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Energy & Fuels 2008, 22, 2734–2742
Screening CaO-Based Sorbents for CO2 Capture in Biomass Gasifiers Nicholas H. Florin* and Andrew T. Harris School of Chemical and Biomolecular Engineering, UniVersity of Sydney, Sydney, Australia ReceiVed December 11, 2007. ReVised Manuscript ReceiVed February 18, 2008
Coupling biomass gasification with in situ CO2 capture significantly enhances the yield of hydrogen. For this process, CaO is the most likely CO2 sorbent. However, the development of a sorbent that is resistant to physical deterioration and maintains reactivity through multiple cycles is a limiting step in the scale-up and commercial operation of a continuous process. To this end, we describe an experimental protocol for screening CO2 sorbents and then use this to assess the performance of six targeted CaO-based sorbents: CaO derived from commercially available CaCO3 (dp ) 25.8 µm), commercially available Ca(OH)2 (dp ) 9.2 µm), precipitated CaCO3 (dp ) 3.2 µm), CaCO3 nanoparticles (dp ) 509.4 nm), and presintered CaO doped with Li2CO3 and with K2CO3. The protocol allows the measurement of reactivity through multiple CO2 captureand-release cycles; this is a significant step in determining the overall consumption of sorbent and, hence, the cost of one of the key parts of the process.
Introduction The use of the reversible reaction between CaO and CO2 (eq 1) has received considerable attention in a diverse range of applications. These include enhanced coal gasification,1 the synthesis of NaCN,2 and high-temperature energy storage (600–700 °C).3,4 CO2 capture has become a topical issue in the context of so-called cleaner coal technologies. The use of CaO for CO2 capture from coal combustion flue gas has been investigated5–7 and coal gasification processes combined with in situ CO2 capture have been proposed.8–11 CaO is the most likely sorbent for in situ CO2 capture in biomass gasifiers, because (i) CaO reacts with CO2 in the temperature range suitable for biomass gasification with steam; (ii) CO2 capture based on a gas–solid absorption reaction is exothermic, providing additional heat to drive the endothermic cracking and reforming reactions; (iii) CaO can be regenerated, producing a pure stream of CO2; and (iv) CaO is derived from a range of cheap and abundant precursors. CaO(solid) + CO2(gas) S CaCO3(solid) ∆H873.15 ° ) 171.2 kJ/mol (for carbonation) (1) Due to the broad potential applicability of the reaction, the characteristics of the reversible reaction have been extensively studied under a range of experimental conditions (the variables include the type of reactor, calcination temperature, calcination atmosphere, carbonation temperature, carbonation atmosphere, number and duration of reaction cycles).1,3,4,12–18,24–32 The relevant studies are summarized in the work of Florin and Harris.32 The reaction between CaO and CO2 is typified by an initial rapidreaction phase, which is chemical-reaction controlled, followed by a slower reaction phase limited by diffusion resistance. The transition from rapid to slow phases is thought to be associated with the formation of a CaCO3 product layer with a critical thickness and/or coverage sufficient to significantly impede fur* Corresponding author. E-mail:
[email protected].
ther conversion. As a result, the conversion of CaO to CaCO3 is usually limited to 70–80% for carbonation of up to 1 h, for example, see refs 13–16. Furthermore, the extent of conversion recedes throughout multiple CO2 capture-and-release cycles.20,21,24–28 A close correlation between the decay in reactivity through multiple reaction cycles and the decline in surface area and pore volume indicates the vulnerability of CaO to sintering.3,4,12,15,16,24,25 Thus, as the number of reaction cycles increases, the ultimate extent of conversion is increasingly dependent on the diffusioncontrolled reaction phase.23 Strategies for enhancing the reactivity of CaO through multiple CO2 capture-and-release cycles have been studied, including: (i) optimized calcination conditions;13,15,28,33 (ii) sorbent hydration techniques;28,33,34 (iii) the incorporation of foreign ions; (iv) the use of nanomaterials;4 and (v) the use of inert porous supports.19,21,22 Baker4 reported a 93% conversion of CaO derived from nanosized CaCO3 sustained through 30 CO2 capture-and-release cycles, in which the carbonation reaction spanned 24 h. However, a 24-h carbonation reaction is not practical in the context of a continuous sorbent regeneration process. Following the work of Li et al.,21 Martavaltzi and Lemonidou22 incorporated CaO derived from Ca(CH3COO)2 in a solid matrix of Ca12Al14O33. They reported 55% conversion of CaO to CaCO3 after 30 min, maintained through 45 cycles. The wide variety of reported experimental conditions and sorbent enhancement strategies points to the need to conduct a single study to compare the performance of CaO sorbents and strategies for enhanced multicycle reactivity under the same experimental conditions. To this end, we have investigated the characteristics of the reversible CO2 capture process and (1) Curran, G. P.; Fink, C. E.; Gorin, E. Fuel Gasification; American Chemical Society: Washington, DC, 1966; pp 141. (2) Dedman, A. J.; Owen, A. J. Trans. Faraday Soc. 1962, 58, 2027– 2035. (3) Barker, R. J. Appl. Chem. Biotechnol. 1973, 23, 733–742. (4) Barker, R. J. Appl. Chem. Biotechnol. 1974, 24, 221–227. (5) Gupta, H.; Fan, L.-S. Ind. Eng. Chem. Res. 2002, 41, 4035–4042. (6) Iyer, M. V.; Gupta, H.; Sakadjian, B. B.; Fan, L.-S. Ind. Eng. Chem. Res. 2004, 43, 3939–394.
