Co-production of Bioethanol and Power

Abstract

Recently, biomass usage for fuel has attracted increased interest in many countries to suppress global warming caused mainly by the consumption of fossil fuels. (Mousdale, 2010). In particular, many researchers expect that bioethanol may be a substitute for petroleum. In fact, bioethanol loses less energy and exergy potential during chemical reactions, saccharification and fermentation for ethanol production, because it is produced merely through energy conversion by chemical reactions (Cardona et al. 2010). However, after fermentation, the product contains a large amount of water, which prevents maximizing the heat value of the product. Therefore, separation of the ethanol-water mixture is required to obtain pure ethanol for fuel (Zamboni et al. 2009a, 2009b, Huang et al. 2008). In practice, distillation is widely used for the separation of this mixture (Fair 2008). However, conventional distillation is well-known to be an energy-consuming process, and also pure ethanol fuel cannot be produced directly from a distillation column, because ethanol and water form an azeotropic mixture. To separate pure ethanol from ethanol-water mixtures by distillation, it is necessary to use an entrainer (azeotrope breaking agent), because the azeotropic mixture is one that vaporizes without any change in composition. Benzene, cyclohexane, or isopropyl alcohol can be used as entrainers for the ethanol-water mixture. Therefore, at least two separation units are required to produce pure ethanol, leading to further increases in energy consumption (Doherty& Knapp 2008). In fact, it is believed that about half of the heat value of bioethanol is required to distill the ethanol from the mixture. To reduce energy consumption during bioethanol production, many researchers have proposed membrane separations (Baker 2008, Wynn 2008) or pressure swing adsorption (PSA) (Modla & Lang, 2008) as alternatives to azeotropic distillation, often successfully developing appropriate membranes or sorbents to achieve an efficient separation. However, in many cases, they have paid little attention to the overall process scheme or have developed heat integration processes based on conventional heat recovery technologies, such as the well known heat cascading utilization. As a result, the minimum energy requirement of the overall process has not been reduced, because changes to the condition of the process stream are constrained in conventional heat recovery technologies (Hallale 2008, Kemp 2007). Moreover, most cost minimization analyses for bioethanol plants have been conducted based on these conventional processes and technologies. Thus, the price of product bioethanol still remains high compared to fossil fuels. Nowadays, by reconsidering the energy and production system from an improvement of energy conversion efficiency and energy saving point of view, the concept of co-production of energy and products has been developed. However, to realize co-production, it is