A New Integration System for Natural Gas Combined Cycle Power Plants with CO 2 Capture and Heat Supply

: Although carbon mitigation in power industry is attracting more and more attention around the world, the large scale application of carbon capture technology is obstructed because of the enormous energy consumption and huge capital investment required. In this study, an integrated system with power generation, CO 2 capture and heat supply are proposed, which adopts three measures to reutilize the waste heat released from the CO 2 capture process, including extracted steam recirculation, a CO 2 Rankine cycle and a radiant ﬂoor heat subsystem. Amongst these measures, the radiant ﬂoor heat subsystem can efﬁciently reuse the relatively low temperature waste energy in the absorbent cooler. Through thermodynamic analysis, it is determined that the power output of the new integrated system is 19.48 MW higher compared with the decarbonization Natural Gas Combined Cycle (NGCC) power plant without system integration. On the other hand, 247.59 MW of heat can be recovered through the radiant ﬂoor heat subsystem, leading to an improved overall energy efﬁciency of 73.6%. In terms of the economic performance, the integration requires only 2.6% more capital investment than a decarbonization NGCC power plant without system integration and obtains extra revenue of 3.40 $/MWh from the simultaneous heat supply, which reduces the cost of CO 2 avoided by 22.3%. The results prove the economic and efﬁciency potential of a NGCC power plant integrated with carbon capture, which may promote the industrial demonstration of carbon capture theology.


