POWER September 2010 - 54

COMBUSTION TURBINES
8. Principles of operation. Alstom's sequential combustion system gives the GT24/
GT26 the ability to maintain a high and constant exhaust temperature for the heat-recovery
steam generator (HRSG) down to a part load of 40%, resulting in much higher part-load combined-cycle
efficiency compared to that derived from a typical single combustion turbine. This
figure illustrates the GT26 low-load hold point. A step-by-step description of the loading and
unloading process is included in the text. The dynamic response of HRSG, steam cycle, and
steam turbine is simplified. Source: Alstom
CT load (%)
110
100
90
80
70
60
50
40
30
20
10
ST load (%)
CCPP load (%)
Temp HP/HRH steam (%)
Small CC power loss
caused by steam attemperation
Steam temp.:
decrease
controlled by
superheaters
ST relative load
CC relative load
CT relative load
CT exhaust
temp.
Steam temp.:
increase
controlled by
desuperheaters
Temp GT exhaust (%)
110
100
90
80
70
60
Time
loading. Low-load operation mode will be
deselected at the operator station. The operator
enters a plant load setpoint. CT and steam
turbine remain in continuous operation.
■ Phase 5: Plant reloading process. In principle,
the reloading sequence reverses the
unloading process. The CT loads at its normal
ramp rate and the water injection valves
After the simulation phase, the LLOC design
was next tested in the GT26 Test Power
Plant. During this test and validation phase,
data across all possible loads in full combinedcycle
mode were recorded in order to verify
the plant's low-load and response capabilities.
Actual results confirmed that the power
plant can be loaded from a " parked " condition
(about 20% of combined-cycle load) to
baseload (100% load) in less than 25 minutes
with standard load gradients, whereas normal
start-up times for hot start of a combinedcycle
plant after an 8-hour shutdown are
typically above 40 minutes. In addition, the
overall combined-cycle efficiency was approximately
60% of the baseload plant thermal
efficiency. ■
-Dr. Robert Peltier, PE is POWER's
editor-in-chief.
of the steam desuperheaters open in order
to control steam temperature gradients. After
approximately 20 minutes, the CT can
reach baseload operation, and >95% of the
combined-cycle baseload output is available.
The steam desuperheaters will gradually
close and increase the HP and HRH
steam temperatures to nominal values.
CIRCLE 27 ON READER SERVICE CARD
54
www.powermag.com
POWER | September 2010
CT, ST, CC relative load (%)
Relative temperatures
steam and CT exhaust (%)

http://www.powermag.com

POWER September 2010

Table of Contents for the Digital Edition of POWER September 2010

Contents
POWER September 2010 - Cover1
POWER September 2010 - Cover2
POWER September 2010 - Contents
POWER September 2010 - 2
POWER September 2010 - 3
POWER September 2010 - 4
POWER September 2010 - 5
POWER September 2010 - 6
POWER September 2010 - 7
POWER September 2010 - 8
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POWER September 2010 - Cover3
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