By Dan Burden, content specialist, AgMRC, Iowa State University, djburden@iastate.edu.

Profile revised August 2009.
 

Overview
One of the attractions of biofuels is they can be used in most internal combustion engines with little or no modification. Ethanol and biodiesel are the two most immediate candidates for adoption into the existing petroleum fuel infrastructure. For decades, ethanol has been used alone and with petroleum-based gasoline in internal combustion engines. Now, the renewable nature of its feedstocks has focused national attention and government assistance on ethanol as a renewable fuel. Present U.S. ethanol production is primarily by the bioconversion of the grain from corn and wheat, and in some instances, sugarcane.

Current grain-based ethanol production systems are an obvious first step in developing an agricultural-based industrial sector that addresses part of our national biofuel need. The industry is based on existing and proven crop production and transportation infrastructure models as well as a proven, workable fermentation production technology. However, grain-based production is limited by available grain feedstocks and their prices as valuable multi-use commodities. Any way to increase the efficiency of the grain-based processing systems increases their profitability for their owners and investors.

For this reason, research engineers and industrialists have sought to make conventional, grain-based ethanol systems more efficient by not just fermenting (fermentation processing) the grain, but by bioprocessing as much of the plant (cellulose and hemicellulose) as possible. This technology looks to digest (enzymatic digestion) much of the plant into usable subunits that can then be efficiently converted (bio-catalysis) to sugars for fermentation processing. The less usable co-products (lignin, ash and hard-to-process proteins) can be combusted to provide power and heat for the ethanol production facility and the residual non-combustible ash and gypsum can become a marketed co-product for field fertilizer. This is the basic system envisioned for cellulosic ethanol production. Cellulosic (plant fiber) "conversion," along with hydrogen, is viewed by many environmental and social policy organizations as being the transportation fuel future of the United States, if not the world.

Conventional Ethanol Versus Cellulosic Ethanol
Cellulosic ethanol exhibits a net energy content three times higher than conventional ethanol from corn kernel grain, and some of the cellulosic production systems emit far lower net levels of greenhouse gases (GHG). Conventional, grain-based ethanol uses fossil fuel to produce heat for fermentation and other aspects of processing and produces GHG emissions. Cellulosic ethanol production uses part of the input-biomass feedstock (lignin, hard-to-process proteins) instead of fossil fuel. This very positively changes the "Well to Wheel" life-cycle analysis model used to calculate overall GHG emissions from fuels and internal combustion engines. Life-cycle analyses look at the environmental impact of a product from its inception to the end of its useful life.

Cellulosic ethanol also may provide additional positive environmental benefits in the form of reductions in GHG emissions and air pollution. Some researchers calculate that since lignin is a renewable fuel with no net GHG emissions, the GHG produced by the combustion of biomass are essentially offset by the CO2 absorbed by the plant material (biomass crop) because it sequesters carbon during its growth.

Conventional ethanol and cellulosic ethanol result in the same product, ethanol, but are produced using different feedstocks and processes. Conventional ethanol is predominantly derived from corn grain. Corn is converted to ethanol in either a dry- or wet-mill process. In dry-milling operations, liquefied corn starch is produced by heating corn meal with water and enzymes. A second enzyme converts the liquefied starch to sugars, which are fermented by yeast into ethanol and carbon dioxide. Wet-milling operations separate the fiber, germ (oil) and protein from the starch before it is fermented into ethanol.

With respect to conventional ethanol, technological research primarily is concentrated in two areas: ethanol yield improvement and overall production efficiency. Both seek to reduce production cost and create a viable technology to replace imported oil, as well as make crop, crop residue and alternative feedstock-based ethanol a sustainable, domestic biofuel. Much of this research is applicable to converting or transitioning existing conventional grain-based production facilities to cellulosic systems.

Cellulosic ethanol is a potential replacement for gasoline and grain-based ethanol in cars and trucks. Additionally, cellulosic ethanol is promoted, as are other biofuels, to reduce the nation's dependence on imported oil, increase energy security and reduce the trade deficit. Rural economies will benefit from new job creation and in the form of increased farm incomes.

