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Impacts of exhaust gas recirculation (EGR) on the natural gas combined cycle integrated with chemical absorption CO2 capture technology
Available online at www.sciencedirect.com
Energy Procedia
Energy Procedia 4 (2011) 1411–1418
Energy Procedia 00 (2010) 000–000
www.elsevier.com/locate/procedia www.elsevier.com/locate/XXX
GHGT-10
Impacts of exhaust gas recirculation (EGR) on the natural gas combined cycle integrated with chemical absorption CO2 capture technology Hailong Li1*, Geir Haugen2, Mario Ditaranto1, David Berstad1, Kristin Jordal1 2
1 SINTEF Energy, 7465 Trondheim, Norway SINTEF Materials and Chemistry, 7465 Trondheim, Norway
Elsevier use only: Received date here; revised date here; accepted date here
Abstract Increasing CO2 concentration in exhaust gas is a potentially effective method to reduce the high electrical efficiency penalty caused by chemical absorption. By varying the exhaust gas recirculation (EGR) ratio, the exhaust gas mass flow and CO2 concentration fed to the chemical absorption unit change. The impacts of EGR applied to a combined gas turbine cycle were investigated quantitatively on the energy demand of MEA-based chemical absorption. Simulations show that compared to a combined cycle without EGR, a recircualtion ratio of 50% could increase CO2 concentration from 3.8mol% to 7.9mol% and reduce the mass flow of the absorber feed stream by 51.0%. Correspondingly, the total thermal energy consumption of the reboiler is reduced by 8.1%. From the aspect of electrical efficiency, the optimized EGR ratio is about 50%, which can increase the overall efficiency by 0.4 percentage point of NG LHV, compared to the system without EGR. In addition, EGR reduces the O2 concentration in exhaust gas. On one hand, the low oxygen concentration may have negative effects on combustion stability and completeness, which can be offset by oxygen enrichment or novel combustor configuration; but on the other hand, it may result in positive effects on the reductions of NOx emission and amine degradation. c 2010 ⃝ 2011 Elsevier PublishedLtd. by All Elsevier © rightsLtd. reserved
Keywords: Exhausted gas recirculation, Chemical absorption, Electrical efficiency, Combustion, Combined cycle
1. Background CO2 capture and storage (CCS) is one of the most important technologies for the CO2 emission mitigation. In particular, the electricity sector, with large point sources of CO2, offers opportunities to apply CCS at a large scale. So far, the main barriers concerning the applications of CCS are high electrical efficiency penalty and high CO2 avoidance cost. Therefore, the integration of novel power generation cycles and variable CO2 capture technologies is an essential topic to bring CO2 capture technologies closer to realization.
* Corresponding author. Tel.: +47-735 91608; fax: +47-735 92889. E-mail address: [email protected].
doi:10.1016/j.egypro.2011.02.006
21412
H. Li et al. // Energy Procedia00 4 (2011) 1411–1418 Author name Energy Procedia (2010) 000–000
Application of exhaust gas recirculation (EGR) has various implications, such as lower exhaust mass flow and higher CO2 concentration in the exhaust gas. The topic has been discussed qualitatively and a common conclusion is that EGR may improve the performance of a power generation system integrated with chemical absorption CO2 capture technology [1, 2]. Peeters et al. [3] predicted the potential future performance of post-combustion CO2 absorption in combination with an NGCC. In this study, EGR was considered as an important technology which can reduce energy penalty of CO2 capture, improve the net cycle efficiency and lower capital cost, cost of electricity (COE) and CO2 avoidance cost in the short, medium and long terms. However, how EGR impacts on the system performance still remains unclear because few quantitative analyses are available. This paper investigated the impacts of EGR on the energy efficiency quantitatively, and on the combustion and turbo-machinery by simulating a natural gas combined cycle (NGCC) integrated with MEA chemical absorption technology in PRO/II [4]. 2. Impacts of CO2 concentration on the energy demand of chemical absorption Currently, the technology of reactive absorption by amines, e.g. monoethanolamine (MEA), methyldiethanolamine (MDEA), has been commercialized [5-8]. Such a process takes advantage of CO2 chemical absorption which enhances absorption rates and hence can be used with relatively low CO2 partial pressure in exhaust gases. The main disadvantages of chemical absorption arise from high amount of thermal energy needed to regenerate the solvent and extract the CO2, problems with corrosion and with solvent degradation. For MEA-based chemical absorption CO2 capture technology, the efficiency penalty is mainly caused by the reboiler duty of the stripping column. The relationship between the specific reboiler duty (Q) and CO2 concentration were investigated by simulating the chemical absorption process with ProTreat [9]. Results were illustrated in Figure 1. 8.0
Specific reboiler duty (MJ/kg CO2)
7.5
MEA (30wt%) Loading 0.25 Stripper pressure 1.8 bar CO2 capture ratio 90% Simulation tool: ProTreat
7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 0
5
10
15
20
25
30
CO2 concentration of flue gas (mol%)
