Nitrogen oxide abatement by distributed fuel addition. Final report

Abstract

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.

Global Rate Expression for Nitric Oxide Reburning. Part 2

“…In this work, these species included C 2 H 4 , C 2 H 3 , C 2 H 2 , C 2 H, CH 4 , CH 3 , CH 2 , and CH. However, throughout the simulations in this study, only CH and CH 2 were predicted in significant concentrations, which was also observed experimentally by Wendt and Mereb …”

Prediction of Nitric Oxide Destruction by Advanced Reburning

“…Increasingly stringent standards imposed on NO x emissions from combustion sources have resulted in a greater effort to develop novel reduction approaches. Among the most recent developments for reducing NO x emissions from coal combustion systems are (1) the reburning technologies, wherein gaseous, liquid, or solid hydrocarbon fuels are injected downstream of the main combustion zone to react with NO and produce HCN and eventually N 2 , − and (2) the advanced reburning technologies, wherein ammonia, urea, or similar substance is injected after hydrocarbon injection to further reduce NO x species . Up to 85∼95% reduction of NO x can be achieved by combining reburning and advanced reburning technologies .…”

A Reduced Kinetic Model for NOx Reduction by Advanced Reburning

“…To meet the requirements of the Clean Air Act Amendments of 1990 (CAA) within budget limitations and scheduling constraints, electric utilities have considered a variety of novel approaches to reduce NO x emissions from powerplants. Among the most recent developments for reducing NO x emissions from coal combustion systems are (1) the reburning technologies wherein gaseous, liquid, or solid hydrocarbon fuels are injected downstream of the main combustion zone to react with NO and produce intermediate HCN, ,, which is subsequently reduced to N 2 , and (2) the advanced reburning technologies wherein ammonia, urea, or similar substances are injected after hydrocarbon injection for reburning to further reduce NO x species…”