Study On Catalytic Combustion Characteristics In A Micro-combustor Filled With Metal Foam

Combustion instability,ignition and low combustion efficiency in a confined space are the key issues which determine the development and application of small or micro-scale energy systems using hydrocarbon as fuels.Herein,a micro combustor partially filled with metal foam was fabricated and the catalytic combustion of premixed methane/air was numerically and experimentally studied in order to improve the stability of combustion and decrease the combustion temperature.By means of Fluent,two types of technology,that is combustion in porous media and catalytic combustion in a micro combustor were simulated.Besides,an experimental system is established to evaluate the performance of monolithic catalysts which were prepared using Ni metal foam as substrate and Pd noble metal as active ingredient.Firstly,the homogeneous combustion of premixed methane/air in the channel with a gap of 3 mm was numerically investigated in order to be compared with the combustion in porous media.The two-step reaction mechanism of CH₄-Air homogenous combustion was used.The effects of inlet temperature,flow rate,equivalent ratio and heat loss on the homogeneous combustion characteristics of methane/air mixture were numerically studied.The results showed that inlet temperature and velocity had important effects on the position and the flame profile because of the inlet temperature and the difference of fluid flow velocity and flame propagation velocity.When the fluid flow rate was equal to the flame propagation velocity,the flame profile was flat;when the inlet flow rate was much greater than the flame propagation velocity,the flame was shifted downstream along with the fluid and the V flame profile was formed.The larger the difference between two velocities,the larger the flame length.Under the conditions of this study,there was no complete flame when the inlet temperature is T₀=300K and inlet velocity is v₀=3.0m/s,due to the flame had been blown out of the burner.Secondly,the homogeneous combustion of premixed methane/air in the3mm height channel that partially filled with metal foam was numerically studied.The porosity,cells diameter and length of the Ni metal foam were 95%,200?m,and 3mm respectively.It was placed at the distance of 1mm downstream the inlet.The single-temperature model in porous media?local thermal equilibrium model?and methane/air two-step reaction mechanism were used.The effects of inlet velocity,equivalence ratio and heat loss on the combustion characteristics in porous media were investigated by using Fluent.The results showed that porous media can effectively stabilize the combustion in confined space and broaden the flammability limits.When the inlet temperature was T₀=300K and the inlet vecolity varied from1.5m/s to 3.0m/s,the flame can be stablized in the porous media and the flame profles were parabolic.Besides,the distribution of temperature in the whole combustor was more uniform and the average temperature was higher than that in the combustor without porous media,due to the strong heat transfer and thermal storage of porous media.The GRI3.0 reaction mechanism of homogenous combustion was usd to calculate the temperature distribution in the combustor with porous media.The results showed the peak temperature was lower and the reaction zone was wider than that obtained by the two-step mechanism.However,it was complex and difficult to converge because of the too many reactions in the detailed mechanism.Then the two-temperature model in porous combustion was used to simulate.It found that the temperature difference between gas and solid skeleton was very small?<5K?for the Ni metal foam.Thirdly,the catalytic combustion of premixed methane/air in the 3mm height channel that partially filled with metal foam were studied based on the model of the combustion in porous media.The Deutschman surface reaction detailed mechanisms was utilized to caculate catalytic reaction,and the two-step reaction mechanism was still used to simulate gas phase reaction.The single-temperature model of combustion in porous media?local thermal equilibrium model?was adopted.Then,the influences of inlet temperature,velocity and equivalence ratio on the catalytic combustion characteristics were investigated.The results showed that catalytic reaction can improve the stability of methane combustion and widen the flammability limits.Compared with the homogenous combustion in porous media,when the inlet velocity is v₀=0.5m/s,not only the flame can be stabilized in the porous region,but also the peak temperature decreased by406K.Besides,with the increase of inlet temperature of premixed mixture,especially the inlet temperatures were less than 900K,the catalytic conversion efficiencies of methane can be improved significantly.However,the gas phase reaction cannot take place at the low inlet temperature?T₀<900K?.Generally,the methane conversion rates of catalytic combustion were higher than that of homogenous combustion.Finally,the monolithic catalyst Pd/Al₂O₃/Ni were prepared by impregnation method with Ni metal foam as the support,the alumina sol as the carrier coating and Pd as the active component.And the catalytic reactivity of the monolithic catalysts in a channel burner with a gap of 2mm was evaluated in the experimental system.The effects of methane concentration,flow rate,temperature and doped Ce,Zr on methane conversion rate were experimentally studied.The experimental results showed that the reaction temperature have very important influence on the catalytic oxidation of methane in the micro channel combustor,when the temperature was more than 450?,the methane conversion rate reached more than 10%;when the temperature increased to 600?,the conversion rate of CH₄ can reach to 89.1%.The methane concentration also influence the CH₄conversion greatly,the methane conversion rate decreased with the increase of methane concentration.The conversion rate of methane got the highest value when the methane concentration was 2%.The light-off temperature(T₁₀)of the monolithic catalyst doped with Zr was reduced by 29?.And T₅₀0 and T₉₀ decreased by 45?and37?respectively.It also can be found that the catalytic reactivity of catalyst doped with Zr was much higher than that doped with Ce under the same conditions.