- Email: [email protected]
Upgrading Low-temperature Steam to Match CO2 Capture in Coal-fired Power Plant Integrated with Double Absorption Heat Transformer
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 4436 – 4443
The 8th International Conference on Applied Energy – ICAE2016
Upgrading low-temperature steam to match CO2 capture in coal-fired power plant integrated with double absorption heat transformer Dandan Wanga,b, Feng Liua,b, Sheng Lib,*, Lin Gaob, Jun Suib a
b
University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District, Beijing100049, China Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11 Beisihuanxi Rd, Beijing 100190, China
Abstract The energy level mismatch between extraction steam from turbines and heat for CO2 regeneration always results in huge exergy destruction and low thermal efficiency in post combustion power plants with CO2 capture. To solve the problem, a new CO2 capture system driven by a double absorption heat transformer is proposed in this paper. Through the absorption heat transformer, low-temperature steam can be upgraded into high level heat to match the temperature of solvent regeneration. Also, flue gas heat is recovered to preheat the circulating water to further decrease system power penalty. Aspen Plus11.0 is used to simulate the system, and the results of the key process are validated by experimental values. It is shown that with 90% CO2 capture, the thermal efficiency of the proposed system is enhanced by 1.25 percentage points. And the efficiency enhancement of proposed system has a trend of increase first and then decrease with the CO2 capture rate growth. For a 350MW coal-fired power plant, the optimum CO2 capture rate is 53.65% and the corresponding efficiency enhancement is 2.06 percentage points. Exergy analysis shows that due to better match between the upgraded steam and CO2 stripping temperature, the exergy destruction in CO2 separation process of proposed system could decrease by 40.3%, and thereby the exergy efficiency of proposed system is 33.76%, which is 1.85 percentage points higher than the conventional method. ©©2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Keywords: CO2 capture˗Double absorption heat transformer˗Flue gas heat recovery˗Process simulation
1. Introduction Carbon dioxide (CO2) has the highest contribution to global warming compared to other greenhouse gases. Since coal-fired power plant is one of the main CO2 emission source, accounting for 43% of total
* Corresponding author. Tel.: +86-10-82543163; fax: +86-10-82543151. E-mail address: [email protected].
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.941
Dandan Wang et al. / Energy Procedia 105 (2017) 4436 – 4443
global CO2 emissions worldwide [1]. Thereby CO2 capture from power plants is of great importance to mitigate global warming. Among many technologies, chemical absorption is thought to be a relatively mature and suitable method [2]. However, the energy consumption of this method is huge, resulting in 1115 percentage points decrease of efficiency in the coal-fired power plants with 90% CO2 capture [3]. One of the main reasons of the high energy penalty for CO2 capture is the large heat requirement for rich solvent decomposition (about 3~4GJ/ton-CO2). To reduce energy consumption, a major way is to develop new absorbents with low decomposition heat and high capacity, such as aqueous piperazine (PZ) [4], tri-solvent containing 2-amino-2-methyl-1-propanol (AMP), piperazine (PZ) and monoethanolamine (MEA) [ 5 ], etc. Reducing water contents in CO2 rich solvent is another solution to decrease heat consumption. Abu-Zahra [ 6 ] found the thermal energy requirement decreased substantially with increasing MEA concentration, and at 40 wt.% MEA solution concentration, the thermal energy requirement is 23% lower than the base case of 30 wt.%. Another reason of the high energy penalty for CO2 capture is the truth that the temperature of the steam extracted from the turbines is usually unmatched with that of the reboiler, especially for capture retrofit of an existing power plant. Typically, the regeneration energy is provided by steam extracted from the intermediate pressure (IP) cylinder of steam turbine, and the temperature of the steam in appropriate pressure is usually over 200ć, which is much higher than the desorption temperature of rich solvent (115~120ć) [7]. The large temperature difference between energy donor (steam extracted) and acceptor (solvent) leads to a significant reduction in power output and thermal efficiency. To solve this mismatch problem, CO2 capture integrated with heat pumps seems to be attractive. Heat pumps can be used to upgrade low temperature heat into high temperature. There are two popular types of heat pumps: compression systems and absorption systems [8]. For a compression system, main power is consumed to compress refrigerant to increase its temperature and pressure. The absorption system doesn’t involve a compression process and therefore it can save a great deal of power consumption compared with compression system. Absorption systems can be divided in two types: absorption heat pumps (AHP) and absorption heat transformers (AHT). In an AHP system, high-level heat source is needed to drive it work, and low temperature heat is recovered and get temperature improved to obtain middle-level heat. The AHT system can be driven by middle- or low-temperature heat source and get part of whose temperature increased, while the penalty is another part of heat will be discharged. Duan [9] used AHP and single-stage absorption heat transformer (SAHT) to recover heat of stripper top gas and lean solvent in a 600MW power plant with CO2 capture, along with a letdown steam turbine and CO2 compression process heat recovery device, making the plant efficiency 1.9 percentage points higher than the normal method. Zhang [10] proposed a new system in which SAHT and