Sunday, August 2, 2009

Small Scale Methane

In my work in the remote villages of Eastern Kenya, I searched for a way to provide cooking fuel and lights that did not degrade the local forest cover and require hours of extreme labor chopping and carrying fire wood. I met a crusty, retired, Prussian cavalry officer from the Eindhoven stove group who was promoting methane fuel from biomass—mostly manure and grass. Already the inhabitants of the huge dumps of Nairobi had discovered that rotting garbage produced a gas that they could cook with—the fire of God they called it. I began to experiment with easy ways to enclose biomass in anaerobic (air-excluded) vessels or bags and extract the gas the evolved. The 3-drum digester you see was a design that was quite successful and produced enough gas each day to cook the villager's biggest meal and provide reading lights at night for school children. There is no loss of water or nutrients in the liquid effluent which went straight to the tree nurseries.

Basically methane is generated from the microbial digestion of properly mixed biomass that has the desired water-nitrogen-carbon and heat ratios that support rapid bacterial growth and consumption of this material. An acid substrate forms quickly and then the methanogens devour this. A byproduct of their digestion is flatulence which is, like in humans, a flammable gas. Think of the problem when we eat too many beans or veggies, and you get the idea. The gas produced is combined with sulfur and carbon dioxide and is very wet. To get pipeline quality natural gas/methane from this gas called bio-gas, you must scrub the sulfur by passing it through an iron filings filter. You remove the carbon dioxide by bubbling it through lime water, and the excess moisture will condense on the pipes and must be led away to U-tubes where it can evaporate.

To understand the entire process, just think of the digester as a huge cow stomach. Chopped nutrients go in and are “ruminated” into a brew that is bacterially digested in the gut. Gas and a slurry are pushed out as new material is fed into it. This is called a plug-flow system—grass in, sludge out in equal volumes. If a floating lid is placed over the digester tank, the bubbles of gas and carbon dioxide will be captured and lift the cover. With a little pressure, this gas can be forced along a pipe to an inverted tank in a water bath/seal, which will float up with the gas under pressure. To push the gas to a point of use, a bucket weighted with water can be used to achieve the desired inches of water column pressure needed to make any device work.

At our farm, as you see in the photo, we have a digester that holds 1,500 gallons of water and slurry. The gas-collector/top of the concrete digester (buried in the ground for insulation) is a floating steel tank 4’ wide x 9’ long x 2’ deep. The single small gas collector tank has now been replaced with 2 fiberglass tanks for added gas collection. We use it for cooking and demonstrations mostly, but it is now at capacity, and when connected to the genset which runs on propane and or natural gas, it will run the 16 kW generator for 1 hour for each 3 cubic meters of gas we make. Our sole substrate for making methane of very high quality is grass clippings which have a perfect C:N ratio and produce such good gas that we do not need to filter it.

The photo of the gas burning reveals a high quality flame of dark blue with no orange tips or smoke. Here is the closest thing to a free lunch I have ever known! The amount of gas we could produce is virtually unlimited and the effluent is perfect for fertilizing trees—a little too low in pH for veggies or grass. There isn’t room to discuss all the safety issues, but I can say that it is much safer to use in the house than propane.

Our system has been running for the last 4 years with very little maintenance. As a demonstration unit, it has surely proven to be worth every dollar invested. If you are interested in a spreadsheet with calculations of each component and productivity, then you can contact me through my profile.

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