Thermodynamic Analysis of Low-Emission Offshore Gas-to-Wire Firing CO 2 -Rich Natural Gas: Aspects of Carbon Capture and Separation Systems

: Despite the growth of renewable energy, fossil fuels dominate the global energy matrix. Due to expanding proved reserves and energy demand, an increase in natural gas power generation is predicted for future decades. Oil reserves from the Brazilian offshore Pre-Salt basin have a high gas-to-oil ratio of CO 2 -rich associated gas. To deliver this gas to market, high-depth long-distance subsea pipelines are required, making Gas-to-Pipe costly. Since it is easier to transport electricity through long subsea distances, Gas-to-Wire instead of Gas-to-Pipe is a more convenient alternative. Aiming at making offshore Gas-to-Wire thermodynamically efficient without impacting CO 2 emissions, this work explores a new concept of an environmentally friendly and thermodynamically efficient Gas-to-Wire process firing CO 2 -rich natural gas (CO 2 > 40%mol) from high-depth offshore oil and gas fields. The proposed process prescribes a natural gas combined cycle, exhaust gas recycling (lowering flue gas flowrate and increasing flue gas CO 2 content), CO 2 post-combustion capture with aqueous monoethanolamine, and CO 2 dehydration with triethylene glycol for enhanced oil recovery. The two main separation processes (post-combustion carbon capture and CO 2 dehydration) have peculiarities that were addressed at the light shed by thermodynamic analysis. The overall process provides 534.4 MW of low-emission net power. Second law analysis shows that the thermodynamic efficiency of Gas-to-Wire with carbon capture attains 33.35%. Lost-Work analysis reveals that the natural gas combined cycle sub-system is the main power destruction sink (80.7% Lost-Work), followed by the post-combustion capture sub-system (14% Lost-Work). These units are identified as the ones that deserve to be upgraded to rapidly raise the thermodynamic efficiency of the low-emission Gas-to-Wire process.


Introduction
Global warming concerns have been the subject of several international agreements.The rising utilization of renewable energies is a remarkable fact; however, fossil fuels still lead the global energy matrix.A large increase in natural gas power generation is expected in the next decades as a result of the expanding natural gas (NG) reserves and considering that NG is the cleanest fossil fuel [1].
Deep-water oil reserves of the Brazilian Pre-Salt offshore layer have a high gas-oil ratio, with CO 2 -rich associated gas (CO 2 > 40%mol) [2].Despite the low-quality gas, large investments in long-distance subsea pipelines are necessary to carry this NG to market [3].
In this scenario, an alternative to bypass both CO 2 and NG transport infrastructures (Gas-to-Pipe) is to install floating Gas-to-Wire (GTW) plants, adopting NG Combined Cycle (NGCC) power plants [4].These plants, placed on the offshore gas field, convert the raw produced gas directly into electricity, which is exported to onshore facilities through long-distance High-Voltage Direct Current (HVDC) cables [5] for less losses [6].
Existing offshore GTW plants (≈600 MW) are not concerned with the destination of CO 2 , sending it into the atmosphere.To mitigate CO 2 emissions, GTW must include carbon capture and storage (CCS) to achieve emission goals [7] by decreasing the carbon footprint of power generation [8].Captured CO 2 reinjection into the reservoir as Enhanced Oil Recovery (EOR) fluid [9] is a solution for CO 2 storage while improving oil production [10], as well as offering additional monetary leverage [11].Hassanpouryouzband et al. [12] showed that more than 90% of the injected stream of CO 2 can be stored.Hassanpouryouzband et al. [13] pointed out that it is essential to control the injection pressure to enhance CO 2 storage efficiency.Hydraulic fracturing increases the permeability of oil and gas reservoirs [14], improving CO 2 storage [15].
Araújo et al. [16] evaluated CCS technologies, such as chemical absorption, physical absorption, membrane permeation, and hybrids.These authors detected that chemical absorption holds the lowest CO 2 emission per ton of injected CO 2 .Hetland et al. [17] performed theorical GTW-CCS research, studying the implementation of post-combustion carbon capture (PCC) downstream a Siemens-NGCC.In their system, the NGCC plant flue gas was sent to a PCC unit using aqueous monoethanolamine (MEA).The authors pointed out that the GTW-CCS concept is feasible, although CO 2 -EOR stream dehydration was not taken into account.
Aiming to achieve the carbon neutrality of GTW-CCS processes, the implementation of gas turbine exhaust gas recirculation (EGR) has been considered in the literature [18].By lowering the exhaust gas flowrate and increasing its CO 2 content, EGR facilitates the CO 2 capture step, because a higher CO 2 content increases the driving force for CO 2 absorption, lowering column height, while a lower flue gas flowrate lowers column diameter.These both reduce the CCS penalty by lowering investment [19].In addition, EGR diminishes NOX emissions, since the circulating oxygen and nitrogen concentrations decrease in the cycle [20].
There is a gap in the literature regarding the GTW-EGR-CCS-CO 2 -DEHY overall process, the singularities of its separation sub-systems-i.e., the PCC-MEA plant and CO 2 -DEHY plant-and the thermodynamic analysis of the overall system and its sub-systems (second law analysis).To fill this gap, the present work assesses GTW-EGR-CCS-CO 2 -DEHY and conducts a thermodynamic analysis of the overall system and sub-systems, as well as exploring the peculiarities of its separation sub-systems.Thermodynamic analysis identifies power destruction sinks and quantifies the Lost-Work of the overall system and its sub-systems, aiming to identify process units that should be improved to increase the overall thermodynamic efficiency.

