Carbon Filter Process for Flue-Gas Carbon Capture on Carbonaceous

Jul 11, 2011 - The gas pure components are purchased from United States Welding, Inc. ... The vacuum pump is a Welch Duo Seal 1401 pump, which is rate...
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Carbon Filter Process for Flue-Gas Carbon Capture on Carbonaceous Sorbents: Steam-Aided Vacuum Swing Adsorption Option Bryce Dutcher, Hertanto Adidharma,* and Maciej Radosz Soft Materials Laboratory, Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States ABSTRACT: A low-pressure carbon filter process can capture carbon dioxide from combustion flue gas. This filter is filled with a carbonaceous sorbent, such as activated carbon, which has a high chemical affinity to and hence high capacity to retain CO2 but not nitrogen. This in turn leads to a high CO2/N2 selectivity, especially at low pressures. While CO2 sorption is spontaneous and rapid, its recovery from the sorbent is a challenge that determines the process efficiency. Among the desorption methods considered in this work, direct steam can produce nearly pure CO2, but it results in condensed water in the bed that reduces the sorbent capacity and requires sorbent cooling, which is slow. Vacuum desorption in the absence of water also works, but it requires low pressures, much lower than 30 Torr, to achieve substantial CO2 yields. However, a hybrid approach, referred to as “steam-aided vacuum swing adsorption” (SA-VSA), proposed in this work, is found to alleviate or eliminate these problems. Water vapor serves as a displacement medium and hence increases the vacuum efficiency at moderate pressures. Vacuum, on the other hand, prevents bulk water condensation and sorbent heating, and hence eliminates the need for cooling. As a result, SA-VSA can completely recover nearly pure CO2 from combustion flue gas, but it also allows for optimizing the CO2 recovery and purity.

’ INTRODUCTION A carbon filter process1 captures CO2 from combustion flue gas by ambient pressure sorption on porous carbonaceous sorbents. Such an adsorption process is therefore applicable to power plants that produce electricity by converting coal or natural gas into carbon dioxide (CO2) and other minor components. In general, separating CO2 from such a flue gas mixture poses no special technical problems for the known separation processes, such as absorption in amine or ammonia, pressureswing adsorption, and membrane technologies.2 5 However, if these technologies require the compression of flue gas with plenty of inert nitrogen in it, for example, to increase the CO2 solubility in a liquid solvent, to increase its sorption in pressureswing adsorption (PSA), or to create a driving force for its transport across the membrane, they tend to be energy intensive. By contrast, the carbon filter requires no expensive materials or exotic equipment, no flue gas compression, or no refrigeration, and it is easy to integrate with an existing power plant or a grassroots plant. A common alternative to carbon sorbents is zeolites. Carbon sorbents have somewhat lower capacity than zeolites, especially for CO2 mixtures, and hence will require larger vessels, but they are less expensive. More important, they are much less moisture sensitive6 and have lower heats of adsorption. In general, they do not heat up during sorption as much as zeolites, preventing a large drop in the sorption capacity, and they require less energy to remove the adsorbed CO2. The latter is important because, while the sorption stage is spontaneous and fast, the desorption step can be slow and energy intensive. In fact, it is the desorption efficiency that is the key challenge as it controls the cost of CO2 capture. The adsorbed CO2 is recovered during a sorbent regeneration (desorption) stage by either heating or reducing the CO2 partial r 2011 American Chemical Society

