Reaction Model Establishment And Experimental Verification Of Ultra-low Concentration Methane Fluidized Combustion

It is common knowledge that ultra-low concentration methane is prevalent in coal bed methane and exhaust gases; its concentration varies depending on the operating conditions and always is not more than 3%. However, its low calorific value makes it difficult to use, and therefore, most untreated low-concentration methane is simply released to the atmosphere in our country. This not only causes air pollution, but also represents a colossal waste of energy resources. Therefore, to study the use of ultra-low concentration methane has an important significance of energy conservation and environmental protection. Fluidized bed catalytic combustion has been widely used for the combustion of inferior grade gases because of its wide scope of application, high heat capacity. However, the reaction model of fluidized combustion need to study and establish while ultra-low methane catalytic combustion in a fluidized bed.For this problem, the partition model was established and coupled on the basis of the characteristics of ultra-low concentration methane catalytic combustion characteristics in the bubbling fluidized bed, then the catalytic combustion characteristics of low concentration(0.15–3 vol. %) methane combustion in a lab-scale fluidized bed with 0.5 wt. % Pd/Al2O3 as catalytic particles were studied experimentally and analyzed theoretically. Finally the axial distribution of low concentration methane in the fluidized bed was predicted using the mathematical model.Based on the known characteristics of low concentration catalytic combustion in a bubbling fluidized bed, the models of dense and freeboard zone were established respectively according to the flow characteristics, reaction and mass balance and gas-solid two phase flow theory. Mass transfer between emulsion phase and bubble phase and the surface reaction of methane absorbed in the catalyst particle was taken into account for dense zone. Based on the characteristics of ultra-low concentration catalytic combustion in the freeboard zone, freeboard zone was divided into two zones: a splash zone and a homogeneous zone, then the models were developed respectively. The reaction model was verified by the experimental study at different conditions. The influences of bed temperature, inlet concentration, fluidized velocity and static bed height on ultra-low concentration methane fluidized catalytic combustion were analyzed, then a comparative analysis was done for the methane conversion predicted by the established model, plug flow, mixed flow and K-L model under different conditions. The research showed the reaction model was agreed with the experimental data, with a deviation within 5%. Meanwhile, it was found that methane conversion reached approximately 100 % as the bed temperature increased to 650 ?.Methane conversion increased and the reactions moved toward the lower part of the bed while bed temperature increased and inlet methane concentration and fluidized velocity decreased. Reactions of methane combustion in dense zone mainly occurs in the emulsion phase. And this variation of methane concentration in the bubble phase is largely dependent on mass transfer between the bubble phase and emulsion phase.Based on the characteristics of ultra-low concentration methane fluidized catalytic combustion in these different zones of the bubbling fluidized bed, the model was developed and verified by the experimental study, the effects of operating conditions on ultra-low concentration methane catalytic combustion were investigated. The results can provide theoretical support and basis for the efficient use and conversion of low calorific value gas.