A LABORATORY STUDY OF THE PARTITIONING OF TRACE-ELEMENTS DURING PULVERIZED COAL COMBUSTION

The extent of vaporization and subsequent recondensation of many trace elements during coal combustion is believed to depend upon the form of occurrence of the trace element in coal. To test this hypothesis, the forms of occurrence of the trace elements arsenic, mercury, selenium, zinc, chromium, and antimony were determined by successive chemical extractions of both a bituminous and a subbituminous coal. Well-controlled laboratory combustion tests were then conducted, and the concentrations of each element measured as a function of flyash particle size. Results indicated that those elements associated with the coal organic matrix, either ion-exchanged or covalently bound, and those associated with the reactive mineral pyrite were highly volatile. Arsenic, selenium, mercury, and antimony provide examples. In contrast, elements that were associated with silicate or oxide minerals (e.g., chromium) were relatively nonvolatile. For all elements, but particularly zinc, differences in form of occurrence between the two coals led to differences in fraction volatilized. For arsenic and selenium, the results indicated that the condensation pathway was dependent upon flyash chemistry. Using thermochemical equilibrium modeling to interpret the experimental data, it was concluded that arsenic vapors reacted to form calcium arsenates during combustion of the high-calcium subbituminous coal, but condensed as arsenic oxide during combustion of the low-calcium bituminous coal. Limited (calcium) selenate formation was also inferred from the data.