Introduction
With the rapid development of the world's economy, the consumption of fossil fuels is expected to increase constantly in the short term future, which makes the environmental issues caused by anthropogenic atmospheric emissions more serious [1,2]. In 2015, a target was set at the Paris Agreement talks, to keep the Earth's temperature rise to only 1.5 degrees Celsius [3]. Therefore, how to meet the requirements of appropriate economic growth and environmentally sustainable fuel utilization have become an important topic at the present stage. As one of the largest CO 2 emitters, the power industry attracts great attention in this context. It is predicted that the world's electricity demand will increase continually up to 34,290 TWh by 2030, 20% of which will be generated from the Natural Gas Combined Cycle (NGCC) power plants [2]. Therefore, it is important to find an efficient approach to realize the carbon mitigation of NGCC power plants.
One potential solution for the carbon mitigation from fossil-fired power plants is the use of CO 2 capture technologies, which can be divided into three different categories: pre-combustion, oxy-fuel, and post-combustion CO 2 capture [1,[4][5][6]. The post-combustion CO 2 capture based on monoethanolamine (MEA) is one of the most feasible approaches and recognized as the first choice for most demonstration projects due to its high technical maturity, large-scale applicability, and strong ability to remove CO 2 from low-CO 2 -concentration flue gas [6][7][8][9].
Despite its advantages, post-combustion CO 2 capture based on MEA also has its own drawbacks. Firstly, the absorbent regeneration consumes large amounts of thermal energy, which is mainly supplied by steam extracted from the steam turbine in the decarbonization power plant, resulting in a considerable decrease of the net plant efficiency and causing safety and operation problems for low pressure turbines (LPTs). Secondly, the CO 2 capture process also requires a large amount of work, leading to a remarkable increase in auxiliary power. Specifically, the CO 2 separated by chemical absorption is nearly at ambient pressure, and must be compressed to a high pressure for transportation and storage, consuming substantial compression work. In addition, the absorbent circulation flowrate is relatively large and requires considerable pump work. The above characteristics of MEA-based CO 2 capture lead to a significant decrease in the net efficiency of power plants (by nearly 10-15 percentage points) [10][11][12].
There are different ways to reduce the energy penalty of the power plant integrated with CO 2 capture, such as the development of new absorbents and the optimization of absorption flowsheets. Some researchers have focused on the system integration between the power plant and CO 2 capture process and considered it as an efficient strategy to improve the net efficiency [13][14][15][16][17][18][19][20][21][22][23]. For example, the temperature of the heat from multi-stages compression intercoolers is relatively high, which makes it feasible to recover and reuse this heat to heat up the water after the turbine condenser [13,14] or the rich amine solution [15], or even generate electricity through a supercritical CO 2 Rankine cycle [16,17]. The energy penalty of carbon capture is not only effected by the energy recovery from the CO 2 capture subsystem, but also the optimization of the steam extraction location [18]. Therefore a new design of a steam cycle using only three-stage LPTs is presented in order to match the temperature between the extracted steam and reboiler requirement while using the surplus heat of the reboiler condensate to increase the feedwater temperature [19]. In addition, the extracted steam flowrate from the steam turbine is generally enormous for CO 2 capture, which affects the operation safety and efficiency of the LPTs. On this account, Bonaventura et al. [20] proposed using medium temperature solar thermal power to supply the regeneration heat for a dry carbonate process, resulting in a fossil fuel energy penalty of 3-4% (including CO 2 compression). The studies mentioned above provide valuable ideas for the application of CO 2 capture processes, reveal the effect of system integration on the overall performance of decarbonization power plants and reduce the energy penalty of CO 2 capture to some extent. However, there are still some improvements which can be done in the system integration of power plants with CO 2 capture. For example, although the waste heat with relative high temperature in CO 2 capture subsystem is recovered efficiently by conventional measures, the wasted heat with relative low temperature and large flowrate cannot be utilized properly, such as absorbent cooling heat (40-65 • C). In addition, the flue gas temperature entering CO 2 capture subsystem in NGCC power plants is generally higher than that in coal-fired power plants, leading to an extra energy penalty of CO 2 capture in NGCC power plants.
Therefore, this study focuses on the relative low temperature heat released from a CO 2 capture subsystem and the sensible heat of flue gas which are always ignored by common heat integration measures. In terms of the above issue, a new integrated system of a NGCC power plant with CO 2 capture and heat supply is presented in this paper and three options are adopted to reduce the energy consumption of CO 2 capture. Specifically: (1) recirculating part of the reboiler condensate and mixing it with the extracted steam to fully utilize the surplus heat of the extracted steam; (2) employing a CO 2 Rankine cycle after the heat recovery steam generator (HRSG) to utilize the remarkable sensible heat in the flue gas; (3) integrating a radiant floor heat subsystem with the decarbonization NGCC power plant to reuse the relative low-temperature heat in the CO 2 capture subsystem. By these efforts, the energy penalty caused by CO 2 capture can be significantly reduced and the overall energy efficiency is greatly improved. A typical NGCC power plant integrated with post-combustion absorption CO 2 capture, as shown in Figure 1, includes three subsystems: a gas turbine (GT) subsystem, a steam turbine subsystem, and a CO 2 capture subsystem. In the GT subsystem, compressed air combusts with the natural gas (NG) at a combustion chamber (CC). The combustion flue gas then enters into a GT to generate electricity and the exhausted gas subsequently passes through a triple pressure HRSG to produce superheated steam for the steam turbine subsystem consisting of high pressure turbine (HPT), medium pressure turbine (MPT) and LPTs. After releasing heat, the flue gas enters a MEA-based CO 2 capture subsystem, where the absorber and stripper columns are interconnected by a cross-flow heat exchanger and the captured CO 2 stream from the top of stripper column is compressed by a multi-stage compressor. Enormous steam, accounting for more than 80% of the steam flow leaving MPT, is extracted from MPT/LPT crossover pipe in the steam turbine subsystem to meet the absorbent regeneration requirements, which implies a significant efficiency penalty for the whole power plant.

Concept Design of a NGCC
Energies 2018, 11, x FOR PEER REVIEW 3 of 13 the energy penalty caused by CO2 capture can be significantly reduced and the overall energy efficiency is greatly improved.

