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April 19, 2014  |  Login
Methane: Another Renewable Fuel Option

By Dan Chiras

By now, you can see that we have lots of clean-burning alternatives to declining fossil fuels, and many of them could be cost-effective, too. One additional fuel that receives very little attention is methane produced by the decay of organic matter-human or livestock wastes and even municipal garbage - under anaerobic conditions (the absence of oxygen).

Methane, of course, is the main component of natural gas, derived from ancient plants and algae. This highly combustible gas is produced naturally in landfills. It is also produced at sewage treatment plants by the natural decomposition of human solid waste. Interestingly, many sewage treatment facilities capture this valuable fuel and put it to good use.

Sewage treatment plants capture the methane produced from the waste they receive each day, and then burn it on-site to generate electricity and heat to power their operations. Landfills sometimes pipe the methane to nearby buildings like schools that use it for heating.

Methane can also be produced in devices called methane digesters. A methane digester is a tank made from metal or concrete that accepts organic waste, for example, from cattle or other livestock. The waste is mixed with water and enters the tank as a slurry. Here it decomposes anaerobically (in the absence of oxygen) to produce methane gas. The methane is then piped out of the system and used for heating or cooking or generating electricity. (To learn more about methane digesters, see below.)

Although methane digesters are a great idea, and are widely used in less developed countries, they're terribly underutilized in the more developed countries. In time, as natural gas supplies fall, though, methane digesters could be used to take up the slack. I even envision the day when septic tanks in rural settings could be used to produce small amounts of methane for homes. Methane produced in the tank could be captured and piped to the house where it would be burned in gas ranges, gas water heaters, and even gas furnaces and boilers.

Methane Digester Designs

Anaerobic digesters are made out of concrete, steel, brick, or plastic. They are shaped like silos, troughs, basins, or ponds, and may be placed underground or on the surface. All designs incorporate the same basic components: a pre-mixing area or tank, a digester vessel(s), a system for using the biogas, and a system for distributing or spreading the effluent (the remaining digested material).

There are two basic types of digesters: batch and continuous. Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.

In a continuous digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. They may be better suited for large-scale operations. more




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