Figure 1 Energy demand of stripper at different feed CO2 concentrations An interesting observation is that for CO2 concentration increase from 1 to 6 mol%, the specific reboiler duty decreases considerably, from 7.5 to 3.76 MJ/kg CO2. The energy demand continues to drop, but mildly until the CO2 concentration reaches about 12 mol%. Above this level the energy demand increases slightly. The drop of energy demand with increasing CO2 concentration is mainly caused by the higher CO2 partial pressure, which increases driving forces and hence favours the capture reaction. The slightly increased specific reboiler duty after 12mol% CO2 concentration is due to the reduced absorber performance inflicted by increasing temperature in the absorber (up to 90°C). Absorption heat is counteracted with water vaporization and at higher CO2 concentrations and less
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H. Li et al.name / Energy Procedia 4 (2011) 1411–1418 Author / Energy Procedia 00 (2010) 000–000
water vaporizes due to smaller gas-to-liquid ratios. Hence, the absorber performance declines due to less driving force in the column at higher temperatures, even though the partial pressure of CO2 was substantially higher. By correlating the calculated results, the following equation is used to predict the energy demand at different feed CO2 concentration:
Q
3.3162 0.0154 yCO 2 2.0383 / yCO 2 2.1432 /( yCO 2 ) 2 (MJ/kg CO2)
(1)
where yCO2 is CO2 concentration in percentage ( yCO2 mol%). 3. Impacts of EGR on combined cycle Exhaust gas recirculation provides an option for increasing CO2 concentration in the CO2 chemical absorption feed and reducing the volume flow at the same time. A system scheme of the gas turbine cycle integrated with EGR and chemical absorption is shown in Figure 2, together with the key input parameters for the simulations. After passing through the bottoming cycle, a partial stream of the exhaust gas is extracted downstream the exhaust gas condenser, and mixed with the inlet air upstream the compressor. In the present study, EGR ratio is defined as: EGR
volume flow of recirculated exhaust gas volume flow of exhaust gas
Compressor
(2) after condensation
Gas Turbine
EGR Cycle + Post-combustion Capture
TIT=1250°C PR=20
Combustor
4bar
27bar
111bar
Generator
Fuel Air CO2 Stream Water Absorbent Coolant
HRSG Ventilation Absorber
To dehydration, compression and transport Stripper
Pump Reboiler Condenser Figure 2 Scheme of a combined cycle integrated with EGR and amine-based CO2 chemical absorption Based on simulation results, Figure 3 shows the CO2 concentration and the mass flow of the exhaust gas entering the absorption column at different recirculation ratios. As can be observed, CO2 concentration increases while the mass flow decreases significantly with increasing EGR ratio. Compared to a combined cycle without EGR, a
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H. Li et al. // Energy Procedia00 4 (2011) 1411–1418 Author name Energy Procedia (2010) 000–000
CO2 concentration in exhaust gas (mol%)
11
52
Pressure ratio of gas turbine: 20 o Turbine inlet temperature: 1250 C
10
48
9
44
8
40
7
36
6
32
5
28
4
24
CO2 concentration Mass flow of exhaust gas going into absorber
3
20
2
16 0
10
20
30
40
50
Mass flow of exhausted gas (kg/s) (per kg/s fuel)
recircualtion ratio of 50% could increase CO2 concentration from 3.8mol% to 7.8mol% and reduce the mass flow of the absorber feed stream by 51.0%. These effects may have advantageous effects on amine-based CO2 capture: the total thermal energy consumption of the reboiler is reduced by 8.1% (Figure 5); moreover, the reduction of mass flow may allow for smaller sizes of absorption and stripper columns, which will contribute to reduced capital cost. According to Røkke et al. [1], an EGR ratio of 50% could reduce CAPEX of an amine plant by 21.1%.
60
EGR ratio (%)
Figure 3 CO2 concentration and mass flow of the exhaust gas going into absorber at different EGR ratios In order to ensure a complete combustion, in this study it is assumed that the minimum allowed excess O2 fraction is 3mol% [10]. This figure is taken from conventional boiler use which ensures complete combustion of the fuel; however, gas turbine combustors have other constraints in terms of geometrical design configuration and aerodynamic cooling, which may make the combustor subject to substantial redesign. Figure 4 shows the O2 concentration at the combustor outlet. As can be observed, CO2 concentration increases while O2 concentration decreases along with the increase of EGR. Therefore, from Figure 4, O2-enriched air is required instead of normal air for an EGR ratio higher than 55%. In this paper, 99mol% O2 is used and the specific energy consumption of the air separation unit (ASU) is assumed to be 0.89 MJ/kg [11].
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H. Li et al.name / Energy Procedia 4 (2011) 1411–1418 Author / Energy Procedia 00 (2010) 000–000 14
Pressure ratio of gas turbine: 20 o Turbine inlet temperature: 1250 C
Excess O2 in exhaust gas (mol%)
12
10
8
6
4
2
0 0
10
20
30
40
50
60
EGR ratio (%)
Figure 4 Excess O2 concentrations at different EGR ratio With oxygen enrichment, the EGR ratio could principally be increased further. Figure 5 shows the calculated CO2 concentrations in exhaust gas and the corresponding energy demand of chemical absorption at EGR up to 75%. When EGR ratio is higher than 65%, CO2 concentration in exhaust gas will exceed 12 mol%, and correspondingly, the energy demand will increase, referring to Figure 1 and Equ. 1.
