Determination of the Critical Product Layer Thickness in the Reaction

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Ind. Eng. Chem. Res. 2005, 44, 5608-5615

Determination of the Critical Product Layer Thickness in the Reaction of CaO with CO2 Diego Alvarez* and J. Carlos Abanades Instituto Nacional del Carbo´ n (INCAR- CSIC), C/Francisco Pintado Fe, No. 26, 33011 Oviedo, Spain

Calcium oxide can be an effective CO2 sorbent at high temperatures. When coupled with a calcination step to produce pure CO2, the carbonation reaction is the basis for several hightemperature separation systems of CO2. The formation of a product layer of CaCO3 is known to mark a sudden change in the reaction regime, from a very fast CO2 uptake to very slow carbonation rates. The critical thickness of this product layer of CaCO3 has been measured in this work on real sorbent materials, using different limestone precursors and submitting them to many repeated carbonation calcination cycles (up to 100). Mercury porosimetry curves of the calcines and their carbonated counterparts have been obtained and their differences interpreted with a simple pore model, from which the thickness of the product layer is derived. An average value of 49 nm ((19% standard deviation) has been obtained, which is quite insensitive to the type of limestone and to the texture of the calcine as long as the model is fulfilled. The implications of this value on our understanding of the sorbent performance in these CO2-capture systems are discussed. Introduction The climate change problem requires new technologies to obtain energy from fossil fuels without emitting CO2 to the atmosphere. CO2-capture and storage systems applied to large stationary sources of CO2 are among a set of existing technologies that could help to stabilize in the next 50 years the CO2 concentration in the atmosphere.1 The idea of capturing and storing CO2 is not new,2 and most components of these systems are commercially available in different industrial applications, but it is widely accepted that there is a large scope for cost reductions and energy-efficient improvements in CO2-capture systems.3 The use of a CaO/CaCO3 chemical loop is emerging as a viable technique for the capture of CO2 from combustion gases4-9 or from fuel gases to obtain H2rich gases.10-15 The CaO is used in these systems to strip the CO2 contained in the gases, by forming CaCO3. CO2 is recovered in a pure form when CaCO3 is regenerated by calcination in a different reactor8 (for instance, by burning a minor part of the fuel with oxygen). A key advantage of these processes is that the separation step is carried out at high temperatures (at atmospheric pressure, carbonation takes place at 600-700 °C and calcination at T > 900 °C), giving rise to lower losses of energy generation efficiency.8,10 The other advantage is that it uses perhaps the cheapest possible regenerable sorbent (crushed stone), which allows high-makeup flows of fresh sorbent at reasonable cost.16 Furthermore, a cement plant may benefit from the purge of solids from the capture system (mainly deactivated CaO) as a precalcined feedstock. One of the main limitations in the system is, however, related to the modest sorbent performance.8 CO2-capture systems are necessarily large-scale systems. Therefore, very fast sorption and desorption rates of CO2 are required to allow for compact capture and * To whom correspondence should be addressed. E-mail: [email protected].

regeneration reactors. In the case of CaO reacting with CO2 (at temperatures over 600 °C and partial pressures of CO2 over 0.02 MPa), it is well-known that a very fast reaction regime takes place initially,4,7,9,11,13,17-20 followed by a slow period controlled by the diffusion of reacting species through the product layer of CaCO3. There is also agreement in that this transition is sudden. It has been established that this diffusioncontrolled reaction process becomes even slower as the conversion increases.21 The consensus from all of these studies suggests that the thickness of the carbonate layer formed on the free surfaces of CaO is a critical parameter to mark the end of the fast reaction period. This critical thickness should be an important parameter to understand the process of lime carbonation during the fast and slow reaction periods. However, little effort has been put into its direct determination in real sorbent materials. Barker17 reported a “critical carbonate layer thickness” of 22 nm before diffusion control takes over in the progress of the reaction. He estimated this value from the conversion achieved by the sorbent at the end of the fast reaction period and assuming an even distribution of the CaCO3 over the entire surface available for reaction in the calcine. Taking into account the expansion of the solid associated with the different molar volumes of CaCO3 and CaO, this thickness is M VCaCO 3 X h) MCaO S0

(1)

