Particulates, removal

Pollutants. The problems posed by ak pollutants are very serious. Within a museum, measures can be taken to remove harmful substances as efficiently as possible by means of the installation of appropriate filter systems in the ventilation equipment. Proposed specification values for museum climate-control systems requke filtering systems having an efficiency for particulate removal in the dioctyl phthalate test of 60—80%. Systems must be able to limit both sulfur dioxide and nitrogen dioxide concentrations <10 /ig/m, and ozone to <2 /ig/m. ... 

Often the required pressure drop through the downstream particulate removal device is not estabUshed until actual operation, and furnaces are sized assuming Httle or no pressure drop across the particulate removal device to allow adequate furnace residence time under all operating conditions. 

The gaseous ammonia is passed through electrostatic precipitators for particulate removal and mixed with the cooled gas stream. The combined stream flows to the ammonia absorber where the ammonia is recovered by reaction with a dilute solution of sulfuric acid to form ammonium sulfate. Ammonium sulfate precipitates as small crystals after the solution becomes saturated and is withdrawn as a slurry. The slurry is further processed in centrifuge faciHties for recovery. Crystal size can be increased by employing one of two processes (99), either low differential controUed crystallization or mechanical size enlargement by continuous compacting and granulation. 

In magnesium casting, sulfur dioxide is employed as an inert blanketing gas. Another foundry appHcation is as a rapid curing catalyst for furfuryl resins in cores. Surprisingly, in view of the many efforts to remove sulfur dioxide from flue gases, there are situations where sulfur dioxide is deHberately introduced. In power plants burning low sulfur coal and where particulate stack emissions are a problem, a controUed amount of sulfur dioxide injection improves particulate removal. 

Removal of Particulate Matter. The amount of particulate entering a cooling system with the makeup water can be reduced by filtration and/or sedimentation processes. Particulate removal can also be accompHshed by filtration of recirculating cooling water. These methods do not remove all of the suspended matter from the cooling water. The level of fouling experienced is influenced by the effectiveness of the particular removal scheme employed, the water velocities in the process equipment, and the cycles of concentration maintained in the cooling tower. 

Cement plants in the United States are now carehiUy monitored for compliance with Environmental Protection Agency (EPA) standards for emissions of particulates, SO, NO, and hydrocarbons. AH plants incorporate particulate collection devices such as baghouses and electrostatic precipitators (see Air POLLUTION CONTROL methods). The particulates removed from stack emissions are called cement kiln dust (CKD). It has been shown that CKD is characterized by low concentrations of metals which leach from the CKD at levels far below regulatory limits (63,64). Environmental issues continue to be of concern as the use of waste fuel in cement kilns becomes more widespread. 

Particulate removal from the coal gas is effected either through a series of dry-sofld and wet-sofld removal steps or through the use of dry soflds filters, so that the gas fed to the combustion turbine is essentially free of suspended particulates. The emissions of total suspended particulates (TSP) from a CGCC plant are about one-third those from a comparable pulverized coal plant equipped with a fabric filter and EGD unit. 

Relatively low operating pressure drops (for degree of particulate removal obtained) in the range of approximately 2- to 6-in water column... 

Preconditioning for Particulates Heavy particulate loading of the inlet gas with dust, grease, oils, or other aerosols can be very dam-aging to the pore structure of the filter bed, resulting in an eventual pressure-drop increase. Oils and heavy metals that are deposited on the filter bed can be poisonous to the microorganisms that live within the biofilm. Particulate APC equipment such as fabric filters and venturi scrubbers are generally adequate for this level of particulate removal. 

Humidification of the gas stream is the preferred method of keeping the filter bed moist. Gas moisture is usually added to the incoming gas stream downstream of the particulate removal APC equipment by either water sprays or steam. Adding moisture directly to the top of the bed in order to maintain filter media moisture is not recommended since this can result in (I) locahzed drying of the substrate, and (2) cold water addition will reduce the ac tivity of the microorganisms until the water becomes warmed to the steady-state filter ed temperature. 

Each is discussed in Sec. 17 of this handbook under Gas-Sohds Separations. The effectiveness of conventional air-poUution-control equipment for particulate removal is compared in Fig. 25-23. These fractional efficiency cui ves indicate that the equipment is least efficient in removing particulates in the 0.1- to 1.0- Im range. For wet... 

The electrostatic effect can be incorporated into wet scrubbing by charging the particulates and/or the scrubbing-liquor droplets. Electrostatic scrubbers may be capable of achieving the same efficiency for fine-particulate removal as is achieved by high-energy scrubbers, but at substantially lower power input. The major drawbacks are increased maintenance of electrical equipment and higher capital cost. 

As with particulate removal systems, it is apparent that many choices are available for removal of gases from effluent streams. Table 29-5 presents some of the factors that should be considered in selecting equipment. 

If an ESP is 90% efficient for particulate removal, what overall efficiency would you expect for two of the ESPs in series Would the cost of the two in series be double the cost of the single ESP List two specific cases in which you might use two ESPs in series The gaseous effluent from a process is 30 m min at 65°C. How much natural gas at 8900 kg cal m would have to be burned per hour to raise the effluent temperature to 820 "C Natural gas requires 10 m of air for every cubic meter of gas at a theoretical air fuel ratio. Assume the air temperature is 20°C and the radiation and convection Iosm s are 10%. 

A variety of particulate removal technologies, with different physical and economic characteristics, are available. Some of these are as follows ... 

List the major types of air pollution control devices used for particulate removal. Visit the Web and make a list of equipment suppliers for these hardware. 

Residuals Produced Exhaust gases require acid gas and particulate removal through the use of a gas scrubber and the ashes may require solidification before landfilling. 

Computer sensitivity studies show that hole size strongly affects the fraction of fission products released from the containment. The failure location determines mitigation due to release into another building in which condensation and particulate removal occur. The quantity released depends on the time of containment fails relative to reactor vessel failure. If containment integrity is maintained for several hours after core melt, then natural and engineered mechanisms (e.g., deposition, condensation, and filtration) can significantly reduce the quantity and radioactivity of the aerosols released to the atmosphere. 

In the true gravity case, pumps are not used. If, however, the liquid is highly viscous, to achieve efficient operation, pumps are required to force the fluid through the pressure filters. The pump can be considered essentially as a press with a plate-and-frame filter. The plate-and-frame filter consists of a series ot frames over which the filter medium is stretched. A centrifugal basket of fine mesh is another method of particulate removal. 

Cilia Fine hairlike structures found in the membranes that line the respiratory tract that assist in particulate removal. 

Venturi Scrubber A scrubber with water velocities of between 60 and 100 m s" or higher, which create shear stresses, breaking up a gaseous air stream to provide ef fective particulate removal. 

Fried lander, S. K., L. Silverman, P. Drinker, and M. W. First, Handbook on Air Cleaning Particulate Removal, United States Atomic Energy Commission, 1952, Washington, D. C. 

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... 

---