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International Journal of Green Energy, 6: 527–539, 2009 Copyright Taylor & Francis Group, LLC ISSN: 1543-5075 print / 1543-5083 online DOI: 10.1080/15435070903231393
SIMULATION AND OPTIMIZATION OF EVAPORATIVE GAS TURBINE WITH CHEMICAL ABSORPTION FOR CARBON DIOXIDE CAPTURE H. Li1, S. Flores1, Y. Hu1, and J. Yan1,2 1
Chemical Engineering and Technology/Energy Processes, Royal Institute of Technology, Stockholm, Sweden 2 Sustainable Development of Society and Technology, Malardalen University, Vasteras, Sweden This article studied the integration of an evaporative gas turbine (EvGT) cycle with chemical absorption for CO2 capture. Two systems of EvGT cycle without CO2 capture and EvGT cycle with CO2 capture were simulated and optimized. The impacts of key parameters such as the water/air ratio (W/A), the stripper pressure, and the flue-gas condensing temperature were studied regarding the electrical efficiency and CO2 reduction. Simulation results show that (1) there always exists an optimum point of W/A for both EvGT and EvGT combined with CCS; (2) although lowering the stripper pressure would lower the heat quality requirement of reboiler, it increases the quantity more obviously. Therefore increasing the operating pressure of stripper would help to increase the total electrical efficiency; but the efficiency improvement becomes smaller if stripper pressure is high; (3) adding a flue-gas condenser to condense out the excessive water is another method to increase the total electrical efficiency. There is also an optimum point of condensing temperature considering the concentration of mono ethanol amine (MEA) and inlet temperature of stripper; and (4) comparatively the combined cycle has a higher gross electricity generation and electrical efficiency than the EvGT cycle no matter if combined with CO2 capture or not. Keywords: Evaporative gas turbines; Humid air turbines; CO2 capture; Chemical absorption; Electrical efficiency
INTRODUCTION To mitigate the global greenhouse gas emissions, reduction of CO2 emissions in the fossil fuel–based power generation is very important. Figure 1 (Stangeland 2007) indicates that the CO2 capture and storage (CCS) may play a significant role for future mitigation targets in line with renewable energy and efficiency improvement. Integrating advanced gas turbine systems with various CO2-capture technologies have been studied as one of the CCS options. For example, the performance analysis has been carried out on the combined cycles (CC) integrated with different CO2-capture technologies (Shao et al. 1995; Bolland and Mathieu 1998; Bolland and Undrum 2003; Kvamsdal et al. 2007). A humidified-gas Address correspondence to H. Li, Chemical Engineering and Technology/Energy Processes, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. E-mail:
[email protected] 527
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Figure 1 Projections of CO2-emission reduction based on different scenarios (Stangeland 2007).
turbine is a novel gas-turbine technology. However few studies are available regarding integration of CCS with it (Li and Yan 2008). Normally humidified-gas turbines include steam injection gas turbines (STIG) and evaporative gas turbines (EvGT), which are also called humid air turbines (HAT). The driving forces for gas turbine humidification have been the potential for high electrical efficiency and specific power output and reduced specific investment cost, decreased formation of nitrogen oxides (NOx) in the combustor, reduced power output degradation caused by high ambient temperatures or low ambient pressure (i.e., at high elevations), and improved part-load performance compared with combined cycles (Jonsson and Yan 2005). In addition, compared with STIG, EvGT has a lower irreversibility because water is injected to the cycle by a humidification tower, which has a small temperature difference between the hot and cold fluids. This study is intended to analyze the integration of CO2 capture with EvGT cycles. Chemical absorption with monoethanolamine (MEA), which is based on a reversible thermal equilibrium, is suitable for dilute systems and low CO2 concentrations and can be easily applied to the existing power plants. So far it is the only commercially available option for CO2 capture (Arnold et al. 1982; Barchas and Davis 1992; Sander and Mariz 1992; Chapel et al. 1999; Mimura et al. 2002). The objectives of this article are to characterize and understand the features of the integration of EvGT with chemical absorption for CO2 capture. The impacts of key parameters such as the water/air ratio, the stripper pressure, and the flue-gas condensing temperature were studied regarding the electrical efficiency and CO2 reduction. Suggestions will be proposed concerning the optimizations of such a system. SYSTEM CONFIGURATIONS Two systems are simulated with natural gas as the fuel input. They are: System I: EvGT cycle without CO2 capture System II: EvGT cycle with chemical-absorption capture System I is the reference system when studying the electrical penalty. To achieve high cycle efficiency, waste heat is recovered for district heating from flue gas and the
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outlet streams of CO2 compressors, strippers, and dehydrators through condensers or heat exchangers.
