Chemkin simulation of mercury oxidation in a condensing heat exchanger.

摘要

Mercury oxidation in a slip stream condensing heat exchanger (CHX) developed by the Energy Research Center (ERC) at Lehigh University was modeled using hundreds of possible chemical reactions and tens of species in coal-fired power plant furnace and flue gas downstream of the furnace. The modeling tools, CHEMKIN, SENKIN and PSR were used to conduct equilibrium calculations in the furnace and the kinetic calculations in the flue gas leading to the CHX. The detailed mechanism of mercury speciation at the furnace and downstream of the furnace has been investigated. Atomic chlorine is generated in the furnace and flows downstream of furnace, where it reacts with elemental mercury (Hg 0). The predicted results suggest oxidation of Hg0 in the CHX occurs by this mechanism. Performance tests of the CHX at Great River Energy's Coal Creek Station were carried out to study moisture, mercury and acid capture abilities of the CHX. The elemental mercury reduction rate and the amount of condensed water were measured in these tests. The measured results show ~35% of Hg0 was oxidized in the CHX. The mercury oxidation results show agreement between simulation and test within a typical range of HCl concentration of flue gas at furnace exit from 20 ppmv to 50 ppmv. The kinetic calculations downstream of the furnace show the oxidation reaction between Hg0 and atomic Cl primarily occurs at temperature 600K to 300K. The flue gas temperature in the CHX affects condensed water formation and mercury oxidation rate. An analytical model of heat and mass transfer processes in the CHX was used to obtain predictions of flue gas temperature profiles with different CHX inlet cooling water temperatures. The simulation results with lower CHX inlet cooling water temperature, which leads to higher flue gas cooling rate and reduced flue gas moisture content in the CHX, suggest higher Hg0 oxidation rates would be obtained. The predicted Hg 0 reduction rate in the CHX increased to ~42% with CHX inlet cooling water temperature of 35°F and a HCl concentration at the furnace exit of 40 ppmv.