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Cellulosic ethanol is a biofuel produced from wood, grasses, or the non-edible parts of plants.

It is a type of biofuel produced from lignocellulose, a structural material that comprises much of the mass of plants. Lignocellulose is composed mainly of cellulose, hemicellulose and lignin. Corn stover, switchgrass, miscanthus, woodchips and the byproducts of lawn and tree maintenance are some of the more popular cellulosic materials for ethanol production. Production of ethanol from lignocellulose has the advantage of abundant and diverse raw material compared to sources like corn and cane sugars, but requires a greater amount of processing to make the sugar monomers available to the microorganisms that are typically used to produce ethanol by fermentation.

Switchgrass and Miscanthus are the major biomass materials being studied today, due to their high productivity per acre. Cellulose, however, is contained in nearly every natural, free-growing plant, tree, and bush, in meadows, forests, and fields all over the world without agricultural effort or cost needed to make it grow.

According to U.S. Department of Energy studies[1] conducted by Argonne National Laboratory of the University of Chicago, one of the benefits of cellulosic ethanol is that it reduces greenhouse gas emissions (GHG) by 85% over reformulated gasoline. By contrast, starch ethanol (e.g., from corn), which most frequently uses natural gas to provide energy for the process, may not reduce GHG emissions at all depending on how the starch-based feedstock is produced.[2] A study by Nobel Prize winner Paul Crutzen found ethanol produced from corn and sugarcane had a "net climate warming" effect when compared to oil.[3]

Contents

History

The first attempt at commercializing a process for ethanol from wood was done in Germany in 1898. It involved the use of dilute acid to hydrolyze the cellulose to glucose, and was able to produce 7.6 liters of ethanol per 100 kg of wood waste (18 gal per ton). The Germans soon developed an industrial process optimized for yields of around 50 gallons per ton of biomass. This process soon found its way to the United States, culminating in two commercial plants operating in the southeast during World War I. These plants used what was called "the American Process" — a one-stage dilute sulfuric acid hydrolysis. Though the yields were half that of the original German process (25 gallons of ethanol per ton versus 50), the throughput of the American process was much higher. A drop in lumber production forced the plants to close shortly after the end of World War I. In the meantime, a small, but steady amount of research on dilute acid hydrolysis continued at the USDA Forest Products Laboratory.[4][5][6] During World War II, the US again turned to cellulosic ethanol, this time for conversion to butanediol to produce synthetic rubber. The Vulcan Copper and Supply Company was contracted to construct and operate a plant to convert sawdust into ethanol. The plant was based on modifications to the original German Scholler process as developed by the Forest Products Laboratory. This plant achieved an ethanol yield of 50 gal/dry ton but was still not profitable and was closed after the war.[7]

With the rapid development of enzyme technologies in the last 2 decades, the acid hydrolysis process has gradually been replaced by enzymatic hydrolysis. However, chemical pretreatment of the feedstock is required to prehydrolyze (separate) hemicellulose for robust enzymatic saccharification of cellulose substrate. The dilute acid pretreatment is developed based on the early work on acid hydrolysis of wood at the USDA Forest Products Laboratory. Recently, the Forest Products Laboratory together with the University of Wisconsin–Madison developed the Sulfite Pretreatment to overcome Recalcitrance of Lignocellulose (SPORL) [8] for robust enzymatic hydrolysis of wood cellulose.

United States President Bush, in his State of the Union address delivered January 31, 2006, proposed to expand the use of cellulosic ethanol. In his State of the Union Address on January 23, 2007, President Bush announced a proposed mandate for 35 billion gallons of ethanol by 2017. It is widely recognized that the maximum production of ethanol from corn starch is 15 billion gallons per year, implying a proposed mandate for production of some 20 billion gallons per year of cellulosic ethanol by 2017. Bush's proposed plan includes $2 billion funding (from 2007-2017?) for cellulosic ethanol plants, with an additional $1.6 billion (from 2007-2017?) announced by the USDA on January 27, 2007.