pressure or both. Temperature swing adsorption (TSA), in which the sorbent bed is heated directly or indirectly, is an example of releasing CO2 from the sorbent primarily with thermal energy. This method can be efficient, as thermal energy is usually cheaper than mechanical energy. If indirect heating alone is not enough to desorb the sorbate completely, reducing its partial pressure by diluting with another component (purge gas) may help. The partial pressure effect can also be accomplished by a direct heating medium, for example steam. However, the direct heating medium may condense, and the purge gas may dilute the final product.7,8 Another issue is that TSA requires a cooling step before the bed is reused for adsorption, which can be slow. Pressure swing adsorption (PSA), in which the CO2 sorption partial pressure is reduced by reducing the total pressure, takes advantage of decreasing sorbent capacity with decreasing pressure. In a typical application, the sorbent saturated at a higher pressure, for example with a feed mixture that already exists at or is compressed to the sorption pressure, is simply vented to atmospheric or other pressure. The problem with a straightforward PSA application to combustion flue gas is that one would have to compress its large fraction of inert nitrogen, which is expensive. Also, it turns out that the sorbent selectivity for CO2 drops with increasing pressure, making it more difficult and costly to achieve a high purity product with PSA.1,9 A version of PSA that involves saturating the sorbent at ambient pressure and sorbate recovery under vacuum, referred to as “vacuum swing adsorption” (VSA), can avoid these pitfalls. An alternative to reducing the total pressure in PSA or VSA is to Received: December 17, 2010 Accepted: July 11, 2011 Revised: July 11, 2011 Published: July 11, 2011 9696

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Table 1. Sorption Properties of ACC and AC111 sorbent

ACC

AC1

CO2 sorption capacity at 1 bar (wt %)

6.7

9.9

CO2 sorption capacity at 10 bar (wt %)

18.6

26.0

N2 sorption capacity at 1 bar (wt %)

0.5

1.0

N2 sorption capacity at 10 bar (wt %)

2.2

5.3

selectivity at 1 bar

13.4

9.9

selectivity at 10 bar

8.5

4.9

BET surface area (m2/g)

1107

821

micropore surface area (m2/g) external surface area (m2/g)

545 562

602 219

micropore volume (cm3/g)

0.246

0.280

mesopore volume (cm3/g)

0.227

0.171

total pore volume (cm3/g)

0.473

0.451

reduce the sorbate (CO2 in this case) partial pressure either with a purge gas that does not get adsorbed much or with a purge gas that can also displace the adsorbed CO2. Such a displacement agent must be easily recoverable from the sorbent and separable from CO2.7,8,10 In our previous work,1 we used TSA/displacement with direct steam as a simple example of a robust sorbent regeneration method. However, we did not address in detail the consequences of significant water condensation, cooling times, and desorption kinetics, including the effluent composition as a function of time, just to mention a few obvious questions. We did not evaluate alternatives to direct steam heating either, such as VSA, displacement, and hybrid approaches. The purpose of this work, therefore, is to explore (1) the impact of direct steam heating on the desorption effluent composition, (2) the vacuum levels required to desorb CO2 without heating the sorbent bed in a plain VSA mode, (3) a hybrid approach referred to as “steam-aided vacuum swing asdorption” (SA-VSA), and (4) the practical advantage or disadvantage of the presence of water. The SA-VSA hypothesis is attractive because it can in principle eliminate the need to cool the sorbent bed following its regeneration, on the one hand, and the need to use very low pressures to desorb remnants of CO2, on the other hand. Furthermore, we will specifically aim at moderate pressures that are relatively inexpensive to generate but low enough to prevent bulk water condensation. The crucial question is if water vapor at such moderate pressures can be an effective displacement agent, and hence result in a nearly complete CO2 recovery, and if it can recover a nearly pure CO2, for example at least 95% pipeline purity. While this work aims at a preliminary SA-VSA proof of concept, we do not attempt to explain the sorption and desorption mechanisms, to select the best sorbent, to do a detailed thermodynamic analysis, or to optimize the process conditions, including the impact of SOx and NOx, not to mention scale-up tests, all of which are among future goals.

’ MATERIALS While different carbon materials were evaluated in the early stages of this work, it turned out that the process concept can be demonstrated with activated carbons derived from coal (ACC) and AC1, which is available commercially. ACC is composed of cylindrical particles, 3.5 mm in diameter and on average 7 mm in length. Its bulk density is 490 kg/m3 with a void fraction of 0.558. AC1 is composed of spherical particles with an average diameter

Figure 1. Steam vacuum hybrid experimental setup.

