2072
Energy & Fuels 2007, 21, 2072-2081
The Effects of Repeated Cycles of Calcination and Carbonation on a Variety of Different Limestones, as Measured in a Hot Fluidized Bed of Sand Paul S. Fennell, Roberta Pacciani, John S. Dennis,* John F. Davidson, and Allan N. Hayhurst Department of Chemical Engineering, UniVersity of Cambridge, Pembroke Street, Cambridge, CB2 3RA, United Kingdom ReceiVed October 10, 2006. ReVised Manuscript ReceiVed April 3, 2007
The capacity of calcined limestone to react repeatedly with CO2, according to CaO(cr) + CO2(g) ) CaCO3(cr) (eq I), and also its regeneration in the reverse reaction have been studied in a small, electrically heated fluidized bed of sand, for five different limestones. The forward step of eq I is a promising way of removing CO2 from the exhaust of, for example, a coal-fired power station, ready for sequestration or as part of a scheme to generate H2 using an enhanced water-gas shift reaction. The reverse step regenerates the sorbent. The uptake of CO2 by CaO, produced by calcining limestone, was measured using a bed of sand fluidized by N2 at ∼1023 K. For each experiment, a small quantity of limestone particles was added to the hot sand, whereupon the limestone calcined to produce CaO. Calcination was completed in ∼500 s for particles of a mean diameter of ∼600 µm. Next, CO2 was added to the fluidizing nitrogen to carbonate the CaO for ∼500 s. Measurements of [CO2] in the off-gases enabled the rates of calcination and the subsequent carbonation to be measured as functions of time. Many successive cycles of calcination and carbonation were studied. The forward step of reaction I is shown to exhibit an apparent final conversion, which decreases with the number of cycles of reaction; the final conversion fits well to a correlation from the literature. The reverse (calcination) reaction always proceeded to completion. Particles of limestone, removed from the reactor after several cycles, in either their partially carbonated or fully calcined state, were studied using X-ray diffraction, gas adsorption analysis, mercury porosimetry, and scanning electron microscopy. It was found that the carrying capacity of CaO for CO2 on the nth cycle of carbonation was roughly proportional to the voidage inside pores narrower than ∼150 nm in the calcined CaO before carbonation began. Thus, morphological changes, including reduction in the volume of pores narrower than 150 nm within a calcined limestone, were found to be responsible for much of the fall in conversion of reaction I with increasing numbers of cycles. The rate of attrition of the particles of limestone in a fluidized bed, while cycling between the calcined and carbonated states, was also studied. It was found that most limestones lost less than 10% of their mass due to attrition over the course of a typical experiment, lasting ∼8 h.
Introduction Recently, concerns over the rapidly increasing threat of global warming have resulted in much attention being paid to so-called “clean coal” technology. This comes in a variety of guises but is generally designed to isolate the carbon from coal, as a pure stream of CO2 suitable for sequestration in the earth, when coal is gasified or combusted to generate either electricity or hydrogen.1 Calcium oxide, CaO, is of interest in some of these schemes, owing to its ability to react with CO2 to produce solid CaCO3 in
CaO(s) + CO2(g) ) CaCO3(s)
(I)
which is readily reversed by the application of heat. A recent economic analysis2 has shown that reversible carbonation of limestone is an economically viable method of removing CO2 * Corresponding author. e-mail:
[email protected]. (1) Slowinski, G. Some Technical Issues of Zero-Emission Coal Technology. Int. J. Hydrogen Energy 2006, 31 (8), 1091-1102. (2) 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, 920.
from the exhaust of a power station. When the CaO is derived from natural sources, for example, limestone (CaCO3), its capacity to take up CO2 diminishes after repeated cycles of calcination,4-8 followed by carbonation, in reaction I. The conversion of reaction I, or its reverse, relative to its theoretical maximum of 1 mol of CO2 reacted per mole of CaO, is the carrying capacity, C, of the limestone. Barker3 showed that, when CO2 is adsorbed by CaO, there is a change from an (3) Barker, K. The Reversibility of the Reaction CaCO3 ) CaO + CO2. J. Appl. Chem. Biotechnol. 1973, 23, 733. (4) Abanades, J. C.; Alvares, D. Conversion Limits in the Reaction of CO2 with Lime. Energy Fuels 2003, 17, 308. (5) Salvador, C.; Lu, D.; Anthony, E. J.; Abanades. J. C. Enhancement of CaO for CO2 Capture in an FBC Environment. Chem. Eng. J. 2003, 96, 187. (6) Abanades, J. C.; Anthony, E. J.; Lu, Y.; Salvador, C.; Alvares, D. Capture of CO2 from Combustion Gases in a Fluidized Bed of CaO AIChE J. 2004, 50, 1614. (7) Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. Improved LongTerm Conversion of Limestone-Derived Sorbents for in Situ Capture of CO2 in a Fluidized Bed Combustor. Ind. Eng. Chem. Res. 2004, 43, 5529. (8) Fennell, P. S.; Pacciani, R.; Davidson, J. F.; Dennis, J. S.; Hayhurst, A. N. The Use of Limestone Particles for the Capture of CO2: Its Initial Reactivity and Loss of Reactivity after Repeated Cycles of Calcination and Carbonation. Proceedings of FBC19, Vienna, Austria, May 21-24, 2006.
