Thaddeus C. Ezeji and Hans P. Blaschek

Abstract

As the worldwide demand for fuels and chemicals surges and petroleum deposits are depleted, producers of ethanol fuel are increasingly looking beyond corn, potatoes, and other starchy crops for substrates for ethanol fuel production. Ethanol is currently the most important renewable liquid biofuel, but it has problems ranging from lesser energy content than gasoline, blending limitations with gasoline, potential for corrosion of pipes, and subsequent inability to be transported using existing pipeline infrastructure, to requir­ing modification of car engines with increasing ethanol concentrations such as E85 or 100% ethanol. Attempts are underway to produce alternative renewable liquid biofuels and chemical feedstocks that are superior to ethanol. Butanol is one such biofuel because it has greater energy content, is more miscible with diesel, is less corrosive, and has a lesser vapor pressure and flash point than does ethanol. Butanol can also be used at greater blend amounts with gasoline or even at 100% concentration in car engines with little or no engine modification, and because of its solubility characteristics, it can be transported in existing fuel pipelines and tanks. One of the major problems associated with bio-based production of butanol is the cost of substrate. The cost of substrate has led to recent interest in the production of butanol from alternative, inexpensive materials. However, much of the proposed alternative substrates, such as corn stover, corn fiber, wheat straw, rice straw, or dedicated energy crops such as switchgrass and Miscanthus, present challenges that need to be overcome before they can be used as commercial substrates for butanol production. This chapter, therefore, details the (1) pretreatment and hydrolysis of various lignocellulosic biomass; (2) generation of lignocellulosic degradation products during pretreatment of biomass; (3) effects of degradation products on growth and butanol production by fermenting microorganisms; and (4) strategies for improving lignocellulosic hydrolysates utilization for butanol production.

Biofuels from Agricultural Wastes and Byproducts Edited by Hans P. Blaschek, Thaddeus C. Ezeji and Ju rgen Scheffran 19 © 2010 Blackwell Publishing. ISBN: 978-0-813-80252-7

Introduction

Increasing energy demand worldwide coupled with a limited supply of fossil fuels and the fluctuating price of oil has generated a strong interest in the bioconversion of agricultural biomass and coproducts into fuels and chemical feedstocks. Biofuels are currently produced mainly from carbohydrates (corn, potato, sugar cane, sugar beets, etc.) and oil (soybean, rapeseed, etc.)-rich crops. Ethanol was once considered to be a replacement for gasoline, a fossil fuel currently used in car engines. A study conducted by the U. S. Department of Agriculture (USDA) in 2007 projected that the average corn price will peak at $3.75 per bushel during the 2009-2010 marketing year and will decline before stabilizing at approxi­mately $3.30 through 2016 (USDA-ERS 2007). An expansion of the ethanol and biodiesel industry has influenced the prices of corn and soybeans in the United States. In 2008, approximately 9.0 billion gallons of ethanol was produced from corn in the United States for use as a fuel supplement (Renewable Fuel Association 2008), which represents approxi­mately 4.2% of total gasoline consumption (150 billion gallons; 1 gallon ethanol = 0.7 gallons of gasoline). An increase in ethanol production from corn would require additional acreage and would potentially have an impact on land available for the production of food crops. Therefore, a further increase in ethanol production will require the use of agricultural materials not directly tied to food, especially lignocellulosic biomass such as corn stover, corn fiber, wheat straw, rice straw, or other energy crops such as switchgrass and Miscanthus. Lignocellulosic biomass, which may contain xylan, arabinan, galactan, glucuronic, acetic, ferulic, and coumaric acids, is the most abundant renewable resource on the planet (Koukiekolo et al. 2005) and has great potential as a substrate for butanol production (Ezeji et al. 2007a, b).

Butanol is a four — carbon alcohol with some very interesting attributes with respect to its use as a fuel and fuel extender that are greater than ethanol (Ezeji et al. 2004b; Ezeji and Blaschek 2007). Prior to 1950, the AB (acetone butanol) or acetone butanol ethanol (ABE) fermentation using corn and molasses as substrates, ranked second only to the ethanol fermentation in its importance and scale of production, but subsequently declined due to increasing substrate (sugars and molasses) costs and availability of much cheaper petrochemically derived butanol (Ezeji and Blaschek 2007; Schwarz et al. 2007). Substrate cost has long been recognized as having a dramatic influence on butanol price (Qureshi and Blaschek 2000). Most bacteria use glucose as a preferred carbon source for growth, and only when glucose is limiting are the pentose sugars utilized, making fermentation of complex mixture of sugars in lignocellulosic hydrolysates challenging (Ezeji et al. 2007a). The solventogenic ABE-producing clostridia have an added advantage over many other cultures in that they can utilize both hexose and pentose sugars, which are released from wood and agricultural residues upon hydrolysis to produce ABE (Ezeji et al. 2007b) .

The solvent-producing clostridia are not able to hydrolyze fiber-rich agricultural residues or lignocellulosic biomass. The lignocellulosic biomass must be pretreated and hydrolyzed to simple sugars using physical (size reduction), chemical, and enzymatic methods. Unfortunately, these treatments can result in the formation of a complex mixture of microbial inhibitors that are detrimental to the growth of fermenting microorganisms (Ezeji and Blaschek 2008a). Examples of inhibitory compounds produced include furfural, hydroxymethylfurfural (HMF), syringaldehyde, and acetic, ferulic, glucuronic, p-coumaric, syringic, levulinic acids, and so on (Zaldivar et al. 1999) Zaldivar and Ingram 1999) Varga et al. 2004) Ezeji et al. 2007b). The reduction or elimination of lignocellulosic degradation products during the pretreatment of biomass, removal of inhibitors from lignocellulosic hydrolysates prior to fermentation, adaptation of strains (to these inhibitors) via the development of inhibitor toler­ant mutants, or a combination of the above approaches have been touted to be the panacea for successful production of biofuels from lignocellulosic biomass. Among these options, the development of inhibitor-tolerant mutants via culture adaptation appears to be most viable approach from an economic standpoint. Many laboratories, including those of the authors, are currently involved in research directed toward development of inhibitor-tolerant butanol- producing microorganisms.