Biomass will ferment naturally in the absence of air to produce a gas that is rich in methane. This is the process that occurs underground in landfill waste disposal sites and it can also be found in the lakes of large hydropower plants when there is a large quantity of biomass immersed beneath the waters. The same process can be harnessed in biomass digesters to produce a methane-rich gas from wastes. The technique is normally applied to agricultural animal wastes.

Biomass digestion is only cost effective for large farming operations, most usually on dairy or pig farms where the slurry produced by the animals must be treated to prevent it causing an environmental hazard. Digesters of differing sophistication are available depending on the size of the farm. For small farms the most suitable is usually a lagoon digester, essentially a pond (the lagoon) into which the slurry is placed. The lagoon is covered with an impermeable membrane cover that is used to collect the emitted gas. The slurry must contain less than 2% solids and the lagoon must usually be maintained above 30 oC, which limits the application to warmer climates since it is not economic to heat a lagoon digester.

A more sophisticated system is the tank digester (Figure 15.5). Slurry is loaded into a tank that is fitted with a stirring mechanism to mix the contents evenly. The tank can be heated to keep the fermentation at the optimum tem- perature. Tank digesters can handle slurries with 3–10% solids. For slurries with higher solids content a plug flow digester is preferable. This has three elements:

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a mixing tank, digester tank, and settling tank. The slurry is first fed into the mixing tank from where it enters the digester tank, which contains heating pipes to maintain the ideal temperature. The material moves slowly across the digester tank, with fermentation proceeding to completion in about 20 days as it crosses the tank. After 20 days it passes into a settling tank where the remaining solid material is removed and can be used as fertilizer.

The gas from an anaerobic digester has a heating content of 22 MJ/m3, suitable to be burned in a reciprocating engine to generate electricity and heat. However, the capital cost of such systems is high and can only be supported when there is a large quantity of waste to ferment. Similar systems can be used to treat municipal sewage waste and they form an effective means of both rendering it harmless and producing a valuable by-product.

Most biomass digester-based power generation plants are relatively small with capacities of 100 kW or less. Landfill gas sites, which can produce large volumes of methane, can sometimes support gas engines with generating capac- ities of 20–30 MW.

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