10.1021/ef060506o CCC: $37.00 © 2007 American Chemical Society Published on Web 07/03/2007
Repeated Cycles of Calcination and Carbonation
Figure 1. Schematic diagram of the experimental apparatus.
initially fast (kinetically controlled) mechanism to a much slower (diffusion controlled) one after deposition of the product, CaCO3. This paper investigates the fall in reactivity of five different limestones [Purbeck, Penrith, (U.K.); Cadomin, Havelock, Glen Morrison (Canada)], after repeated cycles of calcination and carbonation under a variety of different conditions. The experiments involved heating a fluidized bed of sand up to ∼1023 K and manipulating the partial pressure of CO2 in the fluidizing gas to either calcine or carbonate the added Cacontaining particles. The primary concern is with the drop in carrying capacity during the initial, rapid, carbonation of CaO. In addition, the composition of these solids, their morphologies, and the sizes of the pores and so forth have been studied after successive cycles of calcination and carbonation. This work follows previous studies.4-8 Others have noted5 the need for such an investigation of limestones in a fluidized bed, because much of the previous work4,5,7 was done with thermogravimetric analyzers (TGA), where the limestone was not exposed to attrition, as in a fluidized bed. Other work has been on a large pilot scale,5 where it is difficult to control conditions precisely. The aim here was to extend existing studies (i) to carefully controlled conditions in a laboratory-scale fluidized bed and also (ii) to gauge the effect of various additives on carrying capacity. Due to the small size of the fluidized bed employed, it was possible to measure the attrition experienced by the particles; it was also possible to carry out rapidly a large number of experiments. In fact, the fluidized bed technique offers a means of rapidly evaluating promising ways of reducing the degradation in the carrying capacities of limestones, when repeatedly calcined and carbonated. However, since the sample of limestone was significantly bigger than that used in a TGA experiment, it was possible to recover sufficient particles of limestone after each experiment to allow various analytical techniques to be used. Experimental Section Cycling Experiments. Experiments were performed in a laboratory-scale fluidized bed heated by an external electrical furnace, as shown in Figure 1. The bed consisted of a measured volume (20 mL after tapping, ∼30 g) of quartz sand (sieved to 355-425 µm) contained in a quartz tube (i.d. 29.5 mm, length 460 mm) and supported on a sintered quartz plate as the distributor, located 110 mm from the base of the tube. The sand was fluidized by N2 (cold flowrate 80 mL/s), to which a known flowrate (cold) of CO2 between 13 and 32 mL/s was periodically added. The value of
Energy & Fuels, Vol. 21, No. 4, 2007 2073 U/Umf was 8.5 when fluidized with N2 but was up to 11.7 when CO2 was added (note: see the Nomenclature section at the end of this article for abbreviations and variables used). The bed was thus vigorously fluidized. All flowrates were measured using calibrated rotameters, operating at an internal pressure of 101.3 kPa. The temperature inside the fluidized bed was set to a value between 1023 and 1063 K and was measured with a type K thermocouple to be uniform and steady: this thermocouple also controlled the surrounding furnace. A weighed batch of 2.00 ( 0.01 g (weighed to ( 0.001 g) of limestone was added to the hot bed, while it was fluidized by N2. The concentration of CO2 in the off-gas was measured continuously by a nondispersive infrared gas analyzer (ADC 2000 series) via a sampling system containing glass wool to filter out fine particles. Prior to each experiment, calibration gas (containing 15 vol % CO2 in N2, Air Liquide) was passed through the bed to calibrate the analyzer. It was found that the gas-sampling system could be modeled as two continuously stirred tanks in series, both with a measured time constant of ∼2 s; all the measurements were thus corrected, although the correction made only a marginal difference to the measured recovery, the main focus of this investigation. The sampling probe, the tube containing the filter, and the reactor were carefully washed and dried before each experiment to remove adventitious accumulation of CaO powder. The particles were fully calcined in N2 in ∼500-600 s with the bed at, for example, 1023 K. The time for complete calcination was determined by the time taken for [CO2] in the off-gases to return to zero. Next, a known flowrate of CO2 was added to the N2 for a set period of time, generally 500 s, to carbonate the hot CaO back to CaCO3 at the temperature of interest. The flowrate of CO2 in the fluidizing gas was chosen to ensure that its partial pressure was sufficient to convert the CaO to CaCO3. Then, the CO2 was switched off, but the flow of N2 was maintained for another period of calcination, usually 500 s, by a solenoid connected to a timing circuit. This high-temperature cycling of CO2 on and off was repeated until a set number of cycles (up to 30) had been completed. Most of the experiments used Purbeck limestone (Swanworth Quarry, Dorset, U.K.), crushed and then sieved to a particle diameter of 500-710 µm; other particle sizes of Purbeck limestone, from 150-212 µm up to 1000-1180 µm, were also tested. Experiments were also conducted with four other limestones for 500-710 µm particles. Table 1 shows the experimental parameters investigated. Sometimes, particles were recovered from the bed after a certain number of cycles of calcination and carbonation to investigate their morphology and how it had changed over the course of repeated cycling. The procedure involved disconnecting the gas supply to the bed and immediately tipping the entire bed, while still red-hot, into a crucible. The crucible was then cooled to room temperature in a desiccator, to prevent any unreacted CaO from forming Ca(OH)2 by reaction with H2O from the atmosphere. The particles were then sieved from the sand and rapidly transferred to containers with tightly fitting lids. X-ray diffraction (XRD) analysis (Bruker GADDS instrument, calibrated with a silver behenate standard), mercury porosimetry (Micromeretics Autopore IV), and gas (nitrogen) adsorption analysis (to determine the Brunauer-EmmettTeller, BET,9 surface area or Barrett-Joyner-Halenda, BJH,10 pore volume in pores narrower than 150 nm) (Micromeretics Tristar3000) were then used to characterize the particles within a day. In addition, the particles were examined using a scanning electron microscope (SEM; JEOL 5800). Of course, the cycling of calcium compounds in our experiments is somewhat different from what occurs in a commercial process using CaO to remove CO2 (at ∼14-15 vol % CO2) from a combustor; in that case, the CaCO3 produced is transported to a calciner, where the temperature is increased above 1173 K to calcine the particles and produce a high partial pressure of CO2. (9) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309. (10) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. The Determination of Pore Volume and Area Distributions in Porous Substances. 1. Computations from Nitrogen Isotherms. J. Am. Chem. Soc. 1953, 73, 373.
