Recent trends in bioethanol production

Abstract

The rapid depletion of the world petroleum supply and the increasing problem of greenhouse gas effects have strenghtened the worldwide interest in alternative, nonpetroleum sources of energy. Bioethanol accounts for the majority of biofuel use worldwide, either as a fuel or a gasoline enhancer. Utilization of bioethanol can significantly reduce petroleum use and exhaust greenhouse gas emission. The production of this fuel is increasing over the years, and has reached the level of 73.9 billion liters during the year 2009. Even though ethanol production for decades mainly depended on energy crops containing starch and sugar (corn, sugar cane etc.), new technologies for converting lignocellulosic biomass into ethanol are under development today. The use of lignocellulosic biomass, such as agricultural residues, forest and municipial waste, for the production of biofuels will be unavoidable if liquid fossil fuels are to be replaced by renewable and sustainable alternatives. For biological conversion of lignocellulosic biomass, pretreatment plays a central role affecting all unit operations in the process and is also an important cost deterrent to the comercial viability of the process. The key obstacles are: pretreatment selection and optimization; decreasing the cost of the enzymatic hydrolysis; maximizing the conversion of sugars (including pentoses) to ethanol; process scale-up and integration to minimize energy and water demand; characterization and evaluation of the lignin co-product; and lastly, the use of the representative and reliable data for cost estimation, and the determination of environmental and socio-economic impacts. Currently, not all pretreatments are capable of producing biomass that can be converted to sugars in high enough yield and concentration, while being economically viable. For the three main types of feedstocks, the developement of effective continuous fermentation technologies with near to 100% yields and elevated volumetric productivities is one of the main research subjects in the ethanol industry. The application of new, engineered enzyme systems for cellulose hydrolysis, the construction of inhibitor tolerant pentose fermenting strains, combined with optimized process integration promise significant improvements.