March – April 2014
www.gasturbineworld.com
2 ppm limit on NOx, CO
and NH3 while ramping
Clean ramping: the next challenge for
quick start combined cycle operation
By Harry Jaeger
Thanks to flexible quick-start design, today’s CCGT plants can
start fast and produce low start-up emissions. But now there
is a new challenge: how do you keep emissions in compliance
while changing load.
S
iemens has developed what they
call “clean ramp” technology
which uses patent pending features to
integrate the combined cycle plant’s
and emissions reduction system controls so that NOx, CO and ammonia
slip (from an SCR) remain at base
load emission levels even when frequently ramping up or down.
In June 2013, with the cooperation
of NRG, the concept was demonstrated in the field with convincing results
at the El Segundo combined cycle facility which is powered by two “FlexPlant 10” 1x1 SGT6-5000F units.
o Demo test. Gas turbine ramped
up and down at 30 MW/min between
115MW and 180MW base load during two 1-hour test runs.
o Control logic. Load change emissions predicted by control logic system used to adjust SCR ammonia injection flow rate to maintain stack
emissions.
o Test results. Stack NOx and CO
emissions maintained at less than 2
ppmvd (at 15% O2) throughout the
test with ammonia slip also controlled
to under 2 ppmvd.
Most environmental permits in the
U.S. regulate on a time-averaged requirement, typically over a 1-hour
2 GAS TURBINE WORLD March - April 2014
rolling average. When plant load
changes only a few times a day, shortlived transients and emissions spikes
during startup and ramping are seldom a problem.
But with frequent load changing
there is no time to average out the
peak and there is a risk of going out
of emissions compliance.
Today’s quick-start design features
enable the gas turbine side of a combined cycle plant to smoothly ramp
up to base load output like a simple
cycle plant. And do so without holding the gas turbine at low load while
the steam turbine is loaded.
This puts MWs on the grid faster
than before and, importantly, it results
in much lower startup emissions.
Hold-point emissions
Earlier combined cycle plants require
that the gas turbine startup cycle include part-load hold points to enable the bottoming cycle to gradually
warm up prior to loading.
However, as pointed out by Ramesh Kagolanu, Manager of Environmental Engineering for Siemens
Energy, Orlando, low-load gas turbine
operation produces much higher levels of NOx and CO than when operating at full load.
Low-load GT emissions for combined cycles fitted with selective catalytic reduction are especially critical
because a cold SCR (located within
the HRSG) is not very effective at
removing NOx and CO produced during startup.
Typically, conventional F-class
CCGT plants produce about 180 lbs
of NOx and 1,340 lbs of CO (per GT
unit) during a cold startup – compared to about 13 lbs NOx and 340
lbs CO for a quick-start plant.
Clean Ramp does it better
The need for “clean ramp” technology is to further reduce those already
low emission levels during startup and
also to handle variations in gas turbine
emissions during load changes, Kagolanu emphasizes.
The objective is to maintain compliance over the full spectrum of combined cycle operation and without
having to restrict the amount of ramping requested by the system operator.
Without clean ramp, he says, handling of transient emission spikes
would basically depend solely on the
gas turbine emissions controls. This
could mean placing limits on ramp
rates and frequency in order to keep
emissions (on a 1-hour rolling average) in compliance.
DLN flame an issue
This issue is due mainly to an inherent limitation of part-load performance of today’s dry low NOx
(DLN) combustion systems, and regulations that don’t always recognize the
limitations of the hardware.
A fact of life with advanced gas
turbines is they generally produce
more NOx and CO when they are
changing load, points out Kagolanu.
The reason has to do with the complexity and sensitivity of today’s DLN
combustion systems, where the fuel
supply is typically divided into multiple injection points, or stages.
In addition to two (or sometimes
three) lean-premixed flame zones
needed for low NOx, there is an upstream diffusion flame pilot zone
needed to help maintain combustion
stability.
But there is also a less desirable
characteristic of the high-temperature
diffusion flame pilot zone -- it is the
largest contributor to NOx emissions.
During steady state operation, fuel
flow to the pilot is set at a minimum,
as needed to maintain stability in the
downstream lean premixed flame
zones, and to avoid unwanted combustor dynamics.
During transients, however, the
stable nature of the diffusion flame
becomes even more critical, so fuel to
the pilot is increased. But this causes
the engine NOx to increase above the
9 ppm steady state design value and
requires a change in SCR operation
to maintain control over plant NOx
emissions.
A multivariable challenge
“Increasing NOx concentration (ppm)
is not the only problem to contend
with during load reduction,” says
Kagolanu.
“It’s actually a multivariable challenge since the mass flow through the
engine is also changing during load
change, as variable inlet guide vanes
come into play, so the ammonia injection rate must be adjusted to account
for both of these changes happening
at the same time.”
The key and main technical challenge, he says, is integrating the gas
turbine controls with the ammonia
injection system in a way that assures
reliable and consistent results.
By having access to a large amount
of field operating data and, most importantly, being able to change the
way the gas turbine itself is controlled, Siemens was able to develop
such an integrated control scheme
(dubbed “clean ramp”) to keep stack
emission low during transients.
With growing use of intermittent
renewables the expectation is that
plants will be required to change load
more frequently. As a result, says
Kagolanu, it puts a premium on being able to operate with unrestricted
ramping and without increasing emissions over the permit limit.
As already noted, trying to accomplish this simply by addressing this
problem of part-load engine NOx was
Combined cycle test run. Plant was ramped 4 times between 60% and 100% base load during 1-hour test of Clean Ramp
stack emissions control. The GT was ramped at 30 MW/min with the ST load following at about 5 MW/min.
