Three methods are possible for hydrolyzing cellulose into glucose (C6 sugar for fermentation): 1. dilute acid hydrolysis (<1% H2SO4, 215°C, 3 min with 50-70% glucose yield) which is no longer a viable candidate; 2. concentrated acid (30-70% H2SO4, 40°C, a few hours, >90% glucose yield), which has been used in Japan and will be evaluated in a DOE-funded pilot facility (Table 1); and 3. enzymatic hydrolysis (cellulase mixture, ~50°C several days, 75-95% glucose yield).

The efficient enzymatic hydrolysis of cellulose by cellulases requires a coor­dinated and synergistic action of three groups of cellulases: endoglucanase (EG, E. C. 3.2.1.4), exoglucanases like cellodextrinase (E. C. 3.2.1.74) and cellobiohy — drolase (CBH, E. C. 3.2.1.91), and в-glucosidase (BG, E. C. 3.2.1.21). EGs and CBHs act on insoluble cellulose molecules [39]. EGs randomly act internally on the amorphous regions of a cellulose polymer chain and generate oligosaccharides of various lengths and additional free ends (reducing and non-reducing ends) for CBH action. CBHs usually hydrolyze both amorphous and crystalline cellulose and cellooligosaccharide chains from the non-reducing ends in a sequential way with cellobiose as the major product, but some CBHs can hydrolyze cellulose chains from both reducing and non-reducing ends [40-42]. The hydrolysis products of these two groups of enzymes include cellodextrins, cellotriose, cellobiose, and glucose. в — glucosidases hydrolyze soluble cellodextrins and cellobiose into glucose from the non-reducing end and remove the product feedback inhibitory effect of cellobiose on EG and CBH (Fig. 5 ).

Factors impacting the activity of cellulases include enzyme source (e. g. organ­isms and producing conditions), concentration, and combinations. The normal enzyme dose for cellulose hydrolysis study is 10-60 FPU per gram of dry cellulose or glucan; glucanases to в-glucosidase ratio is approximately 1.75-2.0 IU of в — glucosidase for each FPU of glucanase used [29]. Most commercial glucanases are produced by Trichoderma resell and the в-glucosidase is typically from Aspergillus niger [43].

Under research conditions, the reported digestibility or the conversion yield of cellulose from pretreated lignocellulose can be high (Table 4). However, actual glu­cose yield may vary greatly depending on the type of biomass, method/condition of pretreatment, cellulases (composition, source, and dose), solid to liquid ratio of the hydrolysis mixture, and other unspecified factors. The cellulose digestibil­ity of corn stover and corn fiber can reach >90% following dilute acid or liquid hot water pretreatment [44], while the digestibility of rice hulls after similar pre­treatment was about 50% [45]. Similar low digestibility results were obtained on dilute acid pretreated sorghum stubble in our lab (unpublished data). The vari­able digestibility of different biomass sources following dilute acid pretreatment may be an indication that this particular pretreatment is not universally effec­tive. Currently, all the reported results for AFEX [44] and alkaline peroxide [44, 46] treated biomass sources showed consistently high cellulose recovery, and high digestibility, even at lower enzyme concentrations and shorter incubation time (48 h vs normal 96 h) [47] .

Digestibility, or glucose yield, is high when cellulose load is low (1-3% cellu­lose load) in the hydrolysis system. Glucose yield from pretreated biomass typically increases as enzyme load increases [47,48, 49], while digestibility decreases as the cellulose load increases [48, 50]. We are unaware of any reports of >20% cellu­lose load with high digestibility. Starch-based ethanol production involves starch loadings of 20-25% or higher, that results in finished beers with ethanol concen­tration around 10-12% (w/v). Most lignocellulosic ethanol fermentation studies have used hydrolysates with 3-10% cellulose load, which resulted in a finished mash with ~3-5% (V/V) ethanol. Additional research is required to improve the lignocellulose situation. Some non-cellulolytic enzymes (e. g. ferulic acid esterases and various xylanases) have been studied as pretreatment agents and showed promising results in increasing glucose yield from lignocellulose [51].

