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Energy & Fuels 2009, 23, 2797–2803

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CO2 Capture from NGCC Power Stations using Electric Swing Adsorption (ESA) Carlos A. Grande,* Rui P. P. L. Ribeiro, and Alı´rio E. Rodrigues Laboratory of Separation and Reaction Engineering (LSRE), Associate Laboratory LSRE/LCM; Department of Chemical Engineering, Faculty of Engineering, UniVersity of Porto Rua Dr. Roberto Frias, s/n, 4200-465, Porto, Portugal ReceiVed December 9, 2008. ReVised Manuscript ReceiVed February 13, 2009

In this work, we have evaluated the technical feasibility of using Electric Swing Adsorption (ESA) to capture carbon dioxide from flue gases. Simulations under different operating conditions were performed to capture CO2 from Natural Gas Combined Cycle power stations where the CO2 concentration is 3.5%. The objective is to determine a proper cyclic operation to achieve a stream with high CO2 purity, running at high unit productivities to reduce the size and energetic consumption of the capture plant. We propose a novel processscheduling of ESA process to purify the most adsorbed component to very high purities. Using an adsorbent comprising 70% zeolite and 30% of a binder conducting material (with a capacity higher than 1.1 mol of CO2 per kilogram at low CO2 partial pressure), it is not necessary to use a cooling step improving the process productivity. According to our simulations, with the proposed ESA process, it is possible to obtain a concentrated stream with 80% CO2 starting from a stream with 3.5% CO2. The energy consumption was 2.04 GJ/ton CO2, indicating that ESA can be included in the portfolio of CO2 capture technologies.

1. Introduction There is significant evidence showing that the continuous increase in atmospheric greenhouse gases (GHG) is due to anthropogenic emissions. Emissions of carbon dioxide have drastically increased after the discovery and exploration of large reservoirs of fossil fuels. Reduction of GHG concentration in the atmosphere can only be achieved by a blend of strategies involving energy efficiency, renewable and CO2 capture and storage (CCS).1 The use of CCS should be temporary until reaching a fully renewable and sustainable energy production. In the gap, the only way to achieve economic targets while keeping a secure energy supply is using CCS. To put CCS into practice, it is necessary to have economic techniques to capture CO2 from flue gases of power plants and also secure geologic formations to permanently store the captured CO2. Different capture techniques are being developed for different paths to produce energy: postcombustion, precombustion, and oxyfuel. In this work, we have focused on a postcombustion alternative because it can be retrofitted to existing power plants. The time framework to effectively comply with the commitments of capturing CO2 is very short, a reason why “known technologies” were the initial benchmark to start demonstration plants. Scaling-up amine and ammonia scrubbing technology has already started. Other considered technologies are carbonation,2 Vacuum Pressure Swing Adsorption,3 mem* To whom correspondence should be addressed: Telephone: +351 22 508 1618. Fax: +351 22 508 1674. E-mail: [email protected]. (1) International Energy Agency, Energy Technology Perspectives 2008, Scenarios and Strategies to 2050; OECD/IEA, Paris, 2008. (2) Abanades, J. C.; Alonso, M.; Rodrı´guez, N.; Gonza´lez, B.; Grasa, G.; Murillo, R. International conference on Greenhouse Gas Control Technologies (GHGT-9), 2008. (3) Chaffee, A. L.; Knowles, G. P.; Liang, Z.; Zhang, J.; Xiao, P.; Webley, P. A. Int. J. Green. Gas Control 2008, 1, 11–18.

