TRACE-ELEMENT BEHAVIOR DURING COAL COMBUSTION - RESULTS OF A LABORATORY STUDY

The air emission of trace elements from coal combustion processes is dependent upon the high-temperature transformations of the elements. In a laboratory study using several coals of varying rank as well as a physically cleaned bituminous coal, the combustion partitioning of trace elements was examined. Experiments were conducted in an isothermal laminar flow drop tube furnace with a maximum wall temperature of 1750 K. Size segregated and utility grind coal samples were both used. Ash samples were size-fractionated as needed using an on-line cascade impactor, and were chemically analyzed for trace element concentrations by neutron activation analysis. For the coals and conditions analyzed, selenium, arsenic, zinc, and antimony were highly enriched in the smallest ash particles. Chromium was evenly distributed for one coal, but increased with decreasing particle size for another. Chromium concentrations in the submicron ash were dependent upon the size of the parent coal particles, whereas selenium concentrations were not. These results suggest that Se, As, Zn, and Sb are partially volatile during combustion. Chromium behavior is variable and likely dependent upon the form of chromium in the coal.