Magnesia-Stabilized Calcium Oxide Absorbents with Improved

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Magnesia-Stabilized Calcium Oxide Absorbents with Improved Durability for High Temperature CO2 Capture Liyu Li,* David L. King, Zimin Nie, and Chris Howard Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Post Office Box 999, Richland, Washington 99354

Calcium oxide based materials are attractive regenerable absorbents for separating CO2 from hot gas streams because of their high reactivity, high CO2 capacity, and low material cost. Their high carbonation temperature makes it possible to recover and use high quality heat released during CO2 capture, which increases overall process efficiency. However, the performance of all reported CaO-based absorbents deteriorates as the number of carbonation-decarbonation cycles increases. This is caused by absorbent sintering during the highly exothermic carbonation process. We have found that sintering can be effectively mitigated by properly mixing with a modest amount of MgO. A class of CaO-based absorbents with improved durability and CO2 reactivity were prepared by physical mixing of Ca(CH3COO)2 with small MgO particles followed by high temperature calcination. With 26 wt % MgO content, a CaO-MgO mixture prepared by this method gives as high as 53 wt % CO2 capacity after 50 carbonation-decarbonation cycles at 758 °C. Without MgO addition, the CO2 capacity of pure CaO obtained from the same source decreases from 66 wt % for the first cycle to 26 wt % for the 50th cycle under the same test conditions. Introduction The increasing concentration of CO2 in the atmosphere due to fossil fuel burning has been identified as a major contributor to global warming. In the predictable future, fossil fuels will continue to be the dominant energy source, which means that CO2 will continue to be released into the atmosphere.1 To mitigate the related global climate deterioration, large scale CO2 capture and sequestration (CCS) has been proposed and widely studied.2,3 It is well accepted that CaO-containing materials are good candidate absorbents for CO2 capture due to their high reactivity for CO2 absorption, high CO2 capacity, low material cost, and, importantly, their high carbonation temperature (600-700 °C). This makes it possible to efficiently recover as high quality heat the large amount of energy released during CO2 capture (178 kJ/mol CO2).4 With efficient energy recovery, CaO-based absorbents have potentially great advantages over other absorbents that operate at lower temperatures.5,6 However, the carbonation and decarbonation reactions of CaO and CaCO3 are far from complete or reversible.7 Rapid loss of CO2 capacity over many carbonation-decarbonation cycles is always observed due to severe absorbent sintering, which is caused by three major factors: (1) the carbonation process is highly exothermic (CaO + CO2 ) CaCO3, ∆H° ) -178 kJ/mol); (2) there is a large volume increase from CaO to CaCO3 (from 16.9 to 34.1 cm3/mol), which greatly decreases the distance between absorbent particles in the carbonated state; (3) CaCO3 has a Tammann temperature (i.e., the highest treatment temperature before the sintering of a material becomes significant) of 533 °C, lower than normal carbonation temperatures.8 Enhancing the durability of CaO-based CO2 absorbents has been an active research topic for many years. Two major approaches have been used: (1) incorporation of inert materials, such as MgO,9 Al2O3,10-12 ZrO2,8 TiO2,13 SiO2,14,15 and La2O3;9 (2) modification of the stability16 and structure17-19 of CaO. Several promising absorbents have been developed using these * To whom correspondence should be addressed. E-mail: liyu.li@ pnl.gov.

