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260 Fuel Chemistry

up while the gravity pulls the particles down. When the
velolity of the air increases above a certain point, the drag
force exceeds the weight of the particles and the particles
will start to fluidize. Under this condition, the solid bed
material is made to exhibit properties such as static pres-
sure per unit cross section, bed surface level and flow of
solids through a drain etc., similar to that of a fluid in the
bed. This fluidization behavior promotes rapid mixing of
the bed material, thus promoting gas–solid and solid–solid
heat transfer.
In a typical fluidized bed combustion process, solid, liq-
uid, and or/gaseous fuels, together with noncombustible
bed material such as sand, ash, and/or sorbent are flu-
idized. The primary advantage of this mode of combustion
over pulverized and/or fixed-bed combustion is its operat-
ing temperature (800–900 C). At this temperature, sulfur
◦
dioxide formed by combustion of sulfur can be captured
by naturally occurring calcium-based sorbents (limestone
or dolostone). The reactions between the sorbent and
SO 2 are thermodynamically and kinetically balanced in
this range of temperatures. The operating temperature
range of FBC boilers is low enough to minimize thermal
FIGURE 6 A plan view of a tangentially fired furnace. NO x production (which is highly temperature dependent)
compared to other modes of combustion and yet high
enough to achieve good combustion efficiency. In addition
also determines the NO x formation. High volatile bitumi- to these advantages, FBC boilers can be adapted to burn a
nous coals tend to have higher flame velocities compared wide range of fuels such as low-grade, low-calorific-value
to low volatile coals. Swirling flows promote mixing and coal wastes, and high-ash and/or high-sulfur coals in an
esatablish recirculation zones. In swirling flows, the axial environmentally acceptable manner. Fluidized beds can be
flux of angular momentum G and the axial flux of axial
further subdivided into bubbling or circulating fluidized
momentum Gx are conserved.
beds based on the operating characteristics, and into atmo-

r 2 spheric and pressurized fluid beds based on the operating
2
G x = 2πr(p + ρu ) dr = constant
pressure.
r 1
Bubbling fluidized beds characteristically operate with

r 2
2
G φ = 2πρuwr dr = constant, a mean bed particle size between 1000 and 1200 µm (1.0
r 1 to 1.2 mm). The operating velocity in a BFBC boiler is
where u and w are the axial and tangential components of above the minimum fluidizing velocity and less than a
the velocity at radius r, p is the static gage pressure, and third of the terminal velocity (1–4 min/sec). Under the
r 1 and r 2 are radial limits of the burner. Swirl number is a conditions of low operating velocity and large particle
measure of swirl intensity size, the bed operates in the bubbling mode, with a defined
bed surface separating the densely loaded bed and the low
G φ
S = , solids freeboard region. In bubblingfluidized bedsthegoal
G x r b
where r b is the radius of the burner. A strong swirl usually is to prevent solids from carrying over, or elutriating, from
the bed into the convective pass(es). Therefore, particles
has a value of S > 0.6. Stronger swirl is necessary when
smaller than 500–1500 µm are not used in BFBCs so as
burning is difficult to ignite coals.
to avoid excessive entrainment.
Circulating fluidized beds operate with a mean particle
size between 50 and 1000 µm (0.05 to 1 mm). The hydro-
VI. FLUIDIZED BED COMBUSTION dynamics of the fast fluidized beds allow CFBC boilers
to use sorbent particles as fine as 50 µm. CFBC boilers
Fluidization is a process of making solids behave like a operate typically at about twice the terminal velocity of
“fluid.” When a jet of air is passed though a bed of solids the mean size particles, ranging from 3 to 10 min/sec.
from underneath, the force due to drag pushes the particles This high operating gas velocity, recirculation rate, solids