Electrostatic precipitator factors

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 operation of the electrostatic precipitator is based on three major factors ... 

Using the factors from Table 15.2, all the methods recommend the electrostatic precipitator (Table 15.4). The priorities obtained by different methods do not always agree. The EPS method is more sensitive to resource depletion than the other methods. 

Resistivity of dust The electrical resistivity of a dust, which is one of the factors that influences the practical efficiency of an electrostatic precipitator. 

F varies from 0.1, when only 218Po is present, to unity at equilibrium. Figure 5 shows the changes in the aggregate radon decay-product concentration, the concentration of 218Po, and the variation in F achieved by operating the electrostatic precipitators. The radon decay-product concentration was reduced by more than a factor of 10. 

Figure 5. Radon decay-product and RaA concentrations with equilibrium factor during the use of electrostatic precipitators.

Particulate emissions data for 21 studies of coal-fired power plants were compiled for use in receptor models. Enrichment factors were calculated (relative to Al) with respect to the earth s crust (EFcrust) and to the input coal (EFcoai). Enrichment factors for input coals relative to crustal material were also calculated. Enrichment factors for some elements that are most useful as tracers of coal emissions (e.g., As, Se) vary by more than ten-fold. The variability can be reduced by considering only the types of plants used in a given area, e.g., plants with electrostatic precipitators (ESPs) burning bituminous coal. For many elements (e.g., S, Se, As, V), EFcrust values are higher for plants with scrubbers than for plants with ESPs. For most lithophiles, EFcrust values are similar for the coarse (>2.5 ym) and fine (<2.5 ym) particle fractions. 

Equations for estimating the f.o.b. purchase costs of dust collectors are given in Table 16.32, where the size factor is the actual gas flow rate. All collectors are of mild steel construction. For relatively large dust particles, cyclones are adequate and are the least expensive alternative. For gases containing a wide range of particle sizes, a cyclone followed by an electrostatic precipitator is a common combination. 

The cleaning method selected will depend upon the physical state of the contaminant, its physical properties, the airflow in the system, particle characteristics and other factors. Various cleaners exist for collecting dusts. These include cyclones and wet/dry centrifugals, electrostatic precipitators, fabric filters and settling chambers. Filters and baghouse filters operate under the principles of... 

The size of fly ash particles is a very important factor in determining their removal from stack gas and their ability to enter the body through the respiratory tract. Although only a small percentage of the total fly ash mass is in the smaller size fraction of around 0.1 pm size, it includes the vast majority of the total number of particles and particle surface area. Submicrometer particles probably result from a volatilization—condensation process during combustion, as reflected in a higher concentration of more volatile elements such as As, Sb, Hg, and Zn. In addition to their being relatively much more respirable and potentially toxic, the very small particles are the most difficult to remove by electrostatic precipitators and bag houses (see Chapter 8, Section 8.4). 

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