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Emissions

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Circulating Fluidized Bed Boilers

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Abstract

The combustion of fossil fuels in stationary and transportation systems is the main source of man-made (anthropogenic) air pollution. Various boilers, furnaces, and engines burning fossil fuels emit gaseous pollutants, such as SO2, NO X , CO, and fly ash. Sulfur dioxide is a major contributor to acid rain. Nitrogen oxide (NO X ), which represents both nitric oxide (NO) and nitrogen dioxide (NO2), contributes to acid rain and smog. Though nitrous oxide (N2O) is another form of nitrogen oxide, it is not a part of the NO X . Nitrous oxide is a greenhouse gas and contributes to the global warming. Carbon dioxide (CO2) is the most important contributor to global warming and thus its emission intensity (g CO2/kWhe) is a major environmental index of a power plant.

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Notes

  1. 1.

    The IEA conversion for lb/MMBTU (million Btu) for dry flue gas with 6 % O2 is 350 Nm3/GJ for coal.

Abbreviations

A :

Cross section of the furnace, m2

A e :

Area of the furnace exit, m2

a :

Exponent in bed density profile, m−1

ASH:

Weight fraction of ash in fuel

(Ca/S):

Calcium to sulfur molar ratio

C c :

Molar concentration of sorbents in the bed, kmol/m3

\(C_{{{\text{SO}}_{ 2} }}\) :

Concentration of sulfur dioxide, kmol/m3

\(C_{{s_{0} }}\) :

Concentration of sulfur dioxide at x = 0, kmol/m3

\(C_{{{\text{SO}}_{ 2} }} \left( x \right)\) :

Concentration of sulfur dioxide at x, kmol/m3

d :

Diameter, m

E :

Activation energy, kJ/kmol

E e :

Solid collection efficiency of the furnace exit

E c :

Average cyclone efficiency

E sor :

Sulfur capture efficiency

F c :

Coal feed rate, kg/s

F sor :

Sorbent feed rate, kg/s

f c :

Weight fraction of unreacted sorbent in bed materials

G d :

Downward solid flux, kg/m2s

G u :

Upward solid flux, kg/m2s

G s :

Solid recycle rate, kg/m2s

H :

Height of the boiler furnace above the secondary air level, m

HHV:

Higher heating value of fuel, kJ/kg

K(t):

Reactivity of sorbent particles at time t, s−1

K :

Reaction rate of sulfation, m3/kmol s

L :

Emission limit, kg sulfur/kJ heat release

M ca :

Moles of calcium or CaCO3 in sorbent particle, kmol

M cao :

Molecular weight of calcium oxide, 56 kg/kmol

m :

Local rate of SO2 formation, kmol/m3s

Σm d, out :

Sum of all outflows of sorbents of size d and d + Δd, kg/s

\(M_{{{\text{caco}}_{ 3} }}\) :

Molecular weight of limestone, 100 kg/kmol

n :

Index of reaction rate

p :

Moles of calcium per unit volume of sorbent particle, kmol/m3

P e :

Equilibrium partial pressure of CO2

P* :

Proportionality constant in Eq. (5.16), s. kmol/m3

R :

Universal gas constant, 8.314 kJ/kmol K

R 0 :

Initial reaction rate, kmol/s. particle

R(t):

Reaction rate of sulfation at time t, kmol/s. particle

r :

Sorbent radius, m

S :

Sulfur mass fraction in fuel

S′:

Maximum sulfur fraction in fuel to meet regulation without capture

T :

Temperature, K

t :

Time, s

t cs(d):

Total residence time of a particle of diameter d, s

t sf :

Sulfation time, s

t p :

Pore plugging time constant, s

t fs :

Average particle residence time during single trip through the bed, s

U :

Superficial gas velocity, m/s

U s(x):

Net upward velocity of solids at height x, m/s

U s, U d :

Upward and downward velocity of solid particles near the exit of the furnace, m/s

U t :

Terminal velocity of single particle, m/s

V p :

Volume of a sorbent particle, m3

wf(d)Δd :

Mass of bed materials of size between d and d + Δd, kg

x :

Height in the furnace above the secondary air level, m

\(X_{{{\text{caco}}_{ 3} }}\) :

Weight fraction of calcium carbonate in the sorbent particle

ε :

Average bed voidage near the furnace exit

δ :

Current extent of sulfation

δ(t):

Extent of sulfation at time t

δ(∞):

Asymptotic or maximum final extent of sulfation of the sorbent

ρ p :

Density uncalcined sorbent, kg/m3

ρ b(0):

Ρ b (x), ρ b (∞), density of the bed at secondary air level, distance, x above it and the asymptotic value, kg/m3

ρ bav :

Average bed density, kg/m3

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  1. Greenfield Research Incorporated, Halifax, NS, Canada

    Prabir Basu

  2. Dalhousie University, Halifax, NS, Canada

    Prabir Basu

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Basu, P. (2015). Emissions. In: Circulating Fluidized Bed Boilers. Springer, Cham. https://doi.org/10.1007/978-3-319-06173-3_5

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  • DOI: https://doi.org/10.1007/978-3-319-06173-3_5

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