POWER October 2012 - 33

TOP PLANTS
heater hopper material from fly ash and adding
a fly ash treatment system to prevent ash
from settling during unloading and transport.
Altogether, the conversion cost around $70
million and required extensive environmental
permitting and planning. Constellation viewed
this expenditure as prudent both to protect the
generating asset and the personnel who work
there. " The company had the desire to make
sure we did this right, " Butler said.
Unit Characteristics
C.P. Crane Unit 1 is nominally rated at 190
MWnet and began operating in 1961. Unit
2 is nominally rated at 195 MWnet and began
operating in 1963. The boiler units are
cyclone-type, which feed coal in a spiral
manner into the furnace's combustion chamber.
Each unit is equipped with a baghouse
to control particulate matter emissions as
well as overfire air and selective noncatalytic
reduction (in the form of urea injection) to
control NOx emissions. Powdered activated
carbon injection systems are also installed
and provide additional mercury emissions
reductions as needed.
Crane historically burned bituminous coal
from the northern Appalachian region. This
coal has a relatively high heat content (about
12,100 to 13,100 Btu/lb) but typically has a
relatively high sulfur content (about 1.6 to
2.3 wt% sulfur). Crane's permit allows SO2
emissions of up to 3.5 lb/MMBtu, which correlates
to approximately 2.3% sulfur.
Subbituminous coal such as that from
the PRB is typically characterized as having
lower heat content (about 8,500 to 9,400
Btu/lb), but it typically has a much lower
sulfur content (about 0.1 to 0.5 wt% sulfur).
Most subbituminous coals have ash characteristics
that are compatible with cyclone
boilers. The lower sulfur content results in
significantly lower SO2 emission rates, as
low as 0.4 to 0.5 lb/MMBtu.
As part of the Crane Station conversion,
Constellation proposed to change or replace
the rail car dumper to allow for longer
rail cars with a larger volume of coal to be
dumped in the existing railcar dumper building.
Subbituminous coal is less dense than bituminous
coal, so larger rail cars can be used
to transport the higher volumes of coal. As a
result, Crane's rotary railcar dumper had to
be modified to handle the larger cars.
Because subbituminous coals typically
have lower heat content and higher bound
moisture content than bituminous coals, more
of it is needed. The existing coal feeders were
found able to provide the required higher coal
feed rates. After engineering evaluations and
system considerations, the blades on the existing
induced draft fans for Units 1 and 2 were
tipped to help ensure the plant was able to
October 2012 | POWER
handle the increased gas flows that result from
burning PRB coal at current maximum loads.
Constellation tested a range of subbituminous
coals at C.P. Crane during 2008 and
2009 under test burn approvals granted by
state regulators. Crane tested subbituminous
coals from different mines in the PRB as well
as an international subbituminous coal from
Indonesia. During these tests, Crane evaluated
operational and environmental impacts
when burning different blends of subbituminous
coal with bituminous coals. Operators
found that, generally, subbituminous coal
could be burned at the plant, but PRB was
best burned as a blend with bituminous coal.
When high feed rates and/or blends of
PRB were burned, combustion was less efficient,
there were higher levels of unburned
carbon in the fly ash, and full load was difficult
to maintain for extended periods of time.
Increasing the fineness of the coal through
the new crushers had a significantly positive
impact on combustion efficiency and helped
improve furnace tapping as well.
Lessons Learned
The sometimes-unpredictable nature of subbituminous
coal was driven home at the Crane
Station by several teachable moments that
Butler discussed in a presentation at the PRB
Users' Group meeting in May. First, in August
2010 the plant experienced a fire on its
E Belt tail pulley that resulted from hot work
being performed in the area. This ignited coal
in a part of the coal-handling equipment that
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was missed as part of the daily washdown/
cleaning procedure. Plant operators made
structural modifications to gain access to
the area for cleaning and also modified the
plant's hot work procedures.
Then, in September 2011 elevated CO
levels were detected at coal silos, the result
of an exothermic reaction due to wet coal.
Operators addressed this problem by lowering
silo levels and keeping the coal moving.
Butler recommended using CO monitoring
and thermography as useful predictors of
developing problems. Also that month, a fire
broke out on the A-V coal feeder belt, which
stemmed from wet coal hiding out in the belt
roller when the feeder was idled in advance
of a unit shutdown.
Finally, in March 2012, smoldering coal
was discovered on a reclaim feeder, the result
of " old " coal sitting inside a Syntron feed
chute. A degraded chute allowed air ingress,
which led to the smoldering.
Handling subbituminous coal safely and
efficiently " all starts with housekeeping, "
Butler told the PRB Users' Group. He recommended
painting coal-handling areas white
and washing down equipment and surfaces.
Above all, he emphasized, " don't get complacent. "
In addition, Butler recommended
using the PRB Users' Group and other PRB
users as sources for best practices. " They are
willing to help and are tied by the common
bond of safety for any user, " Butler said. ■
-David Wagman is POWER's
executive editor.
33
1. Change in culture. A major undertaking was cleaning the plant and eliminating
coal spillage due to subbituminous coal's highly combustible nature. Shown are conditions
after the PRB coal conversion made cleanliness a part of the plant's new operating culture.
Courtesy: Constellation Energy

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POWER October 2012

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