Page 34 - Impiantistica italiana

ENERGY TRANSITION

vacuum to exploit all the energy contained in the the high temperature systems is not an issue for
steam. The steam, that expands in the turbine, can the CCGT, due to the possibility of operating the
also be tapped (bleeds) or extracted to feed ther- gas turbines also in OCGT mode during transients
mal utilities (i.e. thermal transfer fluid heat exchan- and start/stop. Further, high flexibility in heat avai-
gers) that need heat. lability is provided by the steam let-down valves,
Generally, the steam, extracted and condensed in which allow the heat exchangers to be suddenly
the heat exchangers, returns to the condenser to supplied with low-pressure steam.
close the loop and also the fresh make-up water, This flexibility of steam production and electricity is
necessary to recover the losses of the circuit, is fed very important during the transients of FPSO pro-
into the condenser. cess plant, like start up or partial shutdown, and to
match the product variation of reservoir during oil/
Flexibility in power and heat production. In Of- gas field operational life.
fshore, the split of power/heat demand changes
with the operating conditions of the plant, and the High efficiency and lower CO emission. As
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ability to change these loads is a critical require- shown in Table 1, heat recovery at the gas turbi-
ment. This flexibility is well provided by steam pro- ne exhaust limits the amount of energy lost to the
duction from a waste heat generator and by using atmosphere and increases overall efficiency (i.e.:
steam in a (condensing) steam turbine. In effect, from 35% to 48% in the case considered). As a
the electricity is produced by the driven electrical direct consequence, there is also a significant re-
generator, while the heat comes from the extrac- duction in CO emissions per kWh produced (from
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tion/ bleeding as required. 0.59 to 0.41 kg/kWh).
This system also provides the ability to use more
steam in the turbine when heat demand is low and,
conversely, to use less steam in the turbine, when
the heat demand is higher. Plant architectures
Furthermore, any load change limitation caused by
either the thermal inertia of the steam generator or CCGT Configurations
The overall efficiency of a CCGT, calculated as to-
tal net electric or mechanical power vs heat rate,
increases with the complexity of the bottom cycle
selected. The maximum energy conversion is pos-
sible with Steam generated at three pressure levels
with reheating steam turbine arrangement. Howe-
ver, this is not an optimal solution for power gene-
ration on FPSO plants that must be characterized
by reduced size, high flexibility and powered by
aero-derivative gas turbines. Therefore, the prefer-
red cycle architecture in FPSO is usually with single
steam pressure level that has lower heating reco-
very, higher temperature at the stack, that could
be requested in case of sulphur gas/liquid fuel, as
indicated ahead.
The typical steam generator used in Offshore ap-
plication is the OTSG type, and this is true espe-
cially for FPSO installation. In this type of device,
the water passes to the vapor state inside each cir-
cuit without a controlled interface, and this allows
the removal of the steam drum, with the benefit of
not suffering pitch and roll during operation, lighter
weight, and minimal footprint thanks to vertical flow
arrangement. The adoption of this solution forces
us to design a cycle with lower efficiency becau-
se the arrangement of the coils in such equipment
impedes pushing the pinch point to the minimum.
The generation of steam with multiple pressure le-
vels is also a complication that normally is not re-
commended for systems with OTSG.
Another constraint to be considered during the
design of the CCGT is the selected Gas Turbine
Technology and size. The Gas Turbines normally
installed on FPSO are Aeroderivative in the 25MW
– 54 MW class, for their high efficiency, low weight,
and ease of maintenance.
These turbines are also capable of operating in

30 Impiantistica Italiana - Maggio-Giugno 2024