OCR Text |
Show DISCUSSION Existence of a Second Limit of Combustion/Oxidation In the discussion above qualitative arguements were advanced for the existence of second threshold of combustion/oxidation. While these arguments are supported by the author's previous studies of wet CO oxidation the data in Figures 4, 5, and 6 are the first demonstration of this effect for organic compounds, i.e. these figures all show that below some critical value The empirical rate expressions reported in the literature are effectively representations of the oxidation rate above the second limit and one would expect them to greatly overpredict the extent to which organic compounds oxidize when their initial concentration is low. Thus, for example, reference 1 reports empirical rate expressions for the oxidation of CHaCI, C6H6 and C6H~Cl. The rate expression given for CHaCl predicts that for the experimental conditions of Figure 4 only 5.8x10-7% of the CHaCI going into the reactor will escape oxidation and that this percentage will be independent of the initial concentration of the CHaCl. This is in contrast to the observed result that the amount CHaCl remaining unoxidized varies from 0.298% at [CHaCI]o = 13.5ppm to 0.023% at 694ppm. Similarly the reference empirical rate expression for C6Ho oxidation predicts that for a reaction time of 2.73 seconds and the conditions of Figure 5 only 5.2x10-6% of the input C6H6 will escape oxidation while the observed values range from 28.6% at [C 6 H6 ]o = 46ppm to 50.4% at [C 6H6 Jo = 2.2ppm. For C6H~Cl the prediction is 1.5x10- 7 % for the conditions of Figure 6 while the observed values range from 68.8% at [C6H~Cl]o = O.15ppm to 19.7% at [C6H~ClJo = 26.7ppm. Thus the observed extend of oxidation of the test compounds is orders of magnitude less that that predicted by reference 1 and it appears certain that this difference in the specific rate is due primarily to much lower concentrations (both initial and final) used in the present experiments. Potential for a More Effective Incineration Process The results in Figures 4, 5, and 6 demonstrate that a second threshold for lean fuel combustion exists and may significantly limit incineration effectiveness. Previous efforts to improve incineration technology have tacitly assumed there was nothing basically wrong with the chemistry of incineration and that all that was needed was better mechanical engineering. This meant increasing the three T's: time, temperature, and turbulence. While incinerator effectiveness can indeed be improved by increasing the temperature and intensity of combustion, such an approach involves increased NOx production. Increasing the operating temperature will also cause increased slagging, fouling, and corrosion. The use of exotic materials of construction may become necessary, and, of course, tG achieve the higher temperature the consumption of auxiliary fuel will increase. 7 |