Page 212 - Air Pollution Control Engineering

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Electrostatistic Precipitation 191

formance lines enable interpolations of precipitator performance to be reliably made
for combinations of operating conditions other than those used in the original pilot
tests. Important effects of particle size and of carrier gas additives readily emerge through
the performance line.
High particulate resistivity is one of the principal causes of poor performance by
precipitators. Particle deposits on the surface of collecting electrodes must possess at
least a small degree of electrical conductivity in order to allow transportation of ions
through the dust layer. If the dust is a good conductor, there is little or no disturbance
of the corona discharge. However, as the particulate resistivity increases, a point is
reached at which the corona ions begin to be impeded. A further increase in particulate
resistivity causes the voltage across the dust layer to increase and corona discharge sets
in, which severely reduces the particulate collection efficiency of ESPs.
Most dusts and fumes have dielectric breakdown strengths of about 10 kV/cm; thus,
2
with a typical corona current density of 1 µA/cm , the critical resistivity appears to be
10
around 10 Ω-cm. Loss of precipitator performance increases with increasing particulate
resistivity above the critical value of 10 10 Ω-cm. Here, sparkover voltages are reduced,
back corona may form, and corona currents are disturbed or disrupted; the effects are
limited to reduce operating voltages and currents. When particulate resistivity exceeds
10 11 Ω-cm, it becomes difficult to achieve reasonable collection efficiencies with pre-
cipitators of conventional design. Above 10 12 Ω-cm, precipitator performance drops to
such low levels as to become impracticable for most applications.
Methods for overcoming the high resistivity of dusts can be classified under several
categories and include the following (22):

1. Keeping collecting plate surfaces as clean as possible. Numerous schemes have been
proposed toward this objective, such as moving brushes, scrapers, and belts. The most
commonly used method to keep the collecting plate surface clean is by high-impact
rapping, using accelerations at the plate surfaces of as high as 50g to 100g.
2. Improving the electrical energization of the precipitator. Experience shows that precipita-
tor performance improves considerably with higher operating voltages and currents. In
practice, sparkover voltages limit the maximum operating voltage. Practical methods for
improving electrical energization include greater sectionalization of corona electrodes, use
of pulsating voltages, fast-acting spark-quenching circuits, and automatic control systems.
3. Conditioning of flue gas. Control of particulate resistivity by varying the moisture and chem-
ical conditioning of the carrier gases is achieved by increasing particle conductivity as a result
of adsorption of moisture and the chemical substances from the gas. Adsorption is a surface
effect and is greater at lower temperatures. Moisture (steam) conditioning is effective at
120–150°C. For chemical conditioning, SO , NH , and NaCl have been commonly used as
3 3
conditioning agents.
4. Changing operating temperatures of the precipitators. The particulate resistivity depends on
temperature according to P = A'exp(−E /kT), where A' is a constant, E is an activation ener-
a a
gy, k is the Boltzmann constant, and T is absolute temperature. This curve passes through a
maximum as gas temperature is increased; thus, at low temperatures (100–150°C) and at
high temperatures (300–370°C), the particulate resistivity is below the level at which the
precipitation problems will be encountered.
5. Temperature-controlled electrodes. It is similar to category 4 except that only the temperature
of the deposited dust layer is changed rather than the whole gas steam. The dust layer is
temperature controlled by heating or cooling the collecting electrodes.