Investigation of Heat Transfer in a Large-Scale External Heat Exchanger with Horizontal Smooth Tube Bundle

Artur Blaszczuk,Szymon Jagodzik

doi:10.3390/en14175553

Abstract

In the research work, energy transport between a dense fluidized bed and submerged horizontal tube bundle is analyzed in the commercial external heat exchanger (EHE). In order to investigate the heat transfer behavior, the authors carried out eight performance tests in a fluidized bed heat exchange chamber with a cross-section of 2.7 × 2.3 m in depth and width and a height of 1.3 m. The authors have been developing a mechanistic model for the prediction of the average heat transfer coefficient, which includes the effect of the geometric structure of the tube bundle and the location of the heat transfer surface on the heat transfer rate. The computational results depict that the average heat transfer coefficient is essentially affected by superficial gas velocity and suspension density rather than bed particle size. The empirical correlations have been proposed for predicting heat transfer data since the existing literature data is not sufficient for industrial fluidized bed heat exchangers. On the basis of the evaluated operating conditions of an external heat exchanger, the optimal conditions where heat transfer occurs could be deduced. The developed mechanistic heat transfer model is validated by experimental data under the examined conditions.

Highlights

Fluidized bed combustion technology is the most successful technology for heat and power generation from multi-fuels, such as: fossil fuels, peat, biomass and municipal waste with some challenges
It has been observed that the diffusion, jointly with the dynamic development of circulating fluidized bed (CFB) boilers around the world, has led to the increase in capacity of single-unit CFB boilers [1,2,3]
In the bubbling fluidized bed (BFB) literature, the staggered tube arrangement is primarily chosen for the studies on heat transfer characteristics, as discussed in [13]

Summary

Introduction

Fluidized bed combustion technology is the most successful technology for heat and power generation from multi-fuels, such as: fossil fuels (lignite, bituminous coal, anthracite, sub-bituminous coal and petroleum coke), peat, biomass (agro, wood) and municipal waste with some challenges. The higher steam pressure in subcritical PCC plants results in lower energy efficiency of 33%, compared with 45% for modern CFB combustion systems. The operating temperature of the CFB boiler varies between 760 and 870 ◦ C, and it is half of the temperature inside conventional PC boilers (1300–1700 ◦ C). This relatively low temperature is below the threshold where thermally induced NOx forms. CFB boilers can utilize coal with high ash content in contrast to conventional pulverized coal units, which have to limit ash to a relatively low level. In the bubbling fluidized bed (BFB) literature, the staggered tube arrangement is primarily chosen for the studies on heat transfer characteristics, as discussed in [13]

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Conclusion