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gas generator[′gas ‚jen·ə‚rād·ər]
a device for the thermal processing of solid or liquid fuels into combustible gases; the process is carried out in the presence of air, free oxygen, or bound oxygen (water vapor). The gases produced in a gas generator are called generator gases.
The solid fuel in gas generators, in contrast to ordinary fireboxes, is burned in thick layers. Air is supplied in an amount insufficient for complete combustion of the fuel—for example, with a steam-air blast, 30-35 percent of the air theoretically necessary is supplied. The gases that form in the generator contain the products of complete combustion of the fuel (carbon dioxide and water), as well as the products of their reduction and of the incomplete combustion and pyrogenetic decomposition of the fuel (carbon monoxide, hydrogen, methane, and carbon). Atmospheric nitrogen also passes into the generator gases. The process that occurs in a gas generator is called gasification of fuel.
A gas generator is usually a shaft whose inner walls are covered with refractory material. The fuel is fed from above the shaft, and the blast enters from the lower part. The fuel bed is held in place by a furnace grate. The processes of gas formation in the fuel bed are shown in Figure 1. The blast supplied to the gas generator first passes through the zone of ash and slag, 0, where it is heated slightly, then to the incandescent fuel bed (oxidation zone, or combustion zone I), where its oxygen reacts with combustible elements of the fuel. The products of combustion thus formed move up along the gas generator and, coming into contact with the incandescent fuel bed (gasification zone II), are reduced to carbon monoxide and hydrogen. When the strongly heated products of reduction rise further, thermal decomposition of the fuel occurs (fuel decomposition zone III), and the products of reduction are enriched with the products of decomposition (gases and resinous and water vapor). As the result of the decomposition of fuel, semicoke and then coke are formed. As the semicoke and coke descend (zone II), the reduction of combustion products occurs on their surfaces and, still lower (zone I), the coke burns. In the upper part of the gas generator the fuel is dried by the heat of rising gases and vapors.
The composition of generator gases changes depending on the type of blast oxygen fed into the generator. When atmospheric air is supplied to the generator, air gas is formed, its heat of combustion is 3.8-4.5 megajoules per cu m (MJ/m3), or 900-1,080 kilocalories per cu m (kcal/m3), depending on the fuel being processed. If a blast enriched with oxygen is used, so-called oxygen-vapor gas is formed. It contains less nitrogen than does air gas, its heat of combustion is as high as 5-8.8 MJ/m3 (1,200-2,100 kcal/m3).
When a gas generator operates on air to which a moderate amount of water vapor has been added, a mixed gas is formed, with a heat of combustion of 5-6.7 MJ/m3), (1,200-1,600 kcal/m3), depending on the initial fuel. Finally, if water vapor is supplied to the incandescent fuel bed, water gas is produced. It has a heat of combustion of 10-13.4 MJ/m3 (2,400-3,200 kcal/m3).
In spite of the fact that the idea of a gas generator was advanced as early as the end of the 1830’s in Germany (Bischof in 1839 and Ebelman in 1840), its application in industry was undertaken only after F. Siemens introduced the regenerator principle of heating plant furnaces (1861). This principle made it possible to use generator gas efficiently. The brothers F. and W. Siemens invented the first industrial gas generator; their design was widely used for 40-50 years. More sophisticated designs appeared only at the beginning of the 20th century.
Gas generators are differentiated according to the type of solid fuel processed: generators using lean fuel, with an insignificant yield of volatile substances (coke, anthracite, dry-burning coal); generators using bituminous fuel, with a significant yield of volatile substances (gas and brown coals); generators using firewood and peat; and generators using mineral fuel wastes (coke and coal dirt and the remnants from ore processing). A distinction is also made between gas generators with liquid and solid slag disposal. Bituminous fuels are usually gasified in gas generators having a rotating water pan. Firewood and peat require gas generators with a large internal volume because of their low density. Fine-grained fuel is processed in gas generators operating at high pressure; the fuel enters in a suspended or fluidized bed.
According to their purpose, gas generators may be stationary or portable. They may also be categorized according to the site of air supply and gas bleeding (the direct, back-run, or horizontal process). In a direct-process gas generator (Figure 2), the movement of the oxygen carrier and the forming gases proceeds from bottom to top; in generators with a back-run process (Figure 3), the oxygen carrier and the gas being formed move from top to bottom. To create a back-run flow, the middle part of such gas generators is equipped with tuyeres through which the blast is introduced. Since the gases that are forming are sucked out from below, combustion
zone I (oxidization) is located directly under the tuyeres; below that is reduction zone II. Zone III—the zone of pyrogenetic decomposition of the fuel—is located above combustion zone I; the decomposition takes place owing to the heat of the incandescent burning coke in zone I. The heat transmitted by zone III dries the upper bed of the fuel in gas generators. In horizontal-process generators the oxygen carrier and the forming gas move horizontally.
During the operation of gas generators, pressure and temperature regimes whose levels depend on the fuel being processed, the process of gasification, and the design of the generator are observed.
The vigorous development of the gas industry in the USSR has led to the nearly total substitution of natural and by-product gases for generator gases, since the prime cost of the former is much lower. However, foreign countries with little natural gas, such as the Federal Republic of Germany and Great Britain, use gas generators extensively in various branches of industry.
REFERENCEMikheev, V. P. Gazovoe toplivo i ego szhiganie. Leningrad, 1966.
N. I. RIABTSEV
in a liquid rocket engine, a unit in which hot gas (temperature, 200-900° C) is produced through combustion or decomposition (such as through thermal or catalytic processes) of fuel or its components and which serves as the operating body for driving turbine pump units, pressurizing fuel tanks, operating control units, and so on. In a gas generator the main fuel components are most often jointly used in the instances where the excess oxidant ratios are not equal to 1. Sometimes, one of the main propellant components (oxidant or fuel), for example, asymmetrical dimethyl-hydrazine, is decomposed in a gas generator. Auxiliary propellants are also in use. A distinction is made between deoxidizing gas generators and oxidizing ones, depending on the composition of the gas being generated. The mixing head and the housing are the basic parts of a gas generator.