Clean Coal Technology

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Clean coal technology

Coal is rapidly becoming the world's most popular fuel. Today in the United States, more than half of the electricity produced comes from coal-fired power plants . The demand for coal is expected to triple by the middle of the next century, making it more widely used than petroleum or natural gas .

The unpleasant aspect of this trend is that coal is a relatively dirty fuel. When burned, it releases particulates and pollutants such as carbon monoxide , nitrogen and sulfur oxides into the atmosphere . If the use of coal is to expand continually, something must be done to reduce the hazard its use presents to the environment .

Over the past two decades, therefore, there has been an increasing amount of research on clean coal technologies, methods by which the combustion of coal releases fewer pollutants to the atmosphere. As early as 1970, the United States Congress acknowledged the need for such technologies in the Clean Air Act of that year. One provision of that Act required the installation of flue gas desulfurization (FGD) systems ("scrubbers") at all new coal-fired plants.

More than a dozen different technologies are now available on at least an experimental basis for the cleaning of coal. Some of these technologies are used on coal before it is even burned. Chemical, physical, and biological methods have all been developed for pre-combustion cleaning. For example, pyrite (FeS₂) is often found in conjunction with coal when it is mined. When the coal is burned, pyrite is also oxidized, releasing sulfur dioxide to the atmosphere. Yet pyrite can be removed from coal by rather simple, straightforward physical means because of differences in the densities of the two substances.

Biological methods for removing sulfur from coal are also being explored. The bacterium Thiobacillus ferrooxidans has the ability to change the surface properties of pyrite particles, making it easier to separate them from the coal itself. The bacterium may also be able to extract sulfur that is chemically bound to carbon in the coal.

A number of technologies have been designed to modify existing power plants to reduce the release of pollutants produced during the combustion of coal. In an attempt to improve on traditional wet scrubbers , researchers are now exploring the use of dry injection as one of these technologies. In this approach, dry compounds of calcium, sodium, or some other element are sprayed directly into the furnace or into the ducts downstream of the furnace. These compounds react with non-metallic oxide pollutants, such as sulfur dioxide and nitrogen dioxide, forming solids that can be removed from the system. A variety of technologies are being developed especially for the release of oxides of nitrogen. Since the amount of this pollutant formed is very much dependent on combustion temperature, methods of burning coal at lower temperatures are also being explored.

Some entirely new technologies are also being developed for installation in power plants to be built in the future. Fluidized bed combustion , integrated gasification combined cycle, and improved coal pulverization are three of these. In the first of these processes, coal and limestone are injected into a stream of upward-flowing air, improving the degree of oxidation during combustion. In the second process, coal is converted to a gas that can be burned in a conventional power plant. The third process involves improving on a technique that has long been used in power plants, reducing coal to very fine particles before it is fed into the furnace.

[David E. Newton ]