I've done some more of my own calculations, and it looks like steam is the best option pre-oil-drilling. If I can find a good source of natural gas, making refinery gas from it looks like a good option (although not exactly renewable).
Once I can make a distillation tower, the following look like viable fuel choices:
1. benzene (distilled from wood tar)
2. methane (distilled from fermented biomass)
3. biogas (distilled from ic2 biomass, though probably not as good as using the crops to make methane)
I'm not sure any of the diesel generator fuels can be produced fast enough to make them viable. (edit: to clarify, I mean per machine speed at lowest available voltage; I am aware I can build more of the processing machines or make higher-tier versions of them to produce the fuels faster, but that hardly seems worthwhile if there are alternatives that can be made with fewer and/or lower-tier machines)