Typical Configuration of a NGCC Power Plant with MEA-Based CO2 Capture
A typical NGCC power plant integrated with post-combustion absorption CO2 capture, as shown in Figure 1, includes three subsystems: a gas turbine (GT) subsystem, a steam turbine subsystem, and a CO2 capture subsystem. In the GT subsystem, compressed air combusts with the natural gas (NG) at a combustion chamber (CC). The combustion flue gas then enters into a GT to generate electricity and the exhausted gas subsequently passes through a triple pressure HRSG to produce superheated steam for the steam turbine subsystem consisting of high pressure turbine (HPT), medium pressure turbine (MPT) and LPTs. After releasing heat, the flue gas enters a MEAbased CO2 capture subsystem, where the absorber and stripper columns are interconnected by a cross-flow heat exchanger and the captured CO2 stream from the top of stripper column is compressed by a multi-stage compressor. Enormous steam, accounting for more than 80% of the steam flow leaving MPT, is extracted from MPT/LPT crossover pipe in the steam turbine subsystem to meet the absorbent regeneration requirements, which implies a significant efficiency penalty for the whole power plant.

Energy Distribution Characteristics of the MEA-Based CO2 Capture Process
A typical chemical absorption approach consumes enormous energy for absorbent regeneration and releases amounts of low-grade heat as the exhaust, which is quantitatively equal to the energy input, while its thermal parameters is much lower than that of the input energy [24][25][26][27]. This exhausted

Energy Distribution Characteristics of the MEA-Based CO 2 Capture Process
A typical chemical absorption approach consumes enormous energy for absorbent regeneration and releases amounts of low-grade heat as the exhaust, which is quantitatively equal to the energy input, while its thermal parameters is much lower than that of the input energy [24][25][26][27]. This exhausted low-grade energy aggravates the energy penalty of CO 2 capture and reduces the economic performance of decarbonization NGCC power plant. As shown in Figure 2, there are three main energy loss sources in this CO 2 capture process, including extracted steam cooler, flue gas cooler and coolers in CO 2 capture subsystem. To be specific: • Waste heat from extracted steam The waste energy released by extracted steam cooler accounts for 15-20% of the total energy losses for CO 2 capture, which is attributed to the following two reasons: Firstly, in decarbonization power plants, the absorbent regeneration heat is generally supplied by the extracted steam, whose parameters are generally higher than the requirement of reboiler. Therefore, it must be throttled and cooled to some suitable temperature and pressure before entering into the reboiler, resulting in a large energy wastage. Secondly, the reboiler condensate should be cooled down before entering condenser to ensure the operation safety, which increases the energy penalty of CO 2 capture as well.

• Waste heat in flue gas
The cooling heat of the flue gas constitutes 15-20% of the total energy losses in a CO 2 capture process. It is because the flue gas in NGCC power plants doesn't need to pass through flue gas desulfurizer (FGD) to remove the SO x before entering CO 2 capture subsystem, which leads to the flue gas temperature before entering CO 2 capture subsystem being higher than that in coal-fired power plants with FGD. In order to enhance the exothermic reaction in absorber, the flue gas should be cooled down firstly, which means a great deal of low-temperature heat (40.0-120.0 • C) is dissipated.
• Waste heat in the MEA-based CO 2 capture subsystem The waste heat released by the CO 2 capture subsystem, which mainly comprises absorbent cooling heat (40.0-65.0 • C), CO 2 cooling heat (40.0-100.0 • C) and multi-stage compressor intercooler heat (40.0-165.0 • C), is approximately 55~70% of the total energy losses from CO 2 capture process and is difficult to utilize due to its relative low temperature and large flowrate. Especially, the absorbent cooling heat, accounting for 30-35% of the total energy losses, almost cannot be recovered through conventional integration measures because its temperature is slightly higher than the ambient temperature, resulting in an enormous energy waste. low-grade energy aggravates the energy penalty of CO2 capture and reduces the economic performance of decarbonization NGCC power plant. As shown in Figure 2, there are three main energy loss sources in this CO2 capture process, including extracted steam cooler, flue gas cooler and coolers in CO2 capture subsystem. To be specific: • Waste heat from extracted steam The waste energy released by extracted steam cooler accounts for 15-20% of the total energy losses for CO2 capture, which is attributed to the following two reasons: Firstly, in decarbonization power plants, the absorbent regeneration heat is generally supplied by the extracted steam, whose parameters are generally higher than the requirement of reboiler. Therefore, it must be throttled and cooled to some suitable temperature and pressure before entering into the reboiler, resulting in a large energy wastage. Secondly, the reboiler condensate should be cooled down before entering condenser to ensure the operation safety, which increases the energy penalty of CO2 capture as well.

