Energy for Sustainable Development 13 (2009) 174–182
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Energy for Sustainable Development
Cellulosic ethanol production in the United States: Conversion technologies, currentproduction status, economics, and emerging developments
Puneet Dwivedi a,⁎, Janaki R.R. Alavalapati b, Pankaj Lal a
a Room # 374, Newins Ziegler Hall, School of Forest Resources and Conservation, University of Florida, Gainesville, Florida (32611-0410), USAb 313 Cheatham Hall, The Department of Forest Resources and Environmental Conservation, College of Natural Resources, Virginia Polytechnic Institute and State University, Blacksburg,Virginia (24061), USA
Article history:Received 22 June 2009Accepted 22 June 2009
Keywords:Cellulosic ethanolConversion technologiesCurrent productionEconomicsFuture development
Details of existing conversion technologies for cellulosic ethanol production, both hydrolysis andthermochemical, have been discussed along with their present adoption status. Furthermore, economics ofethanol production by using different conversion technologies has been discussed. Emerging conversiontechnologies and other developments which might affect the cellulosic ethanol production are alsocharacterized. Based on current estimates, it was found that about 400 million gallons of cellulosic ethanolwill be produced in the country in coming years using different conversion technologies. It was noticed thatout of several available conversion technologies, thermochemical-based technologies are gaining popularityand it is projected that the use of these conversion technologies will reduce the cellulosic ethanol productioncost significantly. Similarly, recent advancements in hydrolysis-based technologies have also helped inreducing the production cost of cellulosic ethanol. However, more resources will be needed in coming yearsto meet the policy goal of producing 21 billion gallons of cellulosic ethanol by the year 2022. It is expectedthat this review will be helpful in efficient allocation of resources for facilitating future technologydevelopment and in streamlining the whole initiative of cellulosic ethanol production in the United States.
The United States (U.S.) is the largest consumer of petroleumproducts in the world and is dependent on imports for meeting thisdemand. The U.S. consumed about 20.7 million barrels/day ofpetroleum products in 2007 out of which about 58% i.e., 12 millionbarrels/day was imported (EIA, 2008a). It is predicted that thegasoline consumption will further rise along with the rising popula-tion, as it is a primary energy source for meeting non-commercialtransportation demand (EIA, 2008b). Due to increased use ofpetroleum products like gasoline, the amount of greenhouse gasesreleased into the atmosphere has also shown a rising trend (EIA,2008c). It was found that the transportation sector alone emittedabout 34% of the total carbon dioxide (CO2) released into theatmosphere in 2005 i.e., 2007 million Mg (EIA, 2008d).
Because of rising energy dependency, increasing emissions ofgreenhouse gases, and risks associated with the price fluctuations inthe international energy markets, federal and various state govern-ments have started to evolve new energy strategies in which the roleof various renewable energy sources is emphasized. Out of manyrenewable energy resources (biomass, solar, wind, geothermal, tidal,etc.), biomass is given a high priority as it is the only source which can
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be directly utilized for production of various alternative transporta-tion fuels especially ethanol.
Using food crops for ethanol production raises concerns of foodsecurity (Mitchell, 2008) and environmental degradation (Pimenteland Patzek, 2005). Therefore, majority of the petroleum importingcountries (including U.S.) are interested in utilizing cellulosic biomassas a feedstock for ethanol production. As U.S. has a large cellulosicbiomass production base (Perlack et al., 2005), production of ethanolfrom cellulosic feedstock and utilizing it as a substitute for gasolinecould help in promoting rural development, reducing greenhousegases, and achieving energy independence. Therefore, the federalgovernment has announced various policy targets and incentives topromote the production of cellulosic ethanol in the country. Forinstance, the Energy Independence and Security Act of 2007 set atarget of producing 21 billion gallons of biofuels from cellulosicfeedstocks by2022. Additionally, the recentlyenacted FarmBill of 2008provides a subsidy of $1.01 on every gallon of cellulosic ethanolproduced.
Several technologies have been proposed to convert differentcellulosic feedstocks into ethanol. These technologies range fromfermentation (Lin and Tanaka, 2006) to gasification (Perkins et al.,2008). However, doubts exist among various stakeholders about thecommercial viability of existing conversion technologies (Waltz,2008; Tan et al., 2008; Ruan et al., 2008; Wright and Brown, 2007).As a result, federal and various state governments are providing
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funding support to several private companies and public institutionsto develop a suitable conversion technology using which cost ofethanol production from cellulosic feedstock can be brought downsignificantly. Already, federal government has provided a funding of$1 billion for promoting research in developing a commercial viableconversion technology for producing cellulosic ethanol (Curtis, 2008).It is expected that the successful demonstration of at least oneconversion technology on a commercial scale will help in increasingthe confidence of investors in cellulosic ethanol production and thus,will help in achieving the policy target of producing 21 billion gallonsof cellulosic ethanol by the year 2022.
