Focus on the USA
First Flex-Plant-10
goes into operation
James Varley reports from El Segundo
I
n any competition for the world’s most
attractive power plant locations, NRG’s El
Segundo, on the Pacific Ocean coast, near
Los Angeles, would have to be a contender.
The new 550 MW combined cycle plant there,
inaugurated on 12 September, also establishes
new benchmarks for compactness, reduced
emissions and noise, ultra low water use and,
above all, flexibility, being the first Siemens
Flex-Plant-10 to enter operation.
The new power plant has two identical
multi-shaft power trains, each consisting
of an SGen6-1000A generator driven by an
SGT6-5000F gas turbine upstream of a NEM
DrumPlus HRSG, which provides steam to
an SST-800 steam turbine coupled to its own
SGen6-100A-2P generator. Control is via an
integrated SPPA-T3000 system, with start-up
and shut-down automation and stress limiting
features.
The single-pressure DrumPlus HRSGs (see
last month’s issue), combined with a package
of other measures, including full capacity steam
bypass systems and innovative piping warmup strategies, allow the gas turbines to be run
up to full power without restrictions, bringing
300 MW on to the grid in ten minutes, with full
power achievable in a further 50 minutes or so.
This is in marked contrast to the 12 hour or
more start up time required by the 1950s/early
60s-vintage gas fired boiler technology that the
new combined cycle facility is replacing.
As well as being flexible, permitted for 200
starts per year, the new plant, being combined
cycle, also achieves high baseload efficiency,
around 49%. This is much better than an
November 2013
aeroderivative simple cycle gas turbine peaker,
but not as good as a combined cycle with
three pressure HRSG. NRG opted for a singlepressure non reheat configuration, at the
expense of efficiency, to achieve unrestricted
gas turbine start up and maximum operational
flexibility, and also to make the plant as
compact as possible, as it is a congested site.
The fast start technology means that the
gas turbine is not held at low load, so a lowemissions power level is reached very soon,
resulting in 89% (by weight) less CO per start
and 95% less NOx.
El Segundo is also the first full implementation
of what Siemens calls Clean-Ramp, which
means it is able to maintain low emissions (a
remarkably low 2 ppm of NOx in the El Segundo
case), while ramping steeply, both up and
down – a requirement increasingly demanded
by regulators as they turn their attention to
controlling emissions during transients not just
in steady state conditions.
Clean-Ramp, which has been tested and
partially installed before (eg at Long Island
Energy Center, Lakeside and Marsh Landing)
but never fully implemented, integrates control
of the gas turbine with the SCR so that, for
example, “feed forward logic” is used to trigger
proactive ammonia injection in the event of an
anticipated NOx excursion.
Overall, says Richard Loose of Siemens,
the new plant has “the operating profile of a
peaker, and the emissions profile of a combined
cycle”, with a conventional SCR.
This makes the plant a particularly good
complement for renewables. In fact it could be
Surfer’s eye view of El Segundo, one of a new
wave of flexible, low emissions, gas fired
plants. To the right, unit 3 (retired) and 4.
(photo: NEM)
argued that such plants are a prerequisite for
accommodating large proportions of renewables
on the grid. As Michael Peevey, president of
the California Public Utilities Commission, said
at the 12 September El Segundo dedication
ceremony, “we can’t do what we want to do
with solar and wind without having plants like
this, with quick start and the ability to ‘turn on a
dime’ in terms of output.”
When the El Segundo repowering was first
proposed, back in 2000, the initial plan was
to build a 630 MWe 2-on-1 baseload-oriented
combined cycle plant with once-through
seawater cooling. A permit for such a facility
was received in 2005. But subsequently NRG
decided to radically revise the design and
changed to a rapid response plant with air
cooling.
This was “in anticipation of then future
policies and regulations, such as the California
State Water Resources Control Board oncethrough cooling water policy, which was
approved in 2010, California Assembly Bill 32,
which was adopted in 2006, and California’s
associated Renewable Portfolio Standard”,
www.modernpowersystems.com | MODERN POWER SYSTEMS
19
Focus on the USA
notes George Piantka of NRG. The California
once-through policy, and evolving federal
regulations, namely §316b of the Clean Water
Act, demanded reduced impact on marine life.
This “attached significant risk to the economic
and regulatory viability of new combined cycle
ocean-cooled generation such as originally
licensed in 2005 for El Segundo”, while “the
Renewable Portfolio Standard placed an
increased emphasis on more efficient fast
start conventional generation to help integrate
and complement the expected large amount
of renewable generation capacity rapidly
coming onto the grid.” This expectation proved
well founded, with, for example, 1.8 GW of
renewables added to the California grid in 2011
and 2012 – three times the fossil-fired capacity
constructed.
