Energy Fuels 2010, 24, 4605–4616 Published on Web 07/15/2010
: DOI:10.1021/ef100476d
Mechanism of Particle Breakage during Reactivation of CaO-Based Sorbents for CO2 Capture John Blamey, Nigel P. M. Paterson, Denis R. Dugwell, and Paul S. Fennell* Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, United Kingdom Received April 15, 2010. Revised Manuscript Received June 18, 2010
The calcium looping cycle is being developed as a method for capturing CO2 from both flue and fuel gases. It works by using CaO as a CO2 carrier and through repeated cycles of carbonation and calcination can extract CO2 from a gas with a lower partial pressure of CO2 (e.g., exhaust stream from a power station) and provide a pure stream of CO2 suitable for sequestration. A key problem in the development of calcium looping technology is the decrease in reactivity of the sorbent with an increasing number of cycles of carbonation and calcination. The hydration of calcined sorbent has been shown to be a promising way of periodically regenerating the sorbent, so that its reactivity can be recovered, reducing the requirement to purge material from the cycle. In previous work, the reactivity of sorbents after hydration has been mainly studied by thermogravimetric analysis or in a fluidized bed with an unrealistically low calcination temperature. For this work, a laboratory-scale reactor capable of operation under more realistic conditions has been designed, built, and commissioned. It consists of a computer-controlled, resistance-heated, fluidized-bed reactor capable of temperature cycling, allowing the sorbent to be exposed to repeated cycles of carbonation and calcination within the same vessel. The sorbent is “reactivated” by hydration after a number of cycles and then exposed to further cycles of CO2 capture and release. The reactivity of the sorbent is measured from the CO2 uptake and release during successive cycles of carbonation and calcination. Preliminary tests have been completed, and these show that, for limestone reacted under mild calcination conditions, the ultimate uptake of CO2 (the carrying capacity) of cycled Havelock limestone can be more than doubled upon hydration. As the calcination conditions before hydration become harsher (the temperature is increased), the regeneration technique becomes less effective. This is also observed, although to differing extents, with La Blanca and Purbeck limestones. This is shown to be due to mass loss from the fluidized bed because of the increased friability of the hydrated sorbent. A particle breakage model has been developed to describe this phenomenon.
with separate calcination and carbonation vessels (known as the calciner and carbonator, respectively),4-7 and pilot-plant trials are in operation.8-13 Sorbent is repeatedly cycled between the two vessels, removing CO2 from a flue gas with a low
Introduction The global climate is showing unequivocal signs of warming, and anthropogenic CO2 emissions are considered to contribute significantly to this effect.1 With the objective of reducing future emissions, there has been a drive to develop “clean coal” technologies, allowing for continued use of coal in conjunction with CO2 capture and sequestration.2 One possible sorbent for CO2 capture is CaO derived from limestone, and the crucial reaction for capture is given in eq 1: the forward reaction is known as carbonation, and the reverse reaction is known as calcination.3
(3) Blamey, J.; Anthony, E. J.; Wang, J.; Fennell, P. S. The calcium looping cycle for large-scale CO2 capture. Prog. Energy Combust. Sci. 2010, 36, 260–279. (4) Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K. A twin fluid-bed reactor for removal of CO2 from combustion processes. Chem. Eng. Res. Des. 1999, 77 (A1), 62–68. (5) Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. H. Improved longterm conversion of limestone-derived sorbents for in situ capture of CO2 in a fluidized bed combustor. Ind. Eng. Chem. Res. 2004, 43 (18), 5529– 5539. (6) Abanades, J. C. The maximum capture efficiency of CO2 using a carbonation/calcination cycle of CaO/CaCO3. Chem. Eng. J. 2002, 90 (3), 303–306. (7) Abanades, J. C.; Anthony, E. J.; Alvarez, D.; Lu, D. In-situ capture of CO2 in a fluidized bed combustor. Proceedings of the 17th International American Society of Mechanical Engineers (ASME) Conference on Fluidized Bed Combustion; Jacksonville, FL, 2003; Paper 10. (8) Abanades, J. C.; Alonso, M.; Rodriguez, N.; Gonzalez, B.; Grasa, G.; Murillo, R. Capturing CO2 from combustion flue gases with a carbonation calcination loop. Experimental results and process development. Energy Procedia 2009, 1 (1), 1147–1154. (9) Hughes, R. W.; Lu, D. Y.; Anthony, E. J.; Macchi, A. Design, process simulation and construction of an atmospheric dual fluidized bed combustion system for in situ CO2 capture using high-temperature sorbents. Fuel Process. Technol. 2005, 86 (14-15), 1523–1531. (10) Lu, D. Y.; Hughes, R. W.; Anthony, E. J. Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds. Fuel Process. Technol. 2008, 89 (12), 1386–1395.
