Potential PM2.5 generation mechanism induced by ammonia-based SCR: Nucleation of SO3, H2O and NH3

Air pollution caused by particulate matters with diameter smaller than 2.5 mu m (PM2.5) has aroused widely concern for the past decades. As a main precursor for the PM2.5, NOx emission is strictly control by flue gas denitrification with ammonia-based Selective Catalytic Reduction (SCR). However, the widely application of SCR causes unexpected increase in primary PM concentration in the flue gas, which may induce heavier particle pollutions. Herein, a multicomponent nucleation model with consideration of NH3 spillover and SO2 oxidation is established, to clarify the PM generation dynamics during SCR process and provide effective mitigation strategies. The NH3 slipping rate is decreased by temperature increasing, and is enhanced by XNH3/XNO. While the variation of SO2 oxidation rate is opposite to that of NH3 slipping rate, which may be attributed to their competitive adsorption on the SCR catalyst surface. With temperature varied from 320 degrees C to 420 degrees C and XNH3/ XNO changed from 0.7 to 1.2, the NH3 slipping rate and SO2 oxidation rate falls in the range of 3 %-11 % and 1 %-2%, respectively. Concentration of NH3 and SO2 would be increased in the SCR equipment, leading to gas-toparticle nucleation and PM generation. The nuclei concentration Cnuclei can be sharply increased by 2-5 orders of magnitude with temperature below 320 degrees C. With a constant temperature and increased XNH3/XNO from 0.7 to 1.2, the Cnuclei would be decreased owe to the decreased SO2 oxidation. Therefore, reducing the SO2 oxidation as well as preventing NH3 from spillover during the SCR can be effective way to mitigate the potential PM emission.