How to Generate Power from Garbage
(Page 10 of 12)
May/June 1970
By the Mother Earth News editors
The valve on the pipe to the gas-holder should be closed while digestion is being established. The petcock which precedes it should be open to permit any air in the tank to escape as decomposition starts. After two or three days, gases of decomposition - mostly carbon dioxide - will begin to escape. An attempt to ignite the gas should then be made. The petcock can then be closed. Further attemps to determine whether methane production has started should be made daily by trying to ignite the gas as it escapes when the petcock is opened.
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It will take one or two weeks or often longer to establish the production of a satisfactory gas, the time depends upon the temperature and the success in avoiding development of initial highly acid conditions in the digester. When the escaping gas will provide a continuous flame it is ready for use, and the valve to the gas-holder and distribution lines should be opened, and the petcock closed.
The rate of gas production may be determined when desired by diverting all the gas to the gas-holder for a known time and noting the change of volume, e.g., if one cubic meter of gas were collected in the gas-holder during 6 hours, the gas production would be 4 m 3 per day.
DURATION OF DIGESTION FOR MAXIMUM GAS PRODUCTION
During the digestion period - at any temperature - the rate of gas production in a batch operation will gradually increase at first, reach a maximum-rate plateau, and will finally decrease when a large part of the material has undergone decomposition. At the higher temperatures the rate of gas production will be greater and the digestion cycle shorter. The total amount of gas produced per ton of material in a cycle will, however, be approximately the same for temperatures from 15° to 35°C. At 15°C, the cycle will be about 12 months while at 35°C it would be about one month.
Since the rate of gas production from a single digester is greatest during the middle part of the digestion cycle, it is desirable when two or more digestion tanks are used to stagger the digestion cycles so that the maximum rate of gas production for one tank will occur at the time when the rate for the other tank or tanks is low. When three or more digesters are used, gas production can be maintained at a quite uniform rate. The operation of digesters of the type shown in Figures 3-5 on a more or less continuous basis, by intermittently charging and removing material through the latrine-submerged inlet, will result in a continuous rate of gas production.
In relatively warm areas, or when an outside source of heat is provided, the digestion period will be 2-3 months for the most efficient utilization of plant capacity. In cooler areas where the digester temperature averages about 20°C a digestion period of 4-6 months may be most efficient. At the end of these periods about 70% to 80% of the gas available from the materials has been released.
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