Figure 2. Process conceptual drawing for a steam desorption process.

of 1 mm and a bulk density of 520 kg/m3. Sorption properties of these sorbents are given in Table 1. A model flue gas is obtained by mixing 12% CO2 with a balance of N2. Its composition is verified with the real time gas analyzer (RTGA). The gas pure components are purchased from United States Welding, Inc.

’ EXPERIMENTS A versatile bench unit developed for this work allows for testing three approaches to sorbent regeneration, with direct steam, vacuum, and steam-aided vacuum. A schematic diagram of the apparatus is shown in Figure 1. The vacuum pump is a Welch Duo Seal 1401 pump, which is rated for 20 L/min at STP (in this work, STP is defined as 25 °C and 1 atm). The real time gas analyzer (RTGA) is an Agilent 5975C mass spectrometer. The sorption tank is a vessel 4.6 L in volume, the diameter and height of which are 14 and 34 cm, respectively. The ends are nearly hemispherical. The vessel walls are stainless steel and 1.5 mm thick. A thermocouple is located at the center of the bed to measure the temperature. The steam-generating system is also shown in Figure 1, in which steam is generated in a continuous loop, pumping water through an oven (boiler) and then recycling it back to the water tank through a condenser. 9697

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Figure 3. Process conceptual drawing for a VSA process.

Figure 5. Breakthrough curves for 12% CO2 in N2 using flue gas at different temperatures for AC1 (data taken by Kaspars Krutkramelis).

Figure 4. Process conceptual drawing for an SA-VSA process.

In our preliminary study, the flue gas temperature is varied from 0 to 120 °C to see the effect of gas temperature on the sorption process. All of the other sorption experiments are then run using flue gas at ambient temperature (22 25 °C). In the sorption stage, a fresh bed is contacted continuously with the flue gas mixture, 12% CO2 in N2, at ambient pressure (590 Torr) until the break point is reached, i.e., when the concentration of CO2 in the effluent reaches 2.5%. The sorption time is recorded, and the volume of gas adsorbed is obtained by subtracting the volume of gas collected in the wet test meter from the volume of gas fed to the vessel. The water collected in the dry ice bath, if any, is determined from weighing. This step is unnecessary for the first cycle, when dry sorbent is used. (a) Steam-Driven TSA. For TSA experiments, the sorption tank is uninsulated and filled with AC1. The base-case TSA experiments at ambient pressure are illustrated conceptually in Figure 2. In the sorption stage, the flue gas mixture is introduced at 3.0 L (STP)/min. In the heating stage, CO2 is desorbed with superheated steam at 120 °C at a rate of 3.8 L (STP)/min. This desorption stage is declared to be complete when no CO2 is detected by the RTGA. In the cooling stage, the bed temperature is reduced to ambient temperature with air in preparation for the next sorption stage. Thus, the bed temperature range in a complete cycle is from ambient temperature to steam temperature. Subsequent cycles are performed in the same manner. (b) VSA. For VSA experiments, the sorption tank is also uninsulated and filled with AC1. The two-stage VSA experiments are illustrated conceptually in Figure 3. In the sorption stage, the flue gas mixture is introduced at 3.0 L (STP)/min. In the desorption stage, the pressure is reduced by turning on the vacuum until no CO2 is detected. The bed is allowed to exchange heat with the surroundings, and its temperature, which is initially at ambient temperature, is not controlled. (c) SA-VSA. For SA-VSA experiments, the sorption tank is insulated with 3-cm-thick fiberglass and filled with ACC. Each SA-VSA experiment also consists of two stages, sorption and

