Bag Houses

Dried product is collected in either cyclones or baghouses depending upon the product-particle size. When primary coUection is carried out in cyclones, secondary collection in a baghouse or scrubber is usually necessary in order to comply with environmental regulations. A rotary valve is used to provide an airlock at the discharge point. Screws are utihzed to combine product from multiple cyclones or large bag-houses. If required, a portion of the dried product is separated from the main stream and returned to the feed system for use as backmix. 

Particulate Emissions To meet environmental regulations, AFBC boilers, and some PFBC boilers, use a back-end particulate collector, such as a baghouse or an electrostatic precipitator (ESP). Compared to PC units, the ash from FBCs has higher resistivity and is finer Decause the flue-gas path contains cyclones. Both factors result in reduced ESP collection efficiency with AFBC units, but good performance has been achieved with PFBC units, where the SO3 present in the flue gas lowers the ash resistivity. In general, however, bag-houses are the preferred collection devices for both AFBC and PFBC apphcations. 

The principal technological developments in the control of air pollution by engineering during the nineteenth century were the stoker for mechanical firing of coal, the scrubber for removing acid gases from effluent gas streams, cyclone and bag house dust collectors, and the introduction of physical and chemical principles into process design. 

The electric arc furnace process accounted for about 25% of the 1982 U.S. steelmaking capacity (14). Most of the raw material used for the process is steel scrap. Pollutants generated by the electric furnace process are primarily particulate matter and CO. The furnaces are hooded, and the gas stream containing the particulate matter is collected, cooled, and passed to a bag-house for cleaning. Venturi scrubbers and ESPs are used as control devices at some mills. Charging and tapping emissions are also collected by hoods and ducted to the particulate matter control device. 

Plastic venturis should be avoided wherever there is the potential for isolation of the bag cage or other conductive component. Plastic venturi hazards are noted in [33] a recent dust explosion originating in a bag house might have been due to their use. Criteria for selection of antistatic and conductive filter cloths are given in 6-5.2.1. 

Factory-welded subassemblies are based on separate fabrication of bag housing, dust hopper, inlet plenum, outlet plenums, etc., to form the largest subassemblies that can be shipped over the road. Connections are made to adjacent components by welding or bolting at the job site. Bags can be already... 

Bag house An enclosure containing a series of frame-mounted bag filters and a hopper collection unit. 

New cartridge designs for bag houses will allow improved servicing and cleaning techniques. 

Fabric-filter systems, commonly called bag-filter or bag-house systems, are dust-collection systems in which dustladen air is passed through a bag-type filter. The bag collects the dust in layers on its surface and the dust layer itself effectively becomes the filter medium. Because the bag s pores are usually much larger than those of the dust-particle layer that forms, the initial efficiency is very low. However, it improves once an adequate dust-layer forms. Therefore, the potential for dust penetration of the filter media is extremely low except during the initial period after startup, bag change, or during the fabric-cleaning, or blow-down, cycle. 

Most systems use a time-sequence to control the cleaning frequency. If the particulate load entering the bag-house is constant, this approach would be valid. However, the incoming load generally changes constantly. As a result, the straight time-sequence methodology does not provide the most efficient mode of operation. 

Type I (Grade A), types II and III shall be manufd from primary Mg metal contg not less than 99.80% metallic Mg. Mica fillers or other adulterants shall not be used and bag house dust shall not be included in the Mg powder offered for acceptance. 

Although generally associated with manual cleaning, it is possible to obtain fully automatic bag filter systems. These have a hydraulic backwashing and bag-cleaning device solidly attached to the steel bag housing. 

Bag houses These are employed to follow dry scrubbers and/or precipitators in coal burning plants. 

Another critical part of the incinerator design is the pollution control system.11 Pollution control systems directly influence the levels and kinds of pollutants that are released and that can potentially reach the public. Most modern hazardous waste incinerators are designed with extensive air pollution removal systems. For example, a common pollution control system might include a system that cools or quenches gases produced by burning waste, followed by a system that reduces acid gas emissions, and ultimately followed by a particulate removal system such as fabric filters (bag-houses), electrostatic precipitators, venturi scrubbers, and others.10... 

French Also known as the Indirect process. A process for making zinc oxide, in the form of a white pigment, from metallic zinc. The zinc is melted and vaporized in a current of carbon monoxide. The vapor is oxidized with air in a second chamber, forming zinc oxide and carbon dioxide. The fume passes through a settling chamber, where oversized particles settle out, and from there go to a bag house, where the product is collected. Confusingly, the French process has been operated by North American Oxide Company, at Clarksville, TN. See also American. 

There are three main types of bag filter. The simplest, which is shown in Figure 1.60, consists of a number of elements assembled together in a bag-house . This is the cheapest type of unit and operates with a velocity of about 0.01 m/s across the bag surface. 

Figure 1.60. General view of bag-house (low face-velocity type)...

A more sophisticated and robust version incorporates some form of automatic bagshaking mechanism which may be operated by mechanical, vibratory or air-pulsed methods. A heavier fabric allows higher face velocities, up to 0.02 m/s, to be used and permits operation under more difficult conditions than the simpler bag-house type can handle. 

Cadmium is obtained as a byproduct in zinc recovery processes. The metal volatdizes during roasting of zinc concentrates and collected as dust or fume in bag houses or electrostatic precipitators. The dusts are mixed with coal (or coke) and zinc chloride and calcined. The cadmium chloride formed volatihzes upon calcination and thus separates out from zinc. The chloride then is treated with sulfuric acid in the presence of an oxidizing agent. This converts lead, present as impurity in cadmium ore, to lead sulfate which precipitates out. Cadmium is finally separated from copper by the addition of zinc dust and... 

Cadmium oxide is prepared by the reaction of cadmium vapor with oxygen. The metal is first melted in a steel retort and transported into a heated chamber where it is vaporized. The vapor is reacted with air, and the cadmium oxide formed is collected in a bag house. The particle size of the product depends on the ratio of air to cadmium vapor. The oxide may be further purified and particles of uniform size may be obtained by calcination at low red heat. 

SRM 1649 was originally collected in a bag house over a period of one year in the late 1970s at a site near Washington, DC, screened through a 200-mesh sieve (cutoff point 125 gm), and since then apparently stored in bottles at room temperature (Lewtas et al., 1990a). Understandably, Claxton et al. (1992a) caution While the air particles in SRM 1649 are similar to other air particulate samples, they do not represent a typical air particulate sample as collected by most researchers—they are intended as reference materials and not as samples for assessment of levels of airborne toxicants. ... 

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