Randy S. Lewis
School of Chemical Engineering, Oklahoma State University, Stillwater, Oklahoma, U.S.A.
Rohit P. Datar
Technical Operations, CPKelco, Okmulgee, Oklahoma, U.S.A.
Raymond L. Huhnke
Biosystems and Agricultural Engineering, Oklahoma State Unviersity, Stillwater, Oklahoma, U.S.A.
INTRODUCTION
Since the energy crisis of the 1970s, the development of low-cost, sustainable, and renewable energy sources has been a major focus of research. Fuel-grade ethanol is a transportation energy source that can be produced from biomass. Biomass resources for ethanol include sugar-based crops such as beets and cane, starch-based crops such as corn and potatoes, and lignocellulosic feedstocks such as wood, corn stover, and grasses.
This entry provides an overview on the processes and limitations for making ethanol from biomass using either a fermentable sugar platform or a fermentable producer gas platform. Fig. 1 shows a simplified schematic of the platforms that are described later.
Two of the platforms convert biomass (sugar, starch, or lignocellulose) into fermentable sugars, while the other platform converts lignocellulose into fermentable producer gas. Since an azeotrope occurs in the distillation process (3–5% water remains), further dehydration of ethanol can be accomplished through azeotropic distillation or molecular sieve dehydration.
The advantages and disadvantages of using ethanol as a transportation fuel are also described along with the issues that must be resolved in providing a consis-tent supply of high-quality and low-cost biomass for the production of ethanol. Several technological bar-riers remain in producing ethanol, including high enzyme costs, the inability of microbial strains to simultaneously and efficiently utilize a variety of sugar substrates, and the low ethanol yield in producer gas fermentation. However, with the development of inex-pensive routes for production, low-cost ethanol could soon become a reality.
ETHANOL AS A BIOFUEL
In 2001, 80% of worldwide energy was supplied by fos-sil fuels, with only 1.4% being supplied by sustainable biomass used for electricity, heat production, and
transportation. With regards to transportation energy, 97% was supplied by fossil fuels in industrialized nations.[1]Due to concerns over the depletion of fossil fuels, utilizing renewable resources would be beneficial for supplementing energy needs currently obtained from fossil fuels.
Fuel-grade ethanol (bioethanol) and other valuable commercial products can be produced by utilizing renewable resources (i.e., biomass) as a starting raw material. Over the last few years, there has been a tre-mendous growth in the U.S. ethanol industry fueled by a growing demand for renewable fuels. In 2002, the total ethanol produced in the United States was 2 billion gallons, representing a 20% increase over 2001 and a 45% increase over 1999.[2]The potential world ethanol production from biomass is 130 billion gallons per year, with lignocellulosic biomass contributing to 90% of this amount. This production capacity is 16 times higher than the current ethanol production in the world and could replace 32% of the global gasoline consumption.[3]
As a supplement to gasoline, ranging from 10% (E10) to 95% (E95), ethanol is beneficial for environmental and economic=national security reasons.
Environmental Benefits
The Clean Air Acts Amendments of 1990 mandated the addition of oxygenates to gasoline to reduce the forma-tion of harmful ground-level ozone (photochemical smog) in urban areas. In addition, the reduction of toxic gaseous pollutants and volatile organic compounds from emissions of motor vehicles was mandated. Methyl ter-tiary butyl ether (MTBE) has traditionally been used as an oxygenate in gasoline due to its high octane num-ber, low sulfur content, and relatively low production cost as compared to other high-octane components.[4]
However, in recent years, MTBE has been shown to be a potential carcinogen in humans and animals and a source of ground- and surface-water contamination.
MTBE is currently being eliminated through political
Encyclopedia of Chemical ProcessingDOI: 10.1081/E-ECHP-120039776
Copyright # 2006 by Taylor & Francis. All rights reserved. 143
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means, although efforts are being made to prevent the ban.[5] Ethanol is a very strong candidate to replace MTBE despite higher production costs, since it is envir-onmentally friendly and can be produced domestically from renewable sources.
The addition of ethanol to gasoline reduces harmful vehicle emissions and facilitates the reduction or removal of toxic air components, which are commonly found additives in gasoline.[2] Emissions of carbon dioxide (CO2), the primary greenhouse gas contribu-ting to global warming, have increased steadily, and the transportation sector alone accounts for 33% of total emissions.[6]It is estimated that an 85% ethanol fuel would reduce greenhouse gas emissions between 15% and 30%.[7] Ethanol-blended fuels would reduce particulate emissions, primarily a concern in diesel engines, by as much as 40%.[2]
Economic/National Security Benefits
Currently, the United States imports about 57% of its oil, creating a US$ 66 billion trade deficit; the deficit is expected to rise to US$ 170 billion by 2020.[8] Projec-tions show that oil imports could grow to 68% in 2025, thus increasing the dependence on crude oil from potentially unstable regions around the world.[2] It is estimated that a 10% ethanol blend would reduce oil requirements by 3%, while a 95% ethanol blend could reduce the oil requirement by as much as 44%.[9]Thus, it is beneficial to increase local energy production via fuels from renewable sources to prevent sudden energy disrup-tions, which could have severe economic ramifications.