•
Waste heat in flue gas The cooling heat of the flue gas constitutes 15-20% of the total energy losses in a CO2 capture process. It is because the flue gas in NGCC power plants doesn't need to pass through flue gas desulfurizer (FGD) to remove the SOx before entering CO2 capture subsystem, which leads to the flue gas temperature before entering CO2 capture subsystem being higher than that in coal-fired power plants with FGD. In order to enhance the exothermic reaction in absorber, the flue gas should be cooled down firstly, which means a great deal of low-temperature heat (40.0-120.0 °C) is dissipated.

•
Waste heat in the MEA-based CO2 capture subsystem The waste heat released by the CO2 capture subsystem, which mainly comprises absorbent cooling heat (40.0-65.0 °C), CO2 cooling heat (40.0-100.0 °C) and multi-stage compressor intercooler heat (40.0-165.0 °C), is approximately 55~70% of the total energy losses from CO2 capture process and is difficult to utilize due to its relative low temperature and large flowrate. Especially, the absorbent cooling heat, accounting for 30-35% of the total energy losses, almost cannot be recovered through conventional integration measures because its temperature is slightly higher than the ambient temperature, resulting in an enormous energy waste.
In summary, when the NGCC power plant integrates with MEA-based CO2 capture, huge lowtemperature energy is exhausted in the extracted steam cooler, flue gas cooler and coolers in CO2 capture subsystem, leading to remarkable energy penalty. Therefore, a new design concept of the decarbonization NGCC power plants focusing on the low-temperature energy recovery should be paid special attention.  In summary, when the NGCC power plant integrates with MEA-based CO 2 capture, huge lowtemperature energy is exhausted in the extracted steam cooler, flue gas cooler and coolers in CO 2 capture subsystem, leading to remarkable energy penalty. Therefore, a new design concept of the de-carbonization NGCC power plants focusing on the low-temperature energy recovery should be paid special attention.

Description of the Improved De-Carbonization NGCC Power Plant Integrated with Heat Supply
In view of the energy flow characteristics of CO 2 capture process, especially the three parts of energy loss as mentioned in Section 2.2, a new configuration of the decarbonization NGCC power plant is proposed as shown in Figure 3. In the new system, three targeted measures are adopted to reuse the surplus energy from extracted steam, flue gas and CO 2 capture subsystem, respectively.
• Extracted steam recirculation In the improved system, the steam extracted from the steam turbine for CO 2 capture is mixed with part of the reboiler condensate after going through the throttle to fully utilize the degree of superheating of the extracted steam and avoid absorbent degradation. Then the steam with suitable temperature and pressure subsequently releases the regeneration heat at the reboiler. The remainder of the condensate is recycled to the outlet of HRSG preheater, which further recovers the surplus heat of the extracted steam.
• CO 2 Rankine cycle Aiming to reuse the cooling heat of flue gas whose temperature is relative high (~120 • C), a CO 2 Rankine cycle is introduced because of its lower critical parameters, making it suitable to recover the low-temperature sensible heat in the flue gas, as shown in Figure 3. In the CO 2 Rankine cycle, the working fluid is heated firstly to 148.0 • C by flue gas and then expands to 7.00 MPa in the turbine. The exhausted working fluid is cooled down to 26.0 • C and sequentially pumped to 12.00 MPa to finish the circulation.
• Radiant floor heat subsystem A radiant floor heat subsystem is employed to utilize the low-grade and large flowrate heat in the CO 2 capture subsystem. Specifically, the heating water (40.0 • C) goes through heat exchanger 1 (HE1) to absorb the heat from the absorbent cooler and is then divided into two branches. One is heated by the captured CO 2 in HE2 and the other is heated by a multi-stage compressor intercooler in HE3. Afterwards, the heating water with proper temperature (70.0 • C) flows into the radiant floor system user releasing heat and then goes back to HE1 to finish the circulation.