CO2 concentration in exhaust gas (mol%)
4.2
14 4.1 12 4.0 10 3.9 8 3.8 6 3.7
Specific Reboiler Duty (MJ/kg CO2)
4.3
CO2 concentration Energy demand of chemical absorption
16
4 3.6 0
10
20
30
40
50
60
70
80
EGR ratio (%)
Figure 5 CO2 concentrations in exhaust gas and the corresponding specific reboiler duty at different EGR ratio The electrical efficiency and the efficiency penalty caused by CO2 capture are shown in Figure 6 for different EGR ratios. It is clear that along with the increase of EGR ratio the electrical efficiency rises while the efficiency penalty decreases, as CO2 concentration is increased and the energy demand of chemical absorption is reduced. The maximum efficiency and the minimum penalty appear around EGR=50%. Beyond this point, since O2 enrichment is required to satisfy combustion, the additional energy consumption of O2 production would decrease the efficiency and increase the penalty sharply. Above EGR=65% the increase in specific reboiler duty further increases the efficiency penalty.
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H. Li et al. // Energy Procedia00 4 (2011) 1411–1418 Author name Energy Procedia (2010) 000–000 52
12
Electrical efficiency Efficiency penalty caused by CO2 capture
11
50
10
Bottoming cycle HP inlet pressure: 111bar IP inlet pressure: 27bar LP inlet pressure: 4bar
49
48
9
8
CO2 transport: 150bar
47
7
46
Efficiency penalty (LHV %)
Electrical efficiency (LHV %)
51
6
45
5 0
10
20
30
40
50
60
70
80
EGR ratio (%)
Figure 6 Electrical efficiency and efficiency penalty at different EGR ratio 4. Discussions With EGR, exhaust gas is partially replacing air as combustor temperature moderator. Because exhaust gas has different compositions and properties compared to air, several technical problems may arise, such as flame extinction, onset of thermo-acoustic instability, non-complete combustion, and alteration of the heat transfer distribution. From the viewpoint of combustion, it is also of importance to control oxygen concentration at the flame anchoring region, rather than a global concentration at the exhaust gas station. In general, the presence of EGR always leads to low oxygen concentrations. For example, compared to a cycle without EGR, an EGR ratio of 55% would decrease the O2 concentration at the combustor inlet from 21mol% to ca. 11mol%. In such a vitiated air, severe problems are prone to occur regarding combustion stability, efficiency and emission, particularly in gas turbine combustors that are characterized by high velocity and low residence time. Elkady et al. [12] studied the behaviour of combustion in a 10 bar Dry Low NOx (DLN) combustor with 35% EGR. This combustor was proposed for the plant design in order to avoid potentially challenging gas turbine operating regimes with respect to combustion. The results indicated that low oxygen concentration could ‘reduce the reaction rates, allow for combustion to spread over a large region and reduce the peak flame temperature, which is not in favour of the oxidation of CO to CO2; on the contrary, reduction in oxygen levels combined with supplementary CO2 in the oxidizer leads to changes in the heat release process via CO2 dissociation’. Ditaranto et al. [13] experimentally studied the effect of O2 vitiation in an atmospheric laboratory scale set up. Results showed that even though the flame could be sustained at O2 concentration as low as 14mol% in the oxidizer, the levels of unburned hydrocarbons and CO were excessively high when the O2 concentration reached 16mol%. According to the limit of 16mol%, Figure 7, which shows the O2 concentration after mixing with EGR, indicates that an EGR ratio exceeding 35% could be difficult to achieve for the combustor. This limit can, however, be pushed further by technical adaptation of the engine and combustor arrangement that would allow for additional O2 injection or different feed-stream distribution. For example, as also shown in Figure 7, if fuel is burned in the air without mixing with recirulated exhaust gas, O2 concentration could be maintained at a level above 18mol%. Therefore, if in the combustor, fuel could be ignited in air first and then blended with recirulated exhaust gas, a higher EGR ratio, such as 50%, could be applied, without disturbing combustion stability.
