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bioweb.sungrant.org » Technical » Biofuels » Technologies » Ethanol Production » Ethanol from Cellulose Resources

Ethanol from Cellulose Resources
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Most of the fuel ethanol produced in the United States is made from corn (> 90%) (RFA, 2006). Most analysts agree that the ability to significantly increase ethanol production using corn grain is limited, and that large increases will require the use lignocellulose resources such as agricultural residues, grasses, and wood.

 

Ethanol is produced by fermenting plant carbohydrates with yeast. Plant carbohydrates are grouped as soluble sugars (such as sucrose from sugarcane), storage carbohydrates (such as starch from grains and tubers), and structural carbohydrates which make up the plant cell wall (such as cellulose, hemicellulose, and pectin). The principal carbohydrates contained in lignocellulose resources are the structural carbohydrates. These carbohydrates, along with proteins and lignin, form the complex matrix of plant cell walls that give plants structural stability and protection from the environment (Somerville, 2004).

 

Cellulose is a straight chained (linear) polymer of glucose molecules joined by ß (1-4) glycosidic bonds (figure 1) (Gardner, 1974). The multiple hydroxyl groups (OH) situated along a cellulose chain bond with the hydroxyl groups of other cellulose chains to form tight crystalline structures (microfibrils). Microfibrils have high tensile strength (resistance to being pulled apart, e.g. a rope) and are the major structural components of all plant cell walls (McCann, 2001). Cellulose is far more abundant in nature than is starch, but the high tensile strength and chemical stability of cellulose make it much more difficult to break down into glucose molecules, making the process of converting cellulose to ethanol more complex than for starch.   

 

 

 

In addition to cellulose, plant cell walls contain significant quantities of hemicellulose. Hemicellulose is a heterogeneous polymer composed of a backbone comprised principally of pentose (5 carbon) sugars (figure 2). Xylose is the principal pentose found in grasses and hardwood tree species (up to 40% of the sugars), while mannose is the major pentose in softwood tree species (Somerville, 2004). Other sugars (glucose, galactose, arabinose, or rhamnose) and organic acids (acetic, glucuronic, ferulic and p-coumaric acids) can substitute for some of the xylose and mannose sugars in the hemicellulose backbone.

 

                           

 

Producing fuel ethanol from lignocellulose resources involves four major steps - preparation of the feedstock (pretreatment and hydrolysis), fermentation of the sugars, recovery of the ethanol, and handling of the coproducts.

 
Feedstock preparation

The stability and chemical complexity of cellulose increase the difficulty of breaking it down into glucose, a situation frequently referred to as the recalcintrance of cellulose. A number of pretreatment approaches are being explored to overcome this problem. Pretreatment is the general term used to describe the processing steps preceding hydrolysis of cellulose and hemicellulose into fermentable sugars. The goal of any pretreatment technology is to alter or remove structural and compositional factors present in plant biomass that hinder the breakdown (hydrolysis) of cell wall polysaccharides (polymers of simple sugars) into the fermentable simple sugars (Mosier, 2005a, 2005b; Grohmann, 1984; Lynd, 1999; McMillan, 1994). Pretreatment methods often involve harsh conditions and non-selectively hydrolyze the polysaccharides in the plant material. Sugars that are produced during pretreatment are also subject to further degradation to form compounds that act as inhibitors to the subsequent fermentation process (Klinke, 2004; Olsson, 1996; Palmqvist, 2000; Taherzadeh, 1997). 

 

Pretreatment methods are usually either physical or chemical, although some approaches incorporate both (Hsu, 1996; McMillan, 1994). Chemical approaches use acids or bases to promote the hydrolysis of cellulose by removing the hemicellulose and/or lignin during pretreatment. The most commonly used acids and bases are sulfuric acid (H2SO4), sodium hydroxide (NaOH) or ammonia. Other chemicals used include cellulose solvents such as alkaline H2O2, ozone, organosolv (Lewis acids, FeCl3, and Al2SO4 in aqueous alcohol), glycerol, dioxane, phenol, or ethylene glycol, which disrupt the cellulose structure and promote hydrolysis (Wood, 1988).

 

Dilute sulfuric acid is commonly used as a pretreatment method, but increases cost, due to the need for reactors constructed of expensive steel materials, and results in the formation of unwanted salts that require neutralizing agents (Hinman, 1992; Lynd, 1996; Lynd, 1999;  Thompson, 1979). Liquid hot water pretreatment with pH control effectively dissolves hemicellulose and lignin, while minimizing degradation of monosaccharides without the need for costly and potentially dangerous pretreatments and neutralizing agents (Kim, 2005; Weil, 1998). Other pretreatment methods include steam explosion (Abatzoglou, 1992; Heitz, 1991; Ramos, 1992), ammonia fiber expansion (AFEX) (Gollapalli, 2002; Teymouri, 2005; Wang, 1998), and other chemical solvents (Hsu, 1996; McMillan, 1994). Few of these pretreatment processes have been fully commercialized or tested at industrial scales (Mosier, 2005a, 2005b). Scaling pretreatment processes to commercial sizes and the associated reactor design issues remains a major barrier to the commercial production of fuel ethanol from lignocellulosic biomass (Lynd, 1999; Mosier, 2005a, 2005b). A summary and evaluation of a number of pretreatment processes can be found in Lynd (1999), Mosier (2005a, 2005b), and Kim (2005).

