POWER December 2016 - 42

COAL PLANT O&M
" Who Moved My Btus? " The
Pitfalls of Extended Coal Storage
Many coal power stations have recently been operating at historically low capacity
factors or have even undergone extended economic shutdowns.
This can result in coal stockpiles that are exposed to the elements for
much longer times than anticipated, resulting in a loss of usable coal energy
by several mechanisms. This article explores the severity of the problem
and provides suggestions for preserving the energy that you paid for.
Una Nowling, PE
D
eath came for the boiler, black and
shimmering in the afternoon heat.
It was gathered in piles and dropped
onto conveyors. It was fed to the great beasts
of the coal mills, which growled and complained
as they struggled to grind it. The
boiler tried its best to meet demand but was
starved for energy even as its insides were
increasingly coated with atherosclerotic slag.
A mill tripped and the boiler faltered. Then
it gasped its last breath as the induced draft
fan tripped.
The boiler was dead.
The postmortem was somber, but free of
accusations and recriminations. Everyone
knew the cause: Due to unprecedented rail
line disruptions, the plant had been forced to
reclaim coal from a section of its stockpile
that had sat relatively undisturbed for more
than two years. Just as we all reach for the
milk carton with the longest expiry date at
the grocery store, plant personnel had always
taken the freshest coal for filling the coal
silos. Although operations knew to expect
some problems from the coal, no one was
prepared for exactly how much the coal had
degraded from its original delivered quality.
A week later, at the postmortem, the lab results
delivered a stunning message: Nearly
25% of the original heat content of the coal
had vanished.
Several of the power stations I work
with have faced similar challenges. Fierce
competition from cheap natural gas, milder
summers and winters, and environmental
regulations have reduced coal burn rates
at many sites, leading to greatly extended
storage durations and a greater loss of total
coal stockpile energy. This in turn has exacerbated
operations and maintenance challenges
at these power stations, potentially
reducing the capacity factor even further,
while increasing the total fuel-related cost
of generation.
42
What Happened to My Coal?
Coal begins to lose energy almost the minute
it's mined, through a variety of means.
Upon exposure to the atmosphere, a slow
oxidation process begins, essentially burning
away heat content. Once the coal is handled-through
loading, shipping, unloading,
stockpiling, and reclaiming-it will lose energy
from dust and fines at each step of the
process. In addition, moisture is typically
gained during shipping and storage, which
results in a variety of negative impacts at the
power station. While the coal lies in storage,
the oxidation process will continue, accelerated
by exposure to the elements, potentially
leading to spontaneous combustion.
Although most coal power station personnel
understand that these three mechanisms-loss
of energy through oxidation,
dust and fines, and added moisture-will
affect their coal stockpiles, the magnitude
of the degradation is often unknown. This
article explores the root causes of each of
these energy loss pathways, how they affect a
power station, the magnitude of the degradation
that can occur, and methods for reducing
both coal mass and coal quality losses.
Dust in the Wind
Anyone who has worked in the coal industry
knows that coal is a friable material, and each
time it is handled there is potential for generating
fines and dust. In some cases, a coal terminal,
preparation plant, or power station can
accumulate coal dust in piles deep enough to
wade through. One coal preparation plant I
visited maintained an on-site waste stockpile
of more than 1 million tons of coal fines,
piled in a giant berm and slowly smoldering
from spontaneous combustion in the desert
heat. Although it is a clear nuisance and
safety hazard, we sometimes fail to consider
the magnitude of the coal dust problem (Figure
1).
www.powermag.com
Significant amounts of coal can be lost
via dusting during transport to the power
station. A study along a 500-mile section
of rail for the Norfolk Southern line
found average losses ranging from 0.4 ton
to 0.5 ton per car. When binding agents
were sprayed onto the cars before delivery,
these losses were reduced by 45%. Another
study found that, under windy conditions,
rail transport in uncovered cars can
result in 1 to 2 tons of lost coal per car
per trip. For bottom-dump rail cars, about
400 tons per year were lost through the
gaps in the bottom doors. Dust losses during
unloading of bottom-dump rail cars in
Australia was measured at 0.0027% of the
total annual coal received by power stations.
For a 1 million ton per year site, that
equates to 27 tons per year. In India, the
Central Electricity Regulatory Commission
employs a default assumption of coal
losses from transportation and stockpiling
of 0.2% for mine-mouth plants and 0.8%
for other power stations. Actual power station
data from India, however, reveals that
annual transport and stockpile losses range
from 1.8% to 6.6%.
There are many techniques for reducing
coal dust losses. Coal should be received in
as large a size as practical, thus reducing the
surface area to volume ratio of coal in the
stockpile. This may require operation of a
coal yard crusher upon reclaim in order to
provide properly sized coal to the plant.
Although bulldozers are necessary for
proper compaction and handling operations,
coal dust emissions from these operations
can be significant. One study found that operating
a bulldozer 12 hours a day for five
days a week results in an annual loss of more
than 32 tons of coal.
New coal plant designs should utilize
longer conveyors and fewer transfer points
to minimize coal losses during stockout and
POWER | December 2016
http://www.powermag.com
POWER December 2016

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