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H. Li et al.name / Energy Procedia 4 (2011) 1411–1418 Author / Energy Procedia 00 (2010) 000–000
20
O2 concentration (mol%)
18
16
14
Pressure ratio of gas turbine: 20 o Turbine inlet temperature: 1250 C
12
10
Air O2 concentration, after mixed with EGR Air O2 concentration, before mixed with EGR
8 0
10
20
30
40
50
60
EGR ratio (%)
Figure 7 O2 concentrations before/after mixing with EGR at different EGR ratios Complete and stable combustion is generally optimal at high oxygen concentrations. However, the oxygen decrease could bring other advantages. As the NOx formation is dependent on the oxygen concentration as well as the flame temperature, reducing the oxygen concentration in the combustion air may be advantageous with respect to formation of NOx. Meanwhile, when exhaust gas is recycled, minor species as NOx or CO are also re-introduced into the combustion zone which tends to reduce their concentrations through, for example, the NO reburning mechanism [13]. Also according to the results of Elkady et al. [12], ‘NOx emissions decrease with increasing EGR levels by more than 50% with 35% EGR’. In addition, for a chemical absorption process, the presence of oxygen could cause degradation of amines, and the by-products of which could lead to corrosion problems. Hence, EGR could also minimize the need for chemical inhibitors or process modification involving deoxidation of the CO2-rich amine [14]. EGR can vary the gas turbine inlet mass/volume flows, which decrease with increasing EGR ratio. The decrease of gas turbine inlet mass flow can be explained as following: the recirculated exhaust gas has a higher temperature than ambient air; because the combustion temperature has been assumed as a constant, a smaller mass flow is required when some of the colder air is replaced by the hotter exhaust gas. EGR has larger impacts on volume flow because CO2 has a higher molecular weight than oxygen; exhaust gas has a higher density than air. However, an EGR ratio of 50% reduces the mass flow only by 1.04wt% and correspondingly the volume flow by 1.61vol%. Hence, for gas turbines and compressors, it is likely that the aerodynamics and thermodynamic changes due to the changes in composition and mass/volume flow of the working fluid could be handled without modifications. 5. Conclusions EGR enables the increase of the CO2 concentration in exhaust gas. Compared to a cycle without EGR, a recircualtion ratio of 50% could increase CO2 concentration from 3.8mol% to 7.9mol% and reduce the mass flow of the absorber feed stream by 51.0%. As a result, the total thermal energy consumption of amine-based CO2 capture is reduced by 8.1%. However, the energy demand of chemical absorption is not strictly reduced with increasing CO2 concentration. According to the simulations carried out in this work, the optimized EGR ratio is about 50% from the aspect of electrical efficiency, which can increase the overall efficiency by 0.4 percentage point (from 50.1% to 50.5%) of NG LHV compared to the system without EGR. In addition, with increased CO2 concentration, the mass flow of exhaust gas fed to chemical absorption columns will drop. This could result in smaller sizes of absorber and stripper, which will further reduce CAPEX. Furthermore, EGR reduces the O2 concentration in exhaust gas. On one hand, it may cause negative effects on combustion stability and complete combustion; on the other hand, it may favour the reductions of NOx emission and
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H. Li et al. // Energy Procedia00 4 (2011) 1411–1418 Author name Energy Procedia (2010) 000–000
amine degradation. In order to further raise EGR ratio, technical adaptation of the engine and combustor arrangement that would allow for additional O2 injection or different feed-stream distribution is required. 6. Acknowledgement This publication has been produced with support from the BIGCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The authors acknowledge the following partners for their contributions: Aker Solutions, ConocoPhilips, Det Norske Veritas AS, Gassco AS, Hydro Aluminium AS, Shell Technology AS, Statkraft Development AS, Statoil Petroleum AS, TOTAL E&P Norge AS, and the Research Council of Norway (193816/S60). 7. References [1] Røkke P.E., Barrio M., Austegaard A., Jakobsen J.P., Mejdell T., Hoff K.A., CO2 capture at Tjeldbergodden and transport to Draugen/Heidun, SINTEF Technical Report TR R6390, 2006. [2] Botero C., Finkenrath M., Bartlett M., Chu R., Choi G., Chinn D., Redesign, optimization, and economic evaluation of a natural gas combined cycle with the best integrated technology CO2 capture, Energy Procedia 1: 3835-3842, 2009. [3] Peeters A.N.M., Faaij A.P.C., Turkenburg W.C., Techno-economic analysis of natural gas combined cycles with post-combustion CO2 absorption, including a detailed evaluation of the development potential, Int. J. Greenhouse Gas Control, 1:396-417, 2007. [4] SimSci-Esscor, http://iom.invensys.com/EN/Pages/SimSci-Esscor_ProcessEngSuite_PROII.aspx [5] Barchas R., Davis R., The Kerr-McGee/ABB Lummus Crest Technology for the Recovery of CO2 from Stack Gases. Energy Convers. Mgmt, 33(5-8):333-340, 1992. [6] Sander M.T., Mariz C.L., The Fluor Daniel ® EconamineTM FG Process: Past Experience and Present Day Focus. Energy Convers. Mgmt, 33(5-8):341-348, 1992. [7] Chapel D.G., Mariz C.L., Ernest J., Recovery of CO2 from flues gases: commercial trends. In: Proceedings of annual meeting of the Canadian Society of Chemical Engineering, Saskatoon, Canada, 1999. [8] Mimura T., Nojo T., Ijima M., Yoshiyama T., Tanaka H., Recent developments in flue gas CO2 recovery technology, In: Proceedings of 6th International Conference on Greenhouse Gas Control Technologies (GHGT-6), Kyoto, Japan, 2002. [9] Optimized Gas Treating, Inc., http://www.ogtrt.com/ [10] Shao Y., Golomb D., Brown G., Natural gas fired combined cycle power plant with CO2 capture, Energy Convers. Mgmt, 36(12), 1115-1128, 1995. [11] Bolland O., Mathieu P., Comparison of two CO2 removal options in combined cycle power plants, Convers. Mgmt. 39(16-18), 16531663, 1998. [12] Elkady A., Evulet A., Brand A., Ursin T.P., Lynghjem A., Application of Exhaust Gas Recirculation in a DLN F-Class Combustion System for Post combustion Carbon Capture, Journal of Engineering for Gas Turbines and Power, 131(3), 034505, 2009. [13] Ditaranto M., Hals J., Bjørge T., Investigation on the in-flame NO reburning in turbine exhaust gas, Proc. Combustion Institute, 32:2659-2666, 2009. [14] Chakravarti S., Gupta A., Hunek B., Advanced technology for the capture of carbon dioxide from flue gases, In: Proceedings of 1st National Conference on Carbon Sequestration, 2001.