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System I: EvGT Cycle without CO2 Capture The basic idea of the EvGT cycle is injecting water by evaporation, which will increase the mass flow rate through the turbine and consequently augment the specific power output. It has a high efficiency because the waste heat in the exhaust gas is recovered by humid air in the recuperator and by water in the economizer. The EvGT cycle has been described by Rao (1989). A system sketch of the EvGT cycle without CO2 capture is shown in Figure 2. Water is heated close to saturated by the compressed air in the aftercooler and flue gas in the feedwater heater and economizer. The heated water enters at the top of a humidification tower and is brought into countercurrent contact with the compressed air that enters at the bottom of the tower. The tower is a column with a packing that is either structured or dumped. Some of the water is evaporated and the air is humidified. The water evaporates at the water boiling point corresponding to the partial pressure of water in the mixture (i.e., water evaporates below the boiling point that corresponds to the total pressure in the tower). Therefore, low-temperature heat, which cannot be used to evaporate water in a boiler, can be recovered in an EvGT cycle. Since the water-vapor content in the air increases as the air passes upward through the tower, the boiling temperature also increases. This ensures a close matching of the air and water temperature profiles and small exergy losses compared to evaporation in a conventional steam boiler. System II: EvGT Cycle with Chemical Absorption Capture A system sketch of the EvGT cycle with chemical-absorption capture is shown in Figure 3. Different from System I, instead of being condensed in the feedwater heater, the Compressor
Gas Turbine
Combustor
Generator
Cooling Recuperator Aftercooler Economizer Feedwater Heater
EvGT Cycle Fuel Air CO2 Stream Water
Humidification tower
Coolant
Figure 2 System sketch of System I: EvGT cycle without CO2 capture.
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Gas Turbine
Combustor
Fuel Air CO2 Stream Water Absorbent Coolant
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Dehydrator
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Aftercooler Economizer Stripper Humidification tower
Absorber Condenser Stack Reboiler
To transport
Pump
Figure 3 System sketch of System II: EvGT cycle with chemical absorption capture.
flue gas enters the reboiler of the MEA stripper to support the heat required for MEA regeneration; after that it goes through the recuperator and economizer. Then it is condensed in a heat exchanger in which heat is recovered for district heating as well. After heat recovery, flue gas flows through the absorber countercurrently with the absorbent where the absorbent reacts chemically with CO2. The rich solvent containing chemically bound CO2 is sent to the top of the stripper via a lean/rich cross-heat exchanger in which the rich solvent is heated to a temperature close to the stripper operating temperature. At an elevated temperature, the chemically bound CO2 is released and MEA is regenerated in the stripper. In order to avoid corrosion and the formation of ice or hydrate in CO2 compression and transportation, the CO2 streams captured from all of systems will go through a dehydration process after which the water content would be lower than 0.1%. After dehydration and compression, the recovered CO2 will be transported to the storage reservoir through different means. INPUT DATA AND ASSUMPTIONS The compositions and properties of inlet streams and outlet streams are summarized in Table 1. The input data and assumptions for the simulations of the gas turbine, compressors, chemical absorption, and dehydration are given in Table 2. It should be pointed out that there hasn’t been a type of turbine that can be applied to both EvGT and EvGT integrated with CO2 capture. A new design or significant modification of existing gas turbines is required. This issue has been raised when introducing the EvGT or HAT cycle as a new power generation system (Jonsson and Yan 2005). Further, the study of the turbomachinery of the new cycles is necessary, though this is not the main focus of this article. All systems have been simulated in Aspen Plus 2006 (Aspen 2006). For the chemicalabsorption capture process, the RADFRAC model is used for the absorber and stripper columns. Meanwhile the thermodynamic and transport properties are modeled using a
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Table 1 Compositions and properties of feed streams and outlet streams. Fuel stream CH4 LHV T P Air stream T P Composition N2 O2 Ar CO2 Relative humidity CO2 stream to be transported T P
% MJ/kg C bar C bar
vol% vol% vol% vol% % C bar
100 50 15 1 15 1 76.99 20.65 0.921 0.04 60 20 150
Table 2 Input data and assumptions for the simulations of gas turbine, compressors, chemical absorption, and dehydration. Parameter Turbine Pressure ratio TIT Mechanical efficiency Compressors Type Isentropic efficiency Intercooling T Stage number of air/oxygen compression Stage number of CO2 compression Mechanical efficiency Chemical absorption Solvent Solvent loading Pressure drop in absorption column
Unit
Value
20 1250 99
C %
Isentropic 85 40 2 3 98
% C
%
mbar
MEA (30 wt%) 0.3 150 (Kvamsdal et al. 2007)
Dehydration Dryer Operating P of dehydration Operating P of regeneration Operating T of regeneration
bar bar C
Triethylene glycol (99 wt%) 20 1 204 (Nivargi et al. 2005)
Other assumptions Pump mechanical efficiency Pressure drop in humidification tower Tmin gas/gas Tmin gas/liquid Tmin liquid/liquid Supplying T for district heating CO2 capture ratio of chemical absorption*
% % C C C C %
90 5 (Jonsson and Yan 2003) 30 (Jonsson and Yan 2003) 20 (Jonsson and Yan 2003) 15 (Jonsson and Yan 2003) 60 90
*CO2 capture ratio (CCR) is defined as: CCR ¼ be processed is indicated as Stack in the Figure 3.
