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The transition to oxy-fuel combustion power cycles is a prospective way to decrease carbon dioxide emissions into the atmosphere from the energy sector. To identify which technology has the highest efficiency and the lowest emission level, a thermodynamic analysis of the semiclosed oxy-fuel combustion combined cycle (SCOC-CC), the E-MATIANT cycle, and the Allam cycle was carried out.
- oxy-fuel combustion power cycle
- carbon dioxide capture and storage
- gas turbine coolant
1. Introduction
The International Energy Agency expects a 30% increase in power consumption during 2016–2040 [1]. The annular combustion of hydrocarbons will also grow, which increases the toxic and greenhouse gases’ atmospheric emissions. The anthropogenic pressure aggravates the problem of the 21st century’s local and worldwide stable development.
The power production industry produces a remarkable amount of harmful emissions [2], so the mitigation of power facility emissions is a topical direction. Today, thermal power plants successfully reduce nitrogen and sulfur oxide emissions [3,4,5,6,7]. Organic fuel combustion produces huge amounts of carbon dioxide, and its emissions still are a difficult problem [8,9]. Widely known carbon dioxide capturing technology considerably increases the power production expenses [10,11,12]. The creation of environmentally friendly and financially efficient large power facilities is a valid problem. This is emphasized by the introduction of some international agreements, especially the Paris Climate Agreement, which has been signed by nearly 200 countries.
Oxy-fuel combustion technology is a promising method for harmful emission mitigation in the power production industry [13,14,15]. Unlike traditional technology, oxy-fuel combustion power cycles create less atmospheric harm through the application of a closed gas turbine cycle, oxy-fuel combustion, and carbon dioxide capture and storage. The high efficiency of these cycles is due to the thermodynamic separation of the working fluid that allows for the highly efficient separation of carbon dioxide from the water steam through steam condensing in a cooler-separator.
The first modifications of these power facilities appeared at the end of the 20th century, and now, the USA, Japan, and EU countries are actively developing this direction. Research studies are being carried out, test facilities are being built, and foundations for the zero emission pilot facilities are being create using grant funding, active “green technology” backing, and through the creation of legislation bases. Large power corporations cooperate in a buildup of demonstration facilities with power outputs of up to 50 MW [16].
The widely known oxy-fuel combustion power cycles, such as the semiclosed oxy-fuel combustion combined cycle, the E-MATIANT cycle, the Allam cycle, the Graz cycle, AZEP, and ZEITMOP [17], may be classified as such. The semiclosed oxy-fuel combustion combined cycle (SCOC-CC), the E-MATIANT, and the Allam cycles involve carbon dioxide recirculation, which reduces the combustion temperature. In these cycles, the main operating component is carbon dioxide. In the Graz and the “water” cycles, the temperature is reduced with water, which is the main operating component. The AZEP and ZEITMOP cycles use high temperature membranes integrated into the cycle heat flow circuit.
Numerous papers disclose the results of research and development of methods for increasing efficiency of the existing oxy-fuel combustion power cycles. In particular, a possible way to increase efficiency of the SCOC-CC cycle is an application of the additional recuperator in the heat recovery system [18]. The net efficiency of the R-SCOC-CC is higher compared to SCOC-CC by 0.6 and 1.3% in the F- and H-class gas turbines cases, respectively. Another way to increase the SCOC-CC efficiency is an application of liquid oxygen pump supply of oxidizer to the combustion chamber [19]. Using this method cycle efficiency could be increased by 3%.
In turn, the modified layout of the MATIANT cycle is proposed in [20]. The scheme includes three changes to achieve a more balanced thermal match of the recuperator and to lower the compression power: the reheating process is eliminated, stream split and recompression are added, and the compressor is replaced by a seven-stage one. The optimized efficiency of the new cycle can reach 45.3%, which is 0.35% lower than that of the MATIANT cycle. However, an advantage is the layout’s simplicity.
One of the latest modifications of the Allam cycle is the Allam-Z cycle proposed in [21]. The main modifications are that all the working media are pumped to high pressure by pumps instead of compressors, the cold energy of both liquid oxygen and liquefied natural gas is used for degrading the cooling water for carbon dioxide liquefaction, and a set of regenerative heat exchangers are arranged for turbine exhaust heat recovery.
The selection of the most promising power production technology is an integrated problem that involves numerous factors, including efficiency, environmental harm, and production expenses. The influencing factors must be compared with compatible input data and common simulation methods. Most of the available efficiency assessments presented in the literature involve different simulation approaches. The difference may be seen in assessments of the working fluid thermodynamic parameters, the oxygen production power consumption, the cooled gas turbine model, and the flow analysis algorithm. When modeling oxy-fuel combustion cycles, special attention should be paid to the estimation of gas turbine cooling losses because coolant massflow is significant due to the working fluid and cooling agent thermophysical parameters. The development of methods allowing to decrease carbon dioxide gas turbine coolant flow is of special importance.
