Page 307 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

284 10 Post-combustion Air Emission Control

When the dust cake is to be removed, the pressure drop cross the dust cake is
ð DPÞ ¼ P 1 P 2 ¼ DP DPð Þ ¼ 500 125 ¼ 375 Pa
ck f
The face speed of the gas flow is

3
Q ð 300 m =minÞ 1 min=60 sÞ
ð
U 0 ¼ ¼ ¼ 0:083 m=s
A 60 m 2
(b) The dust cake permeability can be derived from Eq. (10.3)
lU 0 DxÞ ck
ð
k ck ¼
P 1 P 2
1:81 10 5 Pa:s 0:0083 m=s 0:003 m 12 2
¼ ¼ 1:19 10 m
375 Pa
This value is close to that of permeable sandstone.

Dust Cake Removal
There are mainly three types of fabric filter bag-house cleaning methods:
(1) reverse air cleaning,
(2) pulse-jet cleaning, and
(3) shake/deflate systems.
Their respective principles are shown in Fig. 10.4.
During a bag-house cleaning using reverse-gas and shake/deflate methods, the
air emission stream must be temporarily interrupted or bypassed, for example, using
an offline operation. The pulse-jet method operates on-line on a few bags while the
rest of the bags continue working without being interrupted. Reverse gas systems
use cleaned gas from another filter unit to remove the dust cake from the filter.
Shake/deflate systems employ both a mechanical shaking force and reverse air to
remove dust cakes.
The filter face speed or air to cloth ratio depends on the cleaning methods to be
employed. It is about 1, 1.5–2, and 3–4 cm/s for reverse air systems, pulse-jet
systems, and shake/deflate systems, respectively, at a comparable pressure drop.
2
The corresponding dust cake loads also vary from 1 to 2.5 kg/m for shake/deflate
2
2
systems, 2.5–7.5 kg/m for reverse air systems, and 5–10 kg/m for pulse-jet filters.
A typical filter bag has a length of 5–10 m, and a diameter of 0.2–0.3 m, where
2
corresponding surface area is 3–10 m per bag. Pulse-jet units operate with
somewhat smaller bags.