CaOðsÞ þ CO2ðgÞ a CaCO3ðsÞ ΔHr, 298 K ¼ - 178 kJ=mol
ð1Þ
Systems proposed for application of this reaction to postcombustion CO2 capture are frequently dual-fluidized beds *To whom correspondence should be addressed. Telephone: þ44-(0)20-7594-6637. Fax: þ44-(0)-20-7594-5638. E-mail: p.fennell@imperial. ac.uk. (1) Intergovernmental Panel on Climate Change (IPCC). Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; IPCC: Geneva, Switzerland, 2007. (2) Intergovernmental Panel on Climate Change (IPCC). IPCC Special Report on Carbon Dioxide Capture and Storage; IPCC: Geneva, Switzerland, 2005. r 2010 American Chemical Society
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: DOI:10.1021/ef100476d
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industrial processes, and the molar uptake of CO2 during this stage is shown to reduce with an increasing number of cycles.19,20 Reduction in reactivity of a limestone is largely ascribed to sintering of particles during the calcination stage and a corresponding decrease in porosity associated with small pores (1160 and 2 h in air to ensure that it was dry and stable in weight (i.e., any carbonaceous material was burned off). The cold flow rate of gas entering the reactor was set by calibrated rotameters to 47.5 cm3/s at 1 bara, which corresponds to U/Umf = 7 for sand of size fraction 355-425 μm in a N2 atmosphere at 973 K, to ensure vigorous fluidization. Umf was estimated using an equation proposed by Wen and Yu.37 The reactor was heated to the calcination temperature under a N2 atmosphere. When at the desired temperature, the gas was switched to a calibration gas [15.00% (v/v) CO2, balance N2, BOC] to calibrate the gas analyzer, before switching to the gas mixture for the experiment, with the standard being ≈15% (v/v) CO2, balance N2. All flow rates were controlled by calibrated rotameters. Once a stable CO2 concentration was achieved, 4.3 ( 0.1 g of limestone of size fraction 500-710 μm (to allow for easy separation by sieving of the limestone from the sand) was added to the reactor. The mass of 4.3 g was chosen to allow for a shorter experiment time than a pure bed of limestone (which requires a longer time to calcine, because of the high partial pressure of CO2 that builds up during reaction), allowing for more cycles in a day but potentially reducing the similarity to the proposed industrial process. After the addition of the limestone, the temperature control program was switched to cycling mode and the bed was held at the calcination temperature for 900 s before being cycled between the carbonation and calcination temperatures, with the set point being held at 900 s for each. A total of 13 cycles of carbonation and calcination were performed. After the 14th calcination, the gas supply was switched to pure N2 and the sample was removed, placed in a crucible within a desiccator, and weighed when cool. The molar uptake of CO2 (M(CO2),carb) by the sorbent during carbonation was calculated using the formula shown in eq 3 Z t nin ðXin - Xout Þ dt ð3Þ MðCO2 Þ, carb ¼ ð1 - Xout Þ 0
Table 1. X-Ray Fluorescence Analysis of Limestones Used, with Fractions of CaO and MgO Converted to CaCO3 and MgCO3, Respectively CaCO3 MgCO3 SiO2 Fe2O3 Al2O3 P2O5 K2O MnO
Purbeck
