Make a Biogas Generator to Produce Your Own Natural Gas


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Categories: Energy

You can use many household organic “waste” materials to produce your own natural gas for cooking, lighting, and space and water heating. This gas, known as “biogas,” can also replace fossil-based natural gas to fuel an engine or an absorption cooling system, such as a gas refrigerator or chiller. Some gasoline engines are designed for or can be modified for use with natural gas, propane or biogas. Diesel engines can accept up to 80 percent biogas.

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A 200-gallon biogas generator in Oregon turns 15 pounds of food waste into cooking fuel daily. As food and yard waste decompose, methane and carbon dioxide are created, inflating the rubber bladder to create the pressure necessary to supply a gas burner.

Biogas is a mixture of primarily flammable gases — mostly methane — along with carbon dioxide that forms anywhere organic material decomposes anaerobically (without oxygen), such as in water, deep in a landfill, or in the guts of animals, including you.

I prefer the term “generator” for the system, because it conveys the intention of producing something. By constructing a home biogas generator, you can make enough fuel to at least provide your cooking energy. A family with modest daily cooking needs will at a minimum require the output of a warm, well-fed, 200-gallon (27-cubic-foot) generator. This much biogas will allow for about one hour of daily stovetop cooking. Start small to develop an understanding of biogas by making a small generator from a single 55-gallon barrel. Find plans in The Homeowner’s Energy Handbook.

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This methane-producing biogas generator is located at Maitreya Ecovillage in Oregon. The biogas generator turns 15 pounds of the community’s kitchen scraps and garden waste into a day’s worth of cooking fuel for the community kitchen.

Home Much Homemade Biogas Energy Can You Make?

A well-managed methane digester can produce approximately its own volume of biogas each day. Anywhere from 10 to 60 percent of the solids will convert into biogas during digestion, so expect between 3 and 18 cubic feet of available biogas energy for each pound of dry material.

The exact makeup of biogas depends on what you feed to the digester. The main ingredient of biogas is methane. Methane (chemically known as CH4) is the primary component of conventional natural gas, commonly used for cooking and heating, although biogas is not as energy-dense. The methane content of biogas will probably range from 50 to 80 percent, compared with about 70 to 90 percent in utility-supplied natural gas. Natural gas contains up to 20 percent other combustible gases, such as propane, butane and ethane, while biogas does not. Biogas’ primary noncombustible components are carbon dioxide, some water vapor, nitrogen and possibly traces of hydrogen sulphide.

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Organic materials mixed into a slurry and put in an airtight container produce combustible gases that can be used for cooking and heating, along with nitrogen-rich liquids and compostable solids (collectively known as “digestate”) perfect for fertilizing a garden or farm fields.

A good material for producing biogas in terms of both production and availability is freshly cut grass clippings, which can produce about 1-1?2 cubic feet of biogas per pound. At this rate, about 20 pounds of grass clippings will generate one hour of cooking fuel (grass silage is even better, requiring only about 10 pounds to produce this same amount of biogas). Food waste can yield slightly greater amounts of biogas per pound than grass, but most people will have access to grass clippings in larger quantities. If you own a cow, fresh manure is well-suited for on-farm methane production, despite its relatively low yield per dry pound. One cow will produce about 140 pounds (18 gallons) of manure each day, which could ultimately generate, on average, 85 cubic feet of biogas, or about three hours of daily cooking fuel. (Keep in mind that manure produced during hours your cow is on pasture will be difficult to collect.)

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For food waste, chop material into 1-inch or smaller bits. The author set up a chopping station in his garden using an old sink and garbage disposal attached to an extension cord. Other options include using a blender or chipper-shredder.


Producing Biogas in a Methane Digester

If you can compost it, you can digest it. Ideal biogas ingredients are those materials of which you have a plentiful, convenient and consistent supply, so you can make steady and useful quantities of biogas. Nearly any combination of vegetables, food scraps, grass clippings, animal manure, meat, slaughterhouse waste and fats will work as long as your recipe contains the correct ratio of carbon and nitrogen. Avoid using too many woody products, such as wood chips and straw, which contain large amounts of lignin (a part of plant cell walls resistant to microbial breakdown), which tends to clog up the digestion process.

A methane generator usually contains a feeding tube for filling the digester vessel, an effluent outlet to remove digested solids and liquids (called the “digestate”), a gas outlet, and a collection tank for storing the biogas.

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Store biogas in an inverted barrel immersed in water.

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