Nasib Qureshi, Stephen Hughes, and Thaddeus C. Ezeji

Abstract

This is an overview of the emerging technologies that have been developed recently or are in the process of development for ethanol (biofuel) production from agricultural residues. In this direction numerous advances have been made. Problems associated with inhibitor generation and detoxification, fermentation of both hexoses and pentoses to ethanol, and development of efficient microbial strains have partially been addressed. Simultaneous product recovery and process consolidation and/or integration will further improve the economics of production of biofuels from biomass. It is emphasized that numerous domestic and international companies have initiated their programs to commer­cialize conversion of biomass (agricultural residues) to biofuels. Separation and use of byproducts as additional sources of generating revenues can strengthen this fermentation further.

Introduction

Traditional substrates that have been used to produce biofuels such as ethanol and butanol include molasses, corn, and whey permeate. Molasses is a byproduct of the sugarcane­processing industry and contains approximately 50% sugar. This substrate (molasses) has been used to produce ethanol in countries such as Australia, Brazil, and India. Corn has been used in the United States, while whey permeate has been used in New Zealand. The use of these substrates affects the economics of biofuel production (Qureshi and Manderson 1995) and is becoming more challenging due to these subtrates’ use for food and feed. Currently about 140 billion gallons of gasoline is used in the United States annually, of which 15% can be supplemented by corn ethanol (Qureshi and Ezeji 2008). In order to reach a higher level of supplementation, lignocellulosic biomass such as corn stover, wheat straw, barley straw, switchgrass, and reed canary grass has to be used.

Biofuels from Agricultural Wastes and Byproducts Edited by Hans P. Blaschek, Thaddeus C. Ezeji and Ju rgen Scheffran 11 © 2010 U. S. Government. ISBN: 978-0-813-80252-7

Fermentation of lignocellulosic biomass to ethanol requires additional processing steps for the hydrolysis of biomass to simple sugars before these sugars can be fermented. These extra processing steps add to the overall cost of the substrate. Generally, the chemicals that are used to pretreat lignocellulosic substrates include dilute acid (H2SO4), or alkali (NaOH), and their use results in higher sugar yields when compared with pretreatments such as hot water or ammonia. These pretreatments (acid/alkali) generate products that inhibit cell growth and/ or the fermentation process or both. Another challenging problem with respect to fermentation of biomass involves the difficulty by various fermentation microbes for using pentose sugars. Lignocellulosic biomass contains up to 30% pentose sugars, which are not utilized by the traditional ethanol-producing cultures, such as Saccharomyces cerevisae. During the last two to three decades new cultures of Saccharomyces cerevisiae that can utilize both hexose and pentose sugars released from lignocellulosic hydrolysates have been developed (Hahn — Hagerdal and Pamment 2004; Sedlak and Ho 2o04; Hughes et al. 2009a, b). Although such cultures have been developed, the overall productivity and ethanol concentration that can be achieved by these strains are not optimal. This chapter describes recent progress with respect to the development of emerging technologies that have been developed to produce ethanol from lignocellulosic substrates. These challenging technologies include use of lignocellulosic biomass substrates, overcoming (at least partially) inhibitors generated during the pretreat­ment process, development of genetically improved cultures, simultaneous product recovery technologies, and process integration.

Butanol can be used in internal combustion engines. This biofuel can be produced by the fermentation route using renewable biomass (Ezeji et al. 2007a, b). Compared with ethanol, butanol is less volatile, less sensitive to water, less flammable, and has a slightly higher octane number. Its low vapor pressure facilitates its use in existing gasoline supply lines. As opposed to ethanol — producing cultures, butanol — producing cultures (Clostridium acetobutylicum or Clostridium beijerinckii) can use both hexose and pentose sugars released during hydrolysis of cellulosic biomass. During World War I and II there were plants worldwide including those in United States, the former Soviet Union (Russia), Canada, China, Japan, Australia, India, Brazil, Egypt, and Taiwan. As a result of various technological developments, attempts are being made to revive commercial production of butanol from agricultural residues. Details of this fermentation are presented in Chapter 3.