Page 199 - Air Pollution Control Engineering

04_chap_Wang.qxd 05/05/2004 1:15 pm Page 178
178 Chung-Shin J. Yuan and Thomas T. Shen

Fig. 8. Variation of fly ash resistivity with sulfur content and gas temperature. (From ref. 25.)

o
temperatures below 250 F without risking the condensation of sulfuric acid on cold sur-
o
faces. On the other hand, increasing the temperature above 350 F results in unnecessary
loss of heat out the stack, which represents a monetary loss. Moreover, the operation of
o
ESP at a gas temperature of approx 300 F would result in a maximum resistivity of the
particles (see Fig. 7). Therefore, it is recommended to operate the precipitator at gas
o
temperatures either below or above 300 F, which develops cool-side ESP and hot-side
ESP, respctively. Between these two types of ESP, the hot-side ESP is currently more
popular than the cool-side ESP.
Figure 7 also illustrates that particulate resistivity decreases with increasing moisture
content of the gas stream. Therefore, injection moisture into the gas stream has been
practically applied to reduce the particulate resistivity and thus enhance the collecting
efficiency of ESP. However, increasing the moisture level might increase the sparkover
ratio, which contrarily reduces the collecting efficiency of ESP. Also, the sulfur content
of the fuel (e.g., coal) plays an important role in determining the particulate resistivity.
11
9
The maximum resistivity of particles decreases significantly from 4×10 to 8×10 Ω-
cm as the sulfur content of coal increases from 1% to 3% (see Fig. 8). The resistivity
decreases with increasing coal sulfur content because of increased adsorption of conductive
gases by fly ash.