You can make ethanol in the distillation tower however the numbers are very poor. Very low output so i think it needs a bit of a buff to be viable.
I just did a brief look at the numbers and mine didn't seem too bad.
I took 10 fir saplings and converted them to (2500) biomass for 19239eu (Forestry Fermenter)
I then converted that biomass to (~744) ethanol for 23552eu (GT Distillery) (some loss due to trace amounts remaining)
This cost me 42791eu total. I did not include the cost of "fertilizer" or "compost" for fermentation because there's a lot of options, they're mostly dirt cheap or essentially free.
i then tossed that into the following:
a) A gt diesel generator, got 76416 eu
b) A enderio combustion generator, got 327000RF = 81750eu (requires conversion, or use for RF consumers)
c) It should apparently generate 355556 mb of steam in a Railcraft 36LP Boiler. This can be converted into power a dozen ways, but with a Large GT Boiler I believe the average return is to just divide steam by two, for 178k eu.
For Biogas, I spent 660,000eu to convert 4 saplings to plantballs to chaff to biomass to biogas, and it hypothetically generates 1,024,000 EU, but only if you get 100% power (32k eu /bucket) from a gas turbine, which you don't. You actually get 27744 eu/bucket, for 887808 total eu.
For what its worth, you can get around 99% efficiency from that bucket if you use the IC2 Fluid Heat Generator -> Stirling Generator setup.
Which is still a slightly worse cost/output ratio, and a somewhat silly amount of power generated from 4 saplings.
Unless I missed something, the Ethanol route is superior in terms of energy-output, but vastly inferior in terms of saplings->power. The Ethanol route is also way more flexible. I couldn't find any other machines that would work with the IC2 Biomass (The liquid fueled heater thingy probably works
edit: confirmed it works above.). If you have no means of getting fermentables, go Biogas, but otherwise do Ethanol.