Study On The Chemical Kinetics Characteristics Of Methane Combustion Under Low Temperature Condition

As the emphasis on environmental protection and increasing energy demand all over the world,and the increasing shortage of the traditional energy such as oil,LNG also with exploitation and utilization of oxygen-bearing coal-bed methane（CBM）show significant economic and social benefits gradually.However,some research results indicate that in the storage of LNG and liquefaction process of oxygen-bearing CBM,the methane content in gaseous space is within the flammability limit range imposing a risk of explosion,which will cause great economic losses and casualties.Although the explosion process of methane-air mixture at room temperature has been studied systematically by experimental and numerical methods,it is still lack of study on the kinetic characteristics of methane combustion under cryogenic environment.Therefore,in view of the explosion hazard in the storage of LNG and liquefaction process of oxygen-bearing CBM,an experimental setup was built to measure the methane explosion characteristics under low temperatures.Based on the experimental results,the combustion mechanism of methane at low temperatures was studied.And the main elementary reactions and reaction path of the methane combustion under low temperatures were obtained.Numerical method was used to simulate the methane combustion at low temperatures.The critical conditions and the flame propagation characteristics of methane combustion at low temperatures were obtained.Through the entire flow field variation,vortex movement and pressure wave propagation characteristics during the whole process of combustion,the flame propagation mechanism of methane combustion at low temperatures was established finally.The main research works and results are as following:An experimental setup was built to measure the methane explosion characteristics at a temperature as low as 113 K,and the explosion kinetic parameters were obtained using the experimental device.Results indicate that equivalence ratio,initial pressure and temperature have significant effect on both explosion pressure and explosion temperature.When the equivalence ratio is 1,the largest explosion pressure,largest explosion rise rate and largest explosion temperature reach the maximum,respectively.With the increase in initial pressure,explosion pressure,explosion temperature and the largest explosion rise rate increase.The largest pressure rise rate linearly correlates to the initial pressure.Low temperature leads to the increase of explosion pressure and the decrease of the explosion temperature.However,the largest pressure rise rate is unaffected by the initial temperature.During the measurement of the minimum ignition energy（MIE）,the sensitive methane content is when the equivalence ratio is 1,and the sensitive electrode gap is 1 mm.With the increase in initial pressure or temperature,the MIE decreases.MIE linearly correlates to 1/P₀²,and so does the 1/T₀.Principal component analysis and sensitivity analysis were coupled to study the combustion mechanism of methane at low temperatures.12 species and 20 elementary reactions of methane combustion at low temperature are obtained.Among these elementary reactions,R5:CH₃+O₂=CH₂O+OH and R6:CH₂O+OH=CO+H₂O+H are the most important reactions which promote methane combustion.Whereas,elementary reactions R2:CH₄+OH=CH₃+H₂O and R4:CH₄+H=CH₃+H₂ are the most important reactions which inhibit methane combustion.At low temperatures,the whole oxidation burning path is composed of a main path:CH₄→CH₃→CH₂O→CO→CO₂.In addition,several branched and non-important reaction paths are also included.The combustion mechanism at low temperature was closed and coupled with the numerical software FLUENT,the critical conditions and the flame propagation characteristics of methane combustion at low temperatures were obtained.At the beginning of ignition,methane is consumed slowly,and the mass fraction of methane inside the flame ball is not zero.The consumption of methane immediately affects the concentration distribution of carbon monoxide,and the concentration of carbon monoxide presents the opposite relationship with that of methane.Mass fractions of the intermediate products and H,O free radicals are low in the burnt zone and unburnt zone,whereas reach the maximum on the flame front.OH is not only the intermediate free radical which participates in the reaction as the chain carrier,but also the combustion product remained by the intermediate reactions.Ignition radius and locations have large effect on the trigger stage of the methane combustion.At low temperature,the critical radius and temperature of methane ignition are 4.6 mm and 1180 K,respectively.When ignition radius is smaller than the critical value,the failed ignition of methane is cuased by the termination of the elementary reactions:R8,R9,R12 and R13.However,when ignition temperature is lower than the critical value,because of the failed initiation of elementary reaction R1:CH₄=CH₃+H,the combustion of methane is not occurred.With the decrease in initial temperature,the critical ignition radius increases while the critical ignition temperature remains unchanged.At low temperatures,the minimum ignition energy of methane can be predicted very well using the critical ignition radius.At low temperature,five stages are divided during the methane combustion in a closed container:spherical flame propagation,“finger tip”shaped flame propagation,flame“skirt edge”contacts the side wall,“crescent”flame propagation and typical“tulip”flame propagation.In the process of flame propagation,the reverse of the flame front and formation of the“tulip”flame can be immediately contributed to the interaction of the flame front,flame induced reverse flow and vortex motion.However,the pressure wave propagation back and forth along the flame propagation direction has no obvious effect on the formation of“tulip”flame.In the process of flame propagation,after the formation of“tulip”flame,wrinkle will appear in the smooth flame front,which is called the distorted“tulip”flame.The formation mechanism of the distorted“tulip”flame is different from that of the typical“tulip”flame.When the distorted“tulip”flame is formed,vortex motion is not observed.The formation of the distorted“tulip”flame is caused by the superposition of the secondary pressure wave formed by the flame contacting the side wall.However,because of the low intensity of pressure wave,RT instability is weak,and the distortion of flame front is not obvious.Flame propagation velocity and pressure wave are interacted and influenced with each other.In the process of combustion,the change of flame propagation speed and pressure rise rate show almost the same phase.The increase in flame propagation speed directly leads to the increase in pressure rise rate,whereas the pressure wave propagation back and forth in the closed container leads to the vibration of propagation speed.