"They" are working on it.
I have a friend whose father is chairman of some department or other at a major university which is devoting much of it's energy and most of my friend's father's time on just that. I guess that when it's figured out you'll be able to throw in grass clippings and get water and fuel.
The problem is that the bonds between the atoms and molecules in cellulose are so darned strong it's tough to find a way to break them inexpensively.
Arf