Typically, these plants burn
methane. Normally, that methane would simply be released into the atmosphere (which is
bad.)
http://minnesota.publicradio.org/display/web/2008/06/26/methane_digester/ Farmer uses methane to make electricity
by Tim Post, Minnesota Public Radio
June 27, 2008
http://www.publicradio.org/tools/media/player/news/features/2008/06/27_tpost_manure">Listen to feature audio
A small power plant is making electricity on a central Minnesota farm. It's not a lot of electricity, not even enough to power the farm itself. But the fuel source has excited advocates of renewable energy. The plant will make electricity by burning methane from cow manure.
Brooten, Minn. — The 200 cows on Jerry Jennisson's central Minnesota dairy farm make 1,100 gallons of milk every day, and they make even more manure, as much as 3,000 gallons daily.
In the past Jennisson stored the manure in lagoons and used it as fertilizer. But now he's also using it to make electricity.
It's a fairly simple process. The manure is piped from the barn to a 40 foot silo. It's heated to 100 degrees. That's when bacteria naturally present in the manure gets to work and makes methane.
…
Jennison can still use the manure for fertilizer after it's run through the process. Because it no longer contains methane, it's lost much of its odor, and doesn't emit as many greenhouse gases.
…
Think of the methane production as an
improvement in the production of fertilizer.
http://journeytoforever.org/biofuel_library/MethaneDigesters/MDToC.html