Solids back-mixing in the transport zone of circulating fluidized bed boilers
Journal article, 2022

This work investigates the back-mixing of solids in the transport zone of large-scale circulating fluidized bed (CFB) boilers, with the aims of identifying and evaluating the governing mechanisms and providing a mathematical description based on a solid theoretical background rather than on purely empirical correlations. In addition, transient Direct Numerical Simulation (DNS) modeling is used to identify the mechanism that drives migration of the solids from the dilute up-flow in the core region to the down-flow at the furnace walls. Previously published concentration and pressure profiles are collated and analyzed through modeling of the steady-state mass balance of the dispersed solids in the transport zone. The study shows that solids back-mixing at the furnace wall layers is limited (hence governed) by the core-to-wall layer mass transfer transport mechanism rather than by the lateral movement of solids within the core region. The latter is shown by the 3-dimensional (3D) mass balance model, and the transient DNS modeling indicates that this is due to a turbophoresis mechanism. We also show that the use of Pe-numbers to describe the lateral solids dispersion is not straightforward but rather depends on the unit scale, and that Pe-numbers < 26 are needed to yield the solids back-mixing rates measured in large-scale CFB boilers. Finally, we propose a mathematical expression for the core-to-wall layer mass transfer coefficient derived from a Sherwood number (Sh)-correlation fitted to measured values of the characteristic decay constant that result from the solids back-mixing. This expression shows better agreement with the large-scale measurements than do the expressions given in the literature.

Lateral dispersion

Solids back-mixing

Circulating fluidized bed

Deposition velocity

Author

Tove Djerf

Chalmers, Space, Earth and Environment, Energy Technology, Energy Technology 2

David Pallarès

Chalmers, Space, Earth and Environment, Energy Technology

Filip Johnsson

Chalmers, Space, Earth and Environment, Energy Technology

Gaetano Sardina

Chalmers, Mechanics and Maritime Sciences, Fluid Dynamics

Henrik Ström

Chalmers, Mechanics and Maritime Sciences, Fluid Dynamics

Chemical Engineering Journal

1385-8947 (ISSN)

Vol. 428 130976

Subject Categories

Water Engineering

Ocean and River Engineering

Fluid Mechanics and Acoustics

DOI

10.1016/j.cej.2021.130976

More information

Latest update

8/6/2021 2