2. Basic Schemes and Initial Parameters for Simulation of the Oxy-Fuel Combustion Power Cycles
The first investigation object is the SCOC-CC cycle (Figure 1), first published in 1992 by O. Bolland and S. Saether [14]. It differs from the combined cycle facility prototype with the increase of carbon dioxide partial pressure by the oxy-fuel combustion and the recirculation into the combustor of a large part of flue gas from the heat recovery steam generator. This combination reduces the power consumption for the carbon dioxide separation.
Figure 1. The semiclosed oxy-fuel combustion combined cycle (SCOC-CC) flow chart: C—compressor; GT—gas turbine; ASU—air separation unit; PG—power generator; HRSG—heat recovery steam generator; CS—cooler-separator; CC—combustion chamber; ST—steam turbine; CP—condensate pump; M—motor.
The second investigation object is the E-MATIANT cycle (Figure 2), which is a prospective modification of the MATIANT proposed in 1997 by Mathieu and Yantovsky [22]. It differs from the SCOC-CC cycle in that the compressor and turbine consist of a few cells, with the intermediate cooling for the compressor and the working fluid superheating for the turbine. Instead of the heat recovery steam generator, the exhaust gas is cooled in a regenerative heat exchanger. The intermediate cooling and superheating improve the power production efficiency.
Figure 2. The E-MATIANT cycle flow chart: HPCC—high pressure combustion chamber; LPCC—low pressure combustion chamber; HPT—high pressure turbine; LPT—low pressure turbine; R—regenerator; P—pump; ICC—intercooled compressor.
The last investigation object is the Allam cycle (Figure 3). This cycle is free from the two main shortages of the oxy-fuel combustion power plants, which are the large losses of carbon dioxide compression before its storage and the irreversible losses of the hot air low potential heat. R. Allam patented this technology in 2010 [23]. The specific features of this cycle are the high initial and final pressures and the multiflow regenerator.
Figure 3. The Allam cycle flow chart.
Table 1 summarizes the main input data used for the computer simulation of three oxy-fuel combustion power plants.
Table 1. Input data used for the computer simulation of oxy-fuel combustion power cycles.
| Parameter | Value |
|---|---|
| Ambient temperature/pressure/humidity, °C/MPa/% | 15/0.1013/60 |
| Fuel chemical contents | CH4 |
| Fuel combustion temperature/pressure/low heat production, °C/MPa/(kJ/kg) | 15/0.7/50025 |
| O2 produced by air separator temperature/pressure, °C/MPa | 30/1 |
| CO2 storage pressure, MPa | 10 |
| CO2 compressors and turbines internal specific efficiency besides cooling losses, % | 90 |
| Oxygen/fuel compressor internal specific efficiency, % | 85/88 |
| Steam turbine internal specific efficiency, % | 89 |
| Pumps internal specific efficiency, % | 75 |
| Compressors and turbines mechanical efficiency, % | 99 |
| Pumps mechanical efficiency, % | 95 |
| Power generator electric efficiency, % | 98.5 |
| Combustor pressure drop, % | 4 |
| Combustor exit O2 molar content, % | 1 |
| Cycle minimal temperature, °C | 30 |
| High pressure steam parameters, °C/MPa | 560/14 |
| Low pressure steam parameters, MPa | 0.7 |
| Steam turbine condenser/deaerator pressure, MPa | 0.0045/0.121 |
| Minimal high pressure steam under-heating at the gas turbine exit, °C | 20 |
| Heat recovery steam generator gas pressure losses, MPa | 0.002 |
| Pinch point temperature difference in multiflow regenerative heat exchanger, °C | 5 |
3. Structural and Parametric Optimization of the Semiclosed Oxy-Fuel Combustion Combined Cycle
The parametric optimization of the SCOC-CC cycle shows that when the initial fluid temperature increased from 1100 to 1700 °C, the optimal initial pressure, the coolant massflow, and the net efficiency grew from 2 to 7 MPa, from 8.1 to 36%, and from 41.0 to 47.7%, respectively (Figure 7). The fluid heat capacity in the compressor of the SCOC-CC was higher than in the compressor of the combined cycle. In turn, the fluid heat capacity in the gas turbine of the SCOC-CC was lower than in the gas turbine of the combined cycle. Therefore, the SCOC-CC optimal initial pressure was higher. Furthermore, the large SCOC-CC cooling flow causes considerable efficiency losses.
Figure 7. Optimization of the SCOC-CC initial parameters, the cooling losses include the following: (a) optimal turbine inlet pressure; (b) influence of the initial parameters on the net efficiency.
The preliminary cooling of the coolant flow may improve the SCOC-CC thermal efficiency (Figure 8 and Figure 9) [34]. Deep coolant cooling down to 150–250 °C may take place in a surface heat exchanger (SHE; Figure 8). The heat release source may be a part of the steam turbine condensate flow, and the net efficiency improvement may be up to 3.2%. On the contrary, when the coolant cooling to a temperature above 250 °C is smaller, it is reasonable to use the water injection into the coolant flow. The water may be taken from the gas cycle cooler-separator (Figure 9). This scheme improves the net efficiency up to 1.5%.
Figure 8. The SCOC-CC cycle coolant preliminary cooling in a surface heat exchanger.