MolCO2 MolMEA
ðMole flow · CO2 Mole fractionÞCO To be transported 2 ðMole flow·CO2 Mole fractionÞFlue gas to be processed
. Here the flue gas to
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so-called MEA property insert, which describes the MEA-H2O-CO2 system with an electrolyte-NRTL model (Abu-Zahra et al. 2007). In addition, the Predictive-SoaveRedlich-Kwong (PSRK) equation of state (EOS) is used for the calculations on thermodynamic properties in combustion, compression, and other processes based on our previous studies (Li et al. 2007; Li and Yan 2009). Obviously, for the cycles of MEA and the dryer in the chemical-absorption and dehydration processes respectively, input streams are the output streams; therefore the chemical absorption and dehydration processes are simulated using closed-loop flow sheets. The loop structures make the simulations more difficult to converge. However it is important to do so to make the results realistic.
THE IMPACTS OF KEY PARAMETERS ON SYSTEM OPERATION Like other gas turbines, the electrical efficiency of EvGT cycles can be improved by some methods, such as raising turbine inlet temperature (TIT) and lowering the cooling temperature of the intermediate stage of air compression. In order to make our study more specific to EvGT, we investigated the following parameters: Water/Air Ratio (W/A) The water/air ratio is defined as: This is of great importance to the electrical efficiency of an EvGT cycle. For every case with constant combustion air temperature, there always exists an optimum point (Yan et al. 1995). Stripper Pressure (STP) It has been verified that increasing the stripper operating pressure would increase the stripper operating temperature, while decreasing the reboiler specific heat demand required for MEA regeneration (Abu-Zahra et al. 2007). This implies that at a higher operating pressure the reboiler of stripper may need less quantity but higher quality heat. Therefore from the view point of exergy, the heat recovered back to the combustor and the thermal energy requirement shall be carefully arranged with under consideration of temperature matching in the heat exchanging processes, especially those taking place in the humidification tower and stripper, which are in the similar temperature range. It may result in different overall efficiency of the cycle. Flue Gas Condensing Temperature (FCT) Compared to the conventional gas-turbine cycles, more water is contained in stack of the EvGT cycle. The excessive water would dilute the MEA solvent and result in a higher reboiler specific heat requirement. Decreasing the water content in the CO2 stream can help reduce the thermal energy requirement of the reboiler. Therefore the stack should be condensed before entering the absorber. The variation of condenser temperature would first change the reboiler specific-heat requirement of stripper and further affect the distribution of heat recovery in the humidification and economizer. As a result, the total efficiency would change.
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WATER/AIR RATIO Figure 4 shows the results of the optimization of the water/air ratio regarding the EvGT with and without CO2 capture. Both electrical efficiencies first rise and then drop along with the increase of W/A. It has been mentioned that there is an optimum point. This point occurs when both the air temperature after the recuperation reaches the highest value, and, at the same time, the stack temperature is at the lower limit. For the system without CO2 capture, the highest efficiency (52.1%) was reached at the W/A with a value of 0.14; while the electrical efficiency is about 51.6% if W/A is set at the value of the base case. For the system with CO2 capture, the highest efficiency is 41.3%, which appears as W/A is 11.5%. Figure 5 shows the electrical efficiency penalty caused by CCS at different W/A, which rises along with the increase of W/A. This is different from the W/A impacts on the total efficiency. Figure 6 shows the actual CCR at different W/A. As described above, the stack should be condensed to avoid diluting the solvent before it goes into absorber. Some CO2 would dissolve in the condensed water, and consequently it results in a lower actual CCR. At the same condensing temperature, the amount of condensed water varies with different W/A. Therefore, actual CCR decreases along with the increase in W/A. However, from Figure 6, W/A impacts on CCR are rather small. Stripper pressure. Figure 7 shows the specific energy requirement to capture 1 ton CO2 and the reboiler temperature at different STP. Generally along with the rise in stripper pressure, the specific energy requirement of the cycles decreases while the reboiler
52 50 Efficiency (%)
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First the water/air ratio was optimized for EvGT with and without CCS. At the beginning, the flue-gas condensing temperature is set as 40 C (Li and Yan 2008), and the stripper pressure is set as 1 bar (Li and Yan 2008). After a parameter is optimized, the optimal value would be used instead of the initial value in the following simulations.
