Preliminary Investigation Of MILD Combustion Applied For Gas Turbine

Gas turbine, the important power equipment of national defense and economy, has been widely used for its advantages of high efficiency, flexibility of load change and etc. As the core component of chemical energy transformation, the gas turbine combustor has been following the design trend of higher combustion efficiency, fuel flexibility and lower emissions. And the reduction of NOx emission seems to be an important issue as the increase of gas temperature as well as the improvement of emission standard. And the MILD (Moderate or Intense Low-oxygen Dilution) combustion technology may be a sound option to gas turbine combustor for its low peak temperature, high combustion stability, low emissions, fuel flexibility and uniform temperature distribution.Improving the jet momentum of reactants, which will induce the recirculation of enough flue gas, is a common way to establish the MILD combustion applying to gas turbine. And this will help to achieve volumetric reaction while lower reaction rate. The flame front will be blown away by high jet momentum and the NOx emission can be reduced for its lower temperature gradient. The high temperature recirculated gas, which will preheat the reactants, helps to enhance the stability of combustion. Besides, the recirculation of flue gas and the volumetric combustion can also prolong the reaction time which will favor higher combustion efficiency.This study bases on the rated load operating condition of one F class gas turbine, that is operating pressure p=16atm, heat intensity Q=20.5MW/m3-atm, air temperature Ta=723K and the fuel temperature Tf=288K. The fuel is CH4. And the physical model is based on the flame tube. The investigation as well as optimization is based on the method of achieving MILD combustion and carried out with the help the CFD three-dimensional numerical simulation, aiming to design and optimize a MILD combustor that matches up the original combustor casing structure.A model combustor is built firstly and the equivalent ratio φ=0.7. The influence of different configurations to combustion characteristics, like forward or reverse flow of air, the exit diameter of flue gas, the premixed ratio of fuel to air, the injection angle and etc., are studied. Also, the effect of equivalent ratio is also studied. And it is found that the air jet of forward flow distributed that parallel to the combustor wall will utilize the chamber space more fully and acquire higher gas recirculation ratio. The change of exit diameter has little influence on the flow field inside the combustor. And the maximum temperature Tmax is determined by lots of elements like the mixing characteristic of reactants before combustion rather than only equivalent ratio (p. Besides, Tmax and reaction intensity under premixed mode will be lower and the reaction zone will be extended.Then this study designs and optimizes the MILD combustor based on the conclusions obtained from the model combustor. The equivalent ratio φ is 0.625 and the focus here is the effect of jet momentum of single nozzle to MILD combustion characteristic. It is found that high combustion efficiency and the low emissions will be achieved when the nozzle number N is 8 or 12 and jet velocity V varies from 80m/s to 110m/s under premixed mode. The press loss is also quite low.Experiment that under the condition of pressure is 1 atm and air temperature is 293K is carried out to verify the effectiveness of the optimization. The combustor is narrowed to half size of the original configuration while contains nearly the same heat intensity and equivalence ratio. The result shows that CO and NOx will be reduced significantly by improving jet velocity. And MILD combustion can achieve satisfying combustion characteristics at different heat intensity. MILD combustion will achieve more uniform reaction zone and lower pollutant emissions compared with diffusion combustion. Besides, CO is lower than 12ppm@15%O2 and NOx is lower than 2ppm@15%O2 when φ is 0.6-0.65.The possibility of achieving MILD combustion is demonstrated preliminarily.The process and method of numerical simulation, optimization and experiment in this study can also offer a practicable way for the design of MILD combustion.