branes, and physical solvents of CO2.4 In most of the postcombustion capture techniques, the main problem is the low content of CO2 per volume of flue gas. This problem is enhanced in Natural Gas Combined Cycle (NGCC) power stations where the amount of CO2 is between 3.0 to 5.0%. Electric Swing Adsorption (ESA) is a process based on the selective separation of one or more compounds (CO2 in this case) by adsorption on a solid surface and where desorption is achieved by heating a fixed bed using electricity.5 The heating is generated by an electric current passing through a conductor (Joule effect) that can be the adsorbent6-8 or electric wires across the column.9 In another process termed Temperature Swing Adsorption (TSA), the column is also heated for regeneration, but heating is achieved by passing a hot gas through the column. TSA is a well-established technology for recovery of Volatile Organic Compound and gas drying10,11 and has also been suggested to capture CO2 from flue gases.12 In the ESA process, no gas is (4) Ohishi, T.; Kishimoto, S.; Higaki, K.; Hirata, T.; Iijima, M. International Conference on Greenhouse Gas Control Technologies (GHGT-9), 2008. (5) Grande, C. A.; Rodrigues, A. E. Int. J. Green. Gas Control 2008, 2, 194–202. (6) Petkovska, M.; Tondeur, D.; Grevillot, G.; Granger, J.; Mitrovic, M. Sep. Sci. Technol. 1991, 26, 425–444. (7) Sullivan, P. D. Organic Vapor RecoVery Using ActiVated Carbon Fiber Cloth and Electrothermal Desorption, 2003. PhD. Thesis, University of Illinois, Urbana. (8) Moon, S.-H.; Shim, J.-W. J. Col. Interf. Sci. 2006, 298, 523–528. (9) Ettlili, N. , Bertelle, S. , Roizard, D. , Vallieres, C. , Grevillot, G. 8th International Conference on Greenhouse Gas Control Technologies; Elsevier: New York, 2006; ISBN 0-08-046407-6. (10) Basmadjian, D. The Art of Modeling in Chemical Engineering; CRC Press: Toronto, Canada, 1999. (11) Wankat, P. C. Separation Process Engineering; Prentice Hall: London, UK, 2007. (12) Me´rel, J.; Clausse, M.; Meunier, F. EnViron. Prog. 2006, 25, 327– 333.

10.1021/ef8010756 CCC: $40.75  2009 American Chemical Society Published on Web 04/01/2009

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Figure 1. Isotherms of CO2 at low (TL ) 315 K) and high temperature (TH ) 423 K). The square indicates CO2 equilibrium at 1.5 bar (yCO2 ) 0.035). The dotted lines indicate the operation of ESA process in electrification step: move from low temperature to high temperature at constant loading and further depressurization to a discharge pressure, PD.

employed to heat the column and hence, much higher purity of the heavy gas can be obtained.13 The objective of this work is to evaluate the use of ESA as a modular CO2 capture technique in already existing NGCC power stations where the CO2 content in the flue gas is 3.5%. Process simulations were performed with different operating conditions and cyclic ESA schemes. The mathematical model employed was already tested and validated for ESA applications.5 For this simulation studies, we have considered that the adsorbent is composed by a honeycomb monolith with 70% of zeolite 13X and 30% of a binder conducting material (graphite for example) that ensures a continuous medium for electricity across the adsorbent. In this initial analysis, we have also assumed that the adsorption behavior of oxygen and nitrogen is equivalent and that the stream is dry. Water can be adsorbed in a previous layer of adsorbent that can be integrated (or not) in the same ESA unit for CO2 removal. In the design of the cycle, we have removed CO2 counter-currently. With those assumptions, our starting conditions for process evaluation are as follows: 3.5% of CO2 balanced by N2. Temperature within the range 310-325 K will be considered. Outlet pressure will be fixed at 1.05 bar. 2. Process and Adsorbent Material Electric Swing Adsorption Fundamentals. The operation of ESA is quite similar to TSA,11 but heating is carried out without diluting the previously adsorbed gas. As TSA, the ESA process should be more suitable for purifying diluted streams (content of CO2 smaller than 10%). The operating principle of ESA can be explained with the adsorption equilibrium isotherms of the most adsorbed species at low (TL) and high (TH) temperatures which correspond to feed and regeneration temperatures, respectively. The operating principle is shown in Figure 1. A continuous stream with a certain (low) concentration of CO2 is fed to the column at the feed temperature (TL). Assuming that there are no kinetic limitations within the adsorbent, at the end of this feed step, the column will be saturated with CO2 (point A in Figure 1). At this point, the column is rapidly heated (in closed system) to the high temperature of the cycle (TH). The loading of the material moves from point A to point B in Figure 1. In the (13) Place, R. N.; Blackburn, A. J.; Tennison, S. R.; Rawlinson, A. P.; Crittenden, B. U.S. Patent No. 6,964,695, 2005.