Figure 9. The SCOC-CC cycle coolant preliminary cooling water injection preliminary cooling.
These schemes are free from the water production expenses, which is an advantage. It should be noted that preliminary cooling of the coolant flow may also improve the R-SCOC-CC efficiency [18]. However, the increase in efficiency will be lower compared to SCOC-CC due to lower values of the compressor outlet pressure causing the moderate temperature of the coolant.
4. Oxy-Fuel and Combined Cycle with CO2 Storage Facilities Parameters Comparison
Parametric optimization of the oxy-fuel combustion power cycles with carbon dioxide working fluid provides the following dependencies of performance on the cycle initial and final parameters:
-
An increase of the SCOC-CC initial temperature from 1100 to 1700 °C causes the optimal pressure to increase from 2 to 7 MPa, the coolant massflow to increase from 8.1 to 36%, and results in the net efficiency to increase from 41.0 to 47.7%.
-
An increase of the E-MATIANT cycle initial temperature from 1100 to 1400 °C causes the optimal pressure increase from 3 to 4 MPa, the coolant massflow to increase from 14.5 to 40.3%, and causes the results in the net efficiency to increase from 41.1 to 44.0%.
-
An increase of the Allam cycle initial temperature from 1000 to 1100 °C causes the optimal pressure increase from 20 to 31 MPa, coolant massflow increase from 3.9 to 8.5%, and results in the net efficiency increase from 54.6 to 56.3%.
The following combinations of parameters provide the maximal net efficiency of oxy-fuel combustion power cycles with CO2 working fluid:
-
SCOC-CC working fluid initial parameters of 1700 °C/7 MPa.
-
E-MATIANT cycle working fluid parameters at high and low pressure turbine inlets of 1400 °C/4/2 MPa, and the temperature of the working fluid bleeding from the regenerator for the turbine cooling of 400 °C.
-
The Allam cycle working fluid initial parameters of 1083 °C/30 MPa, the turbine exhaust pressure of 3 MPa, and the temperature of working fluid bleeding from the regenerator for the turbine cooling of 200 °C.
The following technical solutions improve the efficiency of the oxy-fuel combustion power cycles with carbon dioxide working fluid:
-
SCOC-CC with cooling in the surface heat exchanger, where the cooling agent is a part of the steam turbine condensate and improves the net efficiency by 3.2%, which is reasonable to apply for the coolant to cool down to 150–250 °C.
-
SCOC-CC with cooling by the water injection: the water is bled from the gas cycle cooler-separator and is injected into the coolant flow to reduce its temperature down to 250 °C. The net efficiency increase is up to 1.5%.
-
The E-MATIANT cycle with the low pressure turbine coolant taken from the regenerator and cooled by injection of the water bleed from the cooler-separator. The net efficiency grows by 0.8%.
At optimal thermodynamic parameters, the Allam cycle net efficiency is 8.8% higher than the SCOC-CC one, and 12.5% higher than the E-MATIANT one (Figure 15a). Besides this, the Allam cycle has a remarkably higher operating pressure and lower initial temperatures (Figure 15b), which provides a minimal turbine cooling massflow (Figure 15c).
Figure 15. Results of oxy-fuel combustion power cycles’ parametric optimization: (a) net efficiency vs. initial parameters; (b) optimal initial pressure vs. initial temperature; (c) Ψ coefficient vs. initial parameters.
Thus, it is possible to conclude that the Allam cycle has a higher efficiency than the competitive oxy-fuel cycles because of a combination of the following thermodynamic features:
-
The working fluid is compressed near the CO2 saturation line, which reduces the compressor drive power consumption.
-
The useful utilization in the regenerator of low potential heat reduces heat losses in the cold source.
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Minimal gas turbine coolant flow caused by the moderate initial temperature.
-
Minimal power consumption for the compression of the separated content due to the high final pressure.
The oxy-fuel combustion power cycles have obvious advantages against the combined cycle of CO2 separation from flue gas. The Allam cycle net efficiency is 8.5% higher than the combined cycle efficiency (Figure 16a) at conditions of CO2 capturing at 98.9% for the oxy-fuel combustion power cycle and 89% for the combined cycle (Figure 16b).
Figure 16. Thermal efficiency and environmental performance of oxy-fuel combustion power cycles and combined cycles with CO2 storage cycles: (a) technology efficiency; (b) technology environmental safety.
The cost parameters of the environmentally safest oxy-fuel Allam cycle are compared with the combined cycle with CO2 storage for the power facility nominal power annual operation of 6000 h for 30 years and CO2 storage at 10 MPa. The Allam cycle installed power cost facility was 46% lower, or $1398/kW against $2423/kW. This difference is caused by the absence of expensive equipment for CO2 separation from flue gas with a high nitrogen content and with steam turbine elements. More than that, the price of the Allam cycle CO2 storage is 5.5% lower due to the smaller mass of produced CO2, or 343 against 413 G/(kW·hr), and utilized CO2 339 against 367 G/(kW·hr).
This entry is adapted from the peer-reviewed paper 10.3390/en14102927
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