Description of the Improved De-Carbonization NGCC Power Plant Integrated with Heat Supply
In view of the energy flow characteristics of CO2 capture process, especially the three parts of energy loss as mentioned in Section 2.2, a new configuration of the decarbonization NGCC power plant is proposed as shown in Figure 3. In the new system, three targeted measures are adopted to reuse the surplus energy from extracted steam, flue gas and CO2 capture subsystem, respectively.

•
Extracted steam recirculation In the improved system, the steam extracted from the steam turbine for CO2 capture is mixed with part of the reboiler condensate after going through the throttle to fully utilize the degree of superheating of the extracted steam and avoid absorbent degradation. Then the steam with suitable temperature and pressure subsequently releases the regeneration heat at the reboiler. The remainder of the condensate is recycled to the outlet of HRSG preheater, which further recovers the surplus heat of the extracted steam. •

CO2 Rankine cycle
Aiming to reuse the cooling heat of flue gas whose temperature is relative high (~120 °C), a CO2 Rankine cycle is introduced because of its lower critical parameters, making it suitable to recover the low-temperature sensible heat in the flue gas, as shown in Figure 3. In the CO2 Rankine cycle, the working fluid is heated firstly to 148.0 °C by flue gas and then expands to 7.00 MPa in the turbine. The exhausted working fluid is cooled down to 26.0 °C and sequentially pumped to 12.00 MPa to finish the circulation. •

Radiant floor heat subsystem
A radiant floor heat subsystem is employed to utilize the low-grade and large flowrate heat in the CO2 capture subsystem. Specifically, the heating water (40.0 °C) goes through heat exchanger 1 (HE1) to absorb the heat from the absorbent cooler and is then divided into two branches. One is heated by the captured CO2 in HE2 and the other is heated by a multi-stage compressor intercooler in HE3. Afterwards, the heating water with proper temperature (70.0 °C) flows into the radiant floor system user releasing heat and then goes back to HE1 to finish the circulation.

Main Assumptions and Case Description
To reveal the impacts of the three integration measures on the thermal performance of de-carbonization NGCC power plant, the following three cases are thoroughly simulated and thermodynamically analyzed in this section: Base case: NGCC power plant without CO 2 capture Case 1: De-carbonization NGCC power plant without system integration Case 2: De-carbonization NGCC power plant with system integration A 555 MW NGCC power plant is selected as the Base case in this study [28]. The pressure ratio of the gas turbine cycle and the flue gas temperature at GT inlet is set to 18.5 and 1371.0 • C, respectively. While the feed NG is formed primarily of methane (93.1 mol %), it also includes ethane (3.2 mol %), propane (0.7 mol %), n-butane (0.4 mol %), CO 2 (1.0 mol %) and nitrogen (1.6 mol %). Other parameters needed for the simulation are listed in Table 1.

Simulation Basis
In this study, three cases are simulated within UniSim Design R450, which can be used to determine energy-efficient solutions (including the identification of the most appropriate operating conditions) [9,29]. The relevant thermodynamic data for the CO 2 capture subsystem was calculated by using the Amine package/Li-Mather method. For the gas turbine subsystem and steam turbine subsystem, the Peng-Robinson package and ASME-Steam package were used respectively. Particularly, the block models "Conversion Reactor", "Expander", "Heat Exchanger", "Absorber Column" and "Distillation Column" are selected for establishing the combustion chamber, the steam/gas turbines, the HRSG, the absorber and the stripper. Meanwhile, some additional block model are adopted to finish the simulation, such as "Compressor," "Heat Exchanger", "Cooler", "Pump" and so on.