Energy Procedia
Energy Procedia 4 (2011) 1411–1418
Energy Procedia 00 (2010) 000–000
www.elsevier.com/locate/procedia www.elsevier.com/locate/XXX
GHGT-10
Impacts of exhaust gas recirculation (EGR) on the natural gas combined cycle integrated with chemical absorption CO2 capture technology Hailong Li1*, Geir Haugen2, Mario Ditaranto1, David Berstad1, Kristin Jordal1 2
1 SINTEF Energy, 7465 Trondheim, Norway SINTEF Materials and Chemistry, 7465 Trondheim, Norway
Elsevier use only: Received date here; revised date here; accepted date here
Abstract Increasing CO2 concentration in exhaust gas is a potentially effective method to reduce the high electrical efficiency penalty caused by chemical absorption. By varying the exhaust gas recirculation (EGR) ratio, the exhaust gas mass flow and CO2 concentration fed to the chemical absorption unit change. The impacts of EGR applied to a combined gas turbine cycle were investigated quantitatively on the energy demand of MEA-based chemical absorption. Simulations show that compared to a combined cycle without EGR, a recircualtion ratio of 50% could increase CO2 concentration from 3.8mol% to 7.9mol% and reduce the mass flow of the absorber feed stream by 51.0%. Correspondingly, the total thermal energy consumption of the reboiler is reduced by 8.1%. From the aspect of electrical efficiency, the optimized EGR ratio is about 50%, which can increase the overall efficiency by 0.4 percentage point of NG LHV, compared to the system without EGR. In addition, EGR reduces the O2 concentration in exhaust gas. On one hand, the low oxygen concentration may have negative effects on combustion stability and completeness, which can be offset by oxygen enrichment or novel combustor configuration; but on the other hand, it may result in positive effects on the reductions of NOx emission and amine degradation. c 2010 ⃝ 2011 Elsevier PublishedLtd. by All Elsevier © rightsLtd. reserved
Keywords: Exhausted gas recirculation, Chemical absorption, Electrical efficiency, Combustion, Combined cycle
1. Background CO2 capture and storage (CCS) is one of the most important technologies for the CO2 emission mitigation. In particular, the electricity sector, with large point sources of CO2, offers opportunities to apply CCS at a large scale. So far, the main barriers concerning the applications of CCS are high electrical efficiency penalty and high CO2 avoidance cost. Therefore, the integration of novel power generation cycles and variable CO2 capture technologies is an essential topic to bring CO2 capture technologies closer to realization.
* Corresponding author. Tel.: +47-735 91608; fax: +47-735 92889. E-mail address: [email protected].
doi:10.1016/j.egypro.2011.02.006
21412
H. Li et al. // Energy Procedia00 4 (2011) 1411–1418 Author name Energy Procedia (2010) 000–000
Application of exhaust gas recirculation (EGR) has various implications, such as lower exhaust mass flow and higher CO2 concentration in the exhaust gas. The topic has been discussed qualitatively and a common conclusion is that EGR may improve the performance of a power generation system integrated with chemical absorption CO2 capture technology [1, 2]. Peeters et al. [3] predicted the potential future performance of post-combustion CO2 absorption in combination with an NGCC. In this study, EGR was considered as an important technology which can reduce energy penalty of CO2 capture, improve the net cycle efficiency and lower capital cost, cost of electricity (COE) and CO2 avoidance cost in the short, medium and long terms. However, how EGR impacts on the system performance still remains unclear because few quantitative analyses are available. This paper investigated the impacts of EGR on the energy efficiency quantitatively, and on the combustion and turbo-machinery by simulating a natural gas combined cycle (NGCC) integrated with MEA chemical absorption technology in PRO/II [4]. 2. Impacts of CO2 concentration on the energy demand of chemical absorption Currently, the technology of reactive absorption by amines, e.g. monoethanolamine (MEA), methyldiethanolamine (MDEA), has been commercialized [5-8]. Such a process takes advantage of CO2 chemical absorption which enhances absorption rates and hence can be used with relatively low CO2 partial pressure in exhaust gases. The main disadvantages of chemical absorption arise from high amount of thermal energy needed to regenerate the solvent and extract the CO2, problems with corrosion and with solvent degradation. For MEA-based chemical absorption CO2 capture technology, the efficiency penalty is mainly caused by the reboiler duty of the stripping column. The relationship between the specific reboiler duty (Q) and CO2 concentration were investigated by simulating the chemical absorption process with ProTreat [9]. Results were illustrated in Figure 1. 8.0
Specific reboiler duty (MJ/kg CO2)
7.5