In light of the importance given to the commercial viability of aconversion technology, it is essential to review the existing conversiontechnologies to ascertain their performance in terms of adoptionstatus and economics. Emerging technological alternatives should alsobe analyzed to understand the future trajectory of technologydevelopment. Such an attempt will help in creating a baseline forthe emerging conversion technologies and in guiding policy makers tostreamline funding and institutional support.
In the next section, the composition of cellulosic feedstock isbriefly discussed. In the third section, two major conversiontechnologies or base technologies that are commonly used forconverting cellulosic feedstocks into ethanol namely hydrolysis andthermochemical conversion are explained. An attempt has also beenmade to capture the existing versions of both the base technologies. Inthe fourth section, the adoption status of existing conversiontechnologies is discussed to evaluate the current status of cellulosicethanol production. In the fifth section, economics in terms of unitethanol production cost of the existing conversion technologies isdiscussed. In the sixth section, emerging trends in the technologydevelopment and alternate uses of cellulosic biomass are discussedand finally study is concluded in the seventh section.
Cellulosic feedstock composition
Cellulosic feedstock is composed of cellulose, hemi-cellulose,lignin, and solvent extractives. Lignin acts as a cementing materialand binds all other constituents together. It is also responsible forproviding structural rigidity to a cellulosic feedstock. Cellulose is apolymer of repeating β-D-glucopyranose units and is a chief constitu-ent of the feedstock. Hemi-cellulose, like cellulose, is a polysaccharidebut is less complex and easily hydrolysable. Soluble materials orextractives in the feedstock consist of those components that aresoluble in neutral organic solvents. The distribution range of differentconstituents in softwood and hardwood is explained in Table 1(Anonymous, 2008; Miller, 1999). Kuhad and Singh (1993) and Olssonand Hägerdal (1996) provide more information on constituents ofother cellulosic feedstocks.
Sugar present in the cellulose is mostly glucose. However, hemi-cellulose is a mixture of different types of sugars. It contains both C6(glucose, mannose, and galactose) and C5 (xylose, arabinose, andrhamnose) sugars. Glucose, mannose, and xylose constitute about 95–97% of the total sugars. For example, distribution of sugars in loblollypine (Pinus taeda), a commercial pine species of the southern U.S., isshown in Table 2 (Frederick et al., 2008a). It is to be noted that being abiological compound, the cellulosic feedstock comprises of three basic
elements i.e., carbon, oxygen, and hydrogen. Different combinations ofthese elements are building blocks of different feedstock componentsi.e., cellulose, hemi-cellulose, lignin, etc.
Base technologies
At present, several technologies are in use for converting cellulosicfeedstocks into ethanol. However, all these technologies can begrouped into two broad categories namely hydrolysis and thermo-chemical conversion. In hydrolysis, the polysaccharides (cellulose andhemi-cellulose) present in a feedstock are broken down to free sugarmolecules (glucose, mannose, galactose, xylose, and arabinose).1
These free sugar molecules are then fermented to produce ethanol. Aslignin cannot be used for ethanol production, it is removed during theconversion process and is generally utilized to meet electricity or heatrequirement of an ethanol mill.2 In thermochemical conversionprocess, the feedstock is gasified to produce syngas (a mixture ofcarbon monoxide, hydrogen, CO2, methane, and nitrogen) and thensyngas is either fermented or catalytically converted to obtainethanol.3 Production of ethanol through thermochemical route isindependent of the sugar quantities originally present in thefeedstock.
Details of specific technologies under each broad category ofconversion technology i.e., hydrolysis and thermochemical conversionare discussed below.
Hydrolysis technology
Hemi-cellulose and lignin present in the feedstock provide aprotective covering to the cellulose. This protective cover should bealtered for ensuring efficient hydrolysis. Therefore, special provisionsare needed to loosen the feedstock structure completely beforeundertaking cellulose hydrolysis. Fig. 1 explains the basic stepsgenerally undertaken while converting cellulosic feedstocks toethanol through hydrolysis.