Evaluation Criteria
For the conventional power generation unit, the net efficiency is commonly considered to evaluate the thermal performance of the whole power plants, which is the ratio of the power output to the total energy input and can be calculated as follows: Here, η e is the net efficiency, P gen is the net power output (MW), M is the total amount of fossil fuel input to the boiler per unit time (kmol/h), Q L is the LHV of fuel (kJ/kmol), 3600 can be considered as 3600 s/h. For the new system, it not only generates electric power, but also supplies heat for the radiant floor user. Therefore, the overall energy efficiency (η f ) is also adopted for comprehensive thermodynamic performance evaluation, which can be defined as follows [29][30][31][32]: where, E q is the valid heat supplied to the radiant floor heat subsystem (MW).

Thermodynamic Performance
Based on the simulation and calculation above, Table 2 shows the main thermodynamic analysis results of these three cases. The net power output of the NGCC power plant is reduced by 115.40 MW due to the implementation of CO 2 capture, which is about 21% of the net power output in NGCC power plant without CO 2 capture. Particularly, 130.56 kg/s of extracted steam is sent to the reboiler, which is~90% of the total steam flowrate at MPT outlet, meaning a 91.43 MW decrease of the LPT power output; secondly, 24.98 MW extra electricity is consumed by auxiliary facilities in CO 2 capture subsystem, including the boost fan, circulating pumps and multi-stage compressor. Due to the above two aspects, the efficiency penalty of CO 2 capture reaches to 11.6 percentage points in the NGCC power plant. In the proposed system, the net power output increases from 445.87 to 465.35 MW by adopting three measures, resulting in 2.0 percentage points reduction in the efficiency penalty of CO 2 capture compared with Case 1.
Specifically, the degree of superheating of the extracted steam is fully utilized through extracted steam recirculation, which reduces the extracted steam flowrate by 18.61 kg/s and simultaneously increases the LPT power output by 10.44 MW. In addition, the rest reboiler condensate is injected into the HRSG preheater outlet to further utilize the surplus heat of extracted steam, increasing the flue gas temperature at HRSG outlet from 122.3 to 161.1 • C. All these waste heat in the flue gas (161.1-40.0 • C) can be recovered to generate electricity in the CO 2 Rankine cycle, which makes the net power output of NGCC increases by 9.09 MW. Therefore, the net power plant efficiency improves from 44.8% to 46.8% without consideration of heat supply.
Moreover, in the heating season, most of the low-grade waste heat in CO 2 capture subsystem can be utilized by the radiant floor heat subsystem. As shown in Figure 3, the energy for the radiant floor heat subsystem includes three parts. Specifically, the heating water flows through HE1 and absorbs 212.58 MW heat from the absorbent cooler, then it is divided into two branches. 76% is delivered to HE2 and absorbs 23.65MW heat from the captured CO 2 stream with the temperature range of 94.7 to 71.0 • C and the rest enters HE3 to recover 11.36 MW heat from the multi-stage compressor intercooler. In this way, 247.59 MW heat can be supplied to the radiant floor users, which could increase the overall energy efficiency from 44.8% to 71.6%.

Energy Analysis for the CO 2 Capture Process
The energy flow diagrams of CO 2 capture process in Case 1 and Case 2 are shown in Figure 4 to reveal the essence of energy saving in the integrated system. extra electricity is consumed by auxiliary facilities in CO2 capture subsystem, including the boost fan, circulating pumps and multi-stage compressor. Due to the above two aspects, the efficiency penalty of CO2 capture reaches to 11.6 percentage points in the NGCC power plant. In the proposed system, the net power output increases from 445.87 to 465.35 MW by adopting three measures, resulting in 2.0 percentage points reduction in the efficiency penalty of CO2 capture compared with Case 1.
Specifically, the degree of superheating of the extracted steam is fully utilized through extracted steam recirculation, which reduces the extracted steam flowrate by 18.61 kg/s and simultaneously increases the LPT power output by 10.44 MW. In addition, the rest reboiler condensate is injected into the HRSG preheater outlet to further utilize the surplus heat of extracted steam, increasing the flue gas temperature at HRSG outlet from 122.3 to 161.1 °C. All these waste heat in the flue gas (161.1-40.0 °C) can be recovered to generate electricity in the CO2 Rankine cycle, which makes the net power output of NGCC increases by 9.09 MW. Therefore, the net power plant efficiency improves from 44.8% to 46.8% without consideration of heat supply.
Moreover, in the heating season, most of the low-grade waste heat in CO2 capture subsystem can be utilized by the radiant floor heat subsystem. As shown in Figure 3, the energy for the radiant floor heat subsystem includes three parts. Specifically, the heating water flows through HE1 and absorbs 212.58 MW heat from the absorbent cooler, then it is divided into two branches. 76% is delivered to HE2 and absorbs 23.65MW heat from the captured CO2 stream with the temperature range of 94.7 to 71.0 °C and the rest enters HE3 to recover 11.36 MW heat from the multi-stage compressor intercooler. In this way, 247.59 MW heat can be supplied to the radiant floor users, which could increase the overall energy efficiency from 44.8% to 71.6%.

