IntroductionEmissions of solid and gaseous pollutants are constanttopics in the cement industry worldwide. Dust is a majorpollutant and is emitted from various sources locatedall over a cement plant, starting with the kiln off gases,including the mills, and ending with conveying systems.

Smaller emission sources, such as powder handlingstations, are also significant polluters. However, theyare difficult to discuss as their number is high and theyare widespread within the plant. The main gas flows areanother main source of particulates and are thereforetraditionally equipped with filter units of various design.

Clinker is burned at 1450 ˚C. The required high inputof thermal energy during the calcining process and thehigh flame temperature generate higher NOx emissionsthan in most other industries. Late developments, suchas low NOx burners, are a primary measure to minimiseNOx emissions. Besides NOx, SOx emissions can alsobe of importance as some raw materials contain a highnatural sulfur content.

GünterGasparin,on behalfof InspecFibres,Austria,

P84 filtermediain a kilndedustingsystem inFrance.

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reports onthe use of

Dedusting

Alternative fuelsIn Europe, especially, secondary fuels increasinglyare replacing the primary fuel to reduce fuel costs.The secondary fuel can be added to the system atvarious positions, starting from the main burner andending in some zone of the precalciner. The use ofalternative fuels also leads to additional modifications

to the system because the higher amount of chlorineand alkalis must be treated by installing bypasses thatavoid concentration processes, caused by evaporationand condensation at different temperature levels.As a side effect of the use of secondary fuels, otherpollutants such as unburned organic carbons can bepart of the flue gas.

Kiln filteringOn the one end of the kiln, the hot clinker is cooledand dedusted in the clinker cooler filter unit. As thegas environment in this filter unit is mainly hot air, themajor point for the filter media is thermal resistance,rather than chemical resistance. The main gas flowpasses through the kiln in reverse flow, comparedto the solid material. It leaves the system after theprecalcining tower and has to be dedusted, as thegas stream carries a significant amount of dust. Thekiln filter unit is exposed to the chemical environment,containing acidic and oxidising components, as well ashigh temperatures, besides the significant dust load.This is the biggest filter unit in a cement plant, withapproximately one third of the total gas flow passingthrough it.

The traditional type of filter unit is an ESP. It iscapable of covering the emission limits of the cementindustry with 20 - 50 mg/Nm³. It is not affected bytemperature excursions. However, the ESP is verysensitive to high CO values, which can occur andcould cause explosions in the filter unit. Therefore,a constant and careful control of the CO is a crucialparameter for the safe operation of an ESP. Wheneverthe CO value exeeds a certain limit, the filter has tobe taken out of operation, with the entire dust loadpassing through the stack.

Bag filters operate differently. The filter elements ina bag filter unit are a physical barrier for dust and arenot affected by high CO values as a bag filter does notneed an electrical field for operation. Emissions of lessthan 5 mg/Nm³ can be achieved by a more economicmethod than an ESP, provided that the filter unit andthe felt material are designed properly. The filter mediais typically based on polymer material and thereforecontrolling the operating temperature of the bag filter isof higher importance than it would be for the ESP. Anupset temperature could cause severe damage to thefilter material, whereas it would cause only a change ofthe electric charging of particles in the ESP.

Non-woven needle felts are the most commonconstruction for pulsejet filter units. They consist oftwo fleece layers, densified by a needling processand a support scrim in between. The outer fleecelayer is exposed to the dust and has to control thefiltration, whereas the inside fibre layer is used for themechanical protection of the filter material during jetpulse cleaning.

The kiln filter unit is a special task for the filtermedia, as it has to be suitable to deal with a widerange of chemical components in the flue gas atelevated temperatures. In many plants, the off gasesfrom the kiln are also used for venting the raw mill. Thefilter unit is downstream of both units. In compoundmode operation, with the combined dedusting of thekiln and raw mill, high dust loads enter the filter unit.

Figure 1. Sketch of impacts on a kiln filter unit.

Figure 2. Needle felt cross-section of a used filter bagafter two years in operation in the Couvrot cementplant. The dust only covers the surface of the filter

fabric and does not penetrate into the felt.

Table 2. Operating and design data at Couvrot

Start up 1999

Filter media P84

Felt weight 550 g/m2

Number of filter bags Approximately 6000

Total filter area Approximately 13 300 m2

Table 1. Comparison of different polymers for kiln dedusting

Filtermedia

Max. peaktemperature(˚C)

Resistanceagainsthydrolysis

Resistanceagainstoxidation

Filtrationefficiency

PAN 140 Good Fair Good

PPS 200 Excellent Good Good

P84 260 Good Very good Excellent

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This means increased physical stress on the filtermedia. Therefore, only a few materials are applicable tobe used in this specific type of filter unit.

