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Electrostatistic Precipitation 177

Fig. 7. Variation of particulate resistivity with moisture content and gas temperature for three
different coals. (From ref. 24.)

For lower resistivity, the particles can be charged quickly to its saturated charge level,
resulting in higher migration velocity, and, accordingly, achieve higher collection
efficiency of ESP. However, if the resistivity of particles is too low (i.e., the particles
7
are considered a good conductor for p<10 Ω-cm), the electrostatic charge could drain
off quickly and the collected particles are then easily re-entrained back into the gas
stream. On the contrary, if the resistivity of particles is too high (i.e., the particles are
considered a good insulator for p>10 11 Ω-cm), the particles become difficult to be
charged in the electrical field and the charges on collected particles do not easily drain
off at the collecting plates. Under this condition, the particles remain strongly attracted
to the collecting plates and are difficult to rap off. Moreover, a “back corona” phe-
nomenon might develop and, accordingly, reduce the migration velocity of particles
in the gas stream.
Major operating parameters influencing particulate resistivity include gas tempera-
ture, moisture levels, and chemical composition of particles (7). The gas temperature of
the maximum resistivity is unfortunate because operators often cannot reduce ESP