Formation of N2 and N2O in industrial combustion of ammonia over platinum

Abstract

The catalytic combustion of ammonia over a 95%Pt/5%Rh gauze has been investigated in an oxygen/steam environment using a flow reactor. Direct measurements of byproduct N2 and N2O selectivities have been made over a wide range of process conditions: temperature 450–815°C; pressure 1–3bar; mass flux 0.12 to 1.12kgm−2s−1; ammonia mole fraction 7–13%; and inlet oxygen-ammonia feed ratio 1.4 to 3. The results show that, while both N2 and N2O selectivities decrease with increasing temperature, the influence of other process variables is important. The formation of N2O is significantly reduced at higher values of the oxygen-ammonia feed ratio so attempts to increase burner throughput by reducing this ratio may lead to increased N2O formation in the burner.A model for product selectivity based on mass-transfer-limited combustion and detailed microkinetic description of the surface N2- and N2O-forming steps is developed. Explicit expressions incorporating all the process and gauze parameters in the system are obtained for the individual products. The model is found to provide excellent descriptions of the entire data set, both for oxygen and temperature dependence. The model also describes available literature data even for data obtained under very different conditions from ours (gauze dimensions, combustion in air, temperatures up to 1000°C and mass fluxes up to 50 times those studied in this work).Fundamental relations between the activation energies for the reactions of surface nitrogen atoms that lead to NO and byproduct formation are extracted from the data. These values provide a target for fundamental studies.The approach developed here is applicable to a wide range of catalysed mass-transfer-limited reactions such as combustion and partial oxidation.