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Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

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Abstract

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O2/N2 and O2/CO2 atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O2/CO2 (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O2/N2 (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O2/N2 condition or the O2/CO2 condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.

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Abbreviations

A :

Surface area/m2

c i :

Mass concentration of species i/(kg · m−3)

c p :

Heat capacity at constant pressure/(J · (kg · K)−1)

C :

Amount of carbon in the coal/%

D :

Gas diffusion coefficient/(m2 · s−1)

h :

Overall heat transfer coefficient between particle and atmosphere/(W · (m2 · K)1)

h g :

Heat transfer coefficient between reaction sheet and atmosphere/(W · (m2 · K)−1)

h in :

Heat transfer coefficient between particle and reaction sheet/(W · (m2 · K)−1)

H :

Amount of hydrogen in the coal/%

k pyro :

Rate constant of devolatilization/(kg · s−1)

k vap :

Rate constant of vaporization/(kg · s−1)

L 0 :

Initial pore length/m

MW c :

Carbon molecular weight/(kg · mol−1)

m :

Mass/kg

m v :

Mass of volatiles/kg

N :

Amount of nitrogen in the coal/%

Nu:

Nusselt number (-)

O :

Amount of oxygen in the coal/%

Pr:

Prandtl number (-)

Q r :

Heat release rate of heterogeneous reaction/(J · s−1)

Q h :

Heat transfer between the fuel particle and environment/(J · s−1)

Q*:

Non-dimensional heat ratio (-)

Re:

Reynolds number (-)

R u :

Universal gas constant

r :

Radius/m

r ef :

Radius of evaporation layer/(m · s−1)

S :

Amount of sulfur in the coal/%

S r :

Source term/(kg · (m3 · s)−1 or W · (m3 · s)−1)

Sc:

Schmidt number (-)

Sh:

Sherwood number (-)

s :

Volume-specific surface area/m−1

T :

Temperature/K

t :

Time/s

U g :

Fluidization velocity/(m · s−1)

U mf :

Minimum fluidization velocity/(m · s−1)

V :

Particle volume/m3

v :

Gas viscosity/(m2 · s−1)

v i,j :

Stoichiometric coefficients of species i in reaction j (-)

w h,j :

Rate of j homogeneous reaction/(103mol ·(m3 · s)−1)

w s,j :

Rate of j heterogeneous reaction (103mol · (m3 · s)−1)

X c :

Carbon conversion rate (-)

ΔH h,j :

Reaction heat of j omogeneous reaction/(J · (kmol)−1

ΔH sj :

Reaction heat of j heterogeneous reaction/(J ·(kmol)−1)

ΔH vap :

Vaporization heat/(J · kg−1)

φ A :

Mass fraction of ash (-)

φ C :

Mass fraction of carbon (-)

φ M :

Mass fraction of moisture (-)

φ V :

Mass fraction of volatiles (-)

ΔM :

Consumption rate of char/(kg · s−1)

[i]:

Mole concentration of species i/(mol · m−3)

α i :

Stoichiometric coefficient of species i in the devolatilization reaction (-)

β g :

Mass transfer coefficient between reaction sheet and atmosphere/(m · s−1)

βin :

Mass transfer coefficient between particle and reaction sheet/(m · s−1)

ε :

Emissivity (-)

ε mf :

Bed voidage at the minimum fluidization state (-)

ε 0 :

Porosity of char (-)

λ :

Thermal conductivity/(W · (m · K)−1)

μ :

Dynamic viscosity/(N · s · m−2)

ρ :

Mass density/(kg · m−3)

σ :

Stefan-Boltzmann constant/(W · (m2 · K4)−1)

ψ :

Structural parameter of the char (-)

Δt :

Time-step/s

par:

Particle

rs:

The reaction sheet

b:

Fluidized bed

ps:

Particle surface

∞:

Infinity far boundary

0:

Initial state

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Acknowledgements

This work was supported by the National Key R&D Program of China (No. 2018YFF0216002) and the Seed Fund of Shanxi Research Institute for Clean Energy, Tsinghua University.

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Authors and Affiliations

  1. Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing, 100084, China

    Xiehe Yang, Yang Zhang, Jiansheng Zhang, Hai Zhang, Junfu Lyu & Guangxi Yue

  2. Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, 210096, China

    Daoyin Liu

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  1. Xiehe Yang
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Correspondence to Yang Zhang.

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Yang, X., Zhang, Y., Liu, D. et al. Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition. Front. Energy 15, 99–111 (2021). https://doi.org/10.1007/s11708-020-0685-0

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