Study on the catalysis of CaCO3 in the SNCR deNO(x) process for cement kilns

摘要

The selective non-catalytic reduction (SNCR) deNO(x) process is influenced by high concentration CaCO3 particles in the pre-calciner when used in cement industry. The influence of CaCO3 on the SNCR deNO(x) process was investigated using a fixed bed reactor with temperature ranging from 873 K to 1123 K. Experimental results showed that CaCO3 catalyzes NH3 conversion to HNCO and N-2 in the absence of O-2, and catalyzes NH3 oxidation to NO and N-2 in the presence of O-2. Both reactions show first order characteristics with respect to NH3 concentration. CaCO3 has no catalytic effect on NO reduction by NH3. The deNO(x) efficiency of the SNCR deNO(x) process is inhibited by CaCO3 mainly by catalyzing NH3 oxidation to NO. Mechanism analysis showed that the reaction between NH3 and CaCO3 is the rate-controlling step of NH3 conversion. In the absence of O-2, the intermediate product decomposed to CaO, HNCO, H2O and N-2, whereas CaO reacted with CO2 to reproduce CaCO3. O-2 did not participate in the rate-coiltrolling step of NH3 conversion. However, in the presence of O-2, the intermediate product of the reaction between NH3 and CaCO3 can be oxidized to NO by O-2 or decomposed into N-2 through dimerization reaction. Product selectivity is determined by the competition of these two reactions. A kinetic model was proposed for the CaCO3-involved SNCR deNO(x) process which well predicted the influence of CaCO3. (C) 2014 Elsevier B.V. All rights reserved.