MEA (30wt%) Loading 0.25 Stripper pressure 1.8 bar CO2 capture ratio 90% Simulation tool: ProTreat
7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 0
5
10
15
20
25
30
CO2 concentration of flue gas (mol%)
Figure 1 Energy demand of stripper at different feed CO2 concentrations An interesting observation is that for CO2 concentration increase from 1 to 6 mol%, the specific reboiler duty decreases considerably, from 7.5 to 3.76 MJ/kg CO2. The energy demand continues to drop, but mildly until the CO2 concentration reaches about 12 mol%. Above this level the energy demand increases slightly. The drop of energy demand with increasing CO2 concentration is mainly caused by the higher CO2 partial pressure, which increases driving forces and hence favours the capture reaction. The slightly increased specific reboiler duty after 12mol% CO2 concentration is due to the reduced absorber performance inflicted by increasing temperature in the absorber (up to 90°C). Absorption heat is counteracted with water vaporization and at higher CO2 concentrations and less
1413 3
H. Li et al.name / Energy Procedia 4 (2011) 1411–1418 Author / Energy Procedia 00 (2010) 000–000
water vaporizes due to smaller gas-to-liquid ratios. Hence, the absorber performance declines due to less driving force in the column at higher temperatures, even though the partial pressure of CO2 was substantially higher. By correlating the calculated results, the following equation is used to predict the energy demand at different feed CO2 concentration:
Q
3.3162 0.0154 yCO 2 2.0383 / yCO 2 2.1432 /( yCO 2 ) 2 (MJ/kg CO2)
(1)
where yCO2 is CO2 concentration in percentage ( yCO2 mol%). 3. Impacts of EGR on combined cycle Exhaust gas recirculation provides an option for increasing CO2 concentration in the CO2 chemical absorption feed and reducing the volume flow at the same time. A system scheme of the gas turbine cycle integrated with EGR and chemical absorption is shown in Figure 2, together with the key input parameters for the simulations. After passing through the bottoming cycle, a partial stream of the exhaust gas is extracted downstream the exhaust gas condenser, and mixed with the inlet air upstream the compressor. In the present study, EGR ratio is defined as: EGR
volume flow of recirculated exhaust gas volume flow of exhaust gas
Compressor
(2) after condensation
Gas Turbine
EGR Cycle + Post-combustion Capture
TIT=1250°C PR=20
Combustor
4bar
27bar
111bar
Generator
Fuel Air CO2 Stream Water Absorbent Coolant
HRSG Ventilation Absorber
To dehydration, compression and transport Stripper
Pump Reboiler Condenser Figure 2 Scheme of a combined cycle integrated with EGR and amine-based CO2 chemical absorption Based on simulation results, Figure 3 shows the CO2 concentration and the mass flow of the exhaust gas entering the absorption column at different recirculation ratios. As can be observed, CO2 concentration increases while the mass flow decreases significantly with increasing EGR ratio. Compared to a combined cycle without EGR, a
41414
H. Li et al. // Energy Procedia00 4 (2011) 1411–1418 Author name Energy Procedia (2010) 000–000
CO2 concentration in exhaust gas (mol%)
11
52
Pressure ratio of gas turbine: 20 o Turbine inlet temperature: 1250 C
10
48
9
44
8
40
7
36
6
32
5
28
4
24
CO2 concentration Mass flow of exhaust gas going into absorber
3
20
2
16 0
10
20
30
40
50
Mass flow of exhausted gas (kg/s) (per kg/s fuel)
recircualtion ratio of 50% could increase CO2 concentration from 3.8mol% to 7.8mol% and reduce the mass flow of the absorber feed stream by 51.0%. These effects may have advantageous effects on amine-based CO2 capture: the total thermal energy consumption of the reboiler is reduced by 8.1% (Figure 5); moreover, the reduction of mass flow may allow for smaller sizes of absorption and stripper columns, which will contribute to reduced capital cost. According to Røkke et al. [1], an EGR ratio of 50% could reduce CAPEX of an amine plant by 21.1%.
60
EGR ratio (%)
Figure 3 CO2 concentration and mass flow of the exhaust gas going into absorber at different EGR ratios In order to ensure a complete combustion, in this study it is assumed that the minimum allowed excess O2 fraction is 3mol% [10]. This figure is taken from conventional boiler use which ensures complete combustion of the fuel; however, gas turbine combustors have other constraints in terms of geometrical design configuration and aerodynamic cooling, which may make the combustor subject to substantial redesign. Figure 4 shows the O2 concentration at the combustor outlet. As can be observed, CO2 concentration increases while O2 concentration decreases along with the increase of EGR. Therefore, from Figure 4, O2-enriched air is required instead of normal air for an EGR ratio higher than 55%. In this paper, 99mol% O2 is used and the specific energy consumption of the air separation unit (ASU) is assumed to be 0.89 MJ/kg [11].
