Electrostatic precipitators dust collection

TABLE VIII Minerals Identified in Electrostatic Precipitator Dusts Collected in a Model Card Room (16) ... 

High-voltage electrostatic precipitator dust diameter 0.01 to 5 pm and conducting particles feed concentration <25 g/m with pressures <7 MPa and temperature <800 °C gas phase Ap = 0.02 to 0.1 kPa mass collection efficiency 99.5% power usage... 

In addition, certain devices are used for pollution control in industry. They either destroy contaminants or remove them (from an exhaust stream) so they are not emitted into the atmosphere. Several examples are provided. An electrostatic precipitator removes (collects) particles from a flowing gas, using the force of an induced electrostatic charge. It can remove fine particles such as dust and smoke from a stream of air. 

Over the years this effect was used in electrostatic precipitators to collect airborne dust in smokestacks,where the dust was collected as a solid cake. Winslow in 1949 found a similar but reversible effect with silica particles mixed in oil. The slurry stiffened when the field of around 2kVmm" was applied but became fluid again when the field was removed. In modem formulations, carbonaceous particles of diameter 3.5 pm and of resistivity 1 M lm are mixed to around 40% volume fraction in a silicone fluid of resistivity lOOTIJm. 

Air cleaning systems are often used to remove dust or vapors from plant or process exhaust streams. Dust collecting systems such as filters or electrostatic precipitators that handle heavy loads of dust are usually designed to be self-cleaning, but it is stiU. necessary to enter the air cleaner periodically for inspection or repair. Dust deposits inside the equipment are likely to be stirred up and inhaled by unprotected workers. Baghouses are particularly likely to cause exposure because large amounts of dust may be retained in the cloth and released when the bags are handled. 

The combined flue dust from waste heat boiler and electrostatic precipitator, including dust from the ventilation system, is collected in a bin and recirculated to the mixing and pelletizing step, where it is used as a binding reagent. 

N2, and traces of PH, CO2, E, and S large furnaces generate off-gas at a rate of about 120—180 m /min. In most installations the off-gas is passed through a series of Cottrell electrostatic precipitators which remove 80—95% of the dust particles. The precipitators ate operated at temperatures above the 180°C dew point of the phosphoms. The collected dust is either handled as a water slurry or treated dry. Einal disposal is to a landfill or the dust is partially recycled back to the process. The phosphoms is typically condensed in closed spray towers that maintain spray water temperatures between 20 and 60°C. The condensed product along with the accompanying spray water is processed in sumps where the water is separated and recycled to the spray condenser, and the phosphoms and impurities ate settled for subsequent purification. 

Handling of alumina and coke presents dusting problems. Hoods and exhaust systems collect the dust, which is then separated from the exhaust air either by cyclones, electrostatic precipitators, filter bags, or a combination of these methods, and recycled to the process (see Air pollution control PffiTHODS). 

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. 

Small solid particles, present in dust and grit emissions, have very low settling velocities (Table 4.4) The collection efficiencies of simple cyclones are tlierefore, as shown in Figure 17.3, relatively low. Fabric filters, electrostatic precipitators or wet scrubbers may be required to remove particles <5 pm in size with an acceptable efficiency. Therefore the cost of pollution control inevitably increases when dealing with particle size distributions skewed towards the lower end. 

Most of the results presented in the previous chapters are based on idealized conditions. In practice, the performance of an electrostatic precipitator can be significantly influenced by the dust layers on discharge and collection electrodes i.e., dust layers may alter the electrical properties of the system. It is also possible that dust layers are not stable i.e., collected particles become loose, increasing the particle concentration in the outlet of the precipitator. These problems play a much smaller role if the surface collection electrode is continuously flushed with water. These wet elearostatic precipitators, however, cannot be used in all applications. 

The two-stage electrostatic precipitators used in light-industry applications are compact devices which can be fitted into the ventilation system. These air cleaners are normally used to clean air from dusts, smokes, and fumes in industrial workplaces. The basic features of these devices are the separate sections for particle charging and collection. The charging section consists of thin metal wires installed between grounded metal plates. The distance... 

Electrostatic precipitators are operated near the sparking limit i.e., corona voltage is continuously adjusted to maximize the collection efficiency. This is normally achieved at the sparking rate of 10-50 sparks per minute. Sparking occurs mostly in the front section(s) of an electrostatic precipitator. In the case of high-resistivity (>10 O cm) dust, special techniques must be... 

A major market which has developed for fabric fillers is for the control ot flue-gas fly ash in the utility industry. This market is primarily at the expen.se of electrostatic precipitators. Fabric filters have the inherent advantage of operating at a high level of collection efficiency tor a wide range of dust and gas conditions. 

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