Methods
Offshore GTW-EGR-CCS-CO 2 -DEHY, using CO 2 -rich NG and exporting CO 2 -to-EOR and power, was designed and simulated in Aspen-HYSYS 10 for technical and thermodynamic assessments.The necessary theory, process descriptions, process complexity, and methods are discussed in this section.

Natural Gas Combined Cycle Plant
The NGCC plant comprises five NGCC elements for adequate electricity output (≈600 MW).As shown in Figure 2, each NGCC element contains four parallel gas turbines united to one Heat Recovery Steam Generator (HRSG), which heats a steam cycle (Rankine cycle).Aero-derivative gas turbines (Table 1) are applicable for offshore rigs considering their high power-to-weight ratio and low footprint [22].Gas turbines fire raw CO2-rich NG (CO2 > 40%mol) without any conditioning.The generated flue gas feeds the HRSG at T = 549 °C [23], producing High-Pressure Superheated Steam (HPS) (T = 524 °C, P = 24 bar) and Low-Pressure Steam (LPS, T = 160 °C, P = 6 bar).Følgesvold et al. [24] presented the HRSG temperature approaches and head losses.The steam turbine receives the HPS and expands it to P = 0.12 bar.The resulting stream is cooled down in the sub-atmospheric condenser with Cooling Water (CW), arriving as condensation to the HRSG (T = 45 °C).The generated LPS heats PCC-MEA and CO2-DEHY reboilers; therefore, the steam cycle power is controlled by LPS demand.The gas turbine model in HYSYS involves the following: (i) adiabatic single-stage air compressor; (ii) combustion chamber modeled as adiabatic conversion reactor; and (iii) adiabatic expander.This model was adjusted to manufacturing data by calibrating the adiabatic efficiencies of its air compressor and expander.

