Low Resistivity

Particles that have low resistivity are difficult to collect because they are easily
charged (very conductive) and rapidly lose their charge on arrival at the collection
electrode. The particles take on the charge of the collection electrode, bounce off the
plates, and become reentrained in the gas stream. Thus, attractive and repulsive electrical
forces that are normally at work at higher resistivities are lacking, and the binding
forces to the plate are considerably lessened. Examples of low-resistivity dusts are
unburned carbon in fly ash and carbon black.
If these conductive particles are coarse, they can be removed upstream of the precipitator
by using a device such as a cyclone. Baffles are often installed on the collection
plates to help eliminate this precipitation-repulsion phenomenon.
The addition of liquid ammonia (NH3) into the gas stream as a conditioning agent has
found wide use in recent years. It is theorized that ammonia reacts with H2SO4 contained
in the flue gas to form an ammonium sulfate compound that increases the resistivity
of the dust. Ammonia vapor is injected into the duct leading to the precipitator at
concentrations of 15 to 40 ppm by volume. The injection of NH3 has improved the
resistivity of fly ash from coal-fired boilers with low flue gas temperatures (Katz
1979).
Table summarized the characteristics associated with low, normal and high resistivity
dusts.