POWER September 2013 - 45

TOP PLANTS
and ML&P (30%) formed a participants
agreement in 2008 to jointly own SPP with
a budget of $369 million. The new 183-MW
plant (200 MW with duct firing) was sized to
replace baseload generation provided by the
1970s vintage Beluga combined cycle Units
6, 7, and 8 (218 MW total). The increased efficiency
of SPP reduces the amount of natural
gas consumed by 25%, which translates into
roughly a $2 million-per-month reduction in
the cost of fuel.
Once the participants approved the project,
Chugach retained the San Francisco
office of Parsons Brinkerhoff (PB) as the
owner's engineer. PB assisted Chugach in
preparing the turnkey contractor bid documents
and purchased equipment specifications.
After competitive bids were evaluated,
an engineering-procurement-construction
(EPC) contract was signed by SNC-Lavalin
on June 20, 2010, with a June 30, 2013, project
completion date.
The official groundbreaking was held in
March 2011, although grading and other site
preparation work was completed in late 2010
to prepare for the 2011 construction season.
Construction was mobilized concurrently
with groundbreaking the following spring,
with priority given to placing the power block
and the enclosed building around it so construction
could continue indoors during the
winter months. Substantial completion of the
project was reached on Jan. 19, 2013, and the
plant was placed into commercial service on
Feb. 1, 2013, five months early and $2 million
under budget.
One means used by Chugach to accelerate
the schedule was to direct purchase the three
General Electric (GE) LM6000PF combustion
turbines (CTs), the three once-through
steam generators (OTSG) manufactured by
IST, and a 57-MW steam turbine from Mitsubishi
Power Systems Americas in advance
of the EPC contract. By doing so, Chugach
assumed the performance risk for the 3 x 1
combined cycle project (balanced with the
commensurate cost savings), but cost and
completion risk remained with SNC-Lavalin.
Provisions for a fourth CT, such as electrical
and piping tie-in points, were included as
part of the original design.
The aeroderivative LM6000PF, nominally
rated at 48 MW, was selected for its high efficiency,
operational flexibility, and ability
to rapidly respond to grid frequency disturbances.
The CT is able to reach full generating
capacity in 10 minutes with guaranteed
15 ppm NOx and 25 ppm CO emissions from
its dry low-emissions combustion system design.
The CT also has a Power Boost capability
that allows operation of each CT up to 2
MW over its full power rating in the event
of a grid disturbance (Figure 1). The steam
September 2013 | POWER
turbine rating was selected to handle the
steam flow produced by a future fourth CT
without duct firing the OTSGs. In the meantime,
duct firing two of the three OTSGs will
produce maximum steam turbine output for
peak load purposes. The official plant rating
is 182.5 MW with a heat rate of 6,382 Btu/
kWh (LHV) at an average annual ambient
temperature of 36F.
The plant's operational flexibility was
considerably improved by using IST's oncethrough
steam generator technology. The
boilers are constructed of stainless steel and
Inconel tubes that can be operated dry at
100% CT load, should the need arise, and
restarted with hot tubes. This design also
eliminates the expense, space, and efficiency
losses of a bypass damper and stack. In
addition, the condensers are sized to accept
100% steam dump should the steam turbine
trip. This capability allows the steam turbine
to be immediately restarted and restored to
full load without disrupting operation of the
steam plant or the CTs.
Also included within each OTSG were selective
catalytic reduction (using 29% aqueous
ammonia) and CO catalysts to further
reduce emissions. Today, the plant operates
with 11 ppm NOx from the CT and 4.5 ppm
from the boiler stack.
An air-cooled condenser was supplied by
GEA Rainey, along with an air-cooled heat
exchanger from GEA Rainey that provides
cooling via an intermediate closed-loop glycol/water
system. Proper water treatment for
the OTSGs is essential. A two-stage reverse
osmosis system eliminated the need for onsite
resin regeneration and bulk chemical
storage. Two crushed resin polishers, each
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sized for 100% of flow, continuously protect
Inconel tubes in the OTSGs by minimizing
potential contamination.
Cool Operator
Plant supervisory controls were provided
by Emerson Process Management's Ovation
distributed control system. The entire
plant is interfaced to the Chugach Dispatch
Center through SCADA with full automatic
generation control capability. The plant has
very high levels of automation and operates
24/7 with only two operators per shift and a
small maintenance staff. A single pushbutton
initiates OTSG startup and brings them to full
load, at which time operators bring the steam
turbine online. A black-start standby generator
enables the plant to independently restart
in the event of a systemwide outage.
The harsh Alaskan winter conditions that
can last over six months a year necessitated
special measures in both plant design and
construction. For example, all building maintenance
access points include roll-up doors,
and large trucks can reach all the CTs via a
drive-through passage. Also, each end of the
CT power train can be accessed through rollup
doors, and each OTSG can be accessed
through removable building panels located at
each end for repairs or catalyst changes.
By building a plant with a very high degree
of operating flexibility and the most
efficient power plant in Alaska, and by taking
significant steps to solve the region's gas
supply problems, Chugach and MP&L have
made Southcentral Power Project a POWER
Top Plant. ■
-Dr. Robert Peltier, PE is POWER's consulting
editor.
45
1. Low-emissions turbine. The dry low-emissions system used on the LM6000PF
combustion turbine that carefully stages combustion within the turbine's annular combustor
is visible with its many pipes and valves. The result is guaranteed NOx emissions of 15 ppm.
Courtesy: SNC Lavalin

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POWER September 2013

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