Emissions Characteristics Of Axial Fuel Staged Combustor For Gas Turbine Applications

With the development of advanced gas turbine,the combustor outlet temperature is approaching 1973 K,and the conventional lean premixed combustion cannot meet the lower NOx emission requirements even with the perfect premixing.Axial fuel staging combustion(AFS)divides the combustor axially into the primary stage and the secondary stage.Small fraction of fuel is injected into the secondary stage to form intense low-oxygen condition,and thereby low NOx emissions in a wider load range was achieved.AFS is already utilized in commercial gas turbines.However,the mechanism for the low NOx generation in AFS combustion are not fully understood yet,and how to combine AFS combustion technology with conventional lean premixed combustion needs to be resolved.Therefore,a 1973 K gas turbine combustor with AFS was studied experimentally and numerically,firstly the influence of the fuel splits and secondary equivalence ratios on emissions and the inhibition of NOx was investigated.and then the coupled influence of the mixing characteristics and secondary key parameters on the AFS combustion characteristics was investigated.At last a model burner was designed to experimentally investigate the combustion characteristics of natural gas premixed combustion with AFS system.The main work of this thesis includes:1)Based on the Chemkin software,this work established several chemical reactor network models for AFS combustors and conventional lean premixed combustors with ideal premixing,and conducted a parametric simulation of emission characteristics.Compared with the conventional lean premixed combustion,AFS combustion has a wider load range of low NOx,especially at higher temperatures.The NOx reburning little affected the NOx emissions in the AFS combustion system and the maximum NOx reburning efficiency is less than 4.5%.Restraining the thermal NOx generation is the main way to achieve low NOx performance in AFS combustors at the firing temperature of 1973 K.With the guarantee of a well-stabilized flame of the primary stage and satisfaction of the CO emissions limitation,reducing the primary stage temperature and the secondary stage residence time are dominant means for low NOx in AFS combustion.2)The work established the two chemical reactor network models for the poor mixing AFS combustor to study the effects of mixing characteristics on emissions.The performance of mixing characteristics includes the premixing characteristics between the secondary inj ector air/fuel and the mixing characteristic of jets in cross flow on NOx and CO emissions.As expected,the NOx and CO emissions increase with the unmixedness degree.The effects of mixing on NOx/CO emissions are more essential in an AFS combustion system,especially with a larger fuel split and a larger secondary inj ector equivalence ratio.Moreover,the NOx emissions decrease with the increase of the fuel splits or the secondary injector equivalence ratios all the way to an optimum value under the two effects of poor mixing and secondary stage parameters.The practical AFS combustors have a better NOx benefit when the secondary stage was under the fuel-lean condition,and the 'lean-lean'mode showed a wider tolerance margin for the design of secondary inj ectors than the 'lean-rich'mode.3)Designed an AFS model burner and conducted experimental research on the combustion characteristics of different fuel splits,secondary equivalent ratios and total equivalent ratios under normal pressure.AFS combustion has the profit of low NOx emissions,and has an optimal fuel split to achieve low NOx emissions,while CO emissions increase slightly.The effects of mixing characteristics between the secondary premixed mixture and primary stage produts increase with the increase of fuel splits.