Exergetic Sustainability Analysis of a Naphtha-Based Combined Cycle Power Plant (CCPP)

In today’s scenario, ensuring sustainability of energy system particularly in the field of power generation is a major concern, and in this regard, exergy analysis is widely accepted tool. For this purpose, first, a comprehensive performance is carried out between two gas turbine plants, i.e., GT2 (case I) and GT1 (case II) using various exergy performance parameters. Further, a comprehensive performance evaluation is carried out between three cases; GT2 (case I), GT1 (case II) and CCPP (case III) using different sustainability indicators such as exergy efficiency, waste exergy ratio, exergy destruction factor and recoverable exergy ratio. Results demonstrate that exergetic sustainability index of combined cycle power plant (CCPP) is 0.45 when compared with GT2 (0.29) and GT1 (0.28). The increased sustainability index is because of the incorporation of a bottoming cycle, which ultimately decreases waste exergy ratio and leads to an increase in exergy efficiency and sustainability index of CCPP.

In present scenario enhancing the energy efficiency of energy systems is the main target, and in this regard, integrated energy systems such as combined cycle power plant (CCPP) have obtained a greater attention (Gowrishankar, Angelides and Druckenmiller, 2013).It has been widely exploited in industry to reduce energy consumption and emission reduction (Ahmadi, Toghraie and Akbari, 2019).
However, it is pretty much clear that to have a clear understanding about the relation between increasing energy efficiency of energy systems and its environmental impact is an interdisciplinary task, and various disciplines present specific methodologies and approaches to make this aspect known to everyone.However, to have a concrete understanding it is fundamental to find a unified approach.To have a unified approach, different indicators are introduced which highlights the economic needs of analysis, engineering optimization, and is also able to consider the social implications (Lucia and Grisolia, 2019).These indicators allow one to evaluate the economic impact, social impact and environmental impact of the energy system.Seeing the perspective of present work, many indicators are introduced based on the thermodynamic approach, to evaluate the process inefficiency, and its consequences on the environment and economy (Sikdar, Sengupta and Mukherjee, 2017).
Energy analysis is the most common indicator carried out to evaluate the performance of energy system (Kaygusuz, Bilgen and Ilgen, 2009).However, the paucity of available resources puts a pressing demand for a rigorous analysis where not only the quantity of energy but also issues like degradation of its quality becomes important (Ersayin and Ozgener, 2015a).Hence, there is an increasing need for considering both energy and exergy as the basis of analysis for the performance assessment.Further, it also gives guidance in achieving sustainability (Bilgen and Sarikaya, 2015).Using the relation between exergy and sustainability many researchers have used exergy analysis as a potential tool in assessing the quality of energy systems.In this regard (Rosen and Dincer, 1997) illustrated that an increase in the exergy efficiency of a process is accompanied by a decreasing environmental impact and increasing sustainability of the process.Further, (Nielson, 2016) introduced some indicators which could connect exergy with the environment like exegetic sustainability analysis, depletion number etc. (Turan et al., 2014) applied these exergetic based indicators such as exergy efficiency, improvement potential, fuel depletion ratio for a turbofan engine and concluded that the combustion chamber requires highest improvement potential.Similarly, (Aydin et al., 2015) performed sustainability analysis for a medium-range commercial aircraft engine with kerosene as fuel, and concluded that the combustion chamber requires highest improvement potential and recoverable exergy rate is zero because all emission from aircraft is left open to atmosphere.(Aydin, 2013) performed the sustainability analysis for gas turbine power plant with steam turbine cycle as bottoming cycle and concluded that it is possible to recover some exergy, as exhaust from the gas turbine is used as fuel in HRSG.The incorporation of steam cycle increases the sustainability index for case B (0.978) when compared with case A (0.651).
Incorporation of steam cycle results in improvement of efficiency and decreases waste exergy ratio.Similarly, (Balli, Ekici and Karakoc, 2021) carried out sustainability analysis of M-CHP fueled by natural gas, and concluded that combustion chamber and heat exchanger have unfavorable exergetic performance parameters.
This sustainability analysis is not only limited to the power generation, rather it has been carried in other sectors also.(Zisopoulos et al., 2017) review the exergetic indicators that are appropriate for food industry such exergy efficiency, exergy losses, improvement potential, exergy destruction ratio.Further, (Midilli and Dincer, 2009) developed some exergetic based indicators for the hydrogen fuel cell.In another paper, (Midilli, Adnan , Kucuk, Haydar, 2014) developed an exergetic sustainability parameter for the aquifer.However, to the best knowledge of the author exergy based sustainability analyses of a naphtha based CCPP has not been carried out.Apart from this, the present CCPP is also configured as cogeneration or CHP mode, where it supplies electricity and heat to entire petrochemical complex (HPL, 2022).The objective of the present work is to address this shortcoming.The present work may serve as a guide for similar systems with detailed performance and environmental assessment by prioritizing increase in sustainability index and decrease in waste exergy ratio.
In the present paper, first a comprehensive comparison between two gas turbine power plants GT2 (case I) and GT1 (case II) is carried out using different performance indicators as they have same exergy efficiency and same power output under ISO condition.Figure 1 and Figure 2 present the schematic representation of GT2 and GT1.Further, the exhaust from GT2 and GT1 is routed to bottoming cycle (BPST and CST) through HRSG2 and HRSG1, making it CCPP (case III). Figure 3 represents schematic representation of CCPP.To understand the effect of incorporating bottoming cycle on the environment, various sustainability indicators are applied, and compared for three cases {GT2 (case I), GT1 (case II) and CCPP (case III)}.