Drawbacks
There have been ongoing arguments over the use of ethanol as a fuel. Ethanol can be corrosive to some
metals, gaskets, and seals in car engines. The heating value of ethanol is 76,000 Btu=gal compared to 115,000 Btu=gal for gasoline; hence, more ethanol is required to travel the same distance. There have been conflicting reports on the preceding statement, since some argue that there is statistically no mileage differ-ence due to the higher combustion efficiency of ethanol compared to gasoline.[10] Some argue that the cost of ethanol is too high for it to be used as an additive or a replacement to gasoline and that the ethanol industry will not survive without tax incentives.[11] Arguably, ethanol as a fuel will not be able to compete with gaso-line unless less expensive routes for ethanol production are investigated and developed.
BIOMASS RESOURCES
The major sources of biomass feedstock are low-cost residues, wastes, and byproducts such as corn stover, wheat and rice straw, municipal solid waste, sawdust, dedicated energy crops such as switchgrass, hybrid poplars, and hybrid willows, and corn starch. The total estimated availability of usable biomass in the world is about 2 billion dry tons per year.[3]Thus, considerable effort has been devoted towards developing energy crops to meet the estimated increase in energy usage.
An estimated 549 million acres of land will be available to cultivate such energy crops (based on estimates by the Oak Ridge National Laboratory—Biofuels Feed-stock Development Program). The use of herbaceous energy crops, such as switchgrass, for the production of ethanol also offers a significant energy gain com-pared to corn. The ratio of total available energy to the total energy input for harvesting and handling is 4.43 and 1.21 for switchgrass and corn, respectively, translating to energy gains of 343% and 21% respec-tively.[12] In addition to its energy gain, switchgrass Acid hydrolysis or
Fig. 1 Processes for biomass conversion to ethanol.
has been chosen as the model crop for biomass utiliza-tion because of its high productivity over a wide geo-graphic range, ability to grow on marginal lands, low water and nutrient requirements, and environmental benefits such as improved soil conservation.[13]
One of the primary reasons for the slow acceptance of biomass to ethanol conversion is the lack of a reli-able and sustainreli-able lignocellulosic supply infrastruc-ture to maintain a commercial conversion facility.
There are many issues that must be resolved in providing a consistent supply of high-quality, low-cost biomass.[14]
Feedstock quality. To operate round the year, most conversion facilities need to utilize a variety of bio-mass feedstocks. Therefore, changes in feedstock quality, including physical and chemical character-istics, must be taken into account in the design, operation, and performance of a commercial plant.
Harvesting and collection. Crop residues must be collected soon after the principal crop has been removed to reduce field losses and avoid contami-nation. Other feedstocks such as perennial grasses are harvested only during selected months of the year. The timing of harvest and collection of crop residues and energy crops often necessitates the use of storage facilities.
Transportation. Movement of biomass from the original source to a processing facility can be very costly because of its relatively low bulk density.
For example, the bulk density of coal is over three times that of most standard biomass packages.
Storage. The quantities required for a commercial conversion facility demand the use of storage ities even if only used as a supply buffer at the facil-ity itself. For dry biomass, protection from the elements is often required to maintain acceptable moisture levels and to minimize storage losses. In addition, fire protection is an important consideration when storing large quantities of biomass at one site.
BIOMASS TO ETHANOL: FERMENTABLE SUGAR PLATFORM
A majority of ethanol produced from biomass utilizes the fermentation of sugars that are obtained from polysaccharides prevalent in the biomass. Feedstocks include sugar-based crops such as beet and cane, starch-based crops such as corn and potato, and ligno-cellulosic plants such as wood, corn stover, and grasses. Fermentation increases in difficulty from sugar crops to starchy crops to lignocellulosic plants as a result of the increasing complexity and heterogeneity of sugar components of these biomass resources. Sugar crops contain the simplest sources of sugars in the form of sucrose (Fig. 2), a disaccharide consisting of
glucose and fructose (both are six-carbon sugars).
Starch is a polysaccharide that serves as the nutritional reservoir in plants and is composed of glucose residues in a-1,4 linkages that must be converted to single glu-cose units for fermentation. Starch consists of either an unbranched form (Fig. 2) or a branched form contain-ing additional a-1,6 linkages of glucose every 20–30 glucose units.