10.1021/ef700751g CCC: $40.75 2008 American Chemical Society Published on Web 05/10/2008
CaO Sorbents for CO2 Capture Table 1. Standard Protocol for Sorbent Reactivity Tests sample size calcination conditions
carbonation conditions
∼2.5mgofCaCO3wasloadedintothesample pan.Thesamplewasevenlydistributedinthe pan tominimizemass transfereffects CaCO3washeatedat20°C/minto700°C.Cal cinationtemperaturewasselectedtominimize theinfluenceofparticlesintering.35,36Thetem peraturewasheldat700°Cfor20minto achievecompletecalcination.EvolvedCO2 wasrapidlyflushedfromthevicinityofthede composingsolid,usingaN2purgewithflowrate 100 mL/min CaOwascooledat20°C/minto600°Cforcar bonation.Thistemperaturewasselectedbe causeitisfavorableforbothCO2capture andbiomassconversionaccordingtothermo dynamicequilibriumtheory37andexperimental datafromtheliterature.38,39Carbonationwas initiatedbyswitchingtheN2purgeto amixtureof15%CO2inN2 (100mL/min).TheCO2concentrationwas selectedasrepresentativeofthetypical CO2concentrationintheproductgasfor biomassgasification.40,41Carbonationtime was 20 min forstandardtests
experimental conditions relevant to a combined biomass gasification/CO2 capture process. This paper describes experimental results underpinning an experimental protocol for assessing the performance of six targeted CaO sorbents for CO2 capture. Experimental Methods The experimental protocol (Table 1), allows the effect of multiple CO2 capture-and-release cycles on the sorbent reactivity to be measured; this is essential for determining the overall consumption of sorbent in a continuous regeneration system, a key component of cost-efficiency.23 The protocol used a modified thermogravimetric analyzer (TGA; Figure 1) to screen likely CaO sorbents and to characterize CaO reactivity through multiple CO2 capture-andrelease cycles. Reactivity is measured by recording the change in sample weight due to CO2 capture-and-release. Thus, an increase in the sample weight corresponds to carbonation and a decrease in the sample weight corresponds to calcination. We used these
Energy & Fuels, Vol. 22, No. 4, 2008 2735 carbonation data to describe the conversion of the CaO sorbent to CaCO3. In eq 2, n(CaCO3)produced is the number of moles of CaCO3 produced, and n(CaO)initial is the initial number of moles of the CaO sorbent. X)
n(CaCO3)produced n(CaO)initial
(2)
Standard protocol development experiments used CaO derived from CaCO3 (Sigma-Aldrich) with total impurities e0.02 wt %. Figure 2a shows the particle size distributionsdetermined by laser diffraction (Malvern Mastersizer S). The mean particle diameter was D(V, 0.5) ) 25.8 µm. The corresponding SEM image shows a fracture surface of the sorbent particles set in resin. We used CaO derived from Ca(OH)2, precipitated CaCO3 and nanosized CaCO3 for sorbent screening experiments. Ca(OH)2 was obtained from Sigma-Aldrich with purity g95.0 wt %. The mean particle diameter was D(V, 0.5) ) 9.2 µm. The precipitated CaCO3 was manufactured by bubbling CO2 through an aqueous solution of Ca(OH)2.23 The particle size distribution for the precipitated CaCO3 and an SEM image of the fracture surface is shown in Figure 2b. The mean particle diameter of the precipitated CaCO3 was D(V, 0.5) ) 3.2 µm. Nanosized CaCO3 was obtained from Nanomaterials Technology Pty. Ltd. The particle size distribution and SEM image of the fracture surface are shown in Figure 2c. A mean particle diameter, 509.4 nm, was determined using a Malvern Zetasizer 3000.
Results and Discussion Sample Mass. We conducted tests to determine the influence of sample mass on the reactivity of the calcine. The mass of CaCO3 (Sigma-Aldrich) was varied between 0.5 and 20 mg. Figure 3a shows the calcination profile for three test runs with CaCO3 weights of 2.5, 5, and 10 mg, represented in terms of the weight percentage (wt %). Calcination was conducted at 700 °C, with a N2 purge flowrate of 100 mL/min. The duration of the calcination was increased for the heavier loads. We observed a small lag in the commencement of weight loss and a decrease in the rate of weight loss, both corresponding to an increase in the sample mass. This trend demonstrates the significance of heat and mass transfer effects, whereby the rate of decomposition is limited by the rate of heating of the sample
Figure 1. Modified TGA for purpose of screening CaO sorbents and characterising CaO reactivity through multiple CO2 capture-and-release cycles.