Natural Gas Combined Cycle Plant
The NGCC plant comprises five NGCC elements for adequate electricity output (≈600 MW).As shown in Figure 2, each NGCC element contains four parallel gas turbines united to one Heat Recovery Steam Generator (HRSG), which heats a steam cycle (Rankine cycle).Aero-derivative gas turbines (Table 1) are applicable for offshore rigs considering their high power-to-weight ratio and low footprint [22].Gas turbines fire raw CO 2 -rich NG (CO 2 > 40%mol) without any conditioning.The generated flue gas feeds the HRSG at T = 549 • C [23], producing High-Pressure Superheated Steam (HPS) (T = 524 • C, P = 24 bar) and Low-Pressure Steam (LPS, T = 160 • C, P = 6 bar).Følgesvold et al. [24] presented the HRSG temperature approaches and head losses.The steam turbine receives the HPS and expands it to P = 0.12 bar.The resulting stream is cooled down in the sub-atmospheric condenser with Cooling Water (CW), arriving as condensation to the HRSG (T = 45 • C).The generated LPS heats PCC-MEA and CO 2 -DEHY reboilers; therefore, the steam cycle power is controlled by LPS demand.The gas turbine model in HYSYS involves the following: (i) adiabatic single-stage air compressor; (ii) combustion chamber modeled as adiabatic conversion reactor; and (iii) adiabatic expander.This model was adjusted to manufacturing data by calibrating the adiabatic efficiencies of its air compressor and expander.Air is provided at stoichiometric proportion for complete NG combustion.To restrict the combustion temperature to factory settings, stoichiometric air is mixed with Exhaust Gas Recycle (EGR).Recycled flue gas is removed after the DCC and before the PCC-MEA, and its flowrate is adjusted to reach the prescribed flue gas temperature at the expander outlet (T = 549 • C).
Air is provided at stoichiometric proportion for complete NG combustion.To restrict the combustion temperature to factory settings, stoichiometric air is mixed with Exhaust Gas Recycle (EGR).Recycled flue gas is removed after the DCC and before the PCC-MEA, and its flowrate is adjusted to reach the prescribed flue gas temperature at the expander outlet (T = 549 °C).

Direct-Contact Column
The DCC (Figure 3) receives flue gas from the five NGCC elements and cools them down to 40 °C via direct contact with CW (T = 30 °C).The cooled flue gas is split by (i) about 65% recycles as EGR and is mixed to the gas turbine air feed, decreasing the flame temperature, and (ii) the rest is forwarded to the PCC-MEA unit.

Direct-Contact Column
The DCC (Figure 3) receives flue gas from the five NGCC elements and cools them down to 40 • C via direct contact with CW (T = 30 • C).The cooled flue gas is split by (i) about 65% recycles as EGR and is mixed to the gas turbine air feed, decreasing the flame temperature, and (ii) the rest is forwarded to the PCC-MEA unit.
Air is provided at stoichiometric proportion for complete NG combustion.To restrict the combustion temperature to factory settings, stoichiometric air is mixed with Exhaust Gas Recycle (EGR).Recycled flue gas is removed after the DCC and before the PCC-MEA, and its flowrate is adjusted to reach the prescribed flue gas temperature at the expander outlet (T = 549 °C).

Direct-Contact Column
The DCC (Figure 3) receives flue gas from the five NGCC elements and cools them down to 40 °C via direct contact with CW (T = 30 °C).The cooled flue gas is split by (i) about 65% recycles as EGR and is mixed to the gas turbine air feed, decreasing the flame temperature, and (ii) the rest is forwarded to the PCC-MEA unit.

Post-Combustion Capture with Aqueous-MEA
The flue gas that arrives at PCC-MEA is divided into four smaller feeds (Figure 4) in order to improve the capture efficiency [25].PCC-MEA is designed to capture 90% of the CO 2 flue gas under two primordial parameters that define solvent recirculation and stripper duty: the Capture Ratio (CR: kg of fresh solvent per kg of captured CO 2 ) and the stripper Heat Ratio (HR: GJ of heat per CO 2 ton).Ideal values for the CR (10-15 kg Solvent /kg CO 2 ) and HR (2.0-4.5 GJ/t CO 2 ) for aqueous-MEA are reported by Araújo et al. [26].
reboiler (T = 103 °C).The stripper condenser operates in total reflux, i.e., it refluxes 100% condensation and distillates water-saturated CO2 (P = 1 atm) through its vent.To maintain CO2 confined in the CO2 loop between the PCC-MEA and CO2-DEHY units, all the condensed carbonated waters (T = 35 °C) from CO2-CMP-1 knock-out vessels and from the TEG stripper condenser (T = 40 °C) are recycled to the PCC-MEA stripper tray#1, while water-saturated CO2 from the TEG stripper condenser vent (T = 40 °C) is recycled to tray#10.This recycling enables a reduction in make-up water and condenser duty, as well as blocks CO2 emissions from CO2-CMP-1 and CO2-DEHY units.After receiving make-up water, a pump recirculates lean-MEA (MEA ≈ 30% w/w) to the PCC-MEA absorber.