2074 Energy & Fuels, Vol. 21, No. 4, 2007
Fennell et al.
Table 1. The Experimental Parameters Investigated
limestone Purbeck
Penrith Cadomin Glen Morrison Havelock
particle sieve size (µm) 106-150, 150-212, 355-500, 500-710, 850-1000, 1000-1180 500-710 500-710 500-710 500-710
temperature (K)
doping (aqueous)
1023-1063
0.002 M Na2CO3 0.01 M Na2CO3 0.01 M Na2CO3 0.5 M Na2CO3 0.5 M NaCl
1023 1023 1023 1023
Weighing. Some experiments determined if there was substantial elutriation or attrition of particles. The bed of sand was heated to 1023 K while fluidized by N2. It was then tipped out into a crucible, which was weighed hot. The sand was then returned to the reactor and reheated to 1023 K. A weighed batch of 2.00 g of limestone was added to the bed, and an experiment was conducted as usual. At the end of the experiment, the bed and calcined particles were rapidly tipped into the crucible and reweighed while hot. The mass of the calcined particles remaining in the bed was determined by difference. Elutriation of fines is discussed below. Doping of Particles. Prior to conducting a cycling experiment, some limestone particles were doped with Na2CO3 or NaCl. For this, an aqueous solution of the salt, of a known molarity, and 10 g of Purbeck limestone (500-710 µm) were added to a flask containing 400 mL of the solution. The flask was gently swirled to mix the contents, before sealing. It was then left for ∼36 h before the contents were gently shaken (to suspend fines) and the liquid was decanted off (including any fines, which were a very small proportion of the total solids). The remaining solids were washed onto a watch glass, using the same solution. The solids were then dried in an oven at 343 K overnight, before being sealed in a container. A control experiment was conducted in which Purbeck limestone particles were simply washed with distilled water according to the procedure outlined above. It showed no deviation from a standard experiment without washing. X-ray Diffraction. For X-ray analysis, particles in the size range 500-710 µm were crushed in a pestle and mortar to an extremely fine powder. The powder was then examined using a Bruker GADDS instrument with a Cu KR X-ray source, operated at 45 mA and 45 kV, with 2D scattering data collection. The equipment was calibrated using a silver behenate standard. Peaks were assigned using the International Center for Diffraction Data (1998) powder diffraction database.11 Equimolar mixtures of analytical-grade CaCO3, with either Ca(OH)2, CaO, or MgCO3, were used as standards to calibrate measurements of the relative proportions of these components in the different limestones, similar to the method of Chung.12 Steetley dolomite (98-99% purity) was used as a calibration standard to determine the proportion of dolomite present in the limestones studied.
Results A typical plot of the mole fraction of CO2, as measured (at atmospheric pressure) at the outlet from the fluidized bed at 1023 K, is shown in Figure 2. When CO2 is being released, the product of the mole fraction of CO2 in the off-gases and the molar flowrate at the outlet gives the rate of production of CO2. For the first ∼510 s, the bed was fluidized with N2 only, so that the added batch of CaCO3 decomposed and released CO2. Next, CO2 was added to the fluidizing N2 to give a mole fraction (11) ICCD Crystallographic Database; International Centre for Diffraction Data: Newtown Square, PA, 1998. (12) Chung, F. H.; Quantitative Interpretation of X-ray Diffraction Patterns of Mixtures. II. Adiabatic Principle of X-ray Diffraction Analysis of Mixtures. J. Appl. Crystallogr. 1974, 7, 526.
reaction time (s) (calcination: carbonation) 500:500 500:1800 1800:500
500:500 500:500 500:500 500:500
[CO2] (vol %) 14, 29
14 14 14 14
(yCO2) of ∼0.14, which is higher than the equilibrium value of ∼0.10 at 1023 K and 101.3 kPa. Figure 2 presents a complete cycle of calcination (0-510 s), followed by carbonation (5101025 s) and finally recalcination (1025-1510 s). The initial addition of ∼2.00 g of limestone to the bed caused the temperature to drop to ∼998 K, although it did rise back to 1023 K within ∼60 s. During this time, yCO2 rose to a maximum of ∼0.017, before slowly dropping to zero. The shaded area in Figure 2, when multiplied by the total molar flowrate of gas through the bed, gives the total number of moles of CO2 absorbed by CaO during this first carbonation. Figure 2 also reveals that the calcination time and the area under the [CO2] profile for the second calcination are less than for the first calcination. Thus, less CO2 was released in the second calcination than the first. In general, the particles took ∼500 s to calcine fully after their addition; however, the Glen Morrison limestone was atypical, in that it took ∼600 s for the first calcination. Within experimental error, the amount of CO2 absorbed in the carbonation step was found to be equal to that released in the subsequent calcination, indicating that the calcination stage always proceeded to completion. This was confirmed by XRD analysis, as described below. Adding smaller batches of limestone led to a smaller drop in the bed’s temperature and, consequently, a slightly shorter calcination time. The undesirable disturbance to the system when adding the particles was considered less important than retrieving sufficient particles after an experiment for, for example, gas adsorption analysis. After the initial drop in temperature, the bed rapidly returned to the setpoint temperature (within less than 1 min) and remained there to within (5 K. Again, when yCO2 in the off-gas reached zero, carbonation was begun by opening the solenoid valve and admitting a known flowrate (here, 13 mL/s) of CO2 into the fluidizing gas. The partial pressure of CO2 was such (>10.13 kPa) that the CaO particles began to carbonate. The short-lived spike in the measured yCO2, shown in Figure 2 at the start of carbonation when t ) 510 s, originates from the imperfect nature of the correction of the measurements for the response time of the analyzer; it makes a trivial difference to the measured recovery. After ∼300 s, yCO2 at the outlet of the reactor was essentially equal to the inlet value. This did not necessarily mean that carbonation had proceeded to completion; it is probable that the reaction had gone from a fast (kinetically limited) regime to a slow (diffusion limited) one. The number of moles of CO2 which had reacted was determined by subtracting the outlet molar flow of CO2 from that at the inlet. Thus, the area shaded in Figure 2 was multiplied by the total molar flowrate of gas passing through the bed. The bed was left fluidizing for a further 200 s, whereupon the flow of CO2 was shut off by the solenoid. After this point (t ) 1040 s), Figure 2 shows that the particles began
