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Energy & Fuels 2008, 22, 3852–3857
Comparison of Different Natural Sorbents for Removing CO2 from Combustion Gases, as Studied in a Bench-Scale Fluidized Bed M. Ives, R. C. Mundy, P. S. Fennell,*,† J. F. Davidson, J. S. Dennis, and A. N. Hayhurst Department of Chemical Engineering, UniVersity of Cambridge, Cambridge CB2 3RA, United Kingdom ReceiVed June 3, 2008. ReVised Manuscript ReceiVed July 23, 2008
The reaction of CO2 with porous particles of CaO in CaO(cr) + CO2(g) f CaCO3(cr) was studied, along with its reverse reaction, for chicken eggshells, mussel shells, and limestone. Reaction I is a promising way of removing CO2, e.g., from the exhaust of a power station, so that a pure stream of CO2 can subsequently be produced for sequestration by calcining (roasting) the solid CaCO3 from reaction I. The reverse of reaction I regenerates the sorbent, which can thus be used cyclically. The forward and reverse steps of reaction I were investigated using a small electrically heated bed of sand at ∼750 °C, fluidized by N2. Typically, a sample (∼2 g) of cleaned calcareous material (sieved to ∼600 µm) was added to the hot bed, and the CO2 produced was measured, while the material was fully calcined. Next, enough CO2 was added to the fluidizing N2 to raise [CO2] to above the value for equilibrium; thus, the CaO was carbonated. This forward step of reaction I is shown to exhibit an apparent final conversion, the carrying capacity of the sorbent, below unity. This carrying capacity reduces after several cycles of calcination and carbonation, because blockage of pores denies access of CO2 to part of the CaO. After several such cycles, particles were removed from the reactor, either in their partially carbonated or fully calcined states, for studies using gas adsorption analysis, X-ray diffraction, and mercury porosimetry. Interestingly, it was found for all three sorbents that the carrying capacity of CaO for CO2 degraded at a similar rate. The carrying capacity was roughly proportional to the volume of pores narrower than ∼100 nm, as measured by Barrett-Joyner-Halenda (BJH) gas adsorption analysis. Evidently, these narrow pores contain both the surface area for CO2 to absorb and the empty volume to accommodate the product, CaCO3. The resistance of eggshells to attrition was broadly comparable to that of Purbeck (U.K.) limestone.
Introduction The use of sorbents based on CaCO3 (usually limestones or dolomites) for the postcombustion capture of CO2 has been investigated previously.1-19 The most important reaction is that * To whom correspondence should be addressed. E-mail: p.fennell@ imperial.ac.uk. † Present address: Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, U.K. (1) Bhatia, S. K.; Perlmutter, D. D. Effect of the product layer on the kinetics of the CO2-lime reaction. AIChE J. 1983, 29, 79–86. (2) 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. (3) Abanades, J. C.; Oakey, J. E.; Alvarez, D.; Ha¨ma¨la¨inen, J.; Gale, J.; Kaya, Y. Novel combustion cycles incorporating capture of CO2 with CaO. In Greenhouse Gas Control Technologies, 6th International Conference; Pergamon: Oxford, U.K., 2003; pp 181-186. (4) Abanades, J. Cost structure of a postcombustion CO2 capture system using CaO. EnViron. Sci. Technol. 2007, 41 (15), 5523–5527. (5) Barker, R. Reversibilty of the reaction CACO3 T CAO + CO2. J. Appl. Chem. Biotechnol. 1973, 23 (10), 733–742. (6) Abanades, J. C.; Anthony, E. J.; Lu, Y.; Salvador, C.; Alvarez, D. Capture of CO2 from combustion gases in a fluidized bed of CaO. AIChE J. 2004, 50, 1614. (7) Abanades Garcia, J. C.; Oakey, J. Combustion method with integrated CO2 separation by means of carbonation. U.S. Patent 20050060985, 2005. (8) Romeo, L. M.; Abanades, J. C.; Escosa, J. M.; Pan˜o, J.; Gime´nez, A.; Sa´nchez-Biezma, A.; Ballesteros, J. C. Oxyfuel carbonation/calcination cycle for low cost CO2 capture in existing power plants. Energy ConVers. Manage. 2008, 49, 2809–2814. (9) 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. (10) Harrison, D. P.; Peng, Z. Low-carbon monoxide hydrogen by sorption-enhanced reaction. Int. J. Chem. Reactor Eng. 2003, 1, A37.