During feedstock preparation, feedstock is first washed to removedust and any other impurities. Then, feedstock is chipped or milledto increase the surface area so that chemicals/enzymes used in thesubsequent steps can easily penetrate the feedstock structure. In thepretreatment process (also called first stage hydrolysis),4 hemi-cellulose is hydrolyzed into the basic sugars (xylose, mannose,arabinose, and galactose). A small amount of cellulose is also
1 This process is also known as saccharification.2 Technologies which hydrolyze the sugars to produce ethanol are also known as
sugar platform technologies.3 Gasification is a process that uses heat, pressure, and steam to convert different
materials including cellulosic biomass into syngas composed primarily of carbonmonoxide and hydrogen (Rezaiyan and Cheremisinoff, 2005).
4 Pretreatment also helps in loosening the wood structure completely. As a result,cellulose present in the wood becomes available for hydrolysis in subsequent step.
Fig. 1. Diagram of hydrolysis-based cellulosic ethanol production technologies.
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hydrolyzed to glucose during the pretreatment. The mixtureobtained at the pretreatment is separated into liquid and solid(lignin + unhydrolyzed cellulose). The liquid is filtered and sent to afermentation column for ethanol production. Solids are sent foranother round of hydrolysis (also called second stage hydrolysis).After hydrolysis, cellulose is converted into glucose. Again, themixture obtained at the end of hydrolysis is separated into liquid andsolid (lignin). After filtration, liquid is sent to a fermentation columnfor ethanol production and lignin is fed into a boiler for heatproduction. Different types of microbes are needed for fermentingsugars obtained from cellulose and hemi-cellulose and converting itto ethanol. After fermentation is over, the mixture of ethanol andwater is distilled to separate ethanol. Ethanol so produced is thendehydrated to produce fuel grade ethanol (b1% of water). Waterproduced as a part of distillation is diverted towards a wastewatertreatment facility. The ethanol obtained is then transported forconsumption purposes.
Currently, several versions of hydrolysis technology exist thoughthe basic framework remains the same. Each version is distinguishedfrom another depending upon the type of inputs used for hydrolyzinghemi-cellulose and cellulose into basic sugars. First, differenttechniques used for feedstock pretreatment (first stage of hydrolysis)are discussed followed by techniques which are commonly used forsecond stage of hydrolysis.
Pretreatment methods (first stage hydrolysis)
Thermal pretreatment. In thermal pretreatment, feedstock is heatedto about 150–180 °C to break down the hemi-cellulose and lignin. Athigher temperatures (above 250 °C), phenolic compounds are formedwhich later retard the fermentation process so care is taken not topretreat the feedstock in severe thermal conditions (Ramos, 2003).Four processes are commonly used for accomplishing thermalpretreatment i.e., steam pretreatment/steam explosion, liquid hotwater, ammonia fiber explosion, and CO2 explosion. During steampretreatment, the feedstock is put in a large vessel and then steamedat a high temperature (up to 240 °C) and pressurized for few minutes.After a set time, the steam is released and the biomass is quicklycooled down. The difference between steam and steam explosionpretreatment is the quick depressurization and cooling down of thebiomass at the end of the steam explosion pretreatment which causesthe water in biomass to explode (Hendricks and Zeeman, 2009; Jeoh,1998). In liquid hot water pretreatment, hot water is added to the
feedstock in a slightly acidic environment to solubilize hemi-celluloseand to prevent formation of any inhibitory compounds (Yang andWyman, 2004). In ammonia fiber explosion, processed feedstock isexposed to liquid ammonia at high temperature/pressure for a smalltime and then the pressure is swiftly reduced. In a typical procedure,the dosage of liquid ammonia is 1–2 kg ammonia/kg dry biomass,temperature is 90 °C, and residence time is about 30 min (Sun andCheng, 2002). Similar to steam and ammonia explosion pretreatment,CO2 explosion is also used for pretreatment of processed feedstock. Itwas found that CO2 explosion is more cost effective than ammoniaexplosion and prevents the formation of inhibitory compounds(Zheng et al., 1998).
Acid pretreatment. In acid pretreatment, dilute sulfuric acid is addedto the feedstock to hydrolyze hemi-cellulose (0.5–1.5%, temperature isgreater than 160 °C). Sometimes, concentrated sulfuric acid is alsoutilized for feedstock pretreatment. The acid must be removed orneutralized before fermentation. Generally lime is used for neutraliz-ing themedium and therefore gypsum is produced as byproduct of thereaction. Dilute acid pretreatment is the most preferred method forfeedstock pretreatment. Recently, nitric acid has also shown positiveresults in terms of better yields and solubility of lignin (Xiao andClarkson, 1997).
