Energy Fuels 2010, 24, 728–736 Published on Web 12/15/2009
: DOI:10.1021/ef900740p
Integration of Carbonate CO2 Capture Cycle and Coal-Fired Power Plants. A Comparative Study for Different Sorbents Pilar Lisbona, Ana Martı´ nez, Yolanda Lara, and Luis M. Romeo* Centro de Investigaci on de Recursos y Consumos Energ eticos (CIRCE). Universidad de Zaragoza. Centro Polit ecnico Superior. Marı´a de Luna, 3, 50018 Zaragoza, Spain Received July 17, 2009. Revised Manuscript Received November 27, 2009
Lately, an outstanding research interest for CO2 capture sorption/desorption looping systems is the improvement of sorbents reactivity and durability. In particular, in calcium-looping cycles the control of sintering processes in the sorbent by thermal pretreatments, doped limestone, or dolomite have deserved excellent works and have shown good experimental results. Also, synthetic sorbents have been tested and demonstrate a lasting capture capacity. Nevertheless, in most cases this long-term conversion enhancement increases the cost of the sorbent and, thus, the system operation and CO2 capture cost. Therefore, any comparison among sorbents will be accurate if both chemical and economical considerations are taken into account in the assessment. In this work, a common basis for sorbent comparison is presented. The integration of the sorbent cost and its chemical and mechanical performance have been studied for different options. The energetic and economical characteristics of several high temperature sorbents have been checked in a CO2 looping cycle applied to an existing coal-fired electrical generation power plant. The aim is to compare the cost of avoided CO2 as a function of average conversion of solid population and cost for different sorbents. Despite excellent conversion results, the unit cost of the sorbent is crucial to maintain the CO2 looping concept as economically attractive. High cost sorbents, even if their residual activity remains at a high level, will be preferred to operate in systems fed with inert-free fuels, that is, natural gas, instead of applications operating with coal. Both their low cost and long-term performance make thermally pretreated limestones competitive sorbents for carbonation/calcination cycles.
main disadvantages, and the energy penalties linked to amine regeneration6 reduce the global thermal efficiency of the system. Among emerging adsorption processes of CO2 capture, high temperature solid sorbents looping is an excellent prospect since no flue gas pretreatments are required and process integration could reduce energy penalties and CO2 capture cost. This CO2 separation process relies on the use of sorption/desorption looping technologies based on equilibrium reactions that allow solid sorbent regeneration. One of the leading options is the Ca-based sorbent looping cycle using
Introduction The need to reduce anthropogenic CO2 emissions acts as the driving force to reconsider the fossil-fuel power generation technologies and to make its use compatible with low CO2 emissions.1 In this context, coal is predicted to be one of the main energy sources during the next decades,2-4 and it has been the fuel with largest growth in 2008.5 Thus, it is essential to develop technologies for clean and sustainable coal-fired power generation. Carbon capture and storage has been pointed out as a suitable technology for mitigating climate change in the midterm.4,6,7 Absorption processes based on amines are commercially available options for CO2 capture since they present high capture efficiency and selectivity at admissible costs.8 Nevertheless, the demand of flue gas pretreatment is one of the
(9) Abanades, J.; Anthony, E.; Lu, D.; Salvador, C.; Alvarez, D. AlChe J. 2004, 50, 1614–1622. (10) Abanades, J.; Anthony, E.; Wang, J.; Oakey, J. Environ. Sci. Technol. 2005, 2861–2866. (11) Rodriguez, N.; Alonso, M.; Grasa, G.; Abanades, J. Chem. Eng. J. 2008, 138, 148–154. (12) Abanades, J. Chem. Eng. J. 2002, 303–306. (13) Abanades, J.; Alvarez, D. Energy Fuels 2003, 17, 308. (14) Hughes, R.; Lu, D.; Anthony, E.; Wu, Y. Ind. Eng. Chem. Res. 2004, 43, 5529–5539. (15) Grasa, G.; Abanades, J. Ind. Eng. Chem. Res. 2006, 45, 8846– 8851. (16) Alvarez, D.; Abanades, J. Ind. Eng. Chem. Res. 2005, 44, 5608– 5615. (17) Grasa, G.; Abanades, J.; Alonso, M.; Gonzalez, B. Chem. Eng. J. 2008, 137, 561–567. (18) Rao, A.; Anthony, E.; Jia, L.; Macchi, A. Fuel 2007, 86, 2603– 2615. (19) Wang, J.; Anthony, E. Ind. Eng. Chem. Res. 2005, 44, 627. (20) Salvador, C.; Lu, D.; Anthony, E.; Abanades, J. Chem. Eng. J. 2003, 96, 187-195. (21) Manovic, V.; Anthony, E. Environ. Sci. Technol. 2007, 41, 1420– 1425. (22) Manovic, V.; Anthony, E. Environ. Sci. Technol. 2007, 41, 4435– 4440.
