Feasibility Study On The Application Of Flameless Combustion In Gas Turbine Combustors

In order to deal with the challenge of NOx emission control created by the increase of the turbine inlet temperature.The flameless combustion has been proposed for the gas turbine application due to the characteristics of low-pollution,and good combustion stability.This article will analyze the difficulties and criterions of flameless combustion in gas turbine combustors with the maintaining performance by the method of numerical simulation.The design of the flameless combustor is based on the experience of the industrial furnace.Different from the furnace,the combustion stability and high efficiency are prerequisites for a flameless combustor.With the comparison of the model combustors investigated by other researchers,it was found that the tangential inlet mode and the conventional swirl combustor can obtain better combustion stability.However,due to the lack of the flue gas recirculation,the characteristics of diffusion flame and the obvious flame front are observed in the latter combustor.In this article,the combustor is modified by adopting a staged combustion method to increase the flue gas recirculation produced by a pre-combustor.The low-oxygen combustion atmosphere is obtained in the primary combustion zone through the dilution of flue gas from the pre-combustor and the central recirculation zone.The simulation of the chemical reactor network model determines the ratio of air in the pre-combustor and the corresponding equivalence ratio.Due to the difference between the gas turbine combustor and the industrial furnace,a low oxygen atmosphere is not easy to be obtained in the flameless combustor.The oxygen concentration of 15% is selected as the design criterion for a flameless combustor with swirl combustion.Finally,the air ratio of 30% and equivalence ratio of 0.6 in the pre-combustor are determined separately based on the characteristics of the temperature rise,ignition delay time and the NOx emission.The structural design of the flameless combustor refers to the ?R-type combustor? of China‘s self-developed heavy-duty gas turbine,which has the features of radial staging and swirl stable combustion.The reason for poor emission performance in the R-type combustor is the premixing non-uniformity of the primary nozzle.In this article,a numerical simulation method is adopted to improve the premixing uniformity by adjusting the location of the swirler upstream the fuel pipes.It was found that the strong turbulence of the swirling flow is beneficial to the premixing of the fuel,and the the size and distribution of the fuel injection holes must be matched with the distribution of the incoming air flow.The advantage of flameless combustion is that diffusion combustion can be used to avoid the flashback and combustion dynamic instability of premixed combustion.In this article,the annular zone of the R-type combustor is applied as a pre-combustor and flue gas is generated to dilute the air upstream the primary combustion zone in a reverse flow configuration.The central recirculation zone is damaged with the reversed flue gas,producing the poor flameless combustion.In the cases using the flue gas jet mixing method,the optimized jet position is the swirling flow upstream the primary zone.The fuel supply should avoid contact with fresh air and high-temperature flue gas before dilution.When the fuel is uniformly premixed with the diluted swirling flow,a more uniform combustion temperature distribution and low NOx emission can be generated,and a better flameless combustion can be obtained.On the other hand,the pre-combustor has the potential to further reduce NOx emissions and reduce pressure losses by operating in flameless mode with the tangential inlet mode.