desorption, as illustrated in Figure 4. Similar to VSA, the bed is also allowed to exchange heat with the surroundings and its temperature is not controlled. For the first few experiments, the bed is initially at ambient temperature, but for all other experiments, to reach cyclic steady state faster, the bed is initially preheated to 50 °C. In the sorption stage, the flue gas mixture is fed at 6.1 L (STP)/min. After the break point is reached, the desorption stage is started by turning on the vacuum. Once the vessel reaches a desired pressure, usually below the water vapor pressure at a given temperature, steam is allowed to enter at a very low flow rate such that the pressure stays constant. Steam is usually introduced at a reference pressure of 30 Torr, which is close to the vapor pressure of water at room temperature, but additional experiments are also done at 15 and 50 Torr to investigate the effects of desorption pressure. The desorption stage is declared to be complete when no CO2 is detected by the RTGA. Subsequent adsorption and desorption cycles are performed in the same manner. Additional experiments with 15-cm-thick fiberglass insulation and without insulation are also performed to probe the heat-loss effects, which may cause slight sorbent cooling and steam condensation, if any. In order to determine the material balance for water, the loss of water in the water tank is assumed to be the amount of the water entering the system, which is equal to the mass of steam used for desorption. For a single sorption desorption cycle, the amount of water retained in the vessel, if any, is calculated by subtracting the amount of water collected in the dry ice bath from the amount of water entering the system.

’ RESULTS AND DISCUSSION In this work, purity is defined as the volume fraction of CO2 in the product on a dry basis and the CO2 recovery is defined as the volume of CO2 in the product divided by the volume of CO2 fed to the vessel, which is verified by a material balance. As mentioned above, in our preliminary study, the flue gas temperature is varied from 0 to 120 °C to see the effect of gas temperature on the sorption process. We find that the flue gas temperature has little if any effect on the bed temperature (or sorbent capacity), as its heat capacity is negligible compared to that of the sorbent. An example is demonstrated for flue gas at 22 and 85 °C in Figure 5, which shows that the breakthrough curve, 9698

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Figure 6. Typical CO2 percent, with balance N2, and midbed temperature as a function of time during a direct-steam regeneration experiment on AC1 (data taken by Kaspars Krutkramelis).

Figure 8. Typical desorption flow rates of CO2 and N2 as a function of time for a VSA experiment on AC1 showing the second fraction, which is essentially pure CO2 (data taken by Kaspars Krutkramelis).

Figure 7. Effects of sorbent water content on the CO2 capacity using direct steam desorption for AC1. In each cycle from left to right, except the first one, the sorbent contains water condensed in the previous cycle (data taken by Kaspars Krutkramelis).

Figure 9. Effect of desorption pressure on the amount of CO2 removed from AC1 (data taken by Kaspars Krutkramelis).

and thus the sorbent capacity, is weakly affected by the gas temperature. Steam-Driven TSA. In a typical TSA regeneration experiment using direct superheated steam, CO2 is removed quickly and completely as soon as the steam front reaches the sorbent. Following a relatively minor fraction that is similar in composition to flue gas, the main fraction is nearly pure CO2, as shown in Figure 6. This can be explained by readsorption of the freshly desorbed CO2-rich sorbate in the cold sorbent that has not been exposed to steam yet, displacing N2 from the bed first. Figure 6 also illustrates a temperature profile measured in the middle of the bed. While this specific example is for AC1, these desorption effluent composition and temperature profiles are typical for other carbon sorbents as well. The rapid desorption and the sharp transition to pure CO2 are the advantages of this approach. However, the condensed steam, which was visible on the surface of and in the space between particles, unless completely removed, causes water to accumulate in a large amount from cycle to cycle, which in effect reduces the sorbent capacity in the subsequent cycles. This reduction in capacity relative to the amount of condensed water is illustrated in Figure 7, which shows a TSA process with four cycles. When the bed is fresh in the first cycle, of course, the sorbent capacity is the highest, and

