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A Thermoelectric Waste-Heat-Recovery System for Portland Cement Rotary Kilns

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Abstract

Portland cement is produced by one of the most energy-intensive industrial processes. Energy consumption in the manufacture of Portland cement is approximately 110–120 kWh ton−1. The cement rotary kiln is the crucial equipment used for cement production. Approximately 10–15% of the energy consumed in production of the cement clinker is directly dissipated into the atmosphere through the external surface of the rotary kiln. Innovative technology for energy conservation is urgently needed by the cement industry. In this paper we propose a novel thermoelectric waste-heat-recovery system to reduce heat losses from cement rotary kilns. This system is configured as an array of thermoelectric generation units arranged longitudinally on a secondary shell coaxial with the rotary kiln. A mathematical model was developed for estimation of the performance of waste heat recovery. Discussions mainly focus on electricity generation and energy saving, taking a Φ4.8 × 72 m cement rotary kiln as an example. Results show that the Bi2Te3–PbTe hybrid thermoelectric waste-heat-recovery system can generate approximately 211 kW electrical power while saving 3283 kW energy. Compared with the kiln without the thermoelectric recovery system, the kiln with the system can recover more than 32.85% of the energy that used to be lost as waste heat through the kiln surface.

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Abbreviations

A :

Cross-sectional area of single thermocouple (m2)

A cold :

Heat transfer area of passive heat sink (m2)

d te :

Diameter of the thermoelectric recovery system (m)

\( h_{\rm{gap}}^{\rm{c}} \) :

Convective heat transfer coefficient from the rotary kiln towards the heat collection plate (W m−2 K−1)

h c :

Convective heat transfer coefficient of the passive heat sink to ambient air (W m−2 K−1)

I i :

Electric current (A)

L :

Height of the thermoelectric module (m)

m :

Side number of the inscribed polygon in an annular element

N te,i :

Number of the thermoelectric module in a annular volume

n :

Number of the thermocouple in a module

P te :

Total power output of the thermoelectric modules (W)

Pr :

Prandtl number of air

Q h,i :

Heat absorbed by the thermoelectric module in an annular volume (W)

Q c,i :

Heat rejection of the thermoelectric module in an annular volume (W)

Q ch,i :

Heat flow from the rotary kiln towards the heat collection plate by convection (W)

Q rh,i :

Heat flow from the rotary kiln towards the heat collection plate by radiation (W)

Q cc,i :

Heat flow from the passive heat sink to the ambient air by convection (W)

Q rc,i :

Heat flow from the passive heat sink to the ambient air by radiation (W)

Q in :

Total input thermal energy of the thermoelectric modules (W)

Q out :

Total heat loss in the TEG system (W)

Q loss :

Heat loss of the bare kiln (W)

Q save :

Energy saved in the TEG system (W)

R shell,i :

Equivalent thermal resistance to the kiln shell and the refractory (KW−1)

R M,i :

Total internal resistance of the thermoelectric module \( \left( \Omega \right) \)

R load :

Load resistance \( \left( \Omega \right) \)

Re:

Reynolds number based on the TEG system

r kiln :

Internal radius of the kiln (m)

r brick :

Inner radius of the refractory (m)

r shell :

Outer radius of the kiln (m)

T kiln,i :

Temperature of the kiln’s inner surface (K)

T surface,i :

Temperature of the kiln’s outer surface (K)

T hot,i :

Hot side temperature of the thermoelectric module (K)

T cold,i :

Cold side temperature of the thermoelectric module (K)

T amb :

Ambient temperature (K)

Ta:

Taylor number based on gap

t gap :

Distance between the surface kiln and the heat collection plate (m)

V wind :

Wind speed of environment (ms−1)

W :

Width of the thermoelectric module (m)

x :

Axial length of the annular volume (m)

α p ,α n :

Seebeck coefficient of the thermoelectric material (VK−1)

α M,i :

Seebeck coefficient of the thermoelectric module (VK−1)

δ :

Gap distance from the near module to the next (m)

ɛ surface :

Emissivity of the kiln shells

ɛ hot :

Emissivity of the heat collection plate coating

ɛ cold :

Emissivity of the passive heat sink coating

η :

Electrical conversion efficiency of the TEG system

λ shell :

Thermal conductance of the kiln shell (W(mK)−1)

λ brick :

Thermal conductance of the kiln refractory (W(mK)−1)

λ M,i :

Thermal conductance of the thermoelectric module (W(mK)−1)

λ air :

Thermal conductivity of air (W(mK)−1)

λ p ,λ n :

Thermal conductance of the thermoelectric material (W(mK)−1)

ρ p ,ρ n :

Electrical resistance of the thermoelectric material \( \left( {\Omega{\hbox{m}}} \right) \)

σ :

Stefan Boltzmann constant \( \left( {\sigma = 5. 6 7\times 10^{ - 8} {\hbox{W m}}^{ - 2} {\hbox{ K}}^{ - 4} } \right) \)

υ:

Kinematic viscosity of air

φ:

Total energy-saving ratio

φ te :

Energy savings ratio due to the electricity generation

φ insulation :

Energy savings ratio due to secondary shell of TEG

ω :

Rotation velocity of the kiln (r(min−1))

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Acknowledgements

This work was financially supported by National Natural Science Foundation of China (no. 51272198), the National High-tech R&D Program of China (863 program, no. 2012AA051104), the International S&T Cooperation Program of China (2014DFA63070), Fundamental Research Funds for the Central Universities (WUT, nos 2014-VII-009 and 2014-zy-063).

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Authors and Affiliations

  1. School of Sciences, Wuhan University of Technology, Wuhan, 430070, China

    Qi Luo, Pengcheng Zhai & Qingjie Zhang

  2. School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan, 430070, China

    Peng Li, Lanlan Cai, Pingwang Zhou & Di Tang

Authors
  1. Qi Luo
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  2. Peng Li
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  3. Lanlan Cai
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  4. Pingwang Zhou
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  5. Di Tang
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  6. Pengcheng Zhai
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  7. Qingjie Zhang
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Correspondence to Peng Li.

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Luo, Q., Li, P., Cai, L. et al. A Thermoelectric Waste-Heat-Recovery System for Portland Cement Rotary Kilns. J. Electron. Mater. 44, 1750–1762 (2015). https://doi.org/10.1007/s11664-014-3543-1

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  • DOI: https://doi.org/10.1007/s11664-014-3543-1

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