Axial-fuel-staged combustion is a promising technology to reduce NOx emission at high turbine inlet temperatures and provide extended gas turbine operational flexibility. To investigate the emissions characteristics of the axial-fuel-staged combustion, a staged model combustor was constructed and a parametric study was performed at atmospheric pressure. Fuel distribution, equivalence ratio, and jet velocity effects on the emissions characteristics have been studied in the present research. Results show that the influence of fuel distribution on emissions is depending on the combustor outlet temperature. The NOx emissions increase with secondary fuel fraction when the combustor outlet temperature is low but decrease when the combustor outlet temperature is high. Investigation of the equivalence ratio on each stage shows that a lower relative NOx increase in secondary combustion zone is achieved at higher first-stage equivalence ratio. Moreover, the secondary stage jet velocity was varied to study the jet mixing influence on the emissions. The results show that a higher jet velocity will enhance the mixing between the secondary jet and the upstream first-stage burnt gases, producing lower NOx emissions. Finally, a simplified axial-fuel-staged combustion chemical reactors network (CRN) model was established to study the mixing of the secondary fresh fuel/air mixture and the first-stage burnt gases. The CRN modeling results show that a poor mixing in the secondary stage will significantly increase the NOx emission, which means that the mixing enhancement at the secondary stage is important for the axial-fuel-staged combustor design.
Axial-fuel-staged combustion is a promising technology that can be applied to lower emission, higher combustor outlet temperature, and extended operational flexibility of gas turbines. In this paper, two simplified chemical reactors network (CRN) models based on the staged combustion were established to investigate the emission characteristics of an axial-fuel-staged MILD (moderate or intense low-oxygen dilution) combustor. Firstly, an atmospheric experiment of a staged MILD combustor was performed to validate the CRN model. Results show that the experimental NOx emission trend is well captured by this CRN model and a NOx reduction of approximately 30% was also achieved by the staged combustion. Subsequently, based on the validated CRN model, a parametric study was conducted and the effects of fuel distribution, residence time, mixing characteristics in the secondary stage and heat losses were systematically analysed. The results show that the NOx emission can be reduced by more than 40% with an unaffected combustion efficiency by both increasing the ratio of secondary fuel and decreasing the ratio of secondary residence time. On the other hand, poor mixing in the secondary stage will significantly increase the thermal NO formation by 140% due to the local high temperature region. Finally, a non-adiabatic CRN model was studied and it was found that the heat loss would exaggerate the NOx-abatement potential of the axial-fuel-staged MILD combustor.
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