Producing Cellulosic Ethanol
In general, the bioconversion of cellulose to ethanol requires three major processing steps: pretreatment, saccharification and fermentation. In order to use woody wastes, it is first necessary to break down the "woody compounds" into fermentable sugars. This has taken the form of a number of different pretreatment (pre-fermentation) strategies. Pretreatment requirements vary with the feedstock and are often substantially less in the case of various paper and hydrolysis waste streams.

Pretreatment
Pretreatment is an essential step for bioconversion of most lignocellulosic materials. Roughly two-thirds of the lignocellulosic materials is present as cellulose and hemicellulose (the two main components of plants that give them structure), and lignin makes up the bulk of the remaining dry mass. To efficiently and economically produce cellulosic ethanol, the complex polymeric structures must be separated into fermentable sugars. The sugars in cellulose and hemicellulose are locked in complex carbohydrates called polysaccharides (long chains of monosaccharides, or simple sugars). Pretreatment breaks apart the structure of biomass to allow for the efficient, effective hydrolysis of cellulosic sugars, but this involves extremely complex chemical engineering. Usually the systems use processes to disrupt the hemicellulose/lignin sheath that surrounds the cellulose in plant material.

Pretreatments maximize subsequent bioconversion yields and minimize the formation of inhibitory compounds. They include acid hydrolysis (a controlled breakdown using dilute acid), alkali treatment, ammonia fiber explosion (liquid ammonia under moderate heat and pressure to separate the biomass components), autohydrolysis, chemical pulping, heat, mechanical size reduction, solvent extraction, steam, steam explosion, weak acid hydrolysis and various combinations of these separate processes. These technologies have different strategies for accessing the cellulose and hemicellulose and then dealing with the lignin, smaller amounts of other proteins, lipids (fats, waxes and oils) and ash.

Acid hydrolysis is often used as a pretreatment because it can be adapted to a wide variety of feedstocks. Except for strong hydrochloric acid hydrolysis, it is generally carried out at an elevated temperature (to 240 100 centigrade degrees) for various times. At higher acid concentrations, it can be carried out at temperatures as low as 30 degrees centigrade. Generally an inexpensive process, acid hydrolysis may also produce large quantities of degradation byproducts and undesirable compounds that inhibit other areas of cellulosic processing.

Autohydrolysis is the process of converting lignocellulose into fermentable sugars by exposure to high-temperature steam. Many lignocellulosic materials contain significant quantities of acetylated hemicellulose. Steam releases these in the form of acetic acid, which subsequently carries out a partial hydrolysis of the hemicellulosic and cellulosic sugars. The principal disadvantage of this approach is that sugar yields are generally very low.

Usually performed in a tumbling reactor, steam treatment is a convenient way to facilitate separation of plastics and fibers while increasing digestibility of municipal solid waste. In this experimental process, garbage is introduced into a large, cylindrical, horizontal autoclave that is slowly rotated on its side while steam treatment takes place. The plastic materials collapse and the fibrous materials form a pulp. Metals and other nonfibrous materials (for example, old shoes, composite material containers, plastic objects) are readily separated on screens after treatment.

The process known as weak acid hydrolysis consists of sulfur dioxide combined with steam. It is particularly effective as a pretreatment for enzymatic cellulose saccharification. Sulfur dioxide is often used in combination with autohydrolysis because it gives better sugar yields and helps to modify lignin for subsequent extraction or recovery.

Saccharification
Cellulose saccharification is the process of turning polymeric lignocellulosic materials into fermentable sugars and can be accomplished by a number of processes including acid and enzymatic hydrolysis.

Acid hydrolysis and enzymatic hydrolysis are currently the main two processes used to create fermentable sugars from cellulosic biomass. Acid hydrolysis processing breaks down the complex carbohydrates into simple sugars. Enzymatic hydrolysis processing uses a complex pretreatment processing stage to reduce the size of the material, making it more efficient than acid hydrolysis. In both processes, enzymes are used to convert the cellulosic biomass into fermentable sugars and then microbial fermentation (as in current corn-based systems) is used to produce ethanol. As with current corn-based systems, carbon dioxide is produced as a co-product in this final stage of production.

Dilute acid hydrolysis with 1 percent to 5 percent sulfuric acid is generally considered the most cost-effective means of hydrolysing wood and agricultural residues. Yields of hemicellulosic sugars can be 80 percent to 95 percent of theoretical. Yields of glucose from cellulose are generally less than 50 percent but can approach 55 percent at elevated temperatures.