Energy Analysis for the CO2 Capture Process
The energy flow diagrams of CO2 capture process in Case 1 and Case 2 are shown in Figure 4 to reveal the essence of energy saving in the integrated system.   The energy input of Case 1 reaches 519.44 MW, including thermal energy brought by extracted steam (387.00 MW), sensible heat brought by flue gas (107.46 MW) and the electric power consumption (24.98 MW). These energy drives the process of CO 2 capture and absorbent regeneration, then degrades into low-temperature waste heat, which is quantitatively equal of the energy input and generally taken away by cooling water in the extracted steam cooler, flue gas cooler and the coolers in CO 2 capture subsystem, leading to a huge energy penalty, remarkable cooling water requirement and potential thermal pollution.
Through system integration, a large amount of waste energy, originally taken away by cooling water in Case 1, can be efficiently recovered and effectively utilized. Firstly, 42.81 MW waste heat of extracted steam is recovered by the extracted steam recirculation, decreasing the extracted steam energy input from 387.00 to 330.19 MW with constant regeneration heat (292.23 MW). Secondly, the waste energy in flue gas cooler (135.75 MW) is utilized to generate electric power by the CO 2 Rankine cycle. It is noted that the exhausted heat from flue gas cooler in the Case 2 is greater than that in Case 1, due to the increase of the inlet temperature of the flue gas cooler. This is because part of the condensate from reboiler is injected to the outlet of HRSG preheater, which means less condensed water is available to recover the heat from the flue gas in preheater of HRSG, leading to an increase of flue gas temperature at HRSG outlet (i.e., the temperature at the inlet of the flue gas cooler). Thirdly, 247.59 MW waste heat from CO 2 capture subsystem is recovered by radiant floor heat subsystem, resulting in~80% reduction in the total cooling heat losses of CO 2 capture subsystem as compared with the Case 1. By utilizing the above three measures, the total energy losses of the CO 2 capture process decrease from 519.44 to 79.29 MW with a reduced energy input by 56.82 MW and the overall energy efficiency of the new system can be improved significantly to 71.6%.

Techno-Economic Criteria
Techno-economic analysis is conducted to reveal the comprehensive performance of these three systems. The cost of electricity (COE) and the cost of CO 2 avoided (COA) are selected as the evaluation criteria. The COE ($/MWh) is calculated as follows: where β is the capital recovery factor; C 0 is the total overnight cost (TOC) ($); C f is the fixed Operation & Maintenance (OM) cost ($); C v is the variable OM cost ($); H is the annual heat supply capacity (GJ); C H is the heat price ($/GJ); P is the annual net power output (kW); CF is the capacity factor. The COA ($/tCO 2 ) is defined by Kreutz et al. [33] as follows: where COE cap is the COE of the system with CO 2 capture ($/kWh); COE ref is the COE of the system without CO 2 capture ($/kWh); Er cap is the CO 2 emission rate of the system with CO 2 capture (kg/kWh); Er ref is the CO 2 emission rate of the system without CO 2 capture (kg/kWh).
To get the TOC of the new system, every added equipment cost should be estimated by utilizing the actual cost and the proper scaling factors, which can be calculated as follows [34]: where Q is the scaling parameter; Q B is the reference parameter; C E is the estimated equipment cost with capacity Q; C B is the known equipment cost with capacity Q B ; M is the scale factor, which is constant specific to unit type.