*To whom correspondence should be addressed. E-mail: luismi@ unizar.es. (1) Herzog, H. Environ. Sci. Technol. 2001, 35, 148A–153A. (2) DoE, Annual Energy Outlook 2008 with projections to 2030; Technical Report, 2008. (3) IEA, World Energy Outlook 2006 (WEO 2006); Technical Report, 2006. (4) EU Commission, Sustainable power generation from fossil fuels: aiming for near-zero emissions from coal after 2020.; Technical Report, 2007. (5) BP, Statistical Review of World Energy 2009. (6) Metz, B.; Davidson, O.; de Connick, H.; Loos, M.; Meyer, L.; Eds., Special Report on Carbon Dioxide Capture and Storage; Technical Report, 2005. (7) European Technology Platform for Zero Emission Fossil Fuel Power Plants. EU Demonstration Programme for CO2 Capture and Storage (CCS). ZEP’s Proposal. Strategic Overview 2007. (8) Rubin, E.; Chen, C.; Rao, A. Energy Policy 2007, 35, 4444–4454. r 2009 American Chemical Society
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raw limestone or improved limestone by sorbent reactivation and sorbent doping.9-37 The use of magnesium, lithium, and potassium-based sorbents have also deserved important research work.38-40 It is evident that the heterogeneus assumptions used to evaluate CO2 capture cost makes difficult an economical comparison between alternatives. In general, results show that Ca-looping systems have lower CO2 avoided cost than other technologies.41,42 The combination of a postcombustion and an oxyfuel system improves the ratio between the amount of CO2 avoided and the capital cost. Several authors present an oxyfuel cost in the vicinity of 25 h/t CO2.43 The integration of both technologies may reduce this figure below 15 h/t CO2 by increasing the CO2 captured and avoided in a carbonator, but there is a wide range of sorbent properties and process variables that influence this value, and it is necessary to focus the interest on the most cost-effective options. The objective of this study is to compare different high temperature sorbents in economical terms to figure out what sorbent leads to lower CO2-avoided costs. This assessment is achieved through the simulation of a postcombustion capture system for the flue gases of an already existing power plant with waste heat integration into a supercritical steam cycle. Those variables, included in the model and which vary from one sorbent to another, are the temperatures in both reactors, the stoichiometry of the reactions taking place, the mechanical properties, the unit cost, and the chemical behavior of the sorbent.
Figure 1. Carbonation-calcination looping.
carbonator, react with the sorbent, and are addressed to the stack as clean gases. The solids are then directed to the calciner where the carbonated sorbent is regenerated, releasing a highly concentrated CO2 stream that can be conducted to compression, transport, and storage. The proposed configuration makes use of two circulating fluidized beds to favor high solid circulation, adequate residence time, proper gassolid mixing, and temperature control.9,10 This concept, in which the calciner operates as an oxyfired combustor, was first published by Shimizu et al.44 Strong energy penalties are associated with the regeneration stage (highly endothermic reaction), the need to heat up solid streams, and the oxygen production for oxyfuel combustion. Waste heat comes out of the CO2 looping cycle as high-quality heat and may be recovered to increase the efficiency of an existing power plant or to produce steam that drives a newdesign steam cycle, increasing the total net power output.42 For a given fuel and process configuration, sorbent properties, sorbent/CO2 ratio, and makeup flow control the carbonation efficiency and influence the waste heat streams of the capture system:45 (1) Many metal oxides show the carbonation and calcination reactions under specific operational conditions. Natural sorbents are basically Ca- and Mg-based, present faster reaction kinetics, and are usually cheaper and widely available, whereas synthetic sorbents show slow carbonation kinetics and high production costs but longer durability. Every material presents its own mechanical and chemical properties and behaves differently in the cycle. Mechanical and chemical properties of each sorbent determine its behavior in the capture cycle and, thus, the optimum values of the operation parameters and the performance of the system. (2) Low sorbent conversion may be compensated by increasing the sorbent/CO2 ratio to achieve a certain level of CO2 capture efficiency from the gas phase. High sorbent/CO2 ratio directly increases the carbonation reaction efficiency but also energy penalties due to larger sorbent circulation and calcination heat requirements. (3) Make-up flow is required to compensate sorbent degradation, inert accumulation, and elutriation of fines. Fresh sorbent flow keeps an adequate level of average carrying capacity of solid population in the reactor. Hence, sorbent durability or reactivation techniques and mechanical resistance may reduce makeup costs.
