The Carbohydrate Economy Newsletter Summer 2002 By David Morris http://www.carbohydrateeconomy.org
Cellulose to Ethanol: A Progress Report Making ethanol in large quantities from lignocellulosic crops is, for many, the equivalent of finding the Holy Grail. After all, lignin, hemicellulose and cellulose are the basic structural materials of plants. Hundreds of millions of tons are contained in everything from wood to grasses to straw to organic wastes. Lignocellulosic feedstocks are cheaper than starch crops. In the late 1990s, the emergence of a commercial process to produce ethanol from these feedstocks appeared imminent. Several companies publicly announced they'd begin producing commercial quantities by 2000. One company held a widely publicized groundbreaking in 1999. Yet midway through 2002, no commercial cellulose-to-ethanol plant is operating. What are the realistic prospects in the near future? First, some background information. The easiest way to make ethanol (alcohol) is to ferment it from simple sugars. Industrial ethanol used to be made from molasses. In Brazil it is made from sugar cane (sucrose). The second easiest way, and the one used to produce the vast majority of ethanol in the United States, is from starch crops, like corn. Starch consists of glucose molecules strung together. Thus one must break down the starch into glucose, an easy process using water. Sugars are also locked up in lignocellulosic materials but they are much harder to access and some of the sugars are harder to ferment using current microbes. The word 'lignocellulosic' describes the three material components of the cell walls of plants: lignin, hemicellulose, cellulose. The proportions can vary significantly. For agricultural residue like straw or stover the breakdown is approximately: 35% cellulose; 35% hemicellulose; 15% lignin; 15% other. Lignin is the portion of plants that is the ancestor of coal. It is a complex substance that supplies a good deal of the structural strength to the plant. It has relatively few current uses. When making paper, lignin is removed from the wood using strong acids. Lignin is not converted into sugars. Hemicellulose is a substance with many branches. It cross links with the lignin to create a complex web of bonds. Hemicellulose is comprised primarily of sugars that have five carbon (C5) atoms (e.g. xylose, arabinose). When treated with dilute sulfuric acid, hemicellulose readily reacts with water (hydrolysis) to produce these sugars. Currently C5 sugars are not converted into ethanol. Finding microbes that can do so is a very high priority for companies and may be the key to making ethanol from lignocellulosic materials competitive with ethanol from starch crops. Cellulose, the third component of cell walls, consists of a long chain of glucose molecules. A concentrated sulfuric acid often is used to access cellulose's sugars. Microbes are readily available to convert these sugars into ethanol. Ethanol was first made from cellulose wood pulp during World Wars I and II using a dilute acid hydrolysis process. But the starch-to-ethanol process has proven more competitive. Lignocellulosic feedstocks are cheaper than starch or sugar feedstocks: $30-35 per ton versus some $80 per ton for crops such as corn. But lignocellulose-to-ethanol facilities are more capital intensive than starch-to-ethanol facilities. Moreover, corn based ethanol facilities produce valuable byproducts (e.g. high protein animal feed) worth, on average, about $38 a ton, thereby reducing the cost of ethanol. The only current large-market for the byproduct of lignocellulose-to-ethanol manufacturing (lignin and hemicellulose) is as a low value fuel to generate steam or electricity. Several technologies are competing in the lignocellulose-to-ethanol area. The first out of the gate uses concentrated or dilute sulfuric acid to break down the cellulose and hemicellulose into sugars. This process is called acid hydrolysis. The economics of acid hydrolysis improve when cellulosic materials are available at a very low cost. The Masada Resource Group plans to build a waste-cellulose-to-ethanol facility in Middletown, New York. The 'tip fee' or fee paid to Masada for garbage unloaded at its facility may be as much as $75 per ton. The revenue from the tip fee plus the additional revenue from sale of recycled materials in the front end of the facility offsets its higher capital cost. Because it is a high temperature, high pressure process that relies on strong acids, it requires a significant capital investment, raising its cost. According to the National Renewable Energy Laboratory (NREL) the acid hydrolysis process currently costs $1.20-1.30 per gallon of ethanol produced and only modest cost reductions are predicted. The second technology entrepreneurs are betting on uses biological processes to convert the cellulose to sugars. This is called enzymatic hydrolysis. In this process the lignocellulosic material is first pretreated by dilute acid to increase the accessibility of the enzymes by solubilizing the lignin and hydrolyzing the hemicellulose. The capital cost of this facility is potentially modest because the biological processes take place at room temperature and atmospheric pressures. The major drawback is the high cost of the enzymes and the fact that one quarter to one third of the total sugars produced (C5) are not converted into high value products. The cost of enzymes to convert cellulose may now be $1 per gallon of ethanol produced, driving the overall price to over $2 per gallon. A feverish race is on to lower the cost of enzymes, as well as to develop microbes to convert the C5 sugars. One challenge is that three enzymes are actually needed to break down the cellulose. One enzyme breaks the cellulose apart. The second chews up the ends of the resulting molecules. The third converts the feedstock into glucose. In 2001, the Department of Energy (DOE) awarded two three year, $15 million contracts to two companies to reduce the cost of enzymes by 90 percent, to about 10 cents per gallon of ethanol. DOE estimates this will be achieved by 2005 and by 2010 an enzymebased commercial cellulose-to-ethanol facility will be producing ethanol at under $1.10 per gallon. That price would be nearly competitive with ethanol produced from starch. Currently, the leader in enzymatic conversion is a Canadian company, Iogen. Founded in the mid 1970s, Iogen began selling cellulase and amylase enzymes to the pulp and paper, textile and animal feed industries in 1991. Iogen was one of the first companies to be able to use glucose rather than the more expensive lactose as the feed material for the organism producing the cellulase enzyme. In 2001, Iogen established a 50 ton per day cellulose to ethanol pilot plant to test its enzymatic hydrolysis process and its steam explosion pretreatment process. The glucose produced is used internally to displace sugars Iogen would otherwise have to buy. Because of the savings from not having to purchase glucose on the open market, the pilot plant reportedly breaks even. A small stream of glucose is fermented into about 1 million gallons of ethanol a year to validate the process. Iogen is also working on converting the C5 sugars to ethanol. Iogen looks to open a large-scale commercial cellulose-to-ethanol facility in 2005. The third technology for converting cellulose into ethanol is a thermochemical process. Under high pressures and high temperatures, the lignocellulosic material becomes a gas. Pearson Technologies uses such a process to produce gas from biomass and then uses proprietary backend catalysts to combine the hydrogen and carbon monoxide components of the gas into alcohols like ethanol, methanol, propanol or butanol. In 2000, Ethxx, a Canadian company purchased Pearson. Pearson/Ethxx expects to have a 1-3 million gallon per year test facility operational by the fall of 2002. A commercial plant could become operational by 2005-2006. A hybrid technology is also available. This technology relies on front-end gasification of the lignocellulosic material and then uses a biological process to convert the components of the gas into alcohols. BioEngineering Resources, Inc. has developed a novel fermentation process to convert carbon monoxide and hydrogen to ethanol. It was originally developed to deal with waste gases from the oil and gas refining industry. The company is seeking to license the technology for the production of ethanol. The pursuit of an inexpensive way to convert our abundant supplies of cellulose into ethanol and other alcohols continues. A lot has been learned. Yet it will still be several years before the first commercial scale plant is in operation. _______________________________________________ Biofuel mailing list [EMAIL PROTECTED] http://wwia.org/mailman/listinfo/biofuel Biofuel at Journey to Forever: http://journeytoforever.org/biofuel.html Biofuel archives at Infoarchive.net (searchable): http://infoarchive.net/sgroup/biofuel/