flash evaporator were used to reduce the heat consumption of CO2 capture processes. The results showed that the new system reduced the heat consumption from 3.873GJ/t-CO2 to 3.772GJ/t-CO2. In the above-mentioned researches, since high-level heat is needed to drive an AHP, making the cost of heat recovery large. SAHTs are also used in CO2 capture, while due to the limit of refrigerant physical properties, the temperature lift is no more than 50ć and available energy is relatively small. To get a higher temperature lift, it is necessary to adopt a multi-stage absorption heat transformer, such as the two-stage absorption heat transformer (TAHT) or the double absorption heat transformer (DAHT). DAHTs have been proved to be usable in many fields, such as water purification, blow tank heat recovery and humid air turbine cycle, etc. [11]. While no relevant studies for CO2 capture combined with DAHTs have been reported in the literature works. So this paper aims to conceive a new system, using a double absorption heat transformer (DAHT) to drive CO2 capture unit in a 350MW coal-fired power plant. Low-level steam is upgraded through the DAHT to match the regeneration temperature in CO2 capture unit, which can avoid the extraction of high-
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Dandan Wang et al. / Energy Procedia 105 (2017) 4436 – 4443
level steam. In addition, the exhaust heat from boiler is utilized to preheat the circulating water from CO2 capture unit, further reducing the energy consumption of CO2 capture system in the power plant. 2. Configuration of the DAHT-CO2 capture system The base system is a 350MW supercritical coal-fired power plant in Jilin province, China. Aspen Plus 11.0 is used to simulate the performance of the plant. The results are validated using the plant operating data. The relative errors of main parameters between the model and the design plant are less than f5%. Table 1 Operating and simulation data of the base system (Operaing/Model) Parameters T, ć
Inlet of HP cylinder
Inlet of IP cylinder
1# heater
3# heater
5# heater
7# heater
566.0/566.0
566.0/566.0
251.6/240.5
176.6/172.1
118.4/113.8
60.6/59.1
24.2/24.2
4.2/4.2
26.3/26.3
30.4/30.4
3.1/3.1
3.1/3.1
P, Mpa
According to the industrial test from Beijing Thermal Power Plant of Huaneng Group [12], the heat requirement for CO2 regeneration is 3.0 GJ/ton-CO2, and the same value is designed in this paper. In order to reduce the energy level difference and irreversibility, a new DAHT-CO2 capture system is proposed. A double absorption heat transformer is integrated to upgrade the energy level of eighth stage extraction steam from low pressure (LP) cylinder into higher level for CO2 regeneration, instead of the reference mode where fifth stage extraction steam is extracted directly. The configuration and solution cycle of the DAHT [13] are shown in Fig. 1(a), (b). Circulating Water Return Steam
6
EV
7
4
P1 2
5
AB/EV P2
17 16 8
14
PAB PEV
13
SHE1 12
9
P3 11
10
CO
To User
SHE2
3 1
H 2O
P
AB 15
PCO
GE
Cool Water
TCO
澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳
(a)
TGE
TAE
TAB
T
(b)
Fig. 1(a) Schematic diagram of DAHT; (b) Solution cycle in DAHT
The simulation result of the DAHT is validated by the experimental data from Waseda University [14] in Japan (shown in Table 2). The relative errors of experimental and simulation data are within f5%. Table 2 Simulation model validation Item
Unit
Experiment
Simulation
Selected
TAB
ć
179.8
179.8
135
TCO
ć
25
25
23
TEV
ć
COP
澳
88.3
88
65
0.278
0.292
0.316
Fig. 2 illustrates the schematic diagram of the proposed DAHT-CO2 capture system. The new system consists of the power generation subsystem, the DAHT subsystem, the CO2 capture subsystem and the
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Dandan Wang et al. / Energy Procedia 105 (2017) 4436 – 4443
flue gas heat recovery subsystem. After heat recovery and desulfurization, the flue gas from the power plant is sent to the CO2 capture subsystem. In this subsystem, CO2 in the flue gas is absorbed in an absorber column and then regenerated in a stripper column. The regeneration heat energy is provided by the steam (0.27Mpa, 130ć) from the DAHT subsystem. Eighth stage extraction steam (65ć, 0.0245Mpa) from LP turbine is used as driving heat source for the DAHT. The steam is divided into two streams and then enters the generator and evaporator, respectively. After deliver heat, the streams are condensed into water and pumped to 7# low-pressure heater in the power generation plant, and then start a new steam cycle. Through the DAHT, circulating water is vaporized into saturated steam and sent to the reboiler of CO2 capture subsystem for rich solvent regeneration. Then it is condensed into water and pumped to the heat recovery subsystem to start a new cycle. The flue gas heat recovery subsystem is used to preheat the circulating water of the DAHT, which can further reduce the steam extraction from turbines. BFPT IP
LP
HP
FLUE GAS HEAT RECOVERY BOIL
CO
8#
6#
5#
DEA
3#
7#
2#
1#
CLEAN GAS SEP
EV
AB SHE2 AB/EV
STRIPPER
SHE1
REBOILER ABSORBER
CO
GE
DAHT
LIQUID CO2
CO2 CAPTURE
Fig. 2 Schematic diagram of the DAHT-CO2 capture system with flue gas heat recovery
3. Results and discussions For the 350MW power plant, the CO2 emission is 285.54 t/h. Fig. 3 illustrates the relationships between the thermal efficiency of the plant, the extraction steam flow rate and CO2 capture rate in the reference CO2 capture system and DAHT-CO2 capture system. In the reference system, when CO2 capture rate increases to 90%, the mass flow rate of the fifth stage extraction steam gradually grows to 367.8 t/h, and the net power output decreases from 332.5MW to 237.6MW. Meanwhile, the thermal efficiency drops from 41.63% to 29.75%. In the proposed system, when the CO2 capture rate increases to 90%, the net power output and the thermal efficiency drop to 247.5MW and 31%, respectively.