Postcombustion Capture System Figure 1 illustrates the process where flue gases from the boiler of an existing power plant (FgasþFCO2) are fed to a (23) Fennell, P.; Davidson, J.; Dennis, J.; Hayhurst, A. J. Energy Inst. 2007, 80, 116–119. (24) Manovic, V.; Anthony, E. Fuel 2008, 87, 1564–1573. (25) Sun, P.; Grace, J.; Lim, C.; Anthony, E. Ind. Eng. Chem. Res. 2008, 47, 2024–2032. (26) Zeman, F. Int. J. Greenhouse Gas Control 2008, 2, 203–209. (27) Li, Y.; Zhao, C.; Qu, C.; Duan, L.; Li, Q.; Liang, C. Chem. Eng. Technol. 2008, 31, 237–244. (28) Manovic, V.; Lu, D.; Anthony, E. Fuel 2008, 87, 3344–3352. (29) Fennell, P.; Pacciani, R.; Dennis, J.; Davidson, J.; Hayhurst, A. Energy Fuels 2007, 21, 2072–2081. (30) Roesch, A.; Reddy, E.; Smirniotis, P. Ind. Eng. Chem. Res. 2005, 44, 6485–6490. (31) Wu, S.; Li, Q.; Kim, J.; Yi, K. Ind. Eng. Chem. Res. 2008, 47, 180– 184. (32) Dobner, S.; Sterns, L.; Graff, R.; Squires, A. Ind. Eng. Chem. Process Design Dev. 1977, 16, 479–486. (33) Chrissafis, K.; Paraskevopoulos, K. J. Therm. Anal. Calorim. 2005, 81, 463–468. (34) Li, Y.; Zhao, C.; Duan, L.; Liang, C.; Li, Q.; Zhou,W.; Chen, H. Fuel Process. Technol. 2008, 89, 1461-1469. (35) Ad anez, J.; de Diego, L.; Garcı´ a-Labiano, F. Fuel 1999, 78, 583– 592. (36) Gupta, H.; Iyer, M.; Sakadjian, B. Int. Environ. Technol. Manage. 2004, 4, 3–20. (37) Gupta, H.; Fan, L. Ind. Eng. Chem. Res. 2007, 46, 35–42. (38) Seo, Y.; Jo, S.; Ryu, C.; Yi, C. Chemosphere 2007, 69 (5), 712–718. (39) Harrison, D. The role of solids in CO2 capture: a mini review. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, 2004. (40) Essaki, K.; Imada, T.; Kato, Y.; Maezawa, Y.; Kato, M. Reproducibility of CO2 Absorption and Emission for Lithium Silicate Pellets. Proceedings of the 22nd Annual International Pittsburgh Coal Conference, 2005. (41) Abanades, J.; Grasa, G.; Alonso, M.; Rodriguez, N.; Anthony, E.; Romeo, L. Environ. Sci. Technol. 2007, 41, 5523–5527. (42) Romeo, L.; Abanades, J.; Escosa, J.; Pa~ no, J.; Gimenez, A.; S anchez-Biezma, A.; Ballesteros, J. Energy Convers. Manage. 2008, 49, 2809–2814. (43) Buhre, B.; Elliott, L.; Sheng, C.; Guptar, R.; Wall, T. Prog. Energy Combust. Sci. 2005, 31, 283–307.
(44) Shimizu, H. K. I. T., Hirama Trans. IChemE, Part A 1999, 77, 6268. (45) Barelli, L.; Bidini, G.; Corradetti, A.; Desideri, U. Energy 2007, 32, 697–710.
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Figure 2. Decay of conversion curves for different natural limestones.
