Synthetic CaO-Based Sorbent for CO2 Capture from Large-Point

Aug 5, 2010 - South Kensington Campus, London SW7 2AZ, United Kingdom. Received April ... to market technologies, e.g., solvent scrubbing. Unfortunate...
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Energy Fuels 2010, 24, 4598–4604 Published on Web 08/05/2010

: DOI:10.1021/ef100447c

Synthetic CaO-Based Sorbent for CO2 Capture from Large-Point Sources Nicholas H. Florin,* John Blamey, and Paul S. Fennell Grantham Institute for Climate Change, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom Received April 9, 2010. Revised Manuscript Received July 6, 2010

The main impetus for future technology development for capturing and purifying CO2 from industrial flue gases is the potential for minimizing the cost of capture and reducing the efficiency penalty that is imposed on the process. Carbonate looping is a very promising future technology, which uses CaO-based solid sorbents, with great potential to reduce the cost of capture and lessen the energy penalty compared to closer to market technologies, e.g., solvent scrubbing. Unfortunately, the CO2-capture capacity of a CaO-sorbent derived from natural limestone decays through long-term capture-and-release cycling; thus, the development of strategies and/or novel sorbents to achieve a high CO2-capture capacity is an important challenge for realizing the cost efficiency of carbonate looping technology. To this end, we report on the development and characterization of a novel synthetic CaO-based sorbent produced via a precipitation method and present experimental results demonstrating improved long-term CO2-capture capacity based on reactivity testing using a thermogravimetric analyzer (TGA) and a bench-scale bubbling fluidized-bed (BFB) reactor. We achieve a capture capacity of about 2.5 times the amount of CO2 after 15 cycles with the synthetic sorbent compared to a natural limestone (Havelock) in the BFB.

temperature, it is proposed that additional fuel be combusted in pure O2, hence, requiring an air separation unit.1,2 A simplified process flow diagram of the carbonate looping technology is presented in Figure 1. Importantly, the energy penalty associated with the air separation is partially offset by the recuperation of heat in the form of the hot CaO and CO2 streams (g650 °C) and heat produced from the exothermic carbonation reaction, which can be used to generate additional steam. On this basis, the efficiency penalty associated with CO2 capture from a power station using carbonate looping is extremely competitive, estimated to be only 6-8% for standard postcombustion carbonate looping3 compared to about 10-12% for amine-based solvent scrubbing.4 In addition to the potential improvements in efficiency, other advantages of carbonate looping include the use of a cheap and environmentally benign sorbent derived from natural limestone or dolomite, the unique prospect for synergy with the cement industry using the exhausted sorbent as a feed stream, relatively low scale-up risks with the use of mature circulating fluidized-bed technology, and broad applicability to a range of advanced clean-energy systems, e.g., for precombustion CO2 capture from fuel or synthesis gas, relevant to integrated gasification combined cycle (IGCC) and hydrogen (H2) production schemes.5-8

1. Introduction The capture and purification of carbon dioxide (CO2) from large-point sources, such as from power stations and energyintensive industries, are a critical challenge with regards to the technical feasibility and cost of carbon capture and storage (CCS) technologies. The major driving force for the development of advanced capture technologies is the potential to improve overall efficiency, reduce the cost of CO2 capture, and minimize adverse environmental impacts, as compared to the more conventional technologies, e.g., amine-based solvent scrubbing. High-temperature sorbents have recently attracted a lot of research attention, and a particularly promising candidate is calcium oxide (CaO), which underpins a process known as carbonate looping. Specifically, carbonate looping exploits the reversible gas-solid reaction between CaO and CO2 to form calcium carbonate (CaCO3), known as carbonation (eq 1). CaOðsÞ þ CO2ðgÞ TCaCO3ðsÞ ¼ - 178 kJ=mol

ΔHr, 298 K ð1Þ

This reaction can be used to selectively separate CO2 from a mixture of gases, such coal-combustion flue gas, and when sufficient heat is applied to CaCO3, CO2 can be released via the reverse reaction, known as calcination, and CaO can be regenerated. Importantly, the release of CO2 from CaCO3 can be carried out under a high CO2 partial pressure, resulting in the production of a pure stream of CO2 suitable for storage or alternative chemical processes. Under these conditions, thermodynamic limitations require that regeneration be conducted at about 900-950 °C; thus, to achieve this high

