Mercury Transformation and NO Emission of Coal Combustion with CaCl2 and NH4Cl Additives in a 6 kW CFB Combustor

Combustion experiments of anthracite with different amounts of CaCl2 and NH4Cl additives were conducted in a 6 kW circulating fluidized bed (CFB) combustor. Mercury transformation resulted by the addition of CaCl2 and NH4Cl, and the side effects on NO emission were explored. Mercury species existing in flue gas and fly ash were determined, respectively, by the Ontario Hydro Method (OHM) and thermal decomposition experiments, to explore the mercury transformation pathway and mechanism. Results show that the initial Hg-0 and He+ emission concentrations in flue gas are 2.276 and 2.572 mu g/Nm(3), respectively, as the anthracite is combusted in the CFB combustor. The proportion of gas-phase mercury is 22.65% and that of particle-bound mercury is 77.35%. The gas-phase mercury emission concentration decreases continuously with the increase of CaCl2 or NH4Cl addition amount and is also affected by the unburned carbon content of fly ash. Mercury compounds species existing in fly ashes are mainly HgCl2 and HgS with little amounts of HgSO4/HgO. The addition of CaCl2 or NH4Cl leads to the significant increase of HgCl2 content in fly ash with a slight change on HgS and HgSO4/HgO contents. Anthracite combustion with CaCl2 or NH4Cl additives can promote more He and He in flue gas transformation to HgCl2 adsorbed on fly ash by homogeneous and heterogeneous reactions. The addition of NH4Cl reduces 5-10% NO emission, but the addition of CaCl2 increases NO emission within 10%.