| Coal is the most important primary energy in China.When mining coal in mine,tens of billions cubic meters of coal mine methane are discharged into the atmosphere without being utilized every year.It not only wastes energy,but also causes serious environmental problems.It is necessary to carry out research on methane oxidation and utilization technologies for low concentration coal mine methane,especially for ventilation air methane.It is of great significance to improve coal mine methane utilization and reduce greenhouse gas emission.In present work,a series of bench scale experiments were conducted and the key design parameters for industrial scale reactor were determined based on the experimental results.The industrial-scale multibed TFRR was developed.The effects of operating parameters,channel structure parameters and material property on the performance of thermal flow-reversal reactor(TFRR)were studied by industrial test and threedimensional numerical simulation.The main contents and results are as follows:(1)A bench scale TFRR with two beds was established.Results show when the temperature of combustion chamber is in the range of 722? to 847?,the methane conversion remains above 96%,and it is not affected by feed methane concentration.For a TFRR with a given inlet flow rate and a constant cycle time,the lower methane concentration limit exists for self-maintained running before the reactor extinguish during the decrease of feed methane concentration.For the case with tc=200s and Fin=1000 Nm3/h,the lower methane concentration limit is 0.3 vol%.When the inlet flow rate decreases to 500 Nm3/h,the lower methane concentration limit increases to 0.32 vol%.(2)The multibed structure was selected for the industrial-scale TFRR by comparing performance parameters of two-bed,three-bed and multibed structures.Key design parameters such as combustion chamber temperature,inlet velocity and the lower feed methane concentration limit were determined based on the experimental research on bench scale reactor.The structure design of industrial-scale TFRR was completed.(3)The industrial-scale experiments on ventilation air methane(VAM)oxidation and utilization by a multibed TFRR were conducted.Results show that the average temperature of ceramic bed,temperature fluctuation range,combustion chamber temperature and heat recovery efficiency increase with increasing inlet flow rate when the feed methane concentration and cycle time are fixed.When the inlet flow rate and cycle time are fixed,the average value and fluctuation range of ceramic bed temperature generally increase with increasing feed methane concentration.The combustion chamber temperature and heat recovery efficiency increase with increase in feed methane concentration.With the increase of cycle time,the temperature fluctuation of ceramic bed increases,and the average temperature of ceramic bed,combustion chamber temperature and heat recovery efficiency decrease slightly when the inlet flow rate and feed methane concentration are fixed.When the combustion chamber temperature is in the range of 718? to 1129?,the multibed TFRR can oxidize methane efficiently,with methane conversion of 97% to 99.8%.The lower feed methane concentration limit decreases with increasing inlet flow rate.When the inlet flow rate is between 102 784 Nm3/h and 107 487 Nm3/h,the pressure drops in the ceramic bed and combustion chamber are between 2950 Pa and 3330 Pa.(4)Three-dimensional numerical studies on the thermal oxidation of methane in a TFRR by using two-step oxidation mechanism were performed by means of the finite volume method.The influences of feed methane concentration,inlet velocity,cycle time,specific heat capacity of honeycomb ceramics,channel length,wall thickness and channel shape on the reactor behavior were analyzed.The lower feed methane concentration limits for self-maintained running under different channel length,inlet velocity and cycle time are determined.The characteristic of pressure distribution in the channel and effects of main parameters on pressure drop were analyzed.Results show that the high temperature zone becomes wide with increases in the channel length and feed methane concentration,and it becomes narrow with increases in inlet velocity,and it is almost not affected by other factors.The temperature distribution in the high temperature zone is significantly affected by the feed methane concentration,inlet velocity,channel length,wall thickness and channel shape,which presents different distribution curves such as straight line,parabola(up or down)and S-type curve.The lengths of the heat-absorbing and heatreleasing zones increase with decrease in the feed methane concentration and increase in the inlet velocity,and they are almost not affected by other factors.The average heat fluxes in the heat-absorbing and heat-releasing zones mainly increase with increases in the feed methane concentration and inlet velocity.The average outlet-inlet temperature difference increases linearly with the increase of methane concentration.The length of methane reaction zone decreases with the increase of the feed methane concentration and cycle time,and it increases with increase in inlet velocity.The specific heat capacity of ceramics,channel length and wall thickness have certain effects on the length of the reaction zone,but there is no obvious rule.The channel shape has no effect on the length of reaction zone.The lower feed methane concentration limit for self-maintained running is strongly influenced by channel length and inlet velocity.The lower feed methane concentration limit decreases dramatically with increasing channel length,rises sharply with increasing inlet velocity,and it is almost not affected by the change in cycle time.The pressure drop in the channel increases significantly with the increases of feed methane concentration,inlet velocity and channel length.The pressure loss expression in the channel was obtained by theoretical analysis.The theoretical calculation results of pressure loss are in good agreement with the numerical simulation and experimental results. |
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