I toured the Maker's Mark bourbon distillery in Kentucky a couple weeks ago.
Their mash tuns hold 11,000 gallons each, so they got the economy of scale thing going. My tour guide (Susan) mentioned they stick all the spent grains in a "bio-reactor" where "anerobic bacteria" break down the spent grains and generate methane. They are able to run the whole plant without any other fuel, apparently.
The methane generator wasn't on the tour, but I am curious to know how such a peice of gear would look or be built.
If I got, say 15# of grain spent in an AG batch, what would it take to save a little propane next time around? I am guessing the microbes work best at or near STP, but the burners will work best with the pressure cranked up to several atmospheres. How would I do that? Where is the $$$ tipping point? How much space would this take up in my garage?