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(a)
(b)
Fig. 3 Relationship between the thermal efficiency of the plant, the extracted steam flow rate and CO2 capture rate. (a) Reference system; (b) Proposed system
It should be noted that because the mass flow of LP cylinder has a limit, when CO2 capture rate is more than 53.65%, the eighth stage extraction steam provided for the DAHT will be up to maximum flow rate of LP. Therefore, when CO2 capture rate continues growing, the fifth and eighth extraction steam flows should be provided at the same time. Through the DAHT, eighth stage extraction steam acts as an indirect heating source for CO2 regeneration, combined with the fifth stage extraction steam as a direct heating source, and both of them provide regeneration heat for the reboiler in CO2 stripping column. While this way will cause the increase of power consumption, compared with the mode only utilizes eighth stage extraction steam. So compared with the reference system, the efficiency enhancement of proposed system has a trend of increase first and then decrease with CO2 capture rate growth, as shown in Fig. 4(a). When CO2 capture rate is less than 53.65%, the efficiency enhancement of the proposed DAHT-CO2 capture system grows up with the increasing capture rate, and the highest promotion can be 2.06 percentage points, saving 28% of power consumption. When CO2 capture rate is greater than 53.65%, the efficiency enhancement of the proposed system will gradually decrease, since the advantage of DAHT is weakened by the addition of fifth stage extraction steam. At 90% CO2 capture rate, the power penalty of the proposed system is 9.9 MW smaller than the reference system, and the efficiency enhancement decreases to 1.25 percentage points.
(a)
(b)
Fig. 4 (a) Efficiency enhancement and CO2 capture power penalty; (b) Upgrading of low-level energy
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Therefore, for the proposed DAHT-CO2 capture system, the optimum CO2 capture rate is 53.65%. Compared with 58.7MW reduced power output of the conventional system, the new method can save 28.07% energy demand. So we choose 53.65% CO 2 capture rate to carry out further analysis. In the proposed system, a large amount of eighth stage extraction steam is required. But due to the relatively lower temperature and pressure, the generation capacity of the extraction steam (32.1MW) is still 32% lower than that of the reference system (47.2MW), which means it has less effect on power output of the whole system. Besides, the exergy of extraction steam (44.6MW) is quite smaller than that of the reference system (51.8MW), so the irreversible loss caused by steam extraction can also decrease. In addition, the flue gas heat recovery can save 11.69 MW heat energy among the whole system. Further, this paper will explain the mechanism of the energy utilization in the CO2 regeneration process of CO2 capture. Fig. 4(b) illustrates the process of low-level energy upgrading. A and H denote the energy level and heat duty of the process, respectively. The energy level A [15] is defined as the ratio of exergy to enthalpy. In a heat transfer process, the energy level can be simplified into A=1-T0/T. T0 is the environmental temperature and T is the temperature of the heat source. The reference system requires fifth stage extraction steam whose temperature is up to 290ć and the energy level is about 0.474. While the desorption temperature required in the reboiler is about 115ć ~120ć (corresponding to an energy level 0.242). It can be seen that the level difference between energy donor and acceptor is large in the reference system, which results in bigger irreversible loss in the regeneration process. In the proposed system, through the DAHT, the energy level of the low-temperature steam is enhanced from 0.118 (65ć) to 0.26 (130ć), reducing the level difference and irreversibility in CO2 regeneration. At the same time, the flue gas heat recovery unit further decreases steam extraction from turbines, and its energy level (0.309) is lower than that of reference system, which also contributes to the reduction of the irreversible loss in CO2 capture process. The results of exergy analysis are presented in Table 3. Table 3 Exergy analysis results of the reference and proposed CO2 capture system Proposed DAHT-CO2 capture system
Reference CO2 capture system
MW
%
MW
%
888.60
100
888.60
100
Boiler
482.26
54.27
482.36
54.28
Turbine
26.06
2.93
25.87
2.91
Regenerative system
25.60
2.88
28.41
3.20
Flue gas heat recovery
0.27
0.03
0
0
Cool and desulfurization
8.12
0.91
11.52
1.30
Flue gas emission
1.05
0.12
1.05
0.12
Steam condensation
2.41
0.27
12.20
1.37
CO2 separation
24.38
2.74
40.82
4.59
CO2 compression
8.62
0.97
8.62
0.97
DAHT
16.56
1.86
0
0
290.28
32.67
273.80
30.81
9.76
1.10
9.76
Exergy input Coal exergy Exergy destruction
Exergy output Net power output Liquid CO2 Exergy efficiency
33.76%
1.10 31.91%