(2) Abanades, J. C.; Anthony, J. E.; Lu, D. Y.; Salvador, C.; Alvarez, D. Capture of CO2 from combustion gases in a fluidized bed of CaO. AIChE J. 2004, 50, 1614–1622. (3) Abanades, J. C.; Anthony, E. J.; Wang, J.; Oakey, J. E. Fluidized bed combustion systems integrating CO2 capture with CaO. Environ. Sci. Technol. 2005, 39 (8), 2861–2866. (4) U.K. Advanced Power Generation Technology Forum (APGTF). Cleaner Fossil Fuel Power Generation in the 21st Century; April 2009; http://www.apgtf-uk.com/. (5) Han, C.; Harrison, D. Simultaneous shift reaction and carbon dioxide separation for the direct production of hydrogen. Chem. Eng. Sci. 1994, 49, 5875–5883. (6) Balasubramanian, B.; Lopez Ortiz, A.; Kaytakoglu, S.; Harrison, D. P. Hydrogen from methane in a single-step process. Chem. Eng. Sci. 1999, 54, 3543–3552.

*To whom correspondence should be addressed. E-mail: n. [email protected]. (1) Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K. A twin fluid-bed reactor for removal of CO2 from combustion processes. Trans. IChemE 1999, 77A, 62–68. r 2010 American Chemical Society

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Figure 2. Flow diagram of the SBC reactor.

that it is theoretically suited to using Ca2þ derived from natural limestone, and the dissolution of CO2 gas eliminates the need for using additives, such as Na2CO3.11 Moreover, previous work has shown CaO-based sorbents derived from CaCO3 precipitated in a SBC to be highly reactive compared to those derived from natural limestones;15,16 however, the precipitated sorbents are highly susceptible to sintering and are extremely friable.17 Thus, to be suitable for carbonate looping technology, they must be combined with a binder and/or pelletized.12 One particularly promising way to improve the long-term performance of CaO-based sorbents is to incorporate inert binders, such as Al2O3.12-14 In this work, to exploit the high reactivity of precipitated CaO-based sorbent and achieve superior resistance to sintering and mechanical stability, we describe a novel mixed precipitation method to produce CaCO3 and Al(OH)3 (as a precursor to synthetic CaO-based sorbent incorporated with mixed calcium-aluminum binder). The paper reports on the development and characterization of the synthetic CaO-based sorbent and presents experimental results demonstrating improved long-term carrying capacity based on CO2-captureand-release experiments conducted in a thermogravimetric analyzer (TGA) and a bubbling fluidized-bed (BFB) reactor.

Figure 1. Simplified process flow diagram of generic application of carbonate looping technology.

Unfortunately, sorbent derived from natural limestone loses its capacity to capture CO2 after multiple CO2-captureand-release cycles, and a large amount of fresh limestone is required to maintain an acceptable CO2-capture efficiency. The main factors influencing this drop-off in capacity are sintering, attrition, and chemical deactivation, owing to the competing chemical reaction with sulfur dioxide (SO2); these factors are discussed in detail in a recent review by Blamey et al.9 Because of the low cost of the raw material (limestone), this does not rule out the economic feasibility of postcombustion carbonate looping;10 however, it does undermine the cost efficiency and future potential of more advanced applications, such as H2 production. This potential can be realized using modified CaO-based sorbents, which incorporate select additives, e.g., alumina (Al2O3), to maintain long-term capacity for CO2 capture and improve their mechanical stability.11-14 To this end, a novel mixed precipitation method using a slurry bubble column (SBC) was developed to produce a synthetic CaO-based sorbent incorporating an inert mixed calcium-aluminum oxide binder. The major premise of this approach was that, via precipitation, the mixing of the sorbent and binder could be achieved at the micrograin scale, thus achieving an even distribution of the binder within the CaO. Precipitation in a SBC represents a relatively simple and scalable procedure, which may be considered a crude mimic of the precipitation of CaCO3 that occurs in natural marine environments based on the reaction of dissolved CO2 gas and calcium ions (Ca2þ). The main advantage of this method is