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H. Li et al.name / Energy Procedia 4 (2011) 1411–1418 Author / Energy Procedia 00 (2010) 000–000 14
Pressure ratio of gas turbine: 20 o Turbine inlet temperature: 1250 C
Excess O2 in exhaust gas (mol%)
12
10
8
6
4
2
0 0
10
20
30
40
50
60
EGR ratio (%)
Figure 4 Excess O2 concentrations at different EGR ratio With oxygen enrichment, the EGR ratio could principally be increased further. Figure 5 shows the calculated CO2 concentrations in exhaust gas and the corresponding energy demand of chemical absorption at EGR up to 75%. When EGR ratio is higher than 65%, CO2 concentration in exhaust gas will exceed 12 mol%, and correspondingly, the energy demand will increase, referring to Figure 1 and Equ. 1.
CO2 concentration in exhaust gas (mol%)
4.2
14 4.1 12 4.0 10 3.9 8 3.8 6 3.7
Specific Reboiler Duty (MJ/kg CO2)
4.3
CO2 concentration Energy demand of chemical absorption
16
4 3.6 0
10
20
30
40
50
60
70
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Figure 5 CO2 concentrations in exhaust gas and the corresponding specific reboiler duty at different EGR ratio The electrical efficiency and the efficiency penalty caused by CO2 capture are shown in Figure 6 for different EGR ratios. It is clear that along with the increase of EGR ratio the electrical efficiency rises while the efficiency penalty decreases, as CO2 concentration is increased and the energy demand of chemical absorption is reduced. The maximum efficiency and the minimum penalty appear around EGR=50%. Beyond this point, since O2 enrichment is required to satisfy combustion, the additional energy consumption of O2 production would decrease the efficiency and increase the penalty sharply. Above EGR=65% the increase in specific reboiler duty further increases the efficiency penalty.
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Figure 6 Electrical efficiency and efficiency penalty at different EGR ratio 4. Discussions With EGR, exhaust gas is partially replacing air as combustor temperature moderator. Because exhaust gas has different compositions and properties compared to air, several technical problems may arise, such as flame extinction, onset of thermo-acoustic instability, non-complete combustion, and alteration of the heat transfer distribution. From the viewpoint of combustion, it is also of importance to control oxygen concentration at the flame anchoring region, rather than a global concentration at the exhaust gas station. In general, the presence of EGR always leads to low oxygen concentrations. For example, compared to a cycle without EGR, an EGR ratio of 55% would decrease the O2 concentration at the combustor inlet from 21mol% to ca. 11mol%. In such a vitiated air, severe problems are prone to occur regarding combustion stability, efficiency and emission, particularly in gas turbine combustors that are characterized by high velocity and low residence time. Elkady et al. [12] studied the behaviour of combustion in a 10 bar Dry Low NOx (DLN) combustor with 35% EGR. This combustor was proposed for the plant design in order to avoid potentially challenging gas turbine operating regimes with respect to combustion. The results indicated that low oxygen concentration could ‘reduce the reaction rates, allow for combustion to spread over a large region and reduce the peak flame temperature, which is not in favour of the oxidation of CO to CO2; on the contrary, reduction in oxygen levels combined with supplementary CO2 in the oxidizer leads to changes in the heat release process via CO2 dissociation’. Ditaranto et al. [13] experimentally studied the effect of O2 vitiation in an atmospheric laboratory scale set up. Results showed that even though the flame could be sustained at O2 concentration as low as 14mol% in the oxidizer, the levels of unburned hydrocarbons and CO were excessively high when the O2 concentration reached 16mol%. According to the limit of 16mol%, Figure 7, which shows the O2 concentration after mixing with EGR, indicates that an EGR ratio exceeding 35% could be difficult to achieve for the combustor. This limit can, however, be pushed further by technical adaptation of the engine and combustor arrangement that would allow for additional O2 injection or different feed-stream distribution. For example, as also shown in Figure 7, if fuel is burned in the air without mixing with recirulated exhaust gas, O2 concentration could be maintained at a level above 18mol%. Therefore, if in the combustor, fuel could be ignited in air first and then blended with recirulated exhaust gas, a higher EGR ratio, such as 50%, could be applied, without disturbing combustion stability.
