Everyone is talking about "green" "sustainable" "bioenergy" these days. One of those alternatives is an anaerobic digester and an engine generator set and then heating the digester with the waste heat from the engine, or even running a drier (partially).
How much power is there in biosolids?
Turning volatile solids into methane yields about 17 cf digester gas/lb volatile solids. Digester gas has a wet value of about 600 BTU/cf. Thats 10,200 BTU/Lb. The most efficient engines are 35-37% efficient. This yields about 1 kilowatt-hour of electricity per pound of volatile solids destroyed. With low grade heat recovery from the engine, your total efficiency can be 75-80%, enough to heat the digester, and enough for other uses as well, including building heat, and , in theory, an air conditioner.
I like the idea of running a dryer with it, though there is not enough enery to run and engine and a dryer without additional gas to close the loop.
How much does an engine generator set and appurtanences cost?
A rule of thumb is at least $1,100/megawatt (1000 kilowatts), plus a building plus installation. In addition, you have gas conditioning equipment ahead of the engine. You need to remove water vapor, particulates, and siloxanes.
Jim what are these siloxanes?
Siloxanes are teeny weenie silicon-based particles used in beauty products becuase it make them flow better, etc. They asre light enough to flow outof the digesgter wsith the gas. When they get burned in an ingine or turbine, they create a ceramic coating on the moving parts (not a good thing). They can be removed by running the digester gas through a gel media, or through a pressure-swing absorption machine. The pressure swing absorption machine removes EVERYTHING inculding carbon dioxide, and produces pipeline quality gas, but is (of course) expensive. The gel is much less expensive. water is removed by refrigerating the gas and having it drop out as water.
So Jim, are we going to make a mint off biogas?
Probably not. There are firms that will put an engine generator seet at your plant and sell you electricity for 9 cents/kWh. In the midwest, one can buy electricity for much less than that. Toledo pays 4.2 cents/kWh. Sidney pays 8 cents, but Brian Shultz says that half of that is demand charge. So, no you are not going to make a lot of money unless you are subsidized?
But Jim, what about all these dairy operations that are making a mint from cow power?
I went to a lecture about Bridgewater dairy out in Montpelier, OH. The dairy has a cow power digester and generator set. They have 3 revenue streams: A methane destruction credit, an alternative energy credit, and cost avoidance/selling electricity to someone. No one said how much the credits were, but I'll bet that Bridgewater gets more revenue from credits (that is subsidies) than from selling or usng their homemade electricity. No one is making a mint without subsidies. Another advantage for the dairies is that the manure is in a more acceptable form and may be easier to use. Fecal bacteria are much reduced as is that "fresh country air" smell
What if we take in food waste?
If you take in food waste you can make more gas, especially if you have lots of fats in it. Fats have twice the energy value of carbohydrates and protein.
How much does it cost to maintain an engine?
I asked someone who keeps good records of engine maintanance, and he said it varies between $3 and $4 per operating hour, plus 75 cents per operating hour for the engine rebuild, usually after 60,000 hours. If you kept one engine on line for a whole year, 8,000 ish hours, your maintenance costs will be $32,000 to $40,000/year. A hat tip to Dapper Dan.
What other hidden problems are you going to warn me about?
Air Permitting You are installing a stationary power source. It will be treated just like a generator running on diesel or natural gas. Depending on where you are and how big your generator set, permitting can be difficult.
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