There are two types of ethanol that can be obtained from biomass: conventional and cellulosic ethanol. The two products are generally the same however the production method differs.

Cellulosic Ethanol

Before conversion can begin, it is necessary to collect large amounts of biomass that is rich in cellulose. Cellulose is an organic compound that is found in the primary and secondary walls of green plant cells as well as in some algae and bacteria. However, plant cellulose is the most commonly used for conversion to what is referred to as cellulosic ethanol due to its widespread availability.

For industrial purposes wood pulp is normally used since 50% of wood is cellulose. Cotton which contains up to 90% of cellulose is also used sometimes. The recent past has seen the introduction and use of energy crops that are high in biomass energy such as corn, poplar trees and switch-grass.
There are three basic stages that biomass goes through in its conversion to cellulosic ethanol: pretreatment, hydrolysis and fermentation to ethanol.

Pretreatment

Cellulose is usually found in the midst of polymers such as lignin and hemicelluloses sugars. The main goal of this process is to make easier the breakdown of cellulose by making it more accessible as well as to make the hemicelluloses sugars present in the biomass soluble.

The plant material is soaked in a mixture of acids and enzymes and in some cases it is also heated. This is done to separate the unwanted polymers from the cellulose in preparation for the hydrolysis stage. This is the most expensive stage due to the complexity of the process involved and there have been many calls for more research to be done in this area to make it more efficient and thus cheaper.

Hydrolysis

This is done to break down the cellulose into its component sugars. There are two types of hydrolysis that can be carried out: chemical and enzymatic.

Chemical Hydrolysis

Cellulose is first exposed to acids under heat and pressure then to water which frees the component sugars. This method is not commonly used because of the toxic by-products that often reduce effectiveness of the next stage.

Enzymatic Hydrolysis

This process is similar to what happens in the stomachs of ruminant animals like cows. Enzymes are similar to cellulose that has been artificially synthesized with the help of bacteria and fungi are used to break down cellulose into its component sugars. This occurs in two stages where the cellulose is first converted into double glucose molecules which are known as cellobiose. Different cellulose is then used to convert the cellobiose into single glucose residues.

Fermentation to Ethanol

This is the final process where the broken down sugars are converted into ethanol using microbes. Baker’s yeast is the most commonly used agent for this process. Distillation is then carried out to separate ethanol from water, microbes and residue.

Conventional Ethanol

This is produced from grains such as corn and wheat whereby starch is first liquefied using enzymes then a different set of enzymes is used to convert the liquefied starch into sugar that is then fermented into ethanol. The rest of the process remains similar with that of Cellulosic Ethanol.