Strong acid hydrolysis, often using a concentrated form of sulfuric acid, usually separates and recycles the acid catalyst, limiting the total acid losses to approximately 3 percent, or the same as the dilute process. Use of the concentrated acid, however, allows lower temperature and pressure hydrolysis with fewer byproducts produced. Concentrated hydrochloric acid at a concentration of about 47 percent is sometimes used for strong acid hydrolysis because it is relatively easy to recover. Hydrolysis with concentrated hydrochloric acid gives one of the highest sugar yields of any acid hydrolysis process. It is carried out at room temperature. The chief drawback is that it is highly corrosive, volatile, expensive and almost complete recovery is essential to make the process economical.

Ultimately, the goal is to get higher sugar yields as efficiently as possible without degrading the feedstock materials. This is easier said than done. Each technology has advantages and disadvantages in terms of costs, yields, material degradation, downstream processing and generation of process wastes.

Feedstocks
Cellulosic ethanol can be produced from a wide variety of cellulosic biomass feedstocks. These include agricultural plant wastes (corn stover, cereal straws, sugarcane bagasse), plant wastes from industrial processes (sawdust, paper pulp, distiller’s grains) and energy crops grown specifically for fuel production, such as switchgrass. Growing energy crops and harvesting agricultural residuals are projected to increase the value of farm crops, potentially eliminating the need for some agricultural subsidies.

Perennial grasses, such as switchgrass and miscanthus, have been discussed as promising feedstocks for cellulosic ethanol production. They use water efficiently and do not need a lot of fertilizers or pesticides. However, their production economics are questionable, and there is little to no production and collection infrastructure in place to drive their acceptance as feedstocks.

With heterogeneous waste streams such as municipal solid waste, it may be necessary to include costly, complex sorting and filtering steps to separate usable material from those that may inhibit the process before carrying out pretreatment. In the case of agricultural residues, particle size reduction can often be done simply with grinding. For dry corn or soybean residues, density is often lower than what is desirable, and particle size reduction is not an issue. Municipal solid wastes present a particular problem because of their extremely heterogeneous nature. Large quantities of plastics, wood, metals and other materials are often present. If certain heavy metals or toxic compounds are present in the mixture when acid is added, they can create severe downstream problems in fermentation and product formation.

Many cellulosic ethanol advocates see cellulosic conversion as a one-stop, one-size-fits-all fuel production system. This is partly due to their lack of understanding of the complexity of the chemical engineering and biological components of the processing system and partly due to their wishful thinking regarding exploitation of agricultural wastes. Agricultural wastes, agricultural residue material that is plowed into the soil for fertilizer, composted, burned or disposed of in landfills, are more abundant and contain greater potential energy than simple starches and sugars. Additionally, the collection, transportation and perhaps processing of these residues would present farmers and agricultural services providers with another lucrative crop-based profit center.

The popular press has promoted cellulosic processing as a system in which any cellulose-containing waste (farm field, industrial and municipal) and waste (newsprint, homeowner leaf and yard waste, sawdust, wild native grasses) can be processed by the same system within the same facility. A range of material this varied would have vast differences in chemical makeup, as well as the contaminant load that each would carry into the system. The biological components of any processing system seldom are amenable to introduced molds, bacteria and other contaminants. In current ethanol production technologies, food production systems and so on, these biological contaminants tend to wreak havoc with production efficiencies and can even curtail operation of a processing facility and force shutdowns, requiring costly restarts.

The problem is that the complex chemical and biological engineering needed to optimally convert (in a manner suitable for profitable, large-scale fuel production) a single homogeneous feedstock, for example, wheat straw, may be very different from another homogeneous feedstock, for example, corn stalks. Add to this the harvest and storage technologies that may not exist at this time (yet have been evolving for over a century for grain-based production systems).

However, cellulosic conversion “dialed-in” to a specific feedstock could be a great solution for taking some waste materials from a landfill and converting them to fuel and usable co-products. This “bio-remediation” of an industrial waste stream has the potential to eliminate a costly expense for a company and turns it into a new profit center. One example is paper sludge, a waste material that goes into landfills at a cost of $60 to $100 a dry ton. Others are various wood wastes and field stovers. Perhaps the most promising application is using cellulosic conversion to “re-use” or “further process” the dried distillers grain component produced as a co-product from conventional grain ethanol production. This system is using an extremely homogeneous pre-processed feedstock, and there is no need for complex new handling and storage systems.

