Page 33 - Impiantistica italiana

COMPARISON OCGT CCGT
Total Power MWe 98.8 98.8
Gas Turbine load % 88.7 87
Turbine Model LM2500+ G4 6stg SAC
Numbers of Units 4 3+ 1 STG
Total Heat Consumption MW 279.67 206.58
Total Exhaust kg/s 304.90 221.35
Total CO Produced kg/s 16.29 11.37
2
kg/h 58647 40928
kg/(kwh) 0.59 0.41
Global Electrical Eff. % 35 48

versus the OCGT case. Thanks to the most recent
carbon pricing schemes, in certain regions set to
achieve or exceed $100/ton of carbon emitted, this
reduction can even become an additional source of
revenue for the project.
The LM2500 family has an efficiency at site condi-
tions usually ranging between 30% and 40%. Mo-
ving to a CCGT configuration with the same gas
turbine technology and the same power output of
approximately 100 MWe, there is a huge electrical
efficiency increase as described in Table 1.
The Rankine Cycles are commonly considered as
bottom cycles to increase the efficiency of a OCGT
system, converting the waste heat available at the
exhaust of the Gas Turbine into electric power, pro-
cess heat, or both. Multiple gas turbine generators
are usually coupled to a steam plant consisting of
associated steam generators for waste heat reco-
very. Steam is expanded in a steam turbine genera-
tor (STG) that discharges into a condenser.
Figure 2 - Combined Cycle Gas Turbines
A recent trend is to consider
Combined Cycle or Cogeneration various types, as described in the dedicated pa-
ragraphs, but for offshore installations it was con-
“Gas Turbines (CCGT) as an sidered that the single-pass type is the most suita-
alternative to the traditional ble, because it avoids problems of weight, size and
thermal inertia, although it involves a more careful
Open Cycle Gas Turbines (OCGT) focus on the control and monitoring of cycle water
quality. Since there is no continuous blow down,
The bottom cycle can have several architectures, to avoid scaling problems within the bundle, the
according to the required purpose as described purity of the water in the loop must be maintained
below. While the CCGT for a FPSO is in principle at extremely high levels, as further described later.
the same as the one used for onshore power ge- The Once Through Steam Generator (OTSG) has
neration, in an Offshore environment the needed the advantage of not having the cylindrical body or
flexibility of heat/electricity production, the physical steam drum, where vaporization takes place in the
dimensions and the weights play a key role in de- recovery boilers, and that constitutes a large depo-
termining the preferred alternative. sit of boiler water, in elevation, which finds difficult
The components of the bottom cycle are much and critical placement on platforms and FPSOs.
more numerous than those of the OCGT, as shown
in Figure 2 and for a typical installation they are Steam turbine and Condenser: the steam tur-
usually: flue gas bypass, steam generator, feed bine is the other fundamental element for the reali-
water pumps, steam turbine generator, conden- zation of the CCGT, since it transforms the energy
ser, condensate extraction pumps and auxiliary contained in the steam into electricity, through the
systems (such as polishing skid, vacuum systems, towed electric generator, and it maintains the pres-
demi-production skid, steam reduction valves). sure in the main steam headers. It discharges into
the condenser, which often in FPSOs is then co-
Steam Generator: Steam generators can be of oled with sea water. The condenser works under

Impiantistica Italiana - Maggio-Giugno 2024 29