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Figure 2. Electric Swing Adsorption (ESA) cycle to capture CO2 from NGCC power stations. The steps are as follows: (1) feed; (2) internal rinse; (3) electrification; (4) depressurization; (5) purge; and (6) optional cooling. CO2 is recovered in steps (4) and (5).

case of one gas adsorbed passing together with an inert gas the loading of point A is the same as the one in point B. In the case of multicomponent adsorption, the loading in point B depends on the new adsorption equilibrium of the mixture. As the system was heated in closed system, the total pressure has increased due to the increase of temperature. After TH is achieved, the column can be depressurized, that means move from point B to point C in Figure 1. Adsorbent Material Properties. In the example shown in Figure 1, we have an example of adsorption isotherms on a material containing 70% zeolite 13X and 30% of a binder conducting material (for example graphite). According to previous works, the heat of adsorption of zeolite 13X is quite high,14,15 which means that by increasing the temperature, a large difference of loading can be obtained. Other adsorbents with high loadings at low partial pressure are available,14,16-20 but we have used zeolite 13X as an example since it is very wellknown. The loading of this material at 315 and 423 K is shown in Figure 1. The Langmuir constants to describe CO2 adsorption equilibrium on this adsorbent are detailed in Table 1. We have assumed that the adsorbent is shaped as a honeycomb monolith to reduce the overall pressure drop of the column.21 A honeycomb monolith provides a continuous medium to conduct electricity, reducing ohmic losses within the column. We have also considered that shaping does not modify the kinetic properties of the zeolite 13X.14 This means that giving a final honeycomb shape will not limit the diffusion of molecules to the micropore structure by any means. We have also assumed that the electrical resistance decreases linearly with temperature according to R(z) ) Ro(1 + Rr(T( z) - Tref)) with Ro ) 290 Ω and Rr ) -5.4 × 10-3 K-1. The resistance obtained with this relation is higher than the one expected for carbonaceous materials. However, we have kept (14) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. J. Chem. Eng. Data 2004, 49, 1095–1101. (15) Shen, D.; Bu¨low, M.; Siperstein, F.; Engelhard, M.; Myers, A. L. Adsorption 2000, 6, 275–286. (16) Harlick, P.; Sayari, J.-E. Ind. Eng. Chem. Res. 2007, 46, 446–458. (17) Harlick, P. J. E.; Tezel, F. H. Microporous Mesoporous Mater. 2004, 76, 71–79. (18) Millward, A. R.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 17998. (19) Kongshaug, K. O.; Heyn, R. H.; Fjellvag, H.; Blom, R. World Patent WO/2007/128994, 2007. (20) Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre´, J. J. Chem. Mat. 2006, 16, 626. (21) Patton, A.; Crittenden, B. D.; Perera, S. P. Trans. Inst. Chem. Eng. A, Chem. Eng. Res. DeV. 2004, 82, 999–1009. (22) Yu, F. D.; Luo, L.; Grevillot, G. Chem. Eng. Process. 2007, 46, 70–81. (23) Subrenat, A.; Bale´o, J. N.; Le Cloirec, P.; Blanc, P. E. Carbon 2001, 39, 707–716.

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Table 1. Process and Adsorbent Properties Used in the Assessment of ESA Technology for CO2 Capture from NGCC Power Stations feed flow rate, m3/s % CO2 in feed feed temperature, K discharge pressure, bar heat capacity, J/mol · kg feed pressure (exit), bar CO2 maximum loading (qm), mol/kg CO2 infinite adsorption constant (K0), bar-1 heat of adsorption of CO2 (-∆H), kJ/mol

330.87 3.5 315 1.05 900 1.05 3.0 4.25 × 10-7 45.0

column length, m column diameter, m column porosity density of the honeycomb walls, kg/m3 adsorbent porosity rinse pressure exit, bar N2 maximum loading (qm), mol/kg N2 infinite adsorption constant (K0), bar-1 heat of adsorption of N2 (-∆H), kJ/mol

11.62 7.38 0.40 1167 0.50 1.5 3.0 1.00 × 10-5 20.0

Table 2. Mathematical Model Employed on ESA Simulations mass balance for the gas phase

(∂Ci)/(∂t) + a′ ((1 - εm)/(εm))kf(Ci - Cpi|R

LDF equation for the macropores

(∂Cpi)/(∂t) +(Fw)/(εw)(∂qi)/(∂t) )(15Dp,i)/(R2p)(Bi)/(Bi + 1)(Ci - Cpi)

LDF equation for the micropore

(∂qi)/(∂t) )(15 Dc,i)/(rc2)(qi* - qi)