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The exergy destruction of boiler in the two systems is of the similar value (482MW), which is the biggest item and account for 54.3% of the whole plant. In the regenerative unit, compared with reference system, the irreversible loss of the proposed system is reduced by 2.81MW, which is mainly because the exergy destruction decreases in the mixture process of circulating water. The flue gas heat recovery subsystem adds 0.03% extra exergy destruction in proposed system, while this process can reduce the exergy loss in the cooling and desulfurization process. It can be seen that 3.4MW destruction (about 0.4%) is avoided because amount of heat is recovered instead of cooled without utilization. The most obvious difference between the two systems is the exergy destruction in steam condensation and CO2 separation process. Since the smaller mass flow is sent into condenser in the proposed system, the exergy destruction of the process is 9.8MW lower than the reference system. In the CO2 separation process, due to the large level difference between energy donor and acceptor in the reference system, the irreversible loss is bigger (40.82MW) in the regeneration process. While in the proposed system, through the DAHT, the energy level of the low-temperature steam is enhanced, which reduces the level difference and exergy destruction (24.38MW). Though the DAHT process leads to extra exergy destruction (about 16.56MW) in the proposed system, owing to the upgrading of low-level energy and change in extraction steam from steam turbine, the proposed system still has advantages of higher exergy output and exergy efficiency. Compared with reference system, the exergy efficiency of proposed system is 32.67%, which is 1.85 percentage point higher than the conventional method. 4. Conclusion The study showed that the integration of the DAHT could reduce the power penalty of CO2 desorption effectively in coal-fired power plants. At 90% CO2 capture rate, the power penalty of proposed DAHTCO2 capture system was 84.98MW with 1.25 percentage points efficiency enhancement, saving 10.5% of power consumption. The efficiency enhancement of proposed system has a trend of increase first and then decrease with CO2 capture rate growth and the optimum CO2 capture rate is 53.65%. At the optimum CO2 capture rate, the capture power penalty is 42.22 MW with an efficiency enhancement of 2.06 percentage points. Compared with 58.7MW power consumption of the conventional system, the proposed system can save 28.07% energy consumption. In addition, flue gas heat recovery unit can save 11.69 MW heat energy among the whole system. The study also carried out the mechanism of energy utilization and exergy analysis. This proposed system could upgrade the steam in low energy level and it matched the regeneration temperature much well. So upgrading of low-level energy and decrease in level difference between energy donor and acceptor are the main ideas of the proposed DAHT-CO2 capture system. The exergy destruction in CO2 separation process of proposed system could decrease to 24.38MW, which is 40.3% lower than that of reference system. In addition, flue gas heat recovery could avoid 3.4MW exergy destruction in the cooling and desulfurization process. Thus, the proposed system provides a new approach for energy saving and CO2 emission reduction, which can effectively improve the thermal and exergy efficiency of the coal-fired power plants with CO2 capture. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51306185) and Youth Innovation Promotion Association CAS.
Dandan Wang et al. / Energy Procedia 105 (2017) 4436 – 4443
References
[1] Statistics, I.E.A., 2012. Electricity Information 2012. International Energy Agency 2012. [2] Rao A B, Rubin E S. A technical, economic, and environmental assessment of amine-based CO2 capture technology for power plant greenhouse gas control. J Environ Sci Technol 2002; 36(20): 4467-75. [3] Freguia S, Rochelle G T. Modeling of CO2 capture by aqueous monoethanolamine. J AlChE J 2003; 49(7): 1676-86. [4] Freeman S A, Dugas R, Van Wagener D H, et al. Carbon dioxide capture with concentrated, aqueous piperazine. J Int J Greenh Gas Con 2010; 4(2): 119-24. [5] Nwaoha C, Saiwan C, Tontiwachwuthikul P, et al. Carbon dioxide (CO 2) capture: Absorption-desorption capabilities of 2amino-2-methyl-1-propanol (AMP), piperazine (PZ) and monoethanolamine (MEA) tri-solvent blends, J J Nat Gas Sci Eng 2016; 33:742-50. [6]Abu-Zahra M R M, Schneiders L H J, Niederer J P M, et al. CO2 capture from power plants: Part I. A parametric study of the technical performance based on mono-ethanolamine. J Int J Greenh Gas Con 2007; 1(1): 37-46. [7] Liu B, Zhang Y, Li C, et al. Simulation and optimization of CO2 capture in power plant based on amine method. J Chem Ind Eng Prog 2009; S1: 293-6. [8] Alabdulkarem A, Hwang Y, Radermacher R. Multi-functional heat pumps integration in power plants for CO2 capture and sequestration. J Appl Energ 2015; 147: 258-68. [9] Duan L, Zhao M, Yang Y. Integration and optimization study on the coal-fired power plant with CO2 capture using MEA. J Energy 2012; 45(1): 107-16. [10] Zhang K, Liu Z, Li Y, et al. The improved CO2 capture system with heat recovery based on absorption heat transformer and flash evaporator. J Appl Therm Eng 2014; 62(2): 500-6. [11] Parham K, Khamooshi M, Tematio D B K, et al. Absorption heat transformers–a comprehensive review. J Renew Sust Energ Rev 2014; 34: 430-52. [12] Huang B, Xu S, Gao S, et al. Industrial test and techno-economic analysis of CO2 capture in Huaneng Beijing coal-fired power station. J Appl Energ 2010; 87(11): 3347-54. [13] Mostofizadeh C, Kulick C. Use of a new type of heat transformer in process industry. J Appl Therm Eng 1998; 18(9): 857-74. [14] Saito K, Inoue N, Nakagawa Y, et al. Experimental and numerical performance evaluation of double-lift absorption heat transformer. J Sci Technol Built En 2015; 21(3): 312-22. [15] Jin H, Lin R. Cascade Utilization of Energy and Gas Turbine Total Energy system. 1st ed. Beijing: Science Press; 2008.