CO2 Dehydration TEG Unit and CO2 Stripping Gas Unit
The CO2 stream arrives at CO2-DEHY (Figure 5) at a high pressure (50 bar), favoring water removal [27].The CO2-to-CO2-DEHY stream (≈2700 ppm-mol H2O) and TEG solvent (TEG = 98.5%w/w) feed the 15-stage TEG absorber, generating Dry-CO2 to STR-CO2 (≈200 ppm-mol H2O).This unit is the top product, and rich-TEG (TEG ≈ 60%mol) is the bottom product.TEG solvent is regenerated in the 10-stage TEG stripper, producing lean-TEG as the bottom product (T = 138 °C), and the top distillates water-saturated CO2 vapor and carbonated liquid water in the partial condenser.Both water and CO2 distillates are recycled to the stripper of the PCC-MEA unit, avoiding CO2 emissions and water losses.STR-CO2 is a small-scale unit that produces stripping gas (1% of Dry-CO2) in order to keep the .This recycling enables a reduction in make-up water and condenser duty, as well as blocks CO 2 emissions from CO 2 -CMP-1 and CO 2 -DEHY units.After receiving make-up water, a pump recirculates lean-MEA (MEA ≈ 30% w/w) to the PCC-MEA absorber.

CO 2 Dehydration TEG Unit and CO 2 Stripping Gas Unit
The CO 2 stream arrives at CO 2 -DEHY (Figure 5) at a high pressure (50 bar), favoring water removal [27].The CO 2 -to-CO 2 -DEHY stream (≈2700 ppm-mol H 2 O) and TEG solvent (TEG = 98.5%w/w) feed the 15-stage TEG absorber, generating Dry-CO 2 to STR-CO 2 (≈200 ppm-mol H 2 O).This unit is the top product, and rich-TEG (TEG ≈ 60%mol) is the bottom product.TEG solvent is regenerated in the 10-stage TEG stripper, producing lean-TEG as the bottom product (T = 138 • C), and the top distillates water-saturated CO 2 vapor and carbonated liquid water in the partial condenser.Both water and CO 2 distillates are recycled to the stripper of the PCC-MEA unit, avoiding CO 2 emissions and water losses.STR-CO 2 is a small-scale unit that produces stripping gas (1% of Dry-CO 2 ) in order to keep the TEG stripper reboiler temperature below the TEG degradation temperature (T ≈ 206 • C) [28,29].Although some operators limit the reboiler temperature to 190-200 • C [30], the present study was more conservative and maintained the reboiler temperature below 140 • C to improve TEG durability, eliminating reposition costs.The residual Dry-CO 2 is forwarded to CO 2 -CMP-2 to reach the EOR pipeline pressure (P = 300 bar).In the STR-CO 2 unit, these two Dry-CO 2 streams feed a countercurrent heat exchanger, allowing for a slight temperature reduction (≈0.5 • C) in the CO 2 -to-EOR stream.
TEG stripper reboiler temperature below the TEG degradation temperature (T ≈ 206 °C) [28,29].Although some operators limit the reboiler temperature to 190-200 °C [30], the present study was more conservative and maintained the reboiler temperature below 140 °C to improve TEG durability, eliminating reposition costs.The residual Dry-CO2 is forwarded to CO2-CMP-2 to reach the EOR pipeline pressure (P = 300 bar).In the STR-CO2 unit, these two Dry-CO2 streams feed a countercurrent heat exchanger, allowing for a slight temperature reduction (≈0.5 °C) in the CO2-to-EOR stream.

CO2 Compression Units
CO2-CMP-1 (Figure 6a) is a four-stage intercooled compression train (stage-compression ratio = 2.85) to increase the CO2 stream pressure to 50 bar for feeding the CO2-DEHY unit.CO2-CMP-2 (Figure 6b) receives the Dry-CO2 from STR-CO2 and pressurizes the stream in order to reach the EOR pipeline pressure (P = 300 bar).