of gaseous CO2 with particles of CaO, formed by calcining CaCO3 in the reverse of (I) CaO(cr) + CO2(g) f CaCO3(cr) Reaction I can be reversed by the application of heat; therefore, the sorbent can be used in many cycles of calcination and carbonation. A flowsheet for a basic process to remove CO2 from combustion off gases is shown in Figure 1. (11) 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, 5529–5539. (12) Manovic, V.; Anthony, E. J. Steam reactivation of spent CaO sorbent for multiple CO2 capture cycles. EnViron. Sci. Technol. 2007, (41), 1420–1425. (13) 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, 187–195. (14) Sun, P.; Grace, J. R.; Lim, C. J.; Anthony, E. J. Removal of CO2 by calcium-based sorbents in the presence of SO2. Energy Fuels 2007, 21, 163. (15) Sun, P.; Grace, J. R.; Lim, C. J.; Anthony, E. J. Co-capture of H2S and CO2 in a pressurized-gasifier-based process. Energy Fuels 2007, 21, 836–844. (16) Fennell, P. S.; Davidson, J. F.; Dennis, J. S.; Hayhurst, A. N. Regeneration of sintered limestone sorbents for the sequestration of CO2 from combustion and other systems. J. Energy Inst. 2007, 80, 116–119. (17) Fennell, P. S.; Pacciani, R. P.; Dennis, J. S.; Davidson, J. F.; Hayhurst, A. N. The effects of repeated cycles of calcination and carbonation on a variety of different limestones, as measured in a hot fluidized bed of sand. Energy Fuels 2007, 21, 2072–2081. (18) Abanades, J. C.; Alvares, D. Conversion limits in the reaction of CO2 with lime. Energy Fuels 2003, 17, 308–315. (19) 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, 920–926.
10.1021/ef800417v CCC: $40.75 2008 American Chemical Society Published on Web 09/11/2008
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Figure 1. Flowsheet for back-end CO2 capture from a combustor.
Off gases from the combustor pass into a carbonator, where reaction I is effected, removing CO2 and forming CaCO3. This is then passed to the calciner, where a high-carbon fuel is burnt in a small quantity of O2 (far lower than that required in an oxy-fuel-fired power station) to reverse reaction I and produce a pure stream of CO2, which, after heat exchange (possibly to preheat the makeup CaCO3 used in the process) and dehydration, is suitable for compression and geological storage. Of course, the crude flowsheet of Figure 1 requires a great deal of heat integration before a viable process can be made. For example, heat integration between the calciner and the combustor, via indirect transfer of heat,7 allows for significantly lower total energy usage for capture and hence lower total cost. The technology is at the advanced demonstration stage, with a number of groups worldwide running pilot-scale rigs. However, a large proportion of the sorbent development work that has been conducted has been at the scale of a thermogravimetric analyser (TGA) (a few milligrams). Here, we present research conducted in a small fluidized bed, the purpose of which is to gain an understanding at an intermediate scale between a TGA9,12,14,15 and a pilot plant.20 When formed from CaCO3, the capacity of CaO for taking up CO2 in reaction I diminishes after being used in a number of cycles. The carrying capacity of a sorbent (measured as described below) is the number of moles of CO2 adsorbed per mole of calcium; its decline after regeneration and subsequent reuse has previously been found1,5,17,18,22 to be largely owing to a loss of the pores in the CaO narrower than ≈150 nm in diameter, as will become apparent in the current work, discussed later. For limestones, it appears18 that the reaction ceases when the product has built up to a depth of ∼50 nm, averaged across the total surface area. Thus, the loss of surface area by sintering is a further contributing factor in the fall in sorbent capacity. However, there is still controversy surrounding the exact mechanism of deactivation. Nonetheless, it is observed that the rate of reaction becomes exceedingly slow after some 500-1000 s. However, economic analyses,8,19 taking account of deactivation with a number of cycles, continue to show that postcombustion capture of CO2, using a limestone-based cycle, is potentially economically viable, with a cost of 16-25 euros (20) Jia, L.; Hughes, R. W.; Lu, D.; Anthony, E. J.; Lau, I. Attrition of calcining limestones in circulating fluidised bed systems. Ind. Eng. Chem. Res. 2007, 46 (15), 5199–5209. (21) Webb, P.; Orr, C. Analytical Methods in Fine Particle Technology, 2nd ed.; Micromeritics: Norcross, GA, 1997. (22) Pacciani, R.; Davidson, J. F.; Hayhurst, A. N.; Dennis, J. S. How does the concentration of CO2 affect its uptake by a Ca-based synthetic solid sorbent? AIChE J. 2008, manuscript accepted for publication.
