Multi-Scale Simulation Of Coke Oven Gas Methanation Process

Natural gas is a kind of clean energy.In recent years,the domestic demand for natural gas has increased dramatically.In view of the energy characteristics of "rich coal,poor oil and little gas" and the huge coking capacity in China,actively developing the production of natural gas from coke oven gas,the by-product of coal coking,is conducive to alleviating the problem of domestic oil and gas shortage,and also has important strategic significance in energy security,energy conservation and emission reduction.Methanation is an important step in the process of natural gas production from coke oven gas.In this paper,the flow,heat transfer,mass transfer and reaction law of methanation process in fixed bed reactor were studied by using COMSOL Multiphysics simulation software at catalyst scale and reactor scale.Firstly,based on the industrial cylindrical methanation catalyst,the single particle catalyst model and reactor model were established respectively,and the effects of catalyst pore structure and operating conditions on the reaction process were explored.The results show that at the catalyst scale,the reactants are mainly distributed in the area near the outer surface of the catalyst,and the methanation process is severely restricted by internal diffusion.Decreasing the temperature and increasing the porosity can improve the internal diffusion effectiveness factor of the catalyst obviously.When the temperature decreases from 673 K to 513 K,the effectiveness factor increases from 0.07 to 0.85;When the porosity increases from 0.2 to 0.8,the effectiveness factor can increase from 0.13 to 0.62.However,the decrease of temperature and the increase of porosity may reduce the reaction rate.In contrast,the change of reaction pressure and pore size has little effect on the performance of the catalyst.At the reactor scale,methanation process is still severely restricted by internal diffusion,and hot spots are formed in the axial direction of the reactor.The increase of pressure and fluid velocity will lead to the decrease of CO conversion and the increase of temperature in the reactor.In addition,the increase of fluid velocity will increase the pressure drop of the bed.When the fluid velocity increases from 0.02 m/s to 0.08 m/s,the pressure drop increases about 7 times.The increase of inlet temperature will lead to the increase of CO conversion,the increase of temperature and the decrease of hot spot temperature difference in the reactor.Furthermore,the influence of catalyst particle shape on the methanation process was investigated on the scale of catalyst and reactor.The results show that the main factor affecting the reaction performance of catalysts with different shapes is their external specific surface area.Compared with spherical and cylindrical catalysts,the multilobular catalyst has higher effectiveness factor and average reaction rate.The effectiveness factor of spherical particle is only 0.13,while that of tetralobular particle is 0.29.At the reactor scale,compared with spherical and cylindrical catalyst beds,the residence time of reactants in the multilobular catalyst bed is longer and the conversion rate is higher,but the hot spot temperature and pressure drop of the bed are also higher.Finally,taking the reactor with cylindrical catalyst particles as an example,a gradient loading model was established to investigate the effect of catalyst activity distribution on the methanation process.The results show that the conversion of CO is about 64%for the three active components distribution and the distribution of active components from inlet to outlet(distribution A)can reduce the hot spot and improve the bed temperature distribution while keeping the conversion almost unchanged.On the basis of distribution A,the inlet temperature,pressure,fluid velocity and other operating conditions were further optimized.After optimization,the hot spot temperature difference can be reduced from 37 K to 19 K.