Innovation in Global Power - Parsons Brinckerhoff

Thermal – Achieving New Efficiencies, Reducing Carbon Emissions
http://www.pbworld.com/news_events/publications/network/
The basic concept would allow a large nuclear power plant
with a typical output of 800 to 1700 MWe to be combined
with a 300 MW CCGT generating unit. Linking the steamcycles
of the two plants enables them to operate as an
integrated power production unit and reduces losses of
potential output, increasing total efficiency. Cycle efficiency
gains enable the CCGT to contribute an increased output for
no additional fuel, with the efficiency of converting the energy
in the gas to electricity increased to about 62 percent.
How NuGas TM Works
Although a CCGT system has a high thermal efficiency, it
relies on using the heat from the exhaust gases of the gas
turbine to boil water to produce steam that drives the
turbine. As the exhaust gases cool, water is evaporated in
the boiler tubes but temperature differences of up to 400ºF
(200ºC) arise in the boiler due to the large amount of heat
needed to evaporate the water. These temperature differences
limit the potential work that can be extracted from the
steam, reducing the output of the steam cycle.
The NuGas TM cycle overcomes this limitation by ‘borrowing’ a
small proportion (typically 10 percent) of the steam from the
nuclear steam cycle (point ‘A’ on Figure 1). The dry saturated
steam is superheated using the exhaust heat of the gas
turbine. The high temperature steam (‘B’) is then used to
drive a separate conventional condensing steam turbine to
provide additional output from the plant. Superheating steam
rather than boiling water enables a much lower temperature
difference to be maintained in the heat recovery system,
maximizing the value of the energy recovered.
The heat in the gas turbine exhaust flow between about
570ºF and 320ºF (300ºC and 160ºC) is recovered via a high
temperature economizer (‘C’) to generate high temperature
Figure 1: Schematic Combination of the Steam Cycles.
feedwater, which is returned to the nuclear cycle (‘D’),
ensuring that the inlet temperature to the steam generator
is maintained close to the design value.
The heat in the gas turbine exhaust below about 320ºF (160ºC)
(‘E’) is used to heat part of the condensate from the high
temperature steam turbine (‘F’) before it is deaerated and
returned to the nuclear cycle feed pumps (‘G’). The remaining
condensate from the high temperature steam turbine is
returned to the nuclear cycle condensate system (‘H’).
The flows of energy around the cycle differ somewhat to
those in a conventional CCGT. Figure 2 shows a simplified
Sankey diagram for the NuGas TM cycle, including the energy
exchanges between the CCGT and PWR cycles shown along
the lower edge of the diagram.
Identifying the separate performance of the CCGT cycle
when it is linked to the PWR cycle requires that the design
PWR energy balance be maintained. Thus, the CCGT returns
power to the PWR to compensate for the reduction in
output due to the ‘borrowed’ steam and returns rejected
heat in the CCGT cooling water to the PWR to account
for the reduced heat rejection from the nuclear turbine
condenser. The diagram therefore shows the additional
energy input, the additional losses and the additional power
generated by the cycle, demonstrating its high efficiency.
Safety Considerations
Downstream failure is limited. The extraction of steam
from the main steam system has the potential to disturb
reactor operating conditions. However, the PWR system is
designed to allow for a 10 percent step change in flow to the
main steam turbine without exceeding the appropriate limits
for a frequent operating condition. It is likely, nevertheless,
Figure 2: Simplified Sankey Diagram for NuGas Cycle.
9 PB Network #68 / August 2008