Darcy equation

3 4R2 )V (∂P)/(∂z) ) -(150 µg (1 - εm)2)/(εm in ch

energy balance for the gas phase

εmFbcV(∂Tg)/(∂t) - εmRgTg(∂Ci)/(∂t) ) λg[(∂2Tg)/(∂z2)] - εmFbcpVch(∂Tg)/(∂z) - (1 - εm) a’ hf(Tg - Ts)

energy balance for the solid channels

Fbcps(∂Ts)/(∂t) )(1 - εm) a′ hf(Tg - Ts) + Fb∑in) 1 (- ∆Hi)(∂qi)/(∂t) + ϑeff((I)/(LcRm))2

Table 3. ESA Simulations of the Process for Various Step Times* sim no. 1 2 3 4 5 6 7 8 (310 K) 9 (310 K) 10 (310 K)

tfeed, s trinse, s telec, s tdepres, s tpurge, s 420 420 420 460 460 460 550 600 600 600

0 0 0 0 10 10 10 10 20 20

50 50 50 50 40 40 40 40 30 30

50 50 50 50 50 50 50 50 50 50

120 120 100 100 100 70 70 70 70 70

ξ 0.15 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.07

purity recovery 39.77 48.90 52.84 54.79 57.68 65.32 68.58 71.41 70.96 79.40

87.64 87.55 87.11 86.15 86.06 83.77 80.77 83.15 82.86 79.50

* Operating conditions are as follows: T ) 315 K (except when indicated); discharge pressure: 1.05 bar; flow rate: 330.87 m3/s; yCO2 ) 0.035; column length: 11.62 m; column diameter: 7.38 m; column porosity: 0.40; heat capacity of adsorbent: 900 J/kg.K; I ) 4000 A. ξ ) purge/Qfeed.

the temperature dependence (Rr) within expected values for activated carbons.22,23 The value of Ro is 2 orders of magnitude higher than the value for activated carbons, provided that only 30% of this conducting material will be mixed with 70% zeolite, which is a nonconducting material. We have assumed this value because this binder material will be mixed with 70% zeolite 13X that is a nonconducting material. The heat capacity of the solid cps ) 900 J/kg · K was taken as an intermediate value between graphite and zeolites. Process Design: Regeneration Protocol. The design of an ESA process to capture CO2 has some constraints that should be taken into account in the initial stages of the design. The most important ones are: (a) The process should be designed to capture as much CO2 as possible at high purity. A value of recovery of 80% should be acceptable resulting from a trade-off simulation with purity and energetic consumption. (b) Low power consumption: the process consumes electricity, which is a high quality source of energy and the product of the plant. Assuming that the plant has an efficiency of 50%, electricity consumption of 1 GJ is equivalent to a heat input of 2 GJ. Heat integration with available sources should be developed to reduce overall penalties. In this work, we have only considered that energy is provided by electricity as the limiting case. The reference value of energy consumption by amine scrubbing is 3 GJ/ton CO2 avoided.24 (c) Small area: The area of the capture plant should be small enough to be retrofitted to existing NGCC plants. This means that the capture plant should have high unit productiv-

) Rin)

) -(∂(VchCi))/(∂z) + Dax((∂2Ci)/(∂z2))

ity. A reference for amine scrubbing process in a 500 MW coal-fired power station is a process with two columns of 35 m length and 12m diameter plus two regenerators and heat exchangers.25 (d) Recycles: the flow rate of a flue gas in a NGCC power station is large reaching several hundreds of cubic meters per second. Thus, recycles of the CO2-free stream should be avoided since they consume power for recompression and also cause a decrease in unit productivity. Generally, any adsorption process can be divided into two fundamental parts: the adsorption step where CO2 is selectively adsorbed in the column(s) and the regeneration, where CO2 is desorbed and the columns are reconditioned to adsorb CO2 again, operating in cyclic mode. More than one column should be used to obtain a continuous feed processing. Considering that we want to concentrate CO2 that is the most adsorbed component of our flue gas, the proposed cycle to capture CO2 from a NGCC power plant is given in Figure 2. This cycle comprise six steps with different functions: Feed. The objective of this step is to remove CO2 from the flue gas. The column was completely or partially regenerated from a previous cycle. The feed stream enters at the lowest temperature of the cycle and the less adsorbed gases (N2 in this case) are obtained at the top of the column. Internal Rinse. The amount of CO2 in the gas phase is very small (e5%) with substantial amount of nitrogen (gas phase and adsorbent voids). It would be desirable to reduce the amount of N2, something that can be achieved by recycling some CO2 (rinse). As mentioned before, recycles should be avoided. A way to circumvent recycling of CO2 is to start the heating of the column without closing the exit valve. As temperature is increasing, some CO2 desorbs from the solid surface to the gas phase, displacing nitrogen to the exit of the column. This step should be stopped before massive breakthrough of CO2 from the column to keep a high recovery. This internal recycle was never reported to our knowledge, in ESA or TSA processes, since its operation was not to concentrate the most adsorbed compound to very high purities in gaseous state. This step should be performed by providing electric power or other source of external heat without being a hot gas. Electrification. Immediately before massive breakthrough of CO2, the valve at the top of the column should be closed and heating should continue until reaching the final temperature (the highest temperature of the cycle). A direct consequence of the increase in temperature is the increase in total pressure. In this

