Experimental Study And Modelling Of Reverse Flow Catalytic Combustion Process For Lean Methane Emissions

In this work, reverse flow catalytic combustion of lean methane inmine ventilation air (≤1.0%CH₄) was researched for exhaust removal andrecycling of chemical energy. The preparation, characterization anddynamic characteristics of catalysts with cordierite carrier for methanecombustion were studied systematically. Based on the optimal catalyst,depth research about simulation of lean methane reverse flow catalyticcoumbusiton reacor with heat exchanger in the middle of the reactor wascarried out. A pilot plant (processing capacity of1000Nm3/h) of leanmethane reverse flow catalytic combusiton was designed and constrctedat Lu'an Group Wuyang Coal Mine. The obtained conclusion can providetechnical basis for the efficient and clean use of the energy of lowconcentrations of methane.The modified experimental results of acid treatment and washcoatloading for γ-AL₂O₃/Cordierite indicated that acid treatment (impregnatedcordierite in10%HNO₃for2h) can improve the adhesion capacity ofcordierite carrier and washcoat carrier under the premise of mechanicalstrength; AlOOH gel prepared on the condition of n(H⁺)/n(AlOOH)0.08, solid content30%, after coating modified, the surface ofγ-Al₂O₃/cordierite was improved from0.81m2/g to50m2/g, and the poresize distribution of the support was between0nm to20nm, which wasfavorable to improve the dispersion of the active component on thesurface of the support.With the γ-Al₂O₃/cordierite above as carrier, a series of γ-Al₂O₃/CORsupported catalysts applied to lean methane combustion were preparedwhich the active component include monometallic palladium (Pd),bimetallic cobalt (Co) and palladium (Pd), and Cu-Mn complex oxides.The catalysts were characterized by SEM,ICP-AES,XPS,TPR/O andN₂-adsorption-desorption.Compared four different Pd loading of monolithic catalyst, on thecatalyst surface of0.1%Pd loading, the active phase of Pd0/PdO scatteredevenly, and the ratio of Pd⁰and PdO achieved better, which wasconducive to the mobility of lattice oxygen on the Pd^O→Pd→PdO redoxprocess, thereby increased the activity of the catalyst. On the condition of430oC and GHSV20000h^-1, the methane conversion was over90%.The methane catalytic combustion on the supported catalyst isgenerally considered to belong to the structure-sensitive reaction: betterdispersion of active component on the surface of the carrier, easier toagglomerate, leading to poor thermal stability. Although the0.1%Pdloading monolithic catalyst performed excellent in activity, in order to improve the synergies between active phase and carrier, the transitionmetals Co was used to modify the supported Pd catalysts. The resultsshowed that, appropriate amount of Co addition can form stable crystalcluster CoAl₂O₄on the catalysts surface which can improve the bindingforce between the active phase and carrier to protect Pd0/PdO active sitenot to be sintered; on the other hand, the Co-O bond cleavage can providemore lattice oxygen (O^2-) to the active phase of Pd0and Pd^O, improvedthe oxidative capacity of the catalyst. Among of which, the0.1%Pd-0.25%Co/γ-Al₂O₃/COR catalyst performed best, on the conditionof410oC and GHSV=20000h^-1, methane conversion was over90%, andthermal stability of the catalyst was relative increased.Taking into account the cost of catalysts, the Cu-Mn complex oxideswas used to instead of Pd as the active component for methanecombustion. And the factors which influenced the catalytic performancesuch as active component loading method, active component loading andadditives were investigated. Characterization and evaluation resultsshowed that the the catalyst prepared by impregnation method with theloading12%performed better for methane combustion. On the conditionof650oC and GHSV=20000h^-1, methane conversion was over80%.Furthermore, in order to improve the low-temperature catalyticcombustion performance of Cu-Mn composite oxide monolithic catalysts,the influence of promoters such as CeO₂,ZrO₂,La₂O₃&CeO₂-ZrO₂ additives was investigated. The results indicated that with theintroduction of CeO₂,ZrO₂,La₂O₃&CeO₂-ZrO₂as the promoter toCu-Mn composite oxide monolithic catalysts, the specific surface area ofthe catalysts increased obviously, resulting in an enhancement of activecomponent