Industry Overview: The Players
In attempting to survey commercial activity in cellulosic digestion and related technologies, the author located a list compiled and provided by the Vandershield Research investment firm (2008) that cites leading companies with extensive cellulosic-digestion research, pilot and production programs, and their strategic alliances. An abridged version with additional information and annotations from the author of this article includes the companies listed below. Please note that the economic downturn of late 2008 and early 2009 has severely curtailed much start-up activity as well as negatively impacted many joint ventures. Also, major petroleum companies, e.g., BP and Chevron, are actively funding comprehensive biofuels research initiatives and joint ventures. However, as a group, petrochemical companies are not included in the list. The information below is for illustrative purposes to give the reader a feel for the landscape of this ever-changing cutting-edge industry and should not be interpreted in any way as a definitive statement.

• Abengoa, Seville, Spain, is constructing the world's first commercial-scale cellulosic ethanol biorefinery in Babilafuente (Salamanca), Spain using components from SunOpta in partnership with Dyadic. They also are involved in conversion of a corn-based ethanol plant they own in York, Nebraska, into a biomass ethanol facility, which would initially use small grain straw and corn stover as the biomass feedstock.  See also Broin and DuPont.
• Alico, a LaBelle, Florida, company, lists itself as a land-management company and is involved in various types of large-scale animal and plant agriculture, for example, cattle ranching, forestry and land leased for farming, grazing, recreation and oil exploration utilize the same acreage in some instances. The company has four wholly owned subsidiaries: Agri-Insurance Company, Ltd. ("Agri"), Alico-Agri, Ltd. (“Alico-Agri”), Bowen Brothers Fruit LLC (“Bowen”), and Alico Land Development Company. In early 2007, the Department of Energy announced it would award the company up to $33 million for a proposed plant in LaBelle (Hendry County), Florida, to produce 13.9 million gallons of ethanol a year and 6,255 kilowatts of electric power, as well as 8.8 tons of hydrogen and 50 tons of ammonia per day. For feedstock, the plant will use 770 tons per day of yard, wood and vegetative wastes and energycane (if and when that resource becomes agronomically viable).
• Archer Daniels Midland (ADM), Decatur, Illinois, is aggressively studying how to produce cellulosic ethanol out of parts of the corn kernel that are traditionally not used for ethanol. They believe this process will boost production of ethanol by 15 percent. With ADM’s extensive research capability, they probably are involved with more varied technologies than those noted in most press releases.
• American process, Inc., Atlanta, Georgia, has stated that it has unique and innovative technology for wood cellulose conversion that will be used at the Flambeau River Biorefinery project, Park Falls, Wisconsin. It will be the first modern U.S.-based pulp-mill biorefinery to produce cellulosic ethanol from spent pulping liquor. Project engineering has commenced with a production of ethanol expected to begin in 2009. This company, with international offices in Greece and Romania, is primarily known for optimizing energy efficiencies within the pulp and paper industries; including on-site energy co-generation systems.
• BRI Energy, New Smyrna Beach, Florida, has developed a gasification process, used with fermentation and distillation to produce ethanol and electricity from a wide array of carbon-based wastes. Their Web site states, “An estimated 1.5 billion tons of municipal solid waste, green waste, sewage sludge, plastics, auto fluff, agricultural, forestry and other waste products, including some 300 million used tires, are generated in the United States each year. 320 million tons are readily available for use in the production of liquid and electric energy.”
• Broin (Broin Companies, renamed Poet), Sioux Falls, South Dakota, See listing for Poet; also in partnership with Abengoa, DuPont and Novazymes listed elsewhere.
• Blue Fire, Irvine, California, plans to use the Arkenol Technology Process (which has been used in Izumi, Japan, since 2002) for creating cellulosic ethanol.
• Ceres, Thousand Oaks, California, is a privately-held plant biotech company investigating novel genomics technologies to develop improved energy crops (switchgrass) for cellulosic ethanol.
• Colusa, Colusa, California, has constructed a California biorefinery for harvested rice straw for conversion to ethanol and has proposed a plant in Arkansas. In March 2008, Colusa’s assets, liabilities and business operations will be transferred to British company Pan Gen Global PLC in exchange for a controlling interest in PGG. Colusa’s business operations will be conducted by Colusa Biomass Inc., a wholly owned subsidiary of PGG headquartered in Reno, Nevada.
