Nitrous Oxide Formation and Destruction by Industrial No Abatement Techniques Including Scr

Abstract

Systematic investigations on N2O emission from full scale stationary combustion units, equipped with primary or secondary NO control techniques, are scarce or inexistent. Recent results obtained from laboratory scale studies are presented, from which it appears that fuel staging, selective non catalytic NO reduction with ammonia and selective catalytic NO reduction in the presence of ammonia, should be considered as potential sources of nitrous oxide emission enhancement. For the two mentioned gas phase NO abatement techniques (fuel staging and NCSR), this N2O emission enhancement is clearly linked with a decrease of the temperature, a result that might have been expected from the known gas phase reactions of N2O formation and destruction. Production of N2O from NCSR is more important than from fuel staging, and increases with ammonia concentration; this probably is related to the fact that ammonia yields N2O precursors (NH, NH2) readily by its decomposition. Separate injection of pure NO or NH3 suggests that N2O is a product of the interaction of those two reactants, whereas NO also is formed as a primary ammonia decomposition product in the presence of oxygen. A kinetic investigation of N2O formation from SCR has been made. It is shown that catalytic decomposition of neat ammonia yields both NO and N2O, the former as a primary product (from adsorbed ammonia and solid bound oxygen), the latter as a secondary product (from NO and adsorbed ammonia). Both NO and N2O subsequently undergo catalytic decomposition. In the presence of molecular oxygen, another NO formation (from O2 and adsorbed ammonia) manifests itself at somewhat higher temperatures, creating the well known optimum temperature window . Comparative tests conducted on a number of metal oxides, tend to show that high efficiency for NO decomposition is often related to high production of N2O within the temperature window .