In March 2007, the US government awarded $385 million in grants aimed at jump-starting ethanol production from nontraditional sources like wood chips, switchgrass and citrus peels. Half of the six projects chosen will use thermo-chemical methods and half will use cellulosic ethanol methods.[9]

The American company Range Fuels announced in July 2007 that it was awarded a construction permit from the state of Georgia to build the first commercial-scale 100-million-gallon-per-year cellulosic ethanol plant in the United States.[10] Construction began in November, 2007.[11]

The U.S. could potentially produce 1.3 billion dry tons of cellulosic biomass per year[12], which has the energy content of four billion barrels of crude oil. This translates to 65% of American oil consumption.[citation needed]

Production methods

There are two ways of producing ethanol from cellulose:

They both include distillation as the final step to isolate the pure ethanol.

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Cellulolysis (biological approach)

There are four or five stages to produce ethanol using a biological approach:

  1. A "pretreatment" phase, to make the lignocellulosic material such as wood or straw amenable to hydrolysis,
  2. Cellulose hydrolysis (cellulolysis), to break down the molecules into sugars;
  3. Separation of the sugar solution from the residual materials, notably lignin;
  4. Microbial fermentation of the sugar solution;
  5. Distillation to produce roughly 95% pure alcohol.
  6. Dehydration by molecular sieves to bring the ethanol concentration to over 99.5%

Pretreatment

Although lignocellulose is the most abundant plant material resource, its susceptibility has been curtailed by its rigid structure. As the result, an effective pretreatment is needed to liberate the cellulose from the lignin seal and its crystalline structure so as to render it accessible for a subsequent hydrolysis step.[13] By far, most pretreatments are done through physical or chemical means. In order to achieve higher efficiency, both physical and chemical pretreatment are required. Physical pretreatment is often called size reduction to reduce biomass physical size. Chemical pretreatment is to remove chemical barriers so that the enzymes can access to cellulose for microbial destruction.

To date, the available pretreatment techniques include acid hydrolysis, steam explosion, ammonia fiber expansion, organosolve, sulfite pretreatment to overcome recalcitrance of lignocellulsoe (SPORL) [8], alkaline wet oxidation and ozone pretreatment.[14] Besides effective cellulose liberation, an ideal pretreatment has to minimize the formation of degradation products because of their inhibitory effects on subsequent hydrolysis and fermentation processes.[15] The presence of inhibitors will not only further complicate the ethanol production but also increase the cost of production due to entailed detoxification steps. Even though pretreatment by acid hydrolysis is probably the oldest and most studied pretreatment technique, it produces several potent inhibitors including furfural and hydroxymethyl furfural (HMF) which are by far regarded as the most toxic inhibitors present in lignocellulosic hydrolysate.[16]. Ammonia Fiber Expansion (AFEX) is a promising pretreatment with no inhibitory effect in resulting hydrolysate.[17]

Most pretreatment processes are not effective when applied to feedstocks with high lignin content, such as forest biomass. Organosolve and SPORL are the only two processes that can achieve over 90% cellulsoe conversion for forest biomass, especially those of softwood species. SPORL is the most energy efficient (sugar production per unit energy consumption in pretreatment) and robust process for pretreatment of forest biomass with very low production of fermentation inhibitors.

Cellulolytic processes

The cellulose molecules are composed of long chains of sugar molecules. In the hydrolysis process, these chains are broken down to free the sugar, before it is fermented for alcohol production.

There are two major cellulose hydrolysis (cellulolysis) processes: a chemical reaction using acids, or an enzymatic reaction.

