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 kWt 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.
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 (-)
kt time constant with dimension (m/s)−1
Lm 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 (-)
ρp 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 (-)