Effect of CaO on the selective non-catalytic reduction deNOx process: Experimental and kinetic study

• CaO catalyzes NH3 decomposition, NO reduction by NH3, and NH3 oxidation by O2.
• NH3 dissociation to NH2 is the rate-controlling step of NH3 conversion.
• NH3 conversion on the CaO surface is mainly influenced by O2.
• The selectivity of NH3 conversion is influenced by both O2 and NO.
• Mechanism and kinetic model are proposed to describe the CaO-involved SNCR process.

The selective non-catalytic reduction (SNCR) deNOx process was influenced by high-concentration CaO particles in the pre-calciners when applied to cement industry. The influence of CaO on the SNCR deNOx process was investigated using a fixed bed reactor at temperature that ranges from 873 K to 1373 K and the mechanism of the process was analyzed. The experimental results demonstrated that CaO has catalytic effects on NH3 decomposition, NO reduction by NH3, and NH3 oxidation. It inhibits the SNCR deNOx process mainly via catalyzing NH3 oxidation to NO. The catalytic effect of CaO can be ignored at temperatures higher than 1273 K. NH3 was found that first adsorbed onto the CaO surface and then dissociated to NH2 by Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The mechanism analysis of CaO catalysis during the SNCR deNOx process showed that NH3 dissociation on the CaO surface is the rate-controlling step, and the intermediate substance, NH2, reacts with O2 or NO to produce NO or N2. The competition of the two reactions determines the product selectivity. A kinetic model including CaO-catalyzed NH3 conversion and gas phase SNCR deNOx reaction was developed, which well predicted the CaO-involved SNCR deNOx process.