Purbeck for different particle sizes, Cadomin and Havelock for large scale results (LS), and a generic theoretical limestone.15 One promising method to improve long-term sorbent performance is steam reactivation of lime.23,26,48 Hydrated limestone presents an increase in the carbonation activity26 as a consequence of the reduced sintering phenomena affecting the sorbent morphology. However, this sorbent is not included in the study since there is a lack of experimental results for a high number of cycles and the reliability of residual conversion given in literature must be further supported by longer tests. In addition, a sensitivity analysis carried out to assess the influence of residual activity in the final cost of avoided CO2 has revealed important fluctuations of the results for small variations in the initial assumption. Dolomite provides a higher surface of active area than limestone, improving the carbonation reaction.49 Although MgO does not absorb CO2 because its carbonation reaction takes place at lower temperatures than for CaO, 385 °C at atmospheric pressure,50 this oxide has a structure-stabilizing effect that provides a higher cyclic stability compared to raw limestone.49 Experimental tests show that the adsorption ratio of limestone doped with alumina after 50 cyclic runs of carbonationcalcination at 650° and 800 °C was still kept at high levels.31 The reason for the more stable durability is that the new substance (CaO)12(Al2O3)7 can be formed even at 800 °C and the pore size of the sorbent can be enlarged. This sorbent presents a great potential for application in high-temperature CO2 sorption. Another interesting sorbent with enhanced carbonation behavior is lithium orthosilicate. This synthetic sorbent presents a residual activity four times higher than generic natural limestone,39 and its high temperature CO2 absorption rate is fast enough for this application. As stated by Yamaguchi et al.,51 33 wt % of CO2 can be absorbed by the material in less than 2 min. All these sorbents also degrade monotonically with the number of cycles, Figure 3. The decay of activity of these
High Temperature Sorbents Ideally, the sorbent introduced in the system could be used for an extremely long number of cycles. Under real operating conditions, several limitations related to kinetics and thermodynamics of the reactions, side sulphation reactions, sintering, and attrition processes lead to the decrease of the reversibility of carbonation reaction. The sorbent conversion capacity decays with the number of sorption-desorption cycles defining the CO2 capture efficiency, optimum makeup flow, and sorbent/CO2 ratio. Several sorbents have been selected to simulate their behavior in the carbonation looping considering the effects already mentioned. They are different kinds of raw limestone (Cadomin, Havelock, and a generic limestone), dolomite, lithium orthosilicate, alumina-doped limestone, and thermally pretreated limestone. Limestone is the most common solid sorbent for looping cycles of CO2 capture and, thus, its performance is considered the baseline case in this study to compare the results obtained in the simulations of the integrated system using other improved sorbents. The loss of sorbent activity due to sintering has been widely reported in the literature, and different operation strategies and sorbents have been proposed to improve the average conversion of the solid population.12-40,46,47 The sorption behavior of a particle of raw limestone after N cycles can be described by the following expression11 1 þ Xr XN ¼ ð1Þ 1 þ kN 1 - Xr Every natural limestone presents different characteristic longterm carrying capacity and initial conversion depending on the initial pore size distribution, sinterization rate, and impurities in the sample. The carrying capacity curves for a number of different natural limestones29 are represented in Figure 2: (46) Lee, S.; Choi, B.; Lee, T.; Ryu, C.; Ahn, Y.; Kim, J. Catal. Today 2006, 111, 385-390; Proceedings of the 10th Japan-Korea Symposium on Catalysis held at Shimane Prefectural Assembly Hall, Matsue, Japan, May 10-12, 2005. (47) Lee, S.; Chae, H.; Lee, S.; Park, Y.; Ryu, C.; Yi, C.; Kim, J. J. Mol. Catal. B 2009, 56, 179-184; Biocatalysis in Biorefinery: Selected papers from the International Conference on Biorefinery, Beijing, China, October 20-23, 2007.
(48) Manovic, V.; Anthony, E.; Lu, D. Fuel 2008, 87, 2923–2931. (49) Chrissafis, K.; Dagounakib, C.; Paraskevopoulos, K. Thermochim. Acta 2005, 428, 193–198. (50) Gupta, H.; Fan, L. Ind. Eng. Chem. Res. 2002, 41, 4035–4042. (51) Yamaguchi, T.; Niitsuma, T.; Nair, B. N.; Nakagawa, K. J. Membr. Sci. 2007, 294, 16–21.