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Figure 2. (a) Particle size distribution for CaCO3 (Sigma-Aldrich) determined by laser diffraction and surface fracture of unreacted sorbent particles (scale bar ) 10 µm). (b) Particle size distribution for precipitated CaCO323 determined by laser diffraction and surface fracture of unreacted particles (scale bar ) 10 µm). (c) Particle size distribution for nanosized CaCO3 (Nanomaterials Technology Pty. Ltd.) and surface fracture of unreacted nanoparticle agglomerates (scale bar ) 10 µm).
and the removal of CO2 from the decomposing solid, respectively.42 A sample mass of about 2.5 mg was selected for all subsequent experiments to minimize the effects of heat and mass transfer. Samples less than 2.5 mg were not practical because it was difficult to ensure an even distribution of very small samples in the sample pan. Minor discrepancies in the sample weight after calcination are likely due to differences in the amount of physically bound water. Figure 3b shows the subsequent reactivity of the sorbents. Carbonation is represented in terms of the sorbent conversion (X) as a function of time (eq 2). The initial reaction rate (0–2 min) appears not to have been influenced by the sample mass;
however, we observed a decline in the rate of conversion during the rapid-reaction-controlled phase with the heavier samples (5 and 10 mg). This demonstrates the importance of mass transfer effects. Consequently, we observed a lag of up to two minutes before the onset of the diffusion-controlled phase with the heaviest sample mass. The minor decrease in the extent of the rapid-reaction phaseswhich ultimately governs the final extent of conversion at 20 minsmay be attributed to the sintering effects that result in a loss of reactive surface area. It has been suggested that sintering may be enhanced by the greater contact between the sorbent particles and increased exposure to CO2.36 The duration of the calcination reaction
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Energy & Fuels, Vol. 22, No. 4, 2008 2737
and the amount of evolved CO2 is greater with larger samples. The impacts of sorbent mass and the removal of evolved CO2 during calcination are important considerations in the design of a regeneration reactor.42 Calcination Time. To determine the influence of the calcination time on the reactivity of CaO, we varied the amount of time at which samples were held at 700 °C. In Figure 4, the reactivity of CaO calcined at 700 °C for 20 min in N2 is compared with CaO calcined at 700 °C for 5 h in N2. In both cases, 2.5 mg of CaCO3 (Sigma-Aldrich) was loaded and complete calcination was achieved in about 15 min. The close alignment of the conversion profiles suggests that the subsequent reactivity of the sorbent was not influenced by the duration. Critically, this result confirms that the CaO particles are not vulnerable to particle sintering at 700 °C under a N2 atmosphere. Calcination Temperature. The calcination temperature has a dramatic influence on sorbent reactivity. The reactivity of CaO, derived from CaCO3 (Sigma-Aldrich), produced at 700–1000 °C is shown in Figure 5. The decomposition occurred nonisothermally for calcination experiments conducted at temperatures greater than 700 °C. Complete calcination was achieved by 800 °C at a heating rate of 20 °C/min; thus, the calcination rate was effectively enhanced by a factor of 3. The conversion profiles indicate a general decreasing trend in the extent of the rapidreaction phase, corresponding with an increase in the calcination temperature. This trend is nonlinearsthat is, the loss of reactivity was more severe at higher calcination temperatures. These experiments clearly indicate the susceptibility of CaO to sintering when exposed to elevated calcination temperatures above 700 °C. However, neither the rates of the rapid-reaction nor diffusion-controlled phases were influenced by calcination temperature. This suggests that although surface area may be lost through sintering, elevating calcination temperature (under
the conditions tested) does not generally exercise a significant influence on pore-size distribution. These distributionssdetermined by N2 adsorption for CaO calcined at 700, 800, and 1000 °C sare compared in Figure 6. Here, similar pore-size distributions are shown for the sorbents, characterized by a narrow range of pores of diameters of 10–30 nm (within the mesoporous range). Pore volumes and surface areas are presented in Table 2. The lower surface area for the CaO calcined at 1000 °C (23.7 m2/g) compared with CaO calcined at 700 °C (27.5 m2/g) indicates the loss of surface area, which may be attributed to sintering. These results have significant implications for the operation of a continuous sorbent regeneration process. The susceptibility of CaO to sintering clearly places a constraint on the maximum temperature suitable for sorbent regeneration. Thus, there is a tradeoff in terms of minimizing surface area loss due to sintering, and achieving a sufficient rate of calcination. Calcination Atmosphere. We investigated the influence of the CO2 concentration on the reactivity of CaO, derived from CaCO3 (Sigma-Aldrich), present during calcination. Due to thermodynamic limitations these experiments were carried out at elevated temperatures.37 Figure 7 shows the conversion profiles for the reaction of CaO calcined at 1000 °C under N2 compared with CaO calcined at 1000 °C with 15% CO2 in N2. The CO2 partial pressure has a dramatic affect on the subsequent reactivity of CaO. The different shape of the conversion profile when additional CO2 was introduced was striking. When additional CO2 was introduced during calcination, the reaction was characterized by a relatively diminished rapid-reactioncontrolled phase. The conclusion of the rapid-reaction phase corresponded with a conversion of only 0.13, compared with 0.46 when the N2 purge was used. Interestingly, the diffusioncontrolled reaction phase proceeded much faster; hence, after
Figure 3. (a) Effect of sample mass on the rate of calcination at 700 °C. (b) Conversion profile for recarbonation at 600 °C with 15% CO2 in N2.
