Oxidation of NH3 and NOx formation during the catalytic combustion of gasified biomasses fuels over Mn-hexaaluminate and alumina-supported Pd catalysts

The reactivity of a BaMnAl11O19 and a PdO/Al2O3 catalyst in the combustion of NH3-containing gasified biomass-derived fuels is investigated. Both cata lysts produce significant amounts of fuel-NOx from ammonia, and particularl y NO. The NO formation increases with temperature and over the BaMnAl11O19 catalyst shows a maximum near 750 degrees C: above this temperature the NO formation is strongly reduced and a corresponding increase in N-2 concentra tion is observed. This phenomenon, that is enhanced by the presence of the fuel components, is not apparent over the PdO/Al2O3 catalyst. The observed reduction in the NO concentration is related to the occurrence of a selecti ve non-catalytic reduction (SNCR) process involving ammonia as reducing age nt and occurring in the first part of the catalyst bed where the simultaneo us presence of oxygen, NO and ammonia is attained. These conditions are not achieved in the case of the PdO/Al2O3 catalyst due to the rapid NH3 oxidat ion. The NO-SNCR reduction process is enhanced by the presence of the bioga s fuel components, that do not directly reduce NO but that enhance the reac tivity of gaseous ammonia through complex effects on the radical pool in th e gasphase. (C) 1999 Elsevier Science B.V. All rights reserved.