POWER October 2019 - 35

COAL ASH HANDLING
lowing for some travel distance above
the water on the incline for dewatering,
it can be appreciated that such a system
is difficult to retrofit in a congested plant.
The addition of a grinder in the grinder2.
Grinder-conveyor system. Courtesy: Babcock
& Wilcox
water and zero-discharge requirements.
In the last couple of years, it has gained
acceptance and has proven successful
on three operating units with an additional
two currently under construction.
A critical and distinguishing feature of
the new conveyor system is that a clinker
grinder is used to crush the ash prior
to feeding into the conveyors. These are
the same types of grinders used in the
sluice systems to reduce size prior to
pipeline transport. In many cases, the old
grinders can be retained in place along
with the ash hopper and gate when the
new conveyor system is installed.
The grinder is a key feature for retaining
the hopper. Because there are
usually only a few feet of headroom beneath
the hopper, the conveyors must
be small. Conventional under-boiler drag
chain conveyors are large to allow sizeable
clinkers to be removed without
crushing. But with the new grinderconveyor
system, the conveyor housing
only needs to be sized to meet the
required volumetric removal rate rather
than being sized to pass a single large
clinker. A typical profile of a grinder-conveyor
housing is about two feet high by
two to three feet wide.
Both the conventional and grinder-conveyor
drag chains have parallel strands
of chain with steel flights spanning the
strands to push ash along the travel path.
Once the ash has been elevated to the
top of the incline, it dumps into a chute
while the chain and flight assembly turns
180-degrees around a head sprocket and
progresses back down the incline and toward
the tail end of the conveyor in an
endless loop.
The conventional conveyor is known
as top-carry because the run of chain
transporting the ash is above the return
run with the floor of the submerged hopper
separating the two. The return run is
called " dry " because it is not within the
submerged bath of water. This arrangement
necessitates that the chain elevates
above the water level in the hopper
at both the head and tail ends. Considering
that the water level is typically about
15 feet above the ground floor, and alOctober
2019 | POWER
conveyor system permits the conveyor
to remain below the water level at both
the head and tail. This provides the flexibility
to transfer to a subsequent conveyor
under water and arrange a train of
conveyors to remain at low elevation until
a clear space is reached to elevate above
the hopper water level, which is typically
done with the final conveyor at the disposal
bunker outside the boiler building.
The grinder makes this possible because
the chain conveyor can be configured
to be bottom-carry with the return
run traveling above the carry run within
the water bath. Because the ash has
been crushed, it can pass through the
return run which is supported only under
the chain with the flights hanging in
suspension, meaning there is no floor
separating the carry and return runs. This
creates a fully enclosed conveyor with
the added benefit of eliminating spillage
of water and residual ash sticking to
the flights in the return run, which is a
nuisance of both the conventional underboiler
and remote conveyors.
The grinder-conveyor design also provides
routing flexibility to work around
existing obstructions in the boiler area.
Often the most direct route from the
grinders to the preferred storage location
is obstructed by structural columns
or coal mills. But because the conveyors
are loaded at individual points rather than
along the entire axis of the furnace hopper,
as with the conventional conveyors,
grinder-conveyors can be oriented in any
direction necessary. So, while the most
direct method is to use a common conveyor
to collect from multiple grinders,
there is flexibility to use a unique conveyor
beneath each grinder and then
transfer to a common. Furthermore,
the ability to " turn " a conveying train by
rotating at a submerged transfer point
provides many routing options to avoid
existing equipment.
Redundant Options
Another attractive feature of a grinderconveyor
system is that redundancy can
often be built into the conveyor layout. For
ash hoppers with dual grinders on each of
the pant-legs, a grinder-conveyor can be installed
to pick up all grinders on one side of
the pant-legs and a separate 100% capacity
conveyor can be installed beneath the
other set (Figure 3). This is not possible in
the conventional under-boiler design with
its single conveyor at the bottom of the
integral trough hopper, which requires a
unit outage to repair a chain break or clear
a jam in the ash collection area.
Even if a single grinder-conveyor arrangement
is used, most maintenance
jobs do not require a unit outage, because
the conveyors can be isolated from the
ash hopper by closing the hopper gate
above the grinder and closing a secondary
isolation gate below the grinder. This
allows the boiler unit to continue to run
as ash is collected in the pre-existing ash
hopper until maintenance is complete.
Plant operators who have selected the
3. Grinder-conveyor system with redundant
conveyors. Courtesy: Babcock & Wilcox
www.powermag.com
grinder-conveyor technology for their bottom
ash conversion projects have primarily
done so because of the substantial savings
in capital expense (equipment and installation)
afforded due to the design features
that have been highlighted. O&M savings
also can be substantial when compared to
competing technologies that either sluice
to remote dewatering equipment or pneumatically
convey the ash dry to a storage
silo. Pumps and blowers used in these
systems are high power consumers compared
to chain conveyors. For example,
comparing two recent projects at plants
each with two operating units and similar
MW output, the plant using the grinderconveyor
technology required only 10%
of the installed horsepower as the plant
that selected a remote mechanical drag
system. Furthermore, eliminating the highpressure
sluice water can provide large
savings on maintenance costs associated
with pump rebuilds.
Coal plant owners and operators are
continuously looking for ways to reduce
cost to stay competitive. Closures and
gas conversions have become common
for plants facing steep compliance costs
associated with new regulations. Grinderconveyor
technology for bottom ash conversions
can offer significant savings in
both capital and operating expenses, and
allow existing coal units to remain operational
in the face of such challenges. ■
-Tyler Little is an engineer in The
Babcock & Wilcox Co.'s advanced
engineering applications department.
35
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POWER October 2019

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