Alkaline pretreatment. Alkaline pretreatment use bases like sodiumhydroxide or calcium hydroxide. All lignin and part of the hemi-cellulose are removed, and the reactivity of cellulose for laterhydrolysis is sufficiently increased. Alkaline-based methods aregenerally more effective at solubilizing a greater fraction of ligninwhile leaving behind much of the hemi-cellulose in an insolublepolymeric form (Hamelinck et al., 2005; Mosier et al., 2005).
Oxidative pretreatment. In oxidative pretreatment, oxidatives likeperacetic acid or hydrogen peroxide are used over the feedstocksuspended in the water (Gould, 1984). It was found that use ofperacetic acid at the ambient temperature increased the ethanolyields by about 98% (Teixeira et al., 1999).
Organosolve pretreatment. In the organosolve process, an organic oraqueous organic solvent mixture with inorganic acid catalysts(hydrochloric acid or sulfuric acid) is used to break the internal ligninand hemi-cellulose bonds (Sun and Cheng, 2002). The organicsolvents used in the process include methanol, ethanol, acetone,
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ethylene, glycol, triethylene glycol, and tetrahydrofurfuryl alcohol(Chum et al., 1998; Thring et al., 1990).
Biological pretreatment. Brown-, white-, and soft-rot fungi arenormally used for degrading lignin and hemi-cellulose present inthe feedstock. Recently, Lee et al. (2007) have evaluated thepretreatment effects of three white rot fungi (Ceriporia lacerata,Stereum hirsutum, and Polyporus brumalis) on the Japanese redpine (Pinus densiflora). Similarly, Zhang et al. (2007) found thatbiological pretreatment with white rot fungi has potential forimproving enzymatic hydrolysis of wood and grass. They foundthat fermentable sugar yield of bamboo culms (Phyllostachyspubescence) pretreated with two fungi (Echinodontium taxodii 2538and Trametes versicolor G20) increased with increasing pretreatmenttime.
Hydrolysis technologies (second stage hydrolysis)
Acid hydrolysis. Acid hydrolysis is only applicablewhen feedstock hasbeen pretreated using dilute acid process. Both dilute and concentratedacid options are available for hydrolyzing pretreated feedstock. At thisstage, higher temperature (about 215 °C) and dilute acid (4%) are usedfor converting cellulose to glucose. The concentrated acid process has avery high sugar yield (90%), can handle diverse feedstock, is relativelyrapid (10–12 h in total), and causes small degradation of sugars.However, the equipment required ismore expensivewhen compared todilute acid hydrolysis (Hamelinck et al., 2005).
Enzymatic hydrolysis. Enzymatic hydrolysis provides many advan-tages over acid hydrolysis. For example, enzymatic hydrolysis takesplace at mild conditions of temperature and pressure. As a resultglucose yields are high, chances of fermentation inhibiting com-pounds are less, equipment requirements are not significant, andthere is a reduction in the total environmental impact of the wholeprocess. Cellulases5 are usually a mixture of several enzymes. At leastthree major groups of cellulases are involved in the hydrolysisprocess: a) endoglucanase (EG, endo-1,4-D-glucanohydrolase, or EC3.2.1.4) which attacks regions of low crystallinity in the cellulosefiber, creating free chain-ends; b) exoglucanase or cellobiohydrolase(CBH, 1,4-β-D-glucan cellobiohydrolase, or EC 3.2.1.91.) whichdegrades the molecule further by removing cellobiose units fromthe free chain-ends; c) β-glucosidase (EC 3.2.1.21) which hydrolyzescellobiose to produce glucose (Coughlan and Ljungdahl, 1988; Sunand Cheng, 2002).
FermentationDuring fermentation, both C5 and C6 sugars are fermented to
ethanol under anaerobic/aerobic conditions. Historically, yeast(Saccharomyces cerevisiae) has been used to ferment C6 sugars i.e.,glucose. Other microbes like Zymomonas mobilis have also beenused. Other engineered microbes like Escherichia coli have also beendeveloped to ferment both C6 and C5 sugars.
Based on the different combinations of technologies adopted at thepretreatment, hydrolysis, and fermentation stages of ethanol synth-esis, several integrated technologies have been developed. Also, fewintegrated technologies have recently evolved due to the develop-ments in the area of biotechnology. In the section below, existingintegrated conversion technologies are discussed.