System description and assumption
The installed capacity of major equipment is provided in Table 1.Fresh air (109.2kg/sec) from the outside environment (1) enters the compressor in GT2 and is compressed non-isentropically (2).After compression, the fuel (37 and 38) is burnt inside the combustion chamber along with air, and further it is routed to turbine (3), where it gets expanded non-isentropically (4) while producing power.Same process is followed in the GT1 (Figure 2) cycle.The combustion gas leaving GT1 and GT2 has 862 K and 846 K respectively.The combustion gases leaving the GT2 (4) and GT1 (8) unit is routed to the respective HRSG2 and HRSG1.The steam generated collectively in HRSG2, HRSG1, AB2 and AB1 is supplied to back pressure steam turbine (BPST) and condensing steam turbine (CST) through headers thus making it a CCPP (Figure 3) of 116 MW.The exhaust gas after producing steam in HRSG2 and HRSG1 leaves through stack having 463 K as temperature.The feedstock naphtha is itself used as the fuel (Lundy, 2000).This CCPP has four steam headers and four steam generating units.Details of four steam headers are presented in Table 2.This After the expansion in the BPST, steam is diverted to the process plant through three headers: -HP (45), MP (46) and LP (47).Steam left in SHP is allowed to expand in the condensing steam turbine (20) and pumped via the condensate pump (22) to a deaerator for preheating, along with the makeup water (23).For preheating, a tapping is made in the LP (24) header.
After the deaerator, the feed water is pumped from 2.24 bar to 150.9 bar through the SHP F/W pump (26) to the economizer section of HRSG2 (27), HRSG1 (28), AB1(29) and AB2 (30), and through the MP F/W pump (32) to HSRG2 (34) and HSRG1 (33).The steam to run the boiler feed pump and turbine-driven fan draft is provided by the HP header, which after providing necessary power goes to the LP header.Before carrying out exergy based sustainability analysis, some assumptions have been taken up.The list of assumptions for calculations are as follows: - The system operates under steady-state conditions, and inert condition assumed is

Airflow
The flow rate of air in the compressor is taken as 109.2kg/sec at ambient temperature of 288 K and ambient pressure of 101.325 kPa.The mass flow rate of naphtha is 2.57 kg/sec in case of GT1, whereas 1.47 kg/sec (residual fuel gas) and 0.8 kg/sec (naphtha) in case of GT2.All the values used for the analysis are presented in Table 3.

Combustion and emission
For 109.2 kg/sec, (2.57kg/sec of naphtha) is taken in case of GT1 and (1.47 kg/sec of residual fuel gas and 0.8 kg/sec of naphtha) is taken in case of GT2.The combustion expression for GT2 and GT1 is expressed in terms of mole fraction of fuel is as follows: Excess air in percentage is expressed as, ( ) ( ) (5) Eq. ( 5) presents the variation of specific heat of air as a function of temperature (Ersayin and Ozgener, 2015b).Variation of specific heat of flue gas considering the composition of the combustion products with temperature for GT2 is given by Eq. (5a) Figure 3: Schematic representation of CCPP (case III).