Chemical hydrolysis

In the traditional methods developed in the 19th century and at the beginning of the 20th century, hydrolysis is performed by attacking the cellulose with an acid.[18] Dilute acid may be used under high heat and high pressure, or more concentrated acid can be used at lower temperatures and atmospheric pressure. A decrystalized cellulosic mixture of acid and sugars reacts in the presence of water to complete individual sugar molecules (hydrolysis). The product from this hydrolysis is then neutralized and yeast fermentation is used to produce ethanol. As mentioned, a significant obstacle to the dilute acid process is that the hydrolysis is so harsh that toxic degradation products are produced that can interfere with fermentation. BlueFire Ethanol uses concentrated acid because it does not produce nearly as many fermentation inhibitors but must be separated from the sugar stream for recycle (simulated moving bed (SMB) chromatographic separation for example) to be commercially attractive.

Enzymatic hydrolysis

Cellulose chains can be broken into glucose molecules by cellulase enzymes.

This reaction occurs at body temperature in the stomach of ruminants such as cows and sheep, where the enzymes are produced by bacteria. This process uses several enzymes at various stages of this conversion. Using a similar enzymatic system, lignocellulosic materials can be enzymatically hydrolyzed at a relatively mild condition (50oC and pH5), thus enabling effective cellulose breakdown without the formation of byproducts that would otherwise inhibit enzyme activity. All major pretreatment methods, including dilute acid pretreatment, require an enzymatic hydrolysis step to achieve high sugar yield for ethanol fermentation.[17] Currently, most pretreatment studies have been laboratory based, but companies are rapidly exploring means to transition from the laboratory to pilot, or production scale.

Various enzyme companies have also contributed significant technological breakthroughs in cellulosic ethanol through the mass production of enzymes for hydrolysis at competitive prices.

The fungus Trichoderma reesei is used by Iogen Corporation, to secrete "specially engineered enzymes" for an enzymatic hydrolysis process.[19] Their raw material (wood or straw) has to be pre-treated to make it amenable to hydrolysis. However, For Fuel Freedom, Inc.'s process uses a blend of lignin consuming bacteria in their patented organic hydrolysis, but the difference is that the microbiological organisms also contain fermentable sugar and when introduced as additional feedstock into the ethanol system, they nearly double the output without the need for additional pretreatment.[20]

Another Canadian company, SunOpta markets a patented technology known as "Steam Explosion" to pre-treat cellulosic biomass, overcoming its "recalcitance" to make cellulose and hemicellulose accessible to enzymes for conversion into fermentable sugars. SunOpta designs and engineers cellulosic ethanol biorefineries and its process technologies and equipment are in use in the first 3 commercial demonstration plants in the world:[21] Verenium (formerly Celunol Corporation)'s facility in Jennings, Louisiana, Abengoa's facility in Salamanca, Spain, and a facility in China owned by China Resources Alcohol Corporation (CRAC). The CRAC facility is currently producing cellulosic ethanol from local corn stover on a 24-hour a day basis using SunOpta's process and technology.

Genencor and Novozymes are two other companies that have received United States Department of Energy funding for research into reducing the cost of cellulases, key enzymes in the production of cellulosic ethanol by enzymatic hydrolysis.

Other enzyme companies, such as Dyadic International,[22] are developing genetically engineered fungi which would produce large volumes of cellulase, xylanase and hemicellulase enzymes which can be used to convert agricultural residues such as corn stover, distiller grains, wheat straw and sugar cane bagasse and energy crops such as switch grass into fermentable sugars which may be used to produce cellulosic ethanol.

Verenium, formed by the merger of Diversa and Celunol, operates a pilot cellulosic ethanol plant in Jennings, Louisiana and is building a 1.4 million gallon per year demonstration plant on adjacent land to be completed by the end of 2007 and begin operation in early 2008. Verenium is the first publicly traded company with integrated, end-to-end capabilities to make cellulosic biofuels.

KL Energy Corporation,[23] formerly KL Process Design Group, began commercial operation of a 1.5 million gallon per year cellulosic ethanol facility in Upton, WY in the last quarter of 2007. The Western Biomass Energy facility is currently achieving yields of 40-45 gallons per bone dry ton. It is the first operating commercial cellulosic ethanol facility in the nation. The KL Energy process uses a thermo-mechanical breakdown and enzymatic conversion process. The primary feedstock is soft wood, however, lab tests have already proven the KL Energy process on wine pomace, sugarcane bagasse, municipal solid waste, and switch grass.