Integrated technologies based on hydrolysis
Simultaneous saccharification and fermentation. In simultaneoussaccharification and fermentation, the feedstock is pretreated by dilute
5 Cellulase refers to a class of enzymes produced chiefly by fungi, bacteria, andprotozoans that catalyze the hydrolysis of cellulose.
acid (1.1% sulfuric acid at 160 °C for 10min) to hydrolyze hemi-celluloseinto sugars. The liquor is vented from the system and then neutralizedusing lime. Then, liquor containing C5 sugars is sent for fermentation.Residual solids comprise of cellulose and lignin. Yeast and enzymes aresubsequently added to the residual solids where the enzymes digestcellulose and produce glucose. Yeast and other microbes fermentglucose to produce ethanol separately (Krishna et al., 2001).
Simultaneous saccharification and co-fermentation. In simultaneoussaccharification and co-fermentation, the pretreated feedstock is exposedto different enzymes/microbes which not only hydrolyze cellulose andhemi-cellulose into different sugars but also ferment sugars into ethanol.This technology is better than the simultaneous saccharification andfermentation technology in terms of cost effectiveness, better yields, andshorter processing time (Wright, 1988; Chandel et al., 2007).
Two stage dilute sulfuric acid hydrolysis. Kadam (2000) hasdescribed the details of two-stage dilute sulfuric acid hydrolysis. Inthis process, dilute sulfuric acid is used at pretreatment phase tohydrolyze hemi-cellulose. In the hydrolysis stage of the process,concentrated sulfuric acid is utilized for hydrolyzing cellulose. Afterboth the stages, the liquid is separated, filtered and then neutralized(using lime) before it is sent for fermentation. Fermentation isconducted in steps. First, glucose is fermented to ethanol by the yeastSaccharomyces cerevisiae. The mixture is then distilled to remove theunconverted xylose. Then second yeast (Pachysolen tannophilu) isadded to the remaining solution to ferment xylose to ethanol. Ethanolproduced from xylose is then distilled (Shleser, 1994).
Biomass fractionation. In biomass fractionation, feedstock is pre-treated using the steam explosion method. The resulting mass iswashed by either water or alkali to separate out the components of thebiomass i.e., hemi-cellulose, cellulose, and lignin (Glasser and Wright,1998). Once separated, components containing sugars are furtherhydrolyzed to produce sugars. Sugars thus produced are fermented toobtain ethanol. Recently, a new technique has been developed usingwhich cellulosic biomass is fractionated in a very small amount of time(Guffey and Wingerson, 2002).
Thermochemical conversion technology
In thermochemical conversion, constituents of feedstock are firstconverted into syngas under intense heat and partial supply of airinside a gasifier. The syngas is then either fermented or catalyticallyconverted to ethanol. Fig. 2 shows the schematic diagram of ethanolformation through thermochemical route. Details of both the technol-ogies i.e., fermentation-based and catalyst-based are discussed below.
Gasification: fermentation-based ethanol productionThe feedstock is washed to remove any impurities and then
chipped to the required size. Chips are dried to attain a desiredmoisture level (5–20% by weight). Sand bed present inside a fluidizedbed gasifier is preheated to a temperature of about 550 °C using anexternal fuel supply. Once optimum temperature is achieved, feed-stock is fed into the bed. On coming in contact with hot sand, thefeedstock decomposes producing syngas. The supply of air issimultaneously controlled to prevent complete combustion of feed-stock and to raise the temperature of the sand bed to approximately800 °C. Once the optimum conditions are achieved, the gasifier canrun on its own without any external supply of additional fuel. Thesyngas so produced is collected from the top of the gasifier. Gas iscleaned to remove tar and ash. Cleaned, gas is cooled to the normalambient temperature and stored at a high pressure. Cooled andcleaned gas is fed into an ethanol conversion chamber wheremicrobes ferment the gas into ethanol and acetic acid. Afterfermentation is complete, the liquid is distilled to separate ethanol
Fig. 2. Diagram of cellulosic ethanol production through gasification technology.
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from other products. The ethanol produced is then dehydrated toproduce fuel quality ethanol. Many microbes have been developedwhich can successfully ferment the syngas into required compoundslike Clostridium carboxidivorans P7 (Liou et al., 2005), Clostridiumljungdahlii, and Clostridium autoethanogenum (Abrini et al., 1994;Vega et al., 1990). Rajagopalan et al. (2002) found that fermentationof the syngas by the microbe (P7) is sensitive to the syngasconstituents and proper cleaning of syngas ensures effectiveness ofthe process. For example, Ahmed et al. (2006) found that presence of
Fig. 3. Installed and under construction cellulosic ethanol production capacity
nitric oxide in the syngas (150 ppm) prevents fermentation of gasconstituents by the P7.
