Cellulosic materials have been touted as the feedstock for the “second generation” of biofuel production and the best alternative for replacing food and feed grains in ethanol production.

Proponents of cellulosic ethanol point out that its production generates a higher net energy gain and a lower level of greenhouse gas emissions relative to grain-based ethanol, due in part to the fact that a higher portion of the feedstock material is converted to fuel. As a result, the past decade has seen a tremendous increase in research related to ethanol production from feedstocks such as corn stover, switchgrass, rice hulls, wheat straw, landscape waste, paper processing waste, wood-processing waste, and sugarcane waste (U. S. Department of Energy, National Renewable Energy Laboratory 2007).

The U. S. Department of Energy committed more than $1 billion toward cellulosic ethanol projects in 2007, with a goal of making the fuel cost competitive at $1.33 per gallon by 2012 (U. S. DOE 2007) . Projects supported by the 2007 DOE commitment range from annual capacities of 11 million gallons of ethanol (Kansas) to 125 million gallons (Iowa) (U. S. DOE

2007) . The technologies utilized by these proposed plants also vary, as do their feedstocks. Feedstocks expected to be used by some of these proposed ventures include corn stover and cobs, rice and wheat straw, milo stubble, switchgrass, yard waste, wood and wood-processing residues, “green” wastes, and other wastes recovered from landfills. Some of the technologies to be utilized also generate coproducts such as electricity, hydrogen, ammonia, and methanol (U. S. DOE 2007).

A wide variety of cellulose-based biomass wastes and byproducts are available for conversion to biofuels. These include:

• Agricultural residues (corn stalks and cobs, straws, cotton gin trash, palm oil wastes, etc.)

• Paper (paper mill sludge, recycled newspaper, sorted municipal solid waste, etc.)

• Wood waste (sawdust, wood chips, prunings, etc.)

• Landscape waste (leaves, grass clippings, vegetable and fruit wastes, etc.)

Most of these materials are available at very low cost, and some even command tipping fees associated with their disposal as wastes.

Unlike grain-based ethanol, where processing technologies have become relatively stan­dardized and feedstock procurement is as simple as participating in the grain marketing system, cellulosic ethanol projects may have a wide range of technical efficiencies, conver­sion rates, and feedstock logistics. Decision makers, including agricultural producers, poten­tial investors, and rural community leaders, are interested in determining whether cellulosic ethanol production could be feasible in their area.

Cellulosic biomass is composed of cellulose, hemicellulose, and lignin. In order to produce ethanol from cellulosic biomass, complex cellulosic carbohydrates must be converted into simple sugars, which can then be fermented to ethanol by a variety of microorganisms. Cellulose conversion to sugars can be catalyzed by a variety of acids, including sulfuric, hydrochloric, hydrofluoric, and nitric acids. A decrystallized cellulosic mixture of acid and sugars reacts in the presence of water to produce individual sugar molecules (hydrolysis). The product from this hydrolysis is then neutralized, and yeast fermentation is used to produce ethanol. When inexpensive dilute acid is used to catalyze the hydrolysis reaction, biomass is impregnated with dilute sulfuric acid solution and treated with steam at tempera­tures ranging from 140 to 260°C (Katzen and Schell 2006). Concentrated acids can also be used to hydrolyze cellulose and hemicellulose to sugars.

Temperatures ranging from 100 to 120°C, which are lower than those with the dilute acid process, are typically used, and high yields of sugars are obtained with little production of degradation products. The economic viability of this process depends, however, on the suc­cessful recovery of the acid at low cost (Katzen and Schell 2006).

Enzymatic conversion of cellulose to sugars offers some promising advantages over acid hydrolysis. Sugar yields are limited during acid hydrolysis because sugars are also converted to degradation products. Cellulase is a multicomponent enzyme system that catalyzes cel­lulose hydrolysis and is 100% selective for conversion of cellulose to glucose; high yields are therefore possible. This enzyme is produced by a variety of microorganisms, most com­monly, the fungus Trichoderma reesei. Cellulose conversion rates are limited by the ability of the enzyme to access the cellulosic substrate. To increase accessibility, biomass is sub­jected to physical and chemical treatments that disrupt the biomass structure, usually by removing a fraction of the hemicellulose and/or lignin. Effective pretreatment is necessary to achieving good cellulose-to-glucose conversion yields (Katzen and Schell 2006).

Figure 8.2 provides a process flow that describes how cellulose processing might be incor­porated into an existing dry-mill ethanol production facility. With this configuration, the plant could continue to process corn feedstocks but could also have the capability of processing cellulosic feedstocks. Cellulosic feedstocks would be cleaned and ground to reduce particle size and expedite processing. The material would then be dried as needed to a moisture content acceptable for acid decrystallization (separation of the cellulose and hemicellulose from the lignin).

