Hydrodynamic Simulation And Pyrolysis Kinetics In A Downer Pyrolyzer For Low Rank Coal

Nowadays,efficient utilization of low rank coal receives more and more attention due to the excessive consumption of high rank coal.Numerous studies have shown that high-efficiency pyrolysis technology is one of the most promising technologies for making full use of low rank coal.The pyrolysis is a very fast reaction with the intermediates as the desired products,which strictly requires the uniform short contact time,therefore,downer was selected as the low rank coal pyrolyzer.Considering the significant role played by computer simulation in the modern chemical engineering industry,this work was mainly carried out from the perspective of numerical calculation.Firstly,based on the concept of volumetric flux,the variation mechanism of the solids holdup in the downer was analyzed.And then,combined with the specific characteristics of the pyrolysis reaction and the actual experimental conditions,the reactor structure was modified to a conical downer,and the corresponding drag model and pyrolysis kinetic model were also established.It is expected to provide effective guidance for the improvement of reactor structure and optimization of operating conditions.The solids holdup achieved in the downer is generally very low,resulting in low heat transfer rate and poor reaction efficiency.Thus,it is necessary to investigate the variation mechanism of solids holdup in the downer to provide some suggestions to guide the enhancement of the solids holdup in a practical process.To more reasonably investigate the solids holdup variation,this work proposed the concept of volumetric flux,which can avoid the overestimation of particle density effect on the solids holdup caused by using mass flux as the comparison base.Meanwhile,this concept also unified the physical meaning of solids flux and operating gas velocity.It is also found that using this concept can better define the high density operation in the downer.By using large numbers of experimental data in response surface methodology analysis,it is found that the effect extent of different factors on the solids holdup was in the order of solids flux > gas velocity >> particle size > particle density,indicating that the most effective way to increase the solids holdup was to increase the solids flux.To further increase the solids holdup when solids flux reaches the bottleneck,the conical downer was proposed in this work by combining the characteristics of coal pyrolysis reaction.The corresponding study was carried out by using computational fluid dynamics(CFD)numerical simulation.The results demonstrate that the conical downer can significantly increase the solids holdup,and is beneficial to the realization of high-density operation.Radial simulation results show that there was also a dense ring in the conical downer,which is more obvious than that in the cylindrical downer.In addition,according to the analysis of the axial distributions of pressure and velocity,the unique hydrodynamics of three axial acceleration regions were found and characterized,which was different from cylindrical downers.Finally,the detailed comparison between cylindrical downer and conical downer are conducted,and it can be found that the cylindrical downer is more suitable for the rapid reaction with gas phase as the reactant,and the conical downer is more suitable for the rapid reaction with solids phase as the reactant,such as coal pyrolysis reaction.The particles used in the actual cold state experiment have a wide particle size distribution(PSD),which leads to the overestimation of the solids holdup by the standard two-fluid model(TFM).To address this issue,a drag model considering the PSD was built here.In this model,the real control volume was divided into a number of sub-grids characterized by different characteristic particle sizes,and the total drag force was obtained by the sum of drag forces of all the sub-grids.The investigation of the subgrid drag force shows that it is necessary to separately consider the slip velocity for each sub-grid to obtain reasonable results.The simulation results obtained by coupling this drag model with TFM show that using this model can more accurately predict the solids holdup and its axial distribution.Also,the unique three-stage flow pattern in the downer can be clearly captured.Compared with other drag models,this model provided a new idea for the construction of drag models.To optimize the operating conditions of the reaction process,and facilitate the hot state CFD numerical simulation in the future,a pyrolysis kinetic model based on the general Arrhenius equation was built in this work,and was further coupled with the heat transfer and mass transfer equations to build a particle scale pyrolysis model.The pyrolysis kinetic model was based on assumptions of multiparallel first order reaction.In this work,the pyrolysis kinetic information of four low rank coal was investigated by fitting thermogravimetric experimental data.Compared with the widely used distribution of activation energies model(DAEM),this model can predict the pyrolysis process more accurately.And since it avoided the complicated integral calculation,the calculation consumption was also significantly reduced,potentially making it fit for being coupled with CFD.It is also found that the same kinetic parameters and different reaction weights can be used to predict the pyrolysis process of different low rank coal,partially verifying the similarity of pyrolysis reactions of different low rank coal.The calculation results of particle pyrolysis model show that the fluidized bed reactor was suitable as a pyrolyzer,and it was recommended that the coal particle size should be smaller than 3 mm.