Approximating a Three-Dimensional Fluidized Bed With Two-Dimensional Simulations

Abstract

Fluidized beds can be used to gasify biomass in the production of producer gas, a flammable gas that can replace natural gas in process heating. Modeling these reactors with computational fluid dynamics (CFD) simulations is advantageous when performing parametric studies for design and scale-up. From a computational resource point of view, two-dimensional simulations are easier to perform than threedimensional simulations, but they may not capture the proper physics. This paper will compare two-and three-dimensional simulations in a 10.2 cm diameter fluidized bed with side air injection to determine when two-dimensional simulations are adequate to capture the bed hydrodynamics. Simulations will be completed in a glass bead fluidized bed operating at 1.5Umf and 3Umf , where Umf is the minimum fluidization velocity. Side air injection is also simulated to model biomass injection for gasification applications. The simulations are compared to experimentally obtained time-averaged local gas holdup values using X-ray computed tomography. Results indicate that for the conditions of this study, two-dimensional simulations qualitatively predict the correct hydrodynamics and gas holdup trends that are observed experimentally for a limited range of fluidization conditions.

Towards accurate three-dimensional simulation of dense multi-phase flows using cylindrical coordinates

“…Both these phenomena act simultaneously and result in excess solid holdup at the center of the bed. Although bed parameters like the pressure drop and bed height are still predicted in reasonable agreement with measurements [9,24], the predicted hydrodynamics at the center of the bed do not compare well.…”

“…Using the 2D cylindrical coordinate system necessitates the use of an axi-symmetric boundary condition, which does not represent a real fluidized bed accurately. Therefore, several researchers have worked with the 3D cylindrical coordinate system [9,[22][23][24][25][26]. Interestingly, modeling a 3D cylindrical bed as an axial slice around the axis of the bed using 2D Cartesian coordinates has predicted bed parameters in good agreement with experimental data for bubbling fluidization.…”

“…However, with increasing inlet superficial gas velocity, there is significant lateral flow variation and hence, increasing disparity between the 2D Cartesian and 3D Cylindrical simulations [22]. Such reduced-dimension scaling may also not be possible for reactive simulations [23] or significantly asymmetric geometries [24]. With growing emphasis on mixing and segregation studies, 2D beds should not be used to validate numerical models.…”

Effects of Mixing Using Side Port Air Injection on a Biomass Fluidized Bed

“…Previous work of Xie et al [20] and Deza et al [21,22] have shown very good agreement using a 2D approach for a cylinder reactor with no side air injection when the flow is limited to the bubbling regime for Geldart B particles. To examine the influence of side port air injection, glass beads were used for the bed material in the preliminary studies by Deza et al [23] and Min et al [24] because the properties of glass are well-characterized. The simulations for both 2D and 3D representations of the reactor compared well with the experiments.…”

Mixing and segregation in 3D multi-component, two-phase fluidized beds

“…Studies have also been conducted using computational fluid dynamics models (CFD) for fluidized beds [3,[7][8][9][10][11][12][13]. However, since experimental data is very limited, the accuracy of these models is too.…”