• DuPont, Wilmington, Delaware, has partnered with Poet (Broin) to bring cost-effective ethanol derived from corn stover to market.
• Dyadic, Jupiter, Florida, has spent over a decade of work on the design and development of enzymes for the increasingly efficient extraction of sugars from biomass. In 2006, a partnership was announced with Abengoa.
• Global Energy Holding Group (formerly Xethanol), Atlanta, Georgia, in 2007 announced aggressive plans for its new BlueRidgeXethanol company to begin producing cellulose ethanol in Spring Hope, North Carolina, using acid hydrolysis and construction of a 50-million-gallon-per-year cellulosic ethanol plant in Augusta, Georgia.
• Globex (Golbex Green Energy), (address unknown). In 2006 and well into 2008, this group claimed to have developed a supercritical fluid (SCF) technology to be used along with enzymatic hydrolysis for the production of cellulosic ethanol. A press release stated that their new supercritical fluid (SCF) technology had a lignin removal rate of more than 50 percent at ambient temperatures into a SCF 4-liter pressure vessel. The material used in the trial was common wood chips. GloA 2009 search for information on this company could not locate it on the Internet; it may have been acquired by another company, the name may have changed or it is no longer in business.
• Green Star Products Inc. (GSPI), Salt Lake City, Utah, has developed a waterless continuous-flow-process reactor system that will be used in cellulose-ethanol plants planned in North Carolina and the Northwest. GSPI revenue stream comes from fabrication and installation of plants, profits generated from joint venture facilities, exclusive licensing arrangements, profit participation in manufacturing and sales of high-tech lubricants, royalty income and engineering service income. In 2007, GSPI also announced the development of an innovative processing system that allows algae to biodiesel conversion.
• Iogen Corp, Ottawa, Ontario, Canada, operates a demonstration-scale facility to convert biomass to cellulose ethanol using enzymatic-hydrolysis technology. Full-scale commercial facilities are being planned. They have publicly discussed an Idaho plant to make ethanol from wheat straw.
• Lignol Energy Conversion, Burnaby, British Columbia, Canada, plans to build biorefineries for ethanol and co-products produced from Canadian forests. The company has acquired and modified a solvent-based pretreatment technology originally developed by a subsidiary of General Electric. Lignol also acquired the original GE pilot plant for cellulose-to-ethanol conversion
• Mascoma, Boston, Massachusetts, is developing bio and process technology for cost-effective conversion of cellulosic biomass with investor and venture-cap support. They have a R&D center and have received a number of major government grants to further develop their technology. Mascoma is aggressively pursuing the development of advanced cellulosic ethanol technologies across a range of cellulosic feedstocks. As part of their strategy of technology discovery, development and deployment, the company is aggressively patenting numerous technologies and forming a broad set of research and commercial partnerships.
• Nova Fuels, (address unknown) is cited in several press releases and their Web site has contact telephone numbers only. They are said to develop mass-to-fuel conversion facilities (that use gasification technology) with joint venture partners. From an Internet search, they do not seem to be engaged in any actual project development.  (Note: Nova Biosource Fuels, Butte, Montana, a biodiesel technology company, appears to be a different company.)
• Novazymes, Bogsvaerd, Denmark, is a major international biotechnology company. They are developing enzymes to convert cellulose into simple sugars, for fermentation into fuel ethanol. They have collaboration/partnerships with Abengoa and Broin.
• Poet, Sioux Falls, South Dakota. The Broin (Broin Companies) changed their name to Poet on March 30, 2007. The largest dry mill ethanol producers in the United States, Poet is collaborating with Novozymes in the research and development of cellulose ethanol technology. Poet will expand their Emmetsburg, Iowa, facility to include cellulosic ethanol production from corn hulls and cobs. Completion is expected in 2009.
• PureEnergy, Paramus, New Jersey, has developed a two-stage dilute-acid-hydrolysis process that will be used in forthcoming projects by Green Star Products, Inc. Their Web site states that they have a biomass conversion technology designed to produce, from urban wastes and agricultural residues, a range of chemicals, including fuel-grade ethanol, in a biorefinery. The process has been engineered and pilot-scale tested and is now ready for commercialization.
