Filtration Fibres and Finishes
In all sections of cement manufacturing like raw material handling, grinding, storage, clinkerisation, calcination, cooling, cement grinding and packing there are several PJBH in use. RABH is used in cement/kiln section only. The fabric filters are positive filtration equipment where the separation of dust takes place on the surface of the filter media or up to a certain depth of the filter media. When the filtration takes place on the surface it is termed as surface filtration and when some depth of filter media is used for holding the primary cake, the process is termed as depth filtration. We shall discuss various filter fabrics made using different fibres used in a cement plant in this article.
There are several synthetic and mineral fibres in use for making filter fabrics. They have different temperature resistance and chemical resistance. Based on these two properties namely, thermal stability and chemical resistance, the choice of filter media is made for a particular application in a cement plant.
The fibres, which are used in cement plant, are mainly polyester, homopolymer acrylic, aramid, PPS, P-84 and glass. Very seldom filter fabrics like polypropylene and PTFE are used in a cement plant. Therefore, we shall restrict the discussion here to the fibres, which are commonly used and to various finishes used for enhancing the filter fabric performance.
Polyester: As can be seen from table, it can withstand a continuous temperature of 150¦C in absence of moisture in the gas stream. It can withstand 160¦C for a short period, but if the temperature goes beyond 160¦C, the felt will shrink and loose its mechanical properties. It will get hardened and loose its flexibility. The colour will get darker and the filter bags will be torn after some time due to continuous pulsing. Polyester is workhorse of industry and has more than 70 per cent share in terms of no of filter bags. It is the cheapest fibre available and therefore, unless it fails in a particular duty condition, polyester remains the first choice.
Homopolymer Acrylic: In presence of water vapour in the gas stream, polyester starts reverse hydrolysing (disintegrates into ester and acid) and holes are formed in the filter bags. Besides, felt also loses its mechanical strength. In the applications like cement grinding, gypsum drying and slag drying homopolymer acrylic is a fibre, which is the preferred material. It can withstand operating temperature of 130¦C continuously and does not hydrolyse. Homopolymer acrylic when blended with polyester resists charging of filter fabric and therefore it is frequently used on coal mill PJBH. It particularly performs well if fly ash is present in the mill and 100 per cent, while homopolymer acrylic fails due to abrasion in such conditions. Polyester renders better abrasion resistance to the felt made using a combination of polyester and homopolymer when compared with 100 per cent homopolymer felt. Use of microdenier fibres for making the felt has further helped in performance of the filter fabrics in cement mill air venting bag-filters. Increased surface area improves filtration efficiency and keeps the operating DP lower than conventional felts.
Aramid/Nomex«: Aramid is a very strong fibre and has excellent resistance to abrasion. It can work at a continuous temperature of 204¦C and can take short surges up to 220¦C. It is fibre of choice in clinker material handling and also handling of fly ash and slag. Nomex is an aramid fibre made by DuPont Inc. Aramid felts can take a continuous temperature of 200¦C. They are rugged and give good life in most of the applications. However, being synthetic fibre, they cannot handle spark OR red hot particles and therefore, having an equipment to arrest hot particles or spark before PJBH must be considered while using these bags. They are very expensive when compared with polyester and homopolymer acrylic.
PPS (Polyphenelyne Sulfide): It is a fibre rarely used in cement plant. It is fibre of choice for boiler flue gas filtration. Therefore most of the captive power plants, which are fitted with PJBH as APC equipment, are using PPS filter bags. PPS has a continuous temperature withstanding capacity up to 180¦C. However, this temperature comes down to less than 140¦C if the oxygen is more than 10 per cent in the flue gases. It is known to interact with oxygen and oxidize at temperatures above 150¦C; therefore, care must be taken to ensure that the O2 levels are always below 10 per cent. It has excellent chemical resistance against both alkali and acids.
P-84« (Polyimide): P-84« has been successfully used for filtering flue gases from cement kiln and one of the largest PJBH installed after kiln at a cement plant in northern India is using the filter bags made of P-84« fibres with excellent operating parameters. It can take a continuous temperature of 200¦C. However, if the NOx content is more than 100 ppm, the fibres may become susceptible to oxidation at elevated temperatures.
Glass: Glass is a mineral fibre and E-glass is the grade of glass fibre, which is used in filtration. The fibres are used for making woven cloth, as it is difficult to needle the glass fibres due to their brittle nature. However, very high temperature withstanding capability (260¦C) has made it a first choice for cement kiln/raw mill bag house. It is used in RABH as well as PJBH. Almost all the PJBH installed in country after cement kiln have woven glass with ePTFE membrane laminated filter bags. RABH bags are made by sewing rings at predetermined distance and do not use cages. The glass fibre filter bags have exceptionally good life and fair resistance to both alkali and acids. They have also been used in power plant bag filters for high temperature flue gas filtration.
Finishes applied to the basic felt
There are various finishes applied to the basic felt to enhance the performance of the filter fabrics. They are described as under:
Heat set: The needle felt must be passed through a temperature higher than its continuous operating temperature during manufacturing to make is shrink to a level when exposed to the heat setting temperature that varies for different fibres. If heat set before, the filter bags dimensions will remain unaffected when a temperature of similar value is reached during filtration. Several times, when not heat set the filter bags shrink to a smaller size and fail prematurely.
Singed: Singeing is a process of reducing the fur like protruding fibre tips from the surface that participates in filtration. Singeing enhances the dust release properties of the felt and improves cleanability. It helps felt operate at relatively lower DP on long term basis. This is accomplished by passing the felt over an open flame, which melts the tip of the fibres. The latest technological advancement in this process has taken place where electrical singeing has replaced open flame singeing. This reduces contamination of felt while singeing is carried out.
Calandering: After singeing, the felt is passed through heavy and hot steel rolls to iron out the fibre tips melted during singeing. Calandering improves the dust release properties further. It is the most basic surface treatment done on a filtration felt to make it suitable for use as a filter fabric. Eggshell like surface is created by heavy calandering when extra smooth surface is desired.
PTFE impregnation: PTFE resins are used in different proportion to apply a chemical coating on the fibres. The impregnation of fibres with PTFE resins enhances the dust release property of the fibres and also increases their chemical resistance as the fibres get encapsulated in PTFE resins.
ePTFE membrane: ePTFE membrane laminated media is termed as the best available control technology for emission control. It promotes surface filtration, increases flow through the media and reduces differential pressure drop. Also, the life of media is enhanced in almost all cases. A thin expanded film of PTFE is thermally OR chemically laminated on the filtration side of the felt or woven filter media. The membrane has holes of less than three microns and promotes surface filtration. The felt remains clean and cleaning force required is extremely less.
Filter fabrics have gained preference over ESP and other technologies for air pollution control due to stringent emission norms imposed on cement industry by CPCB and MOEF. Research in development of new fibres and improvement in filtration efficiency of existing fibres have given great advantage to PJBH/RABH equipment over ESPs. The shift towards installation of new bag-filter and conversion of existing ESP to bag-filter has been the trend in last 10 years.
Filter fabrics have gained preference over ESP and other technologies for air pollution control due to stringent emission norms imposed on cement industry by CPCB and MOEF.