Nitric oxide reduction in the postflame region of pulverized coal flames

Abstract

A computer model was developed to perform three simulations of nitric oxide reduction in the postflame region of a downflow combustor burning Utah bituminous coal at stoichiometric ratios (SR) of 0.8 and 0.4, where SR = 1 Φ , and Φ is fuel equivalence ratio. Comparisons to experiment are made for all three cases. The contributions of both homogeneous and heterogeneous mechanisms to nitric oxide reduction in coal combustion are inferred from the simulations. The first simulation, in which soot was assumed to be the only route for NO reduction, determined the percentage soot yield needed in order for heterogeneous NO reduction on soot to account for the experimental results. The results indicate that required soot yields are consistent with those measured experimentally in pulverized coal flames. The second simulation, in which soot oxidation was added to the first model and where the gas-phase species profiles were calculated using a 119-reaction, 27-species carbon/hydrogen/oxygen reaction set, showed that all the soot was rapidly oxidized fo SR = 0.8, but that no appreciable soot was oxidized at SR = 0.4, due to the much lower OH radical concentrations. Accordingly, soot does not appear to be an important contributor to NO reduction, except under very fuel-rich conditions. The third simulation calculated only the homogeneous NO reduction using a carbon/hydrogen/oxygen/nitrogen reaction set. The model was able to account for all NO reduction at SR = 0.8, but only 40% of the NO reduction could be attributed to homogeneous effects at SR = 0.4, in agreement with the results from the second simulation. The reversible reaction N + CO 2 ↔ NO + CO was found to be an important path for homogeneous NO depletion for both stoichiometries, as was the Zeldovich mechanism. The reaction NH 2 + NO ↔ N 2 + H 2 O was found to be of importance only in the more ammonia-rich case of SR = 0.8.