POWER August 2011 - 50

PLANT OPERATION & MAINTENANCE
Mitigating the Effects of
Flexible Operation on Coal-Fired
Power Plants
As coal-fired power plants increasingly operate in cycling modes, many plants
are confronting the potential for higher levels of component damage and
degraded performance of environmental control equipment. Generators
and EPRI are working together to find ways to mitigate the effects of cycling
operation and to manage the transition of formerly baseload plants
to flexible operation.
By Steve Hesler, Electric Power Research Institute
C
oal-fired power generating units in the
U.S. continue to switch from highcapacity-factor
operation to various
modes of flexible operation. Cycling and
load following are not new. However, over
the past decade, and in particular the past few
years, a perfect storm of factors has gathered
to require even baseload coal plants to prepare
for increased flexible operation.
The first factor is reduced demand caused
by the recent recession. From 2008 to 2009,
overall electricity demand in the U.S. declined
4%, according to the Energy Information
Administration (EIA). An Electric
Power Research Institute (EPRI) study, " Unprecedented
Generation Shifts " (funded by
the Power Technology, Market Analysis, and
Risk Program), found that in 2009, coal-fired
generation declined 11.1%, or about 75 terawatt-hours
(TWh). That was the greatest recorded
one-year decline in coal generation.
The second factor is lower natural gas
prices and increased coal-to-gas switching. In
2008-2009, gas prices declined significantly,
making gas-fired generation a lower-cost option
for economic dispatch than coal-fired
plants. In the same two-year time period, the
net capacity factor for gas-fired combined
cycle plants increased by 5%.
The third factor, which will play a role in
the next decade, even when the economy improves,
is increased deployment of intermittent
renewable generation (principally wind)
in certain regions of the U.S. That increased
variable generation is forcing coal and combined
cycle plants to provide system loadbalancing
services. The EIA reports that the
amount of nondispatchable generation in the
U.S. increased by a factor of four from 2005
to 2009. Wind generation alone increased
from 18 TWh to 71 TWh during this period.
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Contributing to the complications for coalfired
plants, renewable generation is typically
dispatched as " must-take, " and the trends in
both seasonal and daily wind production are
often out of phase with demand.
The overall impact of these trends for coal
plants has been increases in various modes
of flexible operation, with the following operational
impacts: increased load-following
operation, higher unit turndown during low
demand, lower minimum load operating,
two-shifting, frequent unit starts (hot, warm,
and cold), increased load and thermal ramp
rates, frequent reserve shutdown, and longterm
plant layup.
For example, analysis of data from the
North American Electric Reliability Corp.'s
Generating Availability Data System, as
reported by coal units in the 2005-2009
timeframe, indicates an increase in reserve
shutdown hours in 2009. This increase is
observed across a range of unit sizes, in
both supercritical and subcritical designs. In
turn, this trend has produced a reduction in
reported net capacity factor, particularly for
older subcritical units, which are experiencing
high turndown.
From Baseload to Flexible Operation
Few coal-fired units in the existing U.S. coal
fleet are designed specifically for flexible operation.
In 2009, more than 60% of the total
North American subcritical coal-fired generation
was produced by units commissioned
prior to 1980. Those were primarily baseload
units constructed during the 1960s and 1970s
to meet expected trends in demand growth.
Since 1980, power producers have opted to
build large-capacity, more-efficient units
with supercritical steam conditions, which
are also designed for baseload operation.
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New duty cycles force baseload plants
and equipment to operate closer to-
or beyond--nominal design limits and
through more thermal cycles than originally
anticipated. The operational impacts
of flexible operation cited earlier result
in significantly increased occurrences of
thermal transients in the material of critical
high-temperature boiler and turbine
components. These transients, and other
operational factors associated with flexible
operation, have the following effects
on coal-fired generating assets:
■ Increased rate of wear on high-temperature
components.
■ Increased wear-and-tear on balance-ofplant
components.
■ Decreased thermal efficiency at low load
(high turndown).
■ Increased fuel costs due to more frequent
unit starts.
■ Difficulties in maintaining optimum steam
chemistry.
■ Potential for catalyst fouling in NOx
trol equipment.
con■
Increased risk of human error in plant
operations.
Additional wear on plant components requires
increased spending on preventive and
corrective maintenance. This requirement is
often challenging to plants that are placed
lower on the dispatch stack and that, therefore,
receive less revenue and operating budget.
Increased human error is due primarily
to the increased amount of transient operation,
which produces more opportunities for
error. Major plant events caused by human
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POWER August 2011

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