OCR Text |
Show over the proper definition of incinerability the most frequent view is that the incinerability of a given compound is its ranking in a list according to ease of nonflame thermal oxidation. The Measurement of Incinerability Numerous studies of the kinetics of nonflame thermal oxidation pure organic compounds have been carried out (1,2,3,4,5). In reference 1 (and in other studies) the observed oxidation kinetics are used to estimate T99.99, the temperature at which a one second residence time is sufficient to produce 99.99% oxidation of the starting material. Table 1, quoted from reference 1, shows an example of this kind of data. For the compounds studied the values of T99.99 the least incinerable material found was methyl chloride T99.99 = 15970 F (869o C) followed by methane, T99.99 = 15450 F (B41°C), chlorobenzene, T99.99 = 140BoF (764°C), and benzene, T99.99 = 1351 0 F (733o C), and 17 other compounds with even lower values for T99.99. If one takes these data at face value, the normal design for hazardous waste incinerators (a nonflame thermal oxidation zone of one second at 1000°C) would appear to be very conservative, i.e. with incinerators being designed to operate at a much higher temperature than is necessary for 99.99% oxidation, it hardly seems likely that any should fail to meet regulations. The observed fact, however, is that apparently well designed incinerators do sometimes fail to meet specifications and regulations. One possible explanation of this is that the data are not to be taken at face value, that laboratory measurements of incinerability substantially overestimate the ease with which organic materials in trace quantities can be oxidized. The Second Threshold of Combustion It's to be remembered that in many hazardous waste situations one is concerned with the destruction of a material that is initially at a concentration in the ppm range and one wishes to achieve removals in excess of 99.99%. In reference 1, however, the initial concentrations of the organic compound going into the reactor was 1000 ppm in all cases while no attempt was made to observe extents of removal greater than 99%. Similarly in the other studies cited the initial concentrations were orders of magnitude larger than might be found in practice and the extents of reaction studied were much less than one wishes to achieve in practice. Thus reference 1 and the other laboratory studies of incineration involve initial and final concentrations which are orders of magnitude greater than those encountered in practice. This long extrapolation may be invalid because of the existence of a second threshold for lean fuel combustion. While this effect has not been generally recognized, it may be deduced from simple consideration. Gas phase combustion/oxidation processes are rapid for two reasons. Firstly, because oxidation/combustion reactions are exothermic, a reacting mixture can heat itself to the point of forming a flame, but only if it contains enough fuel to cause a large temperature change. Thus the selfheating nature of 3 |