Gasification: catalyst-based ethanol productionIn catalyst-based ethanol production, all the processes remain the
same until the gas enters into an ethanol conversion chamber. Beforeentering the chamber, gas is heated to 300 °C at a pressure of 69 bar.Gas is also mixed with water and methanol to improve yield of higheralcohols. The mixture is passed through the synthetic catalyst
disaggregated by adopted conversion technologies in the United States.
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(molybdenum-disulfide based) to obtain methanol, ethanol, andhigher linear alcohols up to pentanol, water, methane, and minoramounts of other hydrocarbon byproducts. The chamber effluent isfinally cooled to 43 °C using cooling water, and excess syngas isdiverted to the gas cleaning area. The effluent so obtained is sent todistillation for ethanol recovery. Finally, the ethanol obtained isdehydrated to make it suitable for vehicles (Phillips, 2007).
Current production using different conversion technologies
Inspired by the subsidies offered by federal government, manyprivate entrepreneurs have ventured into cellulosic ethanol produc-tion. Fig. 3 shows the details of the total quantities of cellulosic ethanolcurrently or expected to be producedwithin the country by employingdifferent conversion technologies (RFA, 2009).
As observed from Fig. 3, it is expected that about 405 milliongallons of cellulosic ethanol will be produced by the end of 2012 andthree conversion technologies (enzymatic hydrolysis; simultaneoussaccharification and fermentation; and gasification-catalytic conver-sion and distillation) will dominate the total production. Governmenthas so far supported existing cellulosic ethanol producers throughvarious grants and it is thought that the commercial viability of thecellulosic ethanol productionwill be proven to private investors by theend of 2012 (Sandor et al., 2008) and after that the cellulosic ethanolproduction will spike up. In addition to commercial cellulosic ethanolunits, many small scale cellulosic ethanol mills are coming up atvarious places to test the efficacy of newly developed conversiontechnologies or feedstocks. For example, CitrusEnergy, LLC is planningto use citrus waste to produce ethanol in Florida.
It was observed that the majority of cellulosic ethanol mills whichare under construction have plans to meet their cellulosic feedstocksupply either from the agriculture sector (corn stover, corn cob, wheatstraw, rice straw, barley straw, switch grass, sugar cane) or frommunicipal solid wastes. Based on the current trend, it is expected thatthere will be few cellulosic ethanol mills which will utilize forestbiomass as feedstock in the future.
Economics of cellulosic ethanol production
Production of ethanol from cellulosic feedstocks is costly whencompared to its production from starch-based agricultural feed-stocks (McAloon et al., 2000). Therefore the goal of the severalresearch agencies is to bring down the cost of production ofcellulosic ethanol to $1.33/gal by the end of 2012 by improvingoverall efficiency of conversion technologies (EERE, 2009). Sassneret al. (2008) have analyzed the cost effectiveness of three cellulosicfeedstocks (namely salix, corn stover, and spruce) and concludedthat conversion technology used for ethanol production has moreimportant implications for the cost effectiveness of a conversionprocess than the type of feedstock used. Several authors haveattempted to estimate cost of cellulosic ethanol produced throughdifferent conversion technologies. Few such studies are discussedbelow.
Table 3Ethanol plant performance summary for hydrolysis-based conversion technologies.
Processes Biomass cost: $
Ethanol ($/gal)25 Mgal/year
Simultaneous saccharification and fermentation 1.48Concentrated acid hydrolysis, neutralization and fermentation 2.28Ammonia disruption hydrolysis and fermentation 1.81Steam disruption, hydrolysis and fermentation 1.63Acid disruption and transgenic microorganism fermentation 1.86Concentrated acid hydrolysis, acid recycle and fermentation 1.86Acidified acetone extraction, hydrolysis and fermentation 1.7
Hydrolysis-based technologies
Shleser (1994) compared the cost of ethanol produced using sevenintegrated hydrolysis-based conversion technologies. Results of theirstudy are summarized in Table 3.
Table 3 clearly shows that as the scale of the ethanol mill increases,the production cost of ethanol falls. Also, the total production cost ofthe ethanol is directly related to the cost of biomass feedstock. It wasfound that among all the hydrolysis-based conversion technologies,the production cost of ethanol was highest for the concentrated acidhydrolysis, neutralization, and fermentation technology and lowest forsimultaneous saccharification and fermentation technology. Recently,Huang et al. (2009) have stated that for an ethanol mill based onsimultaneous saccharification and co-fermentation technology, theethanol production cost decreases with increasing plant sizes in therange of 1000 dry Mg/day to 4000 dry Mg/day. It was also found thatthe cost of production of ethanol from hybrid poplar increases if theplant size is more than 4000 dry Mg/day as feedstock costs rise fasterthan non-feedstock costs. They also estimated that the cost of ethanolproduction was not variable with the type of feedstock utilized.