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Figure 3. Simulation of CO2 breakthrough curve.

step, as well as in the previous one, the column is connected to an external source of power. Depressurization. Once the highest temperature of the cycle was reached, the gas within the column should be depressurized. This depressurization is done counter-currently to the feed stream (mostly because we are considering that an initial adsorbent is removing water and the adsorbent employed in the CO2-selective layer may be affected). In this step, purified CO2 is produced at high temperature. The amount of CO2 removed in this step depends on the adsorption equilibrium at high temperatures and also on the discharge pressure. Purge. At the end of the depressurization step, there is a substantial amount of CO2 within the column. To improve the process recovery, this CO2 should be recovered. This is done by recycling some of the purified gas counter-currently to the feed direction (keeping in mind a process with simultaneous water removal). It should be noted that the column is still at high temperature and almost no adsorption of the recycled gas is expected; it will breakthrough very fast. The amount of inert gas recycled should be carefully chosen to avoid dilution of CO2, reducing its purity. Cooling. At the end of the purge step, a large portion of the column is still at high temperature. At high temperatures, the capacity of the adsorbent toward CO2 is very small, and thus a cooling step may be necessary to restore the capacity and start a new cycle. It is desirable to avoid this step finding other alternatives to cool down the bed. The main disadvantages of this step are the large amounts of gas that should be recycled to cool down a column filled with adsorbent and the drastic reduction in productivity. The cooling step is not necessary when the adsorbent is able to load large amounts of CO2. In those cases, the cooling can be done with the feed step: the inert gas not adsorbed can cool down the column, cocurrently to feed. This condition is satisfied when the velocity of the thermal wave is faster than the velocity of the adsorption wave. An initial estimate of the minimum capacity that the adsorbent should have can be obtained considering that there are no kinetic limitations and also that the feed step is isothermal:

qCO2

J cps mol kg·K > yCO2,feed kg J cp,gas mol·K

[ ]

[ ] [ ]

(1)

where yCO2,feed is the molar fraction of CO2 in the feed step, cp,s and cp,gas are the adsorbent and gas heat capacities, respectively. This relation should only be considered as an estimative since the process is quite nonisothermal in feed step and some kinetic limitations and axial dispersion may exist.

Mathematical Description of the Process. The cycle proposed in Figure 2 recovers CO2 from flue gases in steps 4 and 5 (depressurization and purge, respectively). The performance parameters from this cycle are:

∫

tdepres+tpurge

PURITY )

0 tdepres+tpurge

∫

0

∫

(yCO2 + yN2)dt

tdepres+tpurge

RECOVERY )

0

∫

tfeed

0

yCO2dt

V| z)0CCO2 | z)0dt

V| z)LCCO2 | z)Ldt

(2)

(3)

We have started the process design by assuming that the flue gas emits 1 million tons of CO2 per year with a molar fraction of 3.5%. The columns were sized to operate 10 min in feed step: we have calculated the number of moles of CO2 entering one column in 10 min and then the needed amount of adsorbent assuming a constant temperature of 315 K. The temperature within the column in the feed step will not remain constant at 315 K, but this was a first approximation. After that, we have fixed the monolith channels velocity of CO2 in 0.30 m/s to determine the diameter of the column. In the initial simulations, we have not considered that the steps should be symmetric to operate in continuous fashion and we have only limited the step times by the required operating conditions to improve process performance. A list with all the parameters employed in this example and the operating conditions is provided in Table 1. The mathematical model used to describe the column behavior is detailed in Table 2. Note that we have considered that the adsorbent is bidisperse with diffusion kinetics described by the Linear Driving Force (LDF) both in macropores and in micropores. The adsorption equilibrium of the binary mixture of gases (CO2 - N2) was described by the Langmuir isotherm: qi )

qm,iKiP 1 + ΣKiP

( )