then it drops for the subsequent cycles as more and more water accumulates in the bed. Although the amount of water accumulated could be reduced if the vessel is insulated, this problem is expected to remain. VSA. By contrast, plain VSA requires no heating or displacement medium and hence leads to a more stable sorbent performance. Following several initial cycles, the temperature of the bed stabilizes and remains constant at ambient temperature, regardless of the flue gas temperature. Similar to TSA, VSA tends to produce two effluent fractions, of which the first fraction is mostly nitrogen and the second fraction is essentially pure CO2, as illustrated in Figure 8. Its purity can be easily controlled by adjusting the cutoff point between the two fractions. In a typical case such as the one shown in Figure 8, almost two-thirds of the total desorbed gas is greater than 95% pure CO2. This is the good news. The bad news, however, is that a relatively deep vacuum is needed to achieve significant desorption yields. As shown in Figure 9, the total adsorbed CO2 that can be recovered at pressures as low as 30 Torr is less than 40%, which is a result of the sorbent equilibrium capacity. What is not shown in Figure 9 is that such a vacuum-driven desorption process is slow and hence likely to become a rate-limiting step for the whole process. SA-VSA. This leads one to a hybrid SA-VSA hypothesis aimed at alleviating or eliminating the steam-TSA and plain-VSA problems, namely the bulk steam condensation and the need 9699

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Figure 10. Mass flow rates of N2 and CO2 during desorption.

Figure 12. Water accumulation in bed per cycle for typical experiment.

Figure 11. CO2 adsorbed in each cycle for typical experiment.

Figure 13. Adsorption time per cycle for typical experiment.

to cool the sorbent in TSA, and the need to pull a very deep vacuum in water-free VSA. This hypothesis is tested in a sorbent regeneration experiment that starts by pulling a vacuum down to 30 Torr and then maintaining a constant pressure of 30 Torr. The pressure is maintained constant by introducing steam at a very low rate, which displaces the sorbate without significant condensation. Typical mass flow rates for CO2 and N2 during the first desorption cycle are shown in Figure 10. The typical average mass flow rate of water vapor, which is not shown in Figure 10, is 0.28 g/min. Similar to steam-TSA and VSA, a clearly defined two-fraction desorption is observed, allowing for capture of a high purity CO2 product by adjusting the cutoff point between the two fractions. Following several initial cycles, the temperature of the bed, which depends on the heats of adsorption, stabilizes and remains constant at about 55 °C, regardless of the initial bed temperature and flue gas temperature. Because of this stable temperature, the sorbent bed is preheated to 50 °C before the first cycle to aid reaching a rapid cyclic steady state. One of the outcomes of the sorption experiments is the sorbent CO2 capacity, for example estimated in weight percent from the actual breakthrough curve and material balance, which can be used as a measure of the sorbent stability from cycle to cycle. Very little CO2, if any, is lost prior to breakthrough because the effluent CO2 concentration is effectively zero, as shown in Figure 5 for dry sorbent. For a series of sorption desorption experiments with 30 Torr sorbent regeneration, following the

first cycle with a fresh dry sorbent, the CO2 capacity significantly drops due to some degree of water adsorption. However, this does not seem to affect the sorbent capacity in the following cycles. Following the first cycle, the CO2 capacity in the subsequent SA-VSA cycles remains relatively stable or fluctuates within a narrow range, as illustrated in Figure 11. This has been confirmed for up to 15 cycles. An attempt at quantifying the mass of water retained in initial cycles is illustrated in Figure 12. Figure 12 suggests a significant decrease in water retained in going from cycle 1 to 2, by a factor of 4 or so, and much less in going from cycle 2 to 3, by a factor of 2 or so. The subsequent lower values suggest slight but quantifiable water accumulation, which however does not seem to affect the process much. For example, Figures 13 and 14 illustrate the sorption and desorption times. The sorption and desorption times substantially decrease from cycle 1 to 2, due to the lower CO2 capacity caused by water sorption, but then they remain steady, and result in a stable recovery of pure CO2. If the water retained by the sorbent is due to steam adsorption alone, one should not see much, if any, accumulation upon reaching a cyclic steady state. One could explain the slight but consistent water retention from cycle to cycle shown in Figure 12, approximately 0.1% of the sorbent mass, on the basis of very slow water vapor sorption or, more likely, trace steam condensation. Such condensation is not detectable visually, but perhaps it can take place in colder spots, which could form due to minor heat loss and the endothermic nature of desorption process. From a 9700

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Figure 14. Desorption time per cycle for typical experiment.

Figure 17. Effect of desorption pressure on the average weight percent CO2 adsorbed on bed after first cycle.