So and Brown (1999) found that the production cost of ethanolfrom a 25 million gallons/year ethanol plant was $1.57/gal, for fastpyrolysis integrated with a fermentation step, $1.28/gal for simulta-neous saccharification and fermentation, and $1.35/gal for dilutesulfuric acid hydrolysis and fermentation conversion technologies.Wooley et al. (1999) have found that to minimize the production costof ethanol produced using corn stover as a feedstock and co-currentdilute acid prehydrolysis and enzymatic hydrolysis as a technology,emphasis should be on increasing the conversion efficiencies of hemi-celluloses and cellulose to fermentable sugars. Similar thoughts wereechoed by Hamelinck et al. (2005). They analyzed four technologiesand three scenarios and found that with an increase in sugarconversion efficiencies; commercial production of cellulosic ethanolwas feasible. Aden et al. (2002) also found that the total cost ofethanol production almost becomes flat i.e., $1.3/gal when plant sizecrosses a threshold value of about 6000 MT of corn stover per day.
Recently, for a 55 million gallons/year ethanol production facility,Aden (2008) found that the total selling price of ethanol is about$2.43/gal when simultaneous saccharification and fermentationtechnique is used for ethanol production. It was also noted that theselling price has shown continuous declining trends since 2001.Feedstock costs were found to be about 40% of the total selling price ofethanol. Feedstock costs have been found to be significant indetermining the final cost of the ethanol especially when theconversion technology costs are falling at a faster pace (Bohlmann,2006). Leistritz et al. (2006) analyzed the production cost of ethanolfrom wheat straw in North Dakota and estimated the production costto be $1.56/gal.
Thermochemical technologies
There are no commercial-scale operating plants using thermo-chemical conversion, so production costs can only be estimated.
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Phillips (2007) modeled cellulosic ethanol production throughgasification technology and catalytic conversion of syngas to ethanol.The minimum selling price was found to be $1.07/gal based on theanticipated and achievable technology parameters by 2012. Tembo etal. (2003) noted that the breakeven cost for the ethanol producedusing thermochemical-fermentation technology will be about $0.76/gal. Recently, Piccolo and Bezzo (2009) have estimated that the cost ofproduction of ethanol produced using gasification-fermentation-based technology will be higher than that of the enzymatic hydrolysistechnology. Wei et al. (2009) have found that the cost of ethanolproduced using gasification-catalytic conversion technology will belower than the cost of ethanol produced using hydrolysis fermentationas processing time is lowest in the former technology.
The economic analysis clearly reveals that the unit cost of ethanolproduction has fallen in recent years due to technological advance-ments and it is expected that the cost of producing ethanol fromcellulosic feedstocks will be comparable to the starch feedstock-basedethanol. It was also observed that the scale of operations has animpact on the production cost of ethanol and optimum size of anethanol mill should be close to 4000 dry Mg/day of feedstockconsumption. Furthermore, the type of feedstock does not appear tobe significant in determining the production cost of ethanol,compared to the conversion technology. It can be inferred that thefuture production cost of ethanol will be lowest for gasification-catalytic conversion followed by hydrolysis and then the gasification-fermentation technology. There also exists a need for reducing thecost of transporting feedstocks to the ethanol mill as feedstock costsconstitute significant portion of the total production cost of ethanol atpresent.
Emerging developments
The importance given to the commercial viability of ethanolproduction from cellulosic feedstocks has attracted many scholars. Ithas been found that irrespective of the technology applied, the costs ofthe plant are correlated with the overall energy loss of the plant(Lange, 2007). Therefore, several new ideas are being tried at differentlevels for ensuring commercial production of ethanol from cellulosicfeedstocks. Some of these emerging technologies are discussed below.
Consolidated bioprocessing
In consolidated bioprocessing, only one microbial community isemployed both for the production of cellulases and fermentation i.e.,cellulose production, cellulose hydrolysis, and fermentation arecarried out in a single step (Cardona and Sánchez, 2007). Lynd et al.(2005) have estimated that consolidated bioprocessing has a potentialto provide the lowest cost route for biological conversion of cellulosicbiomass to fuels and other products among all the hydrolysis-basedprocesses.