 

Following pretreatment, plant cell wall polysaccharides are more susceptible to chemical or enzymatic hydrolysis that breaks them into monomeric (single) sugars (saccharification) that can be fermented into ethanol (Lynd, 1999). Depending on the type and effectiveness of the pretreatment method, hydrolysis takes 24-48 hours to complete (Lin, 2006; Olsson, 1996). In an effort to reduce the overall time needed to produce ethanol, the hydrolysis and fermentation processes are being combined, rather than conducted sequentially, in a process called simultaneous saccharification and fermentation (SSF) (Lynd, 1999). In SSF, the fermenting microorganism (e.g., yeast) and enzymes that hydrolyze the polysaccharides are added at the same time so that the sugars are fermented as soon as they are available in a just-in-time process. SSF reduces ethanol production time and reduces the amount of enzyme used when a separate hydrolysis step is conducted, due to the formation of fewer compounds that inhibit ethanol production (Lin, 2006).

 
Fermentation of sugars

Fermentation involves microorganisms which consume sugars as a food source. Ethanol fermentation results in four major products: additional yeast cells (cell division), ethanol, carbon dioxide, and heat. One molecule of glucose will yield, stoichiometrically, 2 molecules of ethanol plus 2 molecules of carbon dioxide (Figure 3). On a mass basis, one kilogram of glucose will theoretically produce 0.51 kilogram of ethanol and 0.49 kilogram of carbon dioxide. However, glucose consumed to generate additional yeast cells (cell mass) does not result in the production of ethanol. Most industrial fermentation processes operate at 90–95% of the theoretical yield of ethanol from glucose fed to the yeast.

 

             Ethanol Cellulose Fig 3 Fermentation of Glucose 

 

 

The fermentation process for producing fuel ethanol from cellulose is similar to that from corn grain. The fermentation media contains insoluble solids from the plant biomass, yeast, and soluble products from the pretreatment and saccharification steps. Hexoses (the six carbon sugars glucose and galactose) produced by hydrolysis of cellulose and hemicellulose can be fermented to ethanol using existing industrial strains of Saccharomyces cerevisiae.

 

The major difference between using cellulosic feedstocks and starch feedstocks lies in the existence of relatively large amounts of pentose sugars (five carbon sugars such as xylose and arabinose) contained in the hemicellulose. These sugars also need to be fermented to make the overall process economically feasible (Eggeman, 2005; Nagle, 1999). Existing strains of S. cerevisiae cannot directly ferment xylose, but can ferment xylulose through the pentose phosphate pathway (figure 4). Other yeasts and bacteria can ferment xylose or xylitol; efforts utilizing the tools of biotechnology are underway to develop industrial microorganisms capable of efficiently converting xylose to ethanol. A number of approaches are being explored.

 

 

     Ethanol Cellulose Fig 4 

 

S. cerevisiae can ferment xylulose, but lacks the necessary enzymes to convert xylose to xylulose. Early efforts to metabolically engineer S. cerevisiae to be able to ferment xylose focused on inserting a bacterial gene (xylose isomerase XI) capable of converting xylose to xylulose (Ho, 1983a, 1983b; Morgan, 1983). However, the resulting genetically modified yeast strain was not very efficient in fermenting xylose to ethanol. Later attempts focused on inserting genes obtained from yeast species that were more closely related to S. cerevisiae. Successful cloning and over-expression of the yeast xylulokinase (XK) gene in S. cerevisiae was completed in 1987 (Ho, 1989; Xue, 1990) and resulted in a strain that could effectively co-ferment both glucose and xylose. This was accomplished by inserting copies of xylose reductase (XR) and xylitol dehydrogenase (XD) genes from Pichia stipitis, and the xylulokinase (XK) gene, along with the proper promoter sequences into a plasmid which was then used to genetically transform S. cerevisiae. Improvements in this approach have resulted in the development of more than 10 different yeast strains (e.g., 1400(LNH-ST); 1400(pLNH32); 424A (LNH-ST)) that are stable and able to convert glucose and xylose to ethanol (Ho, 1999; Ho, 1998; Ho, 2000; Toon, 1997). The 424A (LNH-ST) strain is being evaluated by companies to ferment cellulosic biomass resources (Corrington, 2003; Dennison, 2003; Helle, 2004). Four other groups (one each in Sweden, Finland, Germany, and Japan) have also genetically engineered S. cerevisiae to ferment xylose using the XR and XD genes from P. stipitis (Kotter, 1990; Tantirungkij, 1993; Walfridsson, 1997). Additional efforts are underway to improve the tolerance of S. cerevisiae to inhibitory compounds found in lignocellulose or generated during processing (Liu, 2004; Olsson, 1996; Palmqvist, 2000; Zaldivar, 2001).

