Effect of limestone addition on radiative heat transfer during co-firing of high-sulfur content lignite with biomass in fluidized bed combustors

ABSTRACT

In this study, influence of limestone addition on radiative heat transfer during co-firing of high-sulfur content lignite with olive residue in the freeboard of 300 kW_t Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) is investigated by using an in-house developed radiation code based on Method of Lines (MOL) solution of Discrete Ordinates Method (DOM). The freeboard is treated as a 3D rectangular enclosure containing gray, absorbing, emitting gas with absorbing, emitting, anisotropically scattering particles surrounded by black/gray diffuse walls. Radiative properties of combustion gases and particles are evaluated by using Leckner’s correlations and Mie theory, respectively. Input data required for the model are provided from four combustion tests which were previously carried out sequentially for lignite only, lignite with limestone addition followed by 30 and 50 wt % shares of olive residue in the fuel mixture for the same Ca/S ratio. The results show that addition of limestone affects the radiative properties of the particles in the freeboard through the change of particle size distribution and load rather than optical properties and decreases the radiative wall heat fluxes along the freeboard. On the other hand, co-firing with biomass compensates this decrease as less limestone is utilized in almost direct proportion to the coal replacement.

KEY WORDS:

• Radiative Heat Transfer
• Limestone
• BiomassCo-Firing
• Fluidized Bed Combustor

Nomenclature

c speed of light (m.s⁻¹)

g asymmetry factor (-)

I radiative intensity (W m⁻² sr⁻¹)

blackbody intensity (W m⁻² sr⁻¹)

k imaginary part of complex refractive index (-)

k_t time constant with dimension (m/s)⁻¹

L_m mean beam length (m)

m complex refractive index (-)

n real part of complex refractive index (-)

position vector (-)

r coordinate axis in cylindrical geometry(-)

T temperature (K)

average temperature (K)

t time

w quadrature weight (-)

Subscripts

g gas

i quadrature point

index for a discrete direction

incoming discrete direction

m ordinate index

m′ incoming ordinate

p particle

λ wavelength (μm)

wavenumber (cm⁻¹)

Superscripts

m ordinate index

m′ incoming ordinate

index for a discrete direction

′ incoming discrete direction

Greek symbols

β extinction coefficient (m⁻¹)

error tolerance (-)

emissivity of the flue gas (-)

direction cosine (-)

scattering angle (rad)

absorption coefficient (m⁻¹)

λ wavelength (μm)

direction cosine (-)

direction cosine (-)

ρ_p particle density (kg/m³)

scattering coefficient (m⁻¹)

τ optical thickness

azimuthal angle (rad)

scattering phase function (sr⁻¹)

ω scattering albedo

direction of radiation intensity (-)