Page 283 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control
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260 9 In-combustion Air Emission Control
Solution
The properties of the air at 1,100 K and 1 atm are
3 5 2
q ¼ 0:3166 kg/m , l ¼ 4:49 10 N.s/m .
p
If we assume Re p \1
s ffiffiffiffiffiffiffiffiffiffiffiffiffi
r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
18lV 5
g 18 4:49 10 3 4
d p0 ¼ ¼ ¼ 4:97 10 m ¼ 0:497 mm
q g 1000 9:81
p
With this size, we can calculate
q V g d p 0:3166 3 4:97 10 4
g
Re p ¼ ¼ 5 1
l 4:49 10
Therefore, our assumption of Re p \1 was invalid.
Now assume 1\Re p 1000 and
8 2
3 q g V g
d p0 ¼ C D
<
4 q p g
Re p 0:687
C D ¼ 1 þ 0:15Re
:
24 p
The calculation becomes complicated, but we can solve it by iteration. Then the
suspended particle size is
d p0 ¼ 6:055mm
And the corresponding particle Reynolds number is Re p ffi 128
Under steady operation condition, a large proportion of the bed materials leave
the chamber via an exit on top of the chamber and are collected by a particle
separator, most likely a cyclone for material recirculation to the bed. Cyclones are
being used at temperatures of 1,000 °C in PFBC systems for solid recycling.
Penetrated fine particles are called fly ash and join the flue gas. Due to the high
particle concentrations in this application, particle agglomeration may occur, which
favor particle separation.
The fuel particle size is larger while the furnace temperature is lower in FBC
(800–950 °C) compared to pulverized coal combustion (>1,000 °C). Due to the
relatively low-combustion temperature, FBC can handle low-grade fuels such as
wet sludge or waste solid fuels with a relatively low-NO x emission. The SO x
emissions can be reduced by addition of sorbent like a limestone or lime to the bed.
On the other hand, the emissions of N 2 O may be high.
Another drawback of CFBC is the increased fly ash, which is mainly the result of
the higher velocity, smaller fuel particle size, and more intense attrition and abra-
sion. The fate of ash-forming material in fluidized bed is much different from that in
a pulverized coal combustion chamber. Again, temperatures in CFBC are much
