The role of coal combustion as a significant global source of nitrous oxide (N2O) emissions was reexamined through on‐line emission measurements from six pulverized‐coal‐fired utility boilers and from laboratory and pilot‐scale combustors. The full‐scale utility boilers yielded direct N2O emission levels of less than 5 ppm. The sub‐scale combustor test data were consistent with full‐scale data, and also showed N2O emission levels not exceeding 5 ppm, although these levels increased slightly when various combustion modifications to lower NO emissions were employed. These on‐line emission measurements are very different from previously published data. The discrepancy is shown to be due to a sampling artifact by which significant quantities of N2O can be produced in sample containers which have been used in establishing the previously employed N2O data base. Consequently, we conclude that N2O emissions bear no direct relationship to NO emissions from these combustion sources, and that this direct source of N2O is negligible. Other indirect routes for the conversion of NO into N2O outside the combustor and other combustion sources not examined by this study, however, cannot be ruled out.

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Reburning is examined as a means of NOx destruction in a 17 kW down-fired pulverized coal combustor. In reburning, a secondary fuel is introduced downstream of the. primary flame to produce a reducing zone, favorable td NO destruction, and air is introduced further downstream to complete the combustion. Emphasis is on natural gas reburning and a bituminous coal primary flame. A parametric examination of reburning • employing a statistical experimental design, is conducted, complemented by detailed experiments. Mechanisms governing the inter-conversion of nitrogenous species in the fuel rich reburn zone are explored. The effect of reburning on N20 emissions, the effect of primary flame mode (premixed and diffusion) and the effect of distributing the reburning fuel, are also investigated. The parametric study allowed the effects of significant reburning variables to be identified and examined, but these effects could not be quantified. Detailed experiments identified optimum reburn zone stoichiometry between 0.8 and 0.9, depending on mixing in the reburn zone. Overall NO reductions, as high as 80%, were possible and depended mainly on reburn zone variables, namely, temperature, residence time and stoichiometry. ,I Exhaust N20 emissions increased after air addition in the final stage of reburning, but were less than 10 ppm. Lower reductions in NO emissions were obtained when the primary flame was of the diffusion type, rather than of the prernixed type, but final NO emissions below 250 ppm (dry, 0% O2) were still possible. Reburning fuel introduction in multiple streams did not enhance NO destruction, relative to single stream injections. Within the reburn zone, rebuming mechanisms ocbarred in two regimes. One regime was in the vicinity of the rebuming fuel flame and was distinguished by fast reactions between NO and hydrocarbons that were limited by mixing. The other regime covered the remainder of the reburn zone and was distinguished by slower reactions, without mixing complications. For the latter regime, a simplified model based on detailed gas phase chemical kinetic mechanisms .and known rate coefficiems was able to predict temporal profiles of NO, HCN and NII 3. Reactions involving hydrocarbons played important roles in both regimes and N2 fixation by hydrocarbons limited reburning effectiveness at low primary NO values. Appropriate corrections for mixing effects in early time scales of the reburn zone allowed the prediction of nitrogenous species profiles from primary NO values, as well as overall reburning effectiveness.

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