Energy and Exergy Analysis on a Blast Furnace Gas-Driven Cascade Power Cycle

Abstract

Blast furnace gas is the major combustible by-product produced in the steel industry, where iron ore is reduced by coke into iron. Direct combustion of blast furnace gas after simple treatment for power generation is a common utilization method nowadays. However, this method suffers from low efficiency and high carbon intensity. The use of gas-steam combined cycle is an excellent method to improve the efficiency of blast furnace gas for power generation. However, there is a problem of insufficient utilization of low product heat, and the addition of CCS system can further reduce the power efficiency. To solve these issues, a new blast furnace gas power generation system with a Brayton cycle with supercritical CO₂ and a Rankine cycle with transcritical CO₂ is proposed in this work. The new system is then thermodynamically simulated by Aspen Plus, after the sub-modules are validated. The effects of molar ratio of steam to carbon, selexol/CO₂ mass ratio, compression ratio, turbine import temperature and turbine inlet pressure on the system are investigated. A comparison is also performed between the new combined cycle system and the traditional combined cycle power generation system. The results show that in the new power generation system, net power efficiency of 53.29%, carbon capture efficiency of 95.78% and sulfur capture rate of 94.46% can be achieved, which is significantly better than the performance of the conventional combined cycle.

1. Introduction

Blast furnace gas (BFG) is an important exhaust gas produced in the steel industry, where iron ore is reduced by coke into iron. BFG has a low calorific value, but has the highest production in steel industry gas. The rational utilization of BFG is of great significance for energy conservation and emission reduction [1]. Campos [2] reported an exergy-based comparison between BFG-fueled combined cycle plants and steam cycle plant configurations, providing a method of direct combustion of blast furnace gas for power generation. Campos [3] compared the efficiency of combined cycle and Rankine cycle in BFG utilization, showing that the combined cycle has higher power generation efficiency. Ryzhkov [4] discussed the worldwide practice of operating combined cycle power plants on BFG to date, and the principles of upgrading a standard gas turbine power plant to a combustion BFG were studied.

Though combined cycle gas turbine (CCGT) is efficient and currently used, its thermal efficiency can be further improved by utilization of low product heat. Wang et al. [5] surveyed the application of gas-steam combined cycle and subcritical gas power generation in the metallurgical industry, and the expected efficiency could reach 43%. Hou et al. [6] adopted Brayton cycle with supercritical CO₂ and organic Ranking cycle in CCGT to recycle residual heat, which resulted in a 62.23% CCGT efficiency, which demonstrated the application of Brayton cycle with supercritical CO₂ and organic Ranking cycle to improve the combined cycle’s efficiency.

The Brayton cycle with supercritical CO₂ has a comparatively high efficiency over a wide temperature range [7]. The efficiency of Brayton cycle with supercritical CO₂ at 550~750 °C is higher than that of the Rankine cycle with steam or helium as working fluid. Thus, the use of supercritical CO₂ as the working fluid in the Brayton cycle can significantly reduce the power consumption of the compressor and achieve a cycle efficiency of at least 35% [8]. Yang [9] studied the effects of inflow pressure and temperature on Brayton cycle with supercritical CO₂, and the highest efficiency of the supercritical CO₂ Brayton cycle can reach 65%. Pan [10] applied supercritical CO₂ Brayton cycle to recover waste heat of combined cycles gas turbine and the energy efficiency of the entire system was increased by 7.03% with the supercritical CO₂ Brayton cycle, compared with the condition that gas turbine without recovery of waste heat.

Power cycle with transcritical CO₂ has many advantages over conventional power cycles for low-grade heat source recovery [11,12]. This is mainly because the temperature profile of transcritical CO₂ can better matches the heat source temperature than other pure working fluids, and its heat transfer performance is better than that of mixed fluids, which makes the cycle more efficient [13]. Chen et al. [14] compared Rankine cycle with transcritical CO₂ with organic Rankine cycle and showed that the power output of the Rankine cycle with transcritical CO₂ is slightly higher than the power of the organic Rankine cycle in utilization of the low-grade heat source with the same average thermodynamic heat rejection temperature. Habibollahzade et al. [15] studied transcritical CO₂, supercritical CO₂ cycle and compared in simple Rankine, which showed that Rankine cycle with transcitical CO₂ is best because they lead to higher efficiency while having a satisfactory total cost rate under well-balanced conditions. These studies showed that Rankine cycle with transcritical CO₂ has advantages in utilizing the low-grade heat source.

