Biological Processing of Cellulosic Biomass to Ethanol

Biological conversion of cellulosic biomass to ethanol is summarized here to provide a context for understanding some of the key features of agricultural residues and their impact on producing fuels, with more details available elsewhere (Valkanas et al. 1977; Fan et al. 1981; Gould 1984; Gusakov et al. 1992; Saddler 1993; Asghari et al. 1996; Baghaei-Yazdi et al. 1996; Belkacemi et al. 1997; Walsh et al. 1998). Typically, cellulosic biomass is com­posed of about 40%-50% cellulose, 20%-30% hemicellulose, 10%-25% lignin, and lesser amounts of minerals, oils, free sugars, starches, and other compounds (Wyman 1996). Enzymes or acids can catalyze the reaction of hemicellulose with water to release sugars, typically arabinose, galactose, glucose, mannose, and xylose, for fermentation to ethanol or other products. Enzymes or acids can also hydrolyze cellulose into glucose in a similar fashion. However, although dilute sulfuric acid can recover sugars from hemicellulose with high yields, yields from dilute acid hydrolysis of cellulose are much lower because higher temperatures are needed to overcome its high crystallinity, resulting in glucose degradation (Grohmann et al. 1985; McMillan 1994; Hsu 1996). Because of their high selectivity, enzymes realize the high yields important to competiveness (Wyman 2007). Concentrated acids can also realize high yields, but costs to recover the high loadings of acid needed are high (Hsu 1996; Yang and Wyman 2008).

Because enzymes cannot penetrate the complex structure of most types of biomass well, a pretreatment step is essential to high yields, and one approach is to employ dilute sulfuric acid to remove much of the hemicellulose and open up the structure for effective sugar release from cellulose by enzymes. In this case, temperatures of about 140-180°C can be employed at acid concentrations of about 0.5%-2.0% and residence times on the order of 10-30 minutes to recover about 80%-90% of the sugars in hemicellulose during pretreatment plus some of the glucose from cellulose. The resulting insoluble solids are enriched in cellulose and lignin, and a small fraction can be used to support growth of the fungus Trichoderma reesei or other aerobic organism that produces enzymes known as cellulase to depolymerize cellulose into glucose and hemicellulases to break down hemicellulose not removed by pretreatment into its component sugars. These enzymes are then added to the pretreated solids to release most of the sugars left in the solids, and an organism can be added to the same vessel to ferment the sugars released to ethanol in a configuration known as the simultaneous saccharification and fermentation (SSF) process, an approach that enhances rates, yields, and concentrations by reducing inhibition by the sugars released and also lowers containment costs (Spindler et al. 1991; Wyman et al. 1992; Katzen and Fowler 1994; Ingram and Zhou 2002) . Sugars released in pretreatment, mostly from hemicellulose, are fermented to ethanol with an organ­ism that has been genetically modified to achieve high yields from the five carbon sugars arabinose and xylose that native organisms could not effectively ferment to ethanol (Ho and Tsao 1995; Zhang et al. 1995; Ingram et al. 1997). The hemicellulose sugar stream could also be left with the pretreated solids and fermented to ethanol in the same vessel in a con­figuration known as simultaneous saccharification and co-fermentation (SSCF). The broth from fermentation is sent to a distillation and dehydration system to remove the ethanol while leaving the unconverted solids (mostly lignin), water, and other leftovers in the column bottoms. Contrary to many incorrect statements, water is not removed from the ethanol, and ethanol recovery is not very energy intensive in a well-engineered process. Rather, we can take advantage of the high volatility of ethanol compared to water to remove high-purity ethanol from a messy fermentation broth and concentrate and burn the residues left in the water to provide more than enough energy to meet all the heat and power needs for the process with significant amounts of electricity left over to export (Wooley et al. 1999a ; Wyman 1999b, 2007).

The pathway outlined above represents the configuration often considered currently. However, as will be discussed later, enzymes are very expensive and are the major showstopper to commercialization. In particular, enzymes suffer from two limitations: (1) they are expensive to make due to the aerobic fermentations used and (2) large doses of enzyme are required to produce sugars with adequate yields (Lynd et al. 1996, 2002; Aden et al. 2002) . An alternate configuration is to employ a single organism or consor­tium of organisms that can both make enzymes and ferment the sugars they produce to ethanol. Several advantages result from this approach. First, less equipment is needed, and fewer transfers are required. In addition, the fermentative organisms are anaerobes, thereby avoiding the high power requirements for making enzymes using fungal systems such as T. reesei. This consolidated bioprocessing (CBP) approach could thus lower costs by reducing capital investments and energy costs. However, although native fermentative organisms can produce enzymes anaerobically, they have a low selectivity for ethanol, resulting in too low yields, and several groups are now working to eliminate competitive pathways so that high yields of ethanol are achieved (Lynd et al. 2005). In addition, these organisms must be hardened to withstand an industrial environment and realize high ethanol concentrations.

Many other pretreatment approaches have been trialed over the years other than dilute acid, and a few such as ammonia fiber expansion (AFEX), controlled pH, ammonia recycle percolation (ARP), lime, and sulfur dioxide technologies can be effective (Mosier et al. 2005b). Those at low pH such as use of sulfur dioxide remove hemicelluloses in the same manner as outlined above, with the primary difference being in the ability to recover and recycle the sulfur dioxide (Schell et al. 1991) . Such low pH pretreatments also produce predominately monomeric sugars that many organisms can ferment to ethanol (Wyman et al. 2007). Use of only water or addition of buffers to maintain the pH nearer to neutral will preserve most of the sugars as short chains that dissolve in water, with the goal of reducing their degradation (Mosier et al. 2005a) . On the other hand, the overall yields of sugars and oligomers are somewhat lower than for dilute acids, and either organisms must be used that can ferment the soluble oligomer chains or additional steps are needed to break them down into fermentable monomers (Eggeman and Elander 2005). Pretreatment through addition of a base such as lime or sodium hydroxide opens up the cellulose to enzymes by removing lignin but can take longer to react or cost more (McMillan 1994) . Ammonia can also be employed to lower pH, but its release for recovery and recycle when the pressure is dropped following pretreatment results in no visible removal of lignin or hemicelluloses (Dale et al. 1996). Yet the resulting solids can be highly susceptible to sugar release by enzymes, and the product stream does not form strong inhibitors of fer­mentation or enzymatic hydrolysis (Wyman et al. 2007). Much more experience has been developed with dilute sulfuric acid than for the other options because of historical limits in funding for such research, but the others can have important advantages and synergies that should be explored.

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