dispersion and oxygen species concentration on the catalystsurface. Therefore, the surface oxidation reactivity of the catalysts isimproved. Besides, the pore structure of the catalysts was also adjustedwell, which was beneficial for matching the diffusion-reaction couplingbehavior on the catalyst. In addition, the promoter can change theinteraction between the active phase and the carrier, re-distribute theelectronic around Cu, Mn, Al and O, improve the mobility of latticeoxygen, and enhance the oxidation of the catalyst. Among the testedpromoters, ZrO₂shows the best promoting effect. The methaneconversion over Cu-Mn-Zr-O/γ-Al₂O₃/COR monolithic catalyst is up to90%under the conditions of570oC and GHSV=20000h^–1.Compared the three different active components of the monolithiccatalysts for lean methane combustionon the condition of space velocity20000h-1and methane concentration1.0vol.%, these catalysts formethane completely oxided with desired temperature:0.1%Pd-0.25%Co/γ-Al₂O₃/COR＜0.1%Pd/γ-Al₂O₃/COR＜12%Cu-Mn--Zr-O/γ-Al₂O₃/COR。In order to reduce the energy consumption of leanmethane catalytic combustion in a reverse flow reactor,0.1%Pd-0.25%Co composition of monolithic catalyst was choosed to use for simulation andexperimental of pilot scale.Based on transient responses to step-change in feed composition, themechanism and the dynamic characteristics of the methane combustionon0.1%Pd-0.25%Co/γ-Al₂O₃/COR catalyst were reasearched. It showedthat the adsorbed CH³⁺was oxidized both by the lattice oxygen (O^2-) andgas oxygen (O₂). And the consumed lattice oxygen (O^2-) was renewed bygas oxygen (O₂). On the condition of lean methane and superfluousoxygen, methane was completely oxidized; the reaction products wereCO₂and H₂O only. Methane catalytic combustion on the catalystcomplied with the Mars and van-Krevelen redox mechanism. Accordingto the redox mechanism, the intrinsic kinetic models of catalyticcombustion of methane over Pd-Co/γ-Al₂O₃/COR catalysts weredeveloped with surface-reaction of CH₃⁺and gas oxygen controlassumption.The intrinsic kinetic experiments of catalytic combustion of methaneover Pd-Co/γ-Al₂O₃/COR catalysts were undertaken in an integralmicro-recator(Φ10×2mm) on the condition of atmospheric pressure,temperature350°C⁴50°C, space velocity40000h-1to50000h^-1, methanevolume concentration0.1%¹%. Before the experiments, the influence ofinternal diffusion,external diffusion and the impact of heat transfer wereeliminated. Model parameters of surface-reaction of CH₃⁺and gas oxygen control assumption were estimated with Simolex methods. Statistical testshowed that the model was highly accepted.At the background of mine ventilation air purification, andmonolithic catalysts used in the flow reactor, a one-dimensionalheterogeneous dynamic mathematical model for RFR was established, inorder to simulatie the pilot-scale device with handling capacity of1000Nm3/h CBM. The influence of switch time, working conditions ofthe adiabatic layer and feed concentration on the reactor performancewere investigated. The results showed that the switch time was animportant operating parameter for the impact of reactor performance, toolong for the switch time would cause the reactor to "turn off"; too shortswitch time would not be conducive to the reactor heat balance operation.The appropriate choice of adiabatic layer temperature can not only ensuregood reactor thermal equilibrium operation, but also to the efficient use ofenergy. In the range of inspection, reactor would be"extinct" at the lowerconcentration of methane (Vol.%≤0.5%). Therefore, the optimization ofoperating conditions control could protect the catalytic reactor is in goodworking condition, while saving energy and reducing operating costs wasof great benefit.According to the simulation results, a pilot plant (processingcapacity of1000Nm³/h) with heat exchanger was designed and constrctedat Lu'an Group Wuyang Coal Mine. The pilot experimental results showed that, on the condition of methane concentration0.34%⁰.54%inmine ventilation air, the reactor can operate normally with appropriatepreheat and proper switch time. The methane content of emissions waslower than0.06%after reverse flow catalytic combustion.The experimental and simulation result can provide technicalsupport for the application of RFR in mine ventilation air.