Carbon capture and storage (CCS) is a technology that separates CO₂ from industrial and energy sector emission sources and transport it for long-term isolation from the atmosphere to safe storage sites [16], which has great significance to Chinese carbon peak and neutrality targets [17,18]. However, it is difficult to balance carbon capture rate and energy consumption. Sun et al. [19] incorporated CCS technology into a combined cycle field, and demonstrated that the carbon capture rate could reach 70%. Chen et al. [20] studied the effect of the CCS system on IGCC and determined that the efficiency of power generation was decreased to 35.16%, and the carbon capture rate was 90% after adding the CCS system. These studies showed that higher carbon capture rates come at the cost of power generation efficiency.

The literature review shows that the existing CCGT systems have high low-grade heat losses, low efficiency after adding the CCS system and waste exergy. The efficiency of CCGT with CCS is usually blow 40%. The Brayton cycle with supercritical CO₂ and the Rankine cycle with transcritical CO₂ have an excellent performance in the utilization of low-grade heat, which can be a substitute for the traditional Rankine cycle. To improve the efficiency of CCGT systems and to rationally use low-grade heat, a new type of combined cycle power generation system is proposed in this paper. The new combined cycle system adopts both a Brayton cycle with supercritical CO₂ and a Rankine cycle with transcritical CO₂ to improve overall efficiency and uses polyethylene glycol dimethyl ether to adsorb CO₂ to reduce energy losses. The main conclusions of this paper are of reference value for future practical operation and performance optimization of similar power plants.

2. New Power Generation System

2.1. Exergy Calculation

In a system, heat energy consists of exergy, which is the usable energy and anergy which is the unusable energy. Exergy represents the maximum work that can be obtained by the whole system consisting of both system and surroundings when the system is balancing with surroundings [21]. Anergy represents the part of energy that cannot be transformed into work even by reversible processes [22]. Therefore, the energy depends on the state and environment of the system. Exergy usually consists of both physical exergy and chemical exergy in thermodynamic system analysis, as Equation (1):

$$ex=e{x}^{ph}+e{x}^{ch }$$

(1)

where $ex$ represents total exergy and $e{x}^{ph}$ and $e{x}^{ch}$ represent the physical exergy and chemical exergy, respectively. The actual physical exergy of a flowing stream can be calculated from Equation (2):

$$e{x}^{ph}=\left(h-h_0\right)-T_0\left(s-s_0\right)$$

(2)

where $h$ and $h_0$ are the represents of the specific molar enthalpies of a flowing stream at practical conditions (T, p) and environmental condition (T₀, p₀), and s and s₀ represents the specific molar entropy of a flowing water stream under actual and ambient conditions. The specific chemical exergy of a gas mixture can be calculated by Equation (3):

$$e{x}_{mix}^{ch}=\sum x_i·e{x}_i^{ch}+R{T}_0\sum x_i·ln{x}_i$$

(3)

where $R$ represents the gas constant and $e{x}^{ch}$ represents the standard chemical molar exergy of species i. In the standard state (298.15 K, 101.325 K Pa), the chemical energies of the different substances can be found in the literature [23].

The basic methods of exergy analysis are based on the exergy balance equation (EXBE). For an environmentally connected system operating at steady state, if kinetic and potential energy are neglected, the EXBE can be written in the following form:

$$E{x}_l=E{x}_{in}-E{x}_{out}+E{x}_q-W_u$$

(4)

where $E{x}_l$ is the loss of exergy in a unit, $E{x}_{in}$ and $E{x}_{out}$ denote the inlet and outlet exergies, $W_u$ is the unit output work. $E{x}_q$ is calculated as in Equation (5):

$$E{x}_q=\left(1-\frac{T_0}{T}\right)Q$$

(5)

So the exergy efficiency of a system can be calculated using Equation (6)

$${\eta }_{ex}=1-\frac{E{x}_l}{E{x}_{in}+E{x}_q}$$

(6)