Havelock
La Blanca
0.939 0.021 0.029 0.004 0.004 0.002 0.001 n/a
0.963 0.019 0.009 0.003 0.004 n/a n/a 0.001
0.994 0.003 0.002 n/a n/a n/a n/a n/a
concentration of CO2 exiting the reactor. The molar evolution of CO2 during calcination (M(CO2),calc) was also calculated and, after the first calcination, found to be similar to the molar uptake of CO2 for the previous carbonation, an expected result because calcination is shown to go to completion. The carrying capacity (CN) of the limestone is presented here as the mass of CO2 taken up by a limestone during the carbonation stage of cycle N per 100 g of the original limestone calcined (as is convention). Various semi-empirical equations have been used to model the decay in reactivity of a sample upon cycling.23,38-40 This work uses the equation shown in eq 4 proposed by Grasa and Abanades,41 which has been found to model experimental carrying capacities at both low and high cycle numbers, modified for use in a mass of CO2 per 100 g of the original sorbentcalcined basis 1 þ a¥ CN ¼ 100 ð1=ð1 - a¥ ÞÞ þ kN 2 3 6 7 6 7 6 7 6 7 RMMCaCO3 FCaCO3 7 ð4Þ 0 1 6 ! 6 7 6 7 F RMM CaCO CaO 3 A 4RMMCO2 @ þ ð1 - FCaCO3 Þ 5 RMMCaCO3 where CN is the carrying capacity in the Nth cycle, while k and a¥ are dimensionless constants, which can be seen as a decay constant and the activity at N = ¥, respectively; they are strongly dependent upon the conditions used (time, temperature, CO2 concentration, gas flow rate at calcination/carbonation, and reactor type). FCaCO3 is the fraction of calcium carbonate in the original limestone, and RMMx is the molar mass of species x. Least squares minimizations have been carried out to obtain best fit values of a¥ and k with experimental data. FCaCO3 was determined by X-ray fluorescence analysis of the original limestone samples using a Bruker AXS S4 Explorer (see Table 1). Experimental Procedure: Standard Hydration Conditions. After weighing, the calcined limestone and sand were transferred to a Petri dish and left in a humidor, a vessel containing water at the bottom, at room temperature. The air within the humidor was assumed to be saturated with H2O, giving a partial pressure of ≈0.023 bar, while the partial pressure of CO2 in ambient air is ≈3.8 10-4 bar. Therefore, both carbonation and (38) Fuertes, A. B.; Alvarez, D.; Rubiera, F.; Pis, J. J.; Marban, G.; Palacios, J. M. Surface area and pore size changes during sintering of calcium oxide particles. Chem. Eng. Commun. 1991, 109, 73–88. (39) Wang, J. S.; Anthony, E. J. On the decay behavior of the CO2 absorption capacity of CaO-based sorbents. Ind. Eng. Chem. Res. 2005, 44 (3), 627–629. (40) Gonzalez, B.; Grasa, G. S.; Alonso, M.; Abanades, J. C. Modeling of the deactivation of CaO in a carbonate loop at high temperatures of calcination. Ind. Eng. Chem. Res. 2008, 47 (23), 9256–9262. (41) Grasa, G. S.; Abanades, J. C. CO2 capture capacity of CaO in long series of carbonation/calcination cycles. Ind. Eng. Chem. Res. 2006, 45 (26), 8846–8851.
where nin is the molar flow rate entering the reactor, Xin is the concentration of CO2 entering the reactor, and Xout is the (37) Wen, C. Y.; Yu, Y. H. A generalized method for predicting the minimum fluidization velocity. AIChE J. 1966, 12 (3), 610–612.
