OCR Text |
Show 7 decreases straightly with increasing 11 T. In the gas temperatures ranging from 900"C to 1600°C, KYM is greater for higher volatile coals. The influence of the temperature on KYM (the slope of curve) for the high volatile coals is relatively small compared with that of the low volatile coals. In other words, the activation energy is greater for lower volatile coals. When KYM values for a given coal are known, we are able to estimate the size of the oxidation zone near the burner (shown in Figure 1). (2) Reaction rate of NOx formation The reaction rate of NOx formation (KN) is calculated from Equation (2) by using data of DTF. Figure 6 shows the variations of NOx, N compound (HCN+NH3) and O2 concentrations with the residence time for coal B. The values of d[NOx ]/dt, [N] and [02] are estimated from the formulas shown in Figure 6. Figure 7 shows Arrhenius plots of the reaction rate of NOx formation (KN) for all coals tested. From this result, KN is found to be expressed by a single equation regardless of the coal type as follows ; KN = (d[NOx]/dt)/([N][02D = 2.51 X 10-2 exp [-13.6/(RT)] frequency factor : AN = 2.51 X 10-2 (l/s·ppm) activation energy : EN = 13.6 (kcal/mol) (5) Obviously, this reaction occurs in gas phase. Therefore, only the nitrogen compounds representedly HeN and NH3 and oxygen concentrations in the gas phase need to be evaluated. This leads to a single equation for all coals, Equation (5). When this KN is clarified, it is possible to estimate the NOx concentration formed in the oxidation zone (Figure 1). (3) Reaction rate of NOx reduction As described in the preceding section, measurements for the reaction rate of NOx reduction (KR) were made under the reduction atmosphere in the furnace. Pulverized coal was supplied to the furnace with N2 ga~ added NO of a known quantity. Then, we measured the concentrations of NOx and [VM] evolved in terms of the residence time. Figure 8 shows these results for coal C. Using the values of -d[NOx ]/dt, [NOx] (=([NOx]1+[NOx]:z)/2) and [VM] (=([VMh+[VM]:z)/2) estimated from Figure 8, KR was calculated from Equation (3). In this calculation, [VM] in unit of volume fraction is estimated by assuming that gasified volatile matter is composed of only methane gas |