 

The use of microorganisms other than S. cerevisiae to ferment xylose, such as the bacteria Escherichia coli, Klebsiella oxytoca, and Zymomonas mobilis is also being explored. E. coli and K. oxytoca are able to metabolize several sugars, but are not efficient producers of ethanol. Z. mobilis is an efficient producer of ethanol, but can only utilize glucose and fructose. All three bacteria have been successfully genetically modified to produce ethanol from xylose (Dien, 2003; Zaldivar, 2001).

 

E. coli has been extensively studied, is easily cultured, and is used commercially to produce recombinant proteins. It can ferment a wide array of sugars without the need for complex growth factors or nutrients. However, it is less tolerant of harsh conditions (heat, salt, shear, acid, etc.) than yeast (Dien, 2003). Genetic modification of E. coli has resulted in the production of the strains K011 and FBR5 which have been shown to ferment sugars derived from a number of biomass resources (pine, sugarcane bagasse, corn stover, and corn hulls and germ meal) (Ingram, 1987; Asghari, 1996; Barbosa, 1992; Dien, 1997; Dien, 2000). Further effort is needed to improve the tolerance of E. coli to high levels of ethanol in order to increase ethanol yields (Gonzalez, 2003).

 

Z. mobilis has several properties that make it appealing, including a native homo-ethanol fermentation metabolism, tolerance to high ethanol concentrations (up to 12% by weight), high yield and productivity of ethanol, and is generally regarded as safe (GRAS) (Dien, 2003; Zaldivar, 2001). However, Z. mobilis can only ferment glucose and fructose to ethanol. It has been successfully modified to ferment xylose to ethanol through the incorporation of genes from E. coli (xylose isomerase, xylulose kinase, transketolase, and transaldolase), which produced a strain capable of co-fermenting glucose and xylose (Zhang, 1995). A similar strategy was employed to engineer a strain capable of fermenting arabinose to ethanol (Deanda, 1996). These initial efforts have led to the development of Z. mobilis AX101 which carries within its chromosomal DNA, all of the genes needed to ferment xylose, arabinose, and glucose (Mohagheghi, 2002; Mohagheghi, 1998). Integration of the genes into the chromosome eliminates the need to use antibiotics to maintain exogenous DNA carried on plasmids (Dien, 2003). Additional efforts are underway to improve the tolerance of Z. mobilis to acetic acid, an organic acid naturally found in lignocellulose (Mohagheghi, 2002; Tao, 2005).

 

Efforts are underway to improve the industrial performance of microorganisms to ferment sugars to ethanol. Efforts include improving the efficiency by which sugars are converted to ethanol (i.e., yields of 90% of theoretical yields are needed) and increasing the rate of productivity (i.e., the rate at which ethanol is produced to be > 1 g L-1 hr-1) (Zaldivar, 2001; Dien, 2003). Additionally, the microorganisms must tolerate high ethanol levels (> 4 wt%) to make distillation and ethanol recovery practical (Ladisch, 1979). The microorganism must also be resistant to fermentation inhibiting compounds (e.g., acetic acid, furfural, and lignin degradation compounds), and be tolerant of acid conditions (pH 4 – 5) (Palmqvist, 1999; Palmqvist, 2000).

 
Ethanol recovery

The recovery of ethanol produced in cellulosic processes is expected to use technology similar to that used in corn ethanol or sugar cane ethanol facilities. These technologies include repeated distillation (vaporization) and condensation of the ethanol-water mixtures produced during fermentation until the azeotrope point is reached (the point at which the difference between the boiling points of water and ethanol ceases to exist, which occurs when the mixture is 95.6% ethanol and 4.4% water by weight). At this point, the ethanol and water both vaporize to the same extent and cannot be further fractionated by distillation and final purification will by performed by the use of molecular sieves (zeolites which adsorb water from a vapor/gas mixture) (Al-Asheh, 2004; Kwiatkowski, 2006; Ladisch, 1979; Wankat, 1988).

 

Co-product production and handling

 

Unlike processes that produce ethanol from corn, the residual solid material produced in lignocellulose processes has little value as an animal feed as it is high in lignin while low in fiber and protein (McAloon, 2000). The material can be used to produce heat, steam, and electricity needed to run the ethanol facility, with the excess electricity sold to the electrical grid (Nagle, 1999).

 
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      Author:  Ryan E. Warner and Nathan S. Mosier      Reviewed:  4/2007
Last Modified: 11/12/2008
Link to Author's Manuscript
  
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