Exergy analysis
The primary step for carrying out exergy based sustainability analysis is to perform the exergy analysis of the entire energy system by employing second law of thermodynamics.Data (mass flow rate, pressure and temperature) used in this analysis are presented in Appendix A. Using the values, the exergy flow of each and every component is calculated by using Eq. ( 6) -( 8) (Ersayin and Ozgener, 2015a).The specific physical exergy of streams can be calculated as For calculating the chemical exergy of liquid fuel (naphtha and carbon black fluid stock), Eq.
2 is applied (Kaushik and Singh, 2014).The value of z is 1.07 according to (Abusoglu and Kanoglu, 2008).(7) In the system analyzed chemical exergies of the fuel and combustion products have important roles.To calculate the chemical exergy of gaseous fuel Eq. ( 3) is used ('Process Waste Heat Recovery Boiler-KCIL', no date) x,chemical mixture , , 0 1 1 RT ln(x ) where i x is the mole fraction of each component and , , x chemical i e is the specific chemical exergy of each component.The value of specific exergy of each component is taken from (Szargut, 2005).The same procedure is followed in the case of flue gas where the molar composition of the combustion gases is known by chemical balance presented by Eq. (2) and Eq. ( 4), and respective values are presented in Table 4. Since rfg is introduced in the combustion chamber at 92 °C, physical exergy is calculated by multiplying mass fraction with Eq. ( 6).In this regard the exergy values of all reference points related to Figure 3 is presented in Appendix A (Table A.1). Further, Figure 3 consists of reference points of GT2 as well as GT1.To understand the environmental effect of energy system various sustainability indicators are applied for three cases {GT2 (case I), GT1 (case II) and CCPP (case III)}.GT2 is powered by a naphtha-RFG mixture, GT1 is powered by naphtha alone and CCPP is powered by naphtha, RFG and CBFS.Before carrying out exergy based sustainability analysis, two gas turbines with same power output is compared using exergetic performance parameters.Some of them are mentioned in Section 4.

Exergy efficiency
Exergy efficiency ( ex h ) can be defined as the ratio of total exergy output to total exergy input (Aydin, 2013), and it is presented by Eq. ( 9).
The exergy destruction ( i X ) presents the exergy destruction of a system unit as a percentage of total exergy destructed of an energy system (Bin, 2009).It is calculated by dividing component destruction exergy to the total destructed exergy of the system, and it is represented by Eq. (10).
, , Eq. ( 11) calculates the fuel depletion ratio ( i d ) by dividing the component's exergy consumption by the engine's fuel exergy rate input (Turan et al., 2014).
Improvement potential (Aydin, 2013) is achieved when ( ) Ex Ex -&& is minimized.The improvement potential, IP & is given by Eq. ( 12) (1 )( ) All these indicators stated above i.e. exergy efficiency, relative exergy ratio, fuel depletion ratio and improvement potential are used to compare the performance of two gas turbines i.e.GT2 and GT1.However, both gas turbines reject heat which could be used as a fuel in bottoming cycle.In the following section i.e. section 3.4, we will see how the application of bottoming cycle increase the sustainability index and decrease waste exergy ratio.

Exergetic sustainability indicators
Sustainability refers to a consistent supply of energy resources that is both affordable and has low environmental impact.In this regard exergy analysis can also be used to examine the energy system environmental impact through various sustainability indicators.Some of them are discussed below: -

Exergy efficiency
The ratio of useable power output, which is electrical power output for cases A and B, to total exergy input is used to calculate a power plant's efficiency.However, for case C, useful exergy is the summation of electrical power and process heat produced which is generated by the entire power plant.In this regard, Eq. ( 13) presents the exergy efficiency of GT2 plant, Eq. ( 13a) presents the exergy efficiency of GT1 plant and Eq.(13b) presents the exergy efficiency of CCPP.In Eq. ( 13

Ex
Ex Ex Ex Ex h

Waste exergy ratio
The power production in any energy systems is accompanied by two things i.e. exergy destroyed in the engine components, and exergy lost through hot exhaust fumes (Aydin, 2013).
As a result, waste exergy can be determined by Eq. ( 14)

Recoverable exergy rate
The "recoverable exergy rate" refers to the amount of energy that can be recovered in the system.Because destructed exergy values are dependent on the design and operating parameters, they cannot be recovered.However, in case I, case II, and case III, some of the exergy that is lost to the environment can be recovered by using it for heating, but this needs more expenditure.With the system in place, it is expected that 90% of exergy can be transformed to thermal energy.(Aydin, 2013).
Recoverable exergy ratio = Recoverable exergy/Total exergy inlet For case I and case II, case loss,out , 0.9

Exergy destruction factor
It is a significant metric that signals that the engine's beneficial effect on exergy-based sustainability is decreasing.The ratio of exergy destruction to total exergy intake is used to determine the exergy destruction factor (Aydin, 2013).
Exergy destruction factor = Exergy destruction/Total exergy input Ex & ) represents chemical and physical exergy of RFG.