per ton of CO2 avoided. Another use for the reversible reaction between CaO and CO2 is to enhance the water-gas shift reaction,10,23 to produce hydrogen. This study investigates the capacity of materials such as chickens’ eggs (hereafter referred to simply as eggshell) or mussel shells to take up CO2. As will be shown below, the initial size distribution of the pores in, e.g., egg shells, is very different from that in limestone. Eggshell is relatively porous even in its carbonated form; also, the distribution of pore sizes in newly calcined eggshell is very different from that in calcined limestone. It was suspected that such very different pore-size distributions might lead to superior characteristics for capturing CO2, although it will be shown below that this is not the case. Had eggshells (or mussel shells) displayed superior characteristics for adsorbing CO2, it might have suggested the optimal pore size distribution to aim in for artificial sorbents for capturing CO2. Experimental Section Preparation of the Shells. Two dozen eggshells were boiled in water for 1 h to loosen the shell from its inner membrane. Next, the shells were dried and crushed to 0-3 mm in a mechanical mixer, and all traces of membrane were removed, before sieving to known size fractions (either 355-500, 500-710, or 710-1180 µm). The final stage of preparation was to heat ∼10 g of the crushed shells in a small tubular furnace (i.d. of 30 mm) with a flow-through of 100 mL/s of air, measured at room temperature and pressure. The heating was from room temperature to ∼600 °C almost linearly with time and took around 10 min in total, producing copious quantities of smoke at around 500 °C. Then, the shells were tipped into a crucible and left to cool in a desiccator, before being transferred into airtight containers for storage. Each batch was analyzed using X-ray diffraction (Phillips PW2773 tube with PW1830 generator) to determine the proportions of CaCO3 (∼99 wt %), CaO, and Ca(OH)2 after burnoff. A similar process was followed with the mussel shells. Adhering meat was removed, and the shells were thoroughly rinsed in water and then crushed to pieces ∼10 mm square. The roughly crushed shells (∼10 g) were then heated in the tubular furnace from room temperature to ∼750 °C; a large amount of white smoke appeared at 400-600 °C, but this time, the shells were calcined during the initial burnoff, by leaving them in the furnace for ∼5 min after a temperature of 750 °C was attained. It was necessary to calcine the mussel shells before finely crushing them, because they were initially much tougher than the eggshells. After burnoff, the shells crushed more easily and were sieved to known size ranges. (23) High temperature CO2 capture using engineered eggshells: A route to carbon management. World Patent WO/2006/099599, The Ohio State University, Columbus, OH.
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Figure 2. Sketch of the fluidized bed of quartz sand and its surrounding furnace.
The general reasoning behind the preparation of both types of shell was to burn off any proteinaceous material and calcine the material to a known extent. This was necessary to prevent damage to the gas analysers (see below) by the above-mentioned smoke. The extent of calcination after these preparatory steps was found, using X-ray diffraction, to be negligible for the eggshells and complete for the mussel shells; thus, the amount of CaO in either material was accurately known. Apparatus. The method used to measure the CO2 carrying capacity of the shells was similar to that described previously16,17 for various limestones. Briefly, the experiments were performed in a laboratory-scale fluidized bed of hot sand (20 mL of silica sand, sieved to 355-425 µm), supported on a sintered quartz plate in a quartz tube (i.d. of 29.5 mm), as shown in Figure 2. Nitrogen (80 mL/s, measured at room conditions by a rotameter) fluidized the bed, whose temperature was held at 750 °C using a type K thermocouple to control the heat input to the furnace and to measure the temperature of the bed. A nondispersive I.R. analyzer (ADC 2000 series) measured [CO2] in the gas leaving the bed.