Ki ) K0i exp

-∆Hi RgT

(4)

(5)

where qm,i is the maximum loading of each gas (CO2 and N2) and the adsorption constant (Ki) is described by an exponential dependence with temperature. Adsorption parameters for CO2 and N2 are listed in Table 1.

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Figure 6. Pressure and temperature history of an ESA cycle to capture CO2 from NGCC power plant. The profiles at two different points of the column are shown (inlet and exit).

Figure 4. Simulation results of the ESA process for CO2 capture. Pressure (a) and temperature (b) history during one cycle at both column ends. CO2 molar fraction (c), CO2 concentration (d), and temperature (e) in cyclic steady state, at the end of each step: 1-feed; 2-internal rinse; 3-electrification; 4-depressurization; and 5-purge.

Figure 7. Internal concentration profiles of CO2 (a) and N2 (b) at the end of each step (1: feed; 2: internal rinse; 3: electrification; 4: depressurization; and 5: purge) in cyclic steady state for ESA process for CO2 capture from NGCC power plants.

Energy balances were performed in gas and solid phases, and the system was considered to operate in adiabatic mode. Pressure drop across the column was described by the Darcy law. The mathematical model was solved using gPROMS (PSE Enterprise, UK). The orthogonal collocation method on finite elements (OCFE) was used with 50 finite elements and two interior collocation points in each element of the adsorption bed. A high number of elements have to be employed to avoid oscillation in the CO2 amount adsorbed due to fast temperature raise. The Figure 5. Cyclic scheme used in Electric Swing Adsorption (ESA) modeling. Steps are as follows: Feed (tfeed ) 10 min); Internal rinse (trinse ) 11 min); Electrification (telec ) 16 min); Depressurization (tdepres ) 1 min); and Purge (tpurge ) 2 min).

(24) Abu-Zahra, M. R. M.; Niederer, J. P. M.; Feron, P. H. M.; Versteeg, G. F. Int. J. Greenhouse Gas Control 2007, 1, 135–142. (25) BP website. “Capturing Carbon Dioxide”. Available at: http://www. bp.com/liveassets/bp_internet/globalbp/globalbp_uk_english/reports_and_ publications/frontiers/STAGING/local_assets/pdf/bpf21_16-24_ccs.pdf. Last accessed on 08/25/2008.

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solvers employed in the simulations use a value of 1 × 10-5 for absolute tolerance. 3. Results and Discussion When analyzing the feed step of the ESA cycle, the first thing noted is the temperature excursion observed. The column operates under adiabatic conditions and the heat generated from CO2 adsorption will result in a temperature rise that will travel within the column together with the concentration front, dispersing it. An example of a CO2 breakthrough curve using the operating conditions described in Table 1 can be observed in Figure 3. It can be observed that CO2 breaks through the column at 280 s and the concentration profile is so spread that more than 50% of the capacity was not employed. The temperature rise in this example is 30 K, significantly reducing the maximum loading of the column. Using an adsorbent with a smaller heat of adsorption, the temperature rise is smaller, but the heat required to obtain the same difference shown in Figure 1 is much higher and thus the energy penalty is higher. A summary of all of the simulations performed is shown in Table 3. Several operating conditions were tested. According to the results obtained, using 15% of the purified gas (N2) as purge gas, we are excessively diluting the CO2 in the purge step and thus, the purity is seriously affected. If the amount of gas used in purge is reduced, then less CO2 is removed in the purge step and the recovery decreases, although within acceptable limits. The effect of the internal rinse is also noted: an increase of 3% in CO2 purity is achieved without modifications in recovery. Another important variable is the temperature of the feed: decreasing 5 K in the inlet temperature, much higher loadings of the column are achieved and thus more CO2 is within the column at the beginning of electrification step, which is the reason why the purity is higher. As an example of the results, we show in Figure 4 some of the important variables of the system when cyclic steady state (CSS) was achieved. The CSS in this system was achieved very quickly, being completed in less than 5 cycles. Figure 4, parts a and b, shows the pressure and temperature history during an ESA cycle, respectively, at both column ends. In the feed step, the heat of adsorption released results in an increase of about 30 K, as can be seen in Figure 4e). This temperature increase along the bed influences the bed capacity for CO2 adsorption, which can be seen in the CO2 concentration profile in Figure 4d). The internal rinse step assumes great importance in the process increasing the CO2 concentration within the column before the electrification step (see figure 4d). In fact, the molar fraction of CO2 increases from 0.035 to 0.87 in the initial part of the column (see Figure 4c), increasing up to almost 1 in the electrification step due to the large amount of CO2 desorbed. The purge step has great importance in reducing the amount of CO2 in the column (Figure 4c) and also decreasing the temperature for the next cycle (Figure 4e). The most important result obtained in the simulations performed is related to CO2 purity. Unfortunately, purity in the order of >98% could not be obtained in the conditions tested and 79% was achieved. However, it should be noted that the CO2 was concentrated more than 23×, which is a very high value. Other process improvements should be performed to increase the purity to those values, although more work is required to achieve the adsorbent in its final form. Another important feature is related to the power consumption of the system. The energy required to capture CO2 using the ESA cycle proposed in this work is 2.04 GJ/ton CO2 captured. This energy consumption does not include either product

