CFD modeling of the hydrodynamics of circulating fluidized bed riser

A two dimensional computational fluid dynamics (CFD) model has been developed to simulate the hydrodynamics of gas-solid flow in a high density circulating fluidized bed (HDCFB) riser using the commercial CFD software, Fluent. The Eulerian-Granular multiphase model was applied, which treats both phases as a continuum while the governing equations of mass and momentum conservation were solved for gas and solid phases. The kinetic theory of granular flow was used to provide the closure relations for the governing equations for the solid phase. CFD modeling of the isothermal multiphase flow of air and fluid catalytic cracking (FCC) particles in a circulating fluidized bed (CFB) riser has been performed and compared to the experimental findings of particle volume fraction, particle axial velocity, and local particle solid flux profiles reported by J. Liu (Liu, PHD thesis, 2001 Department of Chemical and Biological Engineering, The University of British Colombia). The simulated profiles were overall in good qualitative agreement with the experiments, while similarly, the simulated particle axial velocities were in good quantitative and qualitative agreement with the experiments. However, due to the difficulties in modeling the solid segregation toward the wall accurately, the solid volume fraction was under predicted near the walls. The effect of different drag models including Gidaspow, Arastoopour, and Syamlal and O’Brien drag models on modeling results was investigated. All the drag models predicted quite similar flow hydrodynamics; however, the Syamlal and O’Brien drag model, which was modified based on the minimum fluidization velocity of the applied FCC particles, indicated better predictions of the solid volume fraction profiles at the core area. Different wall restitution coefficient values and solid slip conditions have been applied to study their effects on solid volume fraction distribution across the riser. While the wall restitution coefficient did not exhibit a significant effect on the riser hydrodynamics, the appropriate slip condition aided in predicting the solid segregation toward the wall. Using the free solid slip condition resulted in a better agreement with the experimental data of the solid volume fraction distribution near the walls. Finally, the model was evaluated comprehensively by comparing its predictions with experimental results reported for a circulating fluidized bed riser operating at a solid mass flux in the range of 94 to 550 kg/m²s and a superficial gas velocity in the range of 4 to 8 m/s. The model was capable of predicting the main gas-solid flow features in the HDCFB riser operating at a solid mass flux in the range of 254 to 455 kg/m²s. However, the model was incapable of accurately predicting the gas-solid flow behavior in a low density circulating fluidized bed riser with a solid mass flux of 94 kg/ m²s and risers operating in dense suspensipon up-flow regime with a solid mass flux of 550 kg/ m²s.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat

Multifluid modeling of the desulfurization process within a bubbling fluidized bed coal gasifier

The desulfurization process to a two-dimensional (2-D) and three-dimensional (3-D) Eulerian-Eulerian computational fluid dynamic (CFD) model of a coal bubbling fluidized gasifier is introduced. The desulfurization process is important for the reduction of harmful SOx emissions; therefore, the development of a CFD model capable of predicting chemical reactions involving desulfurization is key to the optimization of reactor designs and operating conditions. To model the process, one gaseous phase and five particulate phases are included. Devolatilization, heterogeneous, and homogeneous chemical reactions as well as calcination and desulfurization reactions are incorporated. A calcination-only model and a calcination plus desulfurization model are simulated in 2-D and 3-D and the concentrations of SO2 leaving the reactors are compared. The simulated results are assessed against available published experimental data. The influence of the fluidized bed on the desulfurization is also considered

The influence of multiple tubes on the tube-to-bed heat transfer in a fluidised bed

There have been few studies modelling both flow and heat transfer in fluidised beds. The kinetic theory of granular flow (KTGF) has been used for flow prediction in the past without heat transfer modelling. In the present study, a two-fluid Eulerian-Eulerian formulation incorporating the KTGF was applied first to a tube-to-bed reactor with one immersed tube and compared with the results in the literature. The bed was then modified to introduce two and three heated tubes. The effects on the flow and temperature distribution, local heat transfer coefficients and averaged heat transfer coefficients over a 3.0s time period were carried out. Results showed that increasing the number of tubes promotes heat transfer from tubes to the particles and flow. The heat transfer coefficients extracted from the single-tube to three-tube cases were analyzed in detail, confirming the importance of linking flow/particle and heat transfer calculation

Effects of limestone calcination on the gasification processes in a BFB coal gasifier

An Eulerian-Eulerian computational fluid dynamics (CFD) model of the gasification processes in a coal bubbling fluidised bed (BFB) is presented incorporating the devolatilisation, heterogeneous, homogeneous reactions and limestone calcination. The model considers separate phases for the coal, limestone and char and is carried out for different experimental conditions taken from the literature.The results of the exiting gas compositions have been averaged over time and validated with experimental data. The hydrodynamic behaviour as well as temperature and reaction distributions within the bed are presented. The impact of limestone calcination on the gaseous composition is observe