Repeated Cycles of Calcination and Carbonation
Figure 2. Mole fraction of CO2 measured in the off-gases from the fluidized bed. Purbeck limestone. T ) 1023 K, mo ) 2.00 g, dp ) 500-710 µm, QN2 ) 80 mL/s, QCO2) 13 mL/s, yCO2 ) 0.14, sand ) 355-425 µm, U/Umf ) 8.5 (without CO2), 9.8 (with CO2). Table 2. Mass Fraction of CaCO3 in Bulk Samples of the Various Limestones Used
limestone
(FCaCO3) (wt %) by analysis of bulk samples
Purbeck Cadomin
95 97
Penrith Glen Morrison Havelock
99 94 97
reference this work average of the values of Wu et al.14 and Salvador et al.5 Dennis and Hayhurst15 this work This work
to recalcine. This cycling between the calcined and carbonated states was repeated for up to 30 cycles. Of course, in a real system, it would be necessary to capture all the CO2, requiring a greater ratio of CaO/CO2 than that used here, and also a lower carbonation temperature. Weighing Experiments, and the Initial Mass Fraction of CaCO3 in Each Limestone. The mass fraction of CaCO3 (FCaCO3) in each raw limestone was determined either experimentally (for the Purbeck, Glen Morrison, or Havelock limestones) from their CO2 content according to BS EN 19613 or found from the literature. These contents are listed in Table 2, together with the reference, if appropriate. For comparison with Table 2, the average mass fraction of CaCO3 in Havelock limestone was 96.5 wt % from Wu et al.14 and Salvador et al.,5 in good agreement with our results. However, there is a slight discrepancy between the value of FCaCO3 measured for bulk Purbeck limestone (95 wt %) and the actual value used here (91 wt %). This was because further investigations (including XRD analysis) indicated that tiny particles of flint were present in the Purbeck limestone (but no other). Consideration of the recovery of CO2 from the initial calcination of 500-710 µm particles, and the weighing experiments detailed later, showed that 91 wt % was the most likely fraction of CaCO3 in the Purbeck limestone. X-ray powder diffraction analysis of the Havelock limestone indicated that it was an atypical limestone, mildly dolomitic, containing ∼11 mol % CaMg(CO3)2. The other limestones were also examined using X-ray diffraction; no major components other than CaCO3, and the small quantities of SiO2 as flint in the Purbeck limestone, were detected. (13) BS EN 196. Methods of testing cement. Methods of taking and Preparing Samples of Cement; British Standards Institute: London, 1992. (14) Wu, Y.; Anthony, E. J.; Jia, L. Reactivation Properties of Four Canadian Limestones. Proceedings of FBC19, Vienna, Austria, May 2124, 2006.
Energy & Fuels, Vol. 21, No. 4, 2007 2075
Figure 3. Plots of mm/mt against t. The measurements (together with best fits to eq 2) were as follows: (×) Purbeck (ss), (4) Cadomin (----), ([) Penrith (s•s), (]) Glen Morrison (s s), (+) Havelock (••••). T ) 1023 K, mo ) 2.00 g, dp ) 500-710 µm, QN2 ) 80 mL/s, QCO2) 13 mL/s, [CO2] ) 14.0 vol %, sand ) 355-425 µm, U/Umf ) 8.5 (without CO2), 9.8 (with CO2).
The theoretical weight of the particles (mt) after calcination, assuming no attrition or loss of fines, and also that any material, which was not originally CaCO3, was inert, is given by
[(
m t ) mo
)
FCaCO3MCaO MCaCO3
+ (1 - FCaCO3)
]
(1)
Here, mo is the initial mass of limestone particles added; MCaCO3 and MCaO are the relative molecular masses of CaCO3 (100.8) and CaO (56.1), respectively. Any subsequent drop in the measured mass (mm) of limestone particles is due to the attrition and elutriation of limestone fines, rather than the entrainment of unbroken particles, since all the fluidized particles remained more than 200 mm below the top of the quartz tube. Figure 3 shows (mm/mt) for the five different limestones, plotted against t. Clearly, the ratio (mm/mt) remained within ∼5% of its initial value for the Penrith and Glen Morrison limestones, which do not attrit significantly in 8 h. However, it is clear that Cadomin limestone manifests very significant attrition during the first cycle (each cycle took 0.28 h in total) and continues to do so during further cycles. The Havelock limestone attrits fairly significantly over the first 2 h, but the rate of attrition tails off after 4 h. Several expressions were tested to express the fall in actual mass of limestone as a function of time during an experiment. Minimizing the square of the difference between measurements and theory, using Matlab, indicated that the empirical equation, Equation (2), gave the best fit to the measurements. Here, k1
mm/mt ) k1 + (1 - k1) exp(- t/τ1)
(2)
and τ1 are adjustable constants, with k1 being equivalent to the ultimate fractional recovery as t f ∞. Figure 4 is a plot of the measured mm/mt against t, with eq 2 added for each limestone. Equation 2 fits the experimental measurements best for the Purbeck and Havelock limestones but poorly for the Cadomin, essentially because the Cadomin limestone very significantly attrits in the first cycle (∼0.08 fractional loss). The best values of k1 and τ1 are given in Table 3 for each limestone. It is clear from Table 3 that τ1 is positive for the four limestones which attrited, but that for Penrith limestone τ1 ∼ 0, confirming that it does not attrit after the first cycle. For the Penrith and Cadomin
2076 Energy & Fuels, Vol. 21, No. 4, 2007
Fennell et al.