Figure 15. Effect of insulation thickness on the average water accumulation in the bed per cycle after the first cycle.

Figure 18. Effect of desorption pressure on the average adsorption time after the first cycle.

Figure 16. Effect of desorption pressure on the average water accumulation per cycle after the first cycle.

series of experiments at the reference pressure of 30 Torr with vessel walls insulated with fiberglass of varying thickness, we find that increasing insulation thickness generally does not affect water retention for the first cycle, which is consistent with the steam adsorption conjecture. However, average water retention for the subsequent cycles does not go to zero, but it substantially decreases upon increase of the insulation thickness, as shown in Figure 15. While these data do not prove bulk water condensation,

they can be explained by slight condensation due to heat losses that go down upon increase of the insulation thickness. As in other experiments, it is reassuring to find that these changes in insulation and water retention do not affect the CO2 capacity, adsorption/desorption times, and the cyclic steady state bed temperature, which suggest a robust and stable CO2 capture. Upon increasing the desorption pressure, one should expect a higher water retention, and vice versa. This is confirmed in a series of cycles with uninsulated vessel for desorption pressures of 50 and 15 Torr, in addition to the reference desorption pressure of 30 Torr. As the desorption pressure increases, the average amount of water retained per cycle after the first cycle also increases, as shown in Figure 16, because the increasing pressure increases the sorbent capacity for water and makes the steam easier to condense. With more water in the sorbent, the capacity for CO2 decreases, as shown in Figure 17. As a consequence, the times for adsorption and desorption decrease with increasing pressure, as shown in Figures 18 and 19, respectively. However, neither the CO2 recovery nor its purity is affected by the desorption pressure to a significant degree. Furthermore, it turns out that changing desorption pressure does not affect the bed temperature much, which stabilizes around 55 °C for all three levels of the desorption pressure. 9701

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Figure 19. Effect of desorption pressure on the average desorption time after the first cycle.

Finally, the data presented in this work, for example in Figure 10, suggest that one can control the cutoff between the initial CO2-lean fraction and the final CO2-rich fraction to control and fine-tune the CO2 purity. For example, a cutoff at around 70% CO2 in the desorption stream results in a CO2-lean fraction of about 35% CO2 and a CO2-rich fraction of 95% CO2 or better. If the former is vented and the latter is captured, the CO2 recovery is about 66%. If, however, the CO2-lean fraction is recycled and mixed with the feed flue gas, which is the preferred option, nearly all the CO2 is potentially recoverable. This is because the recycle stream is much richer in CO2 than the original flue gas. While a nearly complete CO2 recovery of nearly pure CO2 is not only feasible but is also likely, it needs to be demonstrated in more systematic tests using a real flue gas, which is a subject of future work. For the record, the flue gas used in these experiments does not contain water vapor while real flue gas can contain some water vapor, which may affect the sorption and regeneration steps in all experiments. However, by using water-free flue gas, in this work we could draw a useful conclusion on whether it is beneficial to have some water in the process. Water in the sorption stage obviously reduces the capacity of the bed, while in the regeneration stage water reduces the CO2 partial pressure and hence allows CO2 recovery at higher pressures (vacuum that is not as low as that required in the absence of water vapor). If we could control or at least limit the amount of water retained in the system, such as in SA-VSA performed in this work, the capacity would not decrease much while the gain of lower energy requirement for vacuum generation could be significant. That said, it is possible, depending on the moisture content in the real flue gas, that the VSA process could become essentially SA-VSA and might or might not need additional steam to maintain higher pressure. This work also suggests intriguing research questions, for example, how to design and optimize future sorbents. This calls for a better understanding of the most fruitful pore size and structure that will maximize the CO2 capacity and selectivity in the presence of water vapor, which is a major experimental and theoretical challenge. Some preliminary work has been done toward this end,11 but much remains to be done. Also, all major process parameters, such as desorption pressure, sorption/desorption times, and hardware, especially the vacuum system, including the vacuum pump energy, need to be optimized toward a realistic cost estimate.

