Study On Combustion And Kinetic Characteristic Of Ultra-low Concentration Methane Catalytic Combustion In A Fluidized Bed

Ultra-low concentration methane (UCM) in which methane volumetric concentration is less than five percent widely exists in coal mine methane and chemical exhaust gases. It is difficult to utilize with regular technologies due to its low methane concentration and large flow variation. Therefore, most of UCM is emitted to atmosphere without any treatments which cause energy wastes and air pollutions. It is significant to utilize UCM in a reasonable way. Fluidized bed combustion technology is widely used for inferior solid fuel according to its large thermal capacity and good fuel flexibility. However, at present the research mostly focused on natural gas in which methane concentration is relatively high. Research involved in combustion zone, bed temperature, combustion products, et al. Kinetic study mainly focused on high methane concentration methane combustion in fluidized bed with inert particles. Few studies have reported on lean methane combustion in a fluidized bed and the influence of operating conditions on combustion such as methane volumetric concentration and flow variation. UCM combustion in a fluidized bed with catalytic particles and its kinetic during combustion require further study. Therefore, the research has an important academic significance and industrial application value.According to the research situations and difficulties of UCM utilization, fluidized bed combustion technology was put forward for UCM combustion. As the experimental fluidized bed system designed, thermal balance analysis and limiting concentration for system operation were conducted. 0.5%Pd/Al2O3 particles were employed in fluidized bed system as catalyst. Research is carried out experimentally and theoretically. The flow in fluidized bed, heat transfer between phases and combustion characteristics were studied. Reaction model for UCM combustion in fluidized bed was established. Kinetics characteristic such as reaction control area and kinetics parameter variations.in fluidized bed during UCM combustion were investigatedFirstly, study on flow and heat transfer between phases in fluidized bed was carried out. The influence of operating conditions on flow and heat transfer was analyzed. Then, the variation pattern on bed expansion height, bubble growth, pressure fluctuation, minimum fluidized velocity, bed temperature and coefficient of heat transfer between phases were obtained. It is found that effects of temperature and fluidized velocity on bed expansion and bubble growth are different. Bed expansion height increases with rising temperature and increasing fluidized velocity. However, increasing temperature has little effects on bubble growth. When fluidized velocity is accelerated, bubble diameter is increased. Pressure fluctuates in fluidized bed and the amplitude in bed surface is little higher than that in bed. The minimum fluidized velocity is reduced when bed temperature increases. Temperatures are uniform in dense zone of fluidized bed. The heat transfer coefficient decreases when rising bed temperature or adding particle size at the same inlet volumetric flow. The heat transfer coefficient increases firstly and then decreases when increasing bubble diameter. There exists a critical bubble diameter, and the critical value is reduced with increasing particle diameter. This work plays a basic role for further study on UCM combustion in fluidized bed and its dynamic characteristics.Secondly, combustion characteristic of UCM combustion in a fluidized bed was studied, and effects of bed temperature, inlet methane concentration, and fluidized velocity on combustion characteristic were also studied for the first time. Axial profile of combustion products were sampled and analyzed. Variation patterns of significant parameters such as combustion products, CO concentration, system conversion and dimensionless methane concentration with operating conditions changing were obtained. The results show that most UCM are converted to CO2 and H2O, with little CO in combustion products. CO concentration increases with rising temperature due to methane steam reform and water gas shift reaction companying with combustion. The dimensionless methane concentration decreases with bed height below bed surface, and increase abruptly at bed surface. Methane conversion increases and combustion reaction moves towards bed bottom when rise bed temperature. Methane conversion decreases and combustion reaction moves towards bed top when inlet methane concentration is added or fluidized velocity is accelerated.Thirdly, based on the flow characteristics, reaction characteristics of fluidized bed and mass balance, mathematical model for UCM combustion in fluidized bed is established. Catalyst efficiency factorη, Damkohler number Da and Stanton number Stm were introduced to this model, which variations could be analyzed to obtain the control factors. Dynamics characteristics of UCM catalytic combustion in fluidized bed was explored and compared with intrinsic kinetics parameters in fixed bed. The results show that reaction is controlled by intrinsic kinetics when temperature below 450℃. Kinetic parameters in fluidized bed are almost the same as in fixed bed in this temperature range. When temperature is over 450℃, the reaction of UCM in fluidized bed is not only controlled by intrinsic kinetics, also influenced by mass transfer between gases and surface of catalyst, diffusion action and mass transfer between bubble phase and emulsion phase. Numerical calculations were introduced with the developed model and acquired kinetic parameters, which showed that calculated values exactly coincided with experimental values. The main reasons for errors are intensified gas-solid flow and bubble growth in fluidized bed.The flow, heat transfer, combustion and kinetic characteristic of UCM combustion in fluidized bed were studied systematically in this thesis. Variations of relevant factor on combustion were obtained. A reaction model for UCM combustion in fluidized bed was established. The kinetic characteristic of UCM combustion in fluidized bed was analysed. It is helpful to promote further studuy on low calorific value fuel catalytic combustion in fluidized bed. Research achievements on gas fuel combustion in fluidized bed were enriched. The research results of this study can provide theoretical basis for ultra-low concentration gas fuel combustion in fluidized bed.