OCR Text |
Show kinetics are well understood, processes which can be accurately modeled by computer. Table 1 shows a list of elementary reactions which are generally accepted as being the important processes which occur during CH 4 and CO oxidation. It is, however, to be noted that a number of reactions which one would expect to be important at high initial concentrations of CH4 and unimportant at low initial CH4 concentrations have been omitted from Table 1 COMPUTER MODELING RESULTS For purposes of a calculational example it is assumed that an incinerator operates in a manner such that the gas exiting the flame contains some amount of CH4 , 5% O2 , 6%He O, 6%C02 , and balance Nc . The residence time of this gas in the post flame oxidation region is 0.5 seconds and the temperature is 900°C. As shown in Figure 2 if the initial concentration of CH4 is 1000 ppm, there's an induction period after which the CH4 rapidly disappears. Before 0.5 seconds pass the CH4 concentration falls below 10-17 ppm and vanishes into the noise of the calculation. If, however, the initial CH4 concentration is 100 ppm, 2xlO-11 ppm survive, if it's 10 ppm, 5x10-7 ppm, if it's 1 ppm, 1.8x10-2 ppm and for an initial concentration of 0.1 ppm, the remaining CH4 is 2.5xlO-2 ppm, fully 25% of the initial concentration. Thus the modeling calculations clearly demonstrate the existence of a second threshold for methane oxidation. Figure 3 compares the oxidation of 0.1 ppm CH4 by itself and with CO added at the levels of 1000, 100 and 10 ppm. Whereas CH4 by itself is virtually inert, with 10 ppm CO present only 7.4xl0-7 ppm of CH4 survive. Increasing the CO to 100 ppm decreases the remaining CH4 to 3x10-10 in only 0.14 seconds. After this the CH4 disappears into the noise of the calculation. At CO = 1000 ppm this disappearance occurs in only 0.02 seconds. EXPERIMENTAL RESULTS Figure 4 shows the results of an experiment in which (CH3 CIJ t /(CH3 ClJ c • was measured for constant reaction conditions over a range of values of (CH3 Cll o ' Figure 5 is a first order rate plot for the oxidation of CoHo for three different initial values of (CoHo). The concentration of CO in the output gas was also measured in these runs and it was found that for (CoHoJ o = 45, 9.5 and 2.2 ppm the average values of {[COlt + 6(CoHoJt}/(CoHo)o were 1.0009 +/- 0.04, 0.985 +/- 0.025 and 0.976 +/- 0.029 respectively. Thus these data show a good mass balance. Figure 6 shows the results of an experiment in which (coH~CIJt/(coH~CIJo was measured for constant reaction conditions over a range of values of (coH~Cllo. Figure 7 shows a first order rate plot for the oxidation of 84 ppm CH3 CI alone and in a mixture with 1940 ppm CO. Figure 6 shows the results of experiments measuring the effect of varying the initial CO concentration on the extent to which CoHo and CoH~CI oxidize. 6 |