Since enzyme cost is a large contributor to the total production cost for lignocel — lulosic ethanol [30, 44], considerable research has been undertaken in attempts to increase the efficiency and reduce the cost of enzymes. Addition of protein (bovine albumen) and other additives (Tween 20 or 80, polyethylene glycerol, etc.) that reduce the affinity between cellulases and lignin all improve the efficiency of cel­lulose hydrolysis [27]. A recycling process using an ultrafiltration membrane to separate hydrolyzed glucose showed that cellulases could be re-used up to 3 times for pretreated low lignin biomass, or until ~50% of the cellulases were bound on accumulated lignin [48].

To help lower enzyme costs and possibly improve effectiveness, a research strategy has been developed to genetically-engineer biomass to express transgenic endocellulases. Microbial cellulose transgenes have been expressed in several crops: tobacco, potato, tomato, alfalfa, rice, maize, and barley [52-54]. Endoglucanase 1 (E1) concentration in some transgenic experiments has reached 1% (corn stover) [55] to 5% (rice straw) [54] of total soluble proteins. In some cases, both treated and non-treated E1 engineered biomass showed higher digestibility than biomass of their wild counterparts. Whether transgenic expression of appropriate enzymes is a viable long-term strategy when used for large-scale production remains under investigation.

3.3 Background

Henry Ford test drove his first prototype automobile called the Ford Quadracycle in July 1896 that ran on pure ethanol. He told the New York Times in 1925 that “The fuel of the future is going to come from fruit like that sumach out by the road, or from apples, weeds, sawdust — almost anything” [41]. Ethyl alcohol, or ethanol, is a two carbon, straight chain alcohol that is found in alcoholic beverages. Ethanol is a renewable, biodegradable, clean burning, alternative fuel that is usually produced by the fermentation of carbohydrates from sugar, corn, or fruits [13]. Ethanol has replaced methyl tert-butyl ether (MTBE) as an emissions reducing additive in gaso­line due to concerns of MTBE ground water contamination that arose in late 2005. Ethanol can be used in current automobiles in blends up to 10% (E10) in gasoline without any engine modifications. Higher percentages of ethanol blends (E85 and E100) can be used in Flex Fuel Vehicles (FFVs).

Sugarcane-based ethanol edges out gasoline at an oil equivalent economic price of $40 per barrel [42]. In contrast, US corn-based ethanol has an edge over gasoline when oil price is $60 or higher. “Flex-fuel” vehicles are designed to run on ethanol, gasoline, or a mixture of the two. Ethanol is made through the fermentation of sug­ars, and sugar cane offers particular advantages. The energetic balance in ethanol production shows that for each unit of energy invested, sugar cane based ethanol yields eight times as much energy as corn [43]. Unlike corn-based fuels, sugarcane requires no fossil fuels to process. Cellulosic ethanol, derived from a range of crops, such as switchgrass and crop waste, is more economical than corn ethanol because it requires far less energy to produce. However, the economics of corn or cellu — losic ethanol has been discussed widely in many articles. A central argument is that corn-based ethanol is literally a waste of energy. Detractors say that it takes more energy to grow the corn, process it, and convert it to ethanol than would be saved by using it. According to Pimentel and Pazek [44] “Ethanol production using corn grain required 29% more fossil energy than the ethanol fuel produced.” Wang et al. dispute this and state that it takes 0.74 BTU of fossil fuel to create 1 BTU of ethanol fuel, compared with a ratio of 1.23 BTUs to 1 BTU for gasoline or 66% more than ethanol [45]. The conclusions of Wang et al. have largely been corroborated by Farell et al. [46]. According to them, “current corn ethanol technologies are much less petroleum-intensive than gasoline but have greenhouse gas emissions similar to those of gasoline.” The authors however opined that cellulosic ethanol would be key to large-scale use of ethanol as a fuel. Hammerschlag compared data from ten dif­ferent studies and used a parameter, rE, defined as the total product energy divided by nonrenewable energy input to its manufacture [47]. Thus, rE > 1 indicates that the ethanol has captured some renewable energy. The corn ethanol studies showed rE in the range 0.84 < rE < 1.65, and three of the cellulosic ethanol studies indicated a range of 4.40 < rE < 6.61.