two approaches, including the Al2O3-CaO system by Li et al.10 and the special pore-structured CaO absorbent (calcined Precipitated Calcium Carbonate, PCC absorbent) by Fan et al.17-19 Table 1 gives the performance of some promising systems. For comparison, the performance of Li2SiO4 is also given. However, no materials have been identified yet which can survive the thousands of carbonation-decarbonation cycles required by practical applications with acceptable capacities. In this paper, we report that the method of incorporating the inert materials has a critical effect on the long-term stability of the CaO-based absorbent. A class of stable MgO-doped CaO absorbents have been developed using mechanical mixing of small MgO particles with Ca(CH3COO)2 followed by high temperature calcination. MgO was selected because of its high stability, lack of CO2 absorption at the CaO reaction temperature, and lack of reaction with CaO or CaCO3 under the operating conditions. MgO nanoparticles can be easily produced by thermal decomposition of many Mg-containing salts. Experimental Section Materials and Preparation Methods. Reagent-grade chemicals CaO (160 nm powder), Ca(OH)2 (∼10 µm), Ca(NO3)2 · 4H2O (∼50 µm), MgO (325 µm), Mg(OH)2, Mg(CH3COO)2 · 4H2O, and Na2CO3 were purchased from Sigma-Aldrich Co. Reagent-grade chemicals CaO (∼10 µm), Ca(CH3COO)2 · 0.4H2O (∼10 µm), calcium oxalate CaC2O4 (∼50 µm), dolomite natural mineral CaMg(CO3)2, magnesium oxalate MgC2O4 · 2H2O, and Mg(CH3COO)2 · 4H2O were purchased from Alfa Aesar. Two nanosized MgO samples, NanoActive Magnesium Oxide (crystallite size ∼8 nm, volume weighted mean aggregate size ∼16 µm) and NanoActive Magnesium Oxide Plus (crystallite size ∼4 nm, volume weighted mean aggregate size ∼16 µm), were ordered from NanoScale Corp. (Manhattan, KS). These chemicals were used as received. Pure CaO samples were prepared by direct thermal decomposition of CaO-containing sources at 800 °C for 2 h in air. Four different methods were used to prepare MgO-stabilized CaO absorbents. Table 2 briefly summarizes these methods. Dry

10.1021/ie901166b CCC: $40.75  2009 American Chemical Society Published on Web 10/07/2009

Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009

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Table 1. Performance of Some Promising Absorbents for High Temperature CO2 Capture

references carbonation T, °C t, min PCO2, atm decarbonation T, °C t, min PCO2, atm cycles end capacity, g of CO2/ 100 g of sorbent

stabilized CaO nanoparticles

PCC-CaO

dolomite (CaO-MgO 1:1 molar)

20, 21

16

17-19

22

9

10, 11

8

600 80 0.2

700 300 0.3

700 30 0.1

800 5 1.0

750 20 0.25

700 30 0.2

700 30 0.3

820 80 0.2 50 26.5

700 30 0 50 39

700 30 0 (N2) 50 40

950 5 1.0 50 18

750 30 0 (N2) 50 30

850 5 0 (N2) 50 41

700 30 0 (He) 100 30

Table 2. Preparation Methods for MgO-Stabilized CaO Absorbents method coprecipitation

solution mixing

dry physical mixing wet physical mixing

details precipitation of aqueous solution containing Ca and Mg acetates with 1 M Na2CO3, followed by filtration, washing, drying, and calcination in air at 800 °C for 2 h direct drying of an aqueous solution containing Ca and Mg acetates, followed by calcination in air at 800 °C for 2 h overnight ball-milling of solid CaO and MgO sources, followed by calcination in air at 800 °C for 2 h overnight ball-milling of 2-propanol slurry containing CaO and MgO sources, followed by drying, and calcination in air at 800 °C for 2 h

physical mixing produced fine absorbent powder. To produce 60-80 mesh (∼175-250 µm) particles that can be tested in a fixed bed reactor, an organic binder (from Ferro Electric Materials, organic binder B75717) was mixed with fine powder samples at 1:1 weight ratio followed by calcination in air at 800 °C for 2 h and then crushing and sieving. Agglomerated absorbents were produced by wet physical mixing. High Temperature Carbonation-Decarbonation Performance Measurement. A Netzsch 409C thermogravimetric analyzer (TGA) was used to screen the performance of the absorbents. Typical measurements employed ∼20 mg of

20 wt %MgO doped CaO

ZrO2-CaO (3:10 molar)

