Kinetics Mechanism Study Of Lean-burn Methane Ignition

Methane is the main component of natural gas and ventilation air of mine, which is widely used in combustion device such as boilers, gas turbines, etc. On the other hand, methane has strong greenhouse effect. Exhaust gas containing methane such as VAM is mostly discharged into air for safety considering, which will bring great pollution to the environment. In gas turbine, combustion is operated in lean-fuel condition to control temperature. Methane oxidation occurred at low equivalence in these fields. It has important significance in both theory and industrial application to research on lean methane ignition.A shock tube experimental platform with pressure control and optical measuring system was designed. Methane/air mixture was pressed by incident shock wave and reflected shock wave, achieved to expected high temperature, then auto-ignition started and OH emission was measured. Methane/air ignition delay defined by maximum of OH emission was obtained from 1500K-2000K in temperature, 10%-0.5% in methane mole fraction,0.9 atm in pressure.Experiments were carried out in tailored condition to prolong available measuring time. Flow field of the shock tube working in tailored condition was simulated through laminar, k-ω, k-ε, RSM and Spalart-Allmaras model. The results calculated through these models were in good agreement with ideal shock relations except k-co model. SA model has the best grid adaptability, secondly for RSM model. Fine grids were required for k-ε model. Local refinement in boundary layer mesh would lead to better results only if the turbulent model has good grid adaptability. According to the simulation and experiment results, perfect tailored condition was difficult to satisfy. However, the temperature and pressure near the test section end of shock tube would keep stable during the time of methane ignition. It was proved the designed experiment system can satisfy the requirements of measurement.The measured ignition delay were compared with simulation results of detailed methane oxidation mechanism GRIMECH 3.0. It was shown that ignition delay time would decrease through reduce of methane mole fraction in low temperature condition and differ little at temperature higher than 1900K. The ignition delay simulated through GRIMECH 3.0 mechanism were shorter than experiments results. The experiments in this paper occurred in weak ignition condition, therefore ignition delay demonstrated great uncertainty at the same temperature. The ignition delay results of both experiments and simulation were fitted by Arrhenius exponential formula, demonstrating similar trend when methane concentration and temperature changed. Simulation could not accurately predict the time between reaction start and OH production reach to maximum, therefore simulated ignition delay was shorter than experiments results. According to the experiment results, empirical relationship was proposed through correcting pre exponential factor and apparent activation energy of original expression.Production rate of important reactants was calculated through GRIMECH 3.0 mechanism. Sensitivity analysis was employed to determine important elementary reactions in oxidation. The results showed that conversion of CH3 was the most important reaction step and reaction CH3+O2<=>CH3O+O dominated the whole reaction rate of methane oxidation. CH3 converted to CH2(s) and compounded to C2H6 or C2H5 were the main branching reactions. H+O2<=>O+OH was the most important reaction to produce active oxidants. A reduced kinetics mechanism for lean methane oxidation was built, including 16 reactants and 31 steps. The reduced mechanism is suitable for lean fuel condition at equivalence ratio lower than 0.2.