POWER November 2014 - 34

HEAT RATE IMPROVEMENT
and each of these projects were focused on
the results of site-specific actions.
Flexible Operation
Flexible operation refers to the ability of a
plant to operate at part load and in load-following
and cycling (on and off) modes, in response
to economic conditions and increased
utilization of non-coal-based generation
(such as renewables and gas). Operating conditions
under flexible operation can result in
reductions in plant efficiency and increased
degradation and maintenance requirements
on components due to constant swings in operating
temperature and pressure (Figure 3).
A 2010 EPRI study (report 1021205)
identified cost-effective capital modifications
and adjustments to plant operating procedures
to improve heat rate during cycling
operation. The study identified 10 upgrade
options, though the practicality of each is
site-dependent:
■ Sliding-pressure operation. With slidingpressure
operation, plant efficiency is
increased by reducing turbine-throttling
losses. This option was further analyzed in
a follow-up project and found to provide a
heat rate improvement at part load in the
range of 2%.
■ Variable-speed drives for main cycle
and auxiliary equipment. Variable-speed
drives reduce auxiliary power consumption
of rotating equipment, thus increasing
plant net output. The amount of
savings available with variable-speed operation
can vary widely. Variable-speed
drives are expensive and can be difficult
to justify for older plants with limited remaining
life.
■ Boiler draft system control schemes and
operating philosophy. Where multiple fans
are operating in parallel, plant efficiency
at low loads and under ramping conditions
can be maintained and improved by the
proper selection of startup/shutdown procedures.
Depending on the load scenario,
this measure will allow auxiliary load reductions
by operating fewer fans, but it
can increase maintenance and reliability
risks.
■ Automated pulverizer supervisory controls
and variations with mill design. Firing
systems and operating procedures can be
optimized for each load level. The goal is
to operate the least number of pulverizers
to maintain stable coal-only flames while
following load.
■ Optimum partial-load operation of air
quality control systems. For a wet FGD
application, the number of operating recycle
pumps can be reduced with load
reductions, resulting in reduced plant par34
4.
Leaking money into the air. Leaks from the steam cycle, like the valves leaking into
a header shown here, can have a direct impact on heat rate. Courtesy: EPRI
asitic loads. With ESPs, once the unit load
has stabilized at the lower load, it may be
possible to reduce ESP power consumption
by turning off specific electrical fields
while maintaining opacity and particulate
emission rates at the regulated levels.
■ Feedwater heater drain system modifications
for cycling. Typically, cycling efficiency
losses occur at low loads when
heater drains are routed to the condenser
as opposed to the deaerator. Plant efficiency
at part load will be improved by ensuring
that drains are directed to their proper
destination, when possible.
■ Cooling system optimization. Where multiple
cooling water pumps and cooling
tower fans are operated in parallel, proper
selection of component startup/shutdown
schemes (dependent on the load scenarios
and ambient conditions) will allow auxiliary
power reduction by removing pumps
and fans from service, but this can increase
maintenance and reliability risks.
■ Performance monitoring. Several tools are
available to display relevant parameters
with respect to plant efficiency at various
loads. These tools can be optimized
to enable operators to prioritize corrective
actions, thereby improving cycling efficiency.
■
Reducing warm-up flow for idle boiler
feed pumps. Heat rate improvement can be
achieved by reducing warm-up water flow
rates from operating pumps to idle pumps.
Less warm-up water flow will reduce the
auxiliary power of the operating pumps.
■ Minimizing flow, pressure, and temperature
oscillations during cycling operation.
Some oscillations of temperature,
pressure, and flow typically occur when
plants are operating at steady-state loads,
but they can be amplified during cyclic operation
and result in a reduction in plant
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efficiency. Commercially available optimizers
contain a forward-looking feature
that minimizes the time that steam temperature
strays from design, reducing attemperation
spray flow and the heat rate
effect of load following.
Cycle Alignment
Cycle alignment, also known as cycle isolation,
refers to the alignment of the cycle by
isolating all, or as much as possible, of the
high-energy fluid leakage from the steam
cycle (Figure 4). Certain leaking valves will
cause a direct loss in generation or an increase
in fuel costs.
When used as part of an overall plant performance
improvement program, cycle alignment
programs have provided large gains at
low costs. Implementing a cycle alignment
program can jump-start a plant performance
program and result in substantial heat rate
improvements that lead to fuel cost savings
and emissions reductions. With improved
cycle alignment, heat rate improvements in
the range of 50 Btu/kWh-about 0.5%-are
common. Units with problematic valves or
no history of maintaining cycle alignment
may experience a large one-time heat rate
improvement upon this program's implementation.
Various
methods have been used to ensure
proper cycle alignment, but an application's
success and costs vary depending upon the
specific valves and unit designs involved. In
2011, an EPRI project assessed cycle alignment
activities and identified their costs
and benefits. The study (report 1024640)
identified methods in use in the field to estimate
or determine the leakage rate through
leaking valves and used several real-life
examples from operating power plants to illustrate
how cycle alignment programs have
been implemented.
POWER | November 2014
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POWER November 2014

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