A permit application for the revised design
was submitted in 2007 and the permit was
received in 2010.
The air cooling system at El Segundo is not
the typical ACC (Air Cooled Condenser), with
a number of large fans mounted on a high
platform, but is described as an ACHE (Air
Cooled Heat Exchanger), which is compact,
with a low profile, and is also relatively quiet
in operation.
The ACHE performs the same function as
a typical ACC, but with one major difference,
explains Richard Loose: “The ACHE condenses
steam exiting the steam turbine at slightly
above atmospheric pressure whereas an
ACC condenses steam at substantially below
atmospheric pressure (vacuum conditions). Steam
at higher pressure occupies a lower volume and
this in turn allows the ACHE to be substantially
smaller in overall size and have a lower profile
than a traditional ACC. For plant sites that are
space constrained, smaller equipment used for
condensing steam is of great benefit.”
The use of this small-footprint form of air
cooling for steam condensing was an important
consideration for NRG because space at the El
El Segundo combined cycle plant: the
main components (source: Siemens)
Segundo site, crammed between a cliff and the
ocean, is indeed fairly restricted, and the new
plant was constructed while the existing plant
continued to operate and other facilities were
being demolished in parallel.
Space restrictions were also an important
factor in the choice of a multi-shaft, rather than
single-shaft, configuration for the combined
cycle plant. In the case of El Segundo the length
of a single-shaft, accommodating gas turbine,
generator, clutch and steam turbine, would
have been problematic. Another consideration,
says Richard Loose, was that “even though
the ACHE used is smaller than an ACC it still
would not have been practical in a single-shaft
arrangement because it would not have fitted
on the site without being located remotely
from the steam turbine exhaust, which would
have resulted in a longer a more geometrically
complex exhaust duct.”
Air cooling combined with the use of
reclaimed waste water and water recycle
results in a 90% reduction in potable water use
at the site, compared with the retired units 1-3.
The plant is also designed to be “zero liquid
Clean-Ramp at El Segundo: low
emissions while ramping (source: Siemens)
10
300
9
8
7
200
6
5
150
4
100
3
2
50
1
0
13:48
20
13:53
13:58
14:03
14:08
Time
14:13 14:18
0
14:23
14:28
14:33
MODERN POWER SYSTEMS | www.modernpowersystems.com
14:38
Stack NOx, CO (ppmvd @ 15% O2)
Combined cycle MW
250
discharge”, with extensive water recycling
and reuse.
The new combined cycle plant has been
constructed on the site of the 1950s vintage El
Segundo units 1 and 2 (290 MW each). These
were seawater-cooled gas-fired steam boiler
units, with oil back up, retired in 2002, and
subsequently demolished, with removal of two
old oil tanks as well.
The repowering has also included improved
site landscaping, a more attractive sea wall,
and better coastal views thanks to the lower
profile of the new plant.
Another key factor in being a “good
neighbour” in the beach community, within
earshot of walkers and cyclists on the Strand,
and surfers, is keeping noise to the minimum.
Measures have included use of low speed fans
in the air cooled heat exchangers, low noise
valves, acoustic enclosures, inlet and exhaust
silencing baffles and acoustic lagging.
Southern California Edison (whose need
for new capacity has been exacerbated by
premature closure of its San Onofre nuclear
station (due to defective replacement steam
generators)) has contracted to take the entire
output of the new plant via a ten-year PPA.
As well as Siemens, the other main
contractor on the project was ARB,
responsible for construction, while NRG
acted as its own EPC contractor, with Worley
Parsons providing engineering services.
El Segundo is the third Siemens F class
based flexible gas fired power plant to have
recently entered operation in California,
the other two being NRG’s Marsh Landing
(open cycle) and NCPA’s Lodi, which is a
multi-shaft Flex-Plant-30 with Benson threepressure reheat HRSG (see MPS, Sept 2012).
With the coming on line of the new El
Segundo combined cycle plant, the 335 MW
unit 3, also a seawater-cooled gas-fired steam
boiler unit, built in 1964, has been retired. Unit
4, of a similar vintage and capacity, continues
to operate, but will close down by the end of
2015 and a second phase of the repowering
programme will be implemented. In this
second phase the plan is to build about 450
MW of combined cycle and peaking capacity
on the sites of units 3 and 4.
MPS
November 2013