Fabric Filter Plant (FFP) Technology is installed in eight of Eskom’s coal fired power stations for particulates removal from the flue gas after combustion in the boiler. Eskom is one of the biggest users of fabric filters globally, with a total number of 676 818 bags currently operating in 42 boilers. The bags are manufactured according to Eskom’s bag specification. Approximately 72% of the installed bags are manufactured from polyphenylene sulphide (PPS) fibres and operate at a temperature of about 140 °C, while 28% of the bags are manufactured from polyacrylonitrile (PAN) fibres and operate at approximately 125 °C. Eskom’s bag specification was modified by blending PAN and PPS fibres with multilobal P84 (Polyimide) fibres to improve filtration efficiency. Approximately 94% of the bags in Eskom’s FFPs are blended with P84 fibres.

The performance monitoring program was introduced whereby the bags are sampled and tested during operation to study failure pattern and to identify parameters of the bag specification that need to be modified for performance improvement. The bags are sampled at 3 000 hour intervals and tested for deviation in air permeability, pH, strength and dust retention. It was observed that the bags in most power stations fail due to acid attack, reduced air permeability and high dust retention.

The performance of the bags differ according to the location of the power station, the type of coal used in the boilers and FFP design. PPS bags are expected to achieve minimum of 32 000 operational hours while PAN bags are expected to operate for minimum of 18 000 operational hours. The bags in the newly built power stations are failing prematurely below 15 000 operational while some of the old power stations are achieving or sometimes exceeding the expected operational life. In one station the bags were achieving approximately 40 000 operational hours, and after analyzing condition monitoring test results it was observed that performance of the bags could be improved by modifying the fabric specification. The specification was modified by constructing the raw gas side of the fabric into two filtration layers instead of one layer (Eskom Fabric Filter Bag Standard, 240 -53113965). The first filtration layer was constructed by ...

The second layer and the inner batt were constructed from 2.0 dtex round PPS fibres. The scrim and inner batt remained unchanged. The air permeability was adjusted from 16 ml/cm²/s to 6 ml/cm².s at 125 Pa by needling tighter and compacting the fabric to reduce dust penetration. The operational life of the FFP increased from 40 000 to 70 000 hours with this modified fabric. For continuous improvement, the condition of the bags will be assessed at the end of life (>70 000) to identify areas of the specification that need to be modified.

Eskom also introduced trial bags program in which different filter media are tested for application in Eskom’s FFPs. The aim of the trial bag program is to identify alternative filter material that can operate efficiently in Eskom’s FFP. Ceramic filters, Laminated fabrics and various blended fabrics were evaluated for Eskom’s application...