Interaction Mechanisms Between Heterogeneous And Homogeneous Reactions During Catalytic Combustion Of Alkane Fuels In Micro Scale

The increasingly popular micro-devices require high-efficiency,environmentally-friendly,long lifetime,and instantly rechargeable power supplies.Due to the advantages,such as high energy density and low pollution emission,micro power systems based on the catalytic combustion of alkane fuels have great research and application value.However,the present researches on the catalytic combustion in micro scale is not deep and systematic enough.It is required to obtain the correlation mechanism between the molecular structure of different alkane fuels and the micro-scale combustion characteristics,the effects of hetero/homogeneous reactions,and the interaction mechanism between heterogeneous and homogeneous reactions.Based on the above scientific problems,experimental and numerical studies have been conducted on the catalytic combustion of alkane fuels in two typical micro combustors-the packed bed combustor and channel combustor,aiming to enrich the understanding of the alkanes catalytic combustion under micro-scale.To acquire combustion characteristics of methane,propane and n-octane,experimental study was conducted in a 4-mm-diameter packed-bed combustor with the Pt/ZSM-5 catalyst particles.It is found out that the stability limits were broadened with the equivalence ratio increasing.The ignition temperature of the gas alkane fuels was reduced in the catalytic combustion while the ignition temperature of propane ignition dropped most.The catalytic combustion of methane showed relatively poor performance,with a narrow self-sustaining combustion region,low conversion rate,high CO selectivity.The combustion cannot sustain in the combustor once equivalent ratio was less than 1.0.The increase of the carbon chain length leads to lower bond energy of C-H in alkane molecule.Thus,the catalytic combustion of propane in the packed-bed combustor showed a wider self-sustaining combustion region,higher conversion rate,with the CO₂ selectivity above 96%.N-octane exhibited similar combustion characteristics with that of propane.The combustion of n-octane exhibits a slightly wider self-sustaining combustion region,higher conversion,slightly lower wall temperature,and more heat loss through the outer wall than that of propane.In the case of propane combustion processes,a homogenous reaction occurred downstream the catalyst section and led to high conversion.The combustion mode was transformed from pure heterogeneous combustion to hetero/homogeneous combustion.To analyze the new phenomenon that heterogeneous reaction and downward homogeneous reaction showed in the upstream catalyst zone and downstream gas-phase zone,a numerical mode of packed bed catalytic combustion was carried out,using Fluent software and the 41-step gas phase reaction mechanism and the 24-step catalytic reaction mechanism.The experimental and simulation data were in good agreement,and the accurate simulation method is obtained.The combustion of methane/air premixed gas in the packed-bed combustor in micro scale was studied with the inlet velocity changing.It was found that the premixed combustion of methane/air showed out three combustion modes:completed heterogeneous combustion under low inlet velocity(u_in)of 0.05?1.53 m/s,hetero-homogeneous combustion under middle inlet velocity of 1.53?8.7 m/s,and uncompleted heterogeneous combustion under middle inlet velocity of 8.7?12.4 m/s.The existence of downstream gas phase combustion was controlled by the thermal and material conditions together at the outlet of the catalytic section.A dimensionless parameter ?=a/(u_in·?r_TZ)·(?Tmax-Tmin?/T_ig) was employed to describe the condition for triggering homogeneous combustion.The homogeneous reaction would not show in the downstream gas phase zone if?is too large too small.A catalytic micro-channel combustor with constant wall temperature was set.The width was 2.62 mm.the ignition and flame characteristics of C₁?C₄ alkane fuel were studied in the micro channel.It was found that the ignition process of the four alkane fuels could be divided into three stages:ignited process,propagation process and stabilization.The flame propagation speed during the ignition process of ethane combustion showed the largest in the four alkanes.With the equivalence ratio increasing,the flame propagation speed increased,and the flame position would move upstream.Under the same equivalent ratio,the concentration of CH and C₂ radicals in ethane combustion was the highest.The maximum concentration of OH radical increased with the increase of carbon chain length and the concentration of OH radical in n-butane flame was the highest.The concentration of the free radicals near the wall was lower due to the absorption of the Pt foils.To analyze the phenomena of the interaction between homogeneous and heterogeneous reaction,the simulation researches were conducted with the gas-phase and catalytic reaction skeletal mechanisms.It was discovered that the methane/air catalytic combustion in the micro channel can be divided into five characteristic phases.In phase I,heterogeneous reaction consumed gaseous H₂ and O₂ slowly,releasing heat and assisting the homogeneous ignition.Homogeneous ignition occurred in phase II,leading to the two evident process of the heat release by heterogeneous reaction phase ? and IV.The additional reaction pathway of CO through heterogeneous reaction was favorable for the completeness of CO to CO₂ during combustion,resulting in lower CO concentration in catalytic combustion than that in non-catalytic combustion.Homogeneous reaction was weakened when the width of micro channel decreased and heterogeneous reaction was enhanced.The effect on combustion characteristics and interaction rules of the inlet parameters was studied Heterogeneous reaction was insensitive to the varying of inlet velocity and its auxiliary effect on homogeneous ignition is weakened when the inlet velocity increased.The assistance of Heterogeneous reaction on homogeneous ignition was most obvious when the equivalence ratio is 1.0.Heterogeneous reaction was suppressed under lean conditions,and the position of gaseous species consumed quickly by heterogeneous reaction was overlapped.Hence,the curve of the heat release by heterogeneous reaction appeared one peak only,which was beneficial to the speed of the finish of combustion.The increase in the inlet temperature would result in drops of amount of the input reactants,leading to the decrease of heat release of the catalytic combustion.Meanwhile,the assistance of heterogeneous reaction on the homogeneous reaction was more obvious.