Mobile fast pyrolysis
Reducing transportation costs will help in ensuring cost-effectiveproduction of cellulosic ethanol because cellulosic biomass is a lightdensity solid, it occupies a large volume resulting in increasedtransportation costs. One way to overcome this problem is to densifythe feedstock at the harvest site and then transport it to the mill sitefor ethanol production (Li and Liu, 2000; Petrolia, 2008; Husain et al.,2002). Recently, establishment of mobile fast pyrolysis plants at thefeedstock source for producing pyrolysis oil has been suggested as onepotential solution (Badger and Fransham, 2006). It was observed thatthe energy density of pyrolysis oil is about 6–7 times higher than theenergy density of green whole tree chips at 45% and 56% moisturecontent, respectively (Czernik and Bridgwater, 2004). Pyrolysis oil canbe gasified and syngas can be utilized for ethanol production.
However, pyrolysis oil is very complex and unstable and therefore,there exists a need of advanced technologies to successfully utilizepyrolysis oil for ethanol production.
Integrated ethanol refineries
Increasing the energy efficiency of the whole conversion process iskey towards ensuring the commercial viability of cellulosic ethanolproduction. Recently, Frederick et al. (2008b) took a holistic approachand analyzed the whole system of producing ethanol from loblollypine and using the residual biomass as a fuel for a combined heat andpower plant. They found that ethanol can be produced at $1.29/galbased on a delivered wood cost of $63.80/dry Mg at 95% conversionefficiency of carbohydrates in wood to sugars for a 93 million gallonsannual plant capacity. Frederick et al. (2008a) also analyzed thefeasibility of ethanol production in a Kraft paper mill. They foundproduction cost of ethanol to be between $1.33/gal and $2.92/galdepending upon process conditions and selectivity of hemi-cellulosesremoval. Leistritz et al. (2006) have also evaluated the integratedbiorefinery concept in North Dakota and found that the production ofcellulose nanowhiskers would be an enhancement to the economicperformance of a wheat straw to ethanol mill.
Alternate uses of cellulosic biomass — biopower
Cellulose used for ethanol production has to compete withalternative uses. One such use is power production. Biopower playsa major role in total renewable electricity produced in the nation asabout 16% electricity produced from renewable sources comes frombiomass alone (EIA, 2008e). Fuelled by several incentives announcedby the government, the interest in using biomass for electricityproduction has gone up and many entrepreneurs are establishing newcellulosic feedstock-based power plants. For example, GainesvilleRegional Utility will establish a 100 MW power plant in Gainesville,Florida which will use various cellulosic feedstocks for electricityproduction. It is possible that the rise in number of such power plantswill increase the competition for available cellulosic biomass and canseverely affect the availability of cellulosic biomass for ethanolproduction.
Conclusions
Production of cellulosic ethanol presents a challenge in terms ofdevelopment of a commercially viable conversion technology. How-ever with the rising interest of policy makers and researchers, it isexpected that such a technology will soon be developed. It is morelikely that the developed conversion technology will be based on thethermochemical platform rather than sugar platform as embeddedtechnologies like gasification and catalytic conversion are alreadyquite mature and only small improvements are needed to customizethe present technology for ethanol production. Similarly, it is alsoexpected that advances in reducing the feedstock costs will help inreducing the total cost of cellulosic ethanol production.
In terms of cellulosic ethanol production using different technol-ogies, only three technologies have gained broader acceptance amongentrepreneurs. This implies that so far industry has accepted onlyhandful of technologies to produce cellulosic ethanol and there existsa need to test more technologies so as to ascertain commercialviability of such technologies. Also, attempt should be made towardsexploring the efficacy of ethanol production from other feedstocks likewoody biomass. This will not only help in diversifying the feedstockportfolio but also add to the energy security of the nation in case of acrop failure.
Cellulosic ethanol holds the promise to supplement the growingenergy needs of the nation. However, at the same time, it is importantto strike a harmonious chord with the other natural processes that are
181P. Dwivedi et al. / Energy for Sustainable Development 13 (2009) 174–182
associated with the production of cellulosic feedstocks. For example,in case of forestry feedstocks, it is important to evaluate the impacts ofbiomass production on the local biodiversity and on the localwatershed. Similarly, in case of agricultural feedstocks, it is importantto evaluate the impact on soil and water conservation and nutrientmanagement of soils. Understanding these relationships will help indeveloping a comprehensive cellulosic feedstock-based bioenergyinfrastructure in the country.
Acknowledgement
This study was a part of Pinchot Institute of Conservation's projecttitled “Wood Bioenergy and Forest Sustainability”.
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