Microbial fermentation

Traditionally, baker’s yeast (Saccharomyces cerevisiae), has long been used in the brewery industry to produce ethanol from hexoses (6-carbon sugar). Due to the complex nature of the carbohydrates present in lignocellulosic biomass, a significant amount of xylose and arabinose (5-carbon sugars derived from the hemicellulose portion of the lignocellulose) is also present in the hydrolysate. For example, in the hydrolysate of corn stover, approximately 30% of the total fermentable sugars is xylose. As a result, the ability of the fermenting microorganisms to use the whole range of sugars available from the hydrolysate is vital to increase the economic competitiveness of cellulosic ethanol and potentially bio-based chemicals.

In recent years, metabolic engineering for microorganisms used in fuel ethanol production has shown significant progress.[24] Besides Saccharomyces cerevisiae, microorganisms such as Zymomonas mobilis and Escherichia coli have been targeted through metabolic engineering for cellulosic ethanol production.

Recently, engineered yeasts have been described efficiently fermenting xylose [25],[26] and arabinose,[27] and even both together.[28] Yeast cells are especially attractive for cellulosic ethanol processes as they have been used in biotechnology for hundreds of years, as they are tolerant to high ethanol and inhibitor concentrations and as they can grow at low pH values which avoids bacterial contaminations.

Combined hydrolysis and fermentation

Some species of bacteria have been found capable of direct conversion of a cellulose substrate into ethanol. One example is Clostridium thermocellum, which uses a complex cellulosome to break down cellulose and synthesize ethanol. However, C. thermocellum also produces other products during cellulose metabolism, including acetate and lactate, in addition to ethanol, lowering the efficiency of the process. Some research efforts are directed to optimizing ethanol production by genetically engineering bacteria that focus on the ethanol-producing pathway.[29]

Gasification process (thermochemical approach)

Fluidized Bed Gasifier in Güssing Burgenland Austria

The gasification process does not rely on chemical decomposition of the cellulose chain (cellulolysis). Instead of breaking the cellulose into sugar molecules, the carbon in the raw material is converted into synthesis gas, using what amounts to partial combustion. The carbon monoxide, carbon dioxide and hydrogen may then be fed into a special kind of fermenter. Instead of sugar fermentation with yeast, this process uses a microorganism named Clostridium ljungdahlii.[30] This microorganism will ingest (eat) carbon monoxide, carbon dioxide and hydrogen and produce ethanol and water. The process can thus be broken into three steps:

  1. Gasification — Complex carbon based molecules are broken apart to access the carbon as carbon monoxide, carbon dioxide and hydrogen are produced
  2. Fermentation — Convert the carbon monoxide, carbon dioxide and hydrogen into ethanol using the Clostridium ljungdahlii organism
  3. Distillation — Ethanol is separated from water

A recent study has found another Clostridium bacterium that seems to be twice as efficient in making ethanol from carbon monoxide as the one mentioned above.[31]

Alternatively, the synthesis gas from gasification may be fed to a catalytic reactor where the synthesis gas is used to produce ethanol and other higher alcohols through a thermochemical process.[32] This process can also generate other types of liquid fuels, an alternative concept under investigation by at least one biofuels company.[33]

Economics

Cellulosic ethanol has the potential to become a competitive energy resource, but currently requires extra financial support to develop the infrastructure necessary to the technology.

