How to Generate Power from Garbage
(Page 4 of 12)
May/June 1970
By the Mother Earth News editors
GAS REQUIREMENTS AND STORAGE
RELATED CONTENT
The methane gas generated by a digester may be used for domestic purposes, such as cooking, food refrigeration, and lighting. The following are some approximate quantities of gas for these different uses: Domestic cooking, 2 m 3 per day for a family of five or six people; water heating, 3m 3 per day for a 100 litre tank or 0.6 m 3 for a tub bath and 0.35 m 3 for a shower bath; domestic food refrigeration, 2.5-3 m 3 per day for a family of five or six; lighting, 0.10-0.15 m 3 per hour per light.
The gas may also be used to provide power for engines, milk cooling and electricity. A two-horsepower stationary engine requires about 0.9 m 3 per hour. For milk cooling on a dairy farm, the following approximate quantities of gas are required to operate a butane cooler unit: 0.8 m 3 for 55 litres; 1.0 m 3 for 90 litres; and 1.25 m 3 for 150 litres.
Since the gas is produced continuously, day and night - but is used largely during the daytime - it is necessary to provide storage facilities so that the methane will not be wasted and will be available when needed. The storage capacity should be estimated to meet peak demands.
For small installations, storage capacity for about one day's requirement of gas should be provided. This will usually be about half, or less, of the total volume of manure actively undergoing digestion. In warm areas the storage capacity might be reduced to half the amount required per day, but it is probable that there will be times when gas will be wasted from the digester because the gas-holder is full. At other times, when a considerable amount of gas is being used, the gas-holder might become empty. The volume of the gas-holder should not be less than about 2 m 3 , even for small installations.
The gas-holder may be circular or square and should be provided with a water seal to prevent escape of gas or admission of air. The weight of the floating cover of the gas-holder provides the gas pressure. The usual pressure for gas-burning equipment is 5-20 cm (2-8 inches) of water. The bottom and walls of the gas-holder - which must be watertight - can be made of concrete, but the cover should usually be metal in order not to produce excessive gas pressure. Center weights on the gas-holder cover may be used to provide the desired pressure for the burning equipment.
Gas may be stored in the digestion tank by using a floating cover as shown in Fig. 9. An additional gas-holder may be used with floating-cover digesters, in which case one gas-holder would serve several individual digesters.
Copper piping is the most satisfactory for gas distribution because it will minimize corrosion problems, but galvanized iron or black iron pipes and, perhaps, plastic or fiberglass pipes can be used.
Valves should be provided for shutting off the gas for the digesters and from the gas-holder. The piping should be arranged so that the gas from the digester can flow directly to the burning fixtures and the gas-holder merely floats on the line producing the pressure, taking up gas when it is produced faster than it is consumed and supplying it when the rate of consumption is greater than the rate of production.
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