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Circulating Fluidized Bed Boilers pp 89–119Cite as

Combustion

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Abstract

The cost of fuel constitutes a major part (15–40 %) of the cost of electricity generation for most fossil fuel power plants (Koornneef et al. 2007). Over the lifetime a boiler plant, a savings of one or one-half a percentage point in the combustion efficiency can save a large amount of money in terms of operating cost. Since the expenditure on fuel is much greater than that on sorbents, the impact of combustion efficiency on the operating cost is greater than that of sorbent utilization performance of the boiler. Furthermore, combustion upset could cause considerable disruption to the operation of a plant. A qualitative as well as quantitative understanding of the process of combustion is, therefore, indeed very important for the rational design as well as optimum operation of the boiler.

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Abbreviations

A?:

Frequency factor in Eq. (4.14), kg/m2 s (kPa)n

A :

External surface area of char, m2

A g :

Pore surface area/weight of char, m2/kg

a :

Constant in Eq. (4.4)

C p :

Specific heat of char, kJ/kg K

C g , C s :

Oxygen concentration away from the char and on its surface, respectively, in partial pressure, kPa

D g :

Molecular diffusivity of oxygen, m2/s

D p :

Pore diffusion coefficient, m2/s

d v :

Initial diameter of a coal particle, mm

d c , d p :

Diameters of char and average bed particles, respectively, m

E :

Activation energy, kJ/kmol

E 0 :

Mean activation energy for the release of volatiles, Eq. (4.2), kJ/kmol

F dg :

Drag force of gas per unit volume of particle, kN/m3

F ds :

Drag force due to solid–solid interaction per unit volume of particle, kN/m3

G s :

Circulation rate in fast bed, kg/m2 s

g :

Acceleration due to gravity, 9.81 m/s2

HHV:

Heating value of char, kJ/kg

h m :

Mass transfer coefficient expressed in terms of carbon removed, kg(C)/m2 kPa s

h gp :

Gas-particle heat transfer coefficient, kW/m2 K

K g :

Thermal conductivity of gas, kW/m K

K a :

Attrition rate constant for combustion-assisted attrition

K 0 :

Pre-exponential factor in Eq. (4.2), s?1

m c :

Mass char particle, kg

m attr :

Rate of combustion-assisted attrition, kg/s

m :

True order of reaction

m d :

Diffusion limit, kg/m2s

n :

Apparent order of reaction

p :

Exponent in Eq. (4.4)

q :

Specific burning rate of char based on external surface, kg(C)/m2 s

q c :

Kinetic reaction rate limit, kg/m2 s

q i :

Reaction rate based on internal surface area, gm(C)/cm2 s

R :

Gas constant, 8.315 kPa m3/kmol K

R c , R i :

Apparent and intrinsic reaction rate based on external and internal surface area, respectively, kg(C)/m2s (kPa)n

T :

Temperature of the coal, K

T b , T s :

Temperatures of the bed and of the surface of the char, respectively, K

T m :

Mean temperature of the diffusion layer around the char particle, K

t :

Time, s

t v :

Time for devolatilization and combustion, s

U c , U g, U p :

Velocity of char, gas, and average bed particles, respectively, m/s

U 0 :

Superficial gas velocity through bed or furnace referred to 300 K, m/s

U mf :

Minimum fluidization velocity, m/s

V :

Amount of volatile yet to be released, %

V*:

Asymptotic value of the extent of volatilization, %

x :

Fraction of the volatile matter released

? p , ? g :

Density of bed particles and gas, respectively, kg/m3

? b :

Bulk density of the fast bed, kg/m3

? c :

Density of carbon, kg/m3

? ch :

Density of char, kg/m3

n :

Ratio of actual combustion rate and rate attainable if no pore diffusion resistance existed

? :

Effectiveness factor (=1, for regime I)

? :

Local voidage

? p :

Emissivity of char particle

? :

Stefan–Boltzman constant, W/m4 K

? :

Mechanism factor (=1 for CO2 2 for CO)

? :

Ratio of volume to surface area of char, m

? :

Time for complete devolatilization, s

? :

Porosity of char

? :

Fraction of combustion heat reaching the char surface

EAC:

Excess air coefficient

? :

Standard deviation of Gaussian distribution of activation energy, kJ/mol

? :

Viscosity of gas, kg/m s

?H 0 :

Standard enthalpy of formation of the reactions, kJ/mol

Nu :

Nusselt number, \(\frac{{h_{\text{gp}} d_{v} }}{{K_{g} }}\)

Re :

Reynolds number, \(\frac{{(U_{g} - U_{c} )d_{c} \rho_{g} }}{\mu }\)

Sc :

Schmidt number, \(\frac{{D_{g} \rho_{g} }}{\mu }\)

Sh :

Sherwood number, as in Eq. (4.8)

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Authors and Affiliations

  1. Greenfield Research Incorporated, Halifax, NS, Canada

    Prabir Basu

  2. Dalhousie University, Halifax, NS, Canada

    Prabir Basu

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Basu, P. (2015). Combustion. In: Circulating Fluidized Bed Boilers. Springer, Cham. https://doi.org/10.1007/978-3-319-06173-3_4

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  • DOI: https://doi.org/10.1007/978-3-319-06173-3_4

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