In Figure 8.2 example, a process using concentrated sulfuric acid is described. The con­centrated acid process offers some advantages over the dilute acid process. Even though the dilute acid is considerably cheaper to purchase, the concentrated acid process can compete successfully if the acid is recycled. While this would add capital expenditure up front, it would significantly reduce operating costs through lower expenditures for acid. Additionally, the concentrated acid process operates at lower temperatures (100-120°C) than the dilute processes, which operate at 140-260°C (Katzen and Schell 2006). Therefore, considerably less energy is required for the concentrated acid process.

In the near term, the concentrated acid process offers higher production rates than enzy­matic processes. However, enzymatic processes are safer and more environmentally friendly than those that use concentrated acid. Consequently, as the productivity of enzymatic hydro­lysis processes are improved, it is likely that the long-term prospects for this technology will be more favorable than either dilute or concentrated acid hydrolysis processes.

Conventional corn-to-ethanol conversion processes, baker’s yeast (Saccharomyces cerevisiae) , is commonly used in the fermentation step to produce ethanol from hexose (six — carbon sugar). The carbohydrates present in lignocellulosic biomass are considerably more complex than those derived from corn. Large quantities of xylose and arabinose, which are five-carbon sugars derived from the hemicellulose portion of the lignocellulose, are also present in the products derived from hydrolysis. For instance, when corn stover is hydrolyzed, approximately 30% of the total fermentable sugars produced are in the form of xylose. Consequently, the fermenting microorganisms used in cellulosic ethanol production must be capable of utilizing the entire range of sugars produced during hydrolysis. This will be vital to increasing the economic competitiveness of cellulosic ethanol and to incorporating cellulosic ethanol processes into existing corn-to-ethanol operations.

Metabolic engineering for microorganisms used in fuel ethanol production has made significant progress in recent years. In addition to S. cerevisiae, microorganisms such as Zymomonas mobilis and Escherichia coli have been targeted through metabolic engineering for cellulosic ethanol production (Jeffries and Jin 2004). Engineered yeasts have also been shown to effectively ferment xylose (Ohgren et al. 2006) and arabinose (Becker 2003) as well as both sugars together (Karhum et al. 2006). Utilizing yeast cells for cellulosic ethanol processes is particularly appealing because they have been used in biotechnology for hun­dreds of years. Yeast is tolerant to high ethanol and inhibitor concentrations because they can grow at low pH values, which prevent bacterial contaminations.

In order to successfully incorporate cellulosic ethanol production using concentrated acid hydrolysis into existing dry-mill processing, several key factors must be addressed. Concentrated acid is expensive; therefore, efficient methods for recovering and reconcentrat­ing acid must be incorporated. The sugars produced through the hydrolysis process must be of high concentration and high purity. Additionally, the process must have the ability to ferment both six-carbon and five — carbon sugars efficiently with conventional microbes. It is anticipated that early attempts to incorporate acid hydrolysis of cellulosic materials into exist­ing dry-mill operations will include dedicated fermentation vessels that are separate from the fermenters that process corn- derived sugars. As yeast strains that are equally capable of fermenting both six — carbon and five — carbon sugars are developed, the need for dedicated fermenters will no longer be necessary.

Several technologies that have been proven in small-scale facilities have been developed to produce cellulosic ethanol. These technologies are moving toward commercial production. However, challenges remain with respect to expanding the technologies to production scale, reducing production costs, and financing large-volume plants. To spur commercial develop­ment, the U. S. DOE announced grants of $385 million for six commercial-scale cellulosic ethanol biorefineries in February 2007. Chesterfield, Missouri-based Abengoa Bioenergy, and Range Fuels in Broomfield, Colorado, were each awarded $76 million; BlueFire Ethanol in Irvine, California, received $40 million; and Sioux Falls, South Dakota-based Poet was granted $80 million. These companies expect to complete commercial-scale facilities between 2009 and 2011. The two remaining firms, Alico, in La Belle, Florida, and Iogen in Ontario, Canada, which were awarded $33 million and $80 million respectively, have dropped out of the program (Greer 2008). The grants were awarded under Section 932 of the Energy Policy Act of 2005, which authorized the DOE to fund commercial demonstration of advanced biorefineries that use cellulosic feedstock to coproduce ethanol, bioproducts, heat, and power. Awards were capped at 40% of the total project cost, up to a maximum of $80 million (Greer 2008 ).

BlueFire Ethanol is developing a $150 million cellulosic ethanol facility in Riverside County, California. The so — called Mecca project will use concentrated acid hydrolysis to convert the green waste portion of a municipal solid waste stream and green agricultural waste, currently sent to landfills, into 17 million gallons per year of ethanol. The firm expects to begin construction in late 2009.