Techno-Economic Assumptions
In this study, some economic assumptions are adopted. Specifically, (1) the NG price is 6.89 $/GJ LHV; (2) the OM costs are fixed at 4% of the TOC; (3) CF is assumed to be 85%; (4) the specific overnight cost of NGCC power plant is 718 $/kW [28]; (5) the CO 2 capture subsystem cost of a 555 MW NGCC power plant with 90% CO 2 capture efficiency is 311 M$ [28]; (6) the heat price is 5.32 $/GJ [35]; (7) the capital recovery factor is 0.105 [28]; and (8) the equipment lifespan is 30 years. Other parameters needed for estimating the added equipment costs of the integration measures are listed in Table 3.

Techno-Economic Results
The techno-economic performances are shown in Figure 5, including the changes of the TOC, net power output, COE, and COA of these three cases. As the results show, the TOC of the NGCC power plant increases by~80% due to the implementation of carbon capture and the net power output of Case 1 is reduced by 115.40 MW because of the extracted steam for absorbent regeneration and the auxiliary power consumption in CO 2 capture subsystem. These two effects result in a 28.54 $/MWh increase of the COE, which is 49% higher than that of the NGCC power plant without CO 2 capture.  Table 3.

Techno-Economic Results
The techno-economic performances are shown in Figure 5, including the changes of the TOC, net power output, COE, and COA of these three cases. As the results show, the TOC of the NGCC power plant increases by ~80% due to the implementation of carbon capture and the net power output of Case 1 is reduced by 115.40 MW because of the extracted steam for absorbent regeneration and the auxiliary power consumption in CO2 capture subsystem. These two effects result in a 28.54 $/MWh increase of the COE, which is 49% higher than that of the NGCC power plant without CO2 capture. In the improved system, three measures are adopted to alleviate the energy consumption for CO2 capture, causing $19.00 M increase of the TOC. The power output of the new integration system is19.48 MW higher than the power output of Case 1. Moreover, in the heating period, 247.59 MW heat In the improved system, three measures are adopted to alleviate the energy consumption for CO 2 capture, causing $19.00 M increase of the TOC. The power output of the new integration system is19.48 MW higher than the power output of Case 1. Moreover, in the heating period, 247.59 MW heat is recovered for the radiant floor heat subsystem, which brings extra benefit for NGCC power plant. Therefore, the COE and COA of the new system decreases to 80.11 $/MWh and 70.39 $/t CO 2 , which are 7.2% and 22.3% lower than that of Case 1 respectively.

Conclusions
With the continuing concern about carbon emissions around the world, CO 2 capture technologies have attracted more and more attention. Not only are large amounts of energy consumed for absorbent regeneration, but the process also causes some operation and safety issues, which hinder its large scale application. Therefore, it is important to pay attention to the system integration between the energy utilization system and CO 2 capture process aiming to recover the low-grade energy and reduce the energy penalty.
In this study, an improved system of NGCC power plant with CO 2 capture and heat supply was developed, which recovers the sensible heat of flue gas to generate electricity through a CO 2 Rankine cycle and reutilizes the relative low heat released from the CO 2 capture subsystem for a radiant floor heating subsystem. Through thermodynamic and techno-economic analysis, the integrated measures cause only 2.6% enhancement of the total investment, but the power output is 4.4% higher than that for the decarbonization NGCC power plant without integration and 247.59 MW heat can be supplied to the radiant floor users as well. Therefore, the COE and COA of the new system are much lower than that of the decarbonization NGCC power plant without integration.
In summary, the integration measures proposed in this study are effective in enhancing the net efficiency of power plants with a reasonable increment of investment, which provides an efficient and economic option for CO 2 removal from NGCC power plants.