Fans, Flows, and Dust Collection
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| View of the bottoms of pulse jet bags. |
A critical component of any cement plant is the dust collector.
Dust, or more appropriately particulate matter, at a cement
plant is typically caused by physical attrition, combustion
particle burnout, or nucleation. Physical attrition occurs
as particles abrade against each other. Particles generated
by physical attrition range from less than 10 micrometers
in size to more than 1,000 micrometers. Combustion particle
burnout refers to the residues remaining from the pyroprocess.
These particles are typically in the 1 to 100 micrometer range.
Nucleation particles are generated when materials that are
in a vapor form condense. These particles are truly very small,
usually between 0.1 and 1.0 micrometers.
Dust collectors have more than one fundamental purpose:
These control systems provide the low particulate matter
emission levels required by regulatory requirements, and
they minimize localized dust emissions that could hinder
maintenance of plant equipment and vehicles. In the case
of the pyroprocessing systems, a significant fraction of
the plant production is captured by the particulate matter
control device and returned to the kiln.
Fabric Filters
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| Open roof monitor of a positive
pressure fabric filter at a cement plant. |
High efficiency fabric filters have been used in the cement
industry for more than 40 years. They are used for controlling
emissions from cement kilns, clinker coolers, alkali bypass
gas streams, finish mills, raw mills, material handling systems,
product bagging, and rail load out. Most cement plants have
between 40 and 80 separate fabric filter control systems ranging
in size from 30 actual cubic meters per minute capacity to
more than 100,000 actual cubic meters per minute capacity.
There are numerous design types in service.
Fabric filter operation can be described as three sequential
steps:
1. Filtration of particles from the gas stream
2. Gravity settling of the dust cake
3. Removal from the hopper
Each of these steps must be performed properly to ensure
high efficiency particulate collection. In fabric filter systems,
particles are removed by 1) inertial impaction, 2) Brownian
displacement, 3) electrostatic attraction, and 4) sieving.
All four of these mechanisms are active in essentially all
fabric filters simultaneously; however, the relative importance
of each mechanism differs among fabric filter systems due
primarily to the characteristics of the filtration media,
the particulate matter size distribution, and the chemical
composition of the particulate matter. The ability of fabric
filters systems to remove particles over the entire size range
of industrial concern of 0.1 to 100 micrometers is achieved
due to the complementary characteristics of these removal
mechanisms. Inertial impaction is highly efficient for large
particles and Brownian displacement is efficient for small
particles. Electrostatic attraction and sieving can be effective
over the entire particle size range. The combined result of
these collection mechanisms is a particle size removal efficiency
curve illustrated in Figure 6.2.2.
Fabric filters have high removal efficiencies over the entire
size range of 0.1 to 100 micrometers. This has important implications
regarding the changing regulatory requirements that have been
applied to all industrial sources. Sources controlled by fabric
filters operate at low PM10 and PM2.5 emission levels. Furthermore,
fabric filter systems have very high removal efficiencies
even in the particle size range of 0.1 µm to 2 µm
subject to the heterogeneous nucleation of vapor phase materials
such as metals and organic compounds.
Proper design, operation, and maintenance are needed to achieve
the very high removal efficiencies shown in Figure 6.2.2.
One of the main design requirements is to provide sufficient
filter media in the fabric filter system. The quantity of
filtration media is expressed in terms of the air-to-cloth
ratio (gross) defined below:
A/C=Gas flow rate, m3/min (actual)/Total filtration media
area, m2
As the air-to-cloth ratio increases, the localized gas velocities
through the dust cake and fabric increase. At high air-to-cloth
values, some small particles can gradually migrate through
the dust layer and fabric. This is possible because dust particles
within the cake are retained relatively weakly. After passing
through the dust cake and fabric, these particles are reentrained
in the clean gas stream leaving the bag. To minimize emission
problems related to excessively high air-to-cloth ratios,
the design levels are limited. As an example, typical air-to-cloth
ratios for plenum pulse fabric filters usually range from
0.6 to 2.4 (m3/min per m2).
A second important design requirement is to provide sufficient
filtration media cleaning capability. Routine cleaning of
the filtration media is needed to ensure that a portion of
the dust is removed from the filtration media surfaces to
prevent excessively high gas flow resistance. In most types
of fabric filters, agglomerated clumps or flakes of particulate
matter are removed from the filter media surface. By allowing
the material to agglomerate on the particle surface, the gravity
settling of material from the vertical filter media to the
hoppers below is facilitated. As indicated earlier, gravity
settling of the collected material is an essential second
step in the filtration process. Optimal cleaning of fabric
filters also requires cleaning on the frequency and intensity
most appropriate for the specific characteristics of the dust
cake. Plant personnel operating and maintaining the fabric
filters have an important role in ensuring proper cleaning.
Bags that are allowed to collect dust have critical impacts
on the entire system. Fugitive emissions increase, pressure
drop across the bag house increases due to higher system resistance,
the flow rate along with the fan current decreases for the
same reason, the fan static pressure increases, and the hood
static pressure decreases along with the decrease in flow
rate.
The third general design area of importance in all fabric
filtration systems is the solids collection and handling systems.
Cement plant sources generate relatively large quantities
of material that must be collected and transported. Continuous
removal of the solids from the fabric filter system is needed
to ensure proper operation.
For more information on dust collectors, check out PCA’s
newest regional program Dust
Collectors: Operation, Maintenance, and Troubleshooting.
This article was excerpted from Chapter 6.2 of PCA’s
Innovations
in Portland Cement Manufacturing by John R. Richards
with additional information provided by the U.S. Environmental
Protection Agency’s Basic Concepts in Environmental
Sciences online learning modules.
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