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dual fuel mode, diesel and natural gas,
the latter produced as by-product from
the process facilities, and therefore with
lower quality (often with high sulphur con-
tent).
The energy conversion of the CCGT is
normally reduced by the quality of the fuel
because the temperature’s margin from
the Acid Dew point to be maintained at
the stack, often above 140°C.
In addition, since the power plant will be
operated in island mode with various am-
bient temperatures and grid loads, the
Gas Turbines unlikely will be operated at
baseload. This is an additional constrain
that need to be considered when desi-
gning the bottoming cycle.
The optimization of a combined cycle
needs a specifi c sensitivity study and is
based on the design envelope given for
each specifi c application.
Here below is represented the effect of
Steam Operating Condition on the Heat
Recovery Steam Generator (HRSG) size
and effi ciency for a typical single pressure
level Combined Cycle installed on fl ue gas F igure 3 - Sensitivity analysis on a single pressure level combined cycle
stream at 550°C, without thermal energy
export. The typical recommended design
zone is highlighted in Figure 3.

Pressure levels
The number of pressure levels to be selected is
always a trade-off between performance and cost/
process constraints.
The single Pressure, non re-heating steam cycle,
shown in Figure 4, has an unfi red HRSG (as
shown) equipped with fi nned tube superheater,
evaporator, and economizer section. Adopting the
alternative with OTSG, instead, each of the three
sections is part of a single bundle without the in-
stallation of the steam drum (see Figure 5).
This is the simplest confi guration that can be ap-
plied in a combined cycle, and it has been used
extensively.
The single pressure CCGT is a lower cost solution Figure 4 - Single Level cycle with HRSG
with minimal footprint (relevant to CCGTs con-
fi guration) and as such it is the fi rst option to be
considered for FPSO application. Although it does
not reach the highest thermal effi ciency it is still re-
commended when the fuel is inexpensive with high
sulphur content and when the GT exhaust tempe-
rature is below 500-550°C.
The adoption of an OTSG with high pinch (lower
effi ciency) increases the plant fl exibility allowing
the operation of the gas turbines without major
constraints, characteristics very stringent in island
mode operation (see Figure 6).
The fi gure represents a theoretical comparison
between HRSG and OTSG installed on the same
fl ue gas stream under the same steam production.
The HRSG resulted with 10.5°C pinch point while
the OTSG resulted with 22.5°C pinch point, which
indicates a lower surface and a higher fl exibility of
the OTSG. Figure 5 - Single Level cycle with OTSG

Impiantistica Italiana - Maggio-Giugno 2024 31