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recompression or water removal. From this value, 1.91 GJ/ton CO2 avoided are employed to heat the adsorbent while the rest is used to heat the molecules (gas + adsorbed phase) inside the column. The energy consumption in the internal recycle step is 1.1 GJ/ton CO2 avoided, while the remaining is consumed in the electrification step. The overall energy required value is smaller than values obtained for CO2 capture using amines in coal-fired power stations, but is subjected to large uncertainties. However, comparing the values in terms of primary energy (heat), the 2.04 GJ/ton CO2 should be doubled for a power station with efficiency of 50%. The energetic consumption should only be taken as indicative because it is based on estimated values of the important parameters of the adsorbent (cp,s, electric resistivity and equilibrium/kinetic properties). Integration with available heat should be studied as a way to reduce overall energetic penalties. Process Scale-up. The simulations carried out allowed us to understand the ESA behavior. In a real ESA unit, a continuous feed should be treated, and the regeneration of the adsorbent should be scheduled with the appropriate number of columns. We have observed that the regeneration in the ESA process can be extremely fast, which will result in very high unit productivities. In fact, the complete regeneration could be done within 3 min, so the column can also be designed to treat 3 min of feed being much smaller and allowing higher flexibility in its design to improve mass and energy transfer. The problem with very fast heating is that the hybrid adsorbent proposed in this study (zeolite 13X + inert binder material) may be damaged or present fast aging if heated cyclically between 310 K and 450 K in 1 min or less.26 In order to keep the heating temperature on the order of 5 K/min, we have to extend the internal rinse and electrification steps. One possible ESA configuration of four columns will provide 10 min for the feed step and 30 min for regeneration, where appropriate heating rate can be accommodated. Also, extending the internal rinse and electrification steps, a process can be envisioned also in terms of continuous electricity consumption. To confirm that the scale-up of the ESA process is possible, we have performed a simulation keeping the feed properties, but increasing the regeneration time according to the following: trinse ) 11 min; telec ) 16 min; tdepres ) 1 min and tpurge ) 2 min, such that the entire regeneration takes 30 min. The cyclic scheme employed is shown in Figure 5. Relevant data such as pressure and temperature are presented in Figure 6. Gas concentration at the end of each step in CSS is shown in Figure 7. It was observed that the performance parameters could be maintained in 78.93% (purity) and 79.50% (recovery). The energy consumption was 2.04 GJ/ton CO2 captured, again with most of the power consumed to heat the adsorbent. The results presented in this work show that there exists potential to consider ESA process as a possible technology to capture CO2 from NGCC power plants, provided that a proper adsorbent material is manufactured. Other alternative adsorbents that can be suitable to use in this configuration are chemically modified activated carbons that can already conduct electricity.27 Electricity consumption is 2.04 GJ/ton CO2 captured, which is equivalent to 4.08 GJ/ton CO2 of heat in a power station with efficiency of 50%. (26) Cavalcante, C. L. J. Lat. Am. Appl. Res. 2000, 30, 357–364. (27) Fauth, D. J.; Filburn, T. P.; Gray, M. L.; Hedges, S. W.; Hoffman, J. S.; Pennline, H. W. DeVelopment of NoVel CO2 Adsorbents for Capture of CO2 from Flue Gas, 2007. A&WMA 100th Annual Conference & Exhibition. Pittsburgh. Available at: http://www.osti.gov/bridge/servlets/ purl/915511-LmLXcj/.