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Figure 3. Decay of conversion curves for different sorbents.
consequent elutriation may result in the disappearance of a certain amount of active material. Therefore, the makeup flow required to compensate both purge at the bottom of the calciner and elutriation has been quantified and included in the model. Limestone attrition in a CFB, represented in Figure 4 for five natural limestones, may be modeled using eq 3. Cadomin is the most fragile limestone; 15% of the material introduced in the calciner will be elutriated after a reduced number of cycles, ka =0.86 and τ=1.0357. Havelock is more resistant to friction and its attrition leads to losses around 8%, ka = 0.92 and τ = 5.7143. This expression estimates the solid percentage not elutriated from the reactor after every carbonation cycle:29
sorbents was modeled following eq 1. The parameters of this equation were fitted to experimental data in open literature11,31,39,49 for a long number of carbonation-calcination cycles. Results obtained by Manovic et al.52 show that thermal pretreatments of limestone can lead to better performance in a longer series of cycles. As a result of the self-reactivation effect, the sorbent activity can even increase during the first cycles. Thus, the carrying capacity of thermally treated limestone follows a different trend with increasing number of cycles as shown in Figure 3. XN cannot be described through eq 1 and the following combination of expressions, so eq 2 has been used to model this sorbent performance. 8 1 > > NeN XN ¼ Xmax > > 1 > > þ ko N < Xmax ð2Þ > 1 > X ¼ > þ Xr N > N N > > 1 > : þ kN 1 - Xr
aN ¼ 1 - ½ka þ ð1 þ ka Þe -N=τ
ð3Þ
There also exist competitive reactions between sulfur compounds from combustion and the solid sorbent that deactivate it partially and convert the active sorbent into inert material. The sulfur content of the coal burnt in the oxyfuel calcinator and the sulfur oxides concentration in the flue gases from the original power plant have a strong effect on the maximum average capture capacity of the sorbent that has been included in the model. Once the stoichiometry, the makeup flow, and the internal solid circulation in the system are set, the maximum average
Although attrition may enhance, in some cases, particle reactivity by continuous removal of the particle surface, (52) Manovic; Anthony Environ. Sci. Technol. 2008, 42, 4170–4174.
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Figure 4. Percentage of nonelutrated sorbent considering attrition phenomena along number of cycles.
Table 1. CO2 Sorbent Properties sorbent
carbonation reaction
ΔH°car kJ/mol
Tcarb °C
Tregen °C
Xr
k
Cs (h/ton)
CaO CaMgO2b Ca12Al14O33c Li4SiO4d Thermally activated CaOe
CaO þ CO2 T CaCO3 CaO þ CO2 T CaCO3 CaO þ CO2 T CaCO3 Li4SiO4 þ CO2 T Li2SiO3 þ Li2CO3 CaO þ CO2 T CaCO3
-182 -182 -182 -142 -182
650 650 650 500 650
930 930 800 710 930
0.075 0.010 0.363 0.309 0.328
0.520 0.150 0.495 4.590 0.460
6-10 6-10 240-310† 27 000‡ 6-10
a
a Reference 11. b Reference 50. c Reference 31. d References 39, 40, and 56-59. e Reference 60. †Corresponding price for the Al2O3.61 ‡Li2SiO3 unit cost provided by commercial suppliers.
conversion of the solid population can be calculated as follows. X rN XN Xave ¼ 0 1 F0 C XB aN -1 - aN FSorb B C XN ¼ ð4Þ C B ka @ F0 N A 1þ FSorb
System Integration Simulations have been carried out considering an original power plant that generates 500 MW with a net efficiency of 38.11% LHV. It consumes high-rank coal with a sulfur content of 0.6%. Flue gases flow is 546.80 kg/s, and its CO2 content is 14.81% v. As shown in Figure 5, a CO2 capture system is fed with flue gases from the original power plant. Waste heat from the Ca-looping cycle and CO2 compression train are integrated into a new supercritical steam cycle. High temperature heats from sorption-desorption processes (carbonation reaction heat, clean flue gases at 650 °C, and CO2 stream at 930 °C from calciner) are used to produce and reheat supercritical steam. Low temperature heat coming from CO2 compression is used to avoid low pressure steam turbine bleedings and increase the net power output in the overall system. Waste heat and bottoming energy from sorbent purge and CO2 stream are exhausted in economizer and high pressure feedwater heater. For the reference plant, a capital cost of 1100 h/kWgross has been assumed.8 According to Abanades et al.,41 a capital investment of 2070 h/kWgross for the capture system comprising an oxy-CFB calciner, a CFB carbonator, and the steam cycle, has been supposed. Thus, the overall capital cost of the whole system ranges between 1250 and 1600 h/kWgross. Main assumptions for these calculations are gathered in Table 3. CO2-avoided cost has been calculated through the definition given by IPCC in the Special Report on Carbon Dioxide Capture and Storage6 which also considers the cost of electricity under both scenarios, with and without CO2 capture.