2. Experimental Section 2.1. Mixed Precipitation Method. Precipitation experiments were carried out using a SBC (50 mm inner diameter) with a sintered glass distributor with average porosity of 70 μm (Figure 2). In a typical experiment, a saturated slurry of Ca(OH)2 (Acros Organics, purity >98 wt %), i.e., 15 times the saturation limit in 250 mL of deionized water, was loaded in the column under a flow of N2. Laboratory reagent-grade Ca(OH)2 was used instead of natural limestone to observe the performance of the modified sorbent without the influence of impurities present in limestone, although theoretically the process can be applied to natural limestone following an initial calcination step, followed by dissolution of the calcine. These conditions were determined on the basis of a previous parametric study.18 Al(NO3)3 3 9H2O (Fisher Scientific, purity >98 wt %) was introduced to achieve a determined CaCO3/Al(OH)3 ratio, and this mixture was held under N2 at 1.5 L/min for 1 min to mix it. Precipitation of CaCO3 was initiated by switching N2 to the same flow rate as CO2, which was bubbled through the slurry for 5 min; the degree

(7) Lin, S.-Y.; Harada, M.; Suzuki, Y.; Hatano, H. Developing an innovative method, HyPr-RING, to produce hydrogen from hydrocarbons. Energy Convers. Manage. 2002, 43, 1283–1290. (8) Gao, L.; Paterson, N.; Dugwell, D.; Kandiyoti, R. Zero-emission carbon concept (ZECA): Equipment commissioning and extents of the reaction with hydrogen and steam. Energy Fuels 2008, 22 (1), 463–470. (9) Blamey, J.; Anthony, E. J.; Wang, J.; Fennell, P. S. The carbonate looping cycle for large-scale CO2 capture. Prog. Energy Combust. Sci. 2010, 36 (2), 260–279. (10) Mackenzie, A.; Granatstein, D. L.; Anthony, E. J.; Abanades, J. C. Economics of CO2 capture using the calcium cycle with a pressurized fluidized bed combustor. Energy Fuels 2007, 21 (2), 920–926. (11) Pacciani, R; Muller, C. R.; Davidson, J. F.; Dennis, J. S.; Hayhurst, A. N. Synthetic Ca-based solid sorbents suitable for capturing CO2 in a fluidised bed. Can. J. Chem. Eng. 2008, 86, 356–366. (12) Manovic, V.; Anthony, E. J. CaO-based pellets supported by calcium aluminate cements for high-temperature CO2 capture. Environ. Sci. Technol. 2009, 43, 7117–7122. (13) Li, Z.-s.; Cai, N.-s.; Huang, Y.-y.; Han, H.-j. Synthesis, experimental studies, and analysis of a new calcium-based carbon dioxide absorbent. Energy Fuels 2005, 19, 1447–1452. (14) Li, Z.-s.; Cai, N.-s.; Huang, Y.-y. Effect of preparation temperature on cyclic CO2 capture and multiple carbonation-calcination cycles for a new Ca-based CO2 sorbent. Ind. Eng. Chem. Res. 2006, 45, 1911– 1917.

(15) Gupta, H.; Fan, L.-S. Carbonation-calcination cycle using high reactivity calcium oxide for carbon dioxide separation from flue gas. Ind. Eng. Chem. Res. 2002, 41, 4035–4042. (16) Iyer, M. V.; Gupta, H.; Sakadjian, B. B.; Fan, L.-S. Multicycle study on the simultaneous carbonation and sulfation of high-reactivity CaO. Ind. Eng. Chem. Res. 2004, 43, 3939–394. (17) Florin, N. H.; Harris, A. T. Screening CaO-based sorbents for CO2 capture in biomass gasifiers. Energy Fuels 2008, 22, 2734–2742. (18) Florin, N. H.; Harris, A. T. Preparation and characterisation of a tailored CO2 sorbent for enhanced H2 synthesis in biomass gasifiers. Ind. Eng. Chem. Res. 2008, 47, 2191–2202.