Because ethanol is made from crops that absorb carbon dioxide, it generally helps reduce greenhouse emissions. Although it is carbon neutral and renewable, the GHG impact depends on farming practices, particularly the use of fertilizers. This is specifically true for ethanol made from corn. When ethanol is made from cellulosic sources there is considerable reduction in GHGs [48]. This is because producers of cellulosic ethanol burn lignin to heat the plant sugars whereas most producers of corn ethanol burn fossil fuels to provide the energy for fermentation. Cellulosic ethanol is a renewable, biodegradable, clean burning, alternative fuel. Cellulosic biomass typically contains 40-50% cellulose, 20-30% hemicellulose, and the remainder, 15-30%, is lignin and other components [49]. Cellulose consists of glucose monomers linked by a в-1,4 bond which forms a linear polymer [50]. Hemicellulose is a highly-branched complex polymer that is composed mainly of xylose and other five-carbon sugars [50]. Lignin is a phenyl propane polymer that acts as a binder, which cannot be converted into useful products. The hemicellulose is randomly acetylated and acts as an interface between the cellulose and lignin. The cellulose and hemicellulose can be broken down into simple sugars that are used to produce ethanol, while the lignin can be burned to produce heat, which helps to increase overall efficiency. What makes cellulosic ethanol promising is the diverse, abundant, low cost feedstock that is readily available. There are two main methods for the production of ethanol from biomass; enzymatic saccharification and fermentation, and fermentation by cellulolytic microorganisms.

However, cellulosic ethanol is not without its challenges and drawbacks. Commercial production of cellulosic ethanol currently requires high initial capital costs and involves risk. In 2002, a DOE study determined that for cellulosic ethanol to be competitive, the production cost would need to be $1.07 per gallon or less [51]. One of the most expensive steps in the production of cellulosic ethanol involves the pretreatment of biomass.

Xiaorong Wu, James McLaren, Ron Madl, and Donghai Wang

Abstract Biomass feedstock, which is mainly lignocellulose, has considerable potential to contribute to the future production of biofuels and to the mitigation of carbon dioxide emissions. Several challenges exist in the production, harvest­ing, and conversion aspects of lignocellulose, and these must be resolved in order to reach economic viability. A broad array of research projects are underway to address the technical hurdles, however, additional research may be required to reach commercial sustainability. Gasification and enzymatic hydrolysis are the main technologies being investigated for the conversion of lignocellulosic biomass into material for the production of biofuels. While each approach has pros and cons, both are being explored to determine the optimum potential commercial method for particular feedstock situations, and to better understand the requirements for the massive scale required to contribute to biofuel volume.

Keywords Lignocellulosic biomass ■ Biofuels ■ Syngas ■ Enzymatic hydrolysis ■ Pretreatment ■ Fermentation ■ Gasification

1 Introduction

As the world population increases from the current 6.7 billion to over 8 billion by 2030 [1], and supporting economic growth expands, energy consumption is pro­jected to increase by 42% to 695 quadrillion (1015) British thermal units (Btu, 1 Btu = 1055 joule) in 2030 [2]. Most of the required energy will still be acquired from fossil fuels, with around 6% being from nuclear sources and about 8% from other renewable energy sources. Carbon dioxide (CO2) emission from such widespread industrial consumption of fossil fuels (coal, oil, and natural gas) is

D. Wang (B)

Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA e-mail: dwang@ksu. edu

O. V. Singh, S. P. Harvey (eds.), Sustainable Biotechnology,

DOI 10.1007/978-90-481-3295-9_2, © Springer Science+Business Media B. V. 2010 likely to continue to be a major contributor to anthropogenic greenhouse gases [3, 4]. Mitigation of CO2-based contributions to the global warming process requires specific actions, including capture and sequestration of CO2 during the consump­tion of fossil fuels and expanded utilization of carbon-neutral and carbon negative renewable energy sources (wind, solar, nuclear, geothermal, and various biomass sources) [3-6]. Most of the types of renewable energy (wind, solar, etc.) can be uti­lized to generate electricity, but not liquid transport fuels. Consequently, biomass has received much attention as a feedstock for biofuels, both in the existing com­mercial industry (e. g. ethanol from grains or sugar) and in the research realm where lignocellulose is the current focal feedstock material [7-11]. To avoid confusion, we adapt the common definition for biomass and biofuels as follows:

• Biomass: Organic, non-fossil material of biological origin (plant parts including grains, tubers, stems/leaves, roots/tubers, agricultural residues, forest residues, animal residues, and municipal wastes arising from biological sources) poten­tially constituting a renewable energy source (basically originating from primary capture of solar energy).

• Lignocellulosic biomass: Organic material derived from biological origin which has a relatively high content of lignin, hemicellulose, cellulose, and pectin com­bined into a molecular matrix with a relatively low content of monosaccharides, starch, protein, or oils. Typically refers to plant structural material with high cell wall content. Sometimes referred to as “cellulosic” biomass, which is techni­cally inaccurate, but is (mis)used due to the typical 40%+ cellulose content in lignocellulose.

• Biofuels: Liquid fuels and blending components produced from biomass (plant) feedstocks, used primarily for transportation. Technically, biogas (e. g. methane from anaerobic digestion of biological residues) is a “biofuel” but tends to be utilized in stationary combustion units and is typically referred to separately as biogas.

Survey reports suggest that the annual world biomass yield contains sufficient inherent energy to contribute 20-100% of the world’s total annual energy con­sumption of 500 EJ (1 EJ = 1 x 1018 Joule), with annual and regional variations [4, 10, 12]. Currently, commercial biofuels are generated from harvestable compo­nents of known crops (starch, sucrose, and oils), while a relatively small amount of the lignocellulosic biomass is used for combustion (cooking/heating fires or co­firing to create steam for electricity generation). The large potential of lignocellulose as an energy feedstock remains to be utilized, and is dependent on the development of economic, sustainable production, and processing systems [11].

Two platforms have been set up to transform the energy in lignocellulosic biomass into liquid fuels or chemicals: the sugar platform and thermochemical platform. In the sugar platform, the lignocellulosic material is first pre-treated to facilitate separation into the major components, then the polymeric celluloses and hemicelluloses are enzymatically hydrolyzed into sugars (hexoses and pentoses), after which these sugars can be fermented into biofuels or converted into other valuable intermediate chemicals. The residual lignin may be utilized as a specialty intermediate or, more commonly, is combusted for heat or power. In the thermo­chemical platform, biomass is degraded into small gas molecules (hydrogen, carbon monoxide, carbon dioxide, methane, etc.) under high temperature and certain pres­sure conditions, then these gas molecules are converted chemically or biologically into Fischer-Tropsch (FT) liquid fuel, alcohols, or other intermediate chemicals. This chapter focuses on the processes, potential, and challenges associated with each of these platforms.

For large-scale, economically viable use of lignocellulose there will be two input streams of sugars, one from hydrolysis of pretreated cellulose (C6 sugars such as glucose) and one from the hydrolysis of pretreated hemicellulose (C5 sugars such as xylose) since the common fermentation yeast (Saccharomyces cerevisiae) can only utilize C6 sugars, an additional technology is required for lignocellulose compared to starch or sucrose based ethanol production. The fermenting process for lignocellulosic ethanol production will include either two fermentation pro­cesses (S. cerevisiae for glucose and bacteria or other yeast for pentoses) or one C5 and C6 co-fermentation process (e. g. genetically-engineered microorganisms with specifically-designed metabolic pathways). To-date, several microbial species have been engineered to ferment both glucose and pentoses, including E. coli, Zymomonas mobilis, Pichia stipitis, Thermoanaerobacterium saccharolyticum and S. cerevisiae [56-58]. While these metabolically-engineered microbes show C6 and C5 fermentation, the ethanol yields have been too low for commercial applica­tions [57]. In addition, many engineered organisms are susceptible to inhibitory compounds generated during pretreatment, and are not as tolerant to high ethanol concentration as the typical S. cerevisiae strains. Research continues to explore the possibilities for economic fermentation of both C6 and C5 sugars.

