Growing any plant takes CO2 out of the atmosphere. So to get a gain in that regard you have to plant in a desert that would normally have NO plant growth. The cornfield would just as soon grow weeds that are just as good for the atmosphere. That said, I'm thinking of growing beets. Lots of sugar for alcohol and less to no fertilizer or pesticide requirements plus they will grow in a cooler clime like mine. The plans for a still look remarkably similiar to the heat exchangers I used to fab at work. (I installed and built solar water heating systems for 15 years) If you are handy with a torch and solder you might enjoy these links:

Building a worldclass distillation apparatus
Home distillation of alcohol
Build a still.com
You should be able to add 15% without any noticeable effects.