Experimental Investigations on Lateral Dispersion Coefficients of Fuel Particles in Large-Scale Circulating Fluidized Bed Boilers with Different Coal Feeding Modes

Jin Yan,Shirong Liu,Xiaofeng Lu,Xiong Zheng,Xiujian Lei,Xuchen Fan,Rui Xue

doi:10.3390/en13236336

Abstract

The dispersion characteristics of fuel particles over the bed have always been a key consideration in the design of fluidized beds. However, the lateral fuel dispersion coefficients (Dsr) reported in the previous literature widely ranged from 10−4 to 10−1 m2/s, which were hard to be directly applied in large-scale CFB (circulating fluidized bed) boilers with wider bed width (close to 30 m) and variable coal feeding modes. To solve this problem, field tests were conducted on the first 600 MW CFB boiler in this paper, and the distributions of flue gas composition and flue gas temperature were obtained near the coal feeding port under four working conditions. The dispersion characteristics of fuel particles were analyzed in detail combined with the dispersion model. In addition, a new method based on bed heat balance was also proposed to estimate Dsr and applied in a 300 MW CFB boiler with a different coal feeding mode. The results revealed that Dsr through the local heat balance method ranged from 0.10–0.35 m2/s, which were 9–18% higher than the values through the dispersion model. Compared with the coal feeding port immersed in the bed, Dsr of the fuel particles fed into the splash zone increased by about 34.7%. Dsr under the coal feeding mode of a screw feeder with pressurized air ranged from 0.12–0.16 m2/s, which were lower than Dsr under the coal feeding method of importing fuel into circulating ash. The method and results in this paper could expand the applicability of fuel dispersion, and provide the most direct guidance for the design and optimization of coal feeding ports in large-scale CFB boilers.

Highlights

In recent decades, the circulating fluidized bed (CFB) boiler has been widely adopted because of the advantages in the treatment of low-grade energy and pollutant control, and is developing towards the direction of large capacity and high parameters [1,2,3,4,5,6,7,8]
Since the center line of the selected coal feeding port was exactly located at the 1500 mm depth, the lateral dispersion process of fuel particles along both sides should be similar, and the measured results showed obvious symmetrical
This indicated that the dense phase zone of a large-scale CFB boiler presented a completely opposite combustion atmosphere at different loads, i.e., it showed an oxidizing condition at low load and the distribution of O2 concentration was determined by that of fuel particles

Summary

Introduction

The circulating fluidized bed (CFB) boiler has been widely adopted because of the advantages in the treatment of low-grade energy and pollutant control, and is developing towards the direction of large capacity and high parameters [1,2,3,4,5,6,7,8]. With the increasing furnace dimensions, a series of non-uniform problems in gas-solid flow and combustion appear to be more and more obviously [9,10]. The uniformity of reactant distribution over the cross-section of the Energies 2020, 13, 6336; doi:10.3390/en13236336 www.mdpi.com/journal/energies. Energies 2020, 13, 6336 bed depends on the reaction rate and lateral particle dispersion. If the dispersion rate is lower than the reaction rate, it shows to be a non-uniform distribution.

Objectives

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Results

Conclusion