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Coal Gasification
Article by E. L. Clark
Coal gasification is a process for converting coal partially or completely to combustible gases. After purification, these gases - carbon monoxide, carbon dioxide, hydrogen, methane, and nitrogen - can be used as fuels or as raw materials for chemical or fertilizer manufacture. From the early 19th century until the 1940s almost all fuel gas distributed for residential or commercial use in the United States was produced by the gasification of coal or coke. In the 1940s the growing availability of low-cost natural gas led to its substitution for gases derived from coal. Interest in coal gasification has been renewed, however, with recent predictions that natural gas reserves in the United States will begin to diminish by 1980. At present, except for by-product gas from the manufacture of coke, no coal gasification plants of any appreciable output are in operation in the United States. Many plants, however, are in operation in other countries that have no reserves of natural gas or petroleum.
Coal may be gasified in a number of ways. The simplest method, and the first used, was to heat coal in a retort in the absence of air, partially converting coal to gas with a residue of coke; the Scottish engineer William Murdock used this technique in pioneering the commercial gasification of coal in 1792.
Murdock licensed his process to the Gas Light and Coke Company in 1813, and in 1816 the Baltimore Gas Company, the first coal gasification company in the United States, was established. The process of heating coal to produce coke and gas is still used in the metallurgical industry.
The most complete conversion of coal or coke to gas that is feasible was achieved by reacting coal continuously in a vertical retort with air and steam. The gas obtained in this manner, called producer gas, has a relatively low thermal content per unit volume of gas (100-150 Btu/cu ft). The development of a cyclic steam-air process in 1873 made possible the production of a gas of higher thermal content (300-350 Btu/cu ft), composed chiefly of carbon monoxide and hydrogen, and known as water gas. By adding oil to the reactor, the thermal content of gas was increased to 500-550 Btu/cu ft; this became the standard for gas distributed to residences and industry. Since 1940, processes have been developed to produce continuously a gas equivalent to water gas; this involves the use of steam and essentially pure oxygen as a reactant. A more recently developed process reacts coal with pure oxygen and steam at an elevated pressure of 3.09 Newtons per sq m (450 psi) to produce a gas that may be converted to synthetic natural gas.
The most common modern process uses lump coal in a vertical retort. The coal is fed at the top with air, and steam is introduced at the bottom. The gas, air, and steam rising up the retort heat the coal in its downward flow and react with the coal to convert it to gas. Ash is removed at the bottom of the retort. Using air and steam as reacting gases results in a producer gas; using oxygen and steam results in a water gas. Increasing operating pressure increases the productivity.
Two other processes currently in commercial use react finely powdered coal with steam and oxygen. One of these, the Winkler process, uses a fluidized bed in which the powdered coal is agitated with the reactant gases. The other, called the Koppers-Totzek process, operates at a much higher temperature, and the powdered coal is reacted while it is entrained in the gases passing through the reactor. The ash is removed as a molten slag at the bottom of the reactor. Both of these processes are being used for fuel gas production and in the generation of gases for chemical and fertilizer production.
As petroleum and natural gas supplies decrease, the desirability of producing gas from coal will increase. It is also anticipated that costs of natural gas will increase, allowing coal gasification to compete as an economically viable process. Research in progress on a laboratory and pilot-plant scale should lead to the invention of new process technology by the end of the century, thus accelerating the industrial use of coal gasification.
Bibliography: Howard-Smith, I., and Werner, G. J., Coal Conversion Technology (1976); Massey, Lester G., ed., Coal Gasification (1974); Schora, Frank C., ed., Fuel Gasification (1967); Schora, Frank C., et al., Fuel Gases from Coal (1976).
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