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Figure 7 O2 concentrations before/after mixing with EGR at different EGR ratios Complete and stable combustion is generally optimal at high oxygen concentrations. However, the oxygen decrease could bring other advantages. As the NOx formation is dependent on the oxygen concentration as well as the flame temperature, reducing the oxygen concentration in the combustion air may be advantageous with respect to formation of NOx. Meanwhile, when exhaust gas is recycled, minor species as NOx or CO are also re-introduced into the combustion zone which tends to reduce their concentrations through, for example, the NO reburning mechanism [13]. Also according to the results of Elkady et al. [12], ‘NOx emissions decrease with increasing EGR levels by more than 50% with 35% EGR’. In addition, for a chemical absorption process, the presence of oxygen could cause degradation of amines, and the by-products of which could lead to corrosion problems. Hence, EGR could also minimize the need for chemical inhibitors or process modification involving deoxidation of the CO2-rich amine [14]. EGR can vary the gas turbine inlet mass/volume flows, which decrease with increasing EGR ratio. The decrease of gas turbine inlet mass flow can be explained as following: the recirculated exhaust gas has a higher temperature than ambient air; because the combustion temperature has been assumed as a constant, a smaller mass flow is required when some of the colder air is replaced by the hotter exhaust gas. EGR has larger impacts on volume flow because CO2 has a higher molecular weight than oxygen; exhaust gas has a higher density than air. However, an EGR ratio of 50% reduces the mass flow only by 1.04wt% and correspondingly the volume flow by 1.61vol%. Hence, for gas turbines and compressors, it is likely that the aerodynamics and thermodynamic changes due to the changes in composition and mass/volume flow of the working fluid could be handled without modifications. 5. Conclusions EGR enables the increase of the CO2 concentration in exhaust gas. Compared to a cycle without EGR, a recircualtion ratio of 50% could increase CO2 concentration from 3.8mol% to 7.9mol% and reduce the mass flow of the absorber feed stream by 51.0%. As a result, the total thermal energy consumption of amine-based CO2 capture is reduced by 8.1%. However, the energy demand of chemical absorption is not strictly reduced with increasing CO2 concentration. According to the simulations carried out in this work, the optimized EGR ratio is about 50% from the aspect of electrical efficiency, which can increase the overall efficiency by 0.4 percentage point (from 50.1% to 50.5%) of NG LHV compared to the system without EGR. In addition, with increased CO2 concentration, the mass flow of exhaust gas fed to chemical absorption columns will drop. This could result in smaller sizes of absorber and stripper, which will further reduce CAPEX. Furthermore, EGR reduces the O2 concentration in exhaust gas. On one hand, it may cause negative effects on combustion stability and complete combustion; on the other hand, it may favour the reductions of NOx emission and
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amine degradation. In order to further raise EGR ratio, technical adaptation of the engine and combustor arrangement that would allow for additional O2 injection or different feed-stream distribution is required. 6. Acknowledgement This publication has been produced with support from the BIGCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The authors acknowledge the following partners for their contributions: Aker Solutions, ConocoPhilips, Det Norske Veritas AS, Gassco AS, Hydro Aluminium AS, Shell Technology AS, Statkraft Development AS, Statoil Petroleum AS, TOTAL E&P Norge AS, and the Research Council of Norway (193816/S60). 7. References [1] Røkke P.E., Barrio M., Austegaard A., Jakobsen J.P., Mejdell T., Hoff K.A., CO2 capture at Tjeldbergodden and transport to Draugen/Heidun, SINTEF Technical Report TR R6390, 2006. [2] Botero C., Finkenrath M., Bartlett M., Chu R., Choi G., Chinn D., Redesign, optimization, and economic evaluation of a natural gas combined cycle with the best integrated technology CO2 capture, Energy Procedia 1: 3835-3842, 2009. [3] Peeters A.N.M., Faaij A.P.C., Turkenburg W.C., Techno-economic analysis of natural gas combined cycles with post-combustion CO2 absorption, including a detailed evaluation of the development potential, Int. J. Greenhouse Gas Control, 1:396-417, 2007. [4] SimSci-Esscor, http://iom.invensys.com/EN/Pages/SimSci-Esscor_ProcessEngSuite_PROII.aspx [5] Barchas R., Davis R., The Kerr-McGee/ABB Lummus Crest Technology for the Recovery of CO2 from Stack Gases. Energy Convers. Mgmt, 33(5-8):333-340, 1992. [6] Sander M.T., Mariz C.L., The Fluor Daniel ® EconamineTM FG Process: Past Experience and Present Day Focus. Energy Convers. Mgmt, 33(5-8):341-348, 1992. [7] Chapel D.G., Mariz C.L., Ernest J., Recovery of CO2 from flues gases: commercial trends. In: Proceedings of annual meeting of the Canadian Society of Chemical Engineering, Saskatoon, Canada, 1999. [8] Mimura T., Nojo T., Ijima M., Yoshiyama T., Tanaka H., Recent developments in flue gas CO2 recovery technology, In: Proceedings of 6th International Conference on Greenhouse Gas Control Technologies (GHGT-6), Kyoto, Japan, 2002. [9] Optimized Gas Treating, Inc., http://www.ogtrt.com/ [10] Shao Y., Golomb D., Brown G., Natural gas fired combined cycle power plant with CO2 capture, Energy Convers. Mgmt, 36(12), 1115-1128, 1995. [11] Bolland O., Mathieu P., Comparison of two CO2 removal options in combined cycle power plants, Convers. Mgmt. 39(16-18), 16531663, 1998. [12] Elkady A., Evulet A., Brand A., Ursin T.P., Lynghjem A., Application of Exhaust Gas Recirculation in a DLN F-Class Combustion System for Post combustion Carbon Capture, Journal of Engineering for Gas Turbines and Power, 131(3), 034505, 2009. [13] Ditaranto M., Hals J., Bjørge T., Investigation on the in-flame NO reburning in turbine exhaust gas, Proc. Combustion Institute, 32:2659-2666, 2009. [14] Chakravarti S., Gupta A., Hunek B., Advanced technology for the capture of carbon dioxide from flue gases, In: Proceedings of 1st National Conference on Carbon Sequestration, 2001.