• Range Fuels, Broomfield, Colorado, formerly known as Kergy, is funded by a venture cap firm. It claims it can produce more cellulosic ethanol for a given amount of energy expended than is possible with any other competing process. Just as noteworthy: The design allows them to "bring systems to sources where biomass is most plentiful, instead of having to transport biomass to a central processing site." It should be known that some in the cellulosic industry tend to scoff at Range's process because they use gasification instead of fermentation. Regardless, the Department of Energy has awarded the company up to $76 million to build a commercial cellulosic ethanol plant at a site near Soperton, Georgia. In August 2009, the company’s Web site stated that Range Fuels began operating a first-of-its-kind fully integrated thermo-chemical conversion pilot plant at its Development Center in Denver, Colorado, in the first quarter of 2008, successfully converting wood from Colorado pine beetle kill and Georgia pine and hardwoods into renewable fuels using a two-step process. The process converts non-food biomass into a synthesis gas or syngas using heat, pressure and steam, after which the syngas is passed over a proprietary catalyst to yield cellulosic biofuels. These cellulosic biofuels can then be separated and processed to yield a variety of low-carbon biofuels, including cellulosic ethanol and methanol, which can be used to displace gasoline or diesel transportation fuels, generate clean renewable energy or be used as low-carbon chemical building blocks. In addition, clean renewable power is produced from energy recovered in the conversion process.
• SunOpta (SunOpta Bioprocess, Inc., formerly Stake Technology Ltd.), Brampton, Ontario, Canada, built the first cellulosic ethanol plant 20 years ago, in France. It has four cellulosic ethanol projects that are or will be operational using SunOpta's technology and equipment to produce ethanol from cellulosic biomass: In September 2006 SunOpta provided its systems and technology to China Resources Alcohol Corporation (CRAC) and the plant began production of ethanol from local corn stover in October 2006. Key components of SunOpta's equipment and technology have recently been shipped to Spain for the Abengoa wheat straw to ethanol facility located in Salamanca, Spain. SunOpta's equipment and technology will be used in the Celunol facility being built in Jennings, Louisiana, to produce ethanol from sugarcane bagasse and wood. Recently announced the formation of a joint venture with GreenField Ethanol Inc., Canada's largest producer of ethanol. The purpose of this joint venture is to design, build and jointly own and operate plants producing ethanol from wood chips.
• Vernium, Cambridge, Massachusetts, was formed in 2007 in a merger between Celunol and Diversa. The company has closely partnered with Dupont, is researching multiple enzyme "cocktails" to break down cellulosic biomass and is positioning itself as a world leader in enzyme and related-technology service-provider to the cellulosic-ethanol industry. They also have purchased biomass-to-ethanol technology from SunOpta to have a system that will complement their own proprietary technology. In February 2007, Verenium broke ground on a 1.4 million gallon per year demonstration facility located adjacent to its cellulosic pilot facility in Jennings, Louisiana. The facility is designated to operate on diverse regional feedstocks including sugarcane bagasse and specially-bred energy cane. The facility was reported to have started limited operations in early 2008. Commercial-scale cellulosic ethanol facilities are slated for completion by 2010.
• Virgin Green Fund (formerly called Virgin Fuels), London, England, and San Francisco, California. This funding entity is looking to invest in projects and emerging technologies created by world-famous entrepreneur and adventurer Sir Richard Branson. He has pledged an estimated $3 billion to fight global warming. A large chunk of that is expected to be invested in cellulosic ethanol research and production.
• Xethanol, Atlanta, Georgia. See Global Energy Holding Group elsewhere in this list.

Conclusion
Research continues to push forward. Attempts are being made to address production system insufficiencies and the feedstock homogeneity challenges. The high cost of cellulose enzymes is a key barrier to economical cellulosic ethanol production. Various initiatives and national research programs are attempting to remedy this situation. Biofuel development is now a national priority. It is clear that technology and economic drivers will eventually take cellulosic conversion from its production system infancy into an adopted fuel production technology. Which feedstocks will go into it and how widespread the application and adoption of the technology are anyone's guess.

Prepared January 2008 and revised August 2009.