Fabric Filters Baghouses

The particulates laden gas normally enters the lower portion of the baghouse near the collection hoppers, then passing upward through the device, either on the outside or the inside of the bags, depending on the specific design. 

Commercially available baghouses employ either felted or woven fabric. A fabric is selected based on its mechanical, chemical, and thermal characteristics. Some fabrics (like nomex) are better suited than others (like polyester) for high temperature operations, some perform well in the presence of acid gases, while others are especially good at collecting sticky particulates because of good release characteristics. 

The two design and operational parameters that determine fabric filter performance are air-to-cloth ratio and pressure drop. The air-to-cloth ratio is the volumetric flowrate of the gas stream divided by the surface area of the fabric. The higher the ratio, the smaller the baghouse and higher the pressure 

The pressure drop across the filter medium is a function of the velocity of the gas stream through the filter and the combined resistance of the fabric and accumulated dust layer. Pressure drop across the filter medium is usually limited to 6-8 in. HjO. 

Cyclones are seldom used as the primary means of particulate collection, but often serve as first-stage air cleaning devices that are followed by other methods of particulate collection. Cyclone collection efficiency is probably more susceptible to changes in particulate characteristics than are other types of devices, therefore, care should be taken if used. Cyclone operation is dependant, generally, on physical parameters such as particle size, density, and velocity, as opposed to the chemical nature or properties of the material being collected. 

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Similar to oil-fired plants, either low NO burners, SCR, or SNCR can be appHed for NO control at PC-fired plants. Likewise, fabric filter baghouses or electrostatic precipitators can be used to capture flyash (see Airpollution controlmethods). The collection and removal of significant levels of bottom ash, unbumed matter that drops to the bottom of the furnace, is a unique challenge associated with coal-fired faciUties. Once removed, significant levels of both bottom ash and flyash may require transport for landfilling. Some beneficial reuses of this ash have been identified, such as in the manufacture of Pordand cement. 

Of the three categories, the packed column is by far the most commonly used for the absorption of gaseous pollutants. Miscellaneous gas-absorption equipment could include acid gas scrubbers that are commonly classified as either wet or diy. In wet scrubber systems, the absorption tower uses a hme-based sorbent liquor that reacts with the acid gases to form a wet/solid by-product. Diy scrubbers can be grouped into three catagories (1) spray diyers (2) circulating spray diyers and (3) dry injection. Each of these systems yields a diy product that can be captured with a fabric filter baghouse downstream and... 

The hot combustion gases rise to enter the boiler, producing superheated steam. Each incinerator has its own boiler, and they both feed steam to the same turbine generator. It produces 14 MW of power, yielding 100 million kilowatt hours each year under normal operations. The power plant includes a full pollution control system, with flue gas desulfurization, thermal de-NOx, and a fabric filter baghouse. 

Whether burning tires or TDF improves or deteriorates emissions appears to depend on the control devices installed. ESP s seem to work the best for controlling emissions while burning tires or TDF. It is believed that the zinc content actually helps the ESP perform better, and this improved performance is seen in reduced emissions. Fabric filters (baghouses) also seem to be well suited for the control of emissions while burning tires or TDF. 

The selection of the optimum type of particulate collection device (i.e., ESP or fabric filter baghouse) is often not obvious without conducting a site-specific economic evaluation. This situation has been brought about by both the recent reductions in the allowable emissions levels and advancements with fabric filter and ESP technologies. Such technoeconomic evaluations can result in application and even site-specific differences in the final optimum choice (see Precip Newsletter, 220, June, 1994 and Fabric Filter Newsletter, 223, June, 1994). 

A portland cement plant ball mill emits particulate matter (PM) emissions that must be controlled to meet state air pollution regulations. Three PM control devices, each of which can control these emissions to the same level, are being evaluated (1) a high-energy wet scrubber (scrubber), (2) an electrostatic precipitator (ESP), and (3) a fabric filter (baghouse). Unlike the wet scrubber, the ESP and the baghouse each recover salable cement dust, and, accordingly, revenue can be attributed to those two options. Two scenarios are visualized, with after-tax hurdle rates of 6% and 18%, respectively. The economic specifications for the three devices are as follows ... 

Particulate matter emissions can be controlled using a variety of APCD that include filtration collectors, including primary fabric filters (baghouses) electrostatic... 

Sorbent injection technology seems to have the highest potential to remove both elemental and oxidized mercury from the flue gas (Yang et al., 2007). Different sorbents such as activated carbon, chemically treated sorbents and coal additives, calcium-based sorbents, petroleum coke, zeolites, fly ash, other chemically treated carbons or carbon substitutes, etc., are injected into the upstream of either an ESP or a fabric filter baghouse to control mercury emissions (Pavlish et al., 2003 Yang et al, 2007). The cost of this process is low-to-moderate and separate injection systems may be required (Pavlish et al., 2003). 

The size of a baghouse is primarily determined by the area of filter cloth required to filter the gases. The choice of a filtration velocity (or its equivalent, the air-to-cloth ratio, ACR, in cubic feet per minute of gas filtered per square foot of filter area) must take certain factors into consideration. The rule of thumb for ACR ratios for conventional fabric filter baghouses with woven cloth is 1.5-3.0 ft /min-ft. When dust is fine or loadings are... 

Levels of particulate emissions from exhaust gases range from 0.3 to 1.0 kg ton , with much of this kiln dust being first collected into fabric filter baghouses and then reintroduced into the kiln feed. The dust is normally very rich in sodium and potassium chlorides that have vaporization temperatures of only 883 °C and 774°C, respectively. In the past, before any concerted efforts were made to capture the particulate emissions, the sodium and potassium plumes from cement plant chimneys settled over the countryside, helping to combat acid rain and also acting as fertiUzers in the soil. Today, however, the dust is mainly carried out in the clinker stream, where it creates problems with alkaU aggregate reactions (Bremner, 2001). 

Air emissions from kilns arise because of the nature and chemistry of the raw material used and from the fuel burned in the kiln. Particulate emissions have traditionally been the main soimce of concern PM emissions generally range from 0.3-1.0 kg/tonne from the combined kiln and clinker cooling facilities. Electrostatic precipitators are commonly employed some humidification may be required to optimize moisture to maintain particle resistivity. Carbon monoxide levels in the ESP must be monitored to avoid explosive conditions. Some plants may also use fabric filter baghouses if flow conditions are appropriate and exhaust temperatures are not too high. Much of the collected kiln dust is often recirculated into the kiln feed. 

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