Maximum average capture capacity of the sorbent (Xave) takes into account the degradation experienced by sintering, sulphation, or attrition related to the number of cycles. However, real average sorption capacity is expected to be lower than the maximum average capture capacity. Actually, ashes derived from coal combustion in the calciner interact with sorbent particles, reducing the amount of available active sorbent in the loop and reactor design (residence time and contact regime) influence kinetics. Experimental results presented by Abanades et al.53 show that real conversion varies from 70 to 80% of the theoretical value. In the model, a correction factor, f = 0.75, has been applied to the calculated value of Xave to better estimate real operation conditions in Xcarb. Sorbent properties were included in the model and their values are gathered in Table 1. (53) Abanades, J.; Alonso, M.; Rodrı´ guez, N.; Gonzalez, B.; Grasa, G.; Murillo, R. Energy Procedia 2009, 1, 1147-1154; Greenhouse Gas Control Technologies 9, Proceedings of the 9th International Conference on Greenhouse Gas Control Technologies (GHGT-9), November 16-20, 2008, Washington DC, USA.
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Figure 5. System integration. Table 2. Coal Ultimate Analysis;Original Power Plant LHV (kJ/kg) 10220
Table 3. Economical Assumptions
C
O
H
N
S
ash
H2O
66.20
6.76
3.75
1.54
0.60
13.05
8.10
interest rate amortization period capacity factor reference plant capital cost capture plant capital cost fixed costs O&M costs fuel cost raw limestone cost auxiliary consumption for reference plant auxiliary consumption for capture plant air separation unit (ASU) consumption
Comparative results The baseline case used to assess the performance and cost of avoided CO2 for different sorbents was calculated by considering raw limestone as CO2 sorbent into an integration model.54 In this model, authors integrate a carbonate looping CO2 capture process with a new supercritical cycle using natural limestone as a sorbent. Limestones present a wide variability in its long-term sorption capacity as presented in Figure 2. In spite of that, expression given by Grasa et al.15 for a generic limestone in Figure 2 may represent a wide range of different raw limestones working for long number of cycles in carbonationcalcination. Its residual activity is quite poor, thus, one of the worst limestone scenarios will be represented. Also mechanical properties may vary strongly from one limestone to another. Therefore, a sensitivity analysis was done for two different attrition behavior, Cadomin and Havelock attrition rate, for large scale. Cadomin parameters have been applied to simulate the attrition of all the tested sorbents to asses the most critical cases. To take into consideration price fluctuations of limestone, the simulations were run for 6 and 10 h/t CaCO3. Figure 6 presents the economical results of the simulation for a generic limestone under different assumptions of
8.78% 25 years 80% 1100 h/kWgross 2070 h/kWgross 1% 2% 1.43 h/GJ 6 h/t 5.5% of gross power output 5.5% of gross power output þ ASU (28-85 MW) From 28 to 85 MW, depending on molar ratio and purge percentage
mechanical properties and price. The minimum value of CO2avoided cost given by the program for the generic natural limestone is 15.80 h/t CO2 under the most advantageous assumptions, 6 h/t CO2 and low attrition of the solid (c). The influence of different attrition rates on CO2-avoided cost is much more relevant than a strong increase of sorbent price. If the price of limestone is increased up to 10 h/t then the CO2 cost becomes 16.71 h/t CO2 (d). The CO2 avoided cost for fragile limestone are 17.53 h/t CO2 (a) and 19.03 h/t CO2 (b). The more fragile limestone, the higher molar ratio sorbent/ CO2 is required to reach the minimum CO2 cost. The reason behind is the extensive elutriation phenomena of sorbent particles during their first cycles in the system when their conversion is higher. Thus, only average carbonation conversions around 10% are reached. From the energetic standpoint, there exists a significant detriment of electrical efficiency due to capture process demand.
(54) Romeo, L.; Lara, Y.; Lisbona, P.; Escosa, J. Chem. Eng. J. 2009, 147, 252–258.
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Figure 6. CO2-avoided cost for different R and purge. Generic limestone. (a) Fragile material and 6 h/t, (b) fragile material and 10 h/t, (c) harder material and 6 h/t, (d) harder material and 10 h/t.