The use of this equation is confronted in practice with the uncertainties surrounding the carbonation model: it is clear that in highly reactive calcines (first cycles) some small pores fill up completely before the maximum thickness has been reached18,19 and some pores may be blocked at an early stage (bottlenecks).19 In these conditions, the determination of the effective sorbent surface at any time during carbonation is not a straightforward task and it requires a detailed knowledge of the

10.1021/ie050305s CCC: $30.25 © 2005 American Chemical Society Published on Web 06/16/2005

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pore model and carbonation pattern across the particle. Alternatively, it requires a choice of experimental conditions that promote simple pore structures and carbonation patterns where eq 1 can be applied with confidence. Mess et al.21 carried out long-duration carbonation tests on nonporous particles of the size range of 15-20 µm. They measured product layer thicknesses of up to 2 µm after very intense carbonation periods (11.74 atm of CO2 and 2000 min at 850 °C), but what is relevant for the discussion here is what they reported as “prompt conversion”, which corresponds (see Figures 1 and 2 in the original reference) to the onset of the slow reaction period, and this was typically 3.5%. With this conversion, a product layer thickness of 220 nm is obtained when considering spherical particles of 15-20 µm, which is reduced to 130 nm when the direct surface-tovolume ratio reported by the authors is used. For spherical particles

{[ (

h ) R0 X

M VCaCO 3

VM CaO

) ]

1/3

-1 +1

- (1 - X)1/3

}

(2)

For the case of porous particles of CaO, Bhatia and Perlmutter18 established a threshold of around 100 nm to distinguish between the “small” pores that led to full carbonation and the “large” pores that allowed the development of a product layer on their surfaces. This followed the argument that attributed the sharp reduction in the carbonation conversion of fresh calcines to the exhaustion of small pores. In a previous work22 investigating pore size effects on the carbonation process of CaO, we have also estimated that a product layer with an equivalent thickness of 100-160 nm had to grow on the surfaces of the large pores observed by scanning electron microscopy (SEM) in highly cycled samples, when submitted to long carbonation stages (30 min, pure CO2, 650 °C), to explain the conversions attained by these samples. The previous estimates of the critical product thickness are therefore subject to uncertainty because they all rely on assumptions on the available surface of the reaction that can be misleading at the relevant conditions for CO2 sorption. Therefore, the main objective of this paper is to determine the critical product layer thickness of CaCO3 from experimental measurements conducted in a wide range of conditions relevant for CO2-capture applications (i.e., natural porous sorbents, samples submitted to many carbonation/calcination cycles, reasonable reaction times, etc.). We shall also discuss the implications of this parameter in our understanding of the carbonation reaction of CaO for its use in these emerging CO2-capture systems. Experimental Section Tests were conducted to produce sorbents with textures and conversions representative of what can be found in a continuous carbonation/calcination loop. The particle size of the parent carbonates was 0.4-0.6 mm in diameter, which was found to be appropriate for fluidized-bed systems. In all of the cases, the sorbents were “CO2-saturated” materials, i.e., those having just reached their slow reaction period, as will be shown below, and these were submitted to varying numbers of calcination/carbonation cycles, between 1 and 100. Both the texture and the carbonate loading were care-

Figure 1. Scheme of the experimental setup used.

fully determined by Hg porosimetry and thermogravimetric analysis (TGA), respectively. The experimental setup used for the calcination/ recarbonation experiments is a small fixed-bed reactor apparatus, represented in Figure 1. The reactor is a 1-m-high alumina tube surrounded by a vertical furnace with two independent heating elements. A basket containing the sample is suspended along the vertical axis. The reacting gas (100% CO2, 2 L/min STP), axially injected at the base of the tube, was forced to pass through the basket and left the furnace by the upper end of the tube. The depth of the particulate bed was limited to about 3 mm (∼5 g of the parent limestone), which yielded enough sample for SEM and Hg porosimetry characterization, as well as the determination of carbonation conversions by TGA. Solids were allowed to react 10 min for calcination (960 °C, 100% CO2) at the bottom part of the furnace and 5 min for carbonation (650 °C, 100% CO2) in the upper part. Both residence times were checked to be sufficient to fully calcine the carbonated samples and to bring the calcines to the slow carbonation regime, respectively. Figure 2 shows the variation of carbonation conversion with the residence time for two different limestones, one in its first cycle (B1) and the other one after being cycled 97 times (K98). In both cases, it can be readily noticed that the CO2 uptake after 4-5 min is very low, corresponding to the onset of the slow, diffusion-controlled, carbonation regime. During cycling experiments, the basket was automatically raised and lowered by means of an engine placed above the furnace. Selected samples were rapidly withdrawn from the basket while in its upper position, either before (calcined samples) or after recarbonation (carbonated samples), using a suction probe. To prevent the recarbonation of the calcined samples, which were intended for further characterization, the engine was stopped in the calcination part of the furnace after the prescribed 10 min and the gas flow was switched to N2 until all of the CO2 was purged off. The basket was then

