Study On The Influence Of Ca-based Particles On The Conversion Of Reducing Agents In The SNCR DeNO_x Process

Selective non-catalytic reduction?SNCR?technology is the most suitable NOx control technology for cement kilns.The high concentration Ca-based particles in pre-calciner affact the reducing agent conversion,thus,inhibit the deNOx performance of the SNCR deNOx process.The influence of Ca-based particles on the conversion of two main reducing agents,NH₃ and urea,was studied in this work,which providesguidance to the development and application of the SNCR deNOx process in cement industry.Firstly,NH₃ conversion was studied using a gas-solid catalytic reaction system and TPD-DRIFTS.NH₃ was found to adsorb onto the Lewis acid sites on the CaO surface.Adsorbed NH₃ dissociation to NH₂ was the rate-controlling step of NH₃ conversion.NO co-adsorbed onto the CaO surface with NH₃,accelerated NH₃ dissociation,and reacted with NH₂ to produce N₂.O₂ increased NH₃ conversion on the CaO surface and oxidized NH₂ to NO.The product selectivity was determined by the competition of NH₂ conversion to NO and N₂.NH₃ can react with CaCO₃ and release HNCO.NO had no influence on NH₃ conversion on the CaCO₃ surface.O₂ had no influence on NH₃ conversion but converted the product between NH₃ and CaCO₃ to NO and N₂.Product selectivity was determined by the competition of NO and N₂ production.The kinetic models established based on the mechanism and experimental results well predicted NH₃ conversion on the CaO and CaCO₃ surface.A kinetic model was also developed to describe NH₃ conversion during CaCO₃ decomposition process based on the research on CaCO₃ decomposition.Secondly,the effect of Ca-based particles on urea pyrolysis and HNCO conversion was studied using pyrolysis/gas-solid reaction system and TPD-DRIFTS.Ca-based particles were found to react with urea and biuret?intermediate product in urea pyrolysis?to produce isocyanate calcium.The reaction and the conversion of isocyanate calcium decreased the pyrolysis rate of urea.Ca-based had little influence on nitrogen element conversion to NH₃ in urea pyrolysis,but decreased the conversion of nitrogen element to HNCO and converted HNCO to NO,N₂O,and CO₂.Ca-based particles reacted with HNCO to produce isocyanate calcium,and isocyanate calcium decomposed to calcium cyanamide and CO₂ at higher temperatures.O₂ promoted the reaction between Ca-based particles and HNCO,oxidized isocyanate calciumto CaO,N₂O,and CO₂,and oxidized calcium cyanamide to CaO,NO,and CO as while.The reaction between Ca-based particles and HNCO inhibited the conversion of HNCO to NH₃,which decreased the utilization of urea as a reducing agent.Lastly,N₂O conversion on the Ca-based particle surface was studied.CaO catalyzed N₂O decomposition,while CaCO₃ had no such catalytic effect.N₂O adsorbed onto the CaO surface and formed cis-dimer nitroso.The dissociation of cis-dimer nitroso was the rate controlling step of N₂O conversion on the CaO surface.NH₃ and O₂ had no influence on N₂O decomposition,but the produced H2 O in NH₃ oxidation inhibited N₂O decomposition on the CaO surface.The kinetic model developed based on the above mechanism well described the N₂O conversion on the CaO surface at different atmospheres.