OCR Text |
Show expenmental observations. It is increasingly being used as a tool to complement and expand the range of expenments and it is in this spint that the present results are presented. In an earlier paper [3] we attempted to develop a gas phase mechanism for the formation of dioxins in terms of a plug flow pre-mixed reaction model. This proved to be unsuccessful and it was concluded that the chlonnated dioxins could only be formed through surface mediated processes. This has been confirmed by subsequent experimental work [5]. These results however do not eliminate the possibility that precursors with some degrees of chlonnation may be formed in gas phase processes. In the case of one and two carbon compounds in laboratory systems there is good evidence that this may in fact be the case[6] . In this paper we attempt to take a more realistic approach to the descnption of combustion in an incinerator. Instead of considering it as a plug flow reactor w e now treat the situation where imperfect mixing may play a role. Specifically, we consider the combustion of lean mixtures of a fuel containing small percentages of chlonnated hydrocarbons. This leads to the production of sufficient chlorine atoms or molecules in an appropriate packet of gas, which after quenching, may then react with unburned or partly burned hydrocarbon fuels that are present in the system. Such a scenano is not unreasonable. Obviously, incinerators do not operate with premixed fuel and waste components and there are violent temperature and concentration gradients. In an actual situation, the proper picture is that of distributions of such packets, The intention of this paper is basically to prove the concept and develop some feeling regarding the physical situations that should be investigated in a more detailed manner. The paper will begin a discussion of the chemistry of hydrocarbon combustion. W e will be interested in both lean and rich conditions. W e will then consider the combustion of chlorinated compounds. Particular attention will be focussed on the stability of chlorinated organic compounds under pyrolytic and oxidative conditions. Data regarding the chlorination of organic compounds will be summarized. The focus will be on the elementary single step reactions that are the fundamental chemical elements in the overall process. All of the above will be brought together in the context of the conditions that can be found during incineration. Some possible scenarios for the formation of polychlorinated organics will be presented. Results from simulations with the chemical kinetic data base will be discussed.. COMBUSTION CHEMISTRY There is currently broad understanding of the mechanism for the combustion of organic fuels. It involves the attack of small radicals, such as O H , H and O atoms on the fuel following initiating processes which releases radical into the system. The overall process is largely controlled by the chain branching reaction [7] H + 02 = O + OH. |