(7) Sun, P.; Grace, J. R.; Lim, C. J.; Anthony, E. J. Energy Fuels 2007, 21, 163–170. (8) Lin, S.-Y.; Harada, M.; Suzuki, Y.; Hatano, H. Energy ConVers. Manag. 2002, 43, 1283–1290. (9) Lin, S.-Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2002, 81, 2079– 2085. (10) Lin, S.-Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2004, 83, 869– 875. (11) Lin, S.-Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2006, 85, 1143– 1150. (12) Aihara, M.; Nagai, T.; Matushita, J.; Negishi, Y.; Ohya, H. Appl. Energy 2001, 69, 225–238. (13) Bhatia, S. K.; Perlmutter, D. D. AIChE J. 1983, 29, 79–86. (14) Mess, D.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1999, 13, 999–1005. (15) Silaban, A.; Narcida, M.; Harrison, D. P. Chem. Eng. Commun. 1995, 138, 149–162. (16) Silaban, A.; Harrison, D. P. Chem. Eng. Commun. 1995, 137, 177– 190. (17) Grasa, G. S.; Abanades, J. C. Ind. Eng. Chem. Res. 2006, 45, 8846– 8851. (18) Lysikov, A. I.; Salanov, A. N.; Okunev, A. G. Ind. Eng. Chem. Res. 2007, 46, 4633–4638. (19) Feng, B.; Liu, W.; Li, X.; An, H. Energy Fuels 2006, 20, 2417– 2420. (20) Manovic, V.; Anthony, E. J. EnViron. Sci. Technol. 2007, 41, 1420– 1425. (21) Li, Z.-S.; Cai, N.-S.; Haung, Y.-Y. Ind. Eng. Chem. Res. 2006, 45, 1911–1917. (22) Martavaltzi, C. S.; Lemonidou, A. A. Microporous Mesoporous Mater. 2007, 110, 119. (23) Florin, N. H.; Harris, A. T. Ind. Eng. Chem. Res. 2008, 47, 2191. (24) Abanades, J. C. Chem. Eng. J. 2002, 90, 303–306. (25) Abanades, J. C.; Alvarez, D. Energy Fuels 2003, 17, 308–315. (26) Abanades, J. C.; Alvarez, D.; Anthony, E. J.; Lu, D. In-Situ Capture of CO2 in a Fluidized Bed Combustor. In Proceedings of the 17th International Fluidized Bed Combustion Conference, Jacksonville, FL, May 18–21, 2003; p. 17. (27) Abanades, J. C.; Rubin, E. S.; Anthony, E. J. Ind. Eng. Chem. Res. 2004, 43, 3462–3466.
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Figure 4. Influence of the duration of the calcination (700 °C) on the reactivity of CaO.
Figure 5. Influence of the calcination temperature on the reactivity of CaO.
60 min of carbonation, the rapid diffusion-controlled reaction phase compensates for the diminished rapid-reaction-controlled phase. The different conversion profiles suggest the evolution of a considerably different porous structure when additional CO2 is introduced during calcination. The diminished rapid-reactioncontrolled phase may be due to a loss of surface area and pore volume. This is attributable to severe sintering catalyzed by the presence of additional CO2.36 The increased rate of reaction during the diffusion-controlled reaction phase, indicating reduced diffusion resistance, is probably due to an increase in the mean pore size. This is consistent with the findings of Bhatia and Perlmutter,13 who investigated the effect of the calcination atmosphere on the resulting pore structure of CaO by calcining limestone in 100% N2, 10% CO2 in N2, and 20% CO2 in N2 at 910 °C. They reported an increase in the mean pore size and a decrease in the breadth of the pore size distribution with an increase in the CO2 content. An increase in the ratio of larger to smaller pores is consistent with a reduction in the extent of the rapid-reaction-controlled phase due to loss of reactive surface area. Larger pores are less susceptible to pore blockage and plugging by the CaCO3 product.5 The pore size distribution (28) Dobner, S.; Sterns, L.; Graff, R. A.; Squires, A. M. Ind. Eng. Chem. Process Des. DeV. 1977, 16, 479–486. (29) Johnsen, K.; Ryu, H. J.; Grace, J. R.; Lim, C. J. Chem. Eng. Sci. 2006, 61, 1195–1202. (30) Wang, J.; Abanades, J. C. Ind. Eng. Chem. Res. 2005, 44, 627–29. (31) Lu, H.; Reddy, E. P.; Smirniotis, G. Ind. Eng. Chem. Res. 2006, 45, 3944–49. (32) Florin, N. H.; Harris, A. T. Chem. Eng. Sci. 2008, 63, 287–316.
