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188 Chung-Shin J. Yuan and Thomas T. Shen

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varies from 5% to 25%; typical particulate emissions range from 4.6 to 16.1 g/m of
stack gas [or 2–7 grains/standard cubic foot (SCF)]. The particle size distribution of fly
ash varies with the type of boiler and the characteristics of coal. The median diameter
of fly ash is around 5–15 µm (9). At gas temperatures of 232°C or above, the particu-
late resistivity is likely to be below the critical value of 10 10 Ω-cm (16).The particulate
collection efficiency has been rated better than 99%. Newer installations can handle
gases up to 370°C, particularly for high-resistivity particles generated from low-sulfur
coals and residual fuel oils.

4.2. Pulp and Paper Industry
Precipitators are used in recovering salt cake from the flue gases of Kraft mill recov-
ery boilers and in collecting acid mist from paper mills. Particulate emissions from the
recovery boiler are extremely fine and hygroscopic. They are composed principally of
sodium sulfate and sodium carbonate with small quantities of sodium chloride, sodi-
um sulfide, and sodium sulfite. Because of its hygroscopic nature, sampling the gas to
determine particle size distribution is quite difficult. The median particle size for
recovery is approx 1.9 µm (9). The particulate collection efficiencies of ESP range
from 90% to 98%.

4.3. Metallurgical Industry
Applications in the ferrous industry have been in the cleaning of gaseous effluent from
steel-making furnaces, blast furnaces, foundry cupolas, sinter machines, and byproduct
coke ovens. The use of precipitators in the nonferrous industry has been standard prac-
tice for copper, lead, and zinc smelters in cleaning the off gases from the extraction pro-
cess. Precipitators are also used in cleaning gases from electrolytic cells in the reduction
of bauxite to produce aluminum (9). The particulate collection efficiencies of ESPs range
from 85% to 99%. The particulate collection efficiencies are relatively low when applied
to electric arc furnaces because of large quantities of high-temperature gas.

4.4. Cement Industry
Precipitator applications to cement kilns have been particularly favorable because
they permit the recovery of cement as well as the control of particulate emissions.
Precipitators have also been used for the cleaning of ventilating gases and dryer gases. The
particulate emission rate for a cement kiln is highly variable because of variation in the
raw feed and kiln design. Particulate matter from cement kilns generally has high resis-
tivity. Early applications in the cement industry were hampered by the resistivity problem,
but newer installations have successfully overcome the problem of resistivity by control-
ling gas temperature, by conditioning with moisture, and by improving electrical
energization. In the wet process, particulate resistivity is less of a problem. The trend
of precipitator designs is toward higher collection efficiency, current precipitators being
designed for collection efficiencies of ESP over 99.5% (9).
4.5. Chemical Industry

Precipitators have been used to collect sulfuric and phosphoric acid mists and to
remove particulates from elemental phosphorus in the vapor phase. In the manufacture