Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 95

A typical gas system for the production of gas by anaerobic digestion comprises the digester cover, pressure and vacuum relief devices, water trap, flame trap, pressure regulator, gas meter, check valve, pressure gauges, waste gas burner, and a gas holder. The continuous stirred tank reactor (CSTR) is an example of a type of anaerobic digester.

The digester is covered to contain odors, maintain temperature, keep air out, and collect the gas. Fixed covers are more usual than floating covers. During normal operation, there is a space for gas collection between the cover and the liquid surface of the digester contents. The cover of a digester has certain unique features that the operating staff must be aware of, for example, how the variation in pressure and the level inside the digester may affect the cover. The biggest danger associated with the operation of fixed cover digesters occurs when the pressure relief device mounted on top of the digester fails or the sludge overflow line blocks and the liquid level in the digester continues to rise. In such a situation, the excess gas pressure inside the digester can exceed the maximum design pressure and damage the cover or its mountings. Fixed covers can also be damaged by excess negative pressure (vacuum) or if the rate of waste sludge withdrawal exceeds the feed rate or the vacuum relief device fails.

Gas leaving the digester is almost saturated with water vapor. As the gas cools, the water vapor condenses causing problems, which are more severe when digesters are heated. It is essential to remove as much of the moisture as possible before the gas comes into contact with the gas system devices. For this reason, water traps should be located as close to the digester as possible. All piping should be sloped a minimum of 1% toward the water trap, which should be situated at a low point in the gas line.

Flame traps are emergency devices installed in gas lines to prevent flames traveling back up the gas line (flashback) and reaching the digester. The flame trap generally consists of a box filled with stone or a metal grid. If a flame develops in the gas line, the temperature of the flame is reduced below the ignition point as it passes through the trap and the flame is extinguished. Many anaerobic digestion waste treatment plants have a means of storing excess gas, either in the form of a floating roof on the digester or a separate gasholder.

A mixture of biogas and air can be explosive. Methane gas in concentrations of between 5% and 15% in air by volume is explosive and all piping and equipment must be sealed properly to prevent gas from escaping to the outside. In addition, all electrical installations, including light switches, and torches must be of the explosion-proof type, as the smallest spark could ignite escaped gases.

See also: Aerobic Digestion, Anaerobic Digestion.