CO 2 Compression Units
CO 2 -CMP-1 (Figure 6a) is a four-stage intercooled compression train (stage-compression ratio = 2.85) to increase the CO 2 stream pressure to 50 bar for feeding the CO 2 -DEHY unit.CO 2 -CMP-2 (Figure 6b) receives the Dry-CO 2 from STR-CO 2 and pressurizes the stream in order to reach the EOR pipeline pressure (P = 300 bar).The main process simulation assumptions and equipment conditions are shown in Table 1.

Equipment Conditions and Process Simulation Assumptions
The main process simulation assumptions and equipment conditions are shown in Table 1.Ordinary NGCC plants at around 500 MW of capacity are feasible and quite simple plants that are already implemented in the offshore scenario of oil and gas production.However, existing ordinary NGCC plants are not concerned with the destination of CO 2 , emitting it freely into the atmosphere.To mitigate CO 2 emissions, one solution is used in its reinjection into reservoirs via EOR.To make this possible, it is necessary to add the PCC-MEA (post-combustion CO 2 capture with aqueous-MEA) and the CO 2 -DEHY (CO 2 dehydration) plants.Moreover, to lower the CCS costs, it is necessary to implement the Exhaust Gas Recycle (EGR), which reduces the flue gas flowrate (about 50% or higher reduction) and increases the flue gas content of CO 2 (about 100% or higher increase).The EGR is practically mandatory if CCS is involved because it raises the driving force for CO 2 absorption and reduces the volume of flue gas to be treated, consequently allowing for a reduction in column height and diameter and the number of absorbing columns.This way, the insertion of PCC-MEA, CO 2 -DEHY, and EGR loop implies that there is a high increase in the number of recycling processes-the EGR itself and several recycles of carbonated waters and CO 2 vapors to the stripper of PCC-MEA-which intrinsically increases the complexity of the process and negatively impacts its controllability.Thus, while a typically ordinary NGCC is a process with a simple direct structure, the proposed offshore GTW-EGR-CCS-CO 2 -DEHY process is a reasonably complex one with a handicapped controllability-besides being a much more expensive process-which demands careful analysis of its global controllability and the possible interactions of different control actions during the design of its control and start-up systems.The problematic controllability configures the main shortcoming and limitation of the new proposed process because it may turn the process into a dynamically unstable system whose operation may entail risks and unexpected extra costs.

Thermodynamic Analysis of Steady-State Processes
Thermodynamic analysis is efficient to pinpoint resource degradation through processes.Steady-state offshore GTW-EGR-CCS-CO 2 -DEHY and its sub-systems are evaluated by the second law analysis of processes.For the second law analysis, systems and their sub-systems are formerly classified as power-producing or power-consuming systems.Figure 7 exhibits a steady-state open system for thermodynamic assessment with numerous feed/product streams (blue/red arrows, respectively) interacting with an infinite isothermal heat reservoir (R 0 ) at temperature T 0 .The overall system and its sub-systems may be power-producing ( .W > 0) or power-consuming ( .W < 0), but they must only have thermal interactions with R 0 either absorbing ( .

Maximum Power
Equations ( 1) and ( 2) depict the first law of thermodynamics applied to a steady-state open system (Figure 7).The system maximum power/work ( MAX W  ) is calculated through the second law at reversible conditions adopting Equations ( 3

Maximum Power
Equations ( 1) and ( 2) depict the first law of thermodynamics applied to a steady-state open system (Figure 7).The system maximum power/work ( .

W MAX
) is calculated through the second law at reversible conditions adopting Equations ( 3)- (6).At reversible conditions, Equation (4) performs the universe entropy balance, making .

Equivalent Power
Being always positive for regular systems, .