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Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005

Figure 2. Variation of the carbonation conversion with carbonation time.

raised to the carbonation position, the calcined sample was readily picked and stored in an inert atmosphere, and CO2 was again admitted into the furnace for further cycling. The actual temperature of the sample was measured with a thermocouple placed in the center of the particulate bed. In carbonated samples, the conversion was determined by weight loss in the TGA in the temperature interval of 30-900 °C under air. Twenty different calcines and their recarbonated counterparts were used in this study. One calcine and its corresponding recarbonated sample were withdrawn from the system in the cycles 1, 4, ∼30, ∼60, and ∼100. The natural variability of the limestones and the drastic morphological changes undergone by the CaO when submitted to these kinds of experiments both provided samples with fairly different pore size distributions and CO2 uptake capacities, as will be shown below. Results and Discussion The Hg intrusion curves of a representative set of samples are presented in Figures 3 and 4 as the pore size distributions of calcined samples (solid lines) and their carbonated counterparts (squares). The dotted lines are calculated curves for the carbonated samples, which were obtained from the application of a model for the pore structure and carbonation pattern of the sorbents. This model will be explained in detail below. All of the plots are derivative curves from the Hg penetration volume (dV/dD) vs pore diameter (D) curves. This graphic representation was chosen in order to give a visual estimate of the total pore volumes (area under the curve) and mean diameters of the samples and also to permit a clear view of the goodness of the model fittings. Figure 3 shows the results obtained from the fourth calcination/carbonation cycles of the four limestones tested, and Figure 4 illustrates the textural changes undergone by one of the limestones along cycling. To illustrate the extent of the textural changes in the sorbents after extensive cycling, the SEM image of Figure 5a shows a fresh fracture surface of a calcined particle, formed by connected spherical grains leaving between a network of intergranular spaces, and a highly cycled counterpart, where the grains have increased in size as a consequence of sintering and the intergranular pores have become accordingly bigger19,22 (Figure 5b). As can be seen in Figures 3 and 4, the pore size distributions measured with Hg porosimetry strongly change between the calcined and carbonated samples because of the growth of a layer of CaCO3 in the CaO-

free surfaces. This provides sufficient information to derive data of the critical product thickness, which is the objective of this work. To interpret the Hg porosimetry data and derive product thicknesses from Figures 3 and 4, a simple textural model is required for the calcines, as well as a reasonable description of the carbonation pattern up to the formation of the product layer. As regards the textural model of the calcines, we have assumed that each volume fraction given by the porosimetry curves corresponds to the volumes occupied by cylindrical pores of that size (in other words, no bottlenecks are present in the calcines). As regards the carbonation patterns inside the particle, we have assumed that an even distribution of carbonate exists in the carbonated particle (no radial conversion profiles). This last observation is fully consistent with the fact that no particle size effects are apparent when comparing the maximum carbonation conversion reported by different authors19 in a wide range of particle size intervals (from below 0.01 to 10 mm). Therefore, the model considers the progress of carbonation in the pores as represented in Figure 6. All of the pores detected by porosimetry are assumed to have a cylindrical geometry, and it is also assumed that they remain cylindrical after carbonation. This last assumption will be revisited later in order to explain some deviations and also to quantify possible asymmetries in the carbonation of CaO particles, which have been described in a previous work22 for some initial textures of calcines (with bottlenecks) and special circumstances (for extended carbonation times). However, it can be anticipated that the conditions chosen to obtain product thickness data in this work were adequate to generate sorbents with a texture and carbonation pattern close to that of the model outlined above, which produced accurate estimations of the real product thickness. The raw data provided by the Hg porosimetry are finite intervals of cumulative pore volumes (Vcum ) at i decreasing pore radii (ri). Thus, the incremental pore volume in a given pore size interval ri-1-ri will be cum ∆Vcum ) Vcum - Vi-1 i i