Results Figure 3 shows the measured [CO2] leaving the bed. After calibration checks (periods 1-2), the eggshells, mass mo, were added at t ) 400 s, and when they had fully calcined (period 3, from ∼400 to ∼800 s), 13 mL/s of CO2 was added to the N2 (at ∼880 s) by opening a solenoid valve. This raised [CO2] in the bed to 14 vol %, i.e., above the equilibrium concentration of 9.9% at 750 °C (period 4). Thus, the particles began to remove CO2, forming carbonate; therefore, the [CO2] leaving the bed fell below 14 vol %. Carbonation took ∼480 s, and at 1380 s (see Figure 3), the CO2 was turned off; therefore, the particles started to calcine again in nitrogen and emit CO2 (period 5). This time the particles only took ∼240 s to fully calcine. The area under the region in Figure 3 labeled “first calcination”, when multiplied by the total molar flow rate of gas at exit from the bed, gives the number of moles of CO2 liberated during this calcination, which proceeded to completion, confirmed by X-ray diffraction analysis of calcined particles. Returning to Figure 3, the solenoid valve was opened again after a further 120 s, i.e., at t ) 1720 s; thus, the particles began to recarbonate. These subsequent cycles took ∼360 s of N2 flow for calcination (i.e., shorter than the zeroth cycle, which took ∼400 s); they were followed by ∼480 s of N2 plus CO2 flow
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for carbonation. This pattern was continued for up to 54 cycles (∼13 h). Sometimes, the bed of sand (including sample) was carefully weighed both before and after the addition of the eggshells, when the eggshell was in the calcined form. This allowed the amount of material lost through the attrition of fines to be determined. Carrying Capacity over Many Cycles of Calcination and Carbonation. The total number of moles of CO2 absorbed by a sorbent was, in general, easier to deduce accurately from the CO2 released during calcination than from the CO2 taken up during carbonation. The carrying capacities, as deduced during calcination of eggshells, mussel shells, and Purbeck limestone are shown in Figure 4. The carrying capacity for the zeroth cycle was obtained from the quantity of CO2 produced in the zeroth calcination of Figure 3 or similar plots. It is clear from Figure 4 that the carrying capacity declines marginally slowest for the eggshells, followed by Purbeck limestone, with mussel shells slightly the worst of all. The mussel shells had a strong and unpleasant odor during preparation; when calcined, their poor mechanical properties gave large quantities of attritted fines clogging filters and depositing on the quartz tube. Therefore, only a limited number of experiments were performed with mussel shells. Figure 5 shows the results of experiments of up to 50 cycles with eggshells and limestone. Also shown in Figure 5 are the best fits of the measured carrying capacities, Cn, to Grasa and Abanades correlation9 (expressed as a percentage) Cn )
1 + Xr 1/(1 - Xr) + kN
(1)
Here, n is the number of cycles of carbonation or the number of cycles of calcination, where calcination of the original limestone is defined as the zeroth calcination. In eq 1, k and Xr are fitting parameters. Xr is the residual fractional capacity for n f ∞, and k is related to the rate at which the carrying capacity approaches this final value. It is clear from Figure 5 that the correlation fits the measurements well. It is interesting that, even with the exceedingly mild calcination conditions applied here, the residual reactivity is only marginally more (0.11-0.12) than the value of 0.075 found by Grasa et al.,9 who employed much more severe conditions within a TGA. It is clear from Figure 5 that, for limestone, the initially measured carrying capacity (n ) 0) is not precisely 100%. This is possibly due to fine material on the surface of the limestone undergoing attrition and elutriation during the first calcination. The rate of attrition for Purbeck limestone has previously8 been shown, over a large number of cycles, to fit
