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Modelling of particle size effect on Equivalence Ratio requirement for wood combustion in fixed beds

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Abstract

A sufficient amount of air is needed for the optimum combustion of fuel. The correct amount of air supply assures optimal flue gas temperature to maximise heat utilisation with fewer pollutants. The quantity of air requirement depends on the fuel type. Equivalence Ratio (ER) requirement has been defined empirically for different fuel types. Wood combustion requires a high degree of excess air requirement compared to other fuels. Data on ER requirement is essential for the industrial operation of wood combustion systems. One of the factors which affect the amount of ER is the size of fuel which has not been given sufficient attention. The effect of particle size on the ER requirement in packed bed combustion of thermally thick wood particles is studied in this research through numerical modelling. Computational fluid dynamics simulations were carried out for the particle sizes of 25 mm, 38 mm, and 63 mm wood particles under air flow velocity of 0.12 ms−1. Simulation results show that the ER value for smaller particle sizes is less than that for the larger particle sizes under the same volumetric air flow rate. CFD simulations were used to decide the optimum ER which maximises the flue gas temperature with minimum possible CO fraction for particle sizes of 25 mm, 38 mm, and 63 mm.

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Data availability

The data used to support the findings of this study are included within the article.

Abbreviations

Aspec :

Volume specific surface area (m−1)

A p :

Projected area (m−1)

ag :

Absorption coefficient of gas (m−1)

as :

Absorption coefficient of solid (m−1)

Ci :

Concentration of gases (mol m−3)

C1,C2 :

Constants used in standard k-epsilon equation

Cpg :

Specific heat capacity of gas (J kg−1 K−1)

Cps :

Specific heat capacity of solid (J kg−1 K−1)

Da,i :

Effective diffusivity of species i through ash layer (m2 s−1)

Di :

Binary diffusivity of species i in air (m2 s−1)

Dk,i :

Knudsen diffusivity of species i (m2 s−1)

Deff,i :

Effective diffusivity of species i (m2 s−1)

dp :

Particle diameter (m)

dp,o :

Initial particle diameter (m)

dpore :

Pore diameter (m)

Ep :

Radiative emission from particles (W m−3)

G:

Radiation intensity (W m−2)

e:

Emissivity

Hg,i :

Heat of reactions gas-phase reactions (J kg−1)

Hm :

Heat of evaporation (J kg−1)

Hs,i :

Heat of reactions solid-phase reactions (J kg−1)

h:

Convective heat transfer coefficient (W m−2 K−1)

hm,a,i :

Mass transfer coefficient of ith gaseous component through ash layer (m s−1)

hmi :

Mass transfer coefficient of ith gaseous species in the boundary layer (m s−1)

hm,ieff :

Effective mass transfer coefficient (m s−1)

k:

Turbulent kinetic energy (m2 s−2)

ki :

Kinetic reaction rate of char with ith gaseous component (m s−1)

ki,eff :

Effective reaction rate (m s−1)

la :

Ash layer thickness (m)

Mair :

Air molar weight (kg mol−1)

Mc :

Carbon molar weight (kg mol−1)

Mi :

Species molar weight (kg mol−1)

ms,i :

Solid-phase species mass in a computational cell (moisture, wood, char, ash) (kg m−3)

ms,w :

Mass of water in a computational cell (kg m−3)

m:

Mass (kg)

Nu:

Nusselt number

n:

Refractive index

P:

Pressure (Pa)

Pr:

Prandtl number

Qrad,g :

Radiation heat source term gas phase (W m−3)

Qrad,s :

Radiation heat source term solid phase (W m−3)

R:

Universal gas constant (J K−1 mol−1)

Re:

Reynolds number

Rg,i :

Gas-phase reaction rate ith species (kgm−3 s−1)

Rki :

Gas-phase kinetic reaction rate of ith species (kgm−3 s−1)

Rmix,i :

Gas-phase mixing rate ith species (kgm−3 s−1)

Rs,i :

Solid-phase reaction rate (kgm−3 s−1)

Rs,w :

Rate of drying (kg m−3 s−1)

Sg,i :

Summed production rate of ith gas species in gas-phase reactions (kg m−3 s−1)

Sh:

Sherwood number

Sij :

Turbulent stress tensor (Pa)

Sm :

Momentum resistance source term (kgm−2 s−2)

Ss,i :

Summed production rate of ith gas species in solid-phase reactions (kg m−3 s−1)

Tevap :

Evaporation temperature (K)

Tg :

Gas-phase temperature (K)

Ts :

Solid-phase temperature (K)

t:

Time (s)

u:

Velocity (m s−1)

ubed :

Bed velocity (m s−1)

Vcell :

Volume of cells (m3)

Xa :

Mass fraction of ash

Yi :

Mass fraction of gas species i

Yb :

Mass fraction of bound water in solid phase

α:

Mass fraction

β:

Solid cell identification factor

γ:

Factor of bed height change

ε:

Turbulent dissipation rate

η:

Permeability

λg :

Thermal conductivity of gas (W m−1 K−1)

λs :

Thermal conductivity of solid (W m−1 K−1)

μ:

Viscosity (kg m−1 s−1)

μt :

Turbulent viscosity (kg m−1 s−1)

ρa :

Ash density (kg m−3)

ρg :

Gas density (kg m−3)

ρs :

Solid density (kg m−3)

σ:

Steffan Boltzmann constant (W m−2 K−4)

σi,air :

Collision diameter (A)

σε,σk :

Model constants in standard k-epsilon equations

σs :

Scattering coefficient (m−1)

τ:

Stress tensor (Pa)

ϕ:

Porosity

ϕa :

Ash layer porosity

Ωc,i :

Stoichiometric coefficient of ith heterogeneous reaction

Ωg,i :

Stoichiometric coefficient of ith gas-phase reactions

Ω c,o :

Stoichiometric coefficient of char oxidation reactions

Ω g,i,o :

Stoichiometric coefficient of ith gas-phase oxidation reaction

Ωi,air :

Collisional integral

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Acknowledgments

The authors are thankful to the University of Moratuwa, Sri Lanka, for providing experimental facilities for this research and Mrs. Dimuthu Rajapaksha for sharing experimental data from her research work for the validation of the presented mathematical model.

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  1. Department of Chemical and Process Engineering, Faculty of Engineering, University of Moratuwa, Moratuwa, 10400, Sri Lanka

    K. Upuli C. Perera & Mahinsasa Narayana

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  1. K. Upuli C. Perera
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Correspondence to K. Upuli C. Perera.

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Perera, K.U.C., Narayana, M. Modelling of particle size effect on Equivalence Ratio requirement for wood combustion in fixed beds. Biomass Conv. Bioref. 9, 183–199 (2019). https://doi.org/10.1007/s13399-018-0348-0

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