POWER September 2015 - 50

FUNDAMENTALS
4. It's quite simple, really. The inspection hatch (A) is left open, and light from the fireball
travels to mirror (B). This light is reflected to mirror (C), which provides verification to plant
operators in the control room (D) that there is indeed fire in the boiler. In this simple and elegant
way, plant operators demonstrated their ingenuity in overcoming the limitations of their equipment.
Courtesy: Una Nowling
coal use could be increased to maintain
the unit generation plan. Last winter onethird
of the utilities within the Midcontinent
Independent System Operator region
had to reduce coal generation and even
take units offline due to both rail delays
and unexpectedly cold temperatures.
These examples may also apply for other
dual-fuel situations, such as coal-oil and
coal-biomass cofiring.
To demonstrate how dual-fuel capability
can reduce operations risk, here's an example
that I investigated for a Pennsylvania coal
power plant that was able to receive a large
number of truck-delivered spot-market coals
to supplement a long-term coal contract supplying
about half of its fuel purchases.
This plant had limited natural gas cofired
derates can sometimes find great values
on the used equipment market, knowing
that if the coal-related equipment isn't up
to the task, natural gas can supplement the
heat input and maintain the desired load.
■ Power stations that are unsure about the
quality of their coal stockpiles or that suffer
from poor or erratic blending practices
can put those fears to rest. If, for some
reason, too much poorer-quality coal is
stocked into the bunkers or silos, then
operators can modulate natural gas use to
make up the difference.
■ Unexpected outages of parallel equipment
(such as coal mills) can be hedged by having
natural gas available to make up lost load.
■ Coal inventory levels can be reduced and
coal supply delays and shortfalls better
managed. Conversely, should natural gas
supply restrictions unexpectedly occur,
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ing capability, in the form of supplemental
burners that could supply up to 12% of the
total heat input of the unit at its maximum
continuous rating. When reviewing a recent
selection of 41 different coal supply bids,
we discovered by use of the aforementioned
Vista program that the potential range of total
fuel-related generation cost was enormous
for these fuels, largely due to three factors:
some of the fuels were ranked as very high
slagging fuels, some possessed a low heating
value that led to a unit derate due to insufficient
pulverizer and induced draft fan capacity,
and some were predicted to exceed the
particulate emissions limit of the unit.
Each of these coal cases was evaluated
again, assuming that natural gas was available
to eliminate derates when one occurred, with
a balance between the cost of natural gas consumption
versus the cost of the derate itself
(as well as the attendant costs of differential
maintenance, unit availability, unit stack emissions,
dispatch ability, and the like). Figure 5
shows the results of this analysis.
Multi-Fuel Power Plants
When we speak of multi-fuel capability at a
coal power plant, we generally are including
coal, natural gas, fuel oil, and biomass or some
sort of opportunity fuel. In the current regulatory
and market climate of most countries there
is little advantage to significant investment in
biomass cofiring, but in the European community,
or nations with a ready supply of biomass,
opportunities could exist for leveraging three
different fuel markets simultaneously.
Yet another, unorthodox, method of increasWhat
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ing fuel flexibility exists for coal-fired assets
within a utility system: capital and operations
upgrades that enable those assets to support wind
and solar generation being added to the grid.
In a 2013 report published by the National
Renewable Energy Laboratory (NREL), the
benefits of upgrading coal and gas-fired power
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POWER September 2015

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