Indeed you can. It's definitely the simplest way of doing it. However, by pulling the fatty acids out, etc, you can also pull out a lot of other useful products, like ammonia and hydrogen sulfide. Additionally, a fire will produce a lot of ash and smoke, which both will need to be mitigated in one way or another.

Yeah, low tech wood fires are an air-pollution disaster:

http://www.samharris.org/blog/item/the-fireplace-delusion

It's possible to burn plants cleanly - you need lots of air, and possibly a catalytic converter.

Not something you would do at home, but a power plant can.

We should, today, switch from burning coal to burning switchgrass.