OCR Text |
Show For a premixd mixture with sufficient oxygen the sequence of reactions involve the rapid breakdown of the fuel to carbon monoxide followed by its final conversion to carbon dioxide through the reaction of O H + C O = H + C 0 2 . In the course of these processes, heat is released leading to drastic temperature rises that further accelerate the reaction and the overall process is driven to completion. Under such conditions the detailed chemistry of hydrocarbon breakdown is not of great importance and flame properties that can be related to energy release such as flame velocity can be simulated with considerable accuracy. A more chemically interesting and indeed challenging situation occurs in fuel nch situations. Under such circumstances complete conversion to carbon dioxide cannot occur. In addition to carbon monoxide, a whole host of unsaturated compounds can be formed. These are the products of incomplete combustion that have become an increasingly important aspect of the validation of incineration operation. The specific unsaturated hydrocarbons are formed sequentially[8]. Initially, the general direction is towards smaller compounds such as ethylene and acetylene. As the concentration of unsaturated compounds and radicals increase, larger entities are formed. The principal reactive intermediate is the hydrogen atom and is in contrast with the situation for fuel lean situations where O H and O atoms are also of p n m e importance. The increasing level of unsaturation also means that hydrogen molecules will be formed. The relative amounts of potentially combustible fuels dunng the vanous stages of combustion is a key issue in the present analysis since they also represent compounds that can be chlonnated. The simulation of combustion systems has been an important research thrust. The consequence is that there are now programs where fundamental chemical information in terms of the thermodynamic and chemical kinetics data pertinent to all the species that are formed and destroyed can be used as input and a variety of global phenomena can be reproduced. This can be used to extend and indeed supplement direct field measurements and clearly represents a major research thrust. Currently, there exist fairly satisfactory models for the combustion of small hydrocarbons such as methane, ethane and propane. The situation for pyrolysis is more complex since as noted earlier at some stage larger molecules are formed. Although there has been considerable recent progress [8], detailed understanding of the chemical transformations of the larger species are much less complete than that for the smaller species. In the present exercise w e will concentrate on the behavior of 1 and 2 carbon compounds. The simulations of such system are more soundly based and some inferences can be drawn with respect to the chlorination behavior of larger compounds. It should be noted that at the late stages of the decomposition process, a great deal of the memory of the initial chemistry is lost and probably the only difference between the systems are variations in the hydrogen to carbon ratios and the physical conditions. CHLORINATION CHEMISTRY We now consider the situation when chlorine compounds are introduced into combustion system. There is considerable data on this issue due to the importance of |