Numerical Study Of Heat Transfer Characteristics And Attrition Behavior Of Coal Tar Pitch Spheres During Fluidization Oxidation

Compared with the traditional fixed bed,the fluidized bed shows many advantages,such as sufficient contact between gas and solid,and relatively uniform temperature distribution in the bed.Up to now,fluidized beds have been widely used in the field of prtrochemical,electric power and other fields.Considering the above advantages,at present,some researchers have introduced fluidization operation into the oxidation process of coal pitch particles.However,this process is still in the laboratory research stage,and the multi-scale coupling and transfer mechanism during the heating process of the bed material is still unclear.The heating rate and temperature uniformity as a function of operating conditions need urgent research.In addition,the problem of bed material attrition has gradually become prominent.Considering that it is difficult to measure the detailed data for such systems with experimental methods,this thesis conducts a more systematic and in-depth study on the heat transfer and attrition of coal tar pitch spheres during fluidization oxidation using numerical simulation.A three-dimensional mathematical model to describe the coupled process of gas-solid heat transfer and attrition in dense gas-solid systems were developed.Finite Volume Method was used to describe the gas phase field,while the description of solid phase was based on the Discrete Element Method.Gas-solid convective heat transfer was realized by introducing a source term into the governing equations.For attrition model,the fragmention process was described using the Particle Replacement Method,and the abrasion was performed using the semi-empirical correlation between particle volume reduction,particle-particle contact force and relative displacement.Parameter calibration experiments were carried out to obtain the model parameters.Subsequently,the gas-solid heat transfer process in fluidized bed and fixed bed,single particle crushing and impact wear process were simulated,and compared with the experimental results,thus improving the model credibility.Based on the numerical method developed in this thesis,the heating process of coal tar pitch spheres in fixed and fluidized bed was simulated.Macro-scale gas-solid flow,mesoscale distribution and particle-scale heat transfer characteristics were compared.The multi-scale law of material temperature rising process in fluidized bed was obtained.Furthermore,systematic research was carried out on the influence of operating parameters,material properties and design parameters.The relationship between above parameters and heating rate and the temperature uniformity was quantitatively analyzed.The results show that the heating rate is related to the bed total heat capacity and the heat carried by the incoming fluidized gas.With the increase of superficial velocity,oxidation temperature and operating pressure,the heat entering the bed increases,so the heating rate increases;the decrease of material density and height diameter ratio reduces the bed total heat capacity,so the bed temperature response is faster;the decrease of material particle diameter does not change the heating rate significantly.Temperature uniformity is closely related to the intensity of particle migration in the bed.The more severe the particle migration is,the better the temperature uniformity is.Correspondingly,the beds with higher superficial velocity,higher pressure,lower material density and smaller material particle diameter show more intense material migration and better temperature uniformity.Based on the numerical method developed in this thesis,the coupling process of heat transfer and attrition of coal tar pitch spheres in fluidized bed was simulated.The results show that the fragmention mechanism caused debris to gradually appear in the bed.Debris gathers in the top spray area,the side wall area and the bubble wake area,and its temperature is much higher than the bed-averaged temperature.The abrasion mechanism makes the particle size of the bed material gradually develop from the initial narrow sieving to the wide sieving.In terms of heating rate and temperature uniformity,the abrasion mechanism increases the heating rate coefficient by about 16%,and the temperature uniformity increases by 6%.The fragmention mechanism reduces the heating rate coefficient by about 33%,the temperature uniformity is reduced by about 21%.This thesis revealed the multi-scale heat transfer mechanism during the fluidization oxidation heating process of coal tar pitch spheres by numerical method,which can provide a certain reference for the actual process optimization.