Twelve of the potentially hazardous air pollutants listed in the 1990 Clean Air Act are elements commonly found in trace amounts in coal. An option for controlling the release of these elements into the atmosphere is to remove them before combustion. Based on commercial-scale tests, conventional physical coal cleaning processes are effective in reducing the concentration of many of these trace elements in coal. In addition, advanced cleaning processes directed toward reduction of various elements may perform better than conventional processes. The degree to which a specific trace element can be reduced by coal cleaning depends on the mode of occurrence of the trace element the method of cleaning employed, and the way in which the cleaning process is operated. For example, higher trace element reductions can often be achieved if the cleaning plant is operated to remove as much ash as possbile. In addition to reducing the concentration of many trace elements, coal cleaning can improve overall boiler performance by increasing thermal efficiency. Also, cleaning changes ash loading and ash chemistry, impacting the performance of particulate collection equipment. For maximum effectiveness, coal cleaning should be considered as part of a system that includes the boiler as well as other emissions control systems.