POWER December 2021 - 35

COAL POWER
able range. Additional control tuning
is almost always required to achieve
elevated ramp rates and several days
of testing per unit are required. Attemperator
tuning and control optimization
provides a positive response in preventing
elevated SH and RH steam temperatures
during elevated ramp rates from
low load to full load, and while ramping
from full load to minimum load, the attemperators
must close completely and
not leak through. These efforts maintain
the required steam temperatures to
stay out of the critical cyclical impact
ranges of the turbine components.
Some boiler configurations/designs
may lack the boiler tube surface area required
to allow for elevated ramp rates.
During testing, these units will experihigher
flue gas exit temperature and thus
minimize or eliminate emissions control
system impacts; however, for plants located
in colder climates, additional steps
may be necessary.
Depending upon the current boiler
configuration, there may be options to
bypass some flue gas around the economizer
section, adjusting SH and RH
bypass dampers, increasing air preheat
system temperature and steam flow.
SCR OEMs are currently re-evaluating
their low-temperature limits and allowing
for lower loads. We have successfully
tested and operated a 650-MW
unit down to 150 MW while maintaining
SCR inlet temperatures around
700F. On single units, the turbine extraction
supply piping and flow control
Some pulverized coal units can experience
about a 25-MW reduction in auxiliary power
consumption while at low loads.
ence a larger temperature change in SH,
RH, or both during elevated ramp rates
to full load. These temperature deviations
can be large enough to fall into a
very low range on cyclical life impacts
and would have a critical impact on O&M
and unit life. Without spending additional
capital to improve the boiler configuration,
these units would have to maintain
a lower ramp rate.
BOP. Unit flexibility impacts to backend
emissions control systems are primarily
impacted due to a decrease in flue
gas temperatures at low loads. Cooler
flue gas exit temperatures can reduce
cold end averages across the air heater,
and if the temperature drops below the
dew point, condensation will occur, and
when exposed to sulfur in the flue gas,
sulfuric acid can form and damage the
air heater baskets, ductwork, and other
equipment downstream.
Select catalytic reduction (SCR) systems
remove NOx
from the flue gas
stream and require an inlet temperature
above 620F to ensure proper reaction
and stay in service. Impacts to electrostatic
precipitators (ESPs) is limited as
the amount of particulate in the flue gas
will be greatly reduced at low loads and
is not temperature dependent. Scrubber
or flue gas desulfurization (FGD) system
operation will not be impacted by
low-load operation outside of how many
modules or atomizers need to be operated
at low load. Operating upper pulverizers/cyclones
will assist in maintaining a
December 2021 | POWER
valves to the SCR hydrolyzer may have
to be resized to allow the necessary
flow at lower loads, or a supply from
the auxiliary steam system could also
be utilized. This is necessary to convert
urea to an ammonia gas for reaction in
the SCR. Properly operating air preheat
systems can also positively impact
backend temperatures.
Turbine-driven boiler feedwater pump
transition can be an area of concern
while reducing load and moving from
two-pump operation to one-pump operation,
or from one-pump operation to twopump
operation while increasing loads.
In some units, this has been the sticking
point to reducing unit load past 50% of
rated full load. The pump transition operation
can be tricky due to the possibility
of the steam source transitioning from
the #7 turbine extraction to the auxiliary
steam supply.
However, if a proper SOP is written
and followed, this boiler feedwater pump
transition can be performed smoothly
and repeatedly without a steam drum
level upset. It is important to ensure
that the boiler feedwater pump recirculation
system is in service and operating
properly. The unit operator needs to pay
special attention to pump discharge pressure
rather than flow, as the pump flow
at low loads and while on recirculation
can be erratic. Most units have an electric
boiler feedwater pump for startups.
This pump can be utilized during the turbine
driven boiler feedwater pump transiwww.powermag.com
tions;
however, this is mainly for control
room operator comfort and should not
be a requirement.
Positive impacts to auxiliary power/
station service can be achieved during
low-load operation. Some pulverized
coal units can experience about
a 25-MW reduction in auxiliary power
consumption while at low loads. This is
primarily due to having several coal mills
out of service. Other areas that help to
reduce the auxiliary power consumption
include taking scrubber modules out
of service, having one forced draft fan
and induced draft (ID) fan in operation,
running one condensate pump, and operating
one circulating water pump. If
low-load operation is going to last over
the weekend or several days, there are
other systems in the plant that could be
taken out of service or operated intermittently.
Every kW or MW you are not
consuming within the unit reduces required
load and associated costs.
Some miscellaneous BOP impacts
during low-load operation include:
■ Increased dissolved oxygen issues
due to air in-leakage.
■ Feedwater heater drain high level issues
due to inadequate pressure to
push condensate through feedwater
heaters to the deaerator.
■ Ash fallout in flue gas ductwork upstream
of the SCR inlet because of reduced
velocity of the flue gas stream.
■ ID fan capacity issues due to control
dampers going completely closed.
Controls tuning. To achieve successful
Unit Flexibility and Asset Optimization,
controls tuning philosophy needs
to adapt to the new modes of operation.
Controls tuning takes several days per
unit; however, it can be completed simultaneously
with low-load and ramp-rate
testing, and this has been successfully
completed on several units. Major areas
of concern have been steam drum level
control, boiler master, fuel master, mill
PA curves, excess O2
range and control,
condensate recirculation control, and SH
and RH attemperators.
On-Site Testing Lessons Learned and
Results
Over the past few years, EAPC has completed
Unit Flexibility and Asset Optimization
testing and implementation on
several units, both pulverized coal and
cyclone units, with very good success.
Most units were operating around 50-
55% of rated full load as their minimum
35
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POWER December 2021

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