CO2 Capture from Power Stations using ESA

Energy & Fuels, Vol. 23, 2009 2803

Mixed utilization of heat and electricity should be used to reduce the consumption of high-quality energy. 4. Conclusions The Electric Swing Adsorption (ESA) process was evaluated for capture CO2 from Natural Gas Combined Cycle (NGCC) power stations. These flow rates are characterized for having very small CO2 concentrations (3.5% in this case). According to our simulations, an ESA process with four columns of 11.62 m length and 7.38 m diameter should be used to treat 1 million tons of CO2. The recovery of CO2 is 80% with a purity of 80% and electricity requirement of 2.04 GJ/ton CO2 captured. Integration with heat sources should be studied to reduce the consumption of high-quality energy. In this work, we have reported a new cyclic operation for ESA processes that allow increasing the purity of the most adsorbed compound. This is done by performing the heating in two sequential steps: the first one with the column open in the product end (releasing the less adsorbed gas by desorption of the most adsorbed gas) and finally a second heating with the column closed until the desired temperature to pressurize the column. Significant improvement of the CO2 purity was observed by including this sequential scheduling and also by adjusting other critical process conditions, like feed temperature and duration and purge flow rate. These values should be taken as indicative since many assumptions were performed on the adsorbent. However, this work shows the large potential of ESA as a CO2 capture technique.

NOTATION a′

area to volume ratio

Bi

Biot number

(m-1)

Lc

column length (m)

P

total pressure (bar)

qi

adsorbed phase concentration of component i (mol/kg)

q*i

adsorbed gas phase concentration in the equilibrium state of component i (mol/kg)

qmax,i

saturation capacity of component i (mol/kg)

Rg

universal gas constant (J/mol.K)

rc

crystal radius (m)

Rin

internal radius of the monolith channels (m)

Rm

radius of the honeycomb (m)

Rp

radius of the monolith wall (m)

R(z)

electric resistance at position z of the column (Ω)

R0

electric resistance at reference temperature (Ω)

t

time (s)

tdepres

depressurization step time (min)

telec

electrification step time (min)

tpurge

purge step time (min)

trinse

rinse step time (min)

T

temperature (K)

Tg

temperature of the gas (K)

Ts

temperature of the solid (K)

Tref

reference temperature (K)

Vch

velocity of the gas in each channel of the monolith (m/s)

yi

molar fraction of species i

z

axial distance along the column (m)

Greek Letters. (mol/m3)

R

coefficient of electric resistance dependence with temperature (K-1)r

Fb

gas density in the bulk (kg/m3)

Ci

concentration of component i in the gas phase

Cpi

concentration of component i in the channel wall (mol/m3)

cp,gas

heat capacity of the gas (J/kg · K)

Fw

monolith density (kg/m3)

cps

heat capacity of the solid(J/mol · K)

λg

thermal conductivity of the gas (W/m2 · K)

cv

heat capacity at constant volume of the gas (J/mol.K)

εm

porosity of the column

Dax

axial dispersion coefficient (m2/s)

εw

porosity of the monolith wall

Dc,i

micropore diffusivity of component i (m2/s)

(-∆Hi)

heat of adsorption of component i (kJ/mol)

Dp,i

pore diffusivity of component i (m2/s)

µg

gas viscosity (Pa · s)

hf

film heat transfer coefficient between the gas and the channel wall (W/m2.K)

ϑeff

electrical resistivity of the solid (Ω · m)

I

intensity of current (A)

Ki

adsorption equilibrium constant of component i (bar-1)

Ki0

adsorption equilibrium constant at the limit T f ∞ of component i (bar-1)

kf

film mass transfer coefficient (m/s)

Acknowledgment. Funding for this research was provided by project PTDC/EQU-EQU/65541 from FCT (Fundac¸a˜o para a Cieˆncia e a Tecnologia).

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