Wrong question. Do you mean ethanol made from irrigated corn grown with heavy fertilization and lots of herbicides and pesticides, trucked to a plant that uses coal to distill the beer and to dry the leftover distillers dried grain so it won’t spoil on the way to the cows that eat it? Not very green; you’re not doing the planet much of a favor using it, but you are subsidizing it 50c per gallon. Or did you mean ethanol from low-input corn, brewed up in a plant that sends wet DDG to a feedlot next door and turns the cow poops into methane that runs the plant? That one, about to hit the ground in Nebraska where I just spent the week hanging out with academics and farmers in the corn and ethanol business, will get the same 50c, but it’s a lot better for the planet than gasoline. (You have to like feedlots, which many do not, to feel good about this one.) Both kinds are nice for corn farmers, Cargill, and Archer Daniels Midland, of course.
Software promised for delivery in six months used to be called “vaporware” in the computer business. Several kinds of vaporol are in the air. One is ethanol from cellulose (grass, wood chips, almost any plant material), which requires another chemical processing step to break the cellulose into pieces yeast considers bite size, but can use whole plants, including varieties that make more sense in many places than corn. Another is butanol, an alcohol that can be transported in pipelines and has more energy per pound than ethanol. Both are between the gleam in the eye stage and real economic products. Going the other way, for simplicity, sugarcane and sweet sorghum produce buckets of sugar water, just what yeast likes, in one mechanical step, and the stalks can be burned or put through a cellulose process. But all of these, like corn, are more complicated than they appear environmentally even though they look like good research and development bets. Sugarcane is actually more of a political than a scientific or engineering issue, having to do with price supports that are Congress’ gift to a few really rich Republicans in Florida (and the corn folks, who sell lots of high-fructose corn syrup that would never survive in a free market against sugar), and import restrictions that forbid the Brazilians to send us cane ethanol at a price that would make us all happy.
Whether it’s petroleum displacement for national security or greenhouse gas emissions you care about, or both, it’s absurd to treat all ethanol the same as a matter of policy. It matters what it’s made from, and how it’s made. Subsidies and regulatory support need to differentiate the good ethanol from bad and mediocre, a task that would be relatively simple if we enact a carbon charge on fossil fuels but will be expensive and complicated if it entails keeping track of multiple streams of ethanol that get mixed together in a commodity market.
In any case, don’t mistake any of this for a solution to the energy crisis, no matter how you understand the latter phrase: if all the corn grown in the US were turned into ethanol, leaving nothing for corn flakes and muffins (but a mountain of DDG for the cows, probably more than they can handle), it would replace less than a fifth of the gasoline we use. Enjoy our current low energy prices while you can, but practice the sure-fire antidote: use less.