Cellulosic Ethanol: How It Works [Cellulolysis Process]

Cellulolysis Process Gasification Process Inputs & Outputs

Cellulolysis Process
There are six stages to produce ethanol using a biological approach (only the first three steps differ materially from crop-based ethanol production):

Pretreatment—There are a number of pretreatment techniques to make the cellulose separable from the lignin seal and crystalline structure, thereby making it accessible in the cellulolytic process. Pretreatment techniques include acid hydrolysis, steam explosion, ammonia fiber expansion (AFEX), alkaline wet oxidation and ozone pretreatment, or a combination of them to ensure better results. The more effective the pretreatment, the less degradation products will be included; these degradation products reduce effectiveness and efficiency of the hydrolysis and fermentation steps by adding inhibitors, including furfural and hydroxymethyl furfural, and require detoxification.
Celluloytic Process—The cellulose is broken down into sugar to be fermented using a chemical reaction or an enzymatic reaction.
Chemical Hydrolysis—Uses acid to break down the cellulose into sugars in the presence of water. Two acid mixtures can be used:
- Diluted Acid—Used under high heat and high pressure can be used, but is so harsh that it comes with the side effect of toxic degradation products which can interfere with the fermentation process.
- Concentrated Acid—Used at lower temperatures with atmospheric pressure can also be used, but the acid needs to be removed and recycled from the sugar to be commercially viable.
Enzymatic Hydrolysis—The cellulose is broken down into sugar by naturally occurring cellulase enzymes, similar to the breakdown of materials in the stomachs of cows. These enzymes work at temperatures around 120°F without forming inhibiting byproducts. To reach high sugar yields, enzymatic hydrolysis is more efficient compared to chemical hydrolysis. However, the problem with this method is the cost of the enzymes and difficulty in transferring from a trial scale to mass production. The enzymes being used/tested are generally those that are secreted by different fungi and bacteria.
Microbial Fermentation—The resulting sugars, xylose and arabinose, are fermented with a one of several yeasts, which consume the sugars and excrete ethanol and carbon dioxide. The yeast used ranges from Saccharomyces cerevisiae (baker’s yeast) to engineered yeasts such as zymomonas mobilis and escherichia coli. The choice of yeast is based on the type of feedstocks and cellulolytic process used.
Distillation—Similar to the production of crop-based ethanol, the fermented liquid moves into a distillation system where the liquid is heated to take advantage of the different boiling points of ethanol and water. These differing boiling points will enable the extraction of ethanol, which is roughly 95% pure.
Dehydration—The alcohol from the top of the column will then pass through a dehydration system, usually a molecular sieve, where the remaining water will be removed. This produces 200 proof, anhydrous (waterless) ethanol.
Denaturing—Ethanol that will be used for fuel in the U.S. and Europe is then denatured with a small amount, 2-5%, of gasoline to make it unfit for human consumption. Brazil uses 100% anhydrous ethanol in its vehicles.

Research is being done on naturally occurring and genetically engineered bacteria which can directly convert the pretreated cellulosic substance into ethanol, thereby combining the Hydrolysis and Fermentation steps. Clostridium thermocellum is an example, however it produces acetate and lactate byproducts which lower the efficiency of the process.