Construction of pilot scale lignocellulosic ethanol plants requires considerable financial support through grants and subsidies. On 28 February 2007, the U.S. Dept. of Energy announced $385 million in grant funding to six cellulosic ethanol plants.[34] This grant funding accounts for 40% of the investment costs. The remaining 60% comes from the promoters of those facilities. Hence, a total of $1 billion will be invested for approximately 140 million gallon capacity. This translates into $7/annual gallon production capacity in capital investment costs for pilot plants (this would work out to $.35/gal over the 20-year life of a facility); future capital costs are expected to be lower. Corn to ethanol plants cost roughly $1–3/annual gallon capacity, though the cost of the corn itself is considerably greater than for switchgrass or waste biomass.[35][36]

As of 2007, ethanol is produced mostly from sugars or starches, obtained from fruits and grains. In contrast, cellulosic ethanol is obtained from cellulose, the main component of wood, straw and much of the structure of plants. Since cellulose cannot be digested by humans, the production of cellulose does not compete with the production of food, other than conversion of land from food production to cellulose production (which has recently started to become an issue, due to rising wheat prices.) The price per ton of the raw material is thus much cheaper than grains or fruits. Moreover, since cellulose is the main component of plants, the whole plant can be harvested. This results in much better yields — up to 10 short tons per acre (22 t/ha), instead of 4 or 5 short tons per acre (9–11 t/ha) for the best crops of grain.[citation needed]

The raw material is plentiful. Cellulose is present in every plant, in the form of straw, grass, and wood.[37] It is estimated that 323 million tons of cellulose containing raw materials that could be used to create ethanol are thrown away each year in US alone. This includes 36.8 million dry tons of urban wood wastes, 90.5 million dry tons of primary mill residues, 45 million dry tons of forest residues, and 150.7 million dry tons of corn stover & wheat straw.[38] Transforming them into ethanol using efficient and cost effective hemi(cellulase) enzymes or other processes might provide as much as 30% of the current fuel consumption in the United States.[citation needed] Moreover, even land marginal for agriculture could be planted with cellulose-producing crops like switchgrass, resulting in enough production to substitute for all the current oil imports into the United States.[39]

Paper, cardboard, and packaging comprise a substantial part of the solid waste sent to landfills in the United States each day, 41.26% of all organic municipal solid waste (MSW) according to California Integrated Waste Management Board's city profiles.[citation needed] These city profiles account for accumulation of 612.3 short tons (555.5 t) daily per landfill where an average population density of 2,413 per square mile persists. Organic waste consists of 0.4% manure, 1.6% gypsum Board, 4.2% Glossy Paper, 4.2% Paper Ledger, 9.2% Wood, 10.5% Envelopes, 11.9% newsprint, 12.3% grass & leaves, 30.0% food scrap, 34.0% office paper, 35.2% corrugated cardboard, and 46.4% agricultural composites, makes up 71.51% of land fill. All these except Gypsum Board contain cellulose which is transformable into cellulosic ethanol[38] because they are the leading cause of methane plumes. Methane, a greenhouse gas, is 21 times more potent than carbon-dioxide.[40]

Reduction of the disposal of solid waste through cellulosic ethanol conversion would reduce solid waste disposal costs by local and state governments. It is estimated that each person in the US throws away 4.4 lb (2.0 kg) of trash each day, of which 37% contains waste paper which is largely cellulose. That computes to 244 thousand tons per day of discarded waste paper that contains cellulose.[41] The raw material to produce cellulosic ethanol is not only free, it has a negative cost — i.e., ethanol producers can get paid to take it away.[42]

In June 2006, a U.S. Senate hearing was told that the current cost of producing cellulosic ethanol is US $2.25 per US gallon (US $0.59/litre). This is primarily due to the current poor conversion efficiency.[citation needed] At that price it would cost about $120 to substitute a barrel of oil (42 gallons), taking into account the lower energy content of ethanol. However, the Department of Energy is optimistic and has requested a doubling of research funding. The same Senate hearing was told that the research target was to reduce the cost of production to US $1.07 per US gallon (US $0.28/litre) by 2012. "The production of cellulosic ethanol represents not only a step toward true energy diversity for the country, but a very cost-effective alternative to fossil fuels. It is advanced weaponry in the war on oil,” said Vinod Khosla, managing partner of Khosla Ventures, who recently told a Reuters Global Biofuels Summit that he could see cellulosic fuel prices sinking to $1 per gallon within ten years.