Figure 4. Carrying capacity for Purbeck limestone in the experiment shown in Figure 2, plotted against the number of cycles: (O) carbonation, (×) calcination. 2 indicates the carrying capacity inferred from X-ray diffraction of carbonated particles. The line is the leastsquares fit of the measurements to eq 3. The “zeroth” calcination is for the original limestone. Table 3. Adjustable Constants in eq 2 for the Five Different Limestones and the Average Pore Volume for the First Calcination of Each Limestone
limestone
k1
τ1 (h)
average pore volume for first calcination (mL/g)
Purbeck Cadomin Penrith Glen Morrison Havelock
0.95 0.86 0.96 0.89 0.92
3.0 0.29 0.01 6.8 1.6
0.19 0.20 0.23 0.22 0.20
limestones, it may be justifiable to treat the attrition on the first cycle separately from that in subsequent cycles and to modify eq 2 so that the “initial” value of mm/mt is that after the first cycle of attrition (rather than unity), and for t to begin after the first cycle of calcination. For the Penrith limestone, such a model indicates a fractional drop of 0.04 on the first cycle, followed by no further attrition. For the Cadomin, the initial fractional loss is 0.09, followed by continued attrition to 0.85 with τ1 ) 2.3 h. The findings on the attrition rates may be summarized as follows: The Penrith, Purbeck, and Glen Morrison limestones were the most attrition-resistant, followed by Havelock, with Cadomin by far the most readily attriting stone. For the Purbeck, Glen Morrison, and Havelock limestones, an exponential decay in the fractional mass of limestone remaining in the bed was observed, from ∼unity to a final value, equivalent to k1 in Table 3. The Penrith limestone lost ∼4% of its calcined mass in the first cycle but then did not appear to attrit further. The Cadomin limestone lost 9% of its calcined mass in the first cycle and continued to lose mass over subsequent cycles but appeared to be reaching an asymptote at 0.85. All of the limestones displayed a rapid initial rate of attrition, but they all displayed a rate of attrition which fell during repeated cycles of calcination and carbonation by at least a factor of 10 from the initial high value. Of course, these findings are for a small fluidized bed; the rate of attrition may be different at a larger scale. However, the rate of mass loss for Cadomin is similar in magnitude to that measured elsewhere16 in a larger fluidized bed with a draft tube. (15) Dennis, J. S.; Hayhurst, A. N. A Simplified Analytical Model for the Rate of Reaction of SO2 with Limestone Particles. Chem. Eng. Sci. 1986, 41, 25 (16) Anthony, E. J. Private communication.
The Reduction in Carrying Capacity after Many Cycles. The percentage carrying capacity of a limestone can be defined as the number of moles of CO2 taken up by a limestone in its carbonation stage × 100, per mole of CaO in the original limestone. It was measured from the number of moles of CO2 released by calcining a particle. It is easier to measure the amount of CO2 released during calcination than that taken up during carbonation, because the latter may depend on the inlet [CO2]. Also, drift in the analyzer’s calibration over ∼8 h, the duration of the longest experiments, and small errors in rotameter readings were less important for the calcination step. Figure 4 shows the carrying capacity for the experiment of Figure 2, from both calcination and carbonation, plotted against the number of cycles of carbonation. Their values are in excellent agreement. For this, and in all subsequent analyses, the recovery of CO2 from the original limestone is the “zeroth” cycle. The nth cycle in each plot shows the amount of CO2 recovered when calcining the product of the nth carbonation (i.e., it actually shows the (n + 1)th calcination). Also shown is the best fit of the semiempirical equation
Cn ) [f nm(1 - fw) + fw] × 100
(3)
to the measurements. Equation 3 has been used4 to describe the carrying capacity, Cn, in terms of the number of cycles, n. Here, fw is the carrying capacity when the number of cycles approaches infinity and fm is another constant, which describes how fast the carrying capacity approaches fw. A least-squares minimization was used to determine the best-fit values of fm and fw; these were used to plot the curve in Figure 5, showing good agreement with the measurements. X-ray powder diffraction was also used to analyze crushed samples of particles sieved from the sand and to determine the proportions of Ca present as CaO, CaCO3, or Ca(OH)2 in the reacted particles. The technique showed that calcination always proceeded to completion, that is, so that no CaCO3 was detected in the calcined material. The measured carrying capacities, plotted in Figure 5, confirm the other measurements. Figure 5 shows the carrying capacities of all five limestones (500-710 µm); in addition, values for six different particle sizes of Purbeck limestone (between 106 and 150 µm and 1000 and 1180 µm) are shown in Figure 5. The volume of pores, determined using BJH analysis, divided by the volume of pores in the initial CaO shown as 100% for the zeroth