Modified lime-based pellet sorbents for high-temperature CO2 capture: Reactivity and attrition behavior

Fuel 96 (2012) 454–461

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Modified lime-based pellet sorbents for high-temperature CO2 capture: Reactivity and attrition behavior Yinghai Wu, Vasilije Manovic, Ian He, Edward J. Anthony ⇑ CanmetENERGY, Natural Resources Canada, 1 Haanel Dr., Ottawa, ON, Canada K1A 1M1

a r t i c l e

i n f o

Article history: Received 14 September 2011 Received in revised form 9 January 2012 Accepted 14 January 2012 Available online 28 January 2012 Keywords: Calcium looping Fluidized bed Pellet Reactivity Attrition

a b s t r a c t Modified lime-based pellets have been developed as potential regenerable high-temperature CO2 sorbents using calcium aluminate cement binders to enhance pellet strength. A mechanical pelletizer was used for granulation of the powdered materials, namely quick lime and hydrated lime, produced from Graymont limestone with the addition of spray water. The CO2 carrying capacity of both the pellet sorbents and the parent limestone was tested in a thermogravimetric analyzer (TGA) at 800 °C with repeated calcination/carbonation cycles. It was found that the CO2 carrying capacity of the pelletized sorbent was higher than that of the parent limestone, and the stability over multiple cycles was improved when cement was added to the pellets. The attrition resistance of these pellets was examined using a bubbling fluidized bed (50 mm ID). The particle size distribution (PSD) of both the calcined pellets and limestone was determined before and after 2 h attrition tests which were performed using air as the fluidizing gas at room temperature and at 800 °C. The results of attrition tests showed that after fluidization for 2 h, particle size distribution changed such that the average particle diameter (d50) of the sample always decreased, but for the pellet sorbents, high-temperature fluidization did not result in significantly more pronounced attrition. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Currently, one-third of all anthropogenic CO2 emissions come from fossil fuel combustion for power generation [1,2]. If fossil fuels, in particular coal, are to continue being used, CO2 capture from such large stationary sources is necessary to stabilize the atmospheric concentration of CO2 in order to reduce the severity of future climate change effects. Unfortunately, low CO2 concentrations from power generation (typically 15 vol.% in flue gas from coal-fired power plants) make it infeasible to directly sequester CO2 from the flue gas [3,4]. Hence, the goal of any fossil fuel CO2 capture process is to produce a concentrated CO2 stream which can be liquefied, transported and stored in geological formations. Currently, the preferred method to achieve this appears to be post-combustion capture, with a solvent like monoethanolamine (MEA) to capture CO2 from the flue gas from power plants. However, the cost of amine-based processes such as MEA scrubbing is high, primarily because of the stripper steam requirements for solvent regeneration. There is, therefore, considerable interest in developing alternatives to amine scrubbing.

CaO þ CO2 ¼ CaCO3

DH < 0

⇑ Corresponding author. Tel.: +1 613 996 2868; fax: +1 613 992 9335. E-mail address: [email protected] (E.J. Anthony). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2012.01.034

ð1Þ

Lime-based calcium looping cycles (CaLC), which utilize reversible carbonation and calcination reactions (Eq. (1)), offer a promising technology to separate CO2 from flue or fuel gas while producing a highly concentrated CO2 stream (>90%) suitable for sequestration. The Ca looping cycle concept (CaLC) can be realized using a dual fluidized bed system [5], where solid sorbents are continuously cycled between the two reactors, a carbonator (where the forward reaction occurs at lower temperature) to absorb CO2 from the flue gas and a calciner (reverse reaction at a higher temperature or lower pressure) to release absorbed CO2 from the sorbent (which regenerates the sorbent) to form a concentrated CO2 stream. Naturally occurring limestones (mostly calcitic) have so far been the subject of intensive research in calcium looping, given their wide availability and low cost. However, natural sorbents rapidly lose their CO2 carrying capacity with increasing numbers of reaction cycles and this effect is most pronounced during initial cycles. Methods to improve the performance of the sorbent include sorbent pre-treatment at high temperature [6], and reactivation of spent sorbent by hydration with steam or moist air [7,8]. Reactivation of spent sorbent has been exhaustively investigated for improved SO2 capture in FBC systems [9,10]. However, in this case, although the reactivated sorbent particles tend to be cracked and mechanically fragile, this poses no major problems for SO2 capture as the sorbents are not extensively cycled given the effectively irreversible nature of the sulfation reaction. Even so it is perhaps

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Al2O3 and 28% CaO), was used as a