The first step in BlueFire’s process, which is described on their Web site, employs con­centrated acid hydrolysis to separate the cellulose and hemicellulose from the lignin and then hydrolyze the cellulose to produce simple sugars for fermentation (BlueFire Ethanol, n. d.) . After hydrolysis, a filtration and pressing process will remove the lignin and other insoluble materials from the sugar mixture.

BlueFire’s process utilizes a chromatographic system to separate the acid from the sugar. The process concentrates and recycles 98% of the sulfuric acid for reuse. The remaining 1%-2% of the acid left in the sugar solution is neutralized with lime, creating hydrated gypsum that can be easily separated from the sugar solution. Specially developed cultures of yeasts are then used to ferment the sugar stream into ethanol.

Byproducts from the process include lignin, which will be burned in solid fuel boilers to satisfy about 70% of the plant’s thermal needs, a yeast stream that can be sold into animal feed markets, and agricultural-grade gypsum that can be used as a soil amendment. The project proximity to Los Angeles is advantageous in reducing the cost of transporting feedstock to the plant and moving the ethanol to market.

Constructing this type of plant in California offers advantages over most other locations because curbside source separation and primary separation of municipal solid waste materi­als has already been implemented to a great extent. Feedstock used in the process includes green agricultural waste, commercial landscaping green waste, clean woody construction and demolition debris, and short paper fibers that cannot be recycled.

Poet and Abengoa Bioenergy plan to utilize enzymatic hydrolysis and fermentation tech­niques to produce cellulosic ethanol. The processes employ enzymes to liberate fermentable sugars locked in the complex carbohydrate structures that form the cell walls of plants. Microbes then ferment the sugars into ethanol (Greer 2008). Poet will convert an existing 50 million gallons per year corn ethanol facility in Emmetsburg, Iowa, into a 125 million gallons per year biorefinery, which will include a 25 million gallons per year cellulosic ethanol pro­cessing system. The new $200 million facility, called Project Liberty, will produce cellulosic ethanol from 770 tons of corncobs and corn fiber. Construction is expected to start in early 2010, with completion and commissioning in the second half of 2011 (Greer 2008).

Utilizing corncobs as a source of feedstock offers a less disruptive source of biomass than most other byproducts, wastes, and crops. In conventional harvesting systems, corn is har­vested by combines and the kernels are removed from the cob. The cobs are discarded and left on the surface, where they are plowed into the soil at a later date. By collecting the cobs separately from the kernels during harvesting, a relatively uniform source of biomass can be removed and stored for conversion to ethanol. The remainder of the crop residues can be left for incorporation into the soil and preservation of organic matter levels.

Poet estimates that farmers will receive between $30 and $60 per ton of corncobs and the average acre of corn yields between three-quarters to a ton of corncobs. Each ton of corncobs will produce approximately 85 more gallons of ethanol per acre. Consequently, about 27% more ethanol can be produced by adding the cellulosic production method to the current corn-to-ethanol technology (Greer 2008).

Colocating the corn and cellulosic ethanol facilities will allow Poet to leverage its relation­ships with the hundreds of farmers that already provide corn to the plant. Those same farmers will provide the cobs as well. The cellulosic ethanol facility will also take advantage of the existing biorefinery infrastructure, including roads, railroads, utilities, and land.

Waste from the cellulose-to-ethanol process will be used to produce steam in a solid fuel boiler and biogas in an anaerobic digester, generating process heat for the entire biorefinery. These alternative fuels will significantly reduce Poet’s usage of natural gas. The company completed a $9 million pilot-scale cellulosic ethanol plant, adjacent to its 9 million gallons per year corn ethanol refinery in Scotland, South Dakota, in late 2008. The facility will produce 20,000gallons of cellulosic ethanol from corncobs and fiber. Lessons learned from the development, testing, and validating of the technology at the Scotland facility will be applied to the design and engineering of the project (Greer 2008).

Abengoa Bioenergy is also building a hybrid biorefinery producing corn and cellulosic ethanol. The new facility in Hugoton, Kansas, will produce 85 million gallons per year of corn ethanol and 11.4 million gallons per year of cellulosic ethanol from 400 dry metric tons of biomass. Total project costs are estimated at $500 million, including $190 million for the cellulosic ethanol plant. Abengoa currently produces 198 million gallons per year of corn ethanol in the United States and 142 million gallons per year in Europe. The company also operates a cellulosic ethanol pilot facility in York, Nebraska.

Initially, biomass feedstock for the Hugoton plant will include corn stover, wheat straw, and milo stubble. In an effort to develop feedstock sources that are diverse and sustainable, the project will work with local farmers to establish energy crops, such as switchgrass, on nonfood-producing acres. A biomass gasification system producing syngas for thermal energy will reduce fossil fuel usage and greenhouse gas emissions.

Abengoa has received $15 million of its $76 million DOE grant to fund the preliminary design, permitting, and environmental review. Construction will begin shortly thereafter, with completion slated for 2011.