cycle, is also shown in Figure 5 and will be discussed later. Figure 5 clearly shows good reproducibility for the results. The fractional error in the carrying capacity is generally only ∼5% for the first calcination but increases to ∼10% for cycle numbers greater than 20. Figure 5 also shows that, when the particle size was increased from 355-500 to 1000-1180 µm, there was little change in carrying capacity. Table 4 lists the values of fm and fw, derived from least-squares fits of eq 3 to the experimental results for various limestones, particle sizes, and experimental conditions. It is clear from Figure 5 that the carrying capacity is reasonably well-described by eq 3 for all five limestones, although it does appear to overpredict marginally the carrying capacity (by up to a relative error of ∼10%) for each limestone after ∼20 cycles. Table 4 shows that the particle size and the type of limestone have a marginal effect on the values of fm and fw, for particle sizes >355 µm, in agreement with previous work,4-6 and even for the mildly dolomitic limestone (Havelock), though this does, overall, display the slowest degradation in carrying capacity with repeated cycling. The faster drop in carrying capacity with increasing numbers of cycles for smaller particles must be
Repeated Cycles of Calcination and Carbonation
Energy & Fuels, Vol. 21, No. 4, 2007 2077
Figure 5. Carrying capacity of five different limestones, initially 500-710 µm from CO2 measurements (Figure 2). For results from the analysis of off-gases: (b) individual measurements of a single cycle of calcination (500-710 µm); the average carrying capacity for a particular cycle is as follows: (O) 106-150 µm, (0) 150-212 µm, (]) 355-500 µm, (×) 500-710 µm, (left-facing 4) 850-1000 µm, and (+) 1000-1180 µm. (s) fit to eq 3. Carrying capacity from X-ray diffraction (2). BJH pore volume in the calcine, relative to that on the first calcination (4). Conditions were T ) 1023 K, mo ) 2.00 g, QN2 ) 80 mL/s, QCO2) 13 mL/s, [CO2] ) 14.0 vol %, sand ) 355-425 µm, U/Umf ) 8.5 (without CO2), 9.8 (with CO2). Table 4. Measured Values of fm and fw (eq 3)a limestone
temperature (K)
size range (µm)
Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Purbeck Cadomin Penrith Glen Morrison Havelock Purbeck
1023 1023 1023 1023 1023 1023 1023 1023 1023 1023 1023 1023 1063 1023 1023 1023 1023 1023 1023 1023 1023-1173
106-150 150-212 355-500 500-710 850-1000 1000-1180 500-710 500-710 500-710 500-710 500-710 500-710 500-710 500-710 500-710 500-710 500-710 500-710 500-710 500-710 500-710
comments
fm
fw
temperature cycling
0.22 0.60 0.80 0.76 0.70 0.77 0.27 0.47 0.63 0.85 0.77 0.75 0.74 0.70 0.65 0.66 0.68 0.74 0.76 0.82 0.61
0.14 0.26 0.27 0.29 0.25 0.29 0.21 0.25 0.35 0.23 0.22 0.22 0.20 0.23 0.20 0.32 0.28 0.28 0.29 0.32 0.24
general correlation comparison to ref 4 large scale large scale
0.77 0.77 0.41 0.66
0.17 0.17 0.12 0.23
M/2 doping NaCl M/2 doping Na2CO3 M/10 doping Na2CO3 M/100 doping Na2CO3 M/500 doping Na2CO3 1023 K, 29% CO2 1063 K, 29% CO2 500 s carb 1000 s cal 500 s carb 1800 s cal 1000 s carb 500 s cal
Values from Literature general correlation4 general correlation5 Cadomin6 Havelock6
a Unless a parameter is given, all experiments were with 14% CO , undoped particles with 500 s of calcination followed by 500 s of carbonation. If 2 coefficients have been proposed for general use, rather than for one specific limestone, they are denoted “general correlation”.
attributed to the tendency of these smaller particles to elutriate. Also shown in Table 4 are values of fm and fw from the literature; these are discussed in the next section. Observations on Table 4. From Table 4, the doping of particles with small quantities (up to 0.1 M) of Na2CO3 may slightly retard the loss of carrying capacity with time and lead to a marginally larger value for fw. However, doping particles with highly concentrated solutions of either NaCl or Na2CO3 actually increased the rate of degradation of a particle’s capacity for CO2. Doubling the flowrate of CO2 to increase yCO2 to 29
vol % had no significant effect on the falloff in carrying capacity. Likewise, increasing the temperature to 1063 K, while doubling the flowrate of CO2, had no significant effect. This was also the case when the carbonation time was increased. However, increasing the calcination time appears to reduce fw. The effect is most probably due to further attrition of the particles, since calcined limestone is much more friable than carbonated limestone. The effect of cycling the temperature, but with a constant concentration of CO2, was investigated when fluidizing the bed
2078 Energy & Fuels, Vol. 21, No. 4, 2007
Figure 6. BET surface areas measured for the limestones in their calcined states, plotted against the number of cycles of calcination. The limestones were as follows: (×) Purbeck, (4) Cadomin, ([) Penrith, (]) Glen Morrison, (+) Havelock. Initial particle size: 500710 µm. T ) 1023 K, mo ) 2.00 g, dp ) 500-710 µm, QN2 ) 80 mL/s, QCO2) 13 mL/s, [CO2] ) 14.0 vol %, sand ) 355-425 µm, U/Umf ) 8.5 (without CO2), 9.8 (with CO2).
