Electrostatic precipitation particle sizing

Systems for sampling of airborne pollutants usually consists of three parts (a) a means of collecting the air sample (b) a device to trap the pollutant and (c) a means of measuring the amount of air sampled. The sampling methods frequently used for this purpose are sedimentation [21,22], centrifugation [23], impaction, filtration and thermal or electrostatic precipitation [24,25], the commonest of which are filtration, impaction and electrostatic precipitation. The size of the collected particles depends on the particular method used. 

Precipitate particles grow in size because of the electrostatic attraction between charged ions on the surface of the precipitate and oppositely charged ions in solution. Ions common to the precipitate are chemically adsorbed, extending the crystal lattice. Other ions may be physically adsorbed and, unless displaced, are incorporated into the crystal lattice as a coprecipitated impurity. Physically adsorbed ions are less strongly attracted to the surface and can be displaced by chemically adsorbed ions. 

An additional method for increasing particle size deserves mention. When a precipitate s particles are electrically neutral, they tend to coagulate into larger particles. Surface adsorption of excess lattice ions, however, provides the precipitate s particles with a net positive or negative surface charge. Electrostatic repulsion between the particles prevents them from coagulating into larger particles. 

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. 

The properties of gas ions are of great importance for the electrical performance of an electrostatic precipitator. They also are very important for particle-charging processes. The size of gas ions is normally such that they can be regarded as gas molecules carrying a single elementary charge. It can even be assumed that ions form a gas component with a very low- partial pressure. Thus, the thermal motion of gas ions is assumed to be similar to that of gas molecules. The most important parameters describing the properties of gas ions are... 

Ash particles produced in coal combustion are controlled by passing the flue gases through electrostatic precipitators. Since most of the mass of particulate matter is removed by these devices, ash received relatively little attention as an air pollutant until it was shown that the concentrations of many toxic species in the ash particles increase as particle size decreases. Particle removal techniques become less efiective as particle size decreases to the 0.1-0.5 pm range, so that particles in this size range that escape contain disproportionately high concentrations of toxic substances. 

Methods of dust removal depend mainly on the particle size of the dust and the temperature and moisture content of the gas. The methods used are broadly divided into dry methods and wet methods. The dry methods involve the use of gravity and baffle chambers, cyclones, filters, and electrostatic precipitators, while the wet methods involve the use of spray towers and venturi scrubbers. In principle, wet cleaning is preferred to dry cleaning because of the excessive wear associated with and the difficulty in handling the fine dusty material removed in the dry methods. The wet methods, however, must be followed by such operations as filtration, drying of filter cakes, and recycling of water. 

Airborne particles, bacteria, fungi spores and allergens will be removed by air filtration, which is often cheaper and more effective than electrostatic precipitators and ionizers. A disposable, high-efficiency filter capable of removing at least 95 percent of particles with size down to 0.3 micron will be installed. A washable coarse filter that removes fifty percent of particles larger than 10 microns will be used as a pre-filter to extend the life of the high efficiency filter by at least a third. An inexpensive sensor will be included to indicate when filter replacement is necessary. 

As cyclones are less effective as the particle size decreases, secondary collection units are frequently required, i.e., filters, electrostatic precipitators, and scrubbers. When dry collection is not required, elimination of cyclones is possible if allowance is made for heavy solids loads in the scrubber (see Gas-Solids Separations see also Sec. 14). 

Electrostatic precipitators devices used to trap fine dust particles (usually in the size range 30 to 60 p,m) that operate on the principle of imparting an electric charge to particles in an incoming airstream which are then collected on an oppositely charged plate across a high-voltage field. 

These data show that the ash content of the dried aqueous extracts tend to be higher than the original for all three types of samples. The increase is especially high for the condenser trash samples and is quite similar to the behavior of plant parts (21). There is also a trend that as the particle size becomes smaller, the ash content increases. The ash content of the electrostatic precipitator dust was particularly high (41.3%). 

Elutriation is important in most industrial fluidized beds and is generally thought of as a disadvantage. In addition to the small particles which may be present in the initial particle size distribution, fines may be created in the course of operation by the attrition of bed particles. Elutriated particles usually need to be collected and recovered either because they represent the loss of product particles of a given size, because they must be separated from the exhaust gas for environmental reasons, or because of safety concerns there is a considerable risk of a dust explosion with very fine particles and perhaps especially so with many food particulates. Therefore the fluidized bed plant will require ancillary gas cleaning equipment such as a cyclone, filter or electrostatic precipitator to separate the fines from the gas. The loss of a particular size fraction from the bed may change fluidized bed behaviour and it then becomes important to return the fines to the bed continuously. 

Figure 2. Concentration vs. size curves of Fe, Al, Sc, V, U, and Se in aerosol particles collected upstream of an electrostatic precipitator of a coal utility boiler.

While electrostatic precipitators have relatively high collection efficiencies (99-100%) over a wide range of particle sizes ( 0.05-5 /im), there are a number of disadvantages. These include the lack of size information, particle reentrainment due to sparking, and practical problems such as high cost and shock hazards. As a result, they have not been widely used in ambient air studies. 

Modem coal combustion employs two principal techniques combustion in a fluidized bed or pulverization, followed by combustion of fine particles suspended in moving air. Figure 1 shows a schematic of pulverized coal combustion, a process much used in steam-raising plants. Each process produces a characteristic residue fluidized bed combustion gives rise mainly to a clinker-like or granular product, whereas pulverization, followed by combustion, produces mainly a much finer, micrometre-sized ash residue. Pulverization also yields a coarser fraction, the so called bottom ash , which is periodically removed without difficulty. However, the finer fly ash has to be recovered by filtration and electrostatic precipitation. Commercially, fly ash has... 

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