Mark Cunningham wrote:
We have tried to utilize grasses and other cellulose / lignin heavy items as feed stock. They produce a lot of gas but the plant structure does not digest easily. Eventually it fills up the digester and you have to do a manual clean out.
I am currently using a method of extracting the digester useful portion of nutrients from finely shredded grass. An Indian professor named P. Sankar Ganesh did his doctoral thesis on extracting volatile fatty acids from water hyacinth. I bastardized his methods to fit our flora and so far it seems to be working out. Search for his name and you will find his paper on line.