FACTORS CONTROLLING TRACE ELEMENTS OF ENVIRONMENTAL CONCERNS FOR POWDER RIVER BASIN COAL, WYOMING

The Powder River Basin coals play a significant role in supplying clean and compliant fuel, and their trace-element concentrations are important in meeting the stringent emission standards of the Environmental Protection Agency (EPA). These coals contribute about 37 percent of the total coal production in the United States and are utilized by 145 coal-fired powder plants to generate electricity.

Twelve trace elements of environmental concerns for the Powder River Basin coals in Wyoming are antimony, arsenic, beryllium, cadmium, cobalt, chromium, lead, manganese, mercury, nickel, selenium, and uranium. Of these, arsenic, mercury, and selenium are the trace elements that seem to be of the greatest environmental concern.

Detailed analysis of the vertical and lateral variations of trace elements in two mineable and one unmineable coal core in the Wyodak-Anderson coal zone show concentrations of these 12 trace elements are within the suggested threshold of compliance. Trace element concentrations for these 12 elements range from a less than value of 0.001 to 105 parts per million (ppm). More specifically, arsenic, mercury, and selenium concentrations range from 0.25 to 64, 0.01 to 2.1, and 0.05 to 8.5 ppm, respectively. Vertical distributions of the trace elements are highly variable and can be related to the ash content and maceral composition of the coal beds. For instance, the vertical variations of mercury with the ash yield have a significant correlation from 0.76 to 0.90. Mercury concentration is also directly related to the woody maceral contents, which accounts for 65 percent of the coal beds. Thus, variation of mercury and associated trace elements is controlled by the paleoenvironment of the raised bogs and the volcanic activity that introduced air-born ash into the accumulating organic material.