| OCR Text |
Show c) The CO/CO ratio in the combustion gases of not pyrolysable (graphite) or hardly pyrolysing carbonaceous solids (soot, highly devolatilized chars) reflects both factors cited in a) and b), but also depends on the nature of the solid and the particle size ( as an example : at 1500 K in air , the CO/CO ratio is about 0.35 for 80/100 micron char particles of a typical low volatile bituminous coal, it reaches 0.51 for gas phase generated soot from ethylene, but is only 0.055 for a typical cenospherical soot from heavy fuel oil). d) In the case of pyrolysable carbonaceous materials, the CO and CO released by pyrolysis as well as those obtained from slow or fast oxidation of pyrolysis products will substantially contribute to the CO/CO ratio measured in their combustion products. Notwithstanding this complexity, it is clearly suggested from the results shown on figures 6 and 7 , that the sudden drop of the CO /CO ratio during the first period of coal combustion is due to CO formation during the oxidation of the pyrolysis products , especially when this oxidation occurs as a gas phase flame (figure 7), the CO formation being then very high if enough oxygen is present. Therefore the total CO evolved during coal combustion over the total CO evolved t t (/" CO dt/./* CO dt) suddenly decreases at these temperature where the pyrolysis products start to burn as a gas phase flame , as shown on figure 8 (which should be compared with figure 9). Further interesting informations on the evolution of V as a function of time may be obtained from the comparison of the combustion behaviour of a coal and the corresponding char. Comparing the results shown on figure 7 (the case of coal) with the ones given on figure 10 (related to the char obtained from this same coal by fast devolatilization), it may be observed that the maximum combustion rate of coal, obtained when the devolatilization and the combustion of the pyrolysis products come to an end, is identical to the maximum value V measured on the c, max 7-16 |