(3)

and assuming a cylindrical geometry for the pores, each incremental intrusion can be ascribed to a single pore with a radius Ri ) (ri-1 + ri)/2 and a length Li equal to

/πRi2 Li ) ∆Vcum i

(4)

The calcined limestone can now be described as a solid with a specific pore volume Vcum arranged in a network of cylindrical pores of varying radii and lengths (Ri and Li). The specific surface area (Si) of such a material would be

Si ) ∆Vcum i

2πRiLi 2

πRi Li

)

2 ∆Vcum Ri i

(5)

It is over this material that the carbonation reaction will proceed, and a product layer will grow on the surfaces of the pores described above. The carbonate layer, with a thickness h and a volume VC, will spread over all of the volume occupied by the reacted oxide (VR) plus the extra volume of the solid (VE) generated as a consequence of the different molar volumes of CaO and CaCO3. This extra volume will grow at the expense of the pore volume. The volume fraction (R) of carbonate


Figure 3. Experimental pore size distributions of calcined samples and their recarbonated counterparts after four calcination/carbonation cycles. Ordinates expressed in cubic centimeters of pore volume (V) per nanometer and per gram of sample. Solid lines: calcined. Squares: carbonated. Dashed lines: model predictions for the carbonated samples.

product layer becomes for the pore considered

invading the former volume of pore i is given by

R)

VEi

π[Ri2 - (Ri - δi)2]Li

Vi

2

) C

2

π[(Ri + h - δi) - (Ri - δi) ]Li

and solving eq 6 for δi yields

δi ) hR + Ri - xh2R2 + Ri2 - Rh2

(7)

The parameter R is obtained from the molar volumes of CaO (16.9 cm3 mol-1) and CaCO3 (36.9 cm3 mol-1) as

R)

M VCaCO - VM CaO 3 M VCaCO 3

hi ) min

(6)

(8)

Using these expressions, the variation of the pore radius with carbonation can be obtained for any given pore and for a specified product layer thickness. This can be simply done by changing Ri into Ri - δi, as obtained from eq 7, and the corresponding ∆Vi into ∆Vi RVCi . Of course, the reduction in the pore radius cannot be bigger than the pore radius itself. If eq 7 predicts such a result, then δi is made equal to Ri (the maximum pore size reduction, implying that the pore gets totally filled with carbonate), and the thickness of the

)

1 R,h R i

(9)

By assigning a value to the maximum product layer thickness, h, in eq 7, we will be able to compute the extra volume of the solid generated during the carbonation of a CaO sample and calculate the carbonation conversion. First, the total volume of the carbonate generated per gram of parent CaO is

VC ) ) 0.54

(x

∑i π[(Ri + hi - δi)2 - (Ri - δi)2]Li

which using eq 3 becomes

VC )

[(

∑i

) ( )]

Ri + hi - δi Ri

(10)

2

-

Ri - δi Ri

2

∆Vcum (11) i

Second, the net CO2 mass gain (∆m) on the partial recarbonation of a unit of mass of CaO is

MCO2 ∆m ) VC M VCaCO3

(12)

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Figure 4. Experimental pore size distributions of calcined samples and their recarbonated counterparts from the Katowice limestone after 1, 4, 29, 63, and 87 cycles. Ordinates expressed in cubic centimeters of pore volume (V) per nanometer and per gram of sample. Solid lines: calcined. Squares: carbonated. Dashed lines: model predictions for the carbonated samples.

Finally, the carbonation conversion (X) is given by

X)

VC

MCaO

MCaO ) M M VCaCO VCaCO 3 3

∑i

[(

) ( )]

Ri + hi - δi

2

-

Ri

Ri - δi Ri

2

∆Vcum (13) i

Using the above expressions, there are two ways to

estimate the product layer thickness, h, from the experimental information available: One method is to look for the optimum value of h in eq 13 that provides a conversion identical with the experimental conversion of the carbonated sample. The second method is to look for the optimum h required to minimize the error between the experimental pore size distribution of the carbonated sample and that calculated from the CaCO3 distribution model. If the model assumptions were correct, both methods should produce similar results.


