An understanding of the capabilities and limitations of the fibres available for filtration is key to their successful application in any given process, writes KN Lakshmipathy.
Dust emissions are a natural result of the cement production process and the means by which they are controlled can be varied with commonly applied fabric filters. With regard to the fabrics employed in these systems, their construction is largely based on the cleaning system used.
Bag design and construction apart, the fibres employed in the filter fabric are selected on the basis of the chemical and thermal conditions in which the filter bags are to operate, along with their ability, when converted into filter fabrics, to deliver the requisite level of filtration efficiency.
Additional consideration is given to the characteristics of the dust itself and the performance level required with respect to particulate emissions and filter bag life. As one might expect, the dusts to be handled during each stage of the cement production process differ significantly and pose their own specific challenges. Equally, the gas chemistry and operating temperatures of these processes vary greatly and heavily influence the selection of the filter fabric to be used.
A thorough understanding of the process itself is vital if an informed filter fabric selection is to be made.This would include:
- Understanding of the process.
- Operating temperature (normal and maximum).
- Filter velocity (air-to-cloth ratio).
- Gas composition (O2, H2O, NOx, SO2, SO3, HCl, HF, etc).
- Dew points – water and acid.
- Inlet dust loading.
- Dust characteristics.
- Particle size distribution.
- Performance expected (life – emissions – DP).
The specific operating parameters for any given process will vary from plant to plant so each application must be considered on a case-by-case.
Kilns / raw mills
A number of media options are available for the high temperature kiln processes. Many systems today utilise woven fibreglass filter bags which display excellent thermal resistance capability; woven glass, when laminated with an ePTFE (expanded Polytetrafluoroethylene) membrane, is able to deliver exceptional filtration efficiency and cleanability and is ideally suited to the chemical environment found in most cement kiln/raw mill systems. Continuous operation at temperatures up to 260oC is possible and, whilst it should be noted that a strong understanding of the requirements of woven glass filter bags is required in order to apply glass successfully, good practice and sound application can see filter bags in woven glass with ePTFE membrane providing a life of up to six years.
Cleaning imparts the greatest mechanical stresses on the filter media and those filter bags which experience fewer cleaning cycles invariably display a higher residual strength over longer periods of operation. Excessive dust loads naturally result in aggressive cleaning regimes. Beyond woven glass, the conditions seen in kiln/mill systems often permit the employment of high temperature polymers such as Polyimide fibre (P84«). In addition to its strong mechanical resistance properties, its ability to withstand elevated temperatures of up to 230oC on a continuous basis (gas conditions permitting) and excellent chemical resistance capability, the multi-lobal cross-sectional structure of P84 fibre increases the effective surface area of the filter media, thus allowing for a more stable dust cake to be maintained on the surface of the filter bags, in turn enhancing filtration efficiency and operational performance.
With a reduced temperature capability to the above-mentioned fabrics, both Polyphenylene Sulphide (PPS) and Homopolymer Acrylic fibre have, to a lesser extent, been applied in cement kiln processes.
Coal mills
Coal remains the fuel of choice for many cement kiln systems around the world and on-site milling of coal for kiln fuel is still a common process. Coal dust, being potentially explosive, must be processed with due care. It is possible for static electrical charge to build up on and within the filter cakes that accumulate on filter bags and such charges must be able to dissipate rapidly to limit the potential for explosion which thereby requires the filter bags and media employed in these systems to be inherently conductive. The addition of a proportion of stainless steel fibres into the fibres of a felted media and/or the use of support scrims which incorporate steel fibres can significantly enhance the conductivity of a filter fabric. As per the requirements of DIN54345-1 and DIN54345-5, filter media for use in potentially explosive environments must display a maximum resistance of 1×108 Ohms (surface and volume).
Operating temperatures in coal mill filters tend to be lower, minimising explosion risks, and typical filter media employed are Polyester and Homopolymer Acrylic or often blends of these two fibres; conductive properties can be added to a variety of filter fabrics to suitably reduce their electrical resistance.
Clinker coolers
The use of ESPs is still common for the exhausting of clinker coolers, but as with the kiln process, the use of fabric filters is on the increase. Exhaust temperatures from the cooler will typically be between 200 and 450oC and it will often be necessary to cool the gases down prior to their being cleaned in the filter. Dust loads are generally low and commonly around 10g/Nm3. A wide variety of filter fabrics are employed in clinker cooler systems, polyester, homopolymer acrylic, aramid, P84, and fibreglass, have all been applied successfully on clinker cooler systems. Attention should be given to gas velocities given the particularly abrasive nature of clinker dust.
Finish mills
The fabric filters employed on finish mills often become a bottleneck in the cement production process; with high dust loads, the filters employed on these systems are often pushed beyond their design capacity so the ability of the filter bags to perform effectively under such demands is of critical value.
Temperatures within finish mill filters are typically below 150oC with humidity being the primary gaseous concern which, with temperature, will influence the filter media options available for such systems. Polyester is commonly employed, but where levels of humidity and temperature are sufficiently high, it may be necessary to consider homopolymer acrylic or PPS fibres. Blends of polyester and acrylic are increasingly common and a variety of fluorocarbon treatments can be employed to afford the filter media enhanced water repellency, dust release and chemical resistance properties.
The use of grinding aids/agents in finish mills can cause some problems for the filters. As the dust becomes less agglomerative so conventional felts, whose efficiency is generally reliant on filter cake formation, can struggle to function effectively; as it becomes difficult to maintain a stable filter cake on the surface of the filter bags, fine particles are more able to penetrate into the structure of the filter fabric, increasing a pressure drop and leading to higher particulate emissions.
In such cases, it may be necessary to consider alternative media constructions and/or finishes better able to handle such situations. P84 fibres can again be utilised to enhance the filtration efficiency of filter fabrics and it is possible to apply a layer of such fibre to the filtration surface of a felt to assist in the maintenance of a stable filter cake. Fine and microdenier fibres have been shown to be better able to collect fine free-flowing dusts with needle felts manufactured from such fibres inhibiting the penetration of fine dust into their structure.
Whilst the selection of high performance, high quality filter media and bags can enhance fabric filter performance, a full system approach is recommended – many factors influence filter operation and minor modifications often bring about significant operational benefits and savings. Andrew Industries (India), a part of the Andrew Industries (UK) group of companies, is well-placed to provide the products and support services to meet the filtration requirements of the cement industry.
KN Lakshmipathy, is Chief Executive Officer, Andrew Industries (India). Email: lpathy@andrewindia.com
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