On Time-Averaged CFD Modeling of Circulating Fluidized Beds

Abstract

CFD simulations of single-phase flows are regularly performed as steady-state utilizing closure models of varying complexity. On the contrary, dense gas-solid flows are usually computed as time dependent. These simulations commonly require a small time step and a fine mesh resulting in costly and time-consuming computations. In case of large industrial circulating fluidized beds (CFB), the steady-state CFD modeling would be an attractive alternative for the transient simulations, if reliable closure models for the time-averaged transport equations were available. The multiphase closure models developed for time-dependent CFB computations are not as such applicable to the steady-state approach. For instance, the fraction of the momentum transfer expressed by the velocities is significantly smaller in the steady-state models than in the transient ones. Therefore, the steady-state simulations rely more on the closure relations and especially on the models for inter-phase momentum transfer and for the Reynolds stress terms.Several attempts to develop closure models for coarsemesh and steady-state simulations have been presented in the literature. In this paper, a novel steady-state simulation approach for a CFB process and a corresponding CFD model are introduced. A successful steady-state simulation for a test case is presented. Compared to the time-dependent simulations, the computing time is reduced by a factor of an order of 1000.

Historical Review on VTT Fast Pyrolysis Bio-oil Production and Upgrading

“…Thus, alternative approaches for faster but more approximative modeling of fluidized beds have been introduced. At VTT, a fast method for simulation of industrial fluidized bed processes has been developed by Taivassalo et al, 115 whose approach is based on time-averaged transport equations and modeling of the steady-state conditions, thus avoiding integration over process time. This method has been successfully used for solving design and emission problems in circulating fluidized bed (CFB) combustors.…”

catchyFOAM: Euler–Euler CFD Simulations of Fluidized Bed Reactors with Microkinetic Modeling of Gas-Phase and Catalytic Surface Chemistry

“…Time-averaging of the simulations is started when the initial transients in these outlet signals have died out and the outlet temperature and CH 4 mass fraction start to fluctuate around a constant value. The time-averaged solution values are reported, indicating the pseudo-steady-state behavior of the reactor.…”

CFD modeling of gas–solids flow in a large scale circulating fluidized bed furnace

“…The boundary conditions for the modeling were based on the measurements and the mass and energy balance that were determined for the period of June [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]2001, during which the boiler unit was operated in stable conditions at full load ( Table 1). The measured furnace core temperature varied between 970 K and 1300 K and the inlet gas temperature was 560 K. In order to simplify the modeling, but to simulate the expansion of colder inlet gas streams inside the hot furnace, the solid phase was set to a fixed temperature of 1177 K, the inlet gas temperatures were set to 560 K, and the heat transfer between the solids and gas was modeled.…”

“…The industrial scale units are usually modeled by transient Eulerian-Eulerian method, which is the approach used in this study as well. Other possible methods include different semiempirical engineering models [7][8][9], hybrid Eulerian-Lagrangian models [10,11], and time-averaged Eulerian-Eulerian methods [12].…”