1.4 Butanol and Other Chemicals

Once hemicelluloses and celluloses in biomass feedstock have been hydrolyzed, the sugar “platform” can be utilized to generate a range of chemicals, including other fuels such as butanol [59]. Butanol has several advantages over ethanol as an alternative fuel (but not as an oxygenate) and may be a better choice for the large volume liquid transport fuel market. However, if other chemicals are produced in an ethanol plant, the final product separation process (distillation and dehydration) would be problematical. Separate down-stream production paths will be required in future biorefineries to accommodate the potential product flows, which may result in different designs and configurations [15].

Pretreatment is required to alter the physical and chemical properties of the biomass to make it easier to process. The methods of pretreatment are similar for either enzy­matic or microbial cellulosic ethanol processing. Removing or altering the lignin allows access to carbohydrates in the biomass. Higher lignin sources require chemi­cal treatment to reduce the level to below 12% to enhance digestibility [50]. To gain access to the cellulose fiber, de-crystallization of the hemicellulose that is cova­lently bound with the lignin via hydrolysis is required [52]. The conversion of all the sugars derived from hemicellulose is highly desired to increase efficiency and minimize by-products. Pretreatment of the biomass is also required to increase the surface area and pore size, thus making it easier to digest. The increase in surface area is from the combination of hemicellulose solubilization, lignin solubilization, and lignin redistribution caused by various methods of pretreatment [53].

There are several methods by which pretreatment is performed: physical, chem­ical, and biological. Physical methods include ball and compression milling that shear or shed the biomass to de-crystallize the cellulose and increase the surface area and digestibility. However, these processes do very little to degrade hemicellulose and lignin polymers. Milling also requires long processing times with high capital and operating costs, thus it is not economical and has not been pursued in scale-up operations [50, 54]. Radiation pretreatment utilizes gamma rays, electron beams, or microwaves to react to weaken and break the chemical bonds between hemicellu — lose and lignin through chemical reactions such as chain scission [55]. However, the high consumption of energy and capital costs makes this process economically unviable.

Dilute-acid pretreatment is a chemical process that increases the solubility of hemicellulose to 80-100%, extensively redistributes the lignin, and depolymerizes some of the cellulose [53]. The process soaks the biomass in a dilute solution of sulfuric, hydrochloric, or nitric acid and then raises the temperature by injecting steam to enhance the pretreatment method [50]. Autohydrolysis generates acids by the introduction of saturated steams into the biomass to breakdown the hemicellu — lose and lignin [50]. The pressure is rapidly released resulting in the breakup of the biomass due to the instant vaporization of the trapped water. This process is known as steam explosion pretreatment and results in 80-100% solubilization into a mix­ture of monomers and oligomers of hemicellulose. It also redistributes the lignin, and depolymerizes some of the cellulose [53]. Similar to steam explosion, ammonia fiber explosion pretreatment (AFEX) uses high temperature and pressure ammonia to de-crystallize cellulose, and increase the solubility of lignin by 10-20%, and of hemicellulose up to 60% while hydrolyzing about 90% to oligomers [53].

Other chemical pretreatment methods include “hydrothermal” processes using liquid hot water, supercritical carbon dioxide, “organosolv” processes that involve organic solvents in an aqueous medium, concentrated phosphoric or peracetic acid treatment, and strong alkali processes using sodium hydroxide or lime [50, 53]. A biological pretreatment process utilizes fungi, such as white rot, brown rot, and soft rot, to hydrolyze the cellulose component of biomass. Filamentous fungi, typically Trichoderma and Penicillium species, can be used directly for cellulose hydroly­sis because of the greater capacity for extracellular protein production than that of cellulolytic bacteria [56]. However, it requires a three-fold reduction in cost for com­mercialization and the reaction rates for the hydrolysis of cellulose are relatively low in comparison to chemical pretreatment methods [56].

