The term coal gasification refers to any process by which coal is converted into some gaseous form that can then be burned as a fuel. Coal gasification technology was relatively well known before World War II, but it fell out of favor after the war because of the low cost of oil and natural gas . Beginning in the 1970s, utilities showed renewed interest in coal gasification technologies as a way of meeting more stringent environmental requirements.
Traditionally, the use of fossil fuels in power plants and industrial processes has been fairly straight-forward. The fuel—coal, oil, or natural gas—is burned in a furnace and the heat produced is used to run a turbine or operate some industrial process. The problem is that such direct use of fuels results in the massive release of oxides of carbon , sulfur, and nitrogen , of unburned hydrocarbons , of particulate matter, and of other pollutants. In a more environmentally-conscious world, such reactions are no longer acceptable.
This problem became much more severe with the shift from oil to coal as the fuel of choice in power generating and industrial plants. Coal is "dirtier" than both oil and natural gas and its use, therefore, creates more serious and more extensive environmental problems.
The first response of utilities and industries to new air pollution standards was to develop methods of capturing pollutants after combustion has occurred. Flue gas desulfurization systems, called scrubbers , were one approach strongly favored by the United States government. But such systems are very expensive, and utilities and industries rapidly began to explore alternative approaches in which coal is cleansed of material that produce pollutants when burned. One of the most promising of these clean-coal technologies is coal gasification.
A variety of methods are available for achieving coal gasification, but they all have certain features in common. In the first stages, coal is prepared for the reactor by crushing and drying it and then pre-treating it to prevent caking. The pulverized coal is then fed into a boiler where it reacts with a hot stream of air or oxygen and steam. In the boiler, a complex set of chemical reactions occur, some of which are exothermic (heat releasing) and some of which are endothermic (heat-absorbing).
An example of an exothermic reaction is the following.
The carbon monoxide produced in this reaction may then go on to react with hydrogen released from the coal to produce a second exothermic reaction.
The energy released by one or both of the reactions is then available to initiate a third reaction that is endothermic.
Finally, the mixture of gases resulting from reactions such as these, a mixture consisting most importantly of carbon monoxide, methane , and hydrogen, is used as fuel in a boiler that produces steam to run a turbine and a generator.
In practice, the whole series of exothermic and endothermic reactions are allowed to occur within the same vessel, so that coal, air or oxygen, and steam enter through one inlet in the boiler, coal enters at a second inlet, and the gaseous fuel is removed through an outlet pipe.
One of the popular designs for a coal gasification reaction vessel is the Lurgi pressure gasifier. In the Lurgi gasifier, coal enters through the top of a large cylindrical tank. Steam and oxygen are pumped in from the bottom of the tank. Coal is burned in the upper portion of the tank at relatively low temperatures, initiating the exothermic reactions described above. As unburned coal flows downward in the tank, heat released by these exothermic reactions raises the temperature in the tank and brings about the endothermic reaction in which carbon monoxide and hydrogen are produced. These gases are then drawn off from the top of the Lurgi gasifier.
The exact composition of the gases produced is determined by the materials introduced into the tank and the temperature and pressure at which the boiler is maintained. One possible product, chemical synthesis gas, consists of carbon monoxide and hydrogen. It is used primarily by the chemical industry in the production of other chemicals such as ammonia and methyl alcohol. A second possible product is medium-Btu gas, made up of hydrogen and carbon monoxide. Medium-Btu gas is used as a general purpose fuel for utilities and industrial plants. A third possible product is substitute natural gas, consisting essentially of methane. Substitute natural gas is generally used as just that, a substitute for natural gas.
Coal gasification makes possible the removal of pollutants before the gaseous products are burned by a utility or industrial plant. Any ash produced during gasification, for example, remains within the boiler, where it settles to the bottom of the tank, is collected, and then removed. Sulfur dioxide and carbon dioxide are both removed in a much smaller, less expensive version of the scrubbers used in smokestacks.
Perhaps the most successful test of coal gasification technology has been going on at the Cool Water Integrated Gasification Combined Cycle plant near Barstow, California. The plant has been operating since June 1984 and is now capable of generating 100 megawatts of electricity. The four basic elements of the plant are a gasifier in which combustible gases are produced, a particulate and sulfur removal system, a combustion turbine in which the synthetic gas is burned, and a stem turbine run by heat from the combustion turbine and the gasifier.
The Cool Water plant has been an unqualified environmental success. It has easily met federal and state standards for effluent sulfur dioxide, oxides of nitrogen, and particulates, and its solid wastes have been found to be non-hazardous by the California Department of Health.
Waste products from the removal system also have commercial value. Sulfur obtained from the reduction of sulfur dioxide is 99.9% pure and has been selling for about $100 a ton. Studies are also being made to determine the possible use of slag for road construction and other building purposes.
Coal gasification appears to be a promising energy technology for the twenty-first century. One of the intriguing possibilities is to use sewage or hazardous wastes in the primary boiler. In the latter case, hazardous elements could be fixed in the slag drawn off from the bottom of the boiler, preventing their contaminating the final gaseous product.
The major impediment in the introduction of coal gasification technologies on a widespread basis is their cost. At the present time, a plant operated with synthetic gas from a coal gasification unit is about three times as expensive as a comparable plant using natural gas. Further research is obviously needed to make this new technology economically competitive with more traditional technologies.
Another problem is that most coal gasification technologies require very large quantities of water. This can be an especially difficult problem since gasification plants should be built near mines to reduce shipping costs. But most mines are located in Western states, where water supplies are usually very limited.
Finally, coal gasification is an inherently less efficient process than the direct combustion of coal. In most approaches, between 30 and 40% of the heat energy stored in coal is lost during its conversion to synthetic gas. Such conversions would probably be considered totally unacceptable except for the favorable environmental trade-offs they provide.
One possible solution to the problem described above is to carry out the gasification process directly in underground coal mines. In this process, coal would be loosened by explosives and then burned directly in the mine. The low-grade synthetic gas produced by this method could then be piped out of the ground, upgraded and used as a fuel. Underground gasification is an attractive alternative for many reasons. By some estimates, up to 80% of all coal reserves cannot be recovered through conventional mining techniques. They are either too deep underground or dispersed too thinly in the earth. The development of methods for gasification in coal seams would, therefore, greatly increase the amount of this fossil fuel available for our use.
A great deal of research is now being done to make coal gasification a more efficient process. A promising breakthrough involves the use of potassium hydroxide or potassium carbonate as a catalyst in the primary reactor vessel. The presence of a catalyst reduces the temperature at which gasification occurs and reduces, therefore, the cost of the operation.
Governments in the United States and Europe, energy research institutes, and major energy corporations are actively involved in research on coal gasification technologies. The Electric Power Research Institute, Institute of Gas Technology, U.S. Department of Energy , Texaco, Shell, Westinghouse, and Exxon are all studying modifications in the basic coal gasification system to find ways of using a wide range of raw materials, to improve efficiency at various stages in the gasification process, and, in general, to reduce the cost of plant construction.
[David E. Newton ]
Douglas, J. "Quickening the Pace in Clean Coal Technologies." EPRI Journal (January-February 1989): 12-15.