48 46
EvGT without CCS EvGT with CCS FCT = 40°C STP = 1bar
44 42 0.09 0.10 0.11 0.12 0.13 0.14 0.15 0.16 0.17 Water/Air Figure 4 Electrical efficiency of EvGT without CO2 Capture at different water/air ratios.
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Efficiency penalty caused by CCS (%)
FCT = 40°C; STP = 1bar
12.0 11.5 11.0 10.5 10.0 9.5 0.11
0.12
0.13 W/A (kg/kg)
0.14
0.15
Figure 5 Electrical efficiency penalty caused by CCS at different water/air ratios.
91.0 90.8 CO2 Capture Ratio (CCR)
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12.5
FCT = 313K; STP = 0.1MPa
90.6 90.4 90.2 90.0 89.8 89.6 89.4 89.2 89.0 0.11
0.12 0.13 W/A (kg/kg)
0.14
Figure 6 Actual CRR at different water/air ratios.
temperature increases. In addition, compared with its reboiler temperature, the specific energy requirement is more sensitive to the variation of STP if STP is lower than 1 bar. Considering the temperature match in the heat exchangers, we have two configurations (Figure 8) for the humidification tower and CO2 capture regarding the different heat quality and quantity requirements. At high STP, for example 2.5 bar, the reboiler temperature is around 128 C, which is higher than the temperature demand of the humidification tower. Thus configuration 1 would be applied. On the contrary, at low STP, for example 0.7 bar, the reboiler temperature is around 93 C, which is lower than temperature demand of humidification tower. Thus configuration 2 would be applied. According to the above strategy, the results of electrical efficiency at different STP are shown in Figure 9. Although low-quality heat can be used when STP is low, and exergy
535
4.5 130 4.4
Reboiler Specific Heat Requirement Reboiler Temperature
125
4.3 120 4.2
115
4.1
110
W/A = 11.5% FCT = 40°C
4.0
105
3.9
100
3.8
95
Reboiler Temperature (°C)
Reboiler Specific Heat Requirement (GJ/ton CO2)
90
3.7 0.5
1.0
1.5 2.0 Stripper Pressure (bar)
2.5
Figure 7 Specific energy requirement and reboiler temperature at different STP. From LP Turbine
To CO2 Capture
Recuperator
Economizer
CO2 Capture District Heating
Configuration 1 To CO2 Capture
From LP Turbine P-16 Recuperator
Economizer I
CO2 Capture Economizer II District Heating
Configuration 2
Figure 8 Configuration of heat exchangers. 42.0 41.5 Electrical Efficiency (%)
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41.0 W/A = 11.5% FCT = 40°C 40.5 40.0 39.5 39.0 0.5
1.0
1.5 2.0 Stripper Pressure (bar)
Figure 9 Electrical efficiency at different STP.
2.5
41.6
3.90
41.4
3.85
41.2
3.80 3.75
41.0
3.70
40.8
3.65 40.6 Reboiler Duty Electrical Efficiency
40.4 25
30
35
40 45 50 55 Flue Gas Condenser T (°C)
3.60
60
3.55 65
Reboiler Specific Heat Requirement (GJ/ton CO2)
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Electrical Efficiency (%)
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Figure 10 Reboiler specific heat requirement at different condenser temperatures.
loss may be reduced by applying different configurations of heat exchangers, as electrical efficiency grows with the rise in stripper pressure. The reason is due to the fast increase of the reboiler specific-heat requirement at low STP. Therefore high STP is helpful to improve the total efficiency. Moreover it is similar to the STP impacts on reboiler specific-heat requirement that efficiency is less sensitive to the variation of STP when it is over 2 bar. Considering the relation between pressure and investment cost, we chose 2 bar in the following simulations. Flue gas condensing temperature. Figure 10 shows the reboiler specific-heat requirement of the stripper at different condenser temperatures. When the condenser temperature drops from 60 C to 50 C, more water is condensed, so the reboiler specific-heat requirement is reduced. However when the condenser temperature drops further, reboiler specific-heat requirements may increase. The reason might be that the low condenser temperature would cause a low input temperature of the reboiler. Although less water is contained, the larger temperature difference between inlet temperature and operation temperature will increase the reboiler specific heat requirement. Figure 10 also shows the electrical efficiency at different condenser temperatures. FCT has a reverse impact on efficiency compared its impact on reboiler specific heat requirement. In this study the highest electrical efficiency appears with a value of 41.6% when FCT is 50 C.