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ScienceDirect Energy Procedia 105 (2017) 4436 – 4443
The 8th International Conference on Applied Energy – ICAE2016
Upgrading low-temperature steam to match CO2 capture in coal-fired power plant integrated with double absorption heat transformer Dandan Wanga,b, Feng Liua,b, Sheng Lib,*, Lin Gaob, Jun Suib a
b
University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District, Beijing100049, China Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11 Beisihuanxi Rd, Beijing 100190, China
Abstract The energy level mismatch between extraction steam from turbines and heat for CO2 regeneration always results in huge exergy destruction and low thermal efficiency in post combustion power plants with CO2 capture. To solve the problem, a new CO2 capture system driven by a double absorption heat transformer is proposed in this paper. Through the absorption heat transformer, low-temperature steam can be upgraded into high level heat to match the temperature of solvent regeneration. Also, flue gas heat is recovered to preheat the circulating water to further decrease system power penalty. Aspen Plus11.0 is used to simulate the system, and the results of the key process are validated by experimental values. It is shown that with 90% CO2 capture, the thermal efficiency of the proposed system is enhanced by 1.25 percentage points. And the efficiency enhancement of proposed system has a trend of increase first and then decrease with the CO2 capture rate growth. For a 350MW coal-fired power plant, the optimum CO2 capture rate is 53.65% and the corresponding efficiency enhancement is 2.06 percentage points. Exergy analysis shows that due to better match between the upgraded steam and CO2 stripping temperature, the exergy destruction in CO2 separation process of proposed system could decrease by 40.3%, and thereby the exergy efficiency of proposed system is 33.76%, which is 1.85 percentage points higher than the conventional method. ©©2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Keywords: CO2 capture˗Double absorption heat transformer˗Flue gas heat recovery˗Process simulation
1. Introduction Carbon dioxide (CO2) has the highest contribution to global warming compared to other greenhouse gases. Since coal-fired power plant is one of the main CO2 emission source, accounting for 43% of total
* Corresponding author. Tel.: +86-10-82543163; fax: +86-10-82543151. E-mail address: [email protected].
