POWER June 2013 - 36

PLANT DESIGN
Repower or Build a New Combined
Cycle Unit?
URS recently performed a combined cycle repowering study to determine the
feasibility and economics of repowering an existing steam turbine that
went into service in the 1950s. The competing option was building a new
combined cycle unit. The results of the study provide insight for others
considering the same alternatives.
By R.B Boulay, PE, and M.S. Massoudi, PE, PhD, URS Corp.
U
RS recently completed an engineering
study for a Midwestern utility
with the goal of identifying the most
cost-effective gas-fired plant that could be
feasibly added to its electricity supply system.
Two design alternatives were identified:
repowering a 1950s-vintage steam
turbine that was originally built as part of
a coal-fired plant or building a 2 x 1 combined
cycle plant based on two different FClass
combustion turbines (CTs). The new
build would have to produce a minimum
560 MW net. The levelized cost of electricity
from the repowered and new plant
options was compared and the best option
was identified.
Repowering Options
Repowering entails removing and replacing
the steam generation portion of an
existing power plant with CTs and heat
recovery steam generators (HRSGs). The
steam produced by the new, high-efficiency
gas-fired equipment is sent to the
existing steam turbine to produce electricity.
The advantage, beyond the obvious
construction cost savings, is that the existing
steam turbine (ST) and its auxiliary
systems are operational, and there is a
generator electrical connection. The existing
ST in this case was a 180-MW-class,
1,800-psig unit with 1,050F main steam
and 1,000F reheat steam.
Site-specific factors often restrict the
number of design options, and this project
was no different. For example, the land
area available for repowering at this site is
very constrained and could accommodate
only a single combustion turbine. A single
Siemens SGT6-8000H was selected for
this evaluation because of its proven design
and high performance, although the
Mitsubishi M501GAC and GE 7FA 7-Series
should be seriously considered during
preliminary design, should the repower36
ing
option be selected. Also, the existing
steam turbine condenser uses once-through
cooling. However, new thermal discharge
limits would limit its reuse, so the repowered
project must also convert the existing
once-through cooling system to a closedloop
cooling system (cooling tower).
Reuse the Existing Steam Turbine.
One major design challenge when repowering
an existing plant is to match the
steam production capability of the CT
and HRSG with the steam needs of the
existing steam turbine. In this project,
the repowering option requires modifications
to the existing steam turbine, such
as blanking off all the existing feedwater
put. These factors can be maximum allowable
reheat pressure, steam pressure at the
crossover, maximum allowable last-stage
bucket (LSB) flow, or the generator rating.
Floating throttle pressure will be lower
than the design value because of the reduced
throttle flow, and it will typically
be lower than the optimum value for combined
cycle operation. This low throttle
pressure can be corrected by replacing
the steam path in the HP turbine. The
intermediate-pressure (IP) steam path is
on the same rotor shaft, so it can be replaced
with minimal additional cost when
replacing the HP steam path. Because turbine
modifications are usually required to
One major design challenge when repowering
an existing plant is to match the steam production
capability of the CT and HRSG with
the steam needs of the existing steam turbine.
heater extractions and introducing a lowpressure
(LP) admission at the crossover
point. Otherwise, the steam paths (essentially
the rotating and stationary parts that
steam contacts as it passes through the
ST) remain unchanged. The steam supply
modifications are required because the repowered
ST is generally not able to pass
the original design high-pressure (HP)
steam flow through the entire turbine because
the extraction ports are closed. The
net effect is a large increase in LP steam
flow that must be accommodated in the
steam cycle design.
The unit may or may not be able to reach
its rated electrical output, depending on
which steam cycle parameter limits its outwww.powermag.com
maintain
the correct axial thrust balance
and the original steam paths may be suffering
degradation from age, replacing the
steam paths is often cost-effective.
Condenser modifications will generally
be required to provide oxygen removal
from the condensate. Adding trays below
the tubes and relocating the makeup header
to the top of the condenser shell will
generally provide an acceptable oxygen
level of 7 ppb.
Duct Fire HRSGs. The HRSG was designed
with duct firing to maximize the
steam cycle output. In many repowering
designs the limiting factor for steam cycle
output is the flow capacity of the LSBs;
steam flow through the LSBs increases
POWER | June 2013
http://www.powermag.com
POWER June 2013

Table of Contents for the Digital Edition of POWER June 2013