W
Eq represents the thermodynamic power equivalence of electricity production (consumption) and utility production (consumption) [31].For example, LPS production (consumption) is equivalent to CW P are assumed constants.Heat power equivalences are calculated through reversible heat engines with the maximum heat work conversion yield, namely, the Carnot Heat Pump (CHP) and the Carnot Engine (CE) [31].The CHP imports power, absorbs heat from a cold source, and rejects heat to a hotter source, while the CE absorbs heat from a hot source, exports power, and rejects heat to a colder one.). Figure 7 shows that the LPS-Loop and CW-Loop are external to the system (Figure 7), R 0 is always a cold heat reservoir (cold source), and Q CW are always positive.It is possible to frame an analogous version of Figure 7 for a power-producing system (i.e., electricity and LPS are exported, and CW is imported).
The steady-state power-consuming system (Figure 7a) absorbs heat ( .Q LPS ) from LPS, making it LPS-condensate, which is restored to LPS via a LPS-Loop, using the CHP, that imports power ( .W Eq LPS ) and absorbs heat ( Similarly, the power-consuming system (Figure 7b) rejects heat ( .Q CW ) to cold-CW, producing hot-CW, which is restored to cold-CW via a CW-Loop using the CE that exports power ( .W CW Eq ) and rejects heat ( . . W Eq LPS is given by Equation ( 10) using Equation (9a,b), and the CHP entropy conservation in Equation (9c).Accordingly, .

W
Eq CW is given by Equation ( 12) using Equation (11a,b), and the CE entropy conservation in Equation (11c).Equation ( 10) also works for .W Eq LPS in power-producing systems, but the LPS-Loop rotates counter-clockwise, the CHP is replaced by the CE, and all the effects are reversed.Equation (13a) provides the equivalent power ( .

W Eq
) produced by a power-producing system that exports .EE and LPS (counter-clockwise LPS-Loop in Figure 7) and consumes CW.Analogously, Equation (13b) gives the equivalent power consumed by a powerconsuming system that consumes .EE, CW, and LPS.The substitution of Equations ( 10) and ( 12) into Equation (13a,b) results in Equation (14a,b) that give, respectively, the equivalent power produced by a power-producing system and the equivalent power consumed by a power-consuming system. . . . . .

Thermodynamic Efficiency
Process resource degradation is calculated via second law analysis, obtaining the thermodynamic efficiency and the Lost-Work (Lost-Power) of the overall system and its sub-systems.With  The Lost-Work (Lost-Power) formulas for power-producing systems and powerconsuming systems are intuitively calculated by Equation (16a,b).Additionally, Lost-Work can be measured through the second law formula (Equation (17a)) that considers all the universe changes caused by system transitions, where .

S UN IV
is the entropy-creation rate of the universe due to the system operation.Thus, Equation (17b,c) denote Lost-Work formulas derived from Equation (17a) for power-producing systems and power-consuming systems, respectively, where .

Results and Discussion
Technical and thermodynamic analyses of offshore GTW-EGR-CSS-CO 2 -DEHY are presented and discussed.

Technical Assessment
Table 2 compiles the technical performance of offshore GTW-EGR-CCS-CO 2 -DEHY.The NGCC plant, comprising five parallel NGCC elements, produces 599.3 MW of gross power (≈92.4% from gas turbines) entailing 534.4 MW of net exported power.Each gas Gases 2024, 4 52 turbine generates ≈ 30 MW at 36.5% LHV-efficiency, firing ≈ 4.76 kg/s of gas.Due to the EGR, each NGCC element produces 370.6 kg/s of flue gas at 17.3%mol CO 2 .The HRSG reduces the gas turbine flue gas temperature from 549 • C to 140 • C, which is the minimum value to maximize HPS output, providing enough LPS for PCC-MEA and CO 2 -DEHY strippers.The PCC-MEA stripper demands 722.2 MW of LPS and releases 144.2 kg/s of watersaturated CO 2 top product.CO 2 -CMP-1 increases the pressure of the CO 2 stream up to 50 bar in order to achieve CO 2 -DEHY ideal conditions.CO 2 -DEHY captures ≈93% of water from its feed and generates 503.8 t/h of Dry-CO 2 (≈193 ppm-mol H 2 O).STR-CO 2 dispatches 5.3 t/h of low-pressure Dry-CO 2 to the TEG stripper reboiler as stripping gas to maintain its temperature below 140 • C, avoiding TEG degradation.The TEG stripper reboiler requires only 0.6 MW of LPS, since the flowrate of captured water from the CO 2 stream is small.CO 2 -CMP-2 sends 498.8 t/h of Dry-CO 2 (P = 300 bar, T = 35 • C) to EOR.The Offshore GTW-EGR-CCS-CO 2 -DEHY power requirement corresponds to 10.8% of its gross power.CO 2 -CMP-1 and CO 2 -CMP-2 units are the major electricity consumers, while PCC-MEA leads LPS and CW consumptions.

