The selective oxidation of ammonia over alumina supported catalysts: Experiments and modelling

Hot gas-clean up will improve the efficiency of emerging gasification technologies. Selective catalytic oxidation (SCO) of ammonia is a promising approach for dealing with the main fixed nitrogen species. The work presented here comprises both laboratory scale experimental measurements of potential SCO catalysts, as well as the development of a simple four-step reaction model to describe the behaviour of one of the more promising catalysts. A range of transition metal oxides supported on γ-alumina were studied for their activity in the SCO of ammonia in a simulated gasification gas mixture containing CO, H₂O, H₂, CO₂, CH₄, H₂S and toluene as model tar species. Both copper and chromium based catalysts demonstrated a window of operating temperature over which they were resistant to poisoning by H₂S; Cu/Al₂O₃ was in fact promoted by this gas for the SCO reaction. The ammonia conversion over 7% Cu/Al₂O₃ was studied in more detail, and this data was further used to develop a kinetic model for the reactions taking place over the temperature range 723–906 K. Excellent conversion and selectivity to N₂ was found in the temperature window 973–1173 K and 2.6 vol% O₂. However, it also catalyses a rapid H₂O₂ reaction. This reaction consumes all remaining available oxygen so that no other oxidation reactions take place (e.g. of methane or ‘tar’). The four-step reaction model was developed using the PLUG application of Chemkin and Surface Chemkin software coupled with the gas-phase mechanism Kilpinen 97. Rates for the heterogeneous oxidation of ammonia and hydrogen are included as well as forward and reverse reactions of the water gas shift. Over the temperature range in question, the surface reaction rates are much faster than the gas-phase reactions. The model is applicable for the 723–906 K temperature range using a gas mixture containing 0.4 vol% ammonia and 0.01 vol% H₂S in the presence of O₂ (0–2.6 vol%).