Figure 7. CO2-avoided cost for different R and purge. Dolomite. (a) Fragile material and 6 h/t; (b) fragile material and 10 h/t.
These losses are relieved after heat integration with a new supercritical cycle. The net global electrical efficiency of the new whole system rises up to 32.33%. Dolomite. As already mentioned, cost of avoided CO2 for low purge flows is affected by inert accumulation in the system. The values of Xave for dolomite at operation conditions;different R;lead to theoretical capture efficiencies from 7 to 100%. The minimum cost of avoided CO2, 18.63 h/ t CO2, is obtained for R=5, fp =3%, and a sorbent unit cost of 6 h/t, Figure 7. Lower purge flows are required to counterbalance the increase in sorbent price with a more limited consumption of fresh sorbent. The optimum in case (b) is found for R=6 and fp =2.5%. Energetic net efficiency of the integrated system ranges between 23.85 and 32.44%. Although carrying capacity during the first 20 cycles is enhanced, the negative effect of the energetic burden associated to inert material in dolomite prevails over.
Lithium Orthosilicate. A different range of purge percentages has been chosen (0.10-0.90%) for molar ratios between 2 and 6 to find the minimum cost of avoided CO2 ton using lithium orthosilicate as sorbent in the capture loop. These conditions have been chosen to offset the high unit costs of the synthetic sorbent with low consumption of fresh material. Data from open literature55,56 point out that new (55) Xiong, R.; Ida, J.; Lin, Y. S. Chem. Eng. Sci. 2003, 58, 4377–4385. (56) Abanades, J.; Rubin, E.; Anthony, E. Ind. Eng. Chem. Res. 2004, 43, 3462–3466. (57) Kleykamp, H. Thermochim. Acta 1996, 287, 191–201. (58) Kleykamp, H. Thermochim. Acta 1994, 237, 1–12. (59) Kourkova, L.; Sadovska, G. Termochimica Acta 2007, 452, 80–81. (60) Manovic, V.; Anthony, E.; Loncarevic, D. Chem. Eng. Sci. 2009, 64, 3236–3245. (61) U.S. Geological Survey. Mineral Commodity Summaries 2009; U.S. Geological Survey: 2009; p 195.
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Figure 8. Solid flow accumulation for different coal compositions varying R and purge. Lithium orthosilicate.
synthetic sorbents should maintain an acceptable performance after hundreds to thousands sorption-desorption cycles to be able to compete with commercially available CO2 capture technologies. In light of the results, purge flows well below 0.10% would be required in a system using lithium orthosilicate to be competitive. Nevertheless, this cannot be carried out in practice with coal as fuel since too poor purge flows lead to dramatic increases in recirculated ashes mass flow. However, depending on the coal and how its ash interacts with the sorbent, it could be possible to separate and remove the ash. Coals with different ash contents, 14.8% db and 0%, have been simulated to assess the influence of inerts accumulation on the behavior of the system with extremely low purge flows. Figure 8 shows that purge flows required to reach acceptable CO2 avoided costs mean unworkable particle fluxes due to the increase in ashes mass flow. Hence, low purge and makeup flows are not limited by Li4SiO4 chemical behavior but by coal ashes content and its accumulation in the loop. To summarize, although a better performance of the system was expected under minimum purge conditions with lithium orthosilicate as sorbent, a minimum threshold limit for purge, depending on the ash content of the coal fed to the power plant and the calciner, has been found. The required purge percentages calculated to offset the current high prices of synthetic sorbents are technically forbidden. To neglect the effect of ash accumulation in the system, the coal with no ashes was introduced and consistent results were obtained for purge values below 0.25%. The minimum cost of avoided CO2, 300.2 h/t CO2, was reached for R = 5 and fp =0.06%. Still, the current price of this sorbent seems to be an insuperable barrier to compete with natural sorbents, raw or modified in coal applications. A new simulation was performed considering a future scenario where orthosilicate lithium unit cost lead to CO2-avoided costs comparable to those obtained for reference case. Costs of avoided CO2 around 25 h/t were obtained for R = 5, fp = 0.2%, and sorbent unit costs 2 orders of magnitude lower than current price (135.8 h/t). Alumina-Doped Limestone. The system operating with alumina-doped limestone as CO2 sorbent has been simulated
Figure 9. CO2 avoided cost for different R and purge. Aluminadoped limestone. (a) Fragile material and 6 h/t; (b) fragile material and 10 h/t.