Li4SiO4

CaO-Ca12Al14O33 (3:1 mass)

powder, and the carbonation-decarbonation test was carried out at a fixed temperature, 758 °C. During each test, 70 mL/ min 100% CO2 (for carbonation) and 130 mL/min pure He (for decarbonation) were introduced into the system alternatively via an automated switch valve every 30 min. The purpose of this test protocol is to carry out uptake and regeneration cycles in an accelerated fashion relative to expected operation with CO2-containing syngas streams. This allows material stability data to be obtained in a reasonable time frame. According to the open literature, this simple test procedure works very well for CaO-based absorbent screening.17-19 The cycle number varied according to the performance of each absorbent. In order to compare the absorbent stability, 50-100 carbonation-decarbonation cycles were normally carried out. CO2 absorption capacity was calculated using the total weight gain during each carbonation cycle divided by the total weight of absorbent in the oxide form. CaO utilization was calculated as the percentage of CaO converted to CaCO3, based on CO2 capacity and CaO concentration in the absorbent. To validate the screening test results, selected absorbents (60-80 mesh particles) were also evaluated in a small fixed bed reactor (Inconel 625 tube reactor, 6.6 mm i.d.), which was heated by a clamshell furnace. Simulated precombustion fuel gas (30% CO2, 10% N2, 50% Ar, and 10% H2O, 550 h-1 gas hourly space velocity (GHSV), N2 and Ar used as H2 simulant) and decarbonation purge gas (95% Ar, 2.5%N2, and 2.5% H2O, 2200 h-1 GHSV) were introduced into the system alternatively

Figure 1. Performances of CaO absorbents obtained from different sources. Carbonation at 758 °C in 100% CO2 for 30 min; decarbonation at 758 °C in 100% He for 30 min.

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Figure 2. SEM images of fresh and used pure CaO absorbents: (a) fresh CaO from Ca(NO3)2; (b) fresh CaO from CaC2O4; (c and d) fresh CaO from Ca(CH3COO)2; (e and f) CaO from Ca(CH3COO)2 after 52 cycles of carbonation and decarbonation at 758 °C.

via two automated switch valves. Absorbent carbonation was carried out in the fuel gas at 650 °C for 60 min, and decarbonation was carried out in the purge gas by heating the absorbent from 650 to 800 °C at 5 °C/min, holding at 800 °C for 20 min, cooling to 650 at 5 °C/min, and holding for 10 min. This test condition allows 90% CO2 removal during the carbonation process and fast decarbonation during the calcination process. Dry gas flows were metered using MKS mass flow controllers. Steam was generated using a small cartridge vaporizer, and steam flow was controlled by an HPLC pump. Downstream of the absorption bed, water was removed with a condenser followed by a 50-tube Nafion membrane dryer (Perma Pure LLC, Toms River, NJ). During the test, the exit gas flow rate was continuously measured using a volumetric digital flow meter (DryCal Definer 220, Bios International, Butler, NY). An Agilent 3000A micro gas chromatograph (micro GC) was also used to analyze the gas composition. Characterization. Scanning electron microscopy (SEM) analysis was carried out with a JEOL JSM-5900LV microscope.

Selected area energy dispersive X-ray spectroscopy (EDS) was performed on regions of interest using a Links EDS system equipped on the microscope. Powder X-ray diffraction (XRD) measurement and analysis were conducted with a Philips PW3050 diffractometer using Cu KR radiation and JADE, a commercial software package. The nitrogen BET surface area was measured with a QUANTACHROME AUTOSORB 6-B gas sorption system with 200 °C degassed samples. Results and Discussion Performance of Pure CaO Absorbents from Different Sources. Four pure CaO absorbents were prepared by thermal decomposition of Ca(OH)2, Ca(CH3COO)2, Ca(NO3)2, and CaC2O4. Their CO2 capture performances, along with those of two commercial CaO samples (10 µm particles and 160 nm particles), were evaluated in a TGA unit at 758 °C. Figure 1 gives the results. Although all these samples do not exhibit the required stability for a long-term carbonation-decarbonation


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Figure 3. Effect of MgO-mixing method on CO2 capture performances of 42 wt % MgO-58 wt % CaO absorbents. Top: CO2 capacity vs cycle number. Bottom: CaO utilization vs cycle number.