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.941
Dandan Wang et al. / Energy Procedia 105 (2017) 4436 – 4443
global CO2 emissions worldwide [1]. Thereby CO2 capture from power plants is of great importance to mitigate global warming. Among many technologies, chemical absorption is thought to be a relatively mature and suitable method [2]. However, the energy consumption of this method is huge, resulting in 1115 percentage points decrease of efficiency in the coal-fired power plants with 90% CO2 capture [3]. One of the main reasons of the high energy penalty for CO2 capture is the large heat requirement for rich solvent decomposition (about 3~4GJ/ton-CO2). To reduce energy consumption, a major way is to develop new absorbents with low decomposition heat and high capacity, such as aqueous piperazine (PZ) [4], tri-solvent containing 2-amino-2-methyl-1-propanol (AMP), piperazine (PZ) and monoethanolamine (MEA) [ 5 ], etc. Reducing water contents in CO2 rich solvent is another solution to decrease heat consumption. Abu-Zahra [ 6 ] found the thermal energy requirement decreased substantially with increasing MEA concentration, and at 40 wt.% MEA solution concentration, the thermal energy requirement is 23% lower than the base case of 30 wt.%. Another reason of the high energy penalty for CO2 capture is the truth that the temperature of the steam extracted from the turbines is usually unmatched with that of the reboiler, especially for capture retrofit of an existing power plant. Typically, the regeneration energy is provided by steam extracted from the intermediate pressure (IP) cylinder of steam turbine, and the temperature of the steam in appropriate pressure is usually over 200ć, which is much higher than the desorption temperature of rich solvent (115~120ć) [7]. The large temperature difference between energy donor (steam extracted) and acceptor (solvent) leads to a significant reduction in power output and thermal efficiency. To solve this mismatch problem, CO2 capture integrated with heat pumps seems to be attractive. Heat pumps can be used to upgrade low temperature heat into high temperature. There are two popular types of heat pumps: compression systems and absorption systems [8]. For a compression system, main power is consumed to compress refrigerant to increase its temperature and pressure. The absorption system doesn’t involve a compression process and therefore it can save a great deal of power consumption compared with compression system. Absorption systems can be divided in two types: absorption heat pumps (AHP) and absorption heat transformers (AHT). In an AHP system, high-level heat source is needed to drive it work, and low temperature heat is recovered and get temperature improved to obtain middle-level heat. The AHT system can be driven by middle- or low-temperature heat source and get part of whose temperature increased, while the penalty is another part of heat will be discharged. Duan [9] used AHP and single-stage absorption heat transformer (SAHT) to recover heat of stripper top gas and lean solvent in a 600MW power plant with CO2 capture, along with a letdown steam turbine and CO2 compression process heat recovery device, making the plant efficiency 1.9 percentage points higher than the normal method. Zhang [10] proposed a new system in which SAHT and flash evaporator were used to reduce the heat consumption of CO2 capture processes. The results showed that the new system reduced the heat consumption from 3.873GJ/t-CO2 to 3.772GJ/t-CO2. In the above-mentioned researches, since high-level heat is needed to drive an AHP, making the cost of heat recovery large. SAHTs are also used in CO2 capture, while due to the limit of refrigerant physical properties, the temperature lift is no more than 50ć and available energy is relatively small. To get a higher temperature lift, it is necessary to adopt a multi-stage absorption heat transformer, such as the two-stage absorption heat transformer (TAHT) or the double absorption heat transformer (DAHT). DAHTs have been proved to be usable in many fields, such as water purification, blow tank heat recovery and humid air turbine cycle, etc. [11]. While no relevant studies for CO2 capture combined with DAHTs have been reported in the literature works. So this paper aims to conceive a new system, using a double absorption heat transformer (DAHT) to drive CO2 capture unit in a 350MW coal-fired power plant. Low-level steam is upgraded through the DAHT to match the regeneration temperature in CO2 capture unit, which can avoid the extraction of high-
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Dandan Wang et al. / Energy Procedia 105 (2017) 4436 – 4443
level steam. In addition, the exhaust heat from boiler is utilized to preheat the circulating water from CO2 capture unit, further reducing the energy consumption of CO2 capture system in the power plant. 2. Configuration of the DAHT-CO2 capture system The base system is a 350MW supercritical coal-fired power plant in Jilin province, China. Aspen Plus 11.0 is used to simulate the performance of the plant. The results are validated using the plant operating data. The relative errors of main parameters between the model and the design plant are less than f5%. Table 1 Operating and simulation data of the base system (Operaing/Model) Parameters T, ć
Inlet of HP cylinder
Inlet of IP cylinder
1# heater
3# heater
5# heater
7# heater
566.0/566.0
566.0/566.0
251.6/240.5
176.6/172.1
118.4/113.8
60.6/59.1
24.2/24.2
4.2/4.2
26.3/26.3
30.4/30.4
3.1/3.1