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maintained through multiple cycles when carbonation is extended to unrealistic lengths of time.20,21 The presence of CO2 during calcination is important because CO2 will be present in the calciner at a concentration >90 vol % and is known to effect the sintering rate of CaO.22 The influence of the CO2 partial pressure on carbonation and calcination is discussed in section 3.4. A heating rate of 50 °C/min was used to vary the temperature from 650 to 900 °C (unless otherwise stated) for up to 30 CO2-capture-and-release cycles, with the reactivity measured by recording the change in sample weight (Figure 3). Thus, an increase in sample weight corresponds to carbonation, and a decrease in sample weight corresponds to calcination. These calcination/carbonation measurements were used to describe the carrying capacity of the sorbent, defined according to eq 2. Reactivity is expressed in terms of grams of CO2 per gram of (calcined) sorbent because this is relevant for sizing equipment and allows for a reasonable comparison between synthetic sorbent with or without inert material. 

 g of CO2 carrying capacity ð%Þ ¼  100 ð2Þ g of calcined sorbent Figure 3. Typical weight-loss-and-gain profile from the TGA corresponding to calcination-carbonation through 30 cycles, including initial dehydration of Al(OH)3 to Al2O3. The inset shows the primary calcination and carbonation phases.

An example of the raw TGA data is shown in Figure 3, for 85 wt % CaO and binder. It is important to note that the material when loaded into the TGA is a mixture of mainly CaCO3 and Al(OH)3, and upon heating to 900 °C, during the first calcination step, four distinct weight loss events were observed (inset in Figure 3). The first weight loss is associated with the removal of physically bound water commencing at about 80 °C. The second decomposition at about 150 °C is likely associated with the decomposition of nitrate residues. This is followed by the decomposition of Al(OH)3 to Al2O3 at around 300 °C. The fourth weight loss is due to the calcination of CaCO3. This stepwise decomposition leads to the formation of a highly porous CaObased sorbent. In addition, the solid-state reaction between CaO and Al2O3 to form mixed calcium-aluminum oxides (e.g., mayenite, Ca12Al14O33), a process not accompanied by a weight change, results in the formation of an inert solid binder.13,14 In the TGA, this process is thought to occur during the first five calcination steps, when the temperature is >800 °C,14 corresponding with an irreversible consumption of a stoichiometric amount of CaO. Here, complete conversion of Al2O3 to mayenite was assumed when determining the initial loading of Al(NO3)3 3 9H2O. Figure 3 clearly shows a decline in the extent of CO2 capture from the 1st to 30th cycle, which demonstrates that the presence of the inert binder does not completely eliminate the effect of sintering. The influence of the binder on the long-term carrying capacity is discussed in detail in section 3. The TGA results are complimented with experiments using a bench-scale BFB because the TGA does not expose the samples to abrasive conditions and mechanical stability is a critical characteristic to evaluate the performance of synthetic sorbents. In contrast, the BFB elutriates any small particles that are formed during cycling experiments. Thus, a laboratory-scale atmospheric pressure BFB reactor consisting of a resistance heated furnace surrounding a quartz reaction vessel (2.1  10-2 m inner diameter) was used. A detailed description of the experimental setup is given elsewhere.23 Consistent with the

of agitation and the ripening period were thereby kept constant throughout all experiments. This mixed precipitation procedure works because of the higher solubility of Ca(OH)2 (0.12 g/ 100 mL) than CaCO3 (0.0013 g/100 mL). When CO2 is bubbled through the slurry, some of the gas dissolves to form carbonate ions (CO32-) that react with Ca2þ and precipitate out as CaCO3; thus, as Ca2þ ions move out of the solution, they are replenished by the continual dissolution of Ca(OH)2 until this supply is depleted. At the beginning of the experiments, when the OHconcentration is high, the Al3þ ions are likely present in complex form [e.g., Al(OH)4-], which reacts with Hþ ions and precipitates out as Al(OH)3 as the pH decreases. The progress of the reaction was monitored by recording the pH, which is initially about 12 and rapidly drops to about 6 when all Ca(OH)2 is converted to CaCO3.The mixed precipitate, consisting mainly of CaCO3 and Al(OH)3, was collected and transferred directly to an oven at 120 °C for drying overnight. For reactivity testing in the TGA, the dried filter cake was crushed into a fine powder using a mortar and pestle; for experiments in the BFB, particles sieved to 500-710 μm were used. Because of the hardness of the synthetic material, it was necessary to place ball bearings in the sieve tray to smash the filter cake. Sorbent preparation for the BFB experiments also included a thermal pretreatment step to decompose the nitrate residues and convert Al(OH)3 to Al2O3 (i.e., steps 2 and 3 in Figure 3), which was conducted at 500 °C under N2 for a duration of 3 h using a tube furnace. 2.2. Reactivity Testing. Reactivity testing was carried out using a TGA (PerkinElmer Pyris 1 TGA) and a bench-scale BFB reactor. Typical TGA tests used 5 mg of sample, which was exposed to the carbonation temperature of 650 °C for 10 min and then calcination at 900 °C for 5 min, both under an atmosphere of 15% CO2 in N2 and He (the TGA was set up with two inert gases: N2 was fed directly to the furnace, and He was directed via the balance before entering the furnace), with a total gas flow rate of about 130 mL/min. The carbonation duration of 10 min was selected for all experiment and is consistent with a lot of published work (e.g., see a recent review9), with likely solid residence times in an industrial-scale carbonator of the order of several minutes.19 Extended carbonation duration is known to delay the rate of decay, with significantly higher conversion

