L

Bypass Valve

Dirty

Flue Gas-Inlet

Clean Flue Gas Outlet

Purge Air Shaking Mechanism

Tubesheet

Disposal

Inlet

Manifold Flltenn9

Null \ Deflation Cleaning Purging ln|et Thimble (Slake)

Valve

FIGURE 6-21. Schematic diagram of the compartments in a shake-deflate baghouse illustrating the flue gas and cleaning air flows during the various cycles of operation. (Source: Bustard, C. J. et al., Fabric Filters for the Electric Utility Industry, Vol. 1, General Concepts, Electric Power Research Institute, Palo Alto, CA, 1988.)

Clean Flue Gas Outlet

Purge Air Shaking Mechanism

Tubesheet

Inlet

Manifold Flltenn9

Null \ Deflation Cleaning Purging ln|et Thimble (Slake)

Valve

Disposal

FIGURE 6-21. Schematic diagram of the compartments in a shake-deflate baghouse illustrating the flue gas and cleaning air flows during the various cycles of operation. (Source: Bustard, C. J. et al., Fabric Filters for the Electric Utility Industry, Vol. 1, General Concepts, Electric Power Research Institute, Palo Alto, CA, 1988.)

Shake-Deflate Fabric Filters Shake-deflate baghouses are another low A/C type system (2 to 4 ft/min), and they collect dust on the inside of the bags similar to the reverse-gas systems [6]. With shake-deflate cleaning, a small quantity of filtered gas is forced backward through the compartment being cleaned, which is done off-line. The reversed filtered gas relaxes the bags but does not completely collapse them. As the gas is flowing or immediately after it is shut off, the tops of the bags are mechanically shaken for 5 to 20 sec at frequencies ranging from 1 to 4 Hz and at amplitudes of 0.75 to 2 in. [67]. The operating cycles of a shake-deflate baghouse are illustrated in Figure 6-21 [67]. Operating experience with shake-deflate baghouses in utility service has been good [8].

Pulse-Jet Fabric Filters In pulse-jet fabric filters, the flue gas flow is from the outside of the bag inward. This is illustrated in Figure 6-22 [67]. The A/C ratio is higher than reverse-air units and is typically 3 to 4 ft/min allowing for a more compact installation, but the ratio can vary from 2 to 5 ft/min [6]. Cleaning is performed with a high-pressure burst of air into the open end of the bag. Pulse-jet systems require metal cages on the inside of the bags to prevent bag collapse. Bag cleaning can be performed on-line by pulsing selected bags while the remaining bags continue to filter the flue gas. Three cleaning methods have evolved for the pulse-jet systems [68]:

• High-pressure (40-100 psig), low-volume pulse;

• Intermediate pressure (15-30 psig) and volume pulse;

FIGURE 6-22. Schematic diagram of the compartments in a pulse-jet baghouse illustrating the flue gas and cleaning air flows during the various cycles of operation. (Source: Bustard, C. J. et al., Fabric Filters for the Electric Utility Industry, Vol. 1, General Concepts, Electric Power Research Institute, Palo Alto, CA, 1988.)

FIGURE 6-22. Schematic diagram of the compartments in a pulse-jet baghouse illustrating the flue gas and cleaning air flows during the various cycles of operation. (Source: Bustard, C. J. et al., Fabric Filters for the Electric Utility Industry, Vol. 1, General Concepts, Electric Power Research Institute, Palo Alto, CA, 1988.)

The first method is used mainly in the United States, while the latter two methods are used primarily in larger boilers in Australia, Canada, and Western Europe [68].

Pulse-jet cleaning results in lower resistance to gas flow than the other two baghouse types, thus allowing smaller baghouses to filter the same volume of flue gas. Despite this, pulse-jet cleaning is not the preferred choice in the United States for utility boilers because of concerns that the more rigorous cleaning method results in lower particulate collection efficiency and shorter bag life. Pulse-jet baghouses are used in the United States, as well as Japan, for industrial boilers [68]. In Canada and Europe, pulse-jet systems are used in industrial plants and some large-sized utility plants. Much work has been done on improving fabrics for the filters, and the pulse-jet technology is becoming more attractive to utilities.

Fabric Filter Characteristics Fabric filters are made from woven, felted, and knitted materials with filter weights that generally range from as low as 5 oz/yd2 to as high as 25 oz/yd2 [6]. Filtration media are selected depending on the type of baghouse, their efficiency in capturing particles, system operating temperature, physical and chemical nature of the fly ash and flue gas, durability for a long bag life, and the cost of the fabric. Tables 6-9 and 6-10 provide some general data of the most commonly used fabrics and the criteria to select them, respectively [57]. Currently, there is a tendency toward using

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