Particle population balance model for a circulating fluidized bed boiler

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Abstract

A two-dimensional particle population balance model has been developed for the particle size and density distributions in a circulating fluidized bed (CFB) boiler furnace based on analysis of particle properties and the core–annulus hydrodynamic model. The model, incorporating modules to consider fuel particle fragmentation, char combustion, particle attrition and gas–solid separation, is part of an overall model developed earlier by the authors to simulate the operation of a 12 MW CFB boiler. The model predictions for particle population in a CFB furnace agree well with the measurement data.

Introduction

The complex hydrodynamics and char combustion behavior of circulating fluidized bed (CFB) boilers have not been fully understood. The properties and size distribution of particles have significant influence on the hydrodynamics and combustion behavior in the CFB furnace [1], [2], [3]. While a number of models of particle size distribution in fluidized beds have been reported, little is known of the particle size distribution in circulating fluidized bed boilers [4], [5], [6], [7], [8], [9]. Most of the published models for the particle population balance in CFB boilers do not consider some important processes occurring in the furnace, such as char burning and attrition while the particles are traveling in the primary loop, particles with different properties undergo different physical and chemical processes. These previous models still have some limitations in describing the particle properties and population balance in the CFB boilers [7], [8].

The present work is intended to develop a particle population balance model for circulating fluidized beds based on the experiments completed in a 12 MW CFB boiler, taking into account major physical and chemical processes that occur in CFB boilers.

Section snippets

Particle properties in CFB furnace

The solids inventory in a CFB furnace consists of spent and reacting fuel and sorbent particles and inert bed materials. The properties of feed particles have significant influence on the particle population balance in a CFB furnace. Additional bed material, like river sand, may be needed when a fuel with low ash content is fired. Even for a given fuel, properties such as the ultimate analysis may be subject to variations from time to time. In some cases, since the minerals content differs from

Particle population balance model

It has been widely accepted that a CFB furnace may be characterized by two flow regimes: a dense bed at the bottom and a dilute region above the solid entry or secondary air inlet. Because there are great differences in the hydrodynamics between the dense bed and the dilute region, the particle population models for the two respective regimes should be developed separately.

Fragmentation of coal particles

Previous studies suggested that primary fragmentation of coal particles in a fluidized bed occurred within a few seconds after injection of the particles into the bed due to build-up of thermal and devolatilization-induced stresses [7], [9]. It may be assumed that primary fragmentation occurs primarily in the zones close to the feed ports. An empirical correlation was proposed by Bellgardt et al. [9]Pnew(dp,new)=Pold(dp,newkf−1/3)kf1/3.

Bellgardt et al. [9] suggest that the fragmentation

Model validation

The overall mathematical model of the CFB boiler has been developed by combining the above-discussed models together with sub-models for other physical and chemical processes [26], [27]. The overall model is used to predict the performance of a 12 MW CFB boiler. The bituminous coal-fired boiler is the first of the type operational in China, which employs an external heat exchanger developed by Zhejiang University. The furnace is 21 m high and has a square cross-section measuring 5.45m×2.45 m in

Conclusions

A particle population balance model for the CFB boiler furnace has been developed. The core–annulus model is employed to describe the hydrodynamics of the circulating fluidized bed furnace. The particles present in the CFB furnace are characterized and grouped by their size and density. The model considers particle fragmentation, char combustion, attrition and gas–solid separation. The model is integrated into a comprehensive model of a 12 MW CFB boiler developed earlier by the authors for

Acknowledgements

The authors gratefully acknowledge the financial support from the Special Funds for China Major State Basic Research Projects (Grant G1999-022105).

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