MadSci Network: Engineering
Query:

Re: Extracting methane gas from pig manure

Date: Thu Jul 23 16:46:12 1998
Posted By: Bob Peeples, Chemical Engineer, Environmental Program Management, U S Postal Service
Area of science: Engineering
ID: 900096178.Eg
Message:

Absolutely. Pig manure is excellent feedstock for an anaerobic digester. Pig manure does not start out with much methane to extract, but the bacteria already housed in the manure are capable of generating plenty of methane. As a result, methane is not extracted as much as it is merely collected.

There are two major justifications for developing methane energy: 1) methane, and 2) energy.

Methane is the second largest contributor to greenhouse gasses that can be attributed to man’s activity. Carbon dioxide emissions are estimated to contribute 75% to global warming effects. Methane is a distant second at 15%, and this relatively small number even accounts for the fact that methane gas is twenty times more effective than carbon dioxide at trapping heat. If you use some other carbon source for your energy, you still generate carbon dioxide as well as releasing the wasted methane from the manure pile. So the end effect of trapping this methane and burning it is to convert one greenhouse gas into another greenhouse gas (but reduce the greenhouse effect of the gas by twenty times) instead of adding greenhouse gasses by burning other chemical energy sources.

Energy utilities throughout the United States are suddenly interested in alternative energy sources. This is in response to government mandates included in the utility deregulation process. The great majority of manmade greenhouse gasses are generated because people need to burn stuff to get energy. We burn coal, oil, natural gas, agricultural byproducts, gasoline, wood, even trash. If it burns, someone has tried to heat their house with it. The few energy sources that do not involve converting various carbon chemicals into carbon dioxide seem to be even less popular than carbon combustion, at least with the portion of our society that is screaming the loudest about global warming. Hydroelectric power exploits gravity in the potential energy of water at one elevation compared to a lower elevation, but it also reroutes natural waterways and loses some heat to the downstream water. This could kill off some endangered mosquito somewhere. Nuclear power is even less popular, even though the nuclear events that represent the greatest threat to the people of United States are usually the protest actions themselves. Go figure.

There is plenty of energy potential in pig manure. Of the 300 million tons of methane that we generate each year, about one fourth is from domestic animals. Most of the methane from domestic animals, about two thirds, is generated during enteric fermentation and immediately lost to the atmosphere from one end or the other of the animal. It would be inconvenient for you to try and collect the gas generated by enteric fermentation, and it would certainly be uncomfortable for your animals. That leaves about 25 million tons of methane generated each year from the anaerobic fermentation of domestic animal manure after it leaves the animal.

The methane generated by the manure fermentation of five pigs is enough to cook three hot meals each day for an average family. The easiest digesters to design are horizontal, plug flow reactors. Fifty feet of 36" culvert pipe laid flat makes a good starting point. Manure is fed into one end, finished fertilizer is drawn from the other, and natural gas rich in methane is collected from the top. I have simplified the description considerably for illustration purposes, please don't consider this a complete design. If you are going to use the gas for indoor cooking, I would suggest a simple gas scrubber such as a drum full of crushed limestone. If you plan to run this process during cold weather, you may find that it takes more energy than you can extract, just to heat the system up to a comfortable temperature for the bacteria (they prefer something around the body temperature of the pig). With no net energy gain, or a possible loss, you can see why these systems aren't in use on every farm yet.


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