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’ CONCLUSIONS In a carbon filter process, CO2 sorption on porous carbons is spontaneous and rapid, but its recovery from the sorbent is a challenge that determines the process efficiency. Among the desorption methods considered in this work, direct steam can produce nearly pure CO2, but it results in condensed water that reduces the sorbent capacity and requires sorbent cooling, which is slow. Water-free vacuum desorption also works, but it requires low pressures, much lower than 30 Torr, to achieve substantial CO2 yields. However, a hybrid approach, referred to as “steamaided vacuum swing adsorption” (SA-VSA), proposed in this work, is found to alleviate or eliminate these problems. Steam serves as a displacement medium and hence increases the vacuum efficiency at moderate pressures. Vacuum, on the other hand, prevents bulk water condensation and sorbent heating, and hence eliminates the need for cooling. As a result, SA-VSA can completely recover nearly pure CO2 from combustion flue gas, and it also allows for optimizing the CO2 recovery and purity. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: (307) 766-2500. Fax: (307) 766-6777.

’ ACKNOWLEDGMENT This work was funded by Wyoming’s Enhanced Oil Recovery Institute, Supercritical Fluids LLC, the state of Wyoming’s Clean Coal Program administered by the University of Wyoming’s School of Energy Resources, and a discretionary fund of one of the authors (M.R.). The authors also thank Mr. Kaspars Krutkramelis, who took the steam and vacuum data, Dr. Xin Hu, who characterized the sorbents, and the reviewers who provided helpful comments. ’ REFERENCES (1) Radosz, M.; Hu, X.; Krutkramelis, K.; Shen, Y. Flue-Gas Carbon Capture on Carbonaceous Sorbents: Toward a Low-Cost Multifunctional Carbon Filter for “Green” Energy Producers. Ind. Eng. Chem. Res. 2008, 47, 3783–3794. (2) Herzog, H. E.; Drake, E. A. CO2 Capture, Reuse, and Storage Technologies for Mitigating Global Climate Change; Technical Report No. DOE/DE-AF22-96PC01257; U.S. Department of Energy: Pittsburgh, PA, 1999. (3) Mimura, T.; Nojo, T.; Iijima, M.; Yoshiyama, T.; Tanaka, H. Recent Developments on Flue Gas CO2 Recovery Technology. Greenhouse Gas Control Technologies—6th International Conference; Elsevier: New York, 2003; Vol. 2, p 1057. (4) White, C. M.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S.; Pennline, H. W. Separation and Capture of CO2 from Large Stationary Sources and Sequestration in Geological Formations—Coalbeds and Deep Saline Aquifers. Air Waste Manage. Assoc. 2003, 53, 645. (5) Aaron, D.; Tsouris, C. Separation of CO2 from Flue Gas. Sep. Sci. Technol. 2005, 40, 321. (6) Sjostrom, S.; Krutka, K. Evaluation of Solid Sorbents as a Retrofit Technology for CO2 Capture. Fuel 2010, 89, 1298. (7) Plaza, M. G.; Garcia, S.; Rubiera, F.; Pis, J. J.; Pevida, C. Postcombustion CO2 Capture with a Commercial Activated Carbon: Comparison of Different Regeneration Strategies. Chem. Eng. J. 2010, 163, 41. (8) An, H.; Feng, B.; Su, S. CO2 Capture by Electrothermal Swing Adsorption with Activated Carbon Fibre Materials. Int. J. Greenhouse Gas Control 2010, 5, 16. 9702

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(9) Kikkinides, E. S.; Yang, R. T.; Cho, S. H. Concentration and Recovery of CO2 from Flue Gas by Pressure Swing Adsorption. Ind. Eng. Chem. Res. 1993, 32, 2714. (10) Rousseau, R. W. Handbook of Separation Process Technology; John Wiley & Sons: New York, 1987. (11) Hu, X.; Shen, Y.; Radosz, M.; Adidharma, H.; Thommes, M. Porous Carbon Materials: Maximizing CO2 Sorption Capacity at Low Pressures. Presented at the Annual AIChE Meeting, 2009.

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