Comparison and Parametric Study of Flameless Oxidation in a Gas Turbine Using Two Kinetics Mechanisms

Mohamed Sassi,Mohamed Hamdi,H Benticha

doi:10.3844/ajessp.2008.535.543

Abstract

The so-called Oxidation is a novel combustion mode, in which combustion products are re-circulated and mixed into the fresh incoming fuel and air streams. This reduces the concentration of the reactants and thereby reducing the reaction rate through avoiding the formation of sharp high temperature zones in the combustion chamber. Flameless combustion has been acknowledged as one of the most interesting combustion technologies to meet both the targets of high energy efficiency and low pollutant emissions. This technology has already been successfully applied and exploited in industrial burners. The present investigation is concerned with the application of the flameless combustion mode to an adiabatic combustor, typically used in gas turbine engines. Detailed chemical kinetics calculations, by means of a specific zero-dimensional loop reactor model, have been performed to analyze its chemical aspects. The model simulates the combustor by a number of reactors that represent different zones in the combustion chamber. The main objective of this study is to increase the understanding of NOx formation from flameless mode, where currently very few experimental data are available. The investigation is focused on a comparison of the influence of pressure, residence time and temperature on the NOx and CO emissions, using two reaction mechanisms of the C/H/O/N system: the Miller-Bowman mechanism and the GRI_MECH3.0 mechanism. Simulation results clearly illustrate that even at high operating temperatures and pressures, NOx emissions could be reduced by flameless combustion to very low levels. A comparison between the predictions obtained by using the two chemical kinetics mechanisms is presented and discussed. It is shown that the predictions of the Miller-Bowman mechanism deviate from the predictions of the GRI3.0 mechanism in many aspects, especially as related to NOx emission results.

Highlights

Environmental concerns and limited resources of fuels have been the major constraints in designing combustion systems
First the computed results are compared with the results of Tomaz et al.[19] for validation purposes and the two reaction kinetics mechanisms predictions are presented and discussed
We can seen a satisfactory agreement on the major species concentrations in the outlet flue gas from the second combustor, except that the concentrations of the NOx (NO, NO2 and N2O) furnished by the CFD calculations are about 100 ppm higher than the ones predicted by our simulations

Summary

Introduction

Environmental concerns and limited resources of fuels have been the major constraints in designing combustion systems. These constraints have triggered researchers and manufacturers of combustion systems to develop low polluting and fuel-efficient combustion systems. Different process alternatives have been proposed to reduce the emissions from combustion devices These include, for example, exhaust gas recirculation, air staging, re-burning and low NOx burners. These methods try to reduce the residence time in high temperature regions, or to avoid high oxygen concentration in such regions. The formation of thermal NO is largely suppressed, since this mechanism is highly dependent on the temperature and requires temperatures above 1800 K to produce a significant amount of NO

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Discussion

Conclusion