3.1/3.1
P, Mpa
According to the industrial test from Beijing Thermal Power Plant of Huaneng Group [12], the heat requirement for CO2 regeneration is 3.0 GJ/ton-CO2, and the same value is designed in this paper. In order to reduce the energy level difference and irreversibility, a new DAHT-CO2 capture system is proposed. A double absorption heat transformer is integrated to upgrade the energy level of eighth stage extraction steam from low pressure (LP) cylinder into higher level for CO2 regeneration, instead of the reference mode where fifth stage extraction steam is extracted directly. The configuration and solution cycle of the DAHT [13] are shown in Fig. 1(a), (b). Circulating Water Return Steam
6
EV
7
4
P1 2
5
AB/EV P2
17 16 8
14
PAB PEV
13
SHE1 12
9
P3 11
10
CO
To User
SHE2
3 1
H 2O
P
AB 15
PCO
GE
Cool Water
TCO
澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳澳
(a)
TGE
TAE
TAB
T
(b)
Fig. 1(a) Schematic diagram of DAHT; (b) Solution cycle in DAHT
The simulation result of the DAHT is validated by the experimental data from Waseda University [14] in Japan (shown in Table 2). The relative errors of experimental and simulation data are within f5%. Table 2 Simulation model validation Item
Unit
Experiment
Simulation
Selected
TAB
ć
179.8
179.8
135
TCO
ć
25
25
23
TEV
ć
COP
澳
88.3
88
65
0.278
0.292
0.316
Fig. 2 illustrates the schematic diagram of the proposed DAHT-CO2 capture system. The new system consists of the power generation subsystem, the DAHT subsystem, the CO2 capture subsystem and the
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Dandan Wang et al. / Energy Procedia 105 (2017) 4436 – 4443
flue gas heat recovery subsystem. After heat recovery and desulfurization, the flue gas from the power plant is sent to the CO2 capture subsystem. In this subsystem, CO2 in the flue gas is absorbed in an absorber column and then regenerated in a stripper column. The regeneration heat energy is provided by the steam (0.27Mpa, 130ć) from the DAHT subsystem. Eighth stage extraction steam (65ć, 0.0245Mpa) from LP turbine is used as driving heat source for the DAHT. The steam is divided into two streams and then enters the generator and evaporator, respectively. After deliver heat, the streams are condensed into water and pumped to 7# low-pressure heater in the power generation plant, and then start a new steam cycle. Through the DAHT, circulating water is vaporized into saturated steam and sent to the reboiler of CO2 capture subsystem for rich solvent regeneration. Then it is condensed into water and pumped to the heat recovery subsystem to start a new cycle. The flue gas heat recovery subsystem is used to preheat the circulating water of the DAHT, which can further reduce the steam extraction from turbines. BFPT IP
LP
HP
FLUE GAS HEAT RECOVERY BOIL
CO
8#
6#
5#
DEA
3#
7#
2#
1#
CLEAN GAS SEP
EV
AB SHE2 AB/EV
STRIPPER
SHE1
REBOILER ABSORBER
CO
GE
DAHT
LIQUID CO2
CO2 CAPTURE
Fig. 2 Schematic diagram of the DAHT-CO2 capture system with flue gas heat recovery
3. Results and discussions For the 350MW power plant, the CO2 emission is 285.54 t/h. Fig. 3 illustrates the relationships between the thermal efficiency of the plant, the extraction steam flow rate and CO2 capture rate in the reference CO2 capture system and DAHT-CO2 capture system. In the reference system, when CO2 capture rate increases to 90%, the mass flow rate of the fifth stage extraction steam gradually grows to 367.8 t/h, and the net power output decreases from 332.5MW to 237.6MW. Meanwhile, the thermal efficiency drops from 41.63% to 29.75%. In the proposed system, when the CO2 capture rate increases to 90%, the net power output and the thermal efficiency drop to 247.5MW and 31%, respectively.
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Dandan Wang et al. / Energy Procedia 105 (2017) 4436 – 4443
(a)
(b)
Fig. 3 Relationship between the thermal efficiency of the plant, the extracted steam flow rate and CO2 capture rate. (a) Reference system; (b) Proposed system
It should be noted that because the mass flow of LP cylinder has a limit, when CO2 capture rate is more than 53.65%, the eighth stage extraction steam provided for the DAHT will be up to maximum flow rate of LP. Therefore, when CO2 capture rate continues growing, the fifth and eighth extraction steam flows should be provided at the same time. Through the DAHT, eighth stage extraction steam acts as an indirect heating source for CO2 regeneration, combined with the fifth stage extraction steam as a direct heating source, and both of them provide regeneration heat for the reboiler in CO2 stripping column. While this way will cause the increase of power consumption, compared with the mode only utilizes eighth stage extraction steam. So compared with the reference system, the efficiency enhancement of proposed system has a trend of increase first and then decrease with CO2 capture rate growth, as shown in Fig. 4(a). When CO2 capture rate is less than 53.65%, the efficiency enhancement of the proposed DAHT-CO2 capture system grows up with the increasing capture rate, and the highest promotion can be 2.06 percentage points, saving 28% of power consumption. When CO2 capture rate is greater than 53.65%, the efficiency enhancement of the proposed system will gradually decrease, since the advantage of DAHT is weakened by the addition of fifth stage extraction steam. At 90% CO2 capture rate, the power penalty of the proposed system is 9.9 MW smaller than the reference system, and the efficiency enhancement decreases to 1.25 percentage points.