Environmental effect factor
Another essential component to consider is the environmental impact factor, which is calculated using the waste exergy ratio divided by the exergy efficiency (Aydin, 2013).It shows whether or not it harms the environment by producing unusable waste exergy and destroying exergy (Aydin, 2013).
Environmental effect factor = waste exergy ratio/ Exergy efficiency

Exergetic sustainability index
The range of this index is between 0 and ¥ (Aydin, 2013) .The higher efficiency of any system means a low waste exergy ratio and low environmental effect as a result of a higher exergetic sustainability index.For determining, exergetic sustainability index Eq.( 18) is used.
Exergetic sustainability index = 1/Environmental effect factor Further, the sustainability indexes for all three cases i.e. case I, case II and case III are presented by Eq. (18a), Eq. (18b) and Eq. ( 18c), where case I esi q presents sustainability index for GT2 plant, case II esi q presents sustainability index for GT1 plant and case III esi q represents sustainability index of CCPP.

Results and discussion
In the present section, first the comparison between two gas turbines i.e.GT2 and GT1 is presented using different exergetic performance parameters.Further, the performance assessment of all three cases i.e. case I, case II and case III has been examined through sustainability indicators also.

Exergetic performance evaluation
The performance evaluation indicators of two cases i.e. case I and case II are presented in this section: case I (GT2), case II (GT1).To evaluate performance evaluation indicators such as exergy efficiency, relative exergy destruction ratio, fuel depletion ratio, and improvement potential rate are used.The corresponding equations are presented from Eq. ( 9) -( 12).From Table 5, it could be observed that both gas turbines (GT1 and GT2) have almost the same exergy efficiency about 20.2% and 20.7% respectively, hence it is quite difficult to understand which gas turbine requires more potential to improve. Figure 4 presents the exergy efficiencies of the various components of GT1 and GT2.It is observed form the Figure 4 that maximum exergy destruction occurs in the combustion chamber (54.79 MW in CC1 and 72.51 MW in CC2).The result of exergy destruction ratio is presented in Figure 5, and it is worth mentioning that tendency in Figure 5 is the same as in Figure 4. Calculations exhibit that the exergy destruction ratio for the combustion chamber appears to be highest (0.79% for CC1 and 0.82% for CC2) among all components in the gas turbine, particularly in case CC2 it is somewhat at a higher side when compared with CC1.It is because of the temperature difference between the two fuels when mixed in CC2: residual fuel gas (92 °C) and naphtha (34 °C).
Figure 4: Exergy destruction of various components in GT1 and GT2.
Fuel depletion ratio which is expressed as a percentage is presented in Figure 6 for GT1 and GT2, and it can be concluded that air compressor (AC1 and AC2) and (Turbine1 and Turbine2) has minimum fuel consumption whereas combustion chamber of both gas turbines have maximum fuel depletion ratio (0.45% and 0.50%) respectively.This is in fact due to high exergy destruction in both combustion chambers (CC2 and CC1), which causes more fuel to deplete.In other words, fuel depletion ratio signifies which component requires high fuel consumption.
Figure 5: Relative exergy ratio of various components of GT1 and GT2.Further, some sort of preheating arrangement may be thought of to minimize the temperature difference between naphtha (34 °C) and residual fuel gas (92 °C).

