Chemical Kinetic Study Of The Effect Of Hydrogen Addition On Methane Ignition And Laminar Burning Characteristics

With the development of economy and industrial technology,energy crisis and environmental pollution become increasingly serious.Natural gas,which is mainly composed of methane,is one of the most promising energy and becomes more and more important in the world energy structure.Due to its high efficiency and low pollution,natural gas has been widely used in combustion industrial,transportation and many other industries.However,natural gas shows unfavorable effect in the application of combustion industrial.For example,there are knocking combustion and misfire in natural gas fueled engines,which are closely related to the combustion characteristics of natural gas.Research shows that natural gas blending hydrogen can effectively overcome the unfavorable combustion characteristics.Therefore,in this paper the natural gas engine was selected to carried out engine test to obtain the pressure range of unburned mixture when the piston is at start of combustion.Based on the pressure range,the Chemkin-Pro code coupling with detailed chemical kinetic mechanism was used to conduct the chemical kinetic study of hydrogen addition on methane ignition and laminar burning characteristics.The main conclusions of this paper are as follows:（1）Combining the experimental data of the ignition and laminar flame speed and the value predicted by the kinetic mechanism,the performance of several chemical reaction mechanisms at high pressure was evaluated.On this basis,the influence of hydrogen fraction and equivalence ratio on the ignition and laminar combustion characteristics of methane under high pressure was obtained.（2）Under high pressure the effect of hydrogen fraction and equivalence ratio on ignition delay of methane at high and low temperature is different.With the addition of hydrogen,at low temperature the reactions（R14,R25）become important and the proportion of reaction path（R86,R96,R104）decreases.At high temperature the inhibition effect of reaction（R104）becomes weakened and the reactions（R2,R3）play an important role.Obviously,the proportion of main reaction path was changed.At low temperature the ignition kinetics of pure methane is more dependent on the fuel concentration while the ignition delay of CH₄/H₂ mixture is very sensitive to equivalence ratio.（3）Based on growth rate of laminar flame speed,it is possible to determine three regimes for CH4/H2 mixtures at high pressures,namely methane chemistry regime,methane and hydrogen combined chemistry regime and hydrogen chemistry regime.Whether adding hydrogen or increasing pressure,the overall sensitivity coefficient increases while the laminar flame speed presents opposite results.The contribution value analysis indicates that it is the results of competition between the chain branching reaction and the chain termination reaction.The influence of equivalence ratio on laminar flame speed of CH₄/H₂ mixtures is mainly affected by the reaction that directly related to the concentration of oxygen and methane.In this paper,the conjoint analysis method was used to deeply reveal the influence mechanism of pressure,equivalence ratio and hydrogen fraction on methane ignition and laminar premixed combustion,which provides theoretical guidance for the control of knocking combustion and fuel design.