2.2. General Description

The flow diagram of the simulation system is shown in Figure 1. SHIFT is the water-gas conversion reaction subsystem. CLEAN is the raw gas clean subsystem with DEPG. CLAUS is the Claus subsystem for sulfur generation. TURB is the gas turbine cycle for power generation. BRATY is the Brayton cycle with supercritical CO₂. RANKI is the Rankine cycle with transcritical CO₂. CCS is the carbon capture and sequestration system. In the gas conversion unit (SHIFT), CO reacts with H₂O to produce high calorific value hydrogen, the purification unit (CLEAN) cleans the syngas by removing most of the H₂S and CO₂, and the CO₂ compression unit (CCS) compresses the CO₂ separated from the CLEAN into a liquid to achieve CO₂ capture. In the gas turbine unit (TURB), the clean syngas from the CLEAN is mixed with N₂ and then air before entering the gas turbine to generate electricity. High-temperature flue gas exiting the TURB is the heat source for the Brayton cycle with supercritical CO₂ (BRAYT) to generate electricity to provide the heat required for the cycle. In the Rankine cycle with transcritical CO₂ unit (RANKI), syngas and flue gas from the Brayton cycle with supercritical CO₂, syngas from the SHIFT, and water from the CCS to absorb waste heat are used as the heat source for the Rankine cycle with transcritical CO₂ to provide the heat required for the cycle.

2.3. SHIFT Module

A schematic of the SHIFT module is shown in Figure 2. The inlet substances of the SHIFT module are gas and water. The high-temperature gas is cooled and enters the latter two stages of the reactor, where it reacts with water vapor under the action of a catalyst. The main reaction equation is as in Equation (7):

CO + H₂O = CO₂ + H₂

(7)

The water in the SHIFT module is passed through a separator proportionally to the two-stage reactor, where it reacts with the gas [24].

The data projected and reported [25] for the SHIFT subunit are shown in

Table 1. Comparison of subunit forecasts with reported data of SHIFT.

Parameters	Reported Data	Prediction
B9 pressure (bar)	54.31	54.31
B10 pressure (bar)	53.59	53.59
B9 temperature (°C)	427.3	427.3
B10 temperature (°C)	270.6	270.6
The mole fractions of syngas	H2 0.26, H2O 0.34, CO 0.27, CO2 0.12, N2 0.01	H2 0.26, H2O 0.34, CO 0.27, CO2 0.12, N2 0.01

.

Table 2. Stream parameters in SHIFT.

Stream	Materials	Mass Flow Rate (kg/sec)	Temperature (K)	Pressure (Bar)	Value (kW)
1	BFG	66.45	417.83	27.00
2	BFG	66.45	429.10	27.00
3	Primary reaction gas	92.06	522.15	27.00
5	H2O	27.24	473.15	30.00
6	H2O	27.24	298.15	30.00
7	Heat				−6052.17
8	Heat				2088.69
9	Heat				−22,498.23
11	Syngas	93.69	498.15	27.00
15				27.00
16				27.00
23	H2O	27.24	473.15	30.00
24	H2O	25.60	473.15	30.00
26	H2O	1.63	473.15	30.00

shows the materials and parameters of all streams in SHIFT module.

2.4. CLAUS Module

A schematic of the CLAUS module is shown in Figure 3. The inlet substances in the CLAUS module are H₂S and air. H₂S is oxidized to SO₂ by air in the burner according to Equation (8):

H₂S + 1.5O₂ = H₂O + SO₂

(8)

The generated SO₂ further reacts with the incompletely reacted H₂S in the converter to form sulfur monomers and water according to Equation (9):

2H₂S + SO₂ = 2H₂O + 3S

(9)

Sulfur crystals generated in the converter are extracted and recovered by the separator as sulfur trapping [26]. The extracted vapor flows into the CLAUS reactor, where the generation and extraction of sulfur crystals are repeated, serving to enhance the CLAUS reaction [27].

Table 3. Comparison of subunit forecasts with reported data of CLAUS.

Parameters	Reported Data	Prediction
BURNER temperature (°C)	1093–1427	1139
Sulfur recovery efficiency	96%	98.40%

shows the predicted and reported data [28] for the CLAUS subunits.

Table 4. Stream parameters in CLAUS.

