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 lignocellulosic 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 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. 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 cellulosic biomass, overcoming its "recalcitance" to make cellulose and hemicellulose accessible to enzymes for conversion into fermenatable sugars. SunOpta designs and engineers cellulosic 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 cellulosic 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 cellulosic 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 cellulosic 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