Gasification of Fuels

the conversion of solid or liquid fuel into combustible gases by incomplete oxidation with air, oxygen, or steam at high temperatures. Gasification of fuels yields mainly combustible products (carbon monoxide and hydrogen).

Any fuel—coals, peat, mazut, coke, wood, and so on—can be gasified. Fuel is gasified in gas producers, and the resultant gas is called generator gas. It is used as a fuel in metallurgical and glass furnaces, pottery kilns, domestic gas equipment, and internal-combustion engines, and also as a raw material for making hydrogen, ammonia, methanol, and artificial liquid fuel.

Despite the wide variety of methods (continuous and batch gasification, fluidized bed gasification, gasification of coal dust and liquid fuel in jets, gasification at normal or high pressure, underground coal gasification, and so on), gasification of fuels is characterized by identical chemical reactions.

In the gasification of solid fuel by oxidation with oxygen or steam, carbon is directly involved: 2C + O₂ = 2CO + 247 megajoules (MJ), or 58,860 kilocalories (kcal); C + H₂O = CO + H₂ − 119 MJ (28,380 kcal). However, it is usually not possible to convert all the carbon to the desired product, CO, since part of it burns completely: C + O₂ = CO₂ + 409 MJ (97,650 kcal). The resultant carbon dioxide reacts in turn with incandescent carbon: CO₂ + C = 2CO − 162 MJ (38,790 kcal).

In high-temperature gasification of liquid fuel the hydrocarbons are split to low-molecular-weight compounds and elementary substances, which are oxidized: CH₄ + 0.5O₂ = CO + 2H₂ 34 MJ (8,030 kcal); CH₄ + H₂O = CO + 3H₂ − 210 MJ (50,200 kcal). The gaseous products of the gasification of fuel react among themselves: CO + H₂O = CO₂ + H₂ 44 MJ (10,410 kcal).

In making generator gas, various forms of oxidants (blast) are used—air, steam plus air or oxygen, air enriched with oxygen, and so on. The blast composition is chosen so that the heat liberated in the exothermic reactions suffices to perform the entire process.

The name of any particular kind of producer gas is often determined by the blast composition—for example, air gas is made by supplying air to the gas producer. Air gas produced from coke contains (by volume) 0.6 percent CO₂, 33.4 percent CO, 0.9 percent H₂, 0.5 percent CH₄, and 64.6 percent N₂. The heat of combustion is 4.53 MJ/m³ (1,080 kcal/m³); gas yield, 4.65 cu m per kg of fuel. Air gas made by gasification of mazut under pressure of 1.5 meganewtons per sq m (MN/m²), or 15 kilograms-force per sq cm (kgf/cm²), contains (by volume) 3.5 percent (CO₂ + H₂S), 21.0 percent CO, 17.5 percent H₂, and 58 percent N₂. The heat of combustion is 5 MJ/m³ (1,200 kcal/m³); gas yield, 6.1 cu m per kg of fuel.

Water gas (synthesis gas, process gas) is made by reacting incandescent fuel with steam. Since the reaction for making water gas is endothermic, air is periodically blown through the fuel layer in the generator to accumulate the amount of heat needed for gasification. (The resultant air gas is a by-product.) Water gas made from carboniferous coke contains (by volume) 37 percent CO, 50 percent H₂, 0.5 percent CH₄, 5.5 percent N₂, 6.5 percent CO₂, 0.3 percent H₂S, and 0.2 percent O₂. Its heat of combustion is 11.5 MJ/m³ (2,730 kcal/m³); gas yield, 1.5 cu m per kg of fuel. Water gas can be made continuously by using a vapor-oxygen blast—for example, gasification of mazut under a pressure of 3 MN/m² (30 kgf/cm²) yields a gas that contains (by volume) 46.8 percent CO, 48.8 percent H₂, 3.8 percent CO₂, 0.3 percent CH₄, and 0.3 percent N₂; the heat of combustion is 12.3 MJ/m³ (2,940 kcal/m³).

Mixed gas (a mixture of air and water gases) is produced by gasification of fuel using a vapor air blast—for example, mixed gas from lump peat contains (by volume) 8.1 percent (CO₂ + H₂S), 28 percent CO, 15 percent H₂, 3 percent CH₄, 45.3 percent N₂, 0.4 percent C_mH_n, and 0.2 percent O₂. Its heat of combustion is 6.9 MJ/m³ (1,660 kcal/m³); gas yield, 1.38 cu m per kg of fuel.

Gas for city use is made from coal using a vapor oxygen blast at a pressure of 2-3 MN/m² (20-30 kgf/cm²). Under these conditions the gas is enriched with methane—for example, gasification of lignite yields a gas that contains (by volume) 23.6 percent CO, 55.7 percent H₂, 14.3 percent CH₄, 5.5 percent N₂, 0.2 percent (CO₂ + H₂S), and 0.7 percent C_mH_n. Its heat of combustion is 16.8 MJ/m³ (4,000 kcal/m³); gas yield, 0.97 cu m per kg of fuel. City gas is made from liquid fuel by a combination of gasification and pyrolysis under pressure. The capacity of plants for the production of gas from solid fuel by pyrolysis is up to 80,000 m³/hr per plant; when liquid fuel is used it is up to 60,000 m³/hr. The present trend in the development of fuel gasification technology is to carry out the process at high pressure (up to 10 MN/m² or higher) in high-capacity plants. The degree of utilization of the heat contained in the fuel (fuel gasification efficiency) is 70-90 percent.

In the 19th century the gasification of fuels became wide-spread because of its advantages as compared to solid and liquid fuels. At the same time, the manufacture of illuminating gas, based on processes involving the thermal breakdown of fuel without access of air (dry distillation, coking), was developed. When fuel is gasified, the entire combustible part becomes gas, whereas in the formation of illuminating gas, only part does so. In the first half of the 20th century water gas was made in order to obtain hydrogen for synthesizing ammonia and artificial liquid fuel. After World War II (1939-45) the intensive development of methods of gasification of liquid fuels under pressure, especially in regions remote from natural-gas resources, was begun. In the USSR methods are being successfully developed for making low-sulfur fuel gases for electric power plants from high-sulfur boiler fuel (mazut). Because of this there is marked reduction in atmospheric pollution and in corrosion of boiler equipment.