Figure 6. (a) N2 adsorption and desorption isotherms for CaO derived from commercially available CaCO3 calcined at 700 °C. (A 20 pointby-point adsorption and desorption isotherm was obtained using a Quantachrome AUTOSORB-1. Sufficiently large samples (0.05–0.1 g) were prepared using a tube furnace.) (b) Influence of the calcination temperature on the pore size distribution of the calcine.
shown in Figure 7, for CaO calcined at 1000 °C with 15% CO2 in N2, supports these conclusions. The pore size distribution for CaO calcined at 1000 °C with a pure CO2 purge is displayed in Figure 8. A comparison between the pore size distributions for CaO calcined in the presence of 15% and 100% CO2 indicates further loss of the pore volume corresponding with an increase in the CO2 content from 0.16 to 0.08 cm3/g, respectively. The influence of the CO2 content in the calcination atmosphere on the subsequent reactivity of CaO presents a significant constraint in terms of the design and operation of a continuous sorbent regeneration process, particularly if the production of a concentrated stream of CO2 is a key process objective. To produce a concentrated stream of CO2, the sorbent regeneration process must be conducted under high CO2 concentrations and thus either at a temperature greater than 900 °C or in the presence of a diluent gas that is easily separated from CO2.42 Both an increase in calcination temperature and CO2 concentration have been shown to diminish the subsequent reactivity of the CaO sorbent due to sintering and the influence on pore size (33) Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. Ind. Eng. Chem. Res. 2003, 43, 5529–5539. (34) Kuramoto, K.; Fujimoto, S.; Morita, A.; Shibano, S.; Suzuki, Y.; Hatano, H.; Lin, S.-Y.; Harada, M.; Takarada, T. Ind. Eng. Chem. Res. 2003, 42, 975–981.
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Energy & Fuels, Vol. 22, No. 4, 2008 2739 Table 2. Morphological Properties of Likely CaO Sorbents
BET surface area [m2/g] total pore volumea [cm3/g]
CaO derived from CaCO3 (Sigma-Aldrich) calcined at 700 °C with N2 purge
CaO derived from CaCO3 (Sigma-Aldrich) calcined at 1000 °C with N2 purge
CaO derived from CaCO3 (Sigma-Aldrich) calcined at 1000 °C with 15% CO2 in N2
CaO derived from nanosized CaCO3 calcined at 700 °C with N2 purge
CaO derived from precipitated CaCO3 calcined at 700 °C with N2 purge
27.49 0.21
23.73 0.22
11.17 0.16
28.55 0.34
17.75 0.17
a The N desorption isotherm was used to derive the total pore volume data because the desorption value of relative pressure corresponds to a more 2 stable adsorbate condition.44
Figure 7. Influence of CO2 partial pressure during calcination on the subsequent reactivity of CaO.
evolution. Thus, there is a tradeoff between the calcination conditions necessary for producing a pure stream of CO2 and those that maximize the reactivity of CaO. Sakadjian et al.42 have investigated a novel subatmospheric calcination process, thus allowing a reduction in the reaction temperature. Steam may be a likely diluent because it can be easily separated from CO2 via condensation. However, further work is needed to determine the merits of these approaches. Screening CaO Sorbent Precursors. We tested six CaO sorbents derived from (i) CaCO3 (Sigma-Aldrich, (ii) Ca(OH)2 (Sigma-Aldrich), (iii) precipitated CaCO3, (iv) CaCO3 nanoparticles (Nanomaterials Technology Pty. Ltd.), (v) CaO doped with Li2CO3, and (vi) CaO doped with K2CO3 (both SigmaAldrich). Figure 9a shows the conversion profiles of CaO derived from four different sorbent precursors (i.e., i-iv). Complete conversion (X ) 1.0) was achieved with CaO derived from the precipitated CaCO3 after 50 min. The superior conversion was due to both a faster reaction rate during the chemical reaction phase (inset Figure 9b) and the faster reaction rate during the diffusion-controlled phase. These reaction characteristics are attributable to a broad pore-size distribution, which is not susceptible to diffusion resistances, and thus a high rate of conversion can be maintained beyond the kinetically controlled phase.23 The second-highest conversion was achieved using the nanosized CaCO3san ultimate conversion of 0.90 after 60 minscompared with only 0.75 and 0.68 for CaO derived from the commercially available CaCO3 (Sigma-Aldrich) and Ca(OH)2, respectively. Surprisingly, the relatively high conversion achieved using the CaO derived from the CaCO3 nanoparticles is attributable to the relatively fast rate of reaction during the diffusion-limited reaction phase. The superior reactivity of nanosized CaO particles was first demonstrated by Barker4 more than three decades ago. Barker hypothesized that if the particle diameter is sufficiently smallsthus limiting the CaCO3 product layer thickness below a critical levelscomplete conversion can
Figure 8. Influence of CO2 during calcination on the pore size distribution of CaO.