with 14 vol % CO2. The temperature of the bed, initially at 973 K, was increased to 1173 K, where equilibrium I was such that the particles calcined, then lowered to 973 K, when the particles carbonated. At 101.3 kPa, with 14 vol % CO2, calcination of carbonated particles begins at 1040 K. The long time (∼22 min) required for the furnace to heat and cool meant that the temperature varied approximately sinusoidally between these two limits, with a period of ∼2600 s. Table 4 shows that the carrying capacity dropped slightly faster for this method of cycling calcination and carbonation. There are three possible explanations for this: the temperature for calcination is higher, or it takes place in the presence of CO2, or the cycle time is longer. With this apparatus, it is not possible to determine which of these factors is dominant. It is also clear from Table 4 that the values of fm and fw are fairly similar to published values.4-5 The dominant term in eq 3 is initially fm, and our “base case” value (for 500-710 µm Purbeck limestone) of ∼0.76 is close to 0.77, the value from the general application of the correlation of Abanades and Alvares,4 though our value of fw ∼ 0.29 means that our “final” carrying capacity is slightly higher than that measured previously,4 probably because our conditions of calcination are less severe than in the previous work.4 Of greater interest are earlier values6 for fm and fw, obtained using a fluidized bed of limestone in a pilot-scale plant: they show a much faster drop in carrying capacity than either the measurements here or those4 using a TGA. The reason for such a swifter drop may be attrition of the particles in the large fluidized bed; it is notable from Figure 3 that the Cadomin and Havelock limestones are very susceptible to attrition. Further research on a larger scale is required for these important factors. Gas Adsorption Analysis. BJH pore volumes (for pores in the size range 3-150 nm) were determined for each limestone; results are shown in Figure 5. With every limestone, the drop in carrying capacity is accompanied by a drop in BJH pore volume. The Glen Morrison limestone shows in Figure 5 a slightly lower relative drop in BJH pore volume with increasing numbers of cycles than the other limestones. The BET surface area was also derived from the adsorption isotherm for each limestone, showing a drop in surface area after repeated cycling, as shown in Figure 6. These results are discussed below.
Fennell et al.
Scanning Electron Microscopy (SEM). Particles of Purbeck limestone were examined using an electron microscope, both before and after a number of cycles of calcination and carbonation. Observations were made of the exteriors of the particles and also their interiors. For this, particles were sunk into silver epoxy and mounted on standard aluminum SEM stubs. When the epoxy was dry, some of the particles were fractured using a clean scalpel, to expose their interiors. The particles were then coated with a thin layer of gold to improve their electrical conductivity. These procedures meant that it was possible to image the particles using secondary electrons, rather than backscattered electrons. It is clear from the typical images in Figure 7 that the particles undergo significant morphological changes during reaction. These changes fall into two categories: (1) changes while a particle calcines or carbonates (i.e., changes over one cycle) and (2) long-term changes over the course of many cycles. Although not shown in Figure 7, a limestone is initially comprised of individual grains, the surface resembling a boulder-strewn landscape. The images of the exterior of a particle, Figure 7a and b, show the differences between a calcined and a carbonated particle; Figure 7c and d show long-term changes in calcined particles. Figure 7a and b show that the size of individual grains visible on the surface is ∼1.3 µm. After particles have been calcined once, the continuous part of the surface appears to become smoother, but large cracks open up, ∼1 µm wide (Figure 7a). Upon recarbonation, the grains appear to grow back out from the surface and to be around the same diameter (Figure 7b). The interior of a particle (Figure 7c), calcined once, appears to be rough on the scale of ∼0.1 µm and lower, almost having the appearance of cotton wool. After 20 calcinations and 19 carbonations, that is, for the calcined material in the 19th cycle (Figure 7d), the interior of a particle looks quite different, with very large pores opening up, but the internal surfaces appear to be smooth on the scale of 0.1 µm. The exteriors of particles after having been cycled up to 16 times show (not seen in Figure 7), in the carbonated state, a large number of grains of roughly the same size as the original grains. There has been some coalescence of individual grains, but not nearly as much as inside a particle. This may be because the grains are able to grow unrestrictedly on the surface, rather than being constricted by the walls of pores. In summary, electron microscopy confirms that the surface morphology of a limestone changes greatly over the course of reaction: the initial CaO looks like cotton wool and has a large number of very tiny pores within it. The surface therefore appears rough. After a large number of cycles of calcination and recarbonation, the internal surfaces of a particle look smooth and almost melted together, indicating a loss of fine structure for the CaO. These findings are confirmed by mercury porosimetry below. Mercury Porosimetry. This technique, in effect, records the volumes inside pores with diameters between 5 and 10 000 nm. Typical results of the distribution of volume inside pores of different widths are shown in Figure 8. Examination of Figure 8 reveals that, after the first calcination, a large volume in pores narrower than 100 nm closes up and disappears, as also does a substantial volume in pores between 100 and 10000 nm. Perhaps the most interesting feature of Figure 8 is that, even in particles which have supposedly been fully carbonated [see (1,1) in Figure 8], there remains a substantial volume of pores between 102 and 105 nm. This indicates that there will always be transport pores throughout the interior of a particle, even when the narrowest pores block, as seen from the (1,1) plot. Actually, it should be noted that the pores supposedly wider than 105 nm
Repeated Cycles of Calcination and Carbonation
Energy & Fuels, Vol. 21, No. 4, 2007 2079
Figure 7. Scanning electron microscope images of the interiors of particles of Purbeck limestone. All images are to the same magnification. The scale bar shown corresponds to 5 µm. Clockwise from the top left: (a) exterior of a particle after being calcined once; (b) exterior of a particle calcined and recarbonated once; (c) interior of a particle calcined once; (d) interior of a particle calcined 20 times and recarbonated 19 times. The particles in all images are mainly CaO.
Figure 8. Differential pore size distribution (V ) volume of pores) derived from mercury intrusion porosimetry. The numbers of calcinations and carbonation are given by the bracketed numbers, the first being the number of calcinations, the second the number of carbonations. N.B., the results for 16 calcinations are repeated.
are in fact interstices between particles, not pores in individual particles. It is also apparent from Figure 8 that the particles after the first calcination (1,0) are different from those after subsequent calcinations, in that a large volume in pores wider than 100 nm in (1,0) is lost, that is, does not reappear upon subsequent calcinations. After the particles have been cycled several times, the peak in Figure 8 for pore diameters less than 100 nm appears to shift downward. However, there is an error17 inherent in the measurements at the lowest pore diameters (∼ 10 nm), because of (17) Webb, P. A.; Orr, C. Analytical Methods in Fine Particle Technology; Micromeretics Instrument Corporation: Norcross, GA, 1997; pp 173174.