DISCUSSIONS Comparison with Combined Cycle Kvamsdal et al. (2007) investigated the performance of combined cycles with various CO2-capture technologies. In Table 3, a comparison of electricity generation and internal electricity consumption is shown between the CC and EvGT cycles. It is clear
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Table 3 Comparison on electricity generation and internal electricity consumption is shown between combined cycle and evgt cycle (in percentage of fuel).
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Without CCS
Gross electricity generation Air/fuel compressors CO2 compressors Pumps Auxiliaries Net electricity efficiency
MEA absorption
CC
EvGT
CC
EvGT
95.2% 37.6% – 0.3% 0.6% 56.7%
90.2% 38.0% – 0.03% – 52.1%
90.9% 37.6% 2.3% 0.6% 2.5% 47.9%
77.9% 34.1% 2.2% 0.03% – 41. 6%
that the CC always has a higher gross electricity generation, net electricity efficiency, and internal power consumption of air/fuel compressors but similar electricity consumptions of CO2 compressors compared with the EvGT cycle. In addition, Kvamsdal (2007) included some electricity consumptions of auxiliaries, like an amine absorption column, while it is ignored in our study. Generally the electrical efficiency penalties caused by CCS are 8.8% and 10.54% for CC and EvGT, respectively. Comparison on the Penalty of Electrical Efficiency with Literature Results Table 4 shows the comparison on the penalty of electrical efficiency caused by CO2 capture between our results and the data from the literature. It can be found that the efficiency penalties of the EvGT cycle are higher than those of CC. Economic Issues As discussed above, increasing the operating pressure of the tripper and adding a fluegas condenser would help to increase the total electrical efficiency. However the increased pressure and additional condenser would raise the investment cost at the same time. So the economic comparison should be carried out in future work to understand the integration of EvGT with CO2 capture more comprehensively.
CONCLUSIONS The integration of evaporative gas turbine (EvGT) cycle with chemical absorption CO2-capture technology has been investigated. Based on the simulation results, the Table 4 Comparison on the penalty of electrical efficiency caused by co2 capture from different sources (%). CO2 capture technology Chemical absorption
Source
Value
IEA (2000) Desideri and Paolucci (1999) Kvamsdal et al. (2007) System II
9.0 7.4 8.8 9.4
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impacts of several important parameters, such as the water/air ratio, stripper pressure, and flue-gas condenser temperature, are analyzed. It is concluded that: 1. There exists an optimum point of W/A for both EvGT and EvGT combined with CCS. As TIT = 1,250 C and PR = 20, the optimal W/A are 14.0% and 11.5%, respectively. 2. Increasing the operating pressure of the stripper would help to increase the total electrical efficiency; but the efficiency improvement becomes smaller if stripper pressure is too high. 3. Adding a flue gas condenser that condenses out the excessive water is another method to increase the total electrical efficiency. There is also an optimum point for the condensing temperature considering the concentration of MEA and inlet temperature of stripper. 4. Based on the optimization results, the EvGT cycle integrated with chemical absorption for CO2 capture has an electrical efficiency of 41.6%, as W/A is 0.115, the stripper pressure is 2 bar, and the flue-gas condenser temperature is 50 C. 5. Comparatively, the combined cycle has a higher gross electricity generation and electrical efficiency than the EvGT cycle no matter if combined with CO2 capture or not.
ACKNOWLEDGMENTS Financial supports from the Swedish Energy Agency (STEM), SINTEF (Norway) and the European Union are gratefully acknowledged.
NOMENCLATURE k P R T W
Specific heat ratio Pressure Specific gas constant Temperature Turbine work Efficiency
Abbreviation CC Combined cycle CCR CO2-capture ratio CHP Combined heat and power EOS Equation of state EvGT Evaporative gas turbine FCT Flue gas condenser temperature HAT Humid air turbine Hum Humidification LHV Lower heating value MEA Mono ethanol Amine PR Pressure ratio PSRK Predictive-Soave-Redlich-Kwong STIG Steam injection gas turbines STP Stripper pressure TIT Turbine inlet temperature W/A Water/Air ratio
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