Filter Media Selection for Dry FGD and Particulate Control was the Hot Topic Yesterday

The importance of filter media selection for dry FGD and particulate control was discussed in the Hot Topic Hour on August 11, 2011.

Zachary Arndt, Environmental Associate with Sargent & Lundy LLC, was the leadoff speaker. He emphasized that recent US EPA policies will require more baghouse installations on utility boilers in order to be in compliance with Utility MACT, the Cross-State Air Pollution Rule and BART (Best Available Retrofit Technology) or Regional Haze Rule. Arndt provided an approach to selecting the appropriate baghouse fabric filter bags when high temperatures, oxygen concentrations and chemical agents are present in the inlet gas flow. He stressed the importance of understanding design conditions before the fabric media is chosen. Fabric filter bag concerns he mentioned were chemical degradation due to acid attack and thermal degradation due to elevated temperature and oxygen levels. He presented an excellent chart showing the effect of high temperature and oxygen concentration level on bag life.

PPS and P84 were the media compared for dry FGD and particulate control. PPS is the choice in most cases for this application. There have been problems with P84. PTFE w/PTFE Membrane and Fiberglass w/PTFE Membrane are seen as overkill. Typical PPS pulse jet fabric filter (PJFF) Bag Specifications are for needle felt, a bag length of 8-10 meters, weight of 16-18 oz/sq yd, thickness of 0.06 inch minimum, density of 0.3g/cm³ minimum and mullen burst of 350 psi.

Tom Anderson, Vice President for Pleated Products at Midwesco/TDC, discussed matching the appropriate media to specific applications and developing pleated bag applications/conversions as a solution to collector operation problems. Anderson also stressed starting with an understanding of temperature and chemistry conditions. He too saw PPS as a good choice and knows of only three or four failures with it. Anderson made a strong case for pleated bags explaining that the greater surface area provides greater residence time and allows for greater capture of mercury. He said we will be hearing more about developments in this area.

The discussion then turned to the CARB ruling which only requires a stack test of baghouses every five years if they are using a fabric approved by ETV testing. Participants seemed to agree that this rule ignores many other problems that could exist in the operation of a baghouse.

Terry Wanta, Pristyne^® Filter Media Business Leader at W.L. Gore & Associates, discussed the impact a dry sorbent injection system (DSI) or dry scrubber can have on the filter media, the various filter material choices available, and how ePTFE membrane media may improve operations while meeting the required regulatory limits. Wanta provided an excellent description and diagrams of the difference between surface and depth filtration. She explained that ePTFE Membrane on PPS felt or woven fiberglass is common in MSW plants and is gaining acceptance with electric utilities. In the case of a DSI or spray dryer retrofit before an existing fabric filter, the operating conditions will change. The higher dust loading may affect cleaning frequency and may affect ash handling system operation. Ash characteristics change in that particle size distribution changes and ash may become more cohesive with higher moisture and lower temperature.

Wanta cited the advantages of a membrane in DSI/DFGD systems. The expanded PTFE membrane is chemically inert. The ePTFE membrane captures fine aerosol particulate matter (PM) on the surface providing the co-benefit of capturing HAP metals and protecting the backing material from acidic PM. The smooth, micro-porous surface provides superior release of sticky ash. The ePTFE membrane is less sensitive to changes in ash size or chemistry and allows more flexibility with the cleaning system to adjust for changes in dust loading and ash characteristics. The ePTFE membrane can recover from process upsets, tube leaks, and start-up/shutdown problems. Higher capture of solid PM may provide some margin for Total PM even if condensable PM increases.

Keith Ogilvie, Account Executive at Hamon Research Cottrell, as the final speaker brought a balance to the session by discussing the collector. He focused on the low pressure high volume pulse jets provided by Hamon Research Cottrell. They have installed collectors on 22,000 MW worldwide and over 12,000 MW in North America. The company has fabric filter application experience on a wide variety of coals, primary particulate control, polishing filters downstream of an ESP, with PAC for mercury control, with DSI for acid gas control, following SDA and/or CFB dry scrubbers and in ESP to fabric filter conversion. Bag life guarantees of five years are being met. Filterable emission guarantees are at 0.01 lb/MMBtu and recent units have averaged less than 0.006 lb/MMBtu. A typical pressure drop is 6 inches.

The Bios, Abstracts and Photos are linked below.
BIOS AND ABSTRACTS - 8-11-11.htm