University of Massachusetts at Amherst researchers have developed a streamlined technique which uses "catalytic fast pyrolysis" (heating to 400–600 °C followed by rapid cooling) and zeolite as a catalyst to produce cellulosic ethanol in about 60 seconds. They estimate improvements in the process should be able to generate ethanol at the equivalent of $1–$1.70/gal of gasoline. As of April 2008, the process has only been developed to work at laboratory scales.[43]

Environmental effects: corn-based vs. grass-based

In 2008, there was only a small amount of switchgrass dedicated for ethanol production. In order for it to be grown on a large-scale production it must compete with existing uses of agricultural land, mainly for the production of crop commodities. Of the United States' 2.26 billion acres (9.1 million km2) of unsubmerged land,[44] 33% are forestland, 26% pastureland and grassland, and 20% crop land. A study done by the U.S. Departments of Energy and Agriculture in 2005 determined whether there were enough available land resources to sustain production of over 1 billion dry tons of biomass annually to replace 30% or more of the nation’s current use of liquid transportation fuels. The study found that there could be 1.3 billion dry tons of biomass available for ethanol use, by making little changes in agricultural and forestry practices and meeting the demands for forestry products, food, and fiber.[45] A recent study done by the University of Tennessee reported that as many as 100 million acres (400,000 km2, or 154,000 sq mi) of cropland and pasture will need to be allocated to switchgrass production in order to offset petroleum use by 25 percent.[46]

Currently, corn is easier and less expensive to process into ethanol in comparison to cellulosic ethanol. The Department of Energy estimates that it costs about $2.20 per gallon to produce cellulosic ethanol, which is twice as much as ethanol from corn. Enzymes that destroy plant cell wall tissue cost 30 to 50 cents per gallon of ethanol compared to 3 cents per gallon for corn.[47] The Department of Energy hopes to reduce this cost to $1.07 per gallon by 2012 to be effective. However, cellulosic biomass is cheaper to produce than corn, because it requires fewer inputs, such as energy, fertilizer, herbicide, and is accompanied by less soil erosion and improved soil fertility. Additionally, nonfermentable and unconverted solids left after making ethanol can be burned to provide the fuel needed to operate the conversion plant and produce electricity. Energy used to run corn-based ethanol plants is derived from coal and natural gas. The Institute for Local Self-Reliance estimates the cost of cellulosic ethanol from the first generation of commercial plants will be in the $1.90–$2.25 per gallon range, excluding incentives. This compares to the current cost of $1.20–$1.50 per gallon for ethanol from corn and the current retail price of over $4.00 per gallon for regular gasoline (which is subsidized and taxed).[48]

One of the major reasons for increasing the use of biofuels is to reduce greenhouse gas emissions.[49] In comparison to gasoline, ethanol burns cleaner with a greater efficiency, thus putting less carbon dioxide and overall pollution in the air. Additionally, only low levels of smog are produced from combustion.[50] According to the U.S. Department of Energy, ethanol from cellulose reduces green house gas emission by 90 percent, when compared to gasoline and in comparison to corn-based ethanol which decreases emissions by 10 to 20 percent.[46] Carbon dioxide gas emissions are shown to be 85% lower than those from gasoline. Cellulosic ethanol contributes little to the greenhouse effect and has a five times better net energy balance than corn-based ethanol.[50] When used as a fuel, cellulosic ethanol releases less sulfur, carbon monoxide, particulates, and greenhouse gases. Cellulosic ethanol should earn producers carbon reduction credits, higher than those given to producers who grow corn for ethanol, which is about 3 to 20 cents per gallon.[47]

