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Numerical Investigation of the Effect of C/O Mole Ratio on the Performance of Rotary Hearth Furnace Using a Combined Model

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Abstract

In a rotary hearth furnace (RHF) the direct reduction of composite pellets and processes of heat and mass transfer as well as combustion in the chamber of RHF influence each other. These mutual interactions should be considered when an accurate model of RHF is established. This paper provides a combined model that incorporates two sub-models to investigate the effects of C/O mole ratio in the feed pellets on the reduction kinetics and heat and mass transfer as well as combustion processes in the chamber of a pilot-scale RHF. One of the sub-models is established to describe the direct reduction process of composite pellets on the hearth of RHF. Heat and mass transfer within the pellet, chemical reactions, and radiative heat transfer from furnace walls and combustion gas to the surface of the pellet are considered in the model. The other sub-model is used to simulate gas flow and combustion process in the chamber of RHF by using commercial CFD software, FLUENT. The two sub-models were linked through boundary conditions and heat, mass sources. Cases for pellets with different C/O mole ratio were calculated by the combined model. The calculation results showed that the degree of metallization, the total amounts of carbon monoxide escaping from the pellet, and heat absorbed by chemical reactions within the pellet as well as CO and CO2 concentrations in the furnace increase with the increase of C/O mole ratio ranging from 0.6 to 1.0, when calculation conditions are the same except for C/O molar ratio. Carbon content in the pellet has little influence on temperature distribution in the furnace under the same calculation conditions except for C/O mole ratio in the feed pellets.

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Abbreviations

a g :

Absorption coefficient for gas medium

C 1ɛ , C 2ɛ , C 3ɛ , C μ :

Constants in standard k–ɛ model

C pm :

Average specific heat at constant pressure, J kg−1 K−1

\( C_{\text{CO}} ,C_{{{\text{CO}}_{2} }} \) :

Concentrations of CO, CO2 in the pellet pores, mol m−3

D effj :

Effective diffusivity of gas species j, m2 s−1

D i,m :

Diffusion coefficient for species i in the mixture, m2 s−1

E :

Total energy per unit mass, J kg−1

E a :

Activation energy, J mol−1

G b :

Production of turbulent kinetic energy by buoyancy, J m−3 s−1

G k :

Production of turbulent kinetic energy by velocity gradient, J m−3 s−1

g i :

Component of the gravitational vector in the ith direction, m s−2

ΔH i :

Reaction heat of the ith reaction, J mol−1

K :

Equilibrium constant

k :

Turbulent kinetic energy, m2 s−2

k eff :

Effective thermal conductivity, W m−1 K−1

k g :

Thermal conductivity of gas phase, W m−1 K−1

k s :

Thermal conductivity of solid phase, W m−1 K−1

k 0 :

Pre-exponential constant, m s−1

M j :

Molecular weight of gas species j, kg mol−1

n :

The number of the particles per unit volume of the pellet, m−3

P :

Total pressure inside the pellet, atm

p :

Pressure, Pa

Pr  t :

Turbulent Prandtl number

R :

Universal gas constant, J mol−1 k−1

R i :

Reaction rate of the ith reaction, mol m−3 s−1

Re:

Reynolds number

r :

Distance from local point to the center of the pellet, m

r p :

Average radius of the particles, m

Sc:

Schmidt number

S chem :

Source term of heat of chemical reaction, J m−3 s−1

Sct :

Turbulent Schmidt number

S f :

Shape factor of the particles

Sh:

Sherwood number

S rad :

Source term of heat of radiation, J m−3 s−1

T :

Pellet temperature, K

T 0 :

Initial temperature of the pellet, K

T g :

Temperature of combustion gas, K

T w :

Temperature of furnace wall, K

t :

Time, s

u i :

Velocity components, m s−1

Y i :

Mass fraction of species i

β :

Coefficient of thermal expansion

δ ij :

Kronecker delta

ɛ :

Dissipation rate of turbulent kinetic energy per unit mass, m2 s−3

ɛ p :

Emissivity of pellet surface

μ :

Molecular viscosity, kg m−1 s−1

μ eff :

Effective viscosity, kg m−1 s−1

μ t :

Turbulent viscosity, kg m−1 s−1

ν i,j :

Stoichiometric coefficient for gas species j in reaction i

ρ :

Density, kg/m3

ρ m :

Average density, kg m−3

ρ j :

Density of gas species i, kg m−3

ρ j,0 :

Initial density of gas species j, kg m−3

ρ j,∞ :

Density of gas species j in bulk flow, kg m−3

σ :

Stefan–Boltzmann constant, W m−2 K−4

σ k :

Turbulent Prandtl number for k in standard k–ɛ model

σ ɛ :

Turbulent Prandtl number for ɛ in standard k–ɛ model

σ ij :

Collision diameter, 10−10 m

τ :

Tortuosity of pellet

ϕ :

Void fraction

ΩD :

Collision integral

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Acknowledgments

The article was supported by the Fundamental Research Funds for the Central Universities (NO. FRF-SD-12-013A) and State Key Laboratory of Advanced Metallurgy.

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Authors and Affiliations

  1. School of Mechanical Engineering, University of Science and Technology Beijing, Beijing, 100083, P. R. China

    Ying Liu (Doctoral Candidate), Zhi Wen (Professor) & Guofeng Lou (Associate Professor)

  2. Beijing Key Laboratory of Energy Saving and Emission Reduction for Metallurgical Industry, University of Science and Technology Beijing, Beijing, 100083, P. R. China

    Zhi Wen (Professor) & Guofeng Lou (Associate Professor)

  3. CISDI Industrial Furnace Co. Ltd., Chongqing, 400013, P. R. China

    Zhi Li (General Manager), Haiquan Yong (Senior Engineer) & Xiaohong Feng (Manager)

Authors
  1. Ying Liu
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  2. Zhi Wen
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  3. Guofeng Lou
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  4. Zhi Li
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  5. Haiquan Yong
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  6. Xiaohong Feng
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Corresponding author

Correspondence to Zhi Wen.

Additional information

Manuscript submitted May 25, 2014.

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Liu, Y., Wen, Z., Lou, G. et al. Numerical Investigation of the Effect of C/O Mole Ratio on the Performance of Rotary Hearth Furnace Using a Combined Model. Metall Mater Trans B 45, 2370–2381 (2014). https://doi.org/10.1007/s11663-014-0160-6

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