The influence of halogen compounds on gaseous oxidation processes

This thesis describes an investigation of the effects of four bromomethanes and hydrogen bromide on the gaseous oxidation of several fuels. In the Introduction the relevant basic theory of chain reactions is first outlined and then the general oxidation mechanisms are considered with particular reference to the compounds studied in this work. Finally the modes of action of various additives in gaseous oxidation reactions are described. In the Experimental Section a full account is given of the vacuum system used and the methods employed for the preparation and purification of the reactants are outlined. The procedures employed for obtaining pressure-time curves and the methods of chemical and gas-liquid chromatographic analysis used are also fully described. The Results Section consists largely of rate measurements for the various fuel-oxygen-additive systems studied. Graphs derived from this basic data are also included showing the effects of the various additives relative to one another. Hydrogen bromide exerts a marked, usually promoting, influence on the oxidation of all the fuels studied while the bromomethanes have a well-defined effect only on oxidation reactions occurring at above 400°C. When the halomethanes do exert an appreciable influence on combustion reactions their effect is qualitatively similar to that of hydrogen bromide. This latter additive retards only the oxidations of formaldehyde and methane while the bromomethanes retard only the oxidation of methane. These retarding effects are confined to the later stages of reaction and to small amounts of additive. In the Discussion an attempt is made to explain these results in terms of the reaction mechanisms involved. Promotion by hydrogen bromide is ascribed either to its direct reaction with oxygen providing an extra mode of chain-initiation, or to its interaction with intermediate peroxides giving rise to enhanced chain-branching. Hydrogen bromide exerts its retarding influence on the oxidation of formaldehyde largely by reacting readily with hydroxyl radicals, thereby significantly reducing the concentration of these important chain-carriers. In the methane oxidation system hydrogen bromide also effectively removes methyl radicals,thereby interrupting the normal chain cycle. The bromomethanes themselves do not appreciably affect the combustion of the fuels studied and it is only their oxidation or pyrolysis products, bromine and hydrogen bromide, which have a pronounced influence as additives.