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Cellulose Ethanol And The Future |
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Cellulose ethanol
Cellulose ethanol is a type of ethanol that is produced from a great diversity of biomass including waste from urban, agricultural, and forestry sources. Unlike normal ethanol, whose original raw material are sugars and starches, cellulose ethanol's starting raw material is cellulose. There are at least two methods of production of cellulose ethanol hydrolysis followed by fermentation of the generated free sugars, and synthesis gas fermentation or catalysis. Neither process generates toxic emissions when it produces ethanol. Cellulose ethanol production currently exists at "pilot" and "commercial demonstration" scale, including a plant in China engineered by SunOpta Inc. and owned and operated by China Resources Alcohol Corporation that is currently producing cellulose ethanol from corn stover (stalks and leaves) on a continuous, 24-hour per day basis.
According to US Department of Energy studies conducted by the Argonne Laboratories of the University of Chicago, one of the benefits of cellulose ethanol is that it reduces greenhouse gas emissions (GHG) by 85% over reformulated gasoline. By contrast, starch ethanol (e.g., from corn), which uses most of the time natural gas to provide energy for the process, reduces GHG emissions by 18% to 29% over gasoline. Sugar ethanol, on the other hand, from sugarcane, reduces greenhouse gas emissions by as much as cellulose ethanol because it uses sugarcane bagasse to provide the energy for the process and the excess to make electricity for the grid.
In April 2004, Iogen Corporation, a Canadian biotechnology firm, became the first business to commercially sell cellulose ethanol, though in very small quantities. The primary consumer thus far has been the Canadian government, which, along with the United States government (particularly the Department of Energy's National Renewable Energy Laboratory), has invested millions of dollars into assisting the commercialization of cellulose ethanol.
Another company which appears to be nearing commercialization of cellulose ethanol is Spain's Abengoa Bioenergy. Abengoa has and continues to invest heavily in the necessary technology for bringing cellulose ethanol to market. Utilizing process and pre-treatement technology from SunOpta Inc., Abengoa is building a 5 million gallon cellulose ethanol facility in Spain and have recently entered into a strategic research and development agreement with Dyadic International, Inc. (AMEX: DIL), to create new and better enzyme mixtures which may be used to improve both the efficiencies and cost structure of producing cellulose ethanol.
On December 21, 2006, SunOpta Inc. announced a Joint Venture with GreenField Ethanol, Canada's largest ethanol producer. The joint venture will build a series of large-scale plants that will make ethanol from wood chips, with SunOpta and GreenField each taking 50% ownership. The first of these plants will be 10 million gallons per year, which appears to be the first true "commercial scale" cellulose ethanol plant in the world. Under 1 million gallons per year (MMgy) is considered "Pilot Scale", greater than 1 MMgy but less than 10 MMgy is defined as "commercial demonstration", while a plant that produces 10 MMgy per year or greater is true "commercial scale". Despite the multiple commercial demonstration cellulose ethanol plants SunOpta has been involved with, media reports continue to state that cellulose ethanol is an unproven, "experimental" technology. The 10 MMgy SunOpta/GreenField cellulose ethanol plant is intended to demonstrate that large-scale cellulose ethanol is commercially viable immediately.
President Bush, in his State of the Union address delivered January 31, 2006, proposed to expand the use of cellulose ethanol. In his State of the Union Adress 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 mandated production of some 20 billion gallons per year of cellulose ethanol by 2017. Bush's plan includes $2 billion dollars funding for cellulose ethanol plants, with an additional $1.6 billion announced by the USDA on January 27, 2007.
Production methods
There are two broad ways of producing alcohol from cellulose. Hydrolysis breaks down the cellulose chains into sugar molecules that are then fermented and distilled. Gasification transforms the lignocellulose raw material into gaseous carbon monoxide and hydrogen that is then fed to a special kind of fermenter or to a catalyst bed.
Hydrolysis processes
The cellulose molecules are composed of long chains of glucose 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 hydrolysis 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. 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 cellulose 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. A significant obstacle to the dilute acid process is that the hydrolysis is so harsh that toxic degradation products are produced which is a hurdle for fermentation. Concentrated acid 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 there are actually at least three enzymes, used at various stages of this conversion. If the enzymatic hydrolysis process takes place with previously isolated enzymes, a steady supply of the cellulase enzymes is needed.
Iogen Corporation is a Canadian producer of enzymes. They are promoting an enzymatic hydrolysis process that uses "specially engineered enzymes". The raw material (wood or straw) has to be pre-treated to make it amenable to hydrolysis. Another Canadian company, SunOpta Inc. markets a patented technology known as "Steam Explosion" to pre-treat cellulose biomass, overcoming its "recalcitance" to make cellulose and hemicellulose accessible to enzymes for conversion into fermenatable sugars. SunOpta designs and engineers cellulose ethanol biorefineries and its process technologies and equipment are in use in the first 3 commercial demonstration scale plants in the world: 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 cellulose ethanol from local corn stover on a 24-hour a day basis utilizing SunOpta's process and technology.
Genencor and Novozymes are two other companies that have received United States government Department of Energy funding for research into reducing the cost of cellulase, a key enzyme in the production of cellulose ethanol by enzymatic hydrolysis.
Other enzyme companies, such as Dyadic International, Inc. (AMEX: DIL), are developing genetically engineered fungi which would produce large volumes of cellulase, xylanase and hemicellulase enzymes which can be utilized 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 cellulose ethanol.
Gasification process
The gasification process does not rely on chemical decomposition of the cellulose chain. 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 yeast, which operates on sugar, this process uses a microorganism named Clostridium ljungdahlii. 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:
Gasification Complex carbon based molecules are broken apart to access the carbon as carbon monoxide, carbon dioxide and hydrogen are produced
Fermentation Convert the carbon monoxide, carbon dioxide and hydrogen into ethanol using the Clostridium ljungdahlii organism
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.
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 as well.
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