Figure 6. Scheme of the carbonation pattern in the pores of a calcined sorbent. Figure 5. (a) Fresh fracture surface of a calcined limestone. (b) Same limestone after 100 calcination/carbonation cycles. Scale bar: 2 µm.

For the purpose of comparison, two simpler methods were also applied in order to estimate h: the direct application of eq 1 and the measurement of the shift in the pore size distribution peaks. Results from these four exercises are summarized in Table 1. The values obtained from peak shift measurements are reasonable in several samples, but they show a large scattering, ranging from 0 to about 100 nm, as illustrated by the large standard deviation (61%), whereas those based on the surface area and conversion diverge far less (22%). The application of the geometrical model described above (optimizing the value of the carbonation conversion, X, or the calculated pore volumes, Vp, of the carbonated samples) led to notably more robust results,

with a mean carbonate layer thickness of 49.1 nm and lower scattering of data (30-64 nm and 19-20% standard deviation). Surprisingly, the value of the layer thickness and the scattering of data obtained from the application of eq 1 were remarkably similar to those found by the application of the model (see Table 1). This is an indication that most of the samples used in this study had a simple pore structure, well represented by the simple pore model outlined above. Also, from the model fitting lines of Figures 3 and 4, it can be readily noticed that no significant peak shifts exist between the calculated and experimental curves of the carbonated sorbents, and this means that the assumption of cylindrical pore shapes, without significant bottlenecks, seems to be quite realistic for most of these samples. This is evidence to support that the pore blockage effects, as a relevant mechanism to prevent carbonation,

Table 1. Carbonation Conversions, Peak Shifts, and Model Predictions for the Samples Studied sample

conversion (%)

B0 B1 B4 B29 B64 B98 H1 H4 H30 H63 H94 P1 P4 P28 P66 P100 K1 K4 K29 K70 K97 mean thickness (nm)a standard deviation (%)a

67.1 64.7 35.2 13.7 13.3 13.1 60.2 35.0 13.9 9.7 9.4 42.2 29.3 16.7 8.5 9.4 53.2 32.0 14.0 10.1 9.7

a

Excluding B0.

h(peak shift) (nm)

h(eq 1) (nm)

h(model X) (nm)

h(model Vp) (nm)

1 42 33 80 48 0 36 64 21 43 52 43 73 37 104 0 33 59 53.7 40 43.1 61

18 33 41 29 43 29 34 48 58 50 60 50 57 64 49 64 45 54 51 49 49 47.9 22

21 41 45 30 46 30 44 53 58 50 58 53 58 64 48 63 50 56 40 48 47 49.1 19

40 46 69 28 45 31 41 52 54 50 39 56 52 54 52 58 54 65 50 44 44 49.1 20

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Figure 8. Comparison between the carbonation conversion experimentally observed and that predicted by the model for a carbonate layer of 49 nm. Figure 7. Pore size distributions of a calcine (800 °C, 100% N2) from La Blanca limestone. Ordinates expressed in cubic centimeters of pore volume (V) per nanometer and per gram of sample. Squares: differential curve from its recarbonated counterpart.

are not so important in marking the transition between the fast and slow reaction regimes, for the experimental conditions and sorbents used here. An important exception to the previous statement was the case of a calcine (B0 in Table 1) prepared under especially mild conditions, so that a smaller sized pore network was deliberately generated in the sorbent. The conditions used were 800 °C and a N2 atmosphere, which after 70 min led to a calcine with much smaller pores, peaking at 50 nm (Figure 6), than those found in the samples of Figures 3 and 4. This material was recarbonated in the same conditions described above, and its conversion and pore size distribution were also determined. According to the model outlined above, most of these pores should have filled up completely, and a very high carbonation conversion for this sorbent, as well as a carbonated counterpart with a very low porosity, would be expected. However, although the porosity was indeed very low (see Figure 7), the conversion was as low as 67.1%, only marginally higher than that found (64.7%) for the calcine prepared from the same limestone at 960 °C and under pure CO2 (Figure 4, cycle 1). On the other hand, the product layer thickness obtained by the application of the model was only 20 nm. Although these data are remarkably similar to those reported by Barker,17 it is conspicuous that a layer thickness of only 20 nm is not sufficient to fill the pore network available in the calcined sample (see Figure 7). The same happens when carbonation conversions are looked at: the total closure of the pores should only occur for a layer thickness of 40 nm (note that the first 20 nm of carbonate corresponds to a conversion of 67.1%, while the second hypothetical 20 nm would translate into a more modest increase in the conversion to 83.0%, as a consequence of the pore geometry effects). Finally, the conversion value that would predict a layer thickness of 40 nm through eq 1 would be 148%. This clearly illustrates that the textural characterization of the sorbents should be more detailed than a simple specific surface area value. It is therefore obvious that none of the models were successful in describing the carbonation behavior of this specific sample. This could be because of an uneven distribution of CO2 across the sorbent particle (bottlenecks) in this highly reactive