Lignocellulosic materials are characterized by vary-ing amounts of cellulose, hemicellulose, lignin, and small quantities of other extractives. Typically, the composition by weight is 40–50% cellulose, 20–40%
hemicellulose, and 10–30% lignin.[15]Cellulose (Fig. 2) is an unbranched polysaccharide that serves a struc-tural role in plants and is composed of glucose residues joined by b-1,4 linkages, which are stronger and more difficult to break than the starch linkages. Hemicel-lulose is a branched polysaccharide, consisting primar-ily of five-carbon sugars such as xylose and arabinose in addition to six-carbon sugars such as glucose and mannose. These complex sugars must be converted to single sugar units for fermentation. Lignin is a group of amorphous, high molecular weight compounds that cannot be fermented.
Fig. 2 Sugar units of biomass.
B
Fermentation of Sugar and Starch Crops
Most current commercial ethanol production is from the fermentation of sugar and starch crops. Yeast can rapidly convert sucrose to ethanol with a theoreti-cal carbon conversion of 67%. The production of ethanol from corn grew to about 1.9 billion gallons in 2001.[16]This accounted for 90% of the total ethanol production and an estimated 615 million bushels of corn (6.2% of total corn produced) were consumed.
The remaining 10% of ethanol production was by fer-mentation of grain sorghum, barley, wheat, cheese whey, and potatoes.
The basic steps for the conversion of sugar and starch crops to ethanol are: 1) The biomass either undergoes a grinding process (dry milling) or is chemically treated (wet milling) to reduce the size of the feedstock; 2) if starch crops are used, the starch is converted to sugars by enzymatic treatment—called saccharification; 3) the sugars are fermented to ethanol by yeast; and 4) the ethanol is purified from the fer-mentation broth by distillation (Fig. 1). The remaining solid residue is used for cattle feed or, in some cases, as a fuel for boilers. The process for the starch=sugar feedstock can either be a dry mill process, in which the entire feedstock is fed to the fermentation unit, or a wet mill process, in which feedstock components are separated and the starch is fed to the fermentation unit. For the dry milling process, a liquefaction step occurs, in which a caustic solution and enzymes are added in a heated environment to obtain a slurry solu-tion.
Such fermentation processes are often used in geo-graphic locations in which the crops are grown in abundance, due to the low transportation costs of the feedstock supply. Low volumetric productivities and long fermentation times are disadvantages of the pro-cesses, which often have a high cost and require federal subsidies.[17] Therefore, to expand ethanol production throughout the United States, alternative raw materi-als, such as underutilized biomass and low-cost cel-lulosic feedstock, are being investigated.
Fermentation of Sugars from Lignocellulosic Biomass
Due to abundant quantity and competitive prices, lig-nocellulosic feedstocks have a greater potential for ethanol production than starch and sugar crops.
Importantly, lignocellulosic feedstocks do not interfere with food security[3]and can be cultivated on marginal lands. The most common lignocellulosic materials are corn stover, grasses, wood chips, paper wastes, and agricultural residues. Since cellulose and hemicellulose
are complex sugars not directly utilized by yeast or bacteria, they need to be reduced to fermentable sugars. Several processes have been utilized to obtain the fermentable sugars, including: 1) acid hydrolysis followed by fermentation; 2) acid and enzymatic hydrolysis followed by fermentation; 3) physical=
chemical disruption and enzymatic hydrolysis followed by fermentation; and 4) pretreatment followed by simultaneous saccharification and fermentation.
Acid hydrolysis followed by fermentation
In the dilute acid process, biomass is treated with dilute sulfuric acid (2–5%) at about 160C under a pressure of about 10 atm[18] to break down the cellulose and hemicellulose components to fermentable sugars. The lignin fraction is separated from the hydrolysate. The five- and six-carbon sugars are then fermented to etha-nol by using genetically engineered organisms such as Escherichia coli,[19] Saccharomyces cerevisiae,[19] and Zymomonas mobilis.[20] A drawback is that this pro-cess results in low glucose yields (50–60%) from cel-lulose and hence gives low ethanol yields.[21] A concentrated acid solution (10–30%) can also be uti-lized at 100C and atmospheric pressure to hydrolyze the hemicellulose and cellulose.[22] The benefits of the concentrated acid hydrolysis process over the dilute acid hydrolysis process are the lower operating tem-peratures and pressures, as well as higher glucose yields. A challenging issue in both the dilute acid hydro-lysis and concentrated acid hydrohydro-lysis processes is the disposal of lignin, although it can be used as a fuel. Also, a major disadvantage is the formation of toxic by-products, such as furfural and hydroxymethyl furfural, that can affect the conversion rates.