Enzymatic saccharification utilizes enzyme blends for recovering carbohydrates from the hydrolyzate generated after pretreatment [51]. Commonly, cellulase and hemicellulase enzymes are used as a “cocktail” with other enzymes to enhance yields and reduce enzyme costs. The products of enzymatic saccharification — the process of breaking a complex carbohydrate into its monosaccharide components — severely inhibit cellulases and hemicellulases [57]. To overcome this difficulty, Simultaneous Saccharification and Fermentation (SSF) of the pretreated hydrolyzate is preferred. Once the structure of the biomass is disrupted, the cellulose and hemicellulose is enzymatically converted to sugars by the saccharification process. During the fermentation process, yeasts such as Saccharomyces cerevisiae, con­vert the sugars to ethanol. The advantage of SSF over Separate Hydrolysis and Fermentation (SHF) is higher yields of ethanol but SSF requires more than dou­ble the fermentation time [58]. However, the hydrolyzate also contains acetic acid and other toxic compounds. Together with increasing ethanol concentrations, this can inhibit the enzymes and fermentation organisms, thus lowering yields. New developments in enzymatic saccharification and fermentation have been developed by Iogen Energy Corporation and the NREL to develop effective “cocktails” of enzymes along with modified strains of yeast that can break down complex sugar molecules, which conventional fermentation yeast cannot.

Recently, Royal Dutch Shell (Shell) announced a partnership with Iogen Energy Corporation to advance cellulosic ethanol from agriculture residues such as cereal straw and corn cobs and stalks. And just recently, Iogen Energy shipped 100,000 L (26,417 gal) to Shell, which is the first installment of the initial order of 180,000 L (47,550 gal) of cellulosic ethanol. Iogen’s demonstration facility located in Ottawa, which first began producing cellulosic ethanol in 2004, is being purchased by Shell for use in upcoming fuel applications [59].

Cellulolytic microorganisms, an alternative to yeast, utilize ethanol fermenting microbes that both hydrolyze and ferment the sugars into ethanol from a milder pretreatment process. Gram-negative bacteria, such as Escherichia coli, Klebsiella oxytoca, and Zymomonas mobilis, are being investigated as potential microor­ganisms for industrial production of ethanol [52]. Using genetic and metabolic engineering, NREL has developed a strain of Z. mobilis (Zymo) that can break down complex sugars like xylose, and tolerate higher concentrations of acetic acid [51]. Other studies have shown that the Z. mobilis strain can produce theoretical yields up to 95% and handle a wider range of feedstocks [52]. High technological costs have impeded the widespread production of cellulosic ethanol by microor­ganisms. Consolidated bio-processing or CBP has been developed to address this problem. This process utilizes cellulolytic microorganisms to perform the hydroly­sis of biomass and the fermentation of sugars into ethanol within a single process, which is a large cost reducing strategy [53]. CBP is expected to reduce overall production costs by eight-fold compared to SSF under similar conditions.

Mascoma Corporation has dedicated their research team to focus on the com­mercialization of CBP, which is seen as the lowest cost configuration for cellulosic ethanol. Mascoma Corporation is in the process of developing a cellulosic fuel pro­duction facility that will use non-food biomass to convert woodchips into fuel. They are predicting that the new facility will produce 40 million gallons of ethanol and other valuable fuel products per year [60].

3.4 Summary

Cellulosic ethanol is ethyl alcohol produced from wood, grass, or the non-edible parts of plants, and is a sustainable and renewable biofuel that is biodegradable. The promising features of cellulosic ethanol are the diverse and abundant feedstock that can utilize existing waste by-products. Iogen Energy Corporation is currently producing cellulosic ethanol for Shell using enzymatic saccharification and fermen­tation in a small-scale commercial facility. Another approach to cellulosic ethanol is via the use of cellulolytic microorganisms. As commercialization of cellulosic ethanol expands, it can be used to increase ethanol production without causing food shortages or demands, and will reduce greenhouse gas emissions and our dependence on fossil fuels.