(a)
(b)
Fig. 4 (a) Efficiency enhancement and CO2 capture power penalty; (b) Upgrading of low-level energy
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Therefore, for the proposed DAHT-CO2 capture system, the optimum CO2 capture rate is 53.65%. Compared with 58.7MW reduced power output of the conventional system, the new method can save 28.07% energy demand. So we choose 53.65% CO 2 capture rate to carry out further analysis. In the proposed system, a large amount of eighth stage extraction steam is required. But due to the relatively lower temperature and pressure, the generation capacity of the extraction steam (32.1MW) is still 32% lower than that of the reference system (47.2MW), which means it has less effect on power output of the whole system. Besides, the exergy of extraction steam (44.6MW) is quite smaller than that of the reference system (51.8MW), so the irreversible loss caused by steam extraction can also decrease. In addition, the flue gas heat recovery can save 11.69 MW heat energy among the whole system. Further, this paper will explain the mechanism of the energy utilization in the CO2 regeneration process of CO2 capture. Fig. 4(b) illustrates the process of low-level energy upgrading. A and H denote the energy level and heat duty of the process, respectively. The energy level A [15] is defined as the ratio of exergy to enthalpy. In a heat transfer process, the energy level can be simplified into A=1-T0/T. T0 is the environmental temperature and T is the temperature of the heat source. The reference system requires fifth stage extraction steam whose temperature is up to 290ć and the energy level is about 0.474. While the desorption temperature required in the reboiler is about 115ć ~120ć (corresponding to an energy level 0.242). It can be seen that the level difference between energy donor and acceptor is large in the reference system, which results in bigger irreversible loss in the regeneration process. In the proposed system, through the DAHT, the energy level of the low-temperature steam is enhanced from 0.118 (65ć) to 0.26 (130ć), reducing the level difference and irreversibility in CO2 regeneration. At the same time, the flue gas heat recovery unit further decreases steam extraction from turbines, and its energy level (0.309) is lower than that of reference system, which also contributes to the reduction of the irreversible loss in CO2 capture process. The results of exergy analysis are presented in Table 3. Table 3 Exergy analysis results of the reference and proposed CO2 capture system Proposed DAHT-CO2 capture system
Reference CO2 capture system
MW
%
MW
%
888.60
100
888.60
100
Boiler
482.26
54.27
482.36
54.28
Turbine
26.06
2.93
25.87
2.91
Regenerative system
25.60
2.88
28.41
3.20
Flue gas heat recovery
0.27
0.03
0
0
Cool and desulfurization
8.12
0.91
11.52
1.30
Flue gas emission
1.05
0.12
1.05
0.12
Steam condensation
2.41
0.27
12.20
1.37
CO2 separation
24.38
2.74
40.82
4.59
CO2 compression
8.62
0.97
8.62
0.97
DAHT
16.56
1.86
0
0
290.28
32.67
273.80
30.81
9.76
1.10
9.76
Exergy input Coal exergy Exergy destruction
Exergy output Net power output Liquid CO2 Exergy efficiency
33.76%
1.10 31.91%
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The exergy destruction of boiler in the two systems is of the similar value (482MW), which is the biggest item and account for 54.3% of the whole plant. In the regenerative unit, compared with reference system, the irreversible loss of the proposed system is reduced by 2.81MW, which is mainly because the exergy destruction decreases in the mixture process of circulating water. The flue gas heat recovery subsystem adds 0.03% extra exergy destruction in proposed system, while this process can reduce the exergy loss in the cooling and desulfurization process. It can be seen that 3.4MW destruction (about 0.4%) is avoided because amount of heat is recovered instead of cooled without utilization. The most obvious difference between the two systems is the exergy destruction in steam condensation and CO2 separation process. Since the smaller mass flow is sent into condenser in the proposed system, the exergy destruction of the process is 9.8MW lower than the reference system. In the CO2 separation process, due to the large level difference between energy donor and acceptor in the reference system, the irreversible loss is bigger (40.82MW) in the regeneration process. While in the proposed system, through the DAHT, the energy level of the low-temperature steam is enhanced, which reduces the level difference and exergy destruction (24.38MW). Though the DAHT process leads to extra exergy destruction (about 16.56MW) in the proposed system, owing to the upgrading of low-level energy and change in extraction steam from steam turbine, the proposed system still has advantages of higher exergy output and exergy efficiency. Compared with reference system, the exergy efficiency of proposed system is 32.67%, which is 1.85 percentage point higher than the conventional method. 4. Conclusion The study showed that the integration of the DAHT could reduce the power penalty of CO2 desorption effectively in coal-fired power plants. At 90% CO2 capture rate, the power penalty of proposed DAHTCO2 capture system was 84.98MW with 1.25 percentage points efficiency enhancement, saving 10.5% of power consumption. The efficiency enhancement of proposed system has a trend of increase first and then decrease with CO2 capture rate growth and the optimum CO2 capture rate is 53.65%. At the optimum CO2 capture rate, the capture power penalty is 42.22 MW with an efficiency enhancement of 2.06 percentage points. Compared with 58.7MW power consumption of the conventional system, the proposed system can save 28.07% energy consumption. In addition, flue gas heat recovery unit can save 11.69 MW heat energy among the whole system. The study also carried out the mechanism of energy utilization and exergy analysis. This proposed system could upgrade the steam in low energy level and it matched the regeneration temperature much well. So upgrading of low-level energy and decrease in level difference between energy donor and acceptor are the main ideas of the proposed DAHT-CO2 capture system. The exergy destruction in CO2 separation process of proposed system could decrease to 24.38MW, which is 40.3% lower than that of reference system. In addition, flue gas heat recovery could avoid 3.4MW exergy destruction in the cooling and desulfurization process. Thus, the proposed system provides a new approach for energy saving and CO2 emission reduction, which can effectively improve the thermal and exergy efficiency of the coal-fired power plants with CO2 capture. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51306185) and Youth Innovation Promotion Association CAS.
Dandan Wang et al. / Energy Procedia 105 (2017) 4436 – 4443
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