Acid and enzymatic hydrolysis followed by fermentation
For the enzymatic process, the feedstock is first pre-treated with a dilute acid to break down the lignocellu-lose into lignin, hemicellulignocellu-lose, and cellulignocellu-lose. Enzymes called cellulases and xylanases are then used to break down the cellulose and hemicellulose fractions into six- and five- carbon sugars, respectively. Without the dilute acid treatment, the enzymes would not be able to come in efficient contact with the cellulose and hemicellulose. The sugars are then fermented to etha-nol using organisms such as E. coli,[19]S. cerevisiae,[23]
and Z. mobilis.[20] The pretreatment of lignocellulose with dilute acid followed by enzymatic breakdown of cellulose and hemicellulose has shown to maximize the overall process yields as compared to processing with dilute acid alone. The disadvantage of this method is its high cost and requirement of enzymes,
although the costs have been greatly reduced over the last several years.[24]Also, this process is complicated by the accumulation of soluble products such as cellobiose and cellotriose, which act as competitive inhibitors of hydrolysis.[19]
Physical=chemical disruption and enzymatic hydrolysis followed by fermentation
Several processes, other than dilute acid hydrolysis, have been used to make biomass accessible to enzy-matic breakdown for the generation of fermentable sugars. For ammonia disruption, the lignocellulose is exposed to ammonia at a high pressure and a tem-perature ranging from 25C to 90C.[25] The elevated pressure and temperature causes swelling and decrys-tallization of the biomass. The pressure is then sud-denly lowered, causing the biomass to explode. This makes the biomass accessible to enzymes that hydro-lyze the cellulose and hemicellulose to fermentable sugars, as noted above. For steam disruption, the bio-mass is fed into a high-pressure cylinder and treated with steam. The biomass is then passed into a flash tank, causing an explosion due to the sudden pressure change. The explosion causes autohydrolysis of the hemicellulose to xylose. The cellulose is treated with enzymes to form glucose. A disadvantage of the pro-cess is that volatile organics (e.g., furfural) are formed, which are toxic to the microbial catalysts. Additional processes that break down the biomass to allow enzy-matic treatment include mild alkaline extraction with Ca(OH)2or NaOH.[26]In all cases, the sugars obtained from the process are fermented to ethanol using the organisms identified in the previous enzymatic method.
Pretreatment followed by simultaneous saccharification and fermentation
To eliminate the separate steps of formation of fermen-table sugars followed by fermentation, the simulta-neous saccharification and fermentation process has been developed following pretreatment with acid or physical=chemical disruption methods.[27] Since glu-cose is an inhibitor of cellulase activity, this process effectively removes the glucose and provides higher yields.
Major disadvantages of the processes mentioned include the current need for economic subsidies, the high cost of enzymes, and the formation of waste streams (such as acid pretreatment materials and toxic compounds found in acidic hydrolysates of biomass), although utmost efforts are being made to eliminate these drawbacks.[24,28]When biomass is utilized, lignin (approximately 10–40 wt% of biomass) cannot be bro-ken down into fermentable components. Thus, 10–40%
of the biomass is not incorporated into products.
However, utilization of the lignin waste stream is currently being explored. Developments in conversion technologies over the last two decades have reduced the cost of bioethanol production from US$ 5.00=gal to approximately US$ 1.20=gal.[29]
BIOMASS TO ETHANOL: FERMENTABLE PRODUCER GAS PLATFORM
A large variety of biomass substrates, including solid municipal waste and waste paper, can be converted to producer gas via gasification. Producer gas can then be converted to ethanol by chemical catalysts, but microbial catalysts offer several advantages, since they require significantly lower temperature and pressure conditions (usually atmospheric conditions) and are less susceptible to varying feed gas compositions.
Chemical catalysts are more susceptible to poisoning, and their specificity is lower compared to microbial processes, although faster conversion times are often possible.
A large number of bacterial strains have been iso-lated that have the ability to ferment producer gas (the CO, CO2, and H2components) to ethanol, acetic acid, and other useful liquid products. Clostridium ljungdahliiwas the first recognized organism to form ethanol from components of producer gas.[30] The organism favors the production of acetate at a higher pH (5–7), but ethanol is the dominant product at pH between 4 and 4.5. Recently, an additional clostridial acetogen was isolated and was shown to produce etha-nol from producer gas generated from biomass.[31,32]
Other organisms that can produce ethanol from producer gas, although not as the major product,
Other organisms that can produce ethanol from producer gas, although not as the major product,