Stream	Materials	Mass Flow Rate (kg/sec)	Temperature (K)	Pressure (Bar)
BURNAIR	Air	0.03	294.26	1.72
H2SFEED	H2S Mixing Gas	0.03	501.68	1.00
2	Primary reaction gas	0.06	1422.04	0.93
1	S-S	0.01	422.04	0.93
3	Secondary reaction gas	0.05	422.04	0.93
5	Triple reaction gas	0.05	533.15	0.82
4	S-S	0.00	438.71	0.82
6	Quadruple reaction gas	0.05	438.71	0.82
11				0.82
8	Final reaction gas	0.05	483.15	0.71
9	H2O	0.05	422.04	0.71
10	S-S	0.00	422.04	0.71
15	S-S	0.01	426.37	0.71

shows the materials and parameters of all streams in CLAUS module.

2.5. CLEAN Module

A schematic of the CLEAN module is shown in Figure 4. In the H₂S absorber, the SELEXOL solution absorb most of the H₂S and some of the CO₂ in the gas [29]. The absorbed H₂S is stripped in the H₂S stripper and the absorbed CO₂ is then stripped in the CO₂ stripper and flows back to the H₂S absorber. The gas from the H₂S absorber then flows to the CO₂ absorber where most of the CO₂ is captured by the SELEXOL solution. The captured CO₂ is then released in the flash tank, producing virtually cleaned CO₂ and renewing the SELEXOL solution.

The absorption efficiency of CO₂ and H₂S of the CLEAN subunit are 97% and 95%, which is proximate to the reported data [30].

Table 5. Stream parameters in CLEAN.

Stream	Materials	Mass Flow Rate (kg/sec)	Temperature (K)	Pressure (Bar)	Value (kW)
1	DEPG	3,250,000.00	260.93	32.50
2	Dehydrated Syngas	93.69	415.00	27.00
3	Desulfurated Syngas	104.68	282.00	27.00
4	DEPG Solution	273.97	306.81	27.00
5	Heated DPEG Solution	273.97	487.26	27.00
6	Recirculated DEPG Mixture	44.57	486.08	27.00
7	Decarbonized DEPG Solution	248.06	479.85	27.00
8	N2	17.65	299.82	27.00
9	Cold DEPG Solution	273.97	306.81	27.00
10	Low Pressure Decarbonized DEPG solution	248.06	478.99	2.00
11	H2S Rich Gas	1.43	501.68	1.00
12	Regenerated DEPG	245.62	508.07	1.00
13	Pressurised Regenerated DEPG	245.62	509.61	27.00
14	Cold Regenerated DEPG Solution	245.62	311.81	27.00
15	Clean Syngas and DEPG Mixture	25.89	297.50	27.00
16	Recirculated DEPF Solution in CO2 Remove Section	5007.13	300.05	27.00
17	Cold Regenerated DEPG Solution	245.62	294.26	27.00
18	Recycled DEPG from CO2 Removal Section	250.36	300.05	27.00
19	Recirculated DEPG Solution in CO2 Remove Section	4756.78	300.05	27.00
20	Syngas and DEPG Mixture	8.55	299.31	6.89
21	Pressurised Regenerated DEPG	245.62	509.61	27.00
22	CO2 DEPG mixture	45.48	297.63	1.72
23	CO2 DEPG solution	4702.75	297.63	1.72
24	CO2 DEPG mixture	28.57	296.58	0.34
25	CO2 DEPG solution	4674.18	296.58	0.34
26	Pressurised CO2 DEPG solution	4674.18	297.18	27.00
27	Dried Syngas	82.33	272.04	27.00
28	CO2 DEPG solution	4748.23	299.31	6.8
29	Liquid Water	11.36	272.04	27.00
30	Pressurised Syngas and DEPG Mixture	8.55	417.60	27.00
31	N2 DEPG Mixture	1.01	553.27	27.00
32	DEPG	247.05	478.88	1.10
33	N2 DEPG Mixture	1.01	478.88	1.10
34	Recycled DEPG from CO2 Removal Section	250.36	272.04	27.00
35					162.54
36					−657.13
37					996.21
38					49,981.85
39	Stage 1 Low Pressure Recirculated DEPG Solution in CO2 Removal Section	4756.78	299.86	6.89
40	Stage 2 Low Pressure Recirculated DEPG Solution in CO2 Removal Section	4748.23	299.26	1.72
41	Low pressure CO2 DEPG solution	4702.75	297.62	0.34
42					−7848.92
43					−2016.42
44					−531.16
45					13,793.59
46					1214.15
47					4611.23
48					5112.86
49					−24,473.56
50	Dried H2S Gas	0.03	501.68	1.00
51	Recycled DEPG from H2S Removal Section	1.40	501.68	1.00
52	CO2 DEPG Mixture	74.05	296.38	0.34
53	CO2	74.05	296.38	0.34
54	Recycled DEPG	0.00	296.38	0.34
55	Clean Syngas	25.89	297.50	27.00
56	Liquid DEPG	0.01	297.50	27.00

shows the materials and parameters of all streams in CLEAN module.

