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Show A>1 232 ^ ) ; o « (fa//7o.i /ai /aos w.01 FIGURE 15: CALCULATED SOURCES ( g cm3 s1) OF THERMAL-NO (TOP) AND PROMPT-NO IN 2 M W NATURAL GAS FLAME FIGURE 16: TYPE-2 FLAME; COMPUTED LOCAL STOICHIOMETRY (TOP) AND CONTOURS OF TOTAL-NO CONCENTRATIONS IN PPM (BOTTOM) 2.9% O2, respectively. The significant differences in predicted and measured temperatures at distances larger than 1.5 m from the swirlcr outlet arc, with respect to the predictability of N O , in all probability of minor importance. This is because the N O formation in that part of the furnace is negligible. Figure 14(boltom) shows the thermal-, prompt- and total-NO calculated along the furnace axis using two values of the s parameter, 0.6 and 0.999. Figure 14 shows a strong sensitivity of the calculated flue N O emissions to the s parameter influencing the temperature fluctuations. The predicted total-NO emissions range, depending on the variance coefficient s, from 6 0 % up to 1 0 4 % of the measured lolal-NO emissions. The model calculates that 20- 23 % of the NO-emissions is prompt-NO. Figure 15 shows isolines of the thermal- and prompt-NO sources for the variance coefficient s equals 0.6. These sources are only significant in the region bounded by the 1600 K-isoline of the mean temperature. The thermal-NO and prompt-NO sources reach their maximum values in positions very near stoichiometric conditions. The prompt*NO source reaches its maximum inside the burner quarl while the thcrmal-NO source just downstream the burner quarl; i.e. prompt- N O is produced somewhat earlier than thermal-NO. This should be the case indeed, because at temperatures less than 1200 K, prompt- N O is produced much faster than thermal. The above NO calculations are performed wilh the maximum value of the fluctuating temperature (Tb ) calculated from Eq.(24). Similar calculations wilh T b value set to the adiabatic flame temperature result in N O emissions around eight times higher than the measured N O flue emissions. It was experienced that when the time-mean temperatures arc increased by 1 0 % throughout the natural gas flame considered, the predicted N O emissions increase from 93 to 770 ppm (for s=0.6). A similar 1 0 % alteration to the oxygen concentration field increases the predicted N O emissions from 93 to 126 ppm (fors=0.6). The above considerations on the sensitivity, call for a high accuracy on temperature and oxygen predictions. The higher the flame and process temperature, the more accurately the in-flame temperatures have to be calculated in order to secure good N O predictions. Thus, all the factors influencing the accuracy of the temperature predictions beginning wilh the rate of combustion, radiation, dissociation of combustion products and ending wilh the formulae for specific heat arc of importance. Nitric Oxide In Pulverized Coal Flames Capabilities of the N O x post-processor for predictions of thermal-, fuel- and total-NO arc shown herewith using both the lype-2 flame shown in Figure 7, and type-1 flame shown in Figure 9. For pulverized coal combustion, the prompt N O mechanism is omitted at this stage of N O modeling. Three computations are performed for each of the two flames considered. The first run is to calculate exclusively the fucl-NO while the second (s=0.5) and third (s=0.999) runs to obtain total-NO. While calculating the fuel-NO only, the thermal-NO source is switched off. In the two total-NO runs, both fuel- and thcrmal-NO mechanisms are activated and some of the N O formed via Zeldovich mechanism is reduced by reactions with C H radicals (see Figure 5). The results summarized in Table 1 show that the total-NO emissions for the low-NOx flame are overpredicted by either 3 5 % or 8 0 % depending on the s value, despite omitting prompt-NO. For the high-NOx type-2 flame the discrepancies are smaller being in the range 1 0 % to 2 5 % . For both flames the major part of the predicted flue emissions is the fuel-NO contributing up to 9 0 % in the typc-2, high N O x flame and up to 7 6 % in the typc-1, low-NOx flame (s=0.5). TABLE 1: MEASURED AND PREDICTED FLUE CONCENTRATIONS OF NO IN 3.4 M W FLAMES OF PULVERIZED COAL (concentrations in ppm, dry, at 0% 02). N O emissions Measured Fuel-NO predictions Total N O predictions s=0.5 s=0.999 Typc-1 flame 470 482 638 846 Type-2 flame 1074 1053 1189 1352 13 A clear difference between emissions of the high and low N O x flames has been correctly identified by the model. Figures 16 and 17 show the computed total-NO contours and the boundary of the fuel rich region for both flames. A comparison between the in- 11-11 |