process, it is interesting to note that CaO absorbents obtained from different sources exhibit quite different CO2 capture performances. CaO prepared by direct decomposition of Ca(CH3COO)2 gives the best performance, even better than that of the commercial 160 nm CaO. Figure 2 gives SEM images of three fresh CaO samples (from decomposition of nitrate, oxalate, and acetate) and one used sample (CaO from acetate, after 52 cycles of carbonation-decarbonation at 758 °C). Decomposition of Ca(NO3)2 gives a very dense CaO sample. As a result, poor CO2 capture performance was observed. Decomposition of Ca(CH3COO)2 produces small CaO crystals with unique porous structure. This special structure contributes to its good long-term CO2 capture performance. Based on this observation, Ca(CH3COO)2 was used as the CaO source throughout the rest of this study. Performance of MgO-Stabilized CaO Absorbent Powders. To improve the long-term performance of the CaO-based absorbent, the addition of inert MgO was extensively studied. Three samples with 42 wt % MgO and 58 wt % CaO were prepared using three different methods described earlier: coprecipitation, solution mixing, and dry physical mixing of Ca(CH3COO)2 with MgO (from decomposition of MgC2O4 at

600 °C). Figure 3 summarizes the CO2 capture results. For comparison, a natural mineral with similar Ca-Mg ratio, dolomite (65 wt % CaO and 35 wt % MgO, as measured by EDS), was also tested and the result is also provided in Figure 3. Despite the similarity of composition, the long-term performances of these samples are surprisingly different. The absorbent obtained from coprecipitation of Ca(CH3COO)2 and Mg(CH3COO)2 with Na2CO3 gave the worst performance, with less than 10 wt % CO2 capacity after 30 cycles. Ca(CH3COO)2 and Mg(CH3COO)2 solution mixing followed by calcination produced an absorbent with a performance similar to that of natural dolomite, indicating that molecular level mixing of CaO and MgO can be achieved with this method. The absorbent obtained from dry physical mixing of Ca(CH3COO)2 with MgO showed the best stability and the highest CO2 capacity (>43 wt %), as well as the highest CaO utilization (>95%) after 50 cycles of carbonation-decarbonation. The results shown in Figure 3 indicate that, when mixing inert MgO into CaO absorbents, molecular level mixing is not the best way to achieve longterm durability. A certain level of heterogeneous mixing appears to have a benefit, probably as a result of providing stable segregation of small CaO particles. Figure 4 shows in greater

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furan (THF) and then the solution was physically mixed with Microna limestone (3.2 µm mean particle size, from Columbia River Carbonates in Washington state) in a mortar. After THF removal, the mixture was reground and then calcined at 900 °C for 3 h. A MgO concentration of 20 wt % was found to be the optimal loading, which gave about 30 wt % CO2 capacity at 750 °C after 50 cycles, and about 26 wt % CO2 capacity after 1250 cycles. The effect of the MgO source was evaluated by physical mixing of Ca(CH3COO)2 with MgO obtained from different precursors. In this case, 26 wt % MgO doped absorbents were used (as will be shown later, use of a lower MgO content results in an accelerated sintering of the CaO). Table 3 compares their performance. In general, the effect of the MgO source is not as large as that of the CaO source. Except for one absorbent containing MgO obtained from decomposition of Mg(OH)2, all the MgO-doped samples show much better long-term performance than the undoped pure CaO absorbent. Among all the tested samples, MgO obtained from thermal decomposition of MgC2O4 at 700 °C shows the best performance and long-term stability when admixed with the calcium oxide precursor. Even after 100 cycles, this absorbent still maintains 45 wt % CO2 capacity, corresponding to 77% total CaO utilization. These results compare favorably with those given in Table 1. However, SEM analysis shows there are dramatic morphology changes along with CaO and MgO particle redistribution after 100 cycles of carbonation and decarbonation (Figure 5). According to the EDS analysis, the MgO and CaO distribution in the fresh 26 wt % MgO doped CaO absorbent was not uniform. A core-shell structure with a CaO-rich region covered by MgO-CaO mixture was observed. Uniform mixing of MgO and CaO particles occurred in the structure after 100 cycles. Although some particle sintering was observed after the cycling test, the absorbent still maintained a porous structure. All the MgO samples obtained from decomposition of MgC2O4 at different temperatures were characterized using XRD analysis, and the MgO crystallite size was roughly estimated using Jade Software based on Scherrer’s equation. Figure 6 shows that very small MgO crystallites can be obtained by direct thermal decomposition of MgC2O4 below 800 °C. The surface area of the MgO sample prepared by calcination of MgC2O4 at 600 °C, as measured by the BET method, is 181 m2/g. If all the small MgO crystallites are considered to be spherical and the theoretical MgO density (3.58 g/cm3) is used, the calculated MgO crystallite size is about 9 nm, which is close to that estimated from XRD analysis (∼8 nm). As shown in Table 3, these small MgO particles, at 26 wt % loading, can effectively stabilize CaO absorbents during carbona-

