POWER September 2010 - 53

COMBUSTION TURBINES
6. How low can you go? A closer look at GT26 emissions at Alstom's Test Power Plant at
very low loads reveals how flat emissions remain. For reference, the current EU statutory limits
are 25 ppm (15% O2) at baseload for NOx and CO emissions below 5 ppm. Source: Alstom
NOx emission
100
80
60
40
20
10
12
14
16
Relative load (%)
Gissi Plant Gets First Commercial Installation of LLOC
However, combined-cycle power plants
The first application of Low Load Operation
Concept (LLOC) technology was at Abruzzoenergia
SpA's 800-MW Gissi combined-cycle plant
located in the Abruzzo region of central Italy.
Alstom was contracted to design, supply, install,
and commission the entire power plant.
The fast-track project was given the green
light in January 2006 and the plant entered
commercial service in early 2008 (Figure 7).
The Gissi plant consists of two GT26 sequential
combustion turbines installed in
a 2 x 410-MW single-shaft combined-cycle
arrangement. Each power train comprises
one GT26 gas turbine, a triple-pressure
reheat, horizontal heat-recovery steam
generator, an STF15C steam turbine using
565C/140 bar high-pressure steam, and a
shared hydrogen-cooled TOGAS turbogenerator,
all manufactured by Alstom.
Initially, Abruzzoenergia planned to
shut down the Gissi plant overnight when
electricity demand was low. The Gissi plant
design assumed two different operating
modes: operate about 8,000 hours per year
at baseload or about 3,000 hours per year
with daily start/stop operation. However,
the possibility of low-load operation using
LLOC presented Abruzzoenergia with a financial
opportunity. The ability to keep the
plant online overnight meant Abruzzoenergia
could provide power for grid support
as well as rapid peak load response. Grid
support at night is especially valuable in
Italy, which suffered a massive blackout in
2003 caused by a lack of spinning reserve
when it was critically required.
September 2010 | POWER
in this region of Italy usually do not operate
at night because emissions regulations
historically have prevented combustion
turbines from operating at less than 60%
of rated load.
Shutting down a combustion turbine
and steam bottoming plant overnight,
only to restart them the next morning, is
a less-than-desirable operating scheme.
Daily cycling causes thermal stresses that
will shorten the life of major plant components
and increase operating costs over
the plant's lifetime as every start-stop
operation avoided will save 10 equivalent
operating hours. The extended time necessary
to start up the plant can also result
in lost revenue in a competitive merchant
power market.
Alstom's LLOC allowed Abruzzoenergia to
park each unit at less than 100 MW during
off-peak hours as an alternative to daily
cycling. Each GT26 combustion turbine
met the baseload plant emissions requirements
while parked.
The units are required to achieve 30 mg/
m3 for NOx and CO in all ambient conditions
down to 30% load.
In addition, when parking the plant at
around 20% combined-cycle load (made
possible by shutting down the second or
Sequential EnVironmental combustor of
the GT26 at low loads, while keeping the
first or EnVironmental combustor operating
at nominal conditions), emissions are
about the same as at full load.
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7. Italian plant design. The first commercial
application of Alstom's Low Load Operation
Capability (LLOC) was with the two
KA26-1 units installed at Abruzzoenergia's
Gissi plant. " KA26-1 " designates a singleshaft
driveline combined-cycle plant based
on Alstom's GT26 combustion turbine. Each
unit is rated at a nominal 400 MW. The LLOC
design feature allows Abruzzoenergia to
" park " the plant during low power demand
periods at about 20% (~80 MW) combinedcycle
load. CO and NOx emissions are kept
below baseload permit limits while the plant
is parked. Courtesy: Alstom
18
20
CO emission
mined target value. Steam turbine power output
and efficiency are marginally decreased
by the temperature reduction.
■ Phase 2: Combustion turbine unloading.
When the new steam target temperature is
reached, the gas turbine reduces load using
its normal unloading ramp rate. The
steam desuperheaters close with decreasing
CT exhaust temperature. Within approximately
20 minutes of starting the GT
unloading, the low-load operation point is
reached. For a plant with two CTs serving
a single steam turbine, the CTs will be
unloaded simultaneously.
■ Phase 3: Plant operation at low load. At
the low-load operation point only the first
EV combustor of the CT is used; the SEV is
switched off to maintain low emission levels.
■ Phase 4: Initiation and release of plant rePlant
tests have confirmed that the
combustion turbine can reach baseload
operation and the plant can achieve over
95% of combined-cycle baseload output
in less than 20 minutes from the " parked "
condition of 20% load.
At a gas price of €0.26/m3 and an offpeak
electricity price of €28/MWh (during
low-load operation), plant owners expect
to save between €10 million and €18 million
over the plant's lifetime. Additional
revenue is expected from providing valuable
grid frequency support and spinning
reserve capacity.
53
NOx / CO emissions relative to permit values (%)

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POWER September 2010

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