Study On Combustion And Emission Performances Of The Micro Gas Turbine With Operational Flexibility

With the development of distributed energy,Micro Gas Turbine(MGT)has been promoted into a global range of researches and applications.It is widely accepted that MGT has the advantages of low pollutant emissions,excellent fuel flexibility,and the capacity of quick turn-on and turn-off.For the places with high thermal and power demands,such as hospitals,schools,paper mills,and textile mills,MGT can also show some advantages on the economy.Different from the stationary power plants with heavy-duty gas turbines,the distributed energy systems with MGTs have to provide intermittent energy,such as cooling,heating and electricity,according to various requests from the demand side.Therefore,MGTs always run under the off-design loads with long durations.In the practical applications,despite the energy storage technologies are adopted to optimize the MGT system operation,a“green”combustor is one of the most economical and reliable solutions to ensure MGT effectively run at the various load conditions.In this paper,the combustion and emission characteristics of micro gas turbine with operational flexibility were discussed from four aspects:numerical simulation approaches,key structure design of combustion chamber,pollutant control of combustion chamber under design state and pollutant control over a wide range of operating conditions.Firstly,the numerical simulation approaches for emission prediction and computational domain construction in full-scale combustion chamber were discussed.In this chapter,the swirling flame experiment,the SMA2 case,of the University of Sydney was took as the research object,and the different turbulence models and methane-air kinetic mechanism are compared.The RKE model and the 40-step skeletal mechanism were proposed to have higher accuracy in predicting the NO_x concentration.Then,the effects of adiabatic model on wall cooling efficiency,flow coefficient and flow field structure in a combustion chamber with advanced effusion cooling hole were studied by numerical simulation on a multi-perforated plate,and the feasibility of ignoring solid wall heat transfer in full-scale combustion chamber simulation was discussed.Secondly,the overall design process and the key structural design points were summarized.Through the structural optimization and numerical simulation of the dilution hole,the liner of the main combustion chamber and the burner of a conventional combustion chamber,the effects of different inlet components on the combustion process were discussed.The results show that the application of weak swirl and large reference cross-sectional area can significantly improve the air/fuel mixing effect in the combustion chamber,increase the residence time of flue gas and improve the combustion efficiency.Thirdly,the pollutant emission performance of DAS 1.0 combustor and its improved DAS 2.1 combustor under design state was studied.The generation of CO and NO_x in the combustion process of micro gas turbine combustor was discussed by core machine operation experiment and numerical simulation.The results show that the pilot fuel ratio has little effect on the internal flow field structure,total pressure loss and air flow distribution.Moreover,it is found that the pilot flame shows a great impact on the NO_x generation,and the pilot fuel ratio of 9.0%can contribute 17.4%NO_x.With the decrease of pilot fuel ratio,the pilot flame is gradually separated from the primary flame,expanding the flame area and reducing the emission concentration of CO and NO_x.When the pilot fuel ratio is 9.0%,the CO and NO_x emission concentrations in DAS 2.1 combustion chamber are 10.1 ppmv and 22.5 ppmv respectively.Finally,this paper took the DAS 2.1 combustor as the research object.The key points and difficulties of pollutant control in micro gas turbine combustor under wide load operation were discussed.The load reduction experiment and research results of DAS 2.1 combustor showed that too high CO emission concentration in flue gas was the key to limit the emission performance of micro gas turbine in a wide range of operating load.The reduction of load reduced the temperature,pressure and natural gas flow of the combustion chamber inlet at the same time,while the residence time in the reaction zone was basically unchanged,which was not conducive to the oxidation of CO.In this paper,it is proposed to control the combustion air flow(External combustion-air adjustment method,ECAM)at off-designed load and maintain the equivalence ratio in the reaction zone to improve the combustion efficiency of the combustion chamber during load reduction.The simulation results show that ECAM is effective in controlling the emission concentration of CO under off-design load.Under off-design load,the combustion air is actively regulated to realize the quasi-baseload state of the reaction zone of the combustion chamber,which can effectively reduce the surge of emission concentration caused by the decrease of CO load.The residence time of fuel gas in the reaction zone is positively correlated with NO_x emission concentration.Under off-design load,the reaction zone residence time can significantly increase the NO_x emission concentration caused by ECAM.Further reduce the pilot fuel ratio,the pilot burned gas reaction zone residence time decreased by 6.72 ms(75%load,pilot fuel ratio of 8.5%)and 3.68 ms(50%load,pilot fuel ratio of 8.7%)respectively.At this time,the NO_x emission concentration is the lowest,which is 20.0 ppmv and 21.3 ppmv respectively.