Figure 4. Carbonation and decarbonation reactions of 42 wt % MgO doped CaO absorbent prepared by dry physical mixing of Ca(CH3COO)2 and MgO. Top: 2nd cycle. Bottom: 50th cycle.

detail the carbonation and decarbonation reactions of this absorbent at the second and 50th cycles. During the carbonation steps of both the second cycle and the 50th cycle, more than 80% of total CO2 capture is achieved within the first 4 min, indicating this absorbent has good CO2 capture kinetics. On the other hand, it takes about 17 min (the second cycle) and about 20 min (the 50th cycle) to completely regenerate the CO2-loaded absorbent. Recently, Albrecht et al. reported that mixing MgO into limestone can improve its cyclic stability for CO2 capture.9 In their work, Mg(NO3)2 salt was first dissolved in tetrahydro-

Table 3. Effect of MgO Source on the Performance of 26 wt % MgO Doped CaO Absorbenta,b,c CO2 capacity at different carbonation-decarbonation cycles, wt % MgO source

5th cycle

25th cycle

50th cycle

100th cycle

no MgO mixing MgO (∼325 µm) NanoActive MgO (∼8 nm) NanoActive MgO Plus (∼4 nm) MgO from decomposition of Mg(OH)2 at 800 °C, 2 h MgO from decomposition of Mg(CH3COO)2 at 700 °C, 2 h MgC2O4 as received MgO from decomposition of MgC2O4 at 500 °C, 2 h MgO from decomposition of MgC2O4 at 600 °C, 2 h MgO from decomposition of MgC2O4 at 700 °C, 2 h MgO from decomposition of MgC2O4 at 800 °C, 2 h MgO from decomposition of MgC2O4 at 900 °C, 2 h MgO from decomposition of MgC2O4 at 1000 °C, 2 h

60.8 47.9 46.2 47.9 49.1 49.0 50.5 55.4 55.9 57.0 53.9 52.2 50.3

36.2 45 44.3 48.0 46.5 44.3 49.6 53.7 55.0 55.0 52.3 53.4 48.9

26.0 42.2 41.1 47.9 34.1 NM 44.7 51.5 52.9 52.9 50.7 49.6 NM

NM NM NM NM NM NM NM 43.6 44.1 45.2 45.2 42.6 NM

All the absorbents were prepared by dry physical mixing of Ca(CH3COO)2 with MgO source, followed by calcination at 800 °C in air for 2 h. Carbonation at 758 °C in 100% CO2 for 30 min; decarbonation at 758 °C in 100% He for 30 min. c NM ) not measured. a

b


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Figure 5. Cross-sectional SEM analysis of fresh and used 26 wt % MgO doped CaO absorbents prepared by dry physical mixing of Ca(CH3COO)2 with MgO. (a-e) Fresh sample; (h-j) after 100 cycles test; (d and e) EDS analysis of selected area in image c; (i and j) Mg and Ca elemental mapping of image h.

tion-decarbonation cycles. Table 3 also shows that two commercial nano MgO samples are not as effective, probably due to MgO particle agglomeration (manufacture-reported volume weighted mean aggregate size is ∼16 µm) in these two products. To optimize the absorbent composition, the effect of MgO concentration was studied. The absorbents were prepared as before by dry physical mixing of Ca(CH3COO)2, with MgO obtained from calcination of Mg2C2O4 at 700 °C. Figure 7 gives the performances of absorbents with 19, 26, and 42 wt % MgO content. Higher MgO

content gives better stability and higher CaO utilization, but relatively lower CO2 capacity. In practical applications, the optimized MgO contenct will be largely decided by the cost of the absorbent, the fuel gas or flue gas composition, and the absorbent replacement ratio during each cycle. An economic evaluation needs to be carried out before a recommendation of absorbent composition can be given. Fixed Bed Testing of MgO-Stabilized CaO Absorbent Particles. The absorbents prepared by dry physical mixing of Ca(CH3COO)2 and MgO are fine powders. To prepare absorbent

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Figure 6. Effect of calcination temperature of MgC2O4 on the crystallite size of produced MgO particles. Crystal size was estimated from XRD pattern using Scherrer’s equation. Duration of calcination at each temperature: 2 h.