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Table 2. Name Identification of Samples for Nitrogen Adsorption Analysis calcination temperature (K)
(a) after first calcination
(b) after calcination and cycling
(c) after calcination, cycling, hydration, and dyhdration
1113 1173 1223
1113Calc 1173Calc 1223Calc
1113C13 1173C13 1223C13
1113C13H 1173C13H 1223C13H
hydration are favored under these conditions.31 After 38 ( 1 h, the sample was removed and weighed. After this time, TGA experiments found samples to be >95% hydrated. Any further cycling of the sample was carried out by cycling under the standard cycling conditions (Tcalc = 1113 K, Tcarb = 973 K, U/Umf = 8, and tcalc = tcarb = 900 s), with the hydrated (reactivated) sample being added with N2 passing through the bed at the calcination temperature. Experimental Procedure: Determining the Extent of Attrition over the Course of an Experiment. Calculations were also carried out to determine the attrition extent during an experiment. Mass lost during an experiment (Δmexp) is expressed as a percentage, as described in eq 5, where mmeas is the mass measured at the end of an experiment and mt is the theoretical mass after calcination of the original limestone. Equation 6 defines mt, where m0 is the original mass of limestone. Δmexp ¼ 100½1 - ðmmeas =mt Þ 2 3 ! F RMM CaCO CaO 3 þ ð1 - FCaCO3 Þ5 mt ¼ m0 4 RMMCaCO3
Figure 3. Results from a typical experiment. Temperature as function of time: bed temperature (;) and temperature set point (- - -). CO2 concentration as a function of time: outlet CO2 concentration (;) and inlet CO2 concentration (- - -).
calcination temperatures of 1113 and 1273 K were monitored using an optical microscope (Seben Stereo Microscope Incognita III). In each experiment, five limestone particles were taken from the sample after cycling and placed on separate platinum pans with a short length of quartz cylinder (diameter of 0.48 mm) to determine the scale. Digital images of the particles were taken before placing the platinum pans into the hydration vessel for the standard length of time. After hydration, digital images were taken to establish the size change during hydration. The size of the particles was determined using ImageJ (developed by the National Institutes of Health, United States Department of Health and Human Services). The sizes of elutriated particles were measured in a similar manner.
ð5Þ ð6Þ
Varying Method of the Addition of Hydrated Sorbent to FBR. As outlined in the Results and Discussion, hydrated sorbent was found to be more likely to be entrained from the bed. To investigate the reasons for this further, two separate experiments were performed: (i) Hydrated sorbent was added to the FBR at room temperature and slowly heated to 1113 K under 47.5 cm3/s (the experimental flow rate) N2 before completing the cycling experiments using the standard procedure. (ii) Hydrated sorbent was added to the FBR at room temperature and heated to 1113 K under a low flow rate (U < Umf) of N2 with the temperature held at 773 K for 600 s, which ensured full dehydration and removal of evolved steam, before completing the cycling experiments under the standard conditions. These were only performed for Havelock limestone with calcination temperatures before hydration of 1113 K. Physical Properties of the Sorbent. In addition to experiments determining the CO2 uptake of the sorbent, further experiments were undertaken to generate particles suitable for pore size, pore distribution, and Brunauer-Emmett-Teller (BET) surface area analysis. Calcined particles were generated and compared for three different stages at each temperature studied: (a) after the first calcination, (b) after the first calcination and a further 13 cycles of carbonation and calcination, and (c) after the first calcination following hydration of particles cycled 13 times. Names of experiments are identified in Table 2 and referred to in the Results and Discussion. BET surface areas and pore volume distributions were determined using a Micromeritics Tristar 3000 N2 sorption analyzer. Skeletal density was determined using a Micromeritics AccuPyc 1330 pycnometer, and envelope density was determined using a Micromeritics GeoPyc 1360 envelope density analyzer. Skeletal (sometimes known as absolute) density is determined by helium displacement and measures the density of particles excluding open pores. Envelope density is determined by fine powder displacement and measures the density of particles including open pores. Mercury porosimetry for verification of the particle breakage model was performed using a Micromeritics Autopore IV mercury porosimeter. Particle Size Change during Hydration. The change in the size of example particles across hydration for the samples cycled at