Thermodynamic Analysis
Thermodynamic and Lost-Work analyses were accomplished for the offshore GTW-EGR-CCS-CO 2 -DEHY overall system and its sub-systems, namely (i) NGCC plant; (ii) DCC; (iii) PCC-MEA; (iv) CO 2 -CMP-1; (v) CO 2 -DEHY; (vi) STR-CO 2 ; and (vii) CO 2 -CMP-2.No sub-system was missed, i.e., the GTW-EGR-CCS-CO 2 -DEHY is correctly partitioned among the sub-systems mentioned, which means that the respective sums of  W LOST for all sub-systems must deliver the same value of the overall system, which is calculated independently of the sub-systems.The comparison of the overall system values with the respective sum over the sub-systems entails an indirect consistency check of the thermodynamic analysis.It is worth mentioning that there is always some divergence between the overall system and the sums over the sub-systems in practice.Thus, divergences below 1% can be accepted to validate the consistency of the thermodynamic analysis.that demands small power consumption ( .W MAX < 0); (ii) the positive power which could be produced by expanding streams through the head losses generated in the absorber, stripping column, and heat exchangers ( .W MAX > 0); and (iii) the positive power which could be produced by the utilization of the thermal approaches in heat exchangers, the stripper reboiler, and stripper condenser.As a result, CO 2 -DEHY has a minimum net power demand to perform water removal from the CO 2 of only 0.00014 MW.On the other hand, CO 2 -DEHY has .W Eq (0.12 MW) calculated as follows: (i)

W
Eq CW are all zero.This means that STR-CO 2 has sufficient spontaneities to produce power, but this potential is wasted, and zero power is produced.Consequently, the STR-CO 2 thermodynamic efficiency is zero.

Lost-Work Analysis
Lost-Work exposes the power potential destroyed in GTW-EGR-CCS-CO 2 -DEHY and in its sub-systems due to spontaneities.Table 3 reveals  in Equation (17b,c) for power-producing and power-consuming systems, respectively.In addition, Table 3 proves a consistency crosscheck in the sum of Lost-Works over sub-systems which, theoretically, should be equal to the overall-system Lost-Work (obtained divergences are smaller than 1%).3).In total, 66.65% of the offshore GTW-EGR-CCS-CO 2 -DEHY available power ( .W MAX = 1602.33MW) is wasted as Lost-Work through process spontaneities, which are mainly (i) combustion spontaneity and mixing in gas turbines; (ii) heat transfer finite thermal approaches; (iii) finite head losses; (iv) mixing of streams in several units; and (v) machine irreversibility, with compressor/expander adiabatic efficiencies lower than 100%.

Conclusions
Technical and thermodynamic analyses of a theoretically environmentally friendly (low-emission) and new offshore GTW-EGR-CCS-CO 2 -DEHY process were conducted.The offshore GTW-EGR-CCS-CO 2 -DEHY burns ≈ 6.5 MMSm 3 /d of CO 2 -rich NG (CO 2 > 40%mol), exports low-emission electricity, and sends dense CO 2 to EOR.The offshore GTW-EGR-CCS-CO 2 -DEHY produces 534.4 MW of net power, abating ≈ 90% of flue gas CO 2 .The offshore GTW-EGR-CCS-CO 2 -DEHY is an intensified power production process, whose major intensification components comprehend the following: (i) Exhaust Gas Recycle (EGR), which reduces the flue gas flowrate by ≈65% while increasing its CO 2 content from ≈7%mol up to ≈17%mol, and (ii) high-pressure CO 2 dehydration in CO 2 -DEHY, which extracts ≈ 93% of water from the CO 2 -to-EOR stream (≈200 ppm-mol H 2 O), avoiding the formation of hydrates in EOR pipelines.The advantage brought about by EGR is that it dismisses air excess (typically ≈ 100%) for gas turbine flame temperature reduction, consequently decreasing ≈ 65% the flue gas volumetric flowrate and raising its CO 2 content from typical ≈ 7%mol (without EGR) to ≈17%mol (with EGR).Thus, EGR drastically lowers investment and the operational cost of the CCS plant by reducing column diameter/height and improving low-emission GTW profitability.
The second law analysis of the offshore GTW-EGR-CCS-CO 2 -DEHY overall system reveals a 33.35% thermodynamic efficiency with 66.65% of Lost-Work, making the NGCC sub-system the greatest Lost-Work sink (80.7%  .W LOST share).Therefore, the NGCC and PCC-MEA are the major GTW-EGR-CCS-CO 2 -DEHY units that need to be upgraded to improve the efficiency of the overall system to attain better economic and environmental benefits.The consistency of the thermodynamic analysis was settled via Lost-Work sum-crosschecks and lateral checks considering the alternative second law formula T 0 • .