2.6. TURB Module

A schematic of the TURB is shown in Figure 5. N₂ is mixed in the synthesizer at the inlet of the TURB model. The mixed gas then enters the combustor for combustion along with one part of the compressed air from the compressor. The high temperature flue gas is then mixed with another part of the compressed air and enters the gas turbine and expands for expansion work to drive the gas turbine to generate power.

The predicted results of TURB subunits are compared against the data reported in the literature [31], which are shown in

Table 6. Comparison of subunit forecasts with reported data of TURB.

Parameters	Reported Data	Prediction
Compression ratio of compressor	17	17
Compression ratio of turbine	17	17
Gas turbine import temperature (°C)	1400	1405
Gas turbine export temperature (°C)	586	573
Net efficiency	38.20%	38.40%

.

Table 7. Stream parameters in TURB.

Stream	Materials	Mass Flow Rate (kg/sec)	Temperature (K)	Pressure (Bar)	Value (kW)
N2	N2	0.02	323.15	17.00
GAS3	Syngas	25.89	297.50	27.00
3	Mixing gas	25.90	393.15	17.00
FUEL	Flue Gas	658.02	1612.00	16.71
AIR2	Air	761.60	293.15	1.01
2	Air	761.60	293.15	1.01
W1					313,617.11
AIR3	Air	761.60	689.76	17.23
AIR3-1	Air	632.13	689.76	17.23
AIR3-2	Air	129.47	689.76	17.23
AIR3-3	Air	129.47	473.15	17.23
W					−547,844.97
GEN	Flue Gas	787.50	695.44	1.02

shows the materials and parameters of all streams in TURB module.

2.7. CCS Module

The CCS module uses a multistage compressor to compress CO₂, which is in a subcritical liquid state, after the multistage compression [32,33]. To maximize the use of the heat produced by the compressor, a heat exchanger is added between the compressors of each stage, and the collected heat can be used to heat the Rankine cycle with transcritical CO₂. A schematic of the CO₂ compression module is shown Figure 6.

Table 8. Stream parameters in CCS.

Stream	Materials	Mass Flow Rate (kg/sec)	Temperature (K)	Pressure (Bar)	Value (kW)
1	CO2	74.05	296.38	0.34
2	CO2	74.05	442.42	4.31
3	CO2	74.05	303.15	4.31
4	CO2	74.05	452.75	18.57
5	CO2	74.05	303.15	18.57
6	CO2	74.05	456.22	80.00
7	CO2	74.05	303.15	80.00
8					9.85
9					10.00
10					9.38
11					29.23
12	H2O	18.90	298.15	10.13
15	H2O	18.90	419.76	10.13
18	H2O	18.90	436.18	10.13
19	H2O	31.50	298.15	10.13
20	H2O	31.50	453.70	10.13
21	H2O	69.30	447.12	10.13
22	CO2	74.05	297.12	1.01

shows the materials and parameters of all streams in CCS module.

2.8. Supercritical CO₂ Brayton Cycle Module (BRAYT Module)

A schematic of the BRAYT module is shown in Figure 7. In the BRAYT module, the supercritical CO₂ is compressed to between 7.63 and 20 MPa, and the high-pressure supercritical CO₂ is heated to 550 °C by the heat exchanger and heat sink. It then expands to 7.63 MPa in the turbine to generate electricity, and the expanded CO₂ from the heat exchanger flows back to the compressor to complete a cycle [34].

The temperatures of CO₂ in streams 2, 3, 5 and 6 in Figure 6 are 71.5 °C, 338.4 °C, 436.1 °C and 81.0 °C, which are close to the reported data of 65.9 °C, 323.9 °C, 434.7 °C and 75.4 °C, respectively [35].