(20) Barker, R. J. The reactivity of calcium oxide towards carbon dioxide and its use for energy storage. J. Appl. Chem. Biotechnol. 1974, 24, 221–227. (21) Grasa, G.; Abanades, C.; Alonso, M.; Gonzalez, B. Reactivity of highly cycled particles of CaO in a carbonation/calcination loop. Chem. Eng. J. 2008, 137, 561–567. (22) Borgwardt, R. H. Sintering of nascent calcium oxide. Chem. Eng. Sci. 1989, 44 (1), 53–60. (23) Blamey, J.; Paterson, N.; Dugwell, D.; Fennell, P. S. The reactivation of CaO-based sorbent for CO2 capture from combustion and gasification plants. Energy Fuels 2010, manuscript submitted.

(19) Grasa, G.; Gonzalez, B.; Alonso, M.; Abanades, C. Comparison of CaO-based synthetic CO2 sorbents under realistic calcination conditions. Energy Fuels 2007, 21, 3560–3562.

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Figure 4. Particle size distribution based on laser diffraction of precipitated particles removed from the SBC after 5 min (90% CaO and binder).

Figure 5. Average carrying capacity of CaO-based sorbent with mayenite support, 80% CaO (wt/wt), performed in the TGA. Error bars show 2 times the standard deviation determined for three samples (carrying capacity defined as grams of CO2 captured per gram of calcined sorbent and expressed as a percentage).

TGA experiments, carbonation was conducted at 650 °C and calcination was conducted at 900 °C, both under 15% CO2 in N2. The gas, with a cold flow rate of 47.5 cm3/s (U/Umf = 8), was delivered at the base of the reactor, passing through a quartz wool plug for gas preheating, before flowing through a sintered quartz distributor. The hot gas then fluidized the particles before it was vented to the atmosphere along with any fine particles that may have resulted from decrepitation (particles “falling apart” in the absence of mechanical stress) and attrition (particles being broken apart, owing to collisions with walls or other particles). A portion of the off-gas was continuously sampled, and the CO2 concentration was measured (using an infrared gas analyzer) to determine CO2-capture capacity. The mechanical stability of the limestone particles was evaluated by measuring the bed loss during an experiment. 2.3. Sorbent Characterization. To obtain a sufficient sample for particle characterization, additional calcination/carbonation cycling experiments were carried out using a tube furnace. These experiments used a constant gas atmosphere of 15 vol % CO2 in N2 and the same reaction temperatures as for the TGA/ BFB experiments; however, the calcination and carbonation times were extended to 20 min, and a ramp rate of 15 °C/min was used, owing to the properties of the furnace. Two calcined sorbents were prepared, after 1 and 10 calcinations, by cooling from 900 °C under pure N2. Brunauer-Emmett-Teller (BET) surface areas and Barrett-Joyner-Halenda (BJH) pore volume distributions were determined by N2 adsorption (Micromeritics Tristar 3000). To assess the degree of mixing, the particle size distribution of the freshly precipitated CaCO3 and Al(OH)3 particles was determined by laser diffraction (Malvern Master Mastersizer S). X-ray fluorescence (Bruker AXS S4 Explorer) was performed to confirm that the particle preparation method used for the BFB experiments did not affect the ratio of active material to support.

