There are at least three reasons for a practical limit to the present ethanol production approach. According to the Energy Information Administration, the 15 billion gallon upper limit projection on ethanol production probably won’t be exceeded if it has to depend on starchy crops such as corn.
This is because such crops are also used for food and animal feed, and only a limited fraction of the available supply could be diverted for fuel use without driving up crop prices to the point where ethanol production would no longer be economical. Corn is currently the primary feedstock for ethanol production in the United States.
There is also a limit to the amount of suitable land available for growing such feedstock crops.
In addition, only a portion of the corn crop feedstock can be used to produce ethanol. For example, corn grain can be used in ethanol production, but the stalks, husks, and leaves (collectively called corn stover) currently cannot be converted at economical costs.
This is why in the long term, greater levels of penetration by ethanol into the transportation fuels markets would require a transition to production of ethanol from cellulosic materials such as corn stover, switchgrass, miscanthus, and wood. But such a switchover is at least a decade away because of technological obstacles to low-cost conversion of cellulosic biomass into ethanol.
— William L. Kovacs