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 CO
2 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 CO
2 and organic Ranking cycle to improve the combined cycle’s efficiency.
The Brayton cycle with supercritical CO
2 has a comparatively high efficiency over a wide temperature range [
7]. The efficiency of Brayton cycle with supercritical CO
2 at 550~750 °C is higher than that of the Rankine cycle with steam or helium as working fluid. Thus, the use of supercritical CO
2 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 CO
2, and the highest efficiency of the supercritical CO
2 Brayton cycle can reach 65%. Pan [
10] applied supercritical CO
2 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 CO
2 Brayton cycle, compared with the condition that gas turbine without recovery of waste heat.
Power cycle with transcritical CO
2 has many advantages over conventional power cycles for low-grade heat source recovery [
11,
12]. This is mainly because the temperature profile of transcritical CO
2 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 CO
2 with organic Rankine cycle and showed that the power output of the Rankine cycle with transcritical CO
2 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 CO
2, supercritical CO
2 cycle and compared in simple Rankine, which showed that Rankine cycle with transcitical CO
2 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 CO
2 has advantages in utilizing the low-grade heat source.
Carbon capture and storage (CCS) is a technology that separates CO
2 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.
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. These data are derived by changing the molar ratio of steam to carbon (
Rhc) in the SHIFT module, the mass ratio of Selexol solution to CO
2 (
Rsc) in the CLAUS module, the compression ratio (
Rcs) and the inlet temperature of turbine (
Tin) in the BRATY module, and the inlet pressure of turbine (
Pin) in the RANKI module. Their effects on the net electrical efficiency (
η), carbon capture rate (
Rc), and sulfur capture rate (
Rs) of the system are presented in
Figure 9. In particular, the system is unstable when
Rcs is less than 2.6 and
Tin is greater than 550, so the
Rcs values are all greater than 2.6, and
Tin is less than 550.
Figure 9a shows that as
Rhc increases,
η increases before decrement. This is because the H
2 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
Rs is more sensitive than that in
Rc, which is due to the amount of CO
2 in the syngas being much larger than the amount of H
2S. From
Figure 9b, it is seen that
η decreases with increasing
Rsc while
Rc 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
Rcs increases,
η decreases, and
Rc and
Rs essentially remain constantly, as shown in
Figure 9c. It can be seen from
Figure 9d that as
Tin increased,
η increases and then decrease, and
Rc and
Rs essentially remain the same. When
Pin is less than 14 MPa,
η rises as
Pin increases, as shown in
Figure 9e. When
Pin is greater than 14 MPa, the rule is reversed, and
Pin has less effect on
Rc and
Rs 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, 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 CO
2 Brayton cycles and transcritical CO
2 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.