Solids back-mixing in the transport zone of circulating fluidized bed boilers

Tove Djerf,David Pallarès,Filip Johnsson,Gaetano Sardina,Henrik Ström

doi:10.1016/j.cej.2021.130976

Abstract

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.

Highlights

The mitigation of anthropogenic global warming and the need for a reliable energy supply are key drivers of technical developments in the heat and power sectors
The back-mixing of solids in the transport zone of large-scale Circulating fluidized bed (CFB) boilers is explored by combining modeling and experimental data from the literature, for 10 different large-scale CFB boilers
The literature data exhibit variability regarding the solids concen tration profile of the transport zone, including how the concentration falls off with height in the furnace, which should be related to the low number of pressure taps in commercial CFB furnaces

Summary

Introduction

The mitigation of anthropogenic global warming and the need for a reliable energy supply are key drivers of technical developments in the heat and power sectors. Higher levels of efficiency, larger shares of renewables, and the implementation of carbon capture and storage processes are among the necessary measures for meeting the goals set in the Paris Agreement [1]. In this context, fluidized bed combustion is a widely used technology with strong fuel flexibility [2,3]. While considerable efforts have been made to develop models from first principles, CFD modeling is still not an efficient tool for describing largescale CFB combustion This is a consequence of either highly uncertain formulations of some terms in the momentum equation (for the Eulerian description of the particulate phase) or limitations linked to computing capacity (for the Lagrangian approach) [11]. The development of CFB combustion relies heavily on semi-empirical models that ensure closure of the mass and heat balances at the macroscopic scale with the support of observations derived from large-scale measurements

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