POWER February 2012 - 32
INSTRUMENTATION & CONTROL
Enhanced Load Dispatch Rate and
Furnace Protection Through Model
Predictive Control
The enhanced plant performance achieved at the 1,477-MW Morgantown Generating
Station shows the value of model predictive control in conjunction
with intelligent distributed control algorithms. This project update looks at
how the project team moved from ramp rate improvements to reducing
tube metal temperatures to improved component life.
By Don Andrasik and John McNulty, GenOn Morgantown and Don Labbe, Invensys Operations Management
M
organtown Generating Station is
on the Potomac River in Charles
County, Maryland. Each unit consists
of a single tandem-compound turbine
generator (one Westinghouse and the other
General Electric) and a single pulverized
coal-fired once-through controlled circulation
supercritical boiler utilizing a single
reheat-regenerative cycle. Each has a
nameplate rating of 572.5 MW with throttle
steam conditions of 3,500 psig, 1,000F
and reheat steam temperature of 1,000F
and seven stages of feedwater heating. The
units are capable of generating 625 MW
each and were placed into service in 1970
and 1971.
The Combustion Engineering steam
generators are a balance draft divided
furnace type, consisting of a tangentially
fired, center wall furnace with economizer,
and superheater and reheater surfaces.
The split furnace design with dual selective
catalytic reduction (SCR) systems
poses unique challenges to balancing steam
temperatures and limiting peak metal temperatures
in the various boiler circuits.
Because high peak metal temperatures are
often a precursor to premature tube failure,
control enhancements that decrease peak
temperatures offer substantial payback to
GenOn, the plant's owner.
Through the expansion of the model
predictive control within the dynamic nitrogen
oxide/heat rate optimization system
and additional distributed control system
(DCS)-based process control algorithms,
the objectives of improved steam temperature
balance in the boiler circuits and lower
peak metal temperatures were achieved.
These control improvements widened
the load range of fast dispatch operation
while protecting boiler tubes from excessive
thermal-induced stress, promoting
32
long boiler tube life. After instrumentation
failures, boiler tube leaks cause more unit
forced outages than any other component
failure in the typical steam plant.
The experiences on Unit 2 are described
in this article. The methodologies were
then applied to Unit 1, and results were
essentially similar, if not better. Earlier
Morgantown control upgrades were described
in " Increasing Generation Ramp
Rate at Morgantown Generating Station's
Coal-Fired Units " in the February 2011 issue,
also found in the POWER archives at
www.powermag.com.
SCR Impact on Boiler Component
Water/Steam Temperatures
Morgantown's units experienced increasing
temperature excursions on the platen
superheater outlet temperatures at both
the lower ramp rates and the newer, higher
ramp rates. Previously, the original temperature
override on the waterwall outlet trim
would suffice to protect the component
tubes in the rare case of high temperatures.
After the SCR startup, the temperature excursion
increased both in severity (peak
temperature) and in frequency. This was
especially true at minimum generation
points and during generation load ramp increases
from lower generation points.
The SCR catalyst requires a minimum
temperature to ensure burn-off of any contaminate
formation (ammonia bisulfate).
To maintain the SCR catalyst temperature
in each furnace in the split furnace design,
an economizer bypass system takes
heat from the gas path flow through the
economizer and diverts it to the SCR inlet
to maintain the minimum temperature
requirement. This greatly decreases the
water side fluid temperature out of the
economizer. The waterwall temperature
www.powermag.com
control then moves to increase the furnace
firing rate to make up this loss and maintain
the waterwall outlet temperature. As
a result of the furnace gas flow redistribution,
significant increases in platen superheater
outlet temperatures threaten the
tube material's thermal limits.
Late-night testing at the lower generation
points illustrated the effect of SCR
temperature control (economizer bypass
dampers with backpressure dampers on
each individual furnace) on platen temperatures.
Manipulation of the economizer
bypass provided only marginal changes on
platen outlet temperatures.
Another test revealed that the burner
tilts had a greater influence in lowering
platen temperature at a minimum generation
point. However, there was an increasing
difference between the individual
split furnace steam temperature outlets as
detected by the boiler throttle (BT) links
feeding the platen superheater. Because
of the split furnace design, one side may
be relatively cooler than the other, but the
main steam sprays will produce the desired
outlet temperature. One side sprays excessively,
while the other sprays with little to
no flow.
The side that is spraying has a high platen
outlet temperature, pushing undesirable
thermal limits of the component tubes. So
this temperature difference control now
becomes a key objective, as it is desirable
to keep both sides near balanced, thereby
lowering peak temperature on the hightemperature
side.
The difference between the furnaces
is attributed to the established SCR temperature
requirement and the economizer
damper arrangement that was placed into
service. There also can be imbalances of
fuel/air distribution at the furnace input
POWER | February 2012
http://www.powermag.com
http://www.powermag.com
POWER February 2012
Table of Contents for the Digital Edition of POWER February 2012
Contents
POWER February 2012 - Cover1
POWER February 2012 - Cover2
POWER February 2012 - Contents
POWER February 2012 - 2
POWER February 2012 - 3
POWER February 2012 - 4
POWER February 2012 - 5
POWER February 2012 - 6
POWER February 2012 - 7
POWER February 2012 - 8
POWER February 2012 - 9
POWER February 2012 - 10
POWER February 2012 - 11
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