Environmental sustainability indicators
Exergy based sustainability indicators for case I, case II and case III are derived from Eq. ( 13) -(18c).The results based on exergy based sustainability analysis are presented in Table 4.2, and further they are elaborated in Figure 8. Seeing Table 6, one can notice that the addition of steam turbines improves the exergy efficiency from 20% to 30%.As noted waste exergy decreases in case of III as compared to case I and case II.We know that waste exergy consists of two components i.e. exergy destruction and exergy loss through stacks.Of course, due to more number of components involved in CCPP, exergy destruction is high when compared with GT1 and GT2, however due to better utilization of exhaust gas from gas turbine in HRSG, the stack loss gets reduced, and hence loss exergy in terms of flue gas become less.Some parts of waste heat can be recovered for heating purposes or cooling purposes provided the addition of extra equipment is economically viable.It can be further understood from the fact that higher is the efficiency of a process less amount of heat is being lost to the environment.
Recoverable exergy rates associated with three cases namely case I, case II and case III are assessed around 0.20, 0.25 and 0.02.The recoverable exergy ratio in case III is less as compared to case I and case II, and it is because of HRSG, as it produces steam for power generation through BPST and CST.The heat is extracted from the flue gas which could be left open to atmosphere, and this heat is utilized to produce extra power from BPST and CST.recovered.(Aydin et al., 2015).Environmental effect factor as shown in Table 6 for case I, case II and case III comes out to be 4.02, 4.18 and 2.26.As implementing of steam turbine has causes less waste exergy ratio, which also led to less recoverable exergy hence environmental effect factor is also less is case III.Further evaluating the last parameter i.e. exergetic sustainability index for three cases (case I, case II and case III), case III has least exergetic sustainability index.

Conclusions
To determine the sustainability and environmental impact, all three cases were assessed through various exergy-based sustainability indicators such as exergetic efficiency, waste exergy ratio, exergy destruction ratio, environmental effect factor, and exergetic sustainability index.Some salient observations from this study are outlined below: - Among GT2 (case I) and GT1 (case II), GT2 requires high improvement potential.
Further GT2 has more fuel depletion ratio.Some of the techniques like increasing turbine inlet temperature could be incorporated however metallurgical aspect has to be considered. When a steam turbine cycle is combined with a gas turbine power plant, the efficiency increases, resulting in a higher exergetic sustainability index.The overall exergy efficiency is 30% as a result of the addition of two turbine back pressure steam turbines (16 MW) and a condensing steam turbine (33 MW).
 Waste exergy from power plants decreases as a result of increasing power output through steam turbine incorporation, resulting in improved exergetic sustainability of CCPP.
 Environmental factor improves in case III with steam cycle implementation.Further, if the comparison could be done between two gas turbines GT1 and GT2, GT2 has high environmental effect factor which could be due to high amount of exergy destruction.
Hence, we could say second law analysis plays an important role in the evaluation of sustainability analysis of energy systems.
 It could be inferred that an increase in efficiency improves the exergetic sustainability parameters.However, any increase in waste exergy ratio and exergy destruction factor results in increasing of environmental effect factor and hence, decreases the sustainability.These parameters are expected to quantify how GT based power plant become more environmentally benign and sustainable.
It could be understood from the above study that second law analysis plays an important role in any evaluation of the sustainability of CCPP.However, it could be also mentioned that to have a more comprehensive conclusion, exergy-based environmental analysis must also be considered.Apart from this there could be various other indicators or indices which could use to access the characteristics of the CCPP such as the Resource Indicator, the Environmental Indicator, the Social Indicators, Economic Indicator (Rösch et al., 2017).

Statement and Declarations
 Funding (Not applicable)


Combustion is assumed to be complete.The fuel naphtha has the following composition: C (0.8392), H2 (0.1583), S (0.001) with a lower heating value of 44079 kJ/kg.

Figure 2 :
Figure 2: Schematic diagram of GT1 with naphtha as fuel (case II).
of specific heat of flue gas with temperature for GT1.

Figure 6 :
Figure 6: Fuel depletion ratio for various components of GT1 and GT2.

Figure 7 :
Figure 7: Improvement potential of various components of GT1 and GT2.

Figure 8 :
Figure 8: Exergetic sustainability indicators of case I, case II and case III.

Table 2 :
Details of different process streams generated.

Table 3
Important data for GT2 and GT1

Table 4
Flue gas composition in GT1 and GT2 assuming complete combustion ),

Table 5 :
Performance evaluation of GT1 and GT2 and their component Comp F Ex & P Ex & d Ex &

Table 6 :
Exergetic sustainability parameters for case I, case II and case III.
Conflicts of interest/Competing interests (Not applicable) Availability of data and material (Not applicable) The participant has consented to the submission of the case report to the journal.Table A.1 Thermophysical properties of streams in CCPP Point Stream Temp Pressure Flow rate Enthalpy Entropy Specific 