Reaction mechanism of NO and SO2 Co-removal by calcium hydroxide

Flue Gas simultaneous Desulfurization and Denitrification (FGDD) is expected to become a new generation of low-cost technology to achieve ultra-low emissions especially in smelting kilns and industrial boilers. The fixed bed system and in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) were used to clarify the reaction mechanism of NO and SO2 co-removal by calcium hydroxide absorbent. The hydroxyl radical on the surface of calcium hydroxide was identified by Protium Nuclear Magnetic Resonance (1H NMR) and radical scavenger 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO). With synergistic effect of hydroxyl radical and oxygen, NO and SO2 are oxidized to NO2 and SO3 respectively, and then absorbed as nitrate and sulfate, according to the three-step chain reactions: SO2 +HO* →HOSO2 *, HOSO2 * +O2→SO3 +HO2 * and HO2 * +NO→HO* +NO2. The key point is the recycling of hydroxyl radical.