Nitric oxide reduction by non-hydrocarbon fuels. Implications for reburning with gasification gases

The ability of non-hydrocarbon fuels such as CO and H-2 to reduce nitric oxide under conditions relevant for the reburning process is investigated experimentally and theoretically. Flow reactor experiments on reduction of NO by CO and H-2 are conducted under fuel-rich conditions, covering temperatures of 1200-1800 K and a range of stoichiometries and reactant levels. Bench and pilot scale results from literature on reburning with CO, H-2, and low calorific value gases are also considered. The experimental data are interpreted in terms of a detailed reaction mechanism, and the reactions responsible for removal of NO are identified. The experimental results indicate that under typical reburn process conditions these non-hydrocarbon fuels may remove 20-30% of the nitric oxide entering the reburn zone. However, results indicate that the process potential increases with temperature and reburn fuel fraction, and at high temperatures and reburn fuel fractions of about 30%, the reduction efficiency approaches that of hydrocarbon gases. If dilution effects and the lowering of the primary zone NO (maintaining the overall load) are accounted for, the reduction potential is further increased. Modeling results indicate that the mixing process may affect the NO reduction in the reducing zone. The modeling predictions are in qualitative agreement with the experimental results but tend to underestimate the reduction of NO. Conversion of NO to N-2 in the reburn zone proceeds primarily through the following sequence: H + NO + M reversible arrow HNO + M, HNO + H reversible arrow NH + OH, NH + NO --> N-2 + .... The implications of the results for reburning with fuels with a low hydrocarbon content are discussed, with special emphasis on gasified fuels.