POWER September 2010 - 52

COMBUSTION TURBINES
SEV inlet air is at a temperature considerably
higher than conventional combustion air, it requires
less heating to reach flame temperature.
Both of these NOx
mitigation phenomena are
known from other combustion technologies
that employ exhaust gas reheating. Although a
large amount of the total fuel is burned in the
SEV combustor, the very low NOx
formation
in the SEV combustor means that emissions
remain low in both combustors.
The LLOC design update takes advantage of
the unique dual-combustor design of the GT24/
GT26 turbines by actually shutting down the
sequential SEV combustor at low part loads
while keeping the EV combustor at nominal
conditions. This operating capability allows
the plant to be operated in combined-cycle
mode at a very low combined-cycle loads (less
than 25%) with the EV combustor operating in
lean premix mode. Lean premix mode ensures
low emissions levels as well as a homogenous
turbine inlet temperature distribution. This
design is significantly different from that of a
typical industrial-sized CT that requires combustor
piloting or staging at part load. A final
advantage: The steam side remains in operation
regardless of CT load (Figure 4).
Favorable Emissions Behavior
The typical measured NOx
emissions across a
GT26 between the LLOC load range and that
of a conventional CT are depicted in Figure 5.
Data were collected from the GT26 Test Power
Plant in Birr, Switzerland, close to Baden, the
headquarters and CT Technology Center of Alstom
Power. The data show that the highest NOx
emissions across the entire load range remain at
baseload, demonstrating that the NOx
emission
guarantee can easily be met when running the
LLOC, unlike similarly sized conventional CTs,
which exhibit high NOx
emissions at low load.
An added advantage is low CO emissions
at low load. The EV combustor is operated in a
stable premix operation mode so that complete
combustion of the fuel occurs independent of
load setpoint. This means that CO emissions,
essentially partially burned fuel, are also very
low and far below typical permit limits across
the entire low load range (Figure 6).
The sequential combustion system design
of the GT24/GT26 CTs produces NOx
emissions
regularly measured well below current
statutory limits (in the EU) of 25 ppm (15%
O2
selective catalytic reduction, and CO emissions
below 5 ppm.
Operating Advantages
Integrating the LLOC into a combined-cycle
plant design began with the requirement to
maintain the bottoming cycle design and operating
requirements as close to standard as possible.
Particular attention was paid to keep the
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LLOC area
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Relative load (%)
POWER | September 2010
70
80
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) at baseload with no additional water or
loading and unloading procedures uncomplicated,
and hence reliable, while minimizing the risk
of forced shutdowns. The plant control scheme
ensures that at any time during a low-load cycle
that the plant is ready for reloading with normal
plant load gradients from the low-load operation
point to a new dispatch load. The LLOC concept
was initially tested using Alstom's cycle simulation
models, ensuring feedback on the selected
functional and control concepts before applying
their findings in a live plant.
From the design and simulation results,
the key elements of the LLOC-equipped
combined-cycle control sequences were developed
(Figure 8). Simplified, the functional
operational sequences are categorized into
the following operating phases:
■ Phase 0: Initiation and release of plant
unloading. The low-load operation mode
is selected by the operator, and the rest of
the control processes (the next three phases
listed below) follow automatically.
■ Phase 1: Steam conditioning. In order to keep
the thermally induced stress of the steam turbine
components within allowable limits, the
temperature of the HP and hot reheat (HRH)
steam will be decreased to a software-deter4.
Low-load operation concept. The low-load limit of a typical combined-cycle plant is
limited by the gas turbine's minimum load. The gas turbine's minimum load may be determined by
emissions permit limits or by mechanical design limits such as combustion temperatures, flame
stability, or acoustics. The typical minimum load of an industrial CT is around 50%. Source: Alstom
Typical minimum CT load given by emission regulations and CT emission characteristic
KA26 LLOC: Reduced CT load at low emission levels
CT shutdown and restart: Thermal stress cycles, starting reliability, no online power reserve
Period of low power demand or reduced power tariff
110
100
90
80
70
60
50
40
30
20
10
Time
5. Low NOx at low loads. NOx emissions at part load from a typical industrial combustion
turbine (CT) will rise well above baseload permit operating limits. The LLOC-equipped CT,
however, can manage the operation of the sequential combustion system to maintain low NOx
emissions even at loads down to 20% of baseload. The low-load data were taken from Alstom's
GT26 Test Power Plant. Source: Alstom
NOx emission
Typical permit level
Typical single combustor engine characteristic
NOx emission
CT relative load (%)

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POWER September 2010

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