five) from the SBC after a ripening time of 5 min. Here, a narrow single-modal distribution, with mean diameter of about 2 μm, indicates that the CaCO3 and Al(OH)3 particles are mixed on a sub-micrometer scale within micrometersized particles. 3.2. Experimental Repeatability. The repeatability of the precipitation procedure was determined by conducting two repeats of the mixed precipitation, and the results are presented in Figure 5; the symbols represent the mean carrying capacity for 80% CaO and mixed oxide, and the error bars show twice the standard deviation determined for the TGA experiments using samples from each precipitation. These repeat experiments demonstrate a reasonable level of experimental repeatability and indicate the reliability of the results. Smaller error bars observed for the first few cycles, i.e., corresponding to the conversion of Al2O3 and CaO to a mixed calcium-aluminum oxide, suggest that structural changes during the solid-state reaction are very significant for the production of a homogeneous synthetic sorbent. 3.3. Enhanced Carrying Capacity. To demonstrate the improvement in carrying capacity of the modified CaObased sorbent compared to that of a natural limestone (Havelock) and CaO derived from precipitated CaCO3 without binder, Figure 6 shows the variation in the carrying capacities over 30 cycles using the TGA. These results clearly show the superior carrying capacity of the supported sorbent, in this case, 85 wt % CaO and binder. Specifically, the capture capacity of the modified sorbent is about 3 times that observed for the natural limestone after 30 cycles. The decay profile observed for the synthetic sorbent follows a different trajectory compared to the natural limestone and the precipitated CaO. This trajectory is characterized by a relatively fast rate of decay during the first 5 cycles, which changes to a more gradual rate of decay. The fast rate of decay is attributable to the irreversible formation of the mixed calcium-aluminum binder, resulting in the consumption of CaO; thus, the transition to the more gradual rate of decay indicates the complete conversion of Al2O3. After about 20 cycles, all of the sorbents decay at a similarly slow rate

3. Results and Discussion 3.1. Mixing at the Micrograin Scale. It was hypothesized that the mixed precipitation methodology would result in mixing of the sorbent and binder at the micrograin scale, thus achieving an even distribution of CaO and binder. To test this hypothesis, the particle size distribution of the freshly precipitated CaCO3 and Al(OH)3 particles was determined. Figure 4 shows an average particle size distribution for a sample removed (and rapidly diluted by a factor of about 4601

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Figure 6. Carrying capacity through 30 CO2-capture-and-release cycles of three CaO-based sorbents derived from natural limestone (Havelock), precipitated CaCO3 (PCC), and CaCO3 precipitated with Al(OH)3, performed using the TGA.

Figure 8. Carrying capacity of CaO with varying amounts of binder from 0 to 25 wt %, performed in the TGA.

because an additional pore volume is produced when a larger amount of CO2 is evolved during the subsequent calcination phase, thus resulting in a slower rate of decay, as seen in Figure 7. However, countering this improvement in the CO2capture capacity is an increase in the rate of sintering during calcination under a high CO2 partial pressure,25 clearly evidenced by the dramatic increase in the carrying capacity after 30 cycles when calcination is conducted under 100% N2. Although not practically important for producing a pure stream of CO2, the highest carrying capacity through 30 cycles is observed under the “mildest” calcination conditions, i.e., 100% N2 and 800 °C. These results clearly demonstrate the effects of the calcination/carbonation conditions and highlight the importance of using consistent and practical reaction conditions for evaluating the performance of novel CaObased sorbents. Not that much previous work using a mixed calcium-aluminum binder has been carried out using lowtemperature calcination under N2, i.e., when significantly improved long-term carrying capacity may be expected.12,13 3.5. Effect of Binder Loading. The role of an inert binder is 2-fold, i.e., to prevent the sintering of the CaO grains by providing an inert framework and to improve the mechanical stability of the sorbent. There is a trade-off because the higher the quantity of binder, the greater the mechanical stability; however, increasing the amount of binder means a reduction in the amount of active CaO per unit mass. To determine the influence of the CaO/binder ratio on the carrying capacity, sorbents were prepared with varied binder loadings from 0 to 25 wt % (Figure 8). Figure 8 shows an increase in the carrying capacity of the sorbents after 30 cycles (performed in the TGA) as the amount of binder used increases to 15 wt %. This indicates an increase in the molar conversion of CaO to CaCO3, which is most likely due to the reduction, although not elimination, of sintering. When the binder loading is increased further to 20 wt %, the carrying capacity is diminished, owing to a reduction in the amount of CaO per unit mass. Significantly, the small decrease in the carrying capacity with loadings above 15 wt % suggests that high binder loadings may be acceptable to enhance mechanical

