The existence of second threshold for combustion and its implications to incineration

Abstract

This paper reports a set of experiments examining the oxidation kinetics of CH 3 Cl, C 6 H 6 and C 6 H 5 Cl. These compounds were chosen because they are typical of the most difficult to incinerate materials encountered in hazardous waste incineration. The specific rate at which these compounds oxidize was measured as a function of the initial concentration of the compound and found to decrease with decreasing initial concentration. At low initial concentrations of CH 3 Cl, C 6 H 6 and C 6 H 5 Cl the specific rates of oxidation found were very low and may represent a previously unrecognized problem in the design and operation of hazardous waste incinerators. Adding CO, however, greatly increased the specific rate at which the organics oxidized and improvements in their extent of oxidation of up to a factor of 10,000 were achieved. The author believes that these experiments demonstrate the existence of a second threshold for lean fuel combustion/oxidation. Gas phase combustion/oxidation reactions are rapid for two reasons: because they are exothermic and because they occur by chain branching mechanisms. The former consideration means that combustion systems can be self-heating. This implies existence of the first threshold of combustion, the familiar flammability limit, i.e., to be flammable a mixture must contain enough fuel to be able to heat itself to flame temperature. Similarly the fact that combustion, oxidation reactions occur by chain branching mechanisms means that combustion/oxidation reactions can generate much higher concentrations of free radical chain carriers than equilibrium would provide. This, then, implies a second threshold, i.e., if a reaction is to occur with the high rate that a superequilibrium free radical concentration can provide, then the initial concentration of the fuel must be high enough to generate that superequilibrium concentration. This second threshold for combustion/oxidation may limit the effectiveness with which incinerators destroy toxic organic materials and it may be possible to improve that effectiveness by injecting some clean fuel to bring the total fuel concentration above this threshold and thus restore a rapid rate of oxidation.