In recent years, fuel ethanol production has been revived for use in gasoline trans­port fuel markets. The main driver for fuel ethanol expansion use has been the need for a gasoline oxygenate, following the issues that were uncovered concern­ing the previous widespread petroleum industry oxygenate, methyl tert-butyl ether (MTBE). Ethanol is biologically safer, biodegradeable, renewable, and carries 88% more oxygen than MTBE (especially useful in the higher compression modern gaso­line engines). A secondary, but nonetheless important, driver for ethanol expansion has been to reduce dependence on foreign oil for those countries that import large volumes of crude oil. The success of ethanol to-date has relied on the harvested por­tions of mainstream agricultural crops, where modern-technology yield increases have allowed increasing harvest volumes [17].

The global production of crop-based renewable ethanol is projected at around 20 billion gallons (77 B liters) for 2008. Figure 1 shows the breakdown by country and main feedstock. In Brazil, fuel ethanol displaces ~20-50% of the transporta­tion petroleum gasoline, with the volume depending on the world price of sugar. Projections are for additional areas to be planted with sugarcane to meet the demand for sugar and fuel, and there are plans to utilize more biotechnology to increase

sugarcane yields by over 10% [2]. The fuel ethanol industry in the USA has grown rapidly since 2000, with over 95% of the ethanol being blended into gasoline as an oxygenate (called E10). Current 2008 production is uncertain due to the volatile economy and sharp commodity fluctuations; however, we project the final volume to be around 9.6 billion gallons (equal to about 7% of the US gasoline volume). The majority of the feedstock for US ethanol is corn (maize) grain, with a small amount (~4%) being generated from sorghum. Unlike sugarcane, which cannot be stored and for which the mills must close for several months each year, grains are easily stored for over a year and can be managed and transported in the existing infras­tructure. Another advantage of grains is that only the starch is consumed in ethanol fermentation. The protein and oil are carried through in the distillers grains (DG) and are available to go back into the livestock feed system. Nevertheless, there will be an upper limit on the land and farm resources that can be used for grain-based ethanol before impacting other commodity food markets (e. g. today the amount of grain exported from the US is about the same as that used for ethanol). Some analysts suggest that there is an impact today, others project that the maximum amount of corn that can be used for ethanol production is approximately 25-30% of the annual corn production [12]. We estimate that the upper limit will depend on how fast the expected biotechnology-driven yield increase is achieved [11, 17]. For example, we can calculate the mathematical outcome for various scenarios:

• Yield is somehow frozen today at 12 B bushels grain. E10 (oxygenate additive value) used in all US gasoline would require 15 B gal ethanol = 5 B bu grain. This would require 41% of the current corn harvest. However, 30% of that goes back into the feed system as DG so the net utilization is 29% of the available corn grain.

• Yields are projected to continue to increase due to various new technologies, with some industry experts projecting 300 bu/acre in 10-15 years: this would generate 24 B bu grain. Again assuming E10 use at 15B gal ethanol = 5 B bu grain, this would result in only 20% of the crop harvest being taken in. Accounting for the DG return, the net corn grain use would be 14%.

In reality, there are many factors which will impact the final scenario. Irrespective of the exact scenario, it seems that corn grain can provide for existing market demands plus enough grain for future oxygenate use (e. g. E10). While this is an excellent contribution, it does not meet requirements for majority replacement of gasoline volume. Obviously, to achieve further energy independence and fur­ther reduce import of foreign oil, additional renewable feedstocks are required to contribute to the total liquid fuel demand.

The main component remaining in the solid residues following cellulose and hemi — cellulose hydrolysis to sugars is lignin (15-20% of the biomass feedstock) which has a heating value just slightly less than coal (~25 GJ/ton vs ~28 GJ/ton for coal). Therefore, lignin could be used as feedstock for co-firing, or gasification, in an integrated biorefinery to generate heat and electricity. Lignin, and associ­ated phenolic compounds, can also be used as chemical intermediates, however, this market volume is probably limited. The main utilization will probably be for heat and electricity: both for internal use in the biorefinery and perhaps to generate surplus electricity that could be sold back to the grid, further capturing the economic benefit [60].

Research on renewable and environmentally sustainable fuels has received a lot of impetus in recent years. With oil at high prices, alternative renewable energy has become very attractive. Many of these technologies are eco-friendly. Besides ethanol, other environmentally sustainable fuels include biodiesel and biobutanol.

