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Research Papers: Radiative Heat Transfer

Theoretical Predictions of Spectral Emissivity for Coal Ash Deposits

[+] Author and Article Information
Dong Liu

Key Laboratory of Thermal Science and Power
Engineering of Ministry of Education,
Beijing Key Laboratory for CO2 Utilization
and Reduction Technology,
Tsinghua University,
Beijing 100084, China
e-mail: liu-d10@mails.tsinghua.edu.cn

Yuan-Yuan Duan

Key Laboratory of Thermal Science and Power
Engineering of Ministry of Education,
Beijing Key Laboratory for CO2 Utilization
and Reduction Technology,
Tsinghua University,
Beijing 100084, China
e-mail: yyduan@tsinghua.edu.cn

Zhen Yang

Key Laboratory of Thermal Science and Power
Engineering of Ministry of Education,
Beijing Key Laboratory for CO2 Utilization
and Reduction Technology,
Tsinghua University,
Beijing 100084, China
e-mail: zhenyang@tsinghua.edu.cn

Hai-Tong Yu

Key Laboratory of Thermal Science and Power
Engineering of Ministry of Education,
Beijing Key Laboratory for CO2 Utilization
and Reduction Technology,
Tsinghua University,
Beijing 100084, China
e-mail: yht09@mails.tsinghua.edu.cn

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received March 23, 2013; final manuscript received July 11, 2013; published online March 17, 2014. Assoc. Editor: Zhixiong Guo.

J. Heat Transfer 136(7), 072701 (Mar 17, 2014) (7 pages) Paper No: HT-13-1160; doi: 10.1115/1.4026907 History: Received March 23, 2013; Revised July 11, 2013

Coal ash inevitably forms deposits as combustion residue on the walls and heat transfer surfaces of coal-fired boilers. Ash deposits decrease the boiler efficiency, reduce the generating capacity, and cause unscheduled outages. The radiative heat transfer is the major heat transfer mechanism in utility boilers; thus, the ash deposit emissivity is critical to boiler efficiency and safety. This paper presents a radiative transfer model to predict the spectral emissivities of coal ash deposits. The model includes the effects of the microstructure, chemical composition, and temperature. Typical ash deposit microstructures are generated using diffusion-limited aggregation (DLA). The radiative properties are then calculated using the generalized multiparticle Mie-solution (GMM). The combined GMM and DLA model predicts spectral emissivity better than the original Mie theory and Tien's dependent scattering theory with the average relative difference between predicted results and experimental data decreasing from 17.8% to 9.1% for sample 1 and from 18.6% to 4.2% for sample 2. Maxwell-Garnett (MG) effective medium theory is used to calculate the ash deposit optical constants based on the chemical compositions to include the effect of chemical composition. Increasing temperatures increase the particle diameters and particle volume fractions and, thus, the spectral emissivities. The spectral emissivity ultimately remains constant and less than one. The homogeneous slab model gives the upper limit of the ash deposit spectral emissivity.

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Figures

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Fig. 1

Representative 510 particle aggregate generated using the DLA method

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Fig. 2

Comparison between experimental data and predicted spectral emissivities modeled using the original Mie theory, Tien's dependent scattering theory and the combined GMM and DLA model. (a) Sample 1 and (b) sample 2.

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Fig. 3

Optical constants of each component in sample 1 (the absorption index of CaO is smaller than 10−6 in the present wavelength range and thus not plotted). (a) Real index and (b) absorption index.

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Fig. 4

Comparison of the measured and optical constants predicted using Maxwell-Garnett theory for sample 1. (a) Real index and (b) absorption index.

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Fig. 5

Comparison of spectral emissivities predicted using measured and calculated optical constants for sample 1

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Fig. 6

Estimate effect of particle diameter on the ash deposit spectral emissivities where the particle diameter increases represent the effect of temperature

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