Numerical Modeling Of Coal Pyrolysis In Externally-heated Fixed-bed/moving-bed Pyrolyzer With Internals

Pyrolysis is one of the important technologies for the use of low-rank coals such as sub-bituminous and lignite,which is particularly important for China.For more than 55%of China's total coal reserve is low-rank coal.However,there are still no successful low-rank coal pyrolysis technologies industrially and economically feasible at home and abroad,which is one of the hottest topics in the research of coal conversion science and technology.A novel indirectly heated moving bed pyrolyzor with specially designed internals is being developed in Institute of Process Engineering,Chinese Academy of Sciences.In this pyrolyzor,internals are mounted to increase the heating rate and regulate the flow of the gaseous products so as to achieve high tar yield and good qualities.A better understanding of the flow behavior of gaseous products,the heat transfer characteristics and the pyrolysis reaction details is critical to the design and scale-up of this pyrolyzor technology.In this work,numerical simulations were conducted to explore the mechanisms of coal pyrolysis reaction,characterize the thermal,hydrodynamic and reaction performance,and understand the regulating principle,so as to guide the future scale-up,optimization and engineering design of this technology.The main research contents and conclusions of this work are as follows:(1)Investigation of the mechanisms of coal pyrolysis.Based on the molecular model of Shinn sub-bituminous coal,the pyrolysis process was simulated though reactive molecular dynamic(ReaxFF MD).The effects of pyrolysis temperature and heating rate on the distribution of pyrolysis products were firstly investigated.It was found that the degree of pyrolysis increased with the increase of pyrolysis temperature.Increasing pyrolysis temperature also strengthened the cracking of tar,leading the fact that tar yield first increased and then decreased.For same targeted pyrolysis temperature,lower heating rate led to serious secondary reaction of tar at later high temperature stage,whereas higher heating rate might lead to incompletion of the primary pyrolysis reaction.Thus,with the increase of heating rate,the tar yield presented a fist increase and then decrease trend.By dividing the temperature-rising process into three stages,the bond-breaking rules and the formation detail of each product was analyzed.It was found that the pyrolysis process of the investigated coal model could be summarized as follow:At the initial pyrolysis stage,the bridge bond such as-C-O-and heteroatom functional groups broke and formed heavy tar and smaller moleculars such as CO2,H2O,CH4.At the fast pyrolysis stage,a large amount of aromatic side chains and cyclic hydrocarbons of the coal model broken and formed large amount of tar and pyrolysis gas.The tar yield at this stage is highest.The third stage is the tar secondary reaction stage.At this stage,the cracking of tar dominated and formed pyrolysis gas and lighter tar.If the temperature was high enough,cross-linking condensation reactions of aromatic ring structure took place,leading to the generation of semicoke.The amount of each pyrolysis product and the generation order were determined by the amount and the bond energies of the corresponding functional group.(2)Evaluation and identification of heat transfer models.A variety of thermal conduction and radiation models for particle-packed beds reported in literature were evaluated and discussed.By comparing the results predicted by the models with the experimental data collected from literature,the sensitiveness of each model to particles size,temperature,emissivity and bed voidage was evaluated.It was found that the Zehner-Bauer-Schlunder(ZBS)model for thermal conduction and Breitbach-Barthels(B-B)model for radiative heat transfer presented the best prediction and broader application region.The accuracy of the identified heat transfer models was illustrated through the comparisons between the CFD simulation results and the temperature rising data measured from the quartz sand heating experiments.(3)CFD modeling of coal pyrolysis in fixed-bed reactor.By facilitating with heat transfer models,water evaporation-condensation model and coal pyrolysis reaction kinetics model,an Eulerian-Eulerian two-fluid simulation framework was built to simulate the coal pyrolysis behavior in fixed bed reactor.The simulation was first validated by comparing the predicted temperature evolution with experimental results.Simulation results indicated that the condensation of vapor formed in the near wall high temperature region led to the appearance of constant temperature platform in the radial interior region,and subsequently postponed the pyrolysis of coal in that region.Increasing the heating rate led to the intenser release of volatiles.The porosity of the bed increased with the evaporation of water and the release of volatile.Due to nonuniform distribution of bed temperature,the bed porosity presented clear nonuniform distribution in the radial direction,which subsequently led to the phenomenon that the generated gaseous products tended to flow through the porous zone adjacent to the heating wall,this would lead to a decrease of pyrolysis tar yield.The results showed the coupling effects of temperature-reaction-flow in the coal pyrolysis bed.(4)Influences of internals.The influences of internals on the pyrolysis performance were investigated by considering the secondary reaction of produced tar in the CFD simulations.The simulation results showed that installing central gas gathering pipe strengthened the contribution of convection heat transfer,and mounting heat transfer plates enhanced the contribution of radiative heat transfer.And both were in favor of increasing the heating rate along radial direction.Relatively,mounting heat transfer plate was more efficient in increasing heating rate.Higher heating rate favored the produce of tar.Thus,for furnace temperature of 973 K,due to the relatively weaker secondary reaction of tar,mounting heat transfer plates was more efficient in increasing tar yield.Nevertheless,for furnace temperature of 1173 K,compared with central gas gathering pipe,the tar yield was notably lower when heat transfer plates were mounted.This was due to the fact that the cracking of tar was intense under this temperature.Because the primary gaseous products generated in the relatively lower temperature zone tended to flow though the higher temperature near heating wall and plates zone before flowing out of the reactor,the yield of tar was significantly reduced.Installing central gas gathering pipe changed the flow direction of primary gaseous products:they tended to flow from the near wall high temperature zone to central low temperature zone,thus notably alleviated the cracking of tar and increased tar yield.The reactor integrated with the heat transfer plates and the gas gathering pipe not only increased the yield of primary tar through a significant increase in the heating rate,but also reduced the secondery cracking of tar through the regulation of primary gaseous products's flow path.Thus,this type of pyrolyzor gave the highest tar yield.(5)Numerical modeling and optimization of pilot scale moving bed pyrolyzor with internals.The temperature field,reaction field,flow structure inside the reactor and the distribution of pyrolysis products were detailedly investigated though CFD simulations,it was found that slightly increasing coal particle size and heating furnace temperature and decreasing the mass flow rate of coal charge were in favor of strengthening heat transfer and increasing tar yield.The influence of the internals was investigated and the results showed that compared with the traditional moving bed pyrolyzor,mounting internals(heat transfer plates and central gas gathering channel)could strengthen heat transfer,decrease bed height and increase tar yield.All these results clearly illustrate the advantage of,the newly developed pyrolyzor.Simulation results also suggest that the number of and the distance between heat transfer plates need to be carefully optimized.Too many plates or closer distance might lead to the decrease of tar yield.