S UN IV
for the Lost-Work (Table 3).The technological innovations associated with the new proposed offshore GTW-EGR-CCS-CO 2 -DEHY process are not related to the units that constitute it because all these units and the adopted intensification strategies (such as EGR and CO 2 dehydration) are well known and individually techno-economically feasible.Instead, one could say that the most important innovation is the overall process configuration and the new possible interactions that emerge among the units that constitute the new process.These interactions obviously occur in the steady-state context as well as in the dynamic and controllability contexts, and they are very different depending on the context.It was demonstrated in the steady-state context, for example, that in spite of the gigantism of the new process and its main complex objective of generating low-emission electricity by firing CO 2 -rich NG at remote offshore sites, its thermodynamic efficiency is still reasonable, and its revenues are improved by exporting tradable CO 2 as an EOR agent, which boosts oil production while being confined in the reservoir.
In other words, the main contribution associated with this study is that it proves that the new process accomplishes its finalities and is thermodynamically, environmentally, and economically feasible.That is, it configures an expensive technological package that is worthwhile of further study, aiming at achieving large-scale implementation.

Suggestions for Future Work
The comparison of the efficiency and other performance (economic, environmental, thermodynamic) aspects of the newly proposed process against conventional counterparts is a very relevant point.It is also relevant to compare our proposed thermodynamic analysis of processes against alternative analyses such as the exergy analysis of processes, which is much more present in the literature.But these recommendations are also somewhat out of the present scope, which is already overburdened.Thus, we recommend that future works are dedicated to these important comparisons.The authors suggest carrying out an exergy analysis of the GTW-EGR-CCS-CO 2 -DEHY process and a subsequent comparison of it against conventional counterparts.Although the exergy analysis of processes is formally different from the thermodynamic analysis of processes, they normally point in the same direction, since both aim at revealing the weak (less thermodynamically efficient) units in the process that mostly require improvement.In addition to the exergy analysis, the efficiency and performance aspects (economic, environmental, and thermodynamic) of the GTW-EGR-CCS-CO 2 -DEHY process should be compared against existing offshore NGCC concepts.

Gases 2024, 4 50Figure 7
Figure7displays a power-consuming system with the following utility effects: absorbs

.
14a,b)), the thermodynamic efficiencies of power-producing systems and power-consuming systems are given by Equation (15a) and Equation (15b), respectively.

CO 2 -=
CMP-1 and CO 2 -CMP-2 are obvious power-consuming systems ( .−4.88 MW, respectively) because they perform non-spontaneous compression.hot-CW from compressor intercoolers (there is no LPS consumption, i.e., . the Lost-Work results and also attests the consistency of the present thermodynamic analysis by comparing Lost-Work values calculated through two thermodynamically independent ways: (i) via .16a,b) and (ii) via T 0 • .

Figure 8 illustrates
Figure 8 illustrates Sankey diagrams for

.
W LOST share) due to the highly spontaneous Gases 2024, 4 56 gas turbine firing process.The PCC-MEA sub-system is the second largest Lost-Work sink (14.0%

Table 2 .
Technical analysis results.