286 ELECTROSTATIC PRECIPITATION
hand high sulfur coal ash does require much more electric
capacity than shown on these curves. Precipitators operating
under these conditions are sometimes referred to as power
“Hogs.”
Certain basic criteria should be met in order to attain
high collection efficiency at a modern power plant installa-
tion. These include:1) Low velocity of the gas passing through the
collection zone.
2) High time of contact of the gas in the precipitator.
3) Reasonably large collecting surfaces.
4) High corona power.The collection efficiency of a precipitator is closely related
to the useful amount of high corona power that can be sup-
plied to the precipitator: the greater the useful power, the
higher the efficiency. For better efficiency this power should
be distributed among many energized collecting sections,
each having an individual control and power supply.
Burning low sulfur coals requires the upgrading of dust
collection equipment. This leaves a company with the fol-
lowing alternatives if physical space exists:1) Add collecting surface and electrical sets to exist-
ing 99.75% collectors for example to maintain the
necessary efficiencies when burning low sulfur
coals.
2) Add new precipitators in series with the existing
precipitators to increase the efficiency for example
from 99% to 99.8% when burning low sulfur coal.
3) SO 2 conditioning and pulsed energization.^7“HOT” PRECIPITATOR CASE STUDYHot gas temperature (>600°F) precipitation offers a feasible
alternative for the low sulfur eastern coal situation.
This approach was used for the newest operating coal
fired unit, at Con Edison’s Ravenswood No. 30. The loca-
tion of this precipitator, between the economizer outlet of
the boiler and the air heater inlet, is shown in Figure 9.
The location of this “hot” precipitator was predicated on
three considerations:1) Anticipated reduced air heater fouling by locating
the precipitator ahead of the air heater.
2) Ability to burn low sulfur coals without affecting
the collection efficiency (see Figure 10).
3) The boiler was designed to burn oil as an alternate
fuel and it was desired to be able to operate the
precipitator when burning this oil. After extensive
tests on a pilot installation at Ravenswood No. 10
while burning oil, it was determined that the pre-
cipitated oil ash caught in a “hot” condition could
be handled. It has been demonstrated that such a
precipitator is effective in collecting oil ash.The collector is made up of four separate combination units,
two double-decked for the north boiler and two double-decked
for the south boiler. Extensive model study was required to
attain the most efficient flue design that would result in proper
gas and dust distribution entering each precipitator. The height
of this precipitator for the 1000 MW unit is over 15 stories. The
latest performance test on this Ravenswood No. 30 collector
when burning coal has met the most optimistic expectations.
As previously mentioned, the design of the precipitator units
for Boiler 30 at the Ravenswood Station featured the “hot gas”
concept primarily because of its more efficient characteristics in
collecting the particulate matter from flue gas while firing either
fuel oil or low sulfur coal. Proposed air-pollution control legis-
lation at the time confirmed the need for such characteristics in
new precipitator equipment. An interesting structural problem
arose in the design of the supporting steel for these precipitator
units which we feel was resolved in a rather unique fashion.
Because the gas to be handled at the higher temperatures
is much greater than would be required at the normal” cold
precipitator” temperature levels, the equipment itself must
be bigger and, therefore, heavier. The four precipitator units
at Ravenswood—Blr 30 required a building volume approxi-
mately 90 243 167 high which included space to
install the large associated flue sections. The decision was
made to enclose the building on all its exposed sides with
uninsulated metal siding (and to provide a roof) for the fol-
lowing reasons:1) To eliminate the need to weatherproof the flue and
equipment insulaton.
2) To eliminate wind loading on the large exposed
surfaces of the flues and precipitator units.
3) To improve the appearance of the installation
which is only 150 from a public street.
4) To reduce the external sound levels around the
installation.
5) To reduce future maintenance costs.FIGURE 9C005_005_r03.indd 286C005_005_r03.indd 286 11/18/2005 10:22:02 AM11/18/2005 10:22:02 AM