Study on the transformation of gaseous selenium during the cooling process of coal-fired flue gas

Understanding the transformation of Se during the flue-gas cooling process is important for controlling Se emissions. By analyzing the thermal stability of Se in ash collected from power plant dust collectors, physical migration was found to be the main pathway by which gaseous Se transformed into ash during the flue-gas cooling process. The dynamic curves were obtained for SeO₂ adsorption by ash at 200-900 ℃. The adsorbed Se amount first decreased, increased, and then decreased with increasing adsorption temperature (400 ℃ and 600 ℃ were the two turning points). By comparing the adsorption characteristics of different ash and minerals and performing a speciation analysis of Se and an elemental distribution characterization after adsorption, the adsorption mechanism of gaseous Se was proposed. In the medium-high temperature range (above 500 ℃), SeO₂ was chemically adsorbed by ash through CaO and CaSO₄, generating CaSeO₃ and CaSeO₄, respectively; the thermal stability of CaSeO₄ was weaker than that of CaSeO₃. In the low temperature range (below 500 ℃), SeO₂ was adsorbed by ash mainly through physical adsorption of unburned carbon, accompanied by a small amount of chemical adsorption of CaO.