The behavior of Se during coal combustion is different from other trace metals because of the high degree of vaporization and high vapor pressures of the oxide (SeO2) in coal flue gas. In a coal-fired boiler, these gaseous oxides are absorbed on the fly ash surface in the convective section by a chemical reaction. The composition of the fly ash (and of the parent coal) as well as the time-temperature history in the boiler therefore influences the formation of selenium compounds on the surface of the fly ash. A model was created for interactions between selenium and fly ash post-combustion. The reaction mechanism assumed that iron reacts with selenium at temperatures above 1200 degrees C and that calcium reacts with selenium at temperatures less than 800 degrees C. The model also included competing reactions of SO2 with calcium and iron in the ash. Predicted selenium distributions in fly ash (concentration versus particle size) were compared against measurements from pilot-scale experiments for combustion of six coals, four bituminous and two low-rank coals. The model predicted the selenium distribution in the fly ash from the pilot-scale experiments reasonably well for six coals of different compositions. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.