Table 9. Stream parameters in BRAYT.

Stream	Materials	Mass Flow Rate (kg/sec)	Temperature (K)	Pressure (Pa)	Value (kW)
1	CO2	639.18	305.15	76.30
2	CO2	639.18	344.69	200.00
3	CO2	639.18	653.34	200.00
4	CO2	639.18	823.15	200.00
5	CO2	639.18	709.25	76.30
6	CO2	639.18	409.50	76.30
7	Power				−79,332.52
8	Power				15,968.77
11 (IN)	Flue Gas	787.50	695.44	1.02
3 (IN)	BFG	66.45	1604.02	27.00
2 (OUT)	BFG	66.45	660.00	27.00
12 (OUT)	Flue Gas	787.50	673.15	1.02

shows the materials and parameters of all streams in BRAYT module.

2.9. Transcritical CO₂ Rankine Cycle Module (RANKI Module)

A schematic of the RANKI module is shown in Figure 8. In the RANKI module, subcritical CO₂ is pressurized to 17 MPa by the pump. This high-pressure CO₂ passes through the heat exchanger and is heated to become supercritical CO₂ which has high temperature and high pressure. The high-temperature and high-pressure CO₂ is further heated by the sensible heat of the CCS, BRATY, and SHIFT after passing through the heat exchanger for further waste heat utilization. After this, the supercritical CO₂ flows into the turbine, expands, and generates electricity. The expanded CO₂ is then cooled to the liquid in the condenser, and flows to the pump again to complete a Rankine cycle with transcritical CO₂.

Comparing the thermal efficiency with the literature [28], the predicted thermal efficiency is 23.8%, which closes to the reported data 23.6%.

Table 10. Stream parameters in RANKI.

Stream	Materials	Mass Flow Rate (kg/sec)	Temperature (K)	Pressure (Pa)	Value (kW)
1	CO2	3023.95	293.15	140.00
2	Heat				168,078.70
3	CO2	3023.95	298.15	70.00
4	CO2	3023.95	411.01	140.00
5	CO2	3023.95	473.15	0.03
6	CO2	3023.95	414.56	70.00
7	BFG	66.45	660.00	27.00
8	BFG	66.45	417.83	27.00
10	Heat				883,086.55
13	Power				34,179.21
14	CO2	3023.95	313.21	140.00
1 (OUT)	Syngas	93.69	415.00	27.00
13 (IN)	Syngas	93.69	498.15	27.00
14 (IN)	H2O	69.30	447.12	10.13
CS	H2O	69.30	415.00	10.13
12 (IN)	Flue Gas	787.50	673.15	1.02
BRT	Flue Gas	787.50	415.00	1.02

shows the materials and parameters of all streams in RANKI module.

3. Characterization of the New System

After its construction, the system was numerically simulated, and the key operating parameters of the system and their variation ranges are shown in

Table 11. Module operation parameters of the system.

Subunits	Essential Parameters	Values
SHIFT	B9/B10 temperature (K)	522.15/498.15
	Molar ratio of steam to carbon, Rhc	1.0–1.3 (1.25)
CLAUS	Burner temperature (K)	1422
	S-split F, 1, 2/ CLAUS 1, 2 temperature (K)	422, 439, 422/533.15, 483.15
	O2/H2S molar ratio	0.5
CLEAN	B1 pressure (MPa)/stage number	2.7/20
	B6 pressure (MPa)/stage number	2.7/10
	B3 pressure (MPa)/stage number)	0.1/10
	B2 pressure (MPa)/stage number	2.7/10
	Mass ratio of Selexol to CO2, Rsc	2.5–3.5 (2.5)
TURB	Burner temperature (K)	1600
	Compression ratio	17
BRATY	Turbine inlet temperature (°C)	500–550 (550)
	Compression ratio, Rcs	2.6–3.2 (2.6)
	Compressor isotropic efficiency	0.89
RANKI	Turbine import temperature (°C)	200
	Turbine import pressure (MPa), Pin	13–14.3 (14)
	Turbine isotropic efficiency	0.91
CCS	CO2 export pressure (MPa)/temperature (°C)	8/30

. These data are derived by changing the molar ratio of steam to carbon (R_hc) in the SHIFT module, the mass ratio of Selexol solution to CO₂ (R_sc) in the CLAUS module, the compression ratio (R_cs) and the inlet temperature of turbine (T_in) in the BRATY module, and the inlet pressure of turbine (P_in) in the RANKI module. Their effects on the net electrical efficiency (η), carbon capture rate (R_c), and sulfur capture rate (R_s) of the system are presented in Figure 9. In particular, the system is unstable when R_cs is less than 2.6 and T_in is greater than 550, so the R_cs values are all greater than 2.6, and T_in is less than 550.

