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(i)

Understanding Our Planet Through Chemistry

IIe. Pollution Prevention

Cleaning up coal burning

While hundreds of abandoned mines across the country are releasing pollutants, active mines can also produce pollutants. Among the best examples of air and water pollution control are advances in coal technology. For years coal has been a major source of both energy and pollution in the United States. Supplies of natural gas and petroleum are dwindling. Alternative energy sources are not expected to contribute significantly to the energy needs of the United States in the near future. Coal will continue to play an important role for energy production through the first half of the 21st century.

Significant improvements in coal processing and burning in modern power plants have dramatically reduced pollution. The process has been improved in three ways. First, sophisticated equipment has significantly reduced fly ash and soot compared to the equipment used many years ago; other specialized equipment greatly reduces sulfur-dioxide emissions.

Photo of coal-burning facility in Ohio.Coal, a major source of energy in the United States, does not have to cause pollution. This coal-burning power facility at Brilliant, Ohio, uses a process wherein sulfur-dioxide emissions are cut by 90 percent, nitrogen oxides by 50 percent, and carbon dioxide by 15 percent. (Photo provided by American Electric Power Service Corporation). [135k]

A second way of reducing coal pollution is by selective mining of low-ash and low-sulfur coals that pollute less. Detailed chemical analyses of coal prior to mining is required to determine the concentrations of ash, sulfur, and other toxic elements. A new multielement analytical technique that introduces the sample in liquid form to an inductively coupled plasma quadrupole mass spectrometer (ICP-QMS) is proving very useful for this purpose. This technique can determine over 70 elements at the ppm to ppb levels. To analyze coal by this method, it must first be converted to ash, fused with a flux, and dissolved. The solution is then sprayed into the 7,000°C thermal environment of an argon plasma where it is ionized. The resulting charged atomic particles are drawn into a high vacuum portion of the instrument where a quadrupole mass spectrometer (shown in Disaster from space section) separates and counts the number of atoms for each different mass.

Detailed mapping of trace elements in a coal seam may be required to locate low-polluting coal resources. The major drawback to selective mining is that only small quantities of clean coals exist, and those that can be found may be too far from power plants or too deep to be economically recovered.

Photomicrograph of a coal sample. The principal source of sulfur emissions from burning coal is pyrite, whose presence is shown in this photomicrograph of a coal sample. [204k]

Coal cleaning is the third method for reducing pollution. Sulfur minerals such as pyrite can be removed by using various techniques. Chemical analysis of the coal and identification of mineral inclusions determines what cleaning procedure will be most effective. This requires looking at the coal under high- power microscopes or performing tests that separate mineral and coal species by using complex physical and chemical techniques. Understanding the chemistry and mineralogy of coals has contributed significantly to the progress that has been made in recent years toward the prevention of coal-burning pollution.

For additional information on environmental geochemistry order a paper copy of Understanding Our Fragile Environment, USGS Circular 1105, a publication in the Public Issues in Earth Science Series.

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