A recent United Nations report urges governments to beware the human and environmental impacts of switching to energy derived from plants. There should a healthy debate about turning food crops or animal feed into fuel and the conse­quences of the switch to biofuels needs to be carefully thought out. Thus the focus of biofuel production needs to be on non-edible and waste sources. In the case of biodiesel, these include restaurant grease, non-edible sources like Jatropha as well as microalgae. Biobutanol is a renewable, biodegradable, alternative fuel, which can be used neat or blended with gasoline. Properties such as energy density, octane value, and Reid vapor pressure (RVP) are similar to gasoline; hence current vehicles can use biobutanol without any engine modifications. Biobutanol can be produced from biomass by the fermentation of sugars and starches or by thermochemical routes using gasification.

Ethanol is made through the fermentation of sugars, and sugar cane offers many advantages. Unlike corn-based fuels, sugarcane requires no fossil fuels to process. Cellulosic ethanol, derived from a range of crops, such as switchgrass and crop waste, is more economical than corn ethanol because it requires far less energy. While there is no single magic bullet that can completely replace our dependence of petroleum, the focus needs to shift on fuels that can not only alleviate our dependence on petroleum but are also renewable and environmentally sustainable.

A 2005 USDA and DOE joint report [12] showed that a combination of crops, agri­cultural residues, trees, forest residues, and bringing conservation reserve land into production could generate up to 1.3 billion dry tons of biomass each year. Given
the assumptions regarding a viable conversion process, the energy inherent in this biomass could produce enough biofuels to replace 30-50% of the annual transporta­tion gasoline in US. Thus, biomass represents considerable potential as a feedstock for biofuels, which is reflected in the Renewable Fuel Standard (RFS) contained in the Energy Independence and Security Act of 2007 [18]. Specific targets are man­dated for lignocellulosic-derived ethanol in the RFS: the initial goal is 0.1 billion gallons by 2010, with increasing milestone targets that reach 16 billion gallons by 2020. The RFS also calls for 15 billion gallons of ethanol from grain, and the mandate then caps that volume from 2015 onwards [2]. Thus, corn and lignocel — lulosic ethanol plants will coexist and since there are common processes on the back-end, it is possible that integrated biorefineries (Fig. 2) may emerge to handle both starch and lignocellulosic feedstocks. The integration of cellulosic and tra­ditional dry grind ethanol plants may reduce the per gallon capital investment of lignocellulosic plants, will certainly smooth the risk of lignocellulosic ethanol, and may also improve ethanol yield on a per acre basis [19, 20].

Besides fuel ethanol or butanol, many other chemicals and value-added products may be produced from lignocellulosic biomass. Once the technologies for biore­fineries are established and commercialized, a wide range of chemicals (e. g. olefins, plastics, solvents, many chemical intermediates) and biofuels (e. g. biogasoline, alcohols, biodiesel, JP-8, and FT liquids) could be produced from lignocellulosic biomass.

Chemicals

Food products

Animal feed

Lignin residue

Heat & power

Fig. 2 Possible integration of different biorefineries

1.5 Current Technology and Commercialization

For over 20 years, a considerable research effort has been made to overcome the technical and economic barriers that currently limit the use of lignocellulosic biomass. Most recently, the DOE has funded the development of several lignocel — lulosic biofuel facilities that will help further define the parameters for potential success. Some aspects of possible systems, such as concentrated acid hydrolysis, dilute acid and steam explosion pretreatment, are relatively well understood at the research level and will benefit from pilot-scale testing. Other aspects, such as fer­mentation inhibitors and fermentation of C5/C6 sugars, require further research to create sufficient improvements for commercial testing. Some technologies, such as biomass gasification, syngas conversion to biofuels by either fermentation or FT process, have been tested at a pilot scale and are ready for further scale-up and integration testing. This is a crucial period of time for lignocellulosic biofuel development: success with the current pilot scale operations will drive the required investment for commercial scale, while poor results in the next 2-3 years may place a prohibitive restriction on future investment.