Figure 9a shows that as R_hc increases, η increases before decrement. This is because the H₂ increased due to increased water. Excessive water consumes more heat and results in an increased conversion ratio of carbon to sulfur. It is also noted that the variation in R_s is more sensitive than that in R_c, which is due to the amount of CO₂ in the syngas being much larger than the amount of H₂S. From Figure 9b, it is seen that η decreases with increasing R_sc while R_c and Rs all increase. This is because the increased amount of SELEXOL requires higher thermal energy consumption and more electricity to circulate the solution. As R_cs increases, η decreases, and R_c and R_s essentially remain constantly, as shown in Figure 9c. It can be seen from Figure 9d that as T_in increased, η increases and then decrease, and R_c and R_s essentially remain the same. When P_in is less than 14 MPa, η rises as P_in increases, as shown in Figure 9e. When P_in is greater than 14 MPa, the rule is reversed, and P_in has less effect on R_c and R_s when it is less than 14 MPa.

Exergy refers to the maximum value of useful work that can be released when the system changes from the state it is into the ambient state. Exergy analysis can reflect the gap between the theoretical and actual useful work that can be released by the system. From

Table 12. Summary of the exergy flux of each module.

Module	Exergy Influx (J)	Exergy Outflux (J)	Heat Exergy (J)	Power Exergy (J)
SHIFT	701,032,804.4	675,995,397.5	10,079,008.78	0
CLAUS	387,385.518	309,927.4635	2587.472084	0
CLEAN	816,414,993.2	652,844,609	−3,972,119.044	−5,112,860.92
TURB	617,866,015.3	135,787,079.2	9,006,064.882	234,227,847
BRATY	916,589,121.8	822,631,334.9	16,618,615.11	63,363,791
RANKI	3,171,549,361	2,929,826,779	2,019,871.956	173,708,352.8
CCS	71,846,695.31	101,030,680.3	0	29,231,205.8

, the exergy flux of each module can be calculated. Heat exergy represents the exergy of the heat input into the unit, and when the system absorbs heat from surroundings, the heat exergy is negative value. Similarly, power exergy is equal to the work output from the unit, and if the unit output works, the power is negative value. The exergy efficiencies of the SHIFT module, the CLAUS module, the CLEAN module, the TURB module, the BRATY module, the RANKI module and CCS module are found to be 95.06%, 79.74%, 79.73%, 59.03%, 98.45%, 87.01%, 99.93%, respectively. Supercritical CO₂ Brayton cycles and transcritical CO₂ Rankine cycles are energy efficient because they operate at relatively low temperatures and use the recovered heat to generate electricity.

4. Conclusions

To improve the efficiency of combined cycle power generation systems and reduce CO₂ emissions, a new power generation system with both supercritical CO₂ Brayton and transcritical CO₂ Rankine cycles is proposed. These cycles improve the efficiency of the power generation system, and the application of CCS reduces carbon emission. To ensure the dependability of the system, the thermodynamic aspects of the system submodules are compared with literature experimental data. Numerical simulations are then undertaken to demonstrate the properties of the system. It was found that the system is stable during operation, indicating that the system is theoretically feasible, and has practical application value. In addition, the net electricity efficiency, carbon capture rate, and sulfur capture rate are 53.29%, 95.78%, and 94.46%, respectively. The electricity efficiency, carbon capture rate, and sulfur capture rate are relatively high compared with CCGT existing. The exergy efficiencies of the SHIFT module, the CLAUS module, the CLEAN module, the TURB module, the BRATY module, the RANKI module and CCS module are found to be 95.06%, 79.74%, 79.73%, 59.03%, 98.45%, 87.01%, 99.93%, respectively. As the consequence, the exergy efficiency of the TURB module can be further improved, potentially contributing to the performance of the new combined cycle.