for both minimum and maximum limestone and alumina prices. A lower range of purge percentages has been chosen (0.20-0.90%) to compensate sorbent price increase due to the high alumina cost. A value of 0.20% was found to be a threshold. Below this purged flow, the model generates inconsistent results. As in the case of using lithium orthosilicate, it was observed that internal solid circulation increased as a consequence of coal ash accumulation in the loop when the capture process is operated at too low makeup flows. This accumulation of ashes in the system generates solid streams whose energetic content is so extremely high that it cannot be handled. A high price of alumina makes purge influence on CO2-avoided cost more significant than CaO/CO2 ratio effect, as shown in Figure 9. Optimum operation is achieved for R = 4 and fp = 0.2%. In this case, CO2 avoided cost ranges between 21.78 h/t CO2 (a) and 735
Energy Fuels 2010, 24, 728–736
: DOI:10.1021/ef900740p
Lisbona et al.
Figure 10. CO2-avoided cost for different R and purge. Thermally pretreated limestone. (a) Fragile material and 6 h/t; (b) fragile material and 10 h/t.
23.47 h/t CO2 (b) depending on sorbent price. Although these costs are higher than those obtained with raw limestone as CO2 sorbent, they remain in the same order of magnitude. Thermally Pretreated Limestone. The better long-term sorption performance of thermally pretreated limestones leads to sorption capacities high enough to reach 100% of capture efficiencies for lower R values than other sorbents. The extra cost related to limestone pretreatment may be compensated by a sorbent cost reduction if the system is operated at low purge flows. As shown in Figure 10, optimum operation is achieved for R = 3 and purge percentages of 2% (a) and 1.5% (b). In these cases, minimum costs of avoided CO2, lower than those obtained using raw limestone as sorbent, make thermally pretreated limestones a competitive sorbent for carbonation/calcination CO2 capture systems.
CCS = carbon capture and storage CFB = circulating fluidized bed COE = cost of electricity f = Xave correction factor Fattrition = flow of elutriated particles FCarbSorb = flow of carbonated sorbent FCO2 = CO2 flow entering the capture system Fgas = gas flow entering the capture system fp = purge percentage Fpurge = purge flow FSorb = sorbent flow F0 = makeup flow k = fitting parameter in eq 1 ka = fitting parameter in eq 3 k0 = fitting parameter in eq 2 LHV = low heating value N = number of cycles N* = number of cycle in which XNtrend of pretreated limestones changes R = CaO/CO2 molar ratio Xave = maximum average capture capacity of sorbent Xmax = maximum capture conversion of pretreated limestone XN = maximum capture conversion of sorbent Xr = residual carbonation activity of sorbent ycomb = fuel fraction to combustor
Conclusions This paper presents the integration within the same model of the unit cost and the chemical and mechanical performance for different high temperature solid sorbents. The cost of avoided CO2 has been compared as a function of average conversion of solid population and cost for several options. The cost of the sorbent is important to maintain the CO2 looping concept economically attractive. However, the effect of attrition in more fragile materials, and the consequent elutriation from the system, has stronger influence on CO2avoided cost. In particular, capture cost increases from 15.8 to 17.5 h/t CO2 if a fragile natural limestone is used instead of a more resistant one. Although this cost remains highly competitive, attrition phenomena should not be neglected in the carbonation/calcination looping model. High cost sorbents, such as alumina-doped limestone and lithium orthosilicate, will be preferred to operate in systems fed with inert-free fuels to avoid ash accumulation for low purge values. Additional research is needed to reduce its cost and improve their economic results. Thermally pretreated limestones have been revealed as competitive sorbents for carbonation/calcination cycles because of their low cost and long-term performance.
Greek Letters η = electrical efficiency ηcap = capture efficiency τ = fitting parameter in eq 2 Subscripts ref = reference capt = capture Acknowledgment. The work described in this paper was supported by the RþD Spanish National Program from Spanish Ministry of Science and Innovation under project ENE200800440/CON and Spanish Ministry of Industry, Tourism and Trade under the project ECC-590000-2008-185. Financial support for A.M. during her Ph.D. studies was provided by the FPU programme of the Spanish Ministry of Science and Innovation.
Nomenclature aN = solid percentage not elutriated after N cycles ASU = air separation unit db = dry basis 736