It takes 0.76 J of energy from fossil fuels to produce 1 J worth of ethanol from corn.[51] This total includes the use of fossil fuels used for fertilizer, tractor fuel, ethanol plant operation, etc. Research has shown that 1 gallon of fossil fuel can produce over 5 gallons of ethanol from prairie grasses, according to Terry Riley, President of Policy at the Theodore Roosevelt Conservation Partnership. The United States Department of Energy concludes that corn-based ethanol provides 26 percent more energy than it requires for production, while cellulosic ethanol provides 80 percent more energy.[46] Cellulosic ethanol yields 80 percent more energy than is required to grow and convert it.[52] The process of turning corn into ethanol requires about 1,700 gallons of water for every 1 gallon of ethanol produced. Additionally, each gallon of ethanol leaves behind 12 gallons of waste that must be disposed.[53] Grain ethanol uses only the edible portion of the plant. Expansion of corn acres for the production of ethanol poses threats to biodiversity. Corn lacks a large root system, which allows extreme soil erosion to take place. This has a direct effect on soil particles, along with excess fertilizers and other chemicals, washing into local waterways, damaging water quality and harming aquatic life. Planting riparian areas can serve as a buffer to waterways, and decrease runoff.

Cellulose is not used for food and can be grown in all parts of the world. The entire plant can be used when producing cellulosic ethanol. Switchgrass yields twice as much ethanol per acre than corn.[46] Therefore, less land is needed for production and thus less habitat fragmentation. Biomass materials require fewer inputs, such as fertilizer, herbicides, and other chemicals that can pose risks to wildlife. Their extensive roots improve soil quality, reduce erosion, and increase nutrient capture. Herbaceous energy crops reduce soil erosion by greater than 90%, when compared to conventional commodity crop production. This can translate into improved water quality for rural communities. Additionally, herbaceous energy crops add organic material to depleted soils and can increase soil carbon, which can have a direct effect on climate change, as soil carbon can absorb carbon dioxide in the air.[54][55] As compared to commodity crop production, biomass reduces surface runoff and nitrogen transport. Switchgrass provides an environment for diverse wildlife habitation, mainly insects and ground birds. Conservation Reserve Program (CRP) land is composed of perennial grasses, which are used for cellulosic ethanol, and may be available for use.

For years American farmers have practiced row cropping, with crops such as sorghum and corn. Because of this, much is known about the effect of these practices on wildlife. The most significant effect of increased corn ethanol would be the additional land that would have to be converted to agricultural use and the increased erosion and fertilizer use that goes along with agricultural production. Increasing our ethanol production through the use of corn could produce negative effects on wildlife, the magnitude of which will depend on the scale of production and whether the land used for this increased production was formerly idle, in a natural state, or planted with other row crops. Another consideration is whether to plant a switchgrass monoculture or use a variety of grasses and other vegetation. While a mixture of vegetation types likely would provide better wildlife habitat, the technology has not yet developed to allow the processing of a mixture of different grass species or vegetation types into bioethanol. Of course, cellulosic ethanol production is still in its infancy, and the possibility of using diverse vegetation stands instead of monocultures deserves further exploration as research continues. [56]

Feedstocks

In general there are two types of feedstocks: forest (woody) Biomass and agricultural biomass. In the US, about 1.4 billion dry tons of biomass can be sustainably produced annually. About 370 million tons or 30% are forest biomass [57]. Forest biomass has higher cellulose and lignin content and lower hemicellulose and ash content than agricultural biomass. Because of the difficulties and low ethanol yield in fermenting pretreatment hydrolysate, especially those with very high 5 carbon hemicellulsoe sugars such as xylose, forest biomass has significant advantages over agricultural biomass. Forest biomass also has high density which significantly reduces transportation cost. It can be harvested year around which eliminates long term storage. The close to zero ash content of forest biomass significantly reduces dead load in transportation and processing. To meet the needs for biodiversity, forest biomass will be an important biomass feestock supply mix in the future biobased economy. However, forest biomass is much more recalcitrant than agricultural biomass. Recently, the USDA Forest Products Laboratory together with the University of Wisconsin–Madison developed efficient technologies [8][58] that can overcome the strong recalcitracne of forest (woody) biomass including those of softwood species that have low xylan content. Short-rotation intensive culture or tree farming can offer an almost unlimited opportunity for forest biomass production.[59]

wood chips from slashes and tree tops and saw dust from saw mills, and waste paper pulp are common forest biomass feedstocks for cellulosic ethanol production [60].

