Fans, Flows, and Dust Collection

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 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.

 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

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.