The formation of nitrogen oxides in gas combustion

This thesis grew out of a practical investigational project, the aim of which was to reduce nitrogen oxides and especially nitrogen dioxide emissions from gas appliances. In the project a number of practical ways of reducing such emissions were identified. This thesis includes much of the work of the project with the addition of a more fundamental component, in which the underlying phenomena have been investigated. Findings of major importance in this study were : (1) The formation of nitrogen oxides in radiant burners is mainly by the prompt mechanism. (2) As a consequence of (1), the level of nitrogen oxide emission from radiant burners is highly dependent on the level of nitrogen oxide emission from radiant burners is highly dependent on the level of primary aeration. (3) Vitiation of combustion air causes the emission of nitrogen oxides from radiant burners to increase. (4) Low concentrations of hydrogen, carbon monoxide and especially unburnt hydrocarbons in combustion and similar gases can cause the oxidation of nitric oxide to nitrogen dioxide (which is undesirable for an unflued gas appliance as nitrogen dioxide is more toxic than nitric oxide). (5) Tests in linear flow reactors have shown that the oxidation of nitric oxide, when promoted by unburnt hydrocarbons, can occur over a small temperature increment. The reaction, which can be quite rapid, occurs more readily with straight chain alkanes than with isomers of the same carbon number. (7) Mathematical modelling using the CHEMKIN package has successfully predicted the behaviour of mixtures of carbon monoxide and nitric oxide in air under conditions similar to those prevailing in combustion gases. Semi-quantitative accuracy was also obtained for models with ethane and ethene, however, the models failed for higher hydrocarbons. This failure was attributed to the absence in the kinetic models of reactions identified in recent literature as being of importance in ignition phenomena. (6) While it was found that the presence of unburnt hydrocarbons promoted the oxidation of nitric oxide, it was also evident that the presence of the nitric oxide caused the hydrocarbon to oxidise. The addition of increasing levels of nitric oxide caused an initial increase in the hydrocarbon oxidation to a maximum level after which further additions decelerated and finally stopped this process