Cellulosic Ethanol as a Fuel Source: A Literature Review
Brittany Hoefener
Abstract
Research has been done on cellulosic ethanol since the oil crisis of the 1970’s. Two dominant methods have evolved for obtaining ethanol from lignocellulosic material over the past three decades: separate hydrolysis and fermentation (SHF), as well as simultaneous saccharification and fermentation (SSF). There are a variety of materials which can be used in these processes that are abundant which include corn stover, switchgrass, hardwoods, softwoods, rice straw, and municipal solid waste. To acquire ethanol from this lignocellulosic material, the lignin barrier must be broken by a pretreatment process. Of the various pretreatment processes, steam-explosion and microwave/alkali have been found beneficial. An enzymatic hydrolysis as well as fermentation processes must also be achieved (either in two steps or simultaneously). There are many pros and cons to each ethanol production method. Technological advances have improved efficiencies of these processes, making it more economically feasible of an alternative to the use of fossil fuels.
Introduction
Two dominant methods of producing ethanol from cellulosic materials exist; separate hydrolysis and fermentation (SHF), as well as simultaneous saccharification and fermentation (SSF). Both methods require a pretreatment in order to disrupt plant cell wall structure to make the cellulose more accessible to hydrolysis. SHF is a multistep process. The pretreated biomass is converted from residual cellulose and hemicelluloses to monomeric sugars using enzymatic hydrolysis. These monomeric sugars are fermented to ethanol with Saccharomyces cerevisiae yeast (Ohgren, 007). SSF carries out enzymatic hydrolysis and fermentation simultaneously, also using Saccharomyces cerevisiae as the fermenting agent (Ohgren, 2006). There are several technological and economic challenges associated with the production of ethanol from cellulosic materials that will be discussed, including; pretreatment, saccharification and fermentation of a mixed stream that contain materials toxic to ethanol producing microorganisms (Gray, 007). SSF and SHF will be compared as methods for making ethanol from lignocellulosic material.
Background
Energy consumption has been increasing steadily with increasing populations, and decreasing poverty. A current worldwide consumption rate of crude oil is approximately 8 million barrels per day (Gray, 007). Ethanol is currently the most widely used biofuel. It is presently produced by the fermentation of sugars derived from various food crops including corn, cereal grains, sugar cane and sugar beet (Gray, 007). Corn ethanol provides between 1. and 1.7 times the energy used to produce it, while cellulosic ethanol provides between 4.4 and 6.1 times the energy used to produce it according to the U.S. Department of Energy. The United States was the largest producer of ethanol in 2006, surpassing Brazil’s ethanol production with approximately 5 billion gallons. This ethanol uses about 18% of the total U.S. corn crop for the production of biofuels, accounting for merely % of U.S. gasoline consumption (Gray, 007). Lignocellulosic ethanol has been researched as a possible fuel source since the 1970’s. Cellulosic materials consist of cellulose, hemicelluloses and lignin, therefore they are sometimes referred to as lignocellulosic biomass (Demirbas, 00 ). Researchers have found it to be more difficult to produce ethanol from lignocelluloses than starches and sugars. The difficulty of the hydrolysis of lignocellulosic materials is due to the presence of lignin. In order to convert the hemicelluloses and cellulose into monomeric sugars, pretreatment must be done, unlike with the conversion of sugars and starches.....continued in print edition.
