Cellulosic Ethanol
Cellulosic ethanol is just ethanol. Instead of coming from the fermentation of starch and sugar, it originates from cellulose. This is the stiff material which makes corn and wheat stalks stand erect, and the major component of wood. Both starch and cellulose are polymers of glucose. The difference is that the connection between glucose molecules in starch is highly digestible – virtually every animal and microbe on earth is capable of dismembering starch into its component glucose units and using these for energy.
The glucose units in cellulose are attached together differently. This bond is extremely hard to break. Only a few fungi, some bacteria plus termites are able to digest cellulose. Cows and other ruminants can digest cellulose too, but they employ bacteria, who live in their rumen, to break down the polymer for them. Then the ruminant absorbs the sugars which the bacteria have left behind and also the bacteria themselves.
Its very durability (Think about antique wood furniture which may be 1000 years old!) makes cellulose an unpromising material from which to obtain fermentable sugars. However, its sheer abundance (crop waste, discarded newsprint, forestry waste, etc) and very low value makes it very attractive.
There are a number of pilot plants which now produce cellulosic ethanol. They are clearly not competitive with corn ethanol or they would now dominate the market. One problem is the high cost of the enzymes which break down cellulose. If bacteria or fungi are used, they work slowly, so require very large vats in which to do their work and they occupy these vats for a very long time.
Cellulosic ethanol is a work in progress . . .