ELECTROSTATIC PRECIPITATION 285
There is another feature of precipitator design that is of
vital importance to collection efficiency, namely the velocity
of the ash laden gas flowing through the collector. The lower
the velocity, the greater treatment time available to thor-
oughly charge the flue gas and the lower the velocity, the less
chance there is for reentraining or sweeping off the fly ash
accumulated on the collecting plates. The lower the sulphur
content of the coal, the higher the resistivity of the ash. Since,
under these conditions the collected ash has difficulty stick-
ing to the collecting plates, lower sulphur coals require lower
gas velocity. Figure 6 indicates the maximum gas velocities
required to insure the required collection efficiencies when
burning 1.0% sulphur coals.
It is known that the resistivity of the fly ash is related to
sulfur content of the coal burned and also to the temperature
of the flue gas.
We have already seen that the lower the sulphur content,
the higher the resistivity of the fly ash but as the flue gas
temperature drops below 300°F, the high resistivity effect of
lower sulfur is substantially reduced. A 30°F decrease in gas
temperature under certain conditions will offset the effect of
a 1.0% decrease in the sulfur content of the coal.
All the data we have previously discussed in this paper
was at a nominal operating temperature of 300°F.
In order to insure that adequate electric power is avail-
able for charging our precipitators we again reviewed the
latest units on our system. Two sets of curves were developed
from this study. Figure 7 shows the total rectifier capacity inmilliamps and Figure 8 shows the total transformer capacity
in kilovolt-amperes.
These curves are based on 2.0% sulfur coal. Having more
electric capacity is of little value with the low sulfur coals
with high resistivity ash because the determining factor is
how much power the ash laden flue gas will absorb, not how
much of a charge can be imposed on the gas. On the other15060708090959697989999.52345 1020 30 40
MA PER 1000 CFM
TOTAL RECTIFIER CAPACITYCOLLECTION EFFICIENCY PERCENT2.0% SULPHURFIGURE 7 Precipitator design curve.105060708090959697989999.520 30 40 50 100 200 300
KVA PER 100,000 CFM
TOTAL TRANSFORMER CAPACITYCOLLECTION EFFICIENCY PERCENT2.0% SULPHURFIGURE 8 Precipitator design curve.1 2 3 4 5 6 7 8 9 10 115060708090959697989999.5VELOCITY FEET PER SECONDCOLLECTION EFFICIENCY PERCENT1.0% SULPHURFIGURE 6 Precipitator design curve.C005_005_r03.indd 285C005_005_r03.indd 285 11/18/2005 10:22:02 AM11/18/2005 10:22:02 AM