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Give your plant a dust control tune-up
Give your plant a dust control tune-up
Because PRB coal is smaller, more friable, and contains more ne particulates
than bituminous coal, controlling the fugitive dust generated as PRB coal
moves from bunker to burner tip is problematic. The challenge for material-
handling systems at power plants that have switched coals is to minimize
this dust and capture it cost-effectively and without compromising safety.
By Bernard H. Schonbach, PE, CDG Engineers
D
ust-collection systems capture fugi-
tive coal dust that would otherwise
escape from the perimeter of equip-
ment areas. Collected fugitive dust is fuel
that should be delivered to the furnace and
burned. Dust control systems come in many
different area-specific configurations (Fig-
ure 1). In the typical power plant, they can
be found between the coal-unloading sta-
tion and the outside stockpile, between the
coal yard and the crusher station, between
the crusher station and the tripper room,
and near the coal storage bunker (Figure 2).
For example, a plants railcar-unloading
system will usually have a ducted dust col-
lection system, a passive enclosure cham-
ber, or some form of control spray. Coal
conveyed to the storage pile typically
receives some form of conditioned water
spray or surfactant. By contrast, coal from
the crushing station contains a higher frac-
tion of ne particulate matter, so it typically
calls for a ducted dust collector. Finally,
dust from conveyor transfers into the tripper
room area is usually controlled by collec-
tion hoods, by secondary control systems
such as fog sprayers, or both.
Dust is ubiquitous in power plants. It can
be found at the fan stack, around convey-
ors, and near coal transfer stations, where
poor or missing seals allow easy escape
paths. Conveyor belt cleaners and poor
loading areas contribute to material spillage
and consequent buildup and dusting.
Improperly designed returns from collector
hoppers can disperse dust into the environ-
ment or feed it back into the dust control
circuit. In these cases, the result is a layer
of coal dust that accumulates on all hori-
zontal surfaces.
Equipment-operating procedures also
can cause dusting problems that necessitate
excessive maintenance. Too-frequent dust-
cleaning cycles can overclean, producing
bleeding through collection bags and,
potentially, shortened bag life. Underclean-
ing is just as bad; loaded collection bags
typically raise the pressure drop across
their baghouse, producing lower fan vol-
BURNING PRB COAL
October 2003
|
POWER
www.powermag.platts.com
53
Region 1
unloading
Region 2
stockpile/reclaim
Region 3
crusher/tripper
2. Dust control.
Dust control systems come in many different congurations. In a typical
power plant, they can be found between the coal-unloading station and the outside stockpile,
between the coal yard and the crusher station, between the crusher station and the tripper
room, and near the coal storage bunker. Each area has unique dust control requirements that
must be met by the use of specic techniques. Source: CDG Engineers
Dust Collector
Reverse
air cleaning
Airlock
Bags
Dust
disposal
Fan
Ducting
Pickup
hoods
Blast
gate
1. Dust collection.
A typical coal dust collection system includes pickup hoods, duct-
ing, branch lines, dust collector, fan, and a dust return system. Experience has shown that
the pickup hoods and ducting design are more problematic than the fan and dust collector.
Source: CDG Engineers umes and reduced collection efficiency at
pickup hoods.
Dust should not be allowed to accumu-
late for long periods in the hopper of a dust
collector. The dust return system should be
sized for peak collection rates, and the col-
lected dust either should be conditioned or
delivered safely back to the conveying sys-
tem beyond the collection perimeter. The
density of coal dust can be half that of coal,
so irregular dumping might be symptomatic
of rotary airlock sizing, bridging, or aera-
tion problems. Factors such as moisture,
condensation, dewpoint, and freezing
weather also affect the performance of the
collection system.
Address the problem,
not the symptom
Each piece of equipment that transports or
processes coal creates some level of partic-
ulate matter. If a piece of equipment is
equipped with a containment device, the
fine particles that are stirred up cannot
escape and can be controlled and collected.
When there are rapid changes in the direc-
tion or speed of equipment motion, fine
particles can be carried by air currents
through openings and gaps.
The experienced designer will locate
pickup hoods and suction points at key
points along the coal delivery route to mini-
mize the escape of dust. System components
must be reasonably sealed, and collection
hoods must be properly sized and located.
For example, when dust is observed stream-
ing from a ducted conveyor transfer point,
the most obvious solution would be to pro-
vide more suction air at the collection hood.
This solution may or may not work, depend-
ing on the situation. The leak may indicate
that the hood or duct may not be the correct
size or positioned in the best place, or that
the equipment generating the dust may not
be sealed or contained properly.
Overdrafting collection hoods to solve a
dust problem often results in the drawing of
unnecessary coal into the dust collection
system without xing the problem. Adjust-
ing blast gates to redirect the flow of air
changes the balance-of-collection ductwork
but may create problems elsewhere in the
circuit. Remember that a dust collection
system is designed to capture only the dust
that would otherwise escape to the environ-
ment. Obviously, hoods with drafts high
enough to pull material off the conveyor
belt should be avoided.
Start with an audit
A dust control audit can be used to assess
the performance of both dust collection and
materials-handling systems. Determining
the performance of a collection system
begins with measurements of its airflows
and pressures. A water manometer and a
series of selected holes in the ductwork are
all that is required to map the static pres-
sure of a system. Velocity pressures in
ducts are best measured by pitot tubes
whose readings reect the volume of air in
the system. Power readings of the collector
fan drive should be evaluated against the
fans performance curve to determine the
fans optimum operating point. When new
lter media are installed and/or a system is
rebalanced, performance data should be
recorded for later use as a benchmark of
future performance. Good recordkeeping
provides insights into system trends and
can aid in troubleshooting (Figure 3).
Evaluations of static pressure readings
can quickly identify any problems and de-
ciencies in the dust control circuit. For
example, a high pressure differential across
the baghouse generally means that its lter
is partially blinded. If the differential is
normal but the inlet suction is high, that
indicates a problem in the ducting circuit.
Some ducts may be partially plugged, or
some hoods may be blocked completely,
forcing higher volumes of air into the
remaining ducting.
Likewise, mapping the pressure readings
at hood inlets can serve as a quick check of
the ducting circuit, although airflow read-
ings would be required for an assessment of
the overall system. Blast gates at inlet hood
locations are a convenient way to adjust
airows to balance a system, but care must
be taken not to optimize one hood and
upset the balance of the rest of the system.
High suction pressures have a big impact on
collector fan operation. As static pressures go up,
fan airow goes down. Ducting can easily be
checked for material buildup by tapping with a
hammer and listening. As ducts plug and bags
blind, air volumes fall and reduce the performance
of the system and the efficiency of dust collection.
BURNING PRB COAL
54
POWER
|
October 2003
5.67
5.59
3.54
4.25
3.71
3.75
10.90
X.XX Static pressure
To
atmosphere
Fan
40 hp
9,000 cfm
26" x 34"
Dust
collector
9,000 cfm
4,000 cfm
13"ø
8"ø
1,400 cfm
400 cfm
4"ø
1,000 cfm
7"ø
800 cfm
6"ø
5,000 cfm
14"ø
1,000 cfm
7"ø
6"ø
800 cfm
2,600 cfm
11"ø
3. Audit.
A dust control audit can be used to assess the performance of dust collection
and materials-handling systems. Determining the performance of a collection system begins
with measurements of its a