Study On Combustion And Emission Of CH₄/H₂ In Conventional And MILD Combustion

Under the severe background of fossil energy shortage and irreversible global temperature rise,it is necessary to carry out the combustion and emission characteristics of alternative fuels for gas turbine equipment in the energy and power field.Methane is a conventional fuel of gas turbines.As a primary energy source,the Earth’s stock is limited and directly produces carbon dioxide emissions,causing rising temperatures.Therefore,the addition of alternative fuels such as hydrogen to conventional methane fuels can reduce the dependence of the power industry on primary hydrocarbon fuels and help achieve the goal of carbon neutrality.Adding hydrogen to methane under gas turbine conditions is a current research hotspot.Scholars have conducted some research on the combustion and emission characteristics under partial hydrogen content and traditional combustion technologies.However,there is still a lack of research on combustion and emission characteristics under the new combustion technology of wide hydrogen mixing ratio range and gentle combustion.In response to the energy situation of primary fuel shortage and high overall carbon emissions,it is necessary to carry out basic and theoretical research on combustion under the new combustion technology of wide hydrogen mixing and green low carbon.In this thesis,the laminar combustion and emission characteristics under mixed hydrogen are studied firstly,and then the combustion and emission characteristics under mixed hydrogen are studied by coupling“high temperature and low oxygen”mild combustion,especially the generation path of carbon emissions.Furthermore,coupled with the research results of laminar combustion and gentle combustion characteristics,an air-induced flue gas recirculation/fuel transverse cross-mixing gentle combustion chamber was developed.The indoor fluid dynamics and carbon and nitrogen emission generation characteristics are analyzed by numerical simulation.The main research contents and conclusions are as follows:1.Study on hydrogen-mixed combustion and emissionsA one-dimensional flame model of laminar premixed flame is established.The effect of hydrogenation（0-100%）on the combustion and emission characteristics of the laminar flame in lean combustion conditions（Φ=0.6,0.8）was studied by Chemkin.The changes of LBV（laminar burning velocity）and AFT（adiabatic flame temperature）withΦ（equivalence ratio）under different hydrogen contents were studied.The effects of increasing hydrogen content on free radicals（H,O and OH）and combustion temperature were discussed.In addition,the effect of hydrogen addition on the NO and CO emissions were investigated.The rate and sensitivity coefficients of NO/CO production were also examined.The results show that the LBV and AFT increase with the hydrogen content.Both the mole fraction of H,O and OH radicals and the flame temperature were observed to increase with the hydrogen content.Furthermore,the hydrogen addition can suppress the formation of CO,but significantly increase the emission of NO.The most important reaction to promote NO formation is NNH+O=NH+NO.The most important reaction to promote CO consumption is H+CO=H+CO₂.Reducing the equivalence ratio（from 1.2 to 0.6）can significantly reduce the NO emission.The addition of hydrogen can also increase the combustion stability of methane fuel under fuel-lean conditions.This is a good hint for gas turbine device to reduce NO emissions at fuel-lean side（Φ<1）.2.Study on combustion and emission of staged-MILD combustionA chemical reaction network model of staged-MILD flame is established.The effects of hydrogen addition（0-80%）,oxidant oxygen concentration（14-16%）and strain rate（10-1000 s-¹）on emissions were studied.The changes of flame temperature and free radicals（H,O and OH）were discussed.The emission and reaction pathways of NO and CO were studied.The main reactions of different NO formation pathways were analyzed.This demonstrates that hydrogen addition can inhibit CO production.However,it will cause the increase of free radicals（H,O and OH）and promote the formation of NO.Under MILD combustion,the increase of oxidant oxygen level will promote the emission of NO and CO.In the CH₄/H₂ flame,the thermal path is still the dominant contributor to NO emission.The change of oxygen level has the greatest influence on the reaction rate of NNH pathway.The reaction rate of NNH route decreases with the increase of oxygen level.The reaction O+C₂H₂=CO+CH₂ is strengthened by increasing the level of oxygen,in order to accumulate a large amount of CO at elevated oxygen levels.Increasing the strain rate can ensure the MILD combustion state and minimize NO and CO emissions.This provides a basis for the design of staged-MILD combustion chamber for gas turbines.3.Study on the combustion field of integral MILD combustionThe grid model of the reverse cross-flow burner was established.The velocity field and temperature field were studied by CFD.The NO_X emission was simulated by code postprocessor.The recirculation combustor helps to realize the internal recirculation of CDC（colorless distributed combustion）.Reducing the oxygen concentration in the oxidant leads to an increase in the speed,and it is possible to promote the CDC of external reflux.The increase of hydrogen content will reduce the flow velocity.However,the flow field of all hydrogen-rich fuels at 15%O₂ is similar to that of methane.Therefore,15%O₂ is beneficial to the distribution of hydrogenated fuels in terms of flow field.All fuel temperature values decrease with the decrease of oxygen concentration.Especially for hydrogen-rich fuel,the temperature field becomes more stable with the decrease of oxygen concentration.Compared with methane,the outlet NO_X value of hydrogen-rich fuel are relatively high.However,in the distributed mode,especially at 15%O₂,the predicted NO_X values of fuels（except H₂）are all lower than 1 ppm,indicating that the ultra-low NO_X goal is met through colorless distributed combustion（CDC）.Therefore,this fuel can replace methane in terms of combustion performance and NO_X emissions.This conclusion can promote the application of hydrogen-containing fuels in gas turbines.