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A numerical approach for the abatement of nox emmissions from gas turbine combustors

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A computer simulation model for use in the design of high pressure industrial combustion chambers has been developed and validated in this project. This numerical model is able to predict nitrogen oxide concentration profiles with an uncertainty in the concentration levels of less than 50 %. The combustion computer code produced has been integrated into commercial computational fluid dynamics packages, and will be the subject of experimental new designs for industrial gas turbines. In order to cope with the simultaneous mixing and combustion process in the high pressure working conditions of the gas turbine, the turbulent combustion model TECK (transient eddy concept with kinetics) has been developed. This model uses the assumption that turbulent microstructures, which have the properties of the mean flow, are created. The basic principle of the model is to look at the fine structures of turbulence as isolated, unsteady, perfectly stirred reactors where chemical reactions occur. The lifetime of these structures at the Kolmogorov scale in which the chemical reactions occur, is supposed to be small compared to the large scale eddies (integral scale). These turbulent microstructures are assumed to remain at the location where they have been created. From literature, it is well-known that the reaction rate of nitrogen oxide formation is relatively slow, therefore the mixing and combustion processes in the turbulent reactive flows have been evaluated first, to generate temperature and concentration fields. A nitrogen oxide formation post processor has then been used to calculate nitrogen oxide concentration in the combustion chamber. Finally, the complete combustion code has been integrated into a commercial computational fluid dynamics package, which allows for large scale validation tests. A description of a detailed kinetics mechanism to model nitrogen oxide formulation at various pressures has been achieved and a detailed mechanism reduction has been obtained.
With growing concern about global warming and other environmental effects, there is a proven need and huge market potential for new means of pollution control. High frequency excitations are known to influence emissions of pollutants from, and also the energy consumption of, fossil fuel burners. However, existing combustor active control studies relate only to noise control; there are few results currently available from which to develop the technology for pollution control. As a first step towards the design of fully controlled "intelligent" combustors, this study aimed to demonstrate the effects of high frequency acoustic excitations on emission characteristics. Emissions of NOx were expected to be reduced around 25%, and energy consumption to be reduced by around 12%.

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