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Show Two Stage Nature of the Incineration Process While there are a great many different kind of incinerators differing in the details of their design, incinerators in general are two stage devices, the first stage being the flame zone in which the bulk of the waste being inci nerated is destroyed. Wastes, whatever their source, all tend to be quite variable fuels which can cause flame instability and other nonidealities. Because of these nonidealities some of the fuel can escape the flame zone. The second stage is the section of the combustion chamber which is downstream of the flame. In this second stage any organic compounds which escaped the flame have an extended residence time at elevated temperature and may be destroyed by nonflame thermal oxidation. In most designs the residence time is at least one second and the average temperature is at least 1000°C. Since, however, 1000°C is only the average, some of the gases going through the thermal oxidation zone will experience temperatures considerably less than 1000~C. Use of the Concept of Incinerability in Permitting Permitting procedures for hazardous waste incinerators are defined by the Resource Conservation and Recovery Act (RCRA). A permit to operate is issued after a trial burn has been executed or other appropriate test data obtained which demonstrate that the incinerator satisfactorily converts hazardous waste into non-hazardous compounds when operated under specified conditions. Satisfactory conversion is defined in terms of destruction and removal efficiency (ORE). However, since most hazardous waste streams contain many compounds, a trial burn which involves the measurement of all of them would be prohibitively expensive. Intuitively it seems obvious that there is no need to measure every compound, i.e. if one measures these compounds which are most difficult to destroy by incineration and finds that they are efficiently removed, it is to be expected that the others are also destroyed. Consequently, the trial burn involves the measurement of a subset of compounds (the principal organic hazardous constituents-POHCs) which are present in the input stream. If the ORE of these POHCs is 99.99 percent or greater, and certain other conditions are met then a permit to operate is granted. The Concept of Incinerability This procedure assumes the existence of a list ranking individual compounds in terms of their relative ease of destruction under incinerator conditions. It can reasonably be argued that the flame zone is not compound selective. Organic materials escape the flame zone because of combustion instabilities and other factors which may be related to the overall properties of the fuel but which do not have any necessary relationship to the individual compounds in the fuel. Thus it seems likely that any difference in the incinerator's ability to destroy compounds in a mixture is a result of selectivity within the second stage of the incineration process. This would mean that in choosing the POHCs to be measured in a test burn the permit writer should specify those compounds in the waste stream which are the most resistant to nonflame thermal oxidation. Consequently, while there is still some discussion 2 |