POWER December 2021 - 36

COAL POWER
Parameter
Pre-test ramp rate
Highest ramp rate,
actual tested
Highest ramp rate,
projected
Superheater
temperature
Reheater
temperature
Economizer gas exit
temperature
O2, avg
CO
NOX
NOX
Units
(MW/min)
(MW/min)
(MW/min)
(F)
(F)
(F)
(%)
(ppm)
(ppm)
Case #1
5
12
15
Case #2
5
10
15
Case #3
Hand
No data
No data
Case #4
3.5
5.5
5
920-996 878-993 825-844 862-887
883-991
8241,008
Case
#5
3
10
15
9931,001
796-802
860-928 970-986
536-709 573-768 No data 627-628 516-542
4.64
27-1,600
(lb/MMBtu) .04-2.49
3.32
15-316
4.44
1-5
.64-.82
3.88
7-18
.72-.84
8.21
50-77
180-403 247-569 417-535 487-569 245-338
.35-.81
.50-.69
Table 3. This table shows ramp rate test data from several coal-fired power plants that EAPC
has assisted. All showed improvement in ramping rates, some by double or triple the previous
rate. Source: EAPC Industrial Services
load setting. Through testing, we were
able to reduce the unit low loads down
to 25% of rated full load, and in some
cases, this allowed large units to improve
their load range by 175 MW or more. This
was accomplished with the unit in a safe
and stable mode of operation and performed
repeatedly over the entire new
load range. Low-load operation would
be held for a minimum of 12-16 hours
overnight to collect data and confirm
that extended periods of low-load operation
could be achieved and that the unit
would remain in a safe and stable mode.
Ramp rate testing showed similar
results with the units originally averaging
around 2-3 MW/minute ramp rate
prior to testing. These same units were
tuned and tested to operate in the 10-
12 MW/minute ramp rate over their entire
load range (Table 3). This requires
several ramp-rate tests, both increasing
and decreasing load over several
days, as controls tuning changes are
required. Increased ramp rates appear
to be less of a concern for many electric
power producers who are not part
of an ISO/RTO, as there is very little
economic incentive to ramp at elevated
rates, and there is increased risk for
operator error and equipment failure.
This risk can be reduced by providing
detailed SOPs, having a well-trained
operations department, and maintaining
the equipment and systems as required
for this new mode of operation.
Some power producers may decide
to utilize increased ramp rates over a
smaller defined unit load range instead
of the entire load range.
36
Sliding Pressure Operation. EAPC
performed tests on units at both full
pressure and sliding pressure operation
with good results. However, there are
several advantages to utilizing sliding
pressure operation and utilizing a sliding
pressure control curve designed for the
unit to make this a fully automated function.
During testing, we developed a sliding
pressure curve designed to reduce
the turbine throttle pressure from 2,400
psi down to 1,300 psi at 10-15 psi/minute
over a load range of 650 MW down
to 150 MW. Below are a few benefits of
utilizing sliding pressure as a part of lowload
operation:
■ Higher/stable turbine metal temperatures.
■
Higher/stable SH and RH steam temperatures.
■
No IP rotor expansion issues.
■ Reduces potential control valve wear
and sticking, and high-energy piping
stress.
■ Balanced steam flow to the turbine.
■ Reduced cyclical life impact due to
unit flexibility operation.
■ Potential for lower loads.
■ SCR inlet temperatures in the 650F to
680F range at 150 MW.
Turbine Full-Arc or Partial-Arc Operation.
EAPC performed tests on units
at both turbine full-arc and partial-arc
control valve configuration. This was performed
in conjunction with sliding pressure
and the data collected indicated the
following advantages to operating the
turbine in a full-arc mode as a function
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Safety Concerns Related to Unit
Flexibility and Asset Optimization
To successfully test and implement
Unit Flexibility and Asset Optimization,
clear goals need to be determined and
a very detailed test plan created. Every
unit can have unique configurations and
systems, and depending upon history
of repairs or upgrades, identical units
can still respond and perform differently.
Small MW adjustments need to be used
while testing to identify system configuration
issues and to ensure the necessary
adjustments can be made without
tripping the unit. Having upfront discussions
about known or suspected issues
at different load points helps eliminate
surprises and unit upsets.
If the plant is burning Powder River Basin
(PRB) coal, then steps need to be taken
to prevent fires during longer periods
of low-load operation. During low-load
operation, several mills/cyclones will be
out of service with bunkers potentially
full of coal. PRB best practices would
include cycling mills/cyclones in and out
of service every five to seven days to
prevent coal from self-combusting and
creating hot spots in the bunkers.
If the existing dust collection system
discharges into the coal bunkers,
this cannot be done into a bunker with
a mill out of service, as this extremely
fine dust may self-combust in only a few
days. If the existing dust collection system
does not provide the flexibility to
always discharge into a live coal bunker,
then the system may have to be updated
or revised to provide this option.
When low-load operation and increased
ramp rates are implemented as
part of a new operation mode, this will
require increased frequency of inspections
to look for new areas of wear or different
O&M issues created by this mode
of operation. The increased inspection
plan should be used to assist with future
outage planning and budgeting. ■
-Bruce Ogden is director of
Business Development with
EAPC Industrial Services.
POWER | December 2021
of low-load operation. Below are a few
benefits of turbine full-arc operation:
■ Stable and responsive control.
■ Improved SH, RH, turbine metal, and
SCR inlet temperatures.
■ Balanced steam flow through turbine.
■ Decreased valve and seat wear.
■ Increased #5 extraction steam pressure
by 5 psi.

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POWER December 2021

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