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ENERGY TRANSITION

plexity on the Steam Turbine selection. The steam
turbine with controlled steam extraction shall be
equipped with additional components with the re-
levant increase of costs, weight and limits the ope-
rating envelope. It is possible to simplify the system
considering a bleed from the steam turbine, but this
is acceptable only when the CCGT plus process
heating are operated with minimal load variation.
When then Process heat demand is low compa-
red with the electric power demand, Baker Hughes
studied the possibility to extract the heat directly
from the Recovery System combining the Steam
Generator Coils with a Waste Heat Recovery sec-
tion. With this solution, the CCGT can be designed
independently from the process heating require-
ments. The heat process requirements will be sati-
sfi ed by the heat recovery section.

By-pass Stack
The need for a bypass stack, that could be integral
or not, is a decision that may be related to the spe-
cifi c design. The adoption of an OTSG gives the
possibility not to install the bypass stack, because
it can be operated in dry run, as it will be discussed
in next chapter.

Figure 6 - Typical Temperature Heat diagram for single
pressure level Waste heat recovery solutions

Process Heat Production Steam Generators for a FPSO need to limit as much
To ensure minimal CO emission, the process heat as possible footprint and weight, that’s why OTSG
2
needed by the production facility can be extracted (see Figure 7) are typically preferred compared to
from the gas turbine exhaust fl ue gases. traditional horizontal or vertical HRSG (See Figure
Heat recovery systems for combined cycles appli- 8) drum type with natural circulation, whereas in-
cation are typically steam generators that produce termediate solutions like forced circulation HRSG
superheated steam for the bottom Rankine cycle, are even less common. A drum-less solution has
although the need of heat for process use lead to the advantage to be less sensitive to sea motion,
destinate part of the available Gas Turbine exhaust that can affect the water level control in the steam
gas enthalpy to a heating medium, that could be drum, based on which HRSG are controlled, even
pressurized water or diathermic oil. Therefore, the if offshore application can mitigate the level fl uc-
possible waste heat recovery systems confi gura- tuations by introducing internal baffl es. The steam
tions are multiple, spanning from a Steam Genera- drum, obviously, partially fi lled of water and located
tor alone, to a mix of Steam Generators and Wa- at highest position of HRSG, with its volume and
ste Heat Recovery Units, that can have each other weight is extremely sensitive to sea motion induced
connected in series, in parallel or both. accelerations and contributes to signifi cantly eleva-
te the overall HRSG centre of gravity.
Baker Hughes is currently Apart from Steam Drum absence, OTSG are typi-
engaged on several CCGT plants cally lighter than HRSG since they are manufactu-
red with exchanging tubes of higher quality metal-
“for FPSO and is executing lurgy, ranging from Austenitic Stainless Steels to

world’s first of such a kind, Nickel Alloys instead of carbon steel or low alloys,
that contributes to have:
a solution which drastically • an increased corrosion resistance to potential
improves electrical efficiency aggressive compounds in the gas turbine fuel

(e.g. Sulphur) that may convert during combu-
stion and following exhaust gas cooling and
The designer shall evaluate the process schematic condensation, generating acid in the coldest
to be adopted depending on the thermal level and sections of the tube bundle;
duty needed by the primary process facilities. • a decreased rate of oxide growth on the ste-
When the thermal duty is high, it is normal practi- am-touched surfaces of boiler tubes, consi-
ce to extract steam from the Steam turbine to be dering that the rate of oxidation of the boiler
condensed against the heat transfer fl uid available tube material increases in a roughly exponen-
on the facility. This choice simplifi es the process tial manner with increasing metal temperature,
design of heat exchangers, but it introduces com- while it generally decreases with increasing

32 Impiantistica Italiana - Maggio-Giugno 2024