Figure 7. Influence of calcination-carbonation conditions on the CO2-capture capacity (all data for 80 wt % CaO and binder, performed in the TGA).

toward a residual decay asymptote, likely because of a reactive sintering mechanism; however, further work is required to better elucidate this observation. 3.4. Influence of the Calcination-Carbonation Conditions. To investigate the influence of the calcination-carbonation conditions on the carrying capacity, experiments were conducted varying the CO2 partial pressure and the calcination temperature. Figure 7 shows the influence of an increase in the CO2 partial pressure from 15 to 50% on the carrying capacity of CaO with 20% binder through 30 cycles using the TGA. Typical of this class of synthetic sorbent24 and distinct from natural limestones, an increase in the CO2 partial pressure during carbonation appears to improve the capture capacity. This improvement affects the decay trajectory,

(25) Borgwardt, R. H. Calcium oxide sintering in atmospheres containing water and carbon dioxide. Ind. Eng. Chem. Res. 1989, 28, 493– 500.

(24) Dennis, J. S.; Pacciani, R. The rate and uptake of CO2 by synthetic CaO-containing sorbent. Chem. Eng. Sci. 2009, 64, 2147–2157.

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bed loss after 15 cycles (%)

PCC, no support

85 wt % CaO

75 wt % CaO

Havelock

3.2

5.7

15

12.5

sorbent with support, there is a large difference in the carrying capacity, despite accounting for bed loss. We note that the X-ray fluorescence (XRF) elemental analysis of these sorbents after cycling confirmed that the bed loss did not change the active material/binder ratio. In addition, the decay profile observed for the synthetic sorbent with binder in the BFB was different from the profile observed from the TGA experiment; i.e., we did not observe the same abrupt decay in the first few cycles associated with the mixed calcium-aluminum oxide formation. Thus, in the case of the BFB, it is assumed that this solid-state process occurred during the initial sample loading in the reactor, which is reasonable given the superior heat-transfer characteristics of the BFB compared to the TGA. The dramatic decrease in the capture capacity of CaO with binder from the TGA to BFB and the different decay trajectory provide further evidence that the structural changes during the solid-state reaction are very significant in terms of the subsequent reactivity of the sorbent. It is suspected that the more gradual heating in the TGA results in more stable formation of crystals of mayenite, owing to the longer time for crystal growth. This is consistent with previous work by Li et al.14 that highlighted the effect of the temperature of a thermal-pretreatment step (during which mayenite is formed) on the subsequent reactivity of their synthetic sorbents. To test the influence of the initial heating rate, we conducted an additional experiment using 85 wt % CaO that was loaded in the BFB at room temperature and heated at 1 °C/s to the calcination temperature (900 °C) and no significant difference in the CO2 carrying capacity nor amount of bed loss was observed (Figure 9a). We note that this heating rate is comparable to the TGA experiments; however, better heat transfer in the BFB is still expected to increase the rate of mixed calcium-aluminum oxide crystal formation, consistent with the different decay trajectories between the TGA and BFB. This result for 85 wt % CaO with a slow initial heating rate indicates that the different performance between the TGA and BFB experiments is not solely due to the decrepitation as a result of loading at 900 °C (i.e., associated with an initial “shock” calcination when the cold sorbent is added to the hot bed). Interestingly, the bed loss measurements (Table 1) indicate that CaO with no support was less susceptible to bed loss during cycling in the BFB. This observation appears contrary to our hypothesis that the mixed calcium-aluminum oxide binder improves the mechanical stability of the synthetic sorbents. Furthermore, we note that the synthetic sorbents with binder are significantly harder; this is most apparent when handling/grinding the materials. Hence, we think that these surprising results are likely due to a greater vulnerability of the synthetic sorbents with binder to decrepitation as well as mechanical stresses associated with calcination/carbonation likely related to the mayenite crystal structure. Furthermore, the relatively small loss of bed material in the case of CaO derived from precipitated CaCO3 without binder may be due to a greater susceptibility to sintering (consistent with the TGA results), which would impart a greater resistance to decrepitation and