be achieved under the rapid-reaction-controlled phase. He tested his hypothesis using CaO derived from nanosized CaCO3 particles. Barker reported a conversion of 0.93 after 24 h of carbonation, maintained through 30 cycles. However, this result did not support his hypothesis, given that the extent of conversion achieved during his kinetically controlled reaction dropped from 0.68 to 0.4. These observations indicate the susceptibility of the nanosized sorbent particles to sintering, and highlight the importance of the diffusion-controlled phase, which appears to be limited only by time. The results in Figure 9 are consistent with Barker’s results. Figure 9b displays the rate and extent of conversion achieved during the rapid-reaction-controlled phase with the nanosized CaO particles. The rate of conversion of the nanosized sorbent in the rapid-reaction-controlled phase compares closely with the rate of conversion achieved with the micrometer-sized CaO derived from both CaCO3 and Ca(OH)2. In the rapid-reactioncontrolled phase, the extent of conversion of CaO derived from nanosized particles is lower than that from the micrometer-sized (35) Borgwardt, R. H. Chem. Eng. Sci. 1989, 44, 53–60. (36) Borgwardt, R. H. Ind. Eng. Chem. Res. 1989, 28, 493–500. (37) Florin, N. H.; Harris, A. T. Int. J. Hydrogen Energy 2007, 32, 4119– 4134. (38) Pfeifer, C.; Puchner, B.; Hofbauer, H. Int. J. Chem. Reactor Eng. 2007, 5, A9. (39) Pfeifer, C.; Puchner, B.; Proll, T.; Hofbauer, H. H2-Rich Syngas from Renewable Sources by Dual Fluidized Bed Steam Gasification of Solid Biomass. In Proceedings of the 12th International Conference on Fluidization: New Horizons in Fluidization Engineering, Vancouver, Canada, 2007; p. 18. (40) Franco, C.; Pinto, F.; Gulyurtlu, I.; Cabrita, I. Fuel 2003, 82, 835– 842. (41) Turn, S.; Kinoshita, C.; Zhang, Z.; Ishimura, D.; Zhou, J. Int. J. Hydrogen Energy 1988, 23, 641–648. (42) Sakadjian, B. B.; Iyer, M. V.; Gupta, H.; Fan, L.-S. Ind. Eng. Chem. Res. 2007, 46, 35–42.
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CaCO3 particles. This may be due to severe sintering of the nanoparticles. It is likely that greater contact between the nanosized particles, on account of their higher surface-to-volume ratio, advances the sintering mechanism, leading to surface area reduction. In contrast to the micrometer-sized sorbent particles, we observed a gradual transition to the diffusion-controlled phase, with a faster reaction maintained. Thus, the relatively high proportion converted at 60 min was due to the faster rate of reaction during the diffusion-controlled phase. It is likely that the open structure of the nanoparticle agglomerates (Figure 2c) leads to lower pore-diffusion resistance than for the micrometer-sized CaO particles. Further characterization of the agglomerate structure is required to reinforce this conclusion. The lowest proportion ultimately converted was with the CaO sorbent derived from Ca(OH)2. This sorbent displayed a comparable rate of conversion during the kinetically controlled regime to the CaO derived from both micrometer- and nanosized CaCO3. However, the proportion converted in this phase was relatively low and an abrupt transition to the diffusion-controlled phase corresponded a conversion of only 0.5. The conversion profile suggests a greater susceptibility to sintering, which is consistent with the work of Borgwardt,36 who showed that CaO derived from Ca(OH)2 sinters at a faster rate than CaO derived from CaCO3. He argued that this was due to the relative particle densities of the calcines, that CaO particles derived from Ca(OH)2 are more densely packed due to the lower molar volume of Ca(OH)2 (32.2 cm3/mol) compared with CaCO3 (37.1 cm3 mol). Thus, logically, there are more contact points facilitating mass transfer. Sorbent Doping. Borgwardt et al.43 recognized an analogous mechanism governing the mass transfer processes associated with the sintering of CaO and the diffusion mechanism that limits the sulfation of CaO. They demonstrated that the rate of conversion of CaO to CaSO4 is dependent on the concentration of defects in the product layer, which they introduced by doping presintered CaO with alkali salts. This observation is equivalent to the enhanced rate of sintering of impure CaO particles compared with pure CaO.35 Thus, they argued that the sulfation of CaO was governed by a solid-state diffusion mechanism. Bhatia and Perlmutter13 argued that the reaction between CaO and CO2 during the diffusion-controlled phase may also be controlled by a solid-state diffusion mechanism. To demonstrate this, we undertook similar experiments to those conducted by Borgwardt et al.43 but instead examined the influence of impurities on the rate of diffusion in the CaCO3 product layer. Precalcined CaOsderived from CaCO3 (Sigma-Aldrich)swas dry-mixed with two alkali salts: Li2CO3 and K2CO3 at 1 wt %. We compared the conversion profiles for the doped sorbents with that of pure CaO (Figure 10). The presence of Li2CO3 had a dramatic influence on the rate of diffusion. An ultimate conversion of 0.97 was achieved at 60 min compared with only 0.75 for the pure sorbent. We found that the presence of K2CO3 does not influence the rate of diffusion. This result may be due to the different melting points of the Li2CO3 (723 °C) and K2CO3 (891 °C); in the case of the CaO doped with Li2CO3, the dramatic influence of such a minor impurity suggests Li2CO3 has long-range effects on the diffusion mechanism, which might be due to the formation of a eutectic containing CaCO3-Li2CO3. In the context of a continuous sorbent regeneration process, the doping of precalcined CaO would not be feasible. To assess the influence of Li2CO3 during the calcination of CaCO3 the (43) Borgwardt, R. H.; Bruce, K. R.; Blake, J. Ind. Eng. Chem. Res. 1987, 26, 1993–1998. (44) Gullet, B. K.; Bruce, K. R. AIChE J. 1987, 33, 1719–1726.
