Coal-fired Boilers. Flyash collection from coal-fired boilers has
been the most common use of electrostatic precipitators. However, these units
are not as well suited to upstream acid gas (HCl, SOx, NOx, and toxics)
treatment systems as are baghouses. For this reason, baghouse/scrubber
combinations are becoming the preferred method in order to comply with
increasingly stringent air emissions requirements.
Solid Waste Boilers. Electrostatic precipitators are suitable
particulate control devices for solid waste incinerators if acid gas control
is not required. Precipitators are better at withstanding the temperature
excursions typical of solid waste combustion, as well as carryover of burning
Advantages. The major advantages of the electrostatic precipitator
are normally smaller physical size than fabric filters, a wide range of
temperature applications and ability to tolerate temperature excursions
outside the normal operating range, low pressure drop with resulting low
energy consumption, and dry continuous disposal of collected dust. A properly
designed and operated precipitator can perform in a reasonably high collection
efficiency range (refer to Figure 20).
Limitations. Electrostatic precipitators are normally more process
sensitive than fabric filters and require tighter control of boiler and
collector operating conditions and fuel selection. Low sulfur coal, selected
for reduced sulfur dioxide emissions, normally produces a high resistivity
ash. The electrical charge retention ability of high resistivity ash makes it
difficult to remove from the collecting plates, thereby causing excessive ash
buildup and electrical arcing. This results in erratic currents and reduced
power to the fields which in turn reduces the collecting ability of the unit.
Figure 19 shows the relationship of sulfur content to ash resistivity.
Unburned carbon in the flyash will reduce resistivity but is an operational
problem. Initial cost is usually higher for an electrostatic precipitator
versus a fabric filter of the same design performance.
Weighted Wire Type. The term weighted wire type comes from the
design of the discharge electrodes. The discharge electrodes, which in this
case are wires, are given the appropriate tension by means of a mass which is
suspended at the bottom of each wire.