Figure 7. Effect of MgO concentration on CO2 capture performance of three CaO-based absorbents prepared by dry physical mixing. Top: CO2 capacity vs cycle number. Bottom: CaO utilization vs cycle number.

particles for fixed bed reactor testing, some of the candidate absorbents were admixed with organic binder (at 1:1 weight

ratio) and then calcined at 800 °C for 2 h. After calcination, the organic binder was completely burned to CO2 and H2O. The


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Figure 8. Effect of MgO concentration on CO2 capture performance of absorbents particles prepared by dry physical mixing followed by agglomeration using organic binder and calcination at 800 °C.

Figure 9. SEM images of 26 wt % MgO doped CaO particles prepared by wet physical mixing method.

Figure 10. Effect of MgO concentration on CO2 capture performances of absorbents prepared by wet physical mixing.

resulting agglomerate was broken into smaller particles, and 60-80 mesh particles were tested in the TGA unit. Figure 8

gives the CO2 capture performance of three absorbent compositions with 26, 32, and 42 wt % MgO. The organic binder likely

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Figure 11. Performance comparison of 42 wt % MgO doped CaO absorbent prepared by wet physical mixing) and dolomite (in both cases using 60-80 mesh particles) in a fixed bed reactor. Carbonation: 650 °C, 60 min, 30% CO2, 10% H2O, 50% Ar, 10% N2, 550 h-1 GHSV. Decarbonation: 800 °C, 20 min, 2.5% H2O, 2.5% N2, 95% Ar, 2200 h-1 GHSV. The 10% CO2 breakthrough capacity is defined as the amount of CO2 absorbed with at least 90% of the CO2 in the fuel gas captured by the absorbent during each cycle.

brought together small MgO and CaO particles and slightly promoted sintering. As a result, the performance is slightly worse than that of the pure powder samples. The highest MgO content sample (42 wt %) shows the most stable performance over the multicycle test. Agglomerated absorbent particles can be directly produced using 2-propanol wet physical mixing as described in the Experimental Section using Ca(CH3COO)2 and MgO (obtained from decomposition of MgC2O4 at 700 °C) as source materials. Figure 9 gives SEM images of fresh 26 wt % MgO doped absorbent prepared by this method, showing the porous structure of this absorbent. EDS analysis shows MgO particles are uniformly distributed in the as-prepared absorbent, which is different from those prepared using the dry physical mixing method. Figure 10 gives the CO2 capture performances of four absorbents prepared by this wet physical mixing method. Again, addition of MgO can significantly improve the durability of CaO-based absorbents. The 60-80 mesh 42 wt % MgO doped absorbent particles, prepared by breaking and sieving the large absorbent pieces produced by direct calcination of Ca(CH3COO) and MgO slurry, were further evaluated in a fixed bed reactor using simulated precombustion fuel gas (30% CO2, 10% N2, 50% Ar, and 10% H2O, at 550 h-1 GHSV). This gas composition was to simulate fuel gas after complete water-gas-shift reaction of CO.23 N2 and Ar were used to simulate H2. The carbonation was carried out at 650 °C, and regeneration was carried out in a CO2-free purge gas at 800 °C (95% Ar, 2.5%N2, and 2.5% H2O, 2200 h-1 GHSV). Figure 11 provides the performance result. For comparison, the performance of dolomite was also tested in the fixed bed reactor under same conditions, and the result is also given in Figure 11. Again, although these two samples have similar MgO loading, their performances are quite different. It is clear that properly mixing MgO into CaO absorbent is critical when preparing stable absorbents. Due to the high porosity, the 42 wt % MgO doped absorbent prepared in this work has a