Figure 9. (a) Carrying capacity of CaO with varying amounts of binder and compared to Havelock in a bench-scale BFB. (b) Comparison to TGA results shown in Figure 8 and corrected for mass loss.

stability. However, later results in the BFB suggest that high binder loadings may not be advisible (section 3.6). 3.6. Carrying Capacity Determined in a Lab-Scale BFB Reactor. On the basis of the TGA results, which identified 85 wt % CaO with binder having the highest carrying capacity through 30 cycles, we conducted experiments using a benchscale BFB with CaO derived from precipitated CaCO3 without binder and samples with 85 and 75 wt % CaO. Contrary to reactivity tests with the TGA, CaO with no binder showed the highest carrying capacity through 15 cycles in the BFB reactor compared to 85 and 75 wt % CaO (Figure 9a) and we observed a significant decrease in the carrying capacity with an increase in the binder loading from 15 to 25 wt %. In comparison to natural limestone (Havelock), the CO2-capture capacity after 15 cycles was about 2.4 and 1.5 times greater for CaO with no binder and 85 wt % CaO, respectively. The capture capacity of CaO with 25% binder was slightly lower than natural limestone after 15 cycles. The TGA and BFB data for CaO with no support (Figure 9b), when a correction is made for bed loss, are in close agreement. In contrast, in the case of the synthetic 4603

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: DOI:10.1021/ef100447c

Florin et al.

area is associated with a reduction in the number of pores with a diameter between about 30 and 90 nm. In addition, a larger proportion of the surface area is associated with larger pores (>100 nm) after 10 calcination cycles. 4. Conclusions Carbonate looping technology using CaO-based sorbents is a promising CO2-capture technology, with the potential to reduce the cost of capture and minimize the parasitic energy penalty imposed on the combustion or gasification process. This future potential may be exploited with the development of synthetic CaO-based sorbents that are resistant to physical deterioration and maintain long-term CO2 carrying capacity. Toward this end, we observed an improved carrying capacity using a synthetic CaO-based sorbent with a mixed calcium-aluminum binder [derived from precipitated CaCO3 and Al(OH)3]. In the TGA experiments, the highest carrying capacity maintained through 30 cycles was achieved with 85 wt % CaO binder, which was 3 times higher than the observed capacity of Havelock limestone. However, in BFB experiments, CaO with no binder showed the highest carrying capacity through 15 cycles (about 2.5 times compared to Havelock) and we observed a significant decrease in the carrying capacity corresponding with an increase in the binder loading from 15 to 25 wt %. This is particularly interesting and suggests that differences in the thermal history of the particles cause significant differences in the subsequent reactivity of the particle, in agreement with previous work. Furthermore, we measured an increase in the amount of bed material that was elutriated with the higher binder loadings. These findings suggest that further investigation into the mechanism and extent of formation of the mixed calcium-aluminum binder in artificial sorbents is appropriate. Further work is also needed to better differentiate between the decrepitating effect of the first calcination phase, the mechanical stresses associated with calcination/carbonation, and the abrasive conditions in the fluidized bed.

Figure 10. Comparison of the incremental surface area based on BJH desorption data for 80 wt % CaO and binder cycled in a tube furnace after 1 and 10 calcinations.

attrition by cementing the CaO grains together. Further work is underway to better differentiate the decrepitating effects of the initial calcination, the attrition effects as a result of abrasive conditions in the fluidized bed, and mechanic stresses during calcination/carbonation experiments in the BFB. 3.4. Morphological Changes during Cycling. To obtain insights into the morphological changes during calcination/ carbonation cycles, N2 adsorption was used to determine the BET surface areas and BJH pore volume distributions for calcines (80 wt % CaO) prepared in a tube furnace after the 1st and 10th calcination. Consistent with the decay in carrying capacity, we observed a small decrease in the BET surface area from 11.85 to 8.44 m2/g. This loss of surface area is most likely attributable to morphological changes associated with the solid-state formation of the mixed calcium-aluminum oxide and the effect of sintering. Figure 10 shows the incremental surface area versus the mean pore diameter based on the desorption isotherm and indicates that the loss in surface

Acknowledgment. The authors are grateful for the financial support of the Grantham Institute and the Engineering and Physical Sciences Research Council (EPSRC).

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