Page 282 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

9.1 Stationary Combustion Devices 259

(CFBC), air velocity is relatively higher than BFBC, being about 5–10 m/s at which
the bed material suspension fills the entire combustion chamber.
For either case, the suspended particle size can be easily estimated from the gas
speed and aerosol dynamics (Sect. 4.2). It can be determined by the balance
between gravity and drag force on the particle

1 2 1 2
F D ¼ C D q V p V g pd p ð9:1Þ
g
2 4
When the particle is suspended in the combustion chamber, the gravitational
force equals the drag on the particle. Assuming the particle is spherical and the
corresponding diameter is d p0 , we have

1 3 1 2 1 2
pd gq ¼ C D q V p V g pd p0 : ð9:2Þ
g
p
p0
6 2 4
When the fuel particle is suspended in the gas, V p ¼ 0, and the equation
becomes
3 q V 2
g g
d p0 ¼ C D : ð9:3Þ
4 q g
p
Fuel particles larger than this size will remain in the chamber; smaller ones will
be carried away by the gas to the downstream unit. Obviously, the greater the gas
speed, the larger particles can penetrate through the combustion reactor and enter
downstream unit. The drag coefficient is described in Eq. (9.4).

24
8
> Re p 1
< Re p

C D ¼ Re p 1 þ 0:15Re 0:687 1\Re p 1000 ð9:4Þ
24 p
>
:
0:44 Re p [ 1000
A simple form can be obtained for Re p 1.
s ffiffiffiffiffiffiffiffiffiffiffiffiffi
18lV g
d p0 ¼ ð9:5Þ
q g
p
Example 9.1: Suspended particle size in fluidized bed combustion
Consider a fluidized bed furnace in a coal fired power plant that operates at
3
atmospheric pressure. Assume the coal particle density of 1,000 kg/m , and the gas
temperature is about 1,100 K. The gas moves upward at a speed of 3 m/s. Use the
gas properties using those of the air at the same temperature, and estiamte the
suspended particle diameter.