The following are a few examples of agricultural biomass:

Switchgrass (Panicum virgatum) is a native tallgrass prairie grass. Known for its hardiness and rapid growth, this perennial grows during the warm months to heights of 2–6 feet. Switchgrass can be grown in most parts of the United States, including swamplands, plains, streams, and along the shores & interstate highways. It is self-seeding (no tractor for sowing, only for mowing), resistant to many diseases and pests, & can produce high yields with low applications of fertilizer and other chemicals. It is also tolerant to poor soils, flooding, & drought; improves soil quality and prevents erosion due its type of root system.[61]

Switchgrass is an approved cover crop for land protected under the federal Conservation Reserve Program (CRP). CRP is a government program that pays producers a fee for not growing crops on land on which crops recently grew. This program reduces soil erosion, enhances water quality, and increases wildlife habitat. CRP land serves as a habitat for upland game, such as pheasants and ducks, and a number of insects. Switchgrass for biofuel production has been considered for use on Conservation Reserve Program (CRP) land, which could increase ecological sustainability and lower the cost of the CRP program. However, CRP rules would have to be modified to allow this economic use of the CRP land.[61]

Miscanthus x giganteus is another viable feedstock for cellulosic ethanol production. This species of grass is native to Asia and is the sterile triploid hybrid of miscanthus sinensis and miscanthus sacchariflorus. It can grow up to 12 feet (3.7 m) tall with little water or fertilizer input. Miscanthus is similar to switchgrass with respect to cold and drought tolerance and water use efficiency. Miscanthus is commercially grown in the European Union as a combustible energy source.

Corn cobs especially corn stovers are the most popular agricultural biomass.

Cellulosic ethanol commercialization

Cellulosic ethanol commercialization is the process of building an industry out of methods of turning cellulose-containing organic matter into fuel. Companies such as Iogen, Broin, and Abengoa are building refineries that can process biomass and turn it into ethanol, while companies such as Genencor, Diversa, Novozymes, and Dyadic are producing enzymes which could enable a cellulosic ethanol future. The shift from food crop feedstocks to waste residues and native grasses offers significant opportunities for a range of players, from farmers to biotechnology firms, and from project developers to investors.[62]

The cellulosic ethanol industry developed some new commercial-scale plants in 2008. In the United States, plants totaling 12 million liters (3.17 million gal) per year were operational, and an additional 80 million liters (21.13 million gal.) per year of capacity - in 26 new plants - was under construction. In Canada, capacity of 6 million liters per year was operational. In Europe, several plants were operational in Germany, Spain, and Sweden, and capacity of 10 million liters per year was under construction.[63]

Commercial Cellulosic Ethanol Plants in the U.S.[64][65]
(Operational or under construction)
Company Location Feedstock
Abengoa Bioenergy Hugoton, KS Wheat straw
BlueFire Ethanol Irvine, CA Multiple sources
Colusa Biomass Energy Corporation Sacramento, CA Waste rice straw
Coskata Warrenville, IL Biomass, Agricultural and Municipal wastes
DuPont Danisco Cellulosic Ethanol (DDCE) Vonore, TN Corn cobs, switchgrass
Fulcrum BioEnergy Reno, NV Municipal solid waste
Gulf Coast Energy Mossy Head, FL Wood waste
KL Energy Corp. Upton, WY Wood
Mascoma Lansing, MI Wood
POET LLC Emmetsburg, IA Corn cobs
Range Fuels[66] Treutlen County, GA Wood waste
SunOpta Little Falls, MN Wood chips
US Envirofuels Highlands County, FL Sweet sorghum
Xethanol Auburndale, FL Citrus peels

See also

References

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