the cane fields, which can be recycled. A preliminary economic evaluation shows that productive use of the subproducts of alcohol distilleries is economically feasible if the price is more than US$30 per MWHh, which is the current sales price in Brazil. Another important advantage of the rational use of by-products is that the generation of electric power has the potential to reduce the emission of CO2 to the atmosphere by 0.8–1.2 t per m3 of alcohol compared to generation using natural gas.