Yep, that is my take as well. Biogas seems to be the simplest and with the least energy inputs. I stayed in a demonstration house once where the stove used gas from animal waste. The setup was pretty simple: put hog/rabbit/etc waste in a tank, run a pipe from the top of the tank to the stove. I'm sure there are ways to make it so smell isn't a risk, but I didn't notice anything since we were burning it anyways. Whatever remains in the tank can eventually be used as fertilizer.