OCR Text |
Show 0.15%. However, this exclusion cannot be justified in reburning, where CH4 values in the rebum zone are relatively high. One source of HCN formation is the destruction of NO by hydrocarbon radicals. In order to explore the significance of other homogeneous gas phase hydrocarbon reactions in forming HCN, the contribution of NO + CHi reactions in the reburn zone was minimized. This was accomplished by using natural gas as the primary fuel to produce low primary NO levels, which would limit the formation of NO in the primary zone mainly to that of Thermal NO. Natural gas was also used as the rebuming fuel. Figure 3 shows the results of two experiments in which primary NO level of 35 and 45 ppm (dry, 0% O2) were obtained. In both cases, HCN levels were relatively high (exceeding 60 ppm) and U%UUL ~ PI.IKAaT lUKE, U%UUL ~ JDUUDI; F11EL nCB sa • 0.86 1800 _----__, r__--------, ~ 1600 ci WOO E t!! 1200 -- 0" 100 ~ o f 80 ~ > E 60 a.. a.. wi at ~ U at a.. en 20 z X / .,.-.l-71-I ---. • lICK o 1L9--~=to~~tMo Bi:3==~to. ..J I!'=---'-_"'--........- ---I 0.0 0.4 0.8 U 0.0 0.4 0.8 U l6 Residence Time, s Residence Tame, s resulted in an increase in total fixed Figure 3: Reburning Fuel Rich Zone- Species nitrogen (TFN = NO + HCN + NH3) Profiles at Low Primary NO. concentrations, relative to the primary NO levels. This is a clear indication that Fenimore12 N2 fixation produces HCN, mainly due to CH + N2 reaction, which increases the fixed nitrogen species pool and depletes the hydrocarbon radical pool. This can limit the effectiveness of combustion modification techniques that involve fuel rich regimes in reducing NO emissions, especially as NO levels decrease. These observations are consistent with the natural gas reburning results of Lanier et al.(1986). Figure 4 shows nitrogenous species profiles in the reburn zone for three tests, in which different reburning fuels were used, namely, natural gas, natural gas doped with NH3 to a nitrogen content of 1.4%, and bituminous coal containing 1.3% nitrogen. Primary NO levels and temperature profiles in the reburn zone were similar in all three cases, and the rebum zone stoichiometry was at 0.9. In all three cases, HeN and NH3 concentrations were less than 100 ppm throughout the reburn zone. Minor differences in NO profiles were observed when gaseous rebuming fuel were used, regardless of the nitrogen content. However, NO decayed at a slower rate in coal reburning and corresponded to low methane levels of less than 0.02%, as compared to levels that were 0.1-0.15% when gaseous reburning fuels were used. In the coal reburning test, HeN values decayed to levels below 20 ppm within 0.6 seconds. These low levels are due to the dominance of HeN destruction reactions in the rebum zone, since HCN formation from hydrocarbons is of minor significance when hydrocarbon concentrations are low. These results further support the hypothesis that HCN formation in long time scales is mostly due to hydrocarbon reactions, and the slow release of nitrogen from the coal residue is a minor contributor to HeN formation. 4 |