sample. In any case, the data from samples such as the above-described B0, where the model is not fulfilled, cannot be used to derive the product layer thickness. Finally, Figure 8 compares the carbonation conversions experimentally observed with those predicted by the model (eq 13) when using the optimum mean layer thickness of 49 nm as the carbonate layer thickness. Only the point B0 has been excluded. The good fitting of the conversion data is an indirect sign of the validation of the pore model adopted, although it is acknowledged that some of the calculated pore size distributions appreciably depart from the experimental ones (as seen in Figures 3 and 4). An understanding of these deviations should add more light to the full range of mechanisms that might give rise to higher sorbent utilizations during the carbonation reaction. For instance, it was found22 that calcines from B limestone, when submitted to extended carbonation periods (30 min in all cycles), show strong evidence of bottleneck formation in the calcines, whereas this effect does not seem to play any role in the carbonation of the same samples when using shorter reaction times (5 min, as in the present work). Experiments carried out with P, K, and H limestones (not included in the reference above22) did not reveal any tendency to bottleneck formation under any of the conditions tested. Obviously, any process(es) leading to a narrowing of the pore entrances would give misleading information about the sample and would lead to the wrong estimates of the product layer thickness if the previous methodology was applied. However, the overall results in Figures 3, 4, and 8 confirm that the main mechanism for the modest conversion of the sorbents after just a few cycles (N > 3) in a carbonation/calcination loop is the formation of a product layer thickness of just around 50 nm on the accessible surfaces of CaO, from which a slow diffusion process governs the progress of the reaction. Conclusions The extensive investigation on the textural characteristics of many calcined and carbonated samples of natural limestones provided sufficient information to determine the critical product layer thickness of CaCO3 that marks the onset of a slow reaction period in the carbonation reaction of CO2 with CaO. By interpreting the Hg porosimetry data with a simple pore model of the calcines, we have estimated a product layer thick-


ness of around 50 nm that seems to be acceptable for a wide range of sorbents and conditions. For some sorbents and/or special calcination conditions, there can be misleading pore effects altering the simple model used to derive this value. However, for a wide range of sorbents obtained at relevant reaction conditions and cycle numbers for a CO2-capture system, this critical product layer seems to be quite a constant characteristic value of the accessible surfaces of CaO reacting with CO2 in the interior of the sorbents. Acknowledgment This work is partially funded by the European Commission (Grant SES6-CT-2003-502743). We thank Dr. E. J. Anthony from CANMET and Dr. H. Kruczec from Wroclaw University for supplying samples of Havelock and Katowice limestones. Notation R ) ratio of the volume expansion upon carbonation to the total carbonate loading δ ) decrease of the pore radius upon carbonation D ) pore diameter r ) pore radius R ) average pore radius S0 ) specific surface area per unit of mass of CaO h ) carbonate layer thickness L ) pore length X ) carbonation conversion VM ) molar volume (used with subscripts CaO or CaCO3) M ) molecular weight (used with subscripts CaO, CaCO3, or CO2) VC ) volume of the carbonate generated per unit of mass of CaO VR ) volume of CaO reacted per unit of mass of CaO VE ) expanded solid volume on the reaction unit of mass of CaO ∆m ) net mass fraction gain upon recarbonation of CaO

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Received for review March 3, 2005 Revised manuscript received April 12, 2005 Accepted May 18, 2005 IE050305S

Determination of the Critical Product Layer Thickness in the Reaction

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