Limiting Dust Pollution

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Electrostatic precipitators are very reliable in normal operation but

have certain shortcomings during special conditions such as kiln start

up, switching from compound operation to direct operation, and

particularly during so-called CO trips when the voltage is switched off

for a short period of time in order to avoid an explosion risk when COconcentrations in the oven exhaust become too high.

Electrostatic precipitators are very reliable in normal operation but

have certain shortcomings during special conditions such as kiln start

up, switching from compound operation to direct operation, and

particularly during so-called CO trips when the voltage is switched off

for a short period of time in order to avoid an explosion risk when CO

concentrations in the oven exhaust become too high.

The performance optimum of EPs lies at 1-2 mg/Nm in routine kiln

operation, with the above-mentioned exceptions under special

conditions.

In many cases, it is advantageous to treat the clinker cooling air jointlywith the oven exhaust: Part of the cooling air is often used as pre-

heated combustion air in the kiln burners, another part can be used for

drying of raw materials in the raw mill. Where such configurations are

possible, joint treatment of clinker cooler air and oven exhaust gas inone EP is economically favourable compared to separate treatment in

two EPs.

Existing EP installations can often be upgraded by fitting more

modern electrodes or by installing automatic voltage control on older

installations [IPTS, 1999]. Addition of an extra electrostatic field for

improved precipitation maybe an option on some but not all existing

installations. Improved conditioning of oven exhaust gas may alsoimprove the performance of an existing EP.

Fabric filters

In the European cement industry, fabric filters are routinely used for

gas cleaning at the cement mill, at the coal mill, at storage silos and

bunkers, and at cement loading into bulk road or rail tankers. In manycases, clinker cooling air is also cleaned by fabric filters. While in

North America cleaning of oven exhaust gas by fabric filters is quite

common, this is a fairly new development in Europe. In early 1999,

one newly built fabric filter for oven exhaust gas is being taken intoroutine operation at an existing rotary kiln (capacity: 3000 Mg per

day) in North Germany; a second one has been recently taken intooperation in Eclepens, Switzerland.

Fabric filters consist of a membrane which is permeable to gas but

which will retain the dust. Initially, dust is deposited both on the

surface fibres and within the depth of the fabric, but as the surface

layer builds up it itself becomes the dominating filter medium.(7)As

the dust cake thickens, the resistance to gas flow increases, making

periodic cleaning of the filter medium necessary in order to control the
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pressure drop over the filter [IPTS, 1999]. The pressure drop over a

new fabric filter is approximately 10 times higher than for an

electrostatic precipitator [SCHOBESBERGER, 1998].

Periodic cleaning of fabric filters can be done either in off-line or inon-line mode. The most common cleaning methods include reverse air

flow, mechanical shaking, vibration and compressed air pulsing [IPTS,1999]. The duration of a cleaning pulse is about a few hundreths of a

second. In order to optimize filter performance and reduce mechanical

stress, cleaning is done in such a way that approximately 10% of the

dust cake remain on the fabric.

Permeability of the fabric for dust is highest during the short cleaning

pulse when it typically lies between 20 mg/Nm3 and 45 mg/Nm3,

depending on the pressure of the cleaning pulse. If the basic

permeability of a fabric filter is 1,5 mg/Nm3, the average dust passage

over time will therefore lie between 1,8 mg/Nm and 2,4 mg/Nm

[VDI, 1997].

Standard fabric materials include polyester fibres and sometimes also

polyacryl nitrile fibres. Required material specifications include

permeability to air, mechanical stability, and resistance towards

temperature and chemicals. Fabric filters are fairly sensitive totemperature drops below the dew point as well as to temperature

peaks.

For oven exhaust gas, polyester fibres do not provide sufficientdurability at high temperatures. Instead, special membrane fibres

(which are more expensive than polyester) are needed in this case

[SCHOBESBERGER, 1998].

Lifetime of the fabric material is crucial for operating costs of the

filters. Depending on the quality of material, gas temperature and

volume, dust quantity and chemical composition, and maintenance of

steady operating conditions, lifetime of the material typically lies

between one and three years.

For new fabric filters, the suppliers routinely guarantee at least 30

mg/Nm as the achievable emission level. Without technical changes,

this guaranteed emission level can be lowered to 20 mg/Nm by a

more frequent exchange of fabric material, leading to increased

operating costs. For a guaranteed emission level below 10 mg/Nm,the filter dimensions would have to be increased in order to reduce the

load level per area of filter medium. This would result in higher

investment costs.

To summarize, today there is wide consensus that average dust

emission levels in oven exhaust from cement kilns below 10 mg/Nm

are achievable with either electrostatic precipitators or fabric filters

[IPTS, 1999]. This is confirmed by numerous European cement kilns


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which are routinely running at that level.

There are, however, certain operating conditions kiln during which

this emission level will be occasionally exceeded. Such situations have

to be taken into account when an emission limit value for a cementkiln is fixed in a permit.

Cement Manufacturing

Cement Manufacturing Home > Dust Collection

Fans, Flows, and Dust Collection

A critical component of any cement plant is the dustcollector. 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.

View of the bottoms of pulse jet

bags.


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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 actualcubic 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 stream2. 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 isefficient 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.

Open roof monitor of a positive pressure

fabric filter at a cement plant.


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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 efficiencieseven in the particle size range of 0.1 m to 2 m subject to the heterogeneous nucleation of vaporphase 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 theair-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 dustlayer 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. Asindicated 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 emissionsincrease, 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 collectionand 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 systemis needed to ensure proper operation.

For more information on dust collectors, check out PCAs newest regional programDust

Collectors: Operation, Maintenance, and Troubleshooting.

This article was excerpted from Chapter 6.2 of PCAsInnovations in Portland Cement

Manufacturingby John R. Richards with additional information provided by the U.S.
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