Polyacrylic (PAN) as an economic solution, issometimes used for kiln filter units. The low acceptableoperating temperature demands a stringent control ofthe temperature by air cooling or conditioning towers.Polyphenylene sulfide (PPS) based filter media arealso sometimes used in this type of installation. Byusing this, a certain effort to cool the gases is alsonecessary, as the maximum allowable temperatureis well below the temperature level at the exit of theprecalcining tower.

Besides needle felts, expanded PTFE membraneson glass fabric are also known for this application andthey are suitable in terms of the thermal and chemicalcomposition of the gases. Their mechanical weaknessregarding abrasion is the limiting factor and so, by usingmembranes, measures have to be taken to limit the dustload in the filter unit. They also show sensitivity againstunburnt hydrocarbons, as PTFE is oleophillic.

P84 polyimide based filter fabrics appear to bea perfect solution. The polymer combines adequatethermal and chemical resistance, and a high dustretention capacity to deal with all possible operatingconditions in a cement kiln filter. The high filtrationefficiency of the felt is caused by the irregular shapeof the P84 fibre, resulting in a high specific surfacearea. As a consequence, during the filtration process,the filter cake is building up only in the surface area.Further more, several effects lead to an extendedlifetime of the filter media. The minimised particleentanglement stabilises the pressure drop of thefilter cloth. Therefore, the pressure drop of the entirefilter unit remains constant. The minimised particlepenetration in the filter media also affects theemissions in a positive way.

Besides the constant monitoring of the operatingparameters of the filter plant, the supplier of the filterbags can determine the mechanical and chemicaldegradation of a filter bag, by lab analysis. Therefore,a used filter bag is removed during a shutdown andinvestigated by standardised methods.

The air permeability measured at standardisedconditions of 200 pa on a sample of 20 cm dia.is a good indication of whether the fabric is inproper condition for operation in the plant. If theair permeability is low, dust could have penetratedthrough the fabric, or a crusty dust cake could havebeen formed on the surface. Therefore, an enlargedview of the cross-section of the felt will indicate whathas happened.

The mechanical strength is determined by meansof tensile strength measurements. Mechanical andchemical degradation will lead to a decrease of thetensile strength of the media. Below a minimumtensile strength, cracks in the felt can occur, causinghigh dust emissions. Further chemical analysis canbe performed on the polymer to determine ageingmechanisms. These investigations have to be donewith selected methods individually adapted for everypolymer.

One example of a kiln filter unit with a longhistory in using P84 needle felts is the plant in

Couvrot, part of the Ciments Calcia group. The useof alternative secondary fuels led to tighter emissionlimits for the plant. The existing ESP was no longercapable of ensuring the emission limits. A bagfilter was installed in 1999, equipped with P84 filterbags. During the entire lifetime, the bag filter couldbe operated at a constant pressure drop level, andemissions requirements were fulfilled. To observethe performance of the filter bags, several bags were

Figure 3. View inside a P84 filter bag at the Couvrotinstallation. The bright yellow colour shows that no

dust penetrated through the fabric.

Table 3. Air permeability and tensile strength of the filterbags

Operating time

12months

24months

39months

Air permeability asreceived from the plant[I/(dm2* min)]

14 17 14

Air permeability aftersimulated pulsejetcleaning in the lab[I/(dm2* min)]

26 24.3 25.2

Tenacity of the material(warp direction)

(N/5 cm)

878 864 850

Tenacity of the material(weft direction)

(N/5 cm)

1474 1211 1470

Table 4. Operating conditions of the filter unit

Operatingtemperature

120 – 150 ˚C

Peak temperature 240 ˚C

Gas flow 554 000 Nm3/h

O2 content 10 Vol.%

H2O content 17 – 18 Vol.%

NOx content 800 mg/Nm3

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investigated in the lab. The values of air permeabilityremain constant and also the decrease of the tensilestrength was rather moderate within the first threeyears in operation (Table 3). For more than eightyears, the plant has been equipped with P84. As thismaterial performed well, it was selected again for thereplacement set of filter bags.

ConclusionParticulate emissions remain a major issue in acement plant. The case history of the Couvrotinstallation shows how a P84 based filter mediacan help to ensure the safe operation of the main

kiln filter unit for a long operating time. Tighter dustemission limits in the future will lead to the useof finer fibres in the fabric. As the public concernincreasingly covers other pollutants, such as SOxand NOx, additional flue gas treatment technologieswill find their way to cement applications. Thesetechnologies containing DeNOx processes, aswell as the absorption of SOx, are available forother applications but are so far not widespreadwithin the cement industry. Again, the kiln filterunit will play a key role in these processes, asthe filter acts as adsorption and absorption unititself.__________________________________________________l

Figure 4. The Couvrot cement plant. Figure 5. View on the plenum of the bag filter unit.

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Evonik Fibres GmbHWerkstraße 3 4860 LenzingAustria

phone +43 7672 701-2891 fax +43 7672 96862www.P84.com