POWER July 2011 - 33

FUELS
of gas turbines in simple-cycle operation
that demonstrate this technology's attractiveness
for supporting variable renewable
generation:
■ GE has a 100-MW gas turbine (LMS 100)
capable of a 10-minute start for lowermegawatt
applications and a recently announced
GE-7FA.05 gas turbine, also with
10-minute start-up capabilities. Similar
start-up and ramping features are offered
by Siemens and Mitsubishi Heavy Industries
(MHI).
■ Due to a unique sequential combustion
design, Alstom turbines' efficiency at part
load is higher than others. The same capability
of shutting off one combustor at
part-load operation offers an additional
advantage for operating in this manner:
continuous operation at close to 30% of
baseload while remaining in compliance
with baseload emissions levels.
■ An alternative to frame gas turbines are
the aero-derivatives. One of the owners
of large wind farms (Westar Energy)
uses GE's LM6000 gas turbines to meet
demand for peak load and to cover for
shortfalls in wind farm generation. The
LM6000 can reach its full load output
from cold start in less than 10 minutes and
operate for 1 hour or less. When a number
of LM6000 gas turbines are on standby,
they can be dispatched immediately and
are able to respond to situations when
high-speed and wide-ranging wind fronts
are cutting wind turbines' production by
hundreds of MW.
Combined-Cycle Developments
The real challenge for the industry is to develop
capabilities for fast start-up and rapid
loading of equipment without affecting its
availability and reliability. Cycling operation
must be performed without increasing the
number of equivalent operating hours or accelerating
the maintenance schedule. To that
end, here are some of the actions initiated by
OEMs:
■ Use high-starting-reliability systems for
the gas turbine and balance of plant.
■ Implement complex control systems capable
of providing adequate ramp rates for
each specific state of the hardware.
■ Employ a high degree of start-up automation
for both gas and steam turbine.
■ Implement measures aimed at heat retention
during shutdowns, such as stack
dampers and the use of auxiliary steam.
■ Provide sophisticated monitoring systems
for major equipment conditions, allowing
operators to evaluate the impact of accelerated
start-up or cycling operation on
July 2011 | POWER
Table 1. Summary of concentrated solar technologies. Source: Bechtel Power
Working fluid
Technology type
Tower direct steam
Tower molten salt
Trough
Linear Fresnel
component life.
■ Allow the gas turbine to rapidly ramp
without the constraints of the heat-recovery
steam generator (HRSG) and steam
turbine.
Heat-Recovery Steam Generator. It
should be remembered that for CCs, the
element requiring the most time to reach
baseload is not necessarily the gas turbine.
For HRSGs, the most appropriate solution
to accelerate the start-up process is the use
of the Benson-type high-pressure (HP) circuit.
Following are some of the well-known
mechanisms affecting the performance and
integrity of the HRSG components in cycling
operation:
■ Low cycle fatigue
■ Creep
■ Thermal shock
■ Oxidation and exfoliation
■ Differential expansion
■ Corrosion fatigue
■ Corrosion in tubes
■ Flow-accelerated corrosion (FAC)
■ Corrosion product migration
■ Deposits
■ Erosion
All components in an HRSG are subject to
the operating-life-affecting mechanisms listed
above. However, some components may
be more vulnerable because of their location,
construction, or exposure. Critical components
in an HRSG generally include these:
■ Superheater and reheater outlets
■ Tube-to-header joints in hot sections
■ Drum to downcomer nozzle in HP drum
■ Bent portion of the heat transfer tubes
■ Attemperators
■ Bypass valves
These need to be designed and monitored
more closely for any kind of life-affecting conditions.
Solutions offered by OEMs include:
■ Designing hot section outlets to minimize
side-to-side variation.
■ Using full-penetration welds, generating a
joint with longer fatigue life.
■ Limiting the use of dissimilar materials.
■ Designing an adequate draining system
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Maximum temperature
Steam
Mixture of salts
Synthetic oil HTF
Steam
550C (1,022F)
575C (1,067F)
395C (743F)
270C (518F) or higher
aimed at reducing quenching effect.
■ Employing various methods to keep the
drums warm during shutdowns.
■ Equipping the stack with a stack damper.
■ Using special alloys to mitigate the exposure
of critical components to FAC.
■ Including special features such cascading
bypass to minimize thermal shock during
start-up.
Steam Turbine. Design and operability
improvements in steam turbines in CC operation
have allowed overall start-up and
turbine rolling to baseload in record times.
As mentioned above, new and complex
control features and stress measurements
for steam turbines permit CCs to respond
much faster than before, particularly during
hot starts and load-following mode. Without
appropriate flexibility for the start-up times
of steam turbines, the viability of CC as a
power-controlling element for renewables
will be significantly reduced.
It should be noted that the most logical
solution for this type of application is a 1 x
1 (one gas turbine, one HRSG, one steam
turbine generator) arrangement. Other configurations
(2 x 1 and 3 x 1) will require
larger turbines with a longer start-up time. A
modern G or H class gas turbine in a 3 x 1
CC configuration might require a 400-MW
to 500-MW steam turbine and therefore be
more suitable for baseload operation.
Considering these constraints, the preferred
configuration of CCs for renewable
back-up power must be selected as a result
of detailed feasibility studies combining the
start-up curves of all CC components and
their control systems.
Integrating Solar and Fossil
Generation
An additional role that a CC can play in the
deployment of renewable power, specifically
solar thermal, is to accept the steam produced
by a solar thermal source into its steam cycle.
This arrangement is called integrated solar
combined cycle (ISCC). By including an additional
source of heat, such as solar energy,
the efficiency of the system is dramatically
increased. Annual electricity production is
increased because the steam turbine is already
in operation, avoiding lost time for
start-up. During solar operation, the steam
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POWER July 2011

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