Study On Conditions And Characteristics Of MILD Combustion Of Gas Turbine

Moderate or Intense Low-oxygen Dilution(MILD)combustion is characterized by its stable and efficient combustion,uniform combustion temperature and low emissions,and is demonstrating the potential of application in the new low-polluting combustion chamber of gas turbines.However,due to the characteristics of the high pressure,lean-fuel and rich-oxygen atmosphere and complex flow inside the combustion chamber of the gas turbine,it is necessary to further study the conditions for the formation of stable and efficient MILD combustion in the combustion chamber of the gas turbine.In this paper,the kinetic characteristics and thermodynamic conditions of the chemical reaction of MILD combustion with CH4 as fuel were studied by means of the combination of numerical simulation and experiment.Based on the concepts of the fuel-staged and MILD combustion,the NOx generation routes in MILD combustion was studied and analyzed,and combining with the concept of fuel-stage and MILD combustion,the emission characteristics of fuel-stage combustion and the blending characteristics of secondary fuel and high temperature flue gas were studied by using axial fuel-staging model burner.The process of the flue gas generation,blending and combustion in MILD combustion was decoupled,and the MILD combustion staged and reflow cycle chemical reaction network model were established.Based on the MILD combustion staged chemical reactor network model,the critical flue gas reflux ratio was determined according to the definition of MILD combustion,and the influence of fuel type,temperature,pressure and residence time on the spontaneous combustion temperature,ignition delay time and chemical reaction rate of the mixture was studied by changing working conditions in the range.The results show that the MILD combustion is diluted with flue gas and fresh reactants to form "high temperature and low oxygen" environment atmosphere,reduce the concentration of reactants,reduce the actual equivalent ratio and reduce the oxidation rate of fuel.The increase in flue gas reflux ratio reduces the spontaneous combustion temperature of the MILD combustion reaction mixture,thereby reducing its ignition delay time.For the analysis of NOx in MILD combustion process,it was found that the NOx generated by the Thermal-NO mechanism became the main source of NOx emissions when the reaction temperature exceeded 1800K,and its formation was positively correlated with the residence time in the high temperature region.The NOx content in the flue gas has a significant effect on the final outlet NOx emissions.In the initial stage of the reaction between the flue gas and the fuel and spontaneous combustion,the NO in the main combustion flue gas is converted into nitrogen-containing compounds such as HNO,HCN and HCNO by NO reburning mechanism,and can be reduced to a certain extent NOx emissions.The fuel-stage technology was applied to MILD combustion,and the secondary fuel was added to the rear of the main combustion zone to form a local MILD combustion reaction zone.The fuel-stage reduces the residence time of the flue gas in the high temperature zone and inhibits the formation of thermodynamic NOx.The experimental results show that the increasing of the fuel-staged ratio in the range of 0～20%of the secondary fuel ratio,the NOx emissions in the outlet flue gas can be reduced.By appropriately arranging the position and form of the secondary fuel,the formation of MILD combustion in the reburning zone helps to adjust the flame morphology and temperature distribution of the combustion chamber and reduce NOx emissions.The fuel jet aperture affects the fuel/flue gas flow ratio,which affects the fuel jet depth.Fuel jet velocity,depth and other factors affect the secondary fuel and high temperature flue gas blending and reaction characteristics.Through the study of the above,the reaction characteristics and the realization conditions of MILD combustion in the combustion chamber of the gas turbine were analyzed,and the mechanism of the formation of MILD combustion pollutants was analyzed in detail.The applicability of fuel-staged technology on MILD combustion was studied.The research results provide a reference for the design and experiment of the high efficiency and low emission gas turbine MILD combustor.