much lower packing density than that of dolomite (0.27 g/cm3 vs 1.34 g/cm3). Conclusion The sintering effect of pure CaO absorbents during high temperature carbonation and decarbonation was examined. Although all the pure CaO materials tested in this work sintered very quickly during multiple-cycle testing, the sintering level varied significantly. Decomposition of Ca(CH3COO)2 gives the best CaO material for long-term high temperature CO2 capture. To improve the long-term performance of CaO-based absorbents, the effect of MgO stabilization was extensively studied. The method of MgO introduction plays an important role in producing stable absorbents. Among the four preparation methods used in this work, i.e., solution mixing, coprecipitation, dry physical mixing, and wet physical mixing, the two physical mixing methods produce the most durable absorbents with high CO2 capacity. The source of MgO also has some effect on the performance of MgO-CaO mixture, although this effect is not as significant as that of the mixing method. CaO doped with MgO nanoparticles prepared by thermal decomposition of MgC2O4 at 700 °C showed the best performance. With 26 wt % MgO content, a CaO-MgO mixture prepared by this method gives as high as 53 wt % CO2 capacity after 50 carbonationdecarbonation cycles at 758 °C, which is better than other stabilized CaO-based absorbents reported in the open literature. Without MgO addition, the CO2 capacity of pure CaO obtained from same source decreases from 66 wt % for the first cycle to 26 wt % for the 50th cycle under the same test conditions. Absorbent particles suitable for fixed bed testing can be prepared by adding organic binder to the absorbent powders followed by high temperature calcination, or by wet physical mixing. These absorbents show performances similar to those in the fine powder form. In a fixed bed reactor test using simulated fuel gas, an absorbent with 42 wt % MgO loading gave 38 wt % CO2 capacity after 45 carbonation-decarbonation cycles, before 10% CO2 breakthrough was observed. This performance is far superior to that of dolomite, which has a similar chemical composition.


Acknowledgment Financial support from Battelle Memorial Institute is gratefully acknowledged. This work was performed in part at the Interfacial and Nano Science Facility in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Office of Biological and Environmental Research of the U.S. Department of Energy and located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the U.S. Department of Energy by Battelle. Literature Cited (1) Raupach, M. R.; Marland, G.; Ciais, P.; Le Quere, C.; Canadell, J. G.; Klepper, G.; Field, C. B. Global and regional drivers of accelerating CO2 emissions. Proc. Natl. Acad. Sci. U.S.A. 2007, 104 (24), 10288–10293. (2) Plasynski, S. I.; Litynski, J. T.; McIlvried, H. G.; Srivastava, R. D. Progress and New Developments in Carbon Capture and Storage. Crit. ReV. Plant Sci. 2009, 28 (3), 123–138. (3) IntergoVernmental Panel on Climate Change (IPCC) Special Report: Carbon Dioxide Capture and Storage; Metz, B., Davidson, O., Coninck, H.; Loos, M., Meyer, L., Eds.; published for IPCC; Cambridge University Press: New York, 2005. (4) Florin, N. H.; Harris, A. T. Enhanced hydrogen production from biomass with in situ carbon dioxide capture using calcium oxide sorbents. Chem. Eng. Sci. 2008, 63, 287–316. (5) Abanades, J. C.; Rubin, E. S.; Anthony, E. J. Sorbent Cost and Performance in CO2 Capture Systems. Ind. Eng. Chem. Res. 2004, 43, 3462– 3466. (6) Romano, M. Coal-fired Power Plant with Calcium Oxide Carbonation for Post-combustion CO2 Capture. Energy Procedia 2009, 1, 1099– 1106. (7) Grasa, G. S.; Abanades, J. C. CO2 capture Capacity of CaO in Long Series of Carbonation/Calcination Cycles. Ind. Eng. Chem. Res. 2006, 45, 8846–8851. (8) Lu, H.; Khan, A.; Pratsinis, S. E.; Smirniotis, P. G. Flame-Made Durable Doped-CaO Nanosorbents for CO2 Capture. Energy Fuels 2009, 23, 1093–1100. (9) Albrecht, K. O.; Wagenbach, K. S.; Satrio, J. A.; Shanks, B. H.; Wheelock, T. D. Development of a CaO-Based CO2 Sorbent with Improved Cyclic Stability. Ind. Eng. Chem. Res. 2008, 47, 7841–7848.

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ReceiVed for reView July 21, 2009 ReVised manuscript receiVed September 22, 2009 Accepted September 23, 2009 IE901166B

Magnesia-Stabilized Calcium Oxide Absorbents with Improved

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