Results and Discussion Results from a Typical Experiment. Results from the first three cycles of a typical experiment are shown in Figure 3. Limestone was added at time = 0 s, with the calcination temperature achieved, and cycling was started. The limestone calcined, releasing CO2, causing an increase in the outlet CO2 concentration to be observed. Upon completion of calcination, the outlet CO2 concentration returned to the inlet CO2 concentration. After 900 s, the temperature set point was switched to the carbonation temperature. When the bed temperature cooled to a value suitable for carbonation to occur, CO2 was absorbed by the sorbent and the outlet CO2 concentration decreased. The fast initial reaction rate of the calcium oxide is observed as the carbonation peak, after which the rate is seen to decrease markedly as it becomes diffusioncontrolled and the outlet CO2 concentration is observed to tend toward the inlet CO2 concentration. After 900 s, the set point was switched back to the calcination temperature. Varying the Temperature of Calcination (Tcalc ) before Hydration. A plot of carrying capacity as a function of the number of cycles is shown in Figure 4 for Tcalc = 1113, 1173, 1223, and 1273 K before hydration, with Figure 4a showing experimental data for Havelock limestone, Figure 4b showing experimental data for La Blanca limestone, and Figure 4c showing experimental data for Purbeck limestone. Also shown are fits of eq 4 to data for the first 13 cycles of experiments for Tcalc = 1113 and 1223 K to allow for a comparison of carrying capacities after hydration with projected values if particles had not been hydrated. It should be noted that the equation is not strictly applicable in the case of 1223 K because the temperature of calcination is different after hydration. However, experiments changing from calcination temperatures of 1113 to 1223 K after 13 cycles using the same 4609
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: DOI:10.1021/ef100476d
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Figure 4. Cycling experiments varying Tcalc before hydration for (a) Havelock limestone, (b) La Blanca limestone, and (c) Purbeck limestone. Experimental data for Tcalc = 1113 K ([), 1173 K (9), 1223 K (2) and 1273 K (), with fits of eq 4 to data for the cycles before hydration of Tcalc = 1113 K (;) and 1223 K (- - -). (d) Carrying capacity of particles after hydration normalized for mass lost at the end of the experiment. Mean of data for 1113, 1173, and 1223 K with error bars for one standard deviation for Havelock limestone ([), La Blanca limestone (b) and Purbeck limestone (2), with a logarithmic line of best fit for each.
limestone showed that there was no appreciable change in the carrying capacity across the transition. It can be expected that the carrying capacity would decay at a slightly decreased rate; however, the projections are plotted as a useful tool of comparison. Error bars are shown for one standard deviation on five experiments, while all other points are the mean of two experiments. Figure 4d shows the carrying capacity of sorbent after hydration adjusted for mass lost at the end of the experiment. In each case, the mean of the experimental data for Tcalc = 1113, 1173, and 1223 K is shown, with error bars for one standard deviation also shown. Table 3 shows the percentage increases in carrying capacity of the hydrated limestone in comparison to the projected value for the 14th cycle, mass loss during the 13 cycles before hydration (Δmexp1), mass loss during the 13 cycles after hydration (Δmexp2), and total mass loss during the whole 26 cycles. Panels a-c of Figure 4 shows that, for each limestone studied, an increased rate of deactivation for the 13 cycles before hydration was observed with increasing Tcalc. This is expected because the molar uptake of CO2 during the fast reaction stage of carbonation is found to decrease as CaO sintering increases at higher temperatures of calcination.24 Purbeck showed the highest reactivity upon cycling followed by Havelock and then La Blanca. Variability of the performance of
limestone upon cycling has been the focus of study elsewhere,22,42 and it is suggested that Purbeck retains its reactivity well because of it has dolomitic character. Data presented in Table 3 show that the “reactivation” extent (increase in carrying capacity after hydration) was largest for each limestone at Tcalc = 1113 K and decreased with increasing Tcalc. Of the limestones studied, the reactivation extents varied significantly, with La Blanca showing the greatest reactivation, followed by Havelock, and followed by Purbeck. Havelock and La Blanca both showed improved reactivity after hydration for Tcalc = 1113 K in comparison to the projections using eq 4, whereas Purbeck showed improved reactivity after hydration for only the first cycle. For the case of Tcalc = 1223 K, only La Blanca showed improved reactivity after hydration in comparison to projections using eq 4, whereas both the reactivity of Havelock and Purbeck decreased after hydration. The reactivation extent will naturally be affected by the reactivity of the limestone before hydration, resulting in an inflation of the hydration extent seen for La Blanca, which showed a very low reactivity before hydration, and a reduction of the hydration extent seen for Purbeck, which showed a high reactivity before hydration. A large factor in the reduced reactivation at higher values of Tcalc is the increased friability of particles, which results in increased production of fine material that elutriates from the freeboard. Table 3 shows that mass lost during the 13 cycles after hydration increases with increasing Tcalc for all
(42) Manovic, V.; Anthony, E. J.; Grasa, G.; Abanades, J. C. CO2 looping cycle performance of a high-purity limestone after thermal activation/doping. Energy Fuels 2008, 22 (5), 3258–3264.