compression of the mercury, the sample, and also the penetrometer used. For this reason, the BJH analysis presented above is preferred for studying pores narrower than 150 nm, whereas mercury porosimetry is more accurate for pores wider than 150 nm. Adding the volumes inside pores narrower (by gas adsorption) and wider (by mercury intrusion) than 150 nm for particles calcined once gives a total porosity of 0.19 + 0.16 ) 0.35 mL/g. This is very close to the theoretical value (Vtot), ∼0.37 mL/g, deduced from the molar volumes of CaCO3 and CaO, assuming the external dimensions of a particle are constant. The total sum of these two measured porosities decreases with the numbers of cycles experienced by a particle, to ∼0.20 mL/g after 16 cycles. This indicates that there may be some closed off pores. Such pores inevitably become inaccessible to the N2 sorbent used in a BJH analysis, after many cycles. This is a matter for further research. Discussion Several models have been proposed previously to explain the drop in both pore volume and reactivity of limestones, after being repeatedly calcined and recarbonated. Among these models are those of Abanades and Alvares,4 essentially an extension of Bhatia and Perlmutter’s.18 This model assumes that the majority of the observed fast part of the carbonation reaction (essentially, most of the conversion of CaO to CaCO3 seen in these experiments) occurs inside pores narrower than ∼150 nm (i.e., in the volume observed using BJH analysis) observed in a limestone. Once these pores have been almost filled, carbonation proceeds by diffusion of CO2 through a layer of CaCO3 formed on the exteriors of grains with diameters of 1-10 µm. The decrease in porosity within the grains leads to the observed fall in carrying capacity, possibly in combination with sintering together of the individual grains, as well as some pores being
2080 Energy & Fuels, Vol. 21, No. 4, 2007
Fennell et al.
Figure 9. Experimental values of the fractional carrying capacity on the nth cycle, (Cn) plotted against VCr/Vtot. The limestones were as follows: (×) Purbeck, (4) Cadomin, ([) Penrith, (]) Glen Morrison, (+) Havelock.
sealed off. This reduces the amount of product which can be laid down on the exteriors of the grains before internal diffusion controls carbonation. The empty volume inside tiny crystallites of CaO, as opposed to that between them, can be assumed to be the measured BJH volume in the particles of CaO (Vcr); Table 3 shows that this is ∼0.19 mL/g for the Purbeck limestone calcined once. This corresponds to a fraction of the voidage contained within the BJH volume of 0.19/0.37 ) 0.51, where Vtot ) 0.37 mL/g is the theoretical total volume of pores in limestone after it has been calcined once. However, even after all the pores within CaO crystallites have been filled with CaCO3, a small amount of further reaction is possible, from a layer of CaCO3 building up on the exteriors of crystallites. It may be that conversion due to the growth of CaCO3 on the surfaces of crystallites, rather than within them, is responsible for the “residual” conversion given above by the value of fw in eq 3. In this case, the theoretical fractional carrying capacity of the CaO in the nth cycle (i.e., for the nth recarbonation, for CaO which has undergone n + 1 calcinations in total, including the zeroth one) is given by
Cn ) Vcr/Vtot + fw
(4)
Figure 9 is a plot of Cn, the measured fractional carrying capacity for CO2 from Figure 5, plotted against measured values of Vcr/Vtot. The slope of the line is 1.16 ( 0.23, and the intercept (fw) is 0.082 ( 0.01 (both error bounds are a 95% CI using Student’s t-test). The value of R2 for all measurements in Figure 9 is 0.76. Regression was carried out on all measurements simultaneously, rather than for each limestone separately, because of the lack of measurements for some of the limestones. It is clear from Figure 9 that the Purbeck limestone tends to have a greater carrying capacity for the same total BJH pore volume than the other limestones. It is clear that eq 4 fits the measurements well. Consequently, the filling of pores narrower than 150 nm strongly influences the transition from rapid to slow reaction and is thereby an important parameter affecting the carrying capacity. The slope of Figure 9 is 1.16 ( 0.23, rather than the value of unity indicated by eq 4. This suggests that parameters other than simply the total BJH pore volume also affect the carrying capacity, as discussed below. The measured drop in total porosity in the calcine (i.e., that in the BJH region 150 nm from mercury porosimetry) also indicates that one mechanism for the loss of measureable porosity is probably sealing over the mouths of some pores, thereby isolating them inside
crystallites, so that no further reaction of CO2 with this sealedoff internal area is possible. This possibility requires further study. Other models exist to explain the drop in carrying capacity with the number of cycles of calcination and carbonation. Alvares and Abanades19 exposed limestones to pure CO2 and cycled the temperature between 923 and 1233 K to successively carbonate and calcine them. That work suggested that, under these more severe calcination conditions, it may be the drop in available surface area which reduces the conversion at which the change from fast to slow carbonation occurs. They assumed that diffusion through the product layer of CaCO3 controls the rate of reaction after a critical thickness (∼49 nm) of the product layer has built up. However, in our work, the measured reduction in surface area was not accompanied by a commensurate fall in BET surface area. For their more severe calcination conditions, the initial BET surface area was ∼8 - 13 m2/g and dropped more rapidly to 2-3 m2/g after 30 cycles. Figure 6 shows that the surface area of Purbeck limestone in this work was initially as high as 25 m2/g and remained at ∼12-15 m2/g after 30 cycles of calcination, during which time the carrying capacity fell to about 20%. Comparison of the pore size distributions from Hg intrusion shown in Alvares and Abanades’s19 Figure 4 with ours (Figure 9) shows that their19 mean pore diameters for particles cycled four times (100-200 nm) were substantially higher than ours would be for four cycles (10-30 nm). Thus, our pores would be full before a layer of product 49 nm in thickness could develop. Of course, in reality, a loss of volume of pores
