Pollution Control

Coal is now used mainly as fuel for the production of electricity. Worldwide about 28% of commercial energy production depends on coal. In the United States it is about 31% and in some coal rich but oil poor countries such as China, Germany, Poland and the Czech Republic the figures are 73%, 56%, 95% and 86%, respectively (1). Because of the ample supply of available coal, dependence on coal as an energy source will probably remain high for some time to come. However, coal is the most polluting of all fuels; its main pollutants are sulfur dioxide and suspended particulate matter (SPM). Depending on its origin, coal contains between 1 and 2.5% or more sulfur. This sulfur comes in three forms: pyrite (FeS2), organic bound sulfur, and a very small amount of sulfates (2). Upon combustion, about 15% of the total sulfur is retained in the ashes. The rest is emitted with flue gases, mostly as SO2 but also, to a lesser extent, as SO3. This mixture is frequently referred to as SOx (2). The three basic approaches to the control of SOx emission are prepurification of coal before combustion, removal of sulfur during combustion, and purification of flue gases. The first approach, referred to as a benefication process, is based on a difference in specific gravity between coal (sp gr = 1.2–1.5) and pyrite (sp gr = 5). Although the technical arrangements may vary, in essence the procedure involves floating the crushed coal in a liquid of specific gravity between that of pure coal and that of pyrite. Coal is removed from the surface while pyrite and other minerals settle to the bottom. Coal benefication can reduce sulfur content by about 40% (2). Although gravity separation is presently the only procedure in use, research was initiated on microbial purification of coal. A research project conducted by the Institute of Gas Technology, with funding from the U.S. Department of Energy, was aimed at the development of genetically engineered bacteria capable of removing organic sulfur from coal. Inorganic sulfur can be removed by the naturally occurring bacteria Thiobacillus ferrooxidans, Thiobacillus thiooxidans, and Sulfolobus acidocaldarius (3).