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Table 3. Reactivation Extents and Mass Changes from Varying Tcalc before Hydration
limestone
calcination temperature (K)
increase in carrying capacity after hydration (%)
mass loss in cycling experiment before hydration (%)
mass loss in cycling experiment after hydration (%)
mass loss over both cycling experiments (%)
Havelock Havelock (slow heat) Havelock (slow heat, low flow) Havelock Havelock Havelock La Blanca La Blanca La Blanca Purbeck Purbeck Purbeck
1113 1113 1113 1173 1223 1273 1113 1173 1223 1113 1173 1223
109 57.8 163 82.4 -1.0 -82.6 368 281 298 9.7 -13.8 0.0
10.3 ( 1.2 10.8 9.5 15.3 14.6 12.1 2.3 3.4 4.5 -3.4 -0.9 1.2
41.3 ( 14.1 60.7 31.3 63.2 84.1 95.7 13.8 50.8 69.5 49.3 59.4 59.2
47.4 ( 12.7 65.1 37.8 68.9 86.3 96.2 15.7 52.5 70.8 47.6 59.0 59.7
limestones studied. At Tcalc = 1113 K, La Blanca limestone was significantly less friable than Purbeck and Havelock, contributing to its very large reactivation extent. Both La Blanca and Havelock showed marked increases in friability with increasing Tcalc; however, Purbeck showed only a small increase in friability with increasing Tcalc. The problem with friability of the sample is observed most strongly for Havelock limestone cycled at Tcalc = 1273 K, where almost no reactivity was observed after hydration because of 96.1% of the sample being lost. These mass losses will be a function of the type of reactor used and would be expected to be higher in a fluidized bed. It is expected that significant particle breakage would be observed on scale up to a similar system with reactivation by hydration (where calcination temperatures of >1223 K are anticipated). Generation of large amounts of fine material could be less of a problem in systems with cyclones and larger freeboards, where fine particles could react (at a slower rate because of the lower temperature) in the freeboard/downstream of the reactor before being collected and recycled. However, cyclones will only collect particles sized within specific limits, and some fine material is likely to not be captured, which will result in undesirable effects on the recycle rate. Plotted in Figure 4d is the carrying capacity for the 13 cycles after hydration for Tcalc = 1113, 1173, and 1223 K adjusted for the mass loss at the end of the whole experiment. A problem with these data is that mass loss is not known for each cycle, and the data cannot be adjusted accordingly. However, it is interesting to note that, after the first few cycles, all of the hydrated samples behave similarly. If it is assumed that most of the mass loss occurs in the first few cycles after hydration, causing the largest deviations from actual mass in bed to mass in bed at the end of the experiment to occur in this period, it is possible to conclude that, for particles large enough to remain fluidized, hydration had reactivated them to the same extent. Figure 4d also shows that each limestone shows different reactivity following hydration. Havelock is reactivated to the highest reactivity, followed by La Blanca, and followed by Purbeck. This could be a result of the irregular behavior of natural limestones, which have different morphological structures and impurities; e.g., Purbeck has mild dolomitic character and contains small particles of flint.22 Data collected from investigations into BET surface area, pore volume, envelope density, and skeletal density of particles are presented in Table 4 (sample identification described in Table 2). For the set of samples calcined once and the samples cycled 13 times, both BET surface areas and pore volumes for small pores are found to decrease as the temperature of calcination increases. BET surface areas and pore volumes are also found to decrease substantially upon
undergoing 13 cycles, following the original calcination. Both of these results can be anticipated as both BET surface area24 and volume of small pores (