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Figure 9. (a) Conversion of CaO derived from a range of likely sorbent precursors. (b) Magnification of area between dotted lines in part a showing conversion only during the rapid-reaction-controlled phase. Reactivity tests were conducted according to the procedure outlined in Table 1.
Figure 10. Influence of alkali carbonates (Li2CO3 and K2CO3) on the reactivity of CaO with CO2.
sorbent precursor was doped prior to calcination. For the CaO derived from the doped CaCO3, the major decline in conversion during the rapid-reaction-controlled phase suggests that the presence of Li2CO3 exercises considerable influence on the sintering rate of CaO calcined at 700 °C. At this temperature,
CaO Sorbents for CO2 Capture
Figure 11. Comparison of the multicycle performance of CaO derived from four CaCO3 precursors.
the melting point of Li2CO3 (723 °C) is approached, thus the severe sintering may be attributable to the presence of the dopant in a liquid state. There was a faster rate of conversion during the diffusion-controlled phase in the presence of Li2CO3. These experiments demonstrate the potential for enhanced CO2 capture during the diffusion-limited phase, using doped sorbents. This supports the hypothesis of Bhatia and Perlmutter13 that the reaction between CaO and CO2 during the diffusionlimited phase is controlled by a solid-state mechanism. It is likely that the presence of impurities in naturally occurring limestones or fuel-bound species contacting the sorbent particles in the gasifier influences the reactivity of CaO with CO2 as well as enhancing particle sintering during calcination. Due to the likely range of impurities, sintering behavior and sorbent reactivity is likely to be highly variable and should be characterized case by case.32 Reactivity of CaO Sorbents through Multiple CO2 Capture-and-Release Cycles. A decay in reactivity through multiple CO2 capture-and-release cycles has been widely reported.20,21,24–28 The primary factor influencing conversion through multiple CO2 capture-and-release cycles has been acknowledged as a loss in pore volume and surface area attributable to particle sintering.3,4,15,16,24,25 CO2 capture-andrelease experiments conducted through a sufficiently large number of reaction cycles (i.e., >50) indicate that there is an asymptotic decay in conversion. Abanades and Grasa17 argued that the asymptote defining the residual conversion level depends on calcination temperature and time. Lysikov et al.18 argued that residual conversion also depends on the duration of the carbonation reaction and the sorbent precursor (Ca(OH)2 and CaCO3). Figure 11 compares the multicycle conversion of four CaO sorbents: CaO derived from commercially available CaCO3 (Sigma-Aldrich), CaO derived from precipitated CaCO3, CaO derived from CaCO3 nanoparticles (Nanoparticles Technology Pty. Ltd.), and CaO derived from CaCO3 (Sigma-Aldrich) doped with Li2CO3 (1.0 wt %). The standard reactivity test (Table 1) was adopted for these multiple cycle reactivity tests. The four sorbents clearly display a significant decay in reactivity through 50 reaction cycles. The rate of decay in terms of reaction cycle number is comparable for the CaO derived from the nanosized CaCO3 and the precipitated CaCO3, which both approached
Energy & Fuels, Vol. 22, No. 4, 2008 2741
asymptotic decay after 50 reaction cycles. We observed more rapid decay in the extent of conversion for the CaO derived from the micrometer-sized CaCO3. Asymptotic decay was approached in only 25 reaction cycles. The most dramatic decay in the extent of conversion through multiple reaction cycles was for the CaO derived from CaCO3 doped with Li2CO3. In all cases without the Li2CO3, although the rate of the decay in conversion appeared to depend on the sorbent precursor, the residual conversion level was independent of the sorbent precursor. Table 3 shows the conversion data for reaction cycles 1, 5, 25, and 50 and compares the ratio of conversion associated with the kinetically controlled reaction phase (Xrapid) in terms of the ultimate extent of conversion after 20 min of carbonation (Xt-20min). For example, in the case of CaO derived from nanosized CaCO3, 74% of the ultimate conversion was associated with the kinetically controlled reaction phase in the first reaction cycle, and only 57% by the 50th cycle. In the case of CaO derived from micrometer-sized CaCO3 (Sigma-Aldrich), 95% of the ultimate conversion was associated with the kinetically controlled reaction phase in the first, and only 48% by the 50th cycle. For all three sorbents, it is clear that as the number of reaction cycles increases, the importance of the diffusion mechanism is enhanced. The dominance of the kinetically controlled phase for CaO derived from micrometer-sized CaCO3 (95% for cycle 