MW and NH3
NOx, CO, NH3
GT Power
flow (lb/hr)
ppmvd
2.0 ppm
– 160
2.0 –
–
–
– 120
1.5 –
–
–
NOx
– 80
1.0 –
ST Power
–
–
NH3 Flow
– 40
0.5 –
CO
–
NH3 Slip
–0
–
0 –
Time
13:45
13:58
14:08
14:18
14:28
14:38
Source: Siemens Energy, March 2014
GAS TURBINE WORLD March - April 2014 3
not going to offer the desired result.
Solution: link SCR and DLN
According to Siemens, their “Clean
Ramp” solution provides an innovative control scheme that enables a
direct link between SCR (selective
catalytic reduction) and DLN combustion systems when operating under
transient conditions.
Power plants in the US commonly
use an ammonia-based SCR, located
within the HRSG, to control stack
emissions to levels substantially below those in the GT exhaust.
For example, if the gas turbine exhaust NOx is 9 ppmvd (current state
of the art for modern DLN burners),
meeting a permit stack limit of 2
ppmvd requires that the exhaust be
further treated to achieve a nearly
80% reduction.
This is done by passing the exhaust
through the catalytically supported reaction zone where ammonia (NH3) is
injected to react with the NOx, “reducing” it to nitrogen and water vapor.
SCR flow regulation
To operate an SCR efficiently, notes
Kagolanu, precisely how much ammonia to inject is critical. Too much
results in excess unreacted ammonia
(ammonia slip) in the exhaust, which
is also usually regulated on a time-average basis. Too little ammonia results
in excess NOx emissions.
The chemistry is simple enough,
but in order to control the optimum
amount of ammonia to inject you
must know how much NOx is in the
exhaust gas.
This is not a problem if the engine is at steady load; the NOx in
the exhaust can be measured and the
amount of ammonia to be injected can
be optimized to minimize any slip.
But under transient conditions the
system must be able to react quickly
to changes and make necessary adjustments in ammonia injection rate.
Unfortunately, the feedback time for
this is measured in minutes, not seconds, according to Kagolanu.
“With changing load”, he cautions,
“the conventional system is too slow
and you can’t reliably maintain stack
emissions, or ammonia slip, while
load is being changed.”
Anticipating with real-time logic
To meet the challenge, Siemens’
“Clean Ramp” technology uses patent
pending features to change how the
gas turbine and SCR are controlled
and interact.
It uses real time anticipatory logic,
effectively in advance of the event, to
processes a number of variables along
Commercial site for Clean Ramp demo testing. El Segundo combined cycle facility is powered by two “Flex Plant” 1x1 SGT65000 combined cycle units.
4 GAS TURBINE WORLD March - April 2014
with stored engine and DLN performance algorithms, to accurately predict gas turbine NOx emissions when
a load change is requested.
The predicted outcome then forward feeds information to the ammonia injection control so that it anticipates the changes in exhaust flow
rate and NOx concentration, and the
precise amount of ammonia to be injected is determined.
As a result the system is able to
maintain base load stack emission
levels even when the gas turbine exhaust emissions are changing.
Demo testing success
Successful demonstration of the Clean
Ramp technology took place in June
2013 during two separate one-hour
test runs conducted at the NRG El
Segundo (California) Flex-Plant 10
combined cycle facility.
During the first run, the steam turbine was kept off-line while the gas
turbine was ramped continuously,
up and down, at more than 30 MW/
min. Gas turbine load ranged between
about 115MW (approximately 65%
load) and 180MW full base load.
During the entire hour, says Kagolanu, NOx emissions, CO emissions
and ammonia slip were each maintained at less than 2 ppmvd.
For the second run the steam turbine was on-line. The gas turbine
was again ramped at 30 MW/min,
followed by the steam turbine ramping at approximately 5 MW/min for a
total plant ramp rate of about 35 MW/
min. Again all emissions were below
2 ppmvd during the entire run.
The key to flexible plant operation with low emissions and frequent
load change is systems control integration, stresses Kagolanu and testing
has clearly confirmed that the system
works better in concert than when its
parts work independently.
Siemens says that Clean Ramp is
now being offered commercially on
all E-, F- and H-class frames, and for
all combined cycle configurations (i.e,
1x1, 2x1, and 3x1). n
El Segundo Energy Center host site
for Clean Ramp demo testing
Clean Ramp transient emissions control technology was demonstration tested in June 2013 on a two-unit Siemens FlexPlant 10 combined cycle facility located near Los Angeles that is owned and operated by NRG Energy.
Siemens supplied the two power islands, each featuring an SGT65000F gas turbine generator and an SST-800 steam turbine generator as well as a heat recovery steam generator, an air-cooled condenser, SPPA-T3000 plant controls and electrical equipment.
The FlexPlant 10 design integrates the fast-start SGT6-5000F gas
turbine with an air-cooled single-pressure non-reheat bottoming cycle
to provide a net efficiency of nearly 49 percent, much higher than
conventional simple cycle peaking units.
This plant type is also claimed to be environmentally friendly, as
compared to conventional combined cycle technology, with a reduction of 95 percent in CO start-up emissions and greatly lower water
consumption.
With Clean-Ramp technology it was further demonstrated that the
plant could comply with tight emissions constraints even while the
gas turbines ramp up and down to meet varied electricity demands.
The 550MW combined cycle facility was commissioned for commercial operation in September 2013 to provide quick-start capacity for
both peaking and intermediate load service. The two units can deliver 300MW to the grid in less than 10 minutes, allowing the plant to
back up intermittent renewable power.
GAS TURBINE WORLD March - April 2014 5