The ability of non-hydrocarbon fuels such as CO and H2 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 H2 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, H2, 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 N2 in the reburn zone proceeds primarily through the following sequence:  H + NO + M ⇌ HNO + M, HNO + H ⇌ NH + OH, NH + NO → N2 + ... The implications of the results for reburning with fuels with a low hydrocarbon content are discussed, with special emphasis on gasified fuels.

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The branching fraction for the NH2 + NO reaction has been determined in the temperature range 1210−1370 K from flow reactor experiments on the NH3/NO/O2 and CO/NH3/NO/O2 systems. The branching fraction is defined as α = k 1/(k 1 + k 2), where NH2 + NO → NNH + OH (1) and NH2 + NO → N2 + H2O (2). The experiments were performed at very low oxygen concentrations to minimize the impact of secondary reactions. The results show that α increases gradually from a value of 0.35 ± 0.04 at 1211 K to 0.45 ± 0.02 at 1369 K. The data blend smoothly with the most recent direct measurements and confirm the significant rise in branching fraction suggested by previous high-temperature determinations in static reactors and flames.

A laboratory study of the hybrid reburning/SNCR technique has been performed under flow reactor conditions. This technique involves the use of a selective reducing agent in the presence of combustibles (mainly CO) arising from the reburning zone. The impact of CO and O2 concentrations, NH3/NO ratio, as well as the presence of HCN has been investigated as a function of the temperature in the range 700−1300 K. The results show that CO in concentrations typical of the rich/lean transition in reburning shift the regime for NO reduction in the SNCR process to temperatures below 1000 K and cause a narrowing of the temperature window. The NO reduction potential is largely unaffected compared to conditions with low CO, and the effect varying the O2 concentration in the range 0.5−4.0% as well as of adding of HCN is found to be insignificant. Synergistic effects between reburning and SNCR are only observed in a narrow range of operating conditions with very low concentrations of CO and O2. Model predictions using a detailed reaction mechanism generally compare favorably with the experimental results. The present results indicate that a reduction of the CO level from the reburn zone is required before the SNCR chemistry is initiated. This can be obtained either by staging the burnout air or by injecting the N-agent in an aqueous solution to delay reaction.

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