1) makes this sorbent highly susceptible to a dramatic decay in reactivity. For CaO derived from CaCO3 doped with Li2CO3 the CO2 capture capacity through multiple reaction cycles was diminished due to the severity of the sintering in the initial reaction cycles (cycles 1–5), despite the enhanced rate of conversion during the diffusion-controlled phase. In all cases, it is apparent that in spite of the mild calcination conditions, a decrease in reactive surface area attributable to particle sintering is a major factor contributing to the overall decline in the sorbent reactivity through multiple reaction cycles. Conclusions The development of a CO2 sorbent that is resistant to physical deterioration and maintains reactivity through multiple cycles is a limiting step in the scale up and commercial operation of a continuous CaO sorbent regeneration process. To this end, we described a protocol for screening likely sorbents, presented data relevant to the design of a continuous sorbent regeneration process, and analyzed the performance of six targeted CaO sorbents. We demonstrated the influence of calcination conditions on CaO reactivity. The rate of decomposition of CaCO3 was limited by both heating rate and the removal of evolved CO2 from the vicinity of CaO. Increases in the calcination temperature and CO2 concentration were shown to diminish the subsequent reactivity of the CaO sorbent due to sintering and a reduction in the total pore volume. The susceptibility of CaO to sintering at elevated temperatures (>700 °C) and the effect of CO2 on the evolution of pore structure constrains the conditions suitable for sorbent regeneration. There is a tradeoff in terms of minimizing the loss of surface area and pore volume while achieving a sufficient rate of calcination, and between calcination conditions necessary for producing a pure stream of CO2 and those optimal for maximizing the reactivity of CaO. This second tradeoff highlights the potential advantages in using a diluent that is easily separable from CO2. In terms of a continuous sorbent regeneration process, the rate of calcination is important for determining the rate of sorbent circulation, the rate of injection of fresh sorbent, and hence the cost of the process.
2742 Energy & Fuels, Vol. 22, No. 4, 2008
Florin and Harris
Table 3. Conversion Data for Reactions between CaO and CO2 through Multiple CO2 Capture-and-Release Cycles CaO derived from nanosized CaCO3
CaO derived from precipitated CaCO3
Cao derived from micrometer-sized CaCO3 (Sigma-Aldrich)
CaO derived from CaCO3 (Sigma-Aldrich) doped with Li2 CO3 (1% by wt)
cyc no.
Xrapida
Xt-20min
Xrapid/Xt-20min
Xrapid
Xt-20min
Xrapid/Xt-20min
Xrapid
Xt-20min
Xrapid/Xt-20min
Xrapid
Xt-20min
Xrapid/Xt-20min
1 5 25 50
0.67 0.46 0.22 0.15
0.90 0.68 0.38 0.27
0.74 0.67 0.58 0.57
0.75 0.45 0.25 0.16
0.93 0.64 0.37 0.26
0.81 0.70 0.67 0.63
0.71 0.33 0.15 0.11
0.75 0.47 0.26 0.24
0.95 0.71 0.55 0.48
0.27 0.12 0.07 0.05
0.53 0.29 0.20 0.15
0.51 0.42 0.36 0.39
a The conversion determined at the point corresponding the transition from the rapid, chemical-reaction-controlled phase to the slow, diffusion-controlled reaction phase.
We observed enhanced conversion for CaO derived from precipitated CaCO3 and nanosized CaCO3, compared with CaO derived from commercially available (micrometer-sized) CaCO3. The superior performance was attributable to a porous structure that was less susceptible to diffusion resistances, and thus a high rate of conversion was maintained beyond the kinetically controlled phase. An increased rate of CO2 capture was achieved using CaO doped with Li2CO3, which indicates that the diffusion reaction phase is controlled by a solid-state mechanism. The reactivity of CaO through multiple CO2 capture-andrelease cycles indicates that the rate of the decay in conversion depends on the sorbent precursor. However, a residual conversion level was shown to be independent of the sorbent precursor under the conditions investigated. The rate of decay in reactivity through multiple reaction cycles corresponds closely with an increase in the relative dominance of the diffusion mechanism
in terms of contributing to the ultimate conversion. Thus, even under mild calcination conditions, a decrease in reactive surface area attributable to particle sintering is a major factor contributing to the overall decline in the sorbent reactivity through multiple reaction cycles. On the basis of these findings, it is recommended that future work be focused on the characterization of optimal sorbent morphology for maximizing sorbent conversion and eliminating diffusion resistance, and on the development of solid supports to eliminate the problem of sintering. Acknowledgment. The authors are grateful for the financial support of the Australian Research Council through DP0666488. EF700751G
