Hydrodynamics And Pyrolysis Behavior Of High-volatile Solid Fuels In The Circulating Fluidized Beds

The coal topping process in circulating fluidized beds(CFBs)is beneficial for efficient conversion and utilization of high-volatile solid fuels,such as biomass and low-rank coal.The hydrodynamics and pyrolysis behavior of high-volatile solid fuels are explored by experimental measurement and computational fluid dynamic(CFD)simulation in this thesis.At the particle-scale,the thermal chemical reaction characteristics are investigated during the pyrolysis of lignite and biomass.The direct numerical simulation method is used to predict the fluid-particle interaction for arbitrary shaped particles.A hydrodynamic model of the fluidized bed is established based on the Eulerian-Eulerian two-fluid model,the effect of drag models,including the correlation derived from direct numerical simulation,the correlation from energy-minimization multi-scale method and the correlations based on experimental data,on the computational results is investigated in the three-dimensional fluidized bed,and then 3-D full loop CFD simulation of detailed hydrodynamics is conducted in gas-solids CFBs.Moreover,a comprehensive CFD model,involving continuity equations,momentum equations,energy equations,species transport equations and related constitutive equations,is established to predict the hydrodynamics and pyrolysis behavior of low-rank coal in the fluidized bed.At the particle-scale,individual pyrolysis and co-pyrolysis characteristics of lignite and energy crop are investigated by the thermogravimetric analyzer together with mass spectrometer.The individual decomposition indicates that ener gy grass possesses higher thermochemical reactivity and shorter devolatilization time than lignite.The maximum decomposition rate increases with increasing the heating rate for both energy grass and lignite.The mass spectrometric analysis reveals that th e emission of sulfur dioxide from energy grass is much lower than that from lignite,whilst higher for carbon dioxide and nitrogen dioxide.The co-pyrolysis of energy grass and lignite blend is characterized by two-stage thermal degradation processes,which is dominant by energy grass content in the first stage but lignite in the second stage.The interaction between energy grass and lignite is explored during the co-pyrolysis process under the operational conditions investigated in this study.Moreover,the distributed activation energy model is applied to determine the activation energy for the pyrolysis of energy grass,lignite and their blends.A noteworthy fact is that non-spherical particles are generally involved in gas-particle system.At the particle-scale,Lattice Boltzmann method is coupled with digital particle to predict the fluid-particle interaction for arbitrary shaped particles.In order to validate the reliability of the present approach,simulation of flow past a single stationary spherical,cylindrical or cubic particle is conducted in a wide range of Reynolds numbers(0.1<Rep<3000).The results indicate that the drag coefficient is closely related to the particle shape,especially at high Reynolds numbers.For non-spherical particles,the drag coefficient is more influenced by the particle morphology at moderate or high Reynolds numbers than at low ones.The inclination angle has an important impact on the pressure drag force due to the change of projected area.Moreover,the Stokes flow past regular arranged or random arranged spheres is conducted in this thesis.The simulated results agree well with the experimental data or empirical correlation for both single particle and spherical particles.A fluidized bed system is built to explore the effect of drag model on the CFD simulation of gas-solids flow.The distribution of solids velocity and pressure are obtained by the wire-mesh electrodes and pressure transducers,respectively.A hydrodynamic model of the fluidized bed is established based on the Eulerian-Eulerian two-fluid model with the kinetic theory of granular flow.Effect of the drag model,including the empirical correlation based on experiment,the correlation based on direct numerical simulation and the correlation based on energy minimization multi-scale method,on the computational results is investigated and validated by the experimental data.Both qualitative and quantitative results indicated that the drag model had a significant effect on the flow behavior.The predictions by empirical correlation are in good agreement with the experiments in this thesis.Three-dimensional full loop simulation of the circulating fluidized bed is conducted to predict the gas-solids hydrodynamics,and the riser and bubbling bed are individually operated in fast and bubbling fluidization regions.The simulated results indicate that the operating condition is strict for the stable operation of circulating fluidized bed.The coupled characteristics of the fluidized bed are predominantly identified by the strong effect of operational gas velocity in the riser on the hydrodynamics in the bubbling bed.The effects of operating gas velocity,particle size and total solids inventory on the solids circulation rate are investigated in the circulating fluidized bed.CFD results indicate that the gas velocity in the riser plays a dominant role in controlling the solids circulation rate,whilst the gas velocity in the pot-seal influences in a narrow operating range.Furthermore,a CFD model,considering the gas-solids flow,heat transfer and chemical reaction,is developed to describe the hydrodynamic and pyrolysis behavior of high-volatile solid fuels in the circulating fluidized bed.