Numerical and Experimental Parametric Study of Emission Characteristics in an Axial Fuel Staging System

Abstract

The axial fuel staging (AFS) combustion is applied in advanced gas turbines as a result of its advantages in emission reduction and operation flexibility at engine turndown conditions. In this study, several chemical reactor network models for AFS combustors were established to conduct simulation at the operating temperature of 1973 K. The investigated parameters include the performance of fuel splits between the primary stage and the secondary injectors, the equivalence ratios of the injected mixture, and the residence time of vitiated products in the secondary stage. The NO x reburning mechanisms, which are proven to have a slight effect on total NO x emissions, were also considered in this model. Simulation showed that the AFS combustor had considerable potential in reducing NO x emissions as a result of the primary stage temperature. Experiments were conducted to compare the simulation results and further verify the advantages of the AFS combustor. Given the mixing characteristics in a practical AFS combustor, the performance of mixing characteristics, including the premixing characteristics between the secondary injector air/fuel and the mixing characteristic of jets in cross-flow on NO x and CO emissions, was investigated. Results show that the mixing characteristics, especially the secondary injector mixing performance, exert a substantial influence on NO x and CO emissions. When the fuel split or the secondary equivalence ratio increases, the NO x emissions decrease first and then increase because the poor premixing of the secondary flow exerts an influence. After optimization, the practical AFS combustors demonstrate an improved NO x benefit when the secondary stage is at the fuel-lean condition, and the “lean–lean” mode shows a wider tolerance margin for the design of secondary injectors than the “lean–rich” mode.