POWER August 2013 - 52

PLANT DESIGN
2. Refurbish instead of rebuild. The existing three conventional units are shown
in the background (outdoor boilers with a single steam turbine building located behind the
boilers) with the two existing 7EA simple cycle combustion turbines (CTs) to the right of
the existing units. The new 7EA CTs are visible in the foreground. Between the new CTs
and the three existing boilers are the two HRSGs being assembled. Each 7EA-HRSG combination
supplies steam to a single, existing steam turbine. The HRSGs are Vogt Power
International's Enhanced Constructability Smart design. The design incorporates pressure
parts, pressure part support steel, interconnecting piping, casing, and structural steel into
only six shop-fabricated module boxes per HRSG, significantly reducing erection labor
expense. The photo was taken during construction in October 2011. Courtesy: Burns &
McDonnell
equipped with evaporative cooling technology,
which increases summer capacity by ~8
MW. At full load, the combustion turbines
will provide a flow of 2,225,000 pounds per
hour of exhaust gas at 1,022F to each HRSG
(Figure 3).
Heat Recovery Steam Generators. The
VPI HRSGs are two-pressure, non-reheat,
natural circulation drum type with horizontal
gas flow and duct firing. A two-pressure nonreheat
HRSG economically maximizes the
combined cycle efficiency of the repowered
1960s technology non-reheat steam turbines.
Given the steam turbine flow limits and the
cost of a new steam turbine, an HRSG with
more than two pressure levels was not cost
justified.
Coen Co. Inc. supplied the duct burner
3. New power block. One of the two new gas-fired GE 7EA combustion turbines is
shown. The ductwork located on top of the CT is the air inlet system that includes an evaporative
cooler. The upper left shows the CT bypass stack during erection with a small portion of the
new HRSG shown at the upper left edge of the photograph. A bypass damper is located at the
base of the bypass stack, which may be closed to direct CT exhaust gas through the bypass
stack or opened to allow the gas to continue to the HRSG. Courtesy: Burns & McDonnell
system that is located after the finishing highpressure
(HP) superheater section within the
casing of the HRSG. The system includes
pressure-reducing and flow measurement stations,
burner runners mounted horizontally
across the HRSG duct, a burner management
system, and a cooling air blower skid for
flame scanners. Duct-firing the HRSG can
increase the HP superheater outlet flow rate
up to 60%, thus adding the ability to quickly
respond to load fluctuations.
In addition to the CT's DLN1 system, a
selective catalytic reduction (SCR) system
is located downstream of the HP evaporator
to further reduce NOx
emissions. A 19%
aqueous ammonia reducing agent is injected
onto the catalyst as part of the NOx
reduction
process. In sum, the system is capable
of reducing NOx
emissions by 98%. A space
allocation was provided upstream of the SCR
to accommodate a future CO catalyst.
To maximize HRSG heat recovery, a condensate
preheater is provided just upstream
of the HRSG exhaust stack, which heats condensate
to near saturation temperature prior
to entering the existing deaerator. A recirculation
system is used to maintain a minimum
condensate inlet temperature to the HRSG to
prevent external condensation and corrosion
on preheater tubes. The system has two 100%
recirculation pumps that take hot condensate
exiting the preheater and mixes it with cold
condensate entering to maintain a minimum
inlet temperature of 140F (Figure 4).
Stack Bypass System. The stack bypass
Design Characteristics
The engineering and design of the repowering
project was performed with two goals in
mind: increased operational flexibility and
reuse of existing equipment, where feasible,
to minimize project cost. Reused equipment
included the steam turbine, boiler feed
pumps, condensate pumps, condenser, cool52
ing
towers, deaerator, plant air system, and
flash evaporator. Details about the major
components and equipment used on the repowering
project follow.
Combustion Turbines. The two new
natural gas-fired GE 7EA CTs are equipped
with dry low-NOx
technology (DLN1) and
each is rated at ~85 MW. Each CT is also
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system adds operating flexibility by allowing
each CT to operate independently of the
HRSG, such as during a steam turbine outage
or trip. However, the number of hours each
year the bypass may be used is limited by the
plant's operating permit. The combustion gas
bypass system also allowed SME to generate
power in simple cycle mode during construction
of the combined cycle portion of
the project. For additional safety, a blanking
POWER | August 2013

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POWER August 2013

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