(mm/mt) ) β + (1 - β)exp(-nτ)
(2)
for the ratio of the mass, mm, of calcined CaO remaining in the bed to the theoretical amount, mt, for no attrition. Also, n is the number of cycles. β and τ are fitting parameters: β is the final asymptotic value for (mm/mt), and τ characterizes the rate of approach to this final value, while attrition occurs. As mentioned above, the accurate measurements of the total weight of CaO remaining within the bed allowed for the mass loss by attrition to be determined, so that eq 2 gives the contribution to the loss of reactivity caused by attrition. The loss of mass was determined for a number of cycles (up to 55), and the results are shown in Table 1. The loss is up to 8% (and can be higher for very soft materials17 and is not readily detectable in a TGA. To convert eq 2 to one based on time, it is necessary to know that here the cycle time was 0.23 h. Equation 2 was found to be true for eggshells as well as Purbeck limestone. However, the loss of material during the
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Figure 3. Concentrations of CO2 at exit from the fluidized bed containing 30 g of sand (355-425 µm) plus 2 g of eggshell (500-710 µm) at 750 °C. Calcination in N2 was followed by carbonation with CO2 in N2 and then a further calcination, the start of a calcination/carbonation sequence. Periods 1 and 2 were calibration with a standard containing 15 vol % CO2 in N2. Period 3, calcination with N2 flow QN2 ) 80 mL/s, measured at room temperature. Period 4, carbonation, using QCO2 ) 13 mL/s CO2 plus 80 mL/s N2, i.e., 14 vol % CO2 and 86 vol % N2. Period 5, calcination, using 80 mL/s N2. Similar experiments used 1.16 g of mussel shells, giving the same CaO inventory as 2 g of eggshell.
Figure 4. Carrying capacity after a number of cycles as measured for (]) eggshells, (4) Purbeck limestone, and (×) mussel shells, in a bed of sand (355-435 µm) at 750 °C. The sorbent particles were sieved to 500-710 µm. Initial mass of eggshells, limestone, or mussel shells, mo ) 2.00 g, QN2 ) 80.0 mL/s, and QCO2 ) 13.0 mL/s.
first cycle was generally higher than predicted by eq 2. Table 1 gives values of β and τ, as measured here; they show that eggshells are mildly more friable than Purbeck limestone, because the shell has a greater ultimate loss of material (a smaller value for β), although it does take longer to achieve its final value (a smaller value for τ). In fact, the attrition characteristics of the eggshells in this small reactor are very similar to those previously measured17 for Havelock limestone. Of course, the rates of attrition in a large-scale fluidized bed may well be bigger than those measured here, although a comparison of the results of Fennell et al.17 with recent work at CanMet Energy Technology Centre20 shows that small-scale results at least appear to correctly predict the relative attrition rates of the limestones. It should be noted that, with both eggshells and Purbeck limestone, attrition is such that the mass of calcium in the bed has certainly fallen to its ultimate value (of β) after about 10 cycles. This fact means that the plots in
Figure 5. Measured carrying capacity for (]) eggshells and (4) Purbeck limestone. Sorbent sieved to 500-710 µm. Sand (355-435 µm) at 750 °C, with mo ) 2.00 g, QN2 ) 80.0 mL/s, and QCO2 ) 13.0 mL/s. The continuous curves are best fits of eq 1 to the measurements over 54 cycles for (-) eggshells and (- - -) Purbeck limestone. Table 1. Values of k, Xr, β, and τ for eggshells and Purbeck limestonea Xr
k
β
τ (per cycle)
eggshell 0.12 ( 0.02 0.27 ( 0.05 0.92 ( 0.01 0.17 ( 0.04 Purbeck limestone 0.11 ( 0.02 0.36 ( 0.05 0.95 ( 0.01 0.88 ( 0.20 mussel shells 0.18 ( 0.08 1.02 ( 0.2 / / a nb that τ is per cycle and not per unit time. No measurements were taken for parameters marked with an asterisk. Values of β and τ were made using masses measured over the first 30 cycles only.
Figures 4 and 5, showing a falling carrying capacity after 10 cycles, cannot be explained solely by attrition. Pore Size Distributions (PSDs) for Eggshells and Purbeck Limestone. The pore size distributions in eggshells and Purbeck limestones were measured using both Hg intrusion porosimetry (Micromeritics Autopore IV) and Barrett-Joyner-
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Figure 6. Pore size distributions measured by Hg intrusion porosimetry for Purbeck limestone particles and eggshells, both in the calcined state. The numbers (x, y), respectively, indicate the total number of calcinations (x) (including the “zeroth” calcination) and carbonations (y) undergone by the particles, all originally 500-710 µm. The area underneath each curve, when plotted logarithmically, is the volume inside the pores.
Halenda (BJH) analysis (Micromeritics Tristar 3000) following the adsorption of N2 on the solids. Figure 5 shows results for calcined samples of eggshell and Purbeck limestone, which were sieved from the sand while hot and carefully protected from contact with atmospheric moisture and CO2. It is clear from Figure 6 that the PSDs for eggshells and limestone, calcined once, are quite dissimilar. Eggshells begin with a broad distribution of pore sizes between 20 and 1000 nm, whereas Purbeck limestone is bimodal, with one sharp peak between 20 and 50 nm and one broader peak between 50 and 5000 nm. After 10 cycles of calcination, the broad peak for calcined eggshells has resolved into two peaks: a sharp one at ∼15 nm and a broader peak at ∼250 nm. The peak for the smallest pores in Purbeck limestone has shifted downward in diameter and reduced in total volume. Interestingly, although the heights of the peaks for calcined limestone and eggshells are different, the local maxima in the pore volumes for both the wider pores (300 nm) and narrower ones (15 nm) occur at the same width
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of pore for both sorbents. Finally, it is clear from Figure 6 that the volume inside the pores of the calcined eggshells decreases more after several cycles of carbonation and calcination than is the case for the limestone. Figure 7 shows the effects of large numbers of cycles of calcination and carbonation on eggshells, in both the calcined and carbonated forms. These measurements were made with Hg intrusion porosimetry. It is clear from Figure 7 that the pores with widths up to ∼77 nm completely disappeared during carbonation, whereas the volume in pores ∼200 nm in diameter remained essentially unchanged during carbonation. This makes it clear that during carbonation CO2 is adsorbed in pores narrower than ∼77 nm. Presumably, this is because the majority of the surface area of a calcined particle is to be found on the walls of these narrowest pores. The results from BJH analysis, described below, indicate that there is a proportion of the total volume in pores narrower than ∼10 nm, which is not observed by Hg porosimetry. A comparison of the pore size distributions from Hg porosimetry with those from BJH analysis yielded a smaller diameter for the pores in the lower (10-20 nm) size range. This is because of the differences in the way the two techniques work21 and is discussed further in the Supporting Information. Comparison of the Volume of Small Pores with the Carrying Capacity. It has previously been shown17 that, for experiments similar to those described here, the carrying capacity of a calcined limestone is proportional to that fraction of the volume of all of the pores in CaO (νtot, 0.37 mL/g17), which resides in pores narrower than 100 nm, i.e., is measured as νcr using the BJH technique. This was also found by Bathia and Perlmutter;1 it also occurs with a range of synthetic sorbents and dolomite22 and thus appears to be rather generally applicable. Figure 8 is a plot of the measured carrying capacity of the eggshells against vcr/Vtot for both Hg intrusion into pores narrower than 77 nm and BJH analysis of pores narrower than 100 nm. It is clear that the carrying capacity of the eggshells is not proportional to Vcr/Vtot. A straight best fit line has a slope and intercept of 1.20 ( 0.6 and 18 ( 16% for the BJH analysis
Figure 7. Pore size distribution measured by Hg intrusion porosimetry for eggshells (originally 500-710 µm). The numbers (x, y) indicate the total number of calcinations (x) (including the “zeroth” calcination) and carbonations (y) that the particles have undergone. The area underneath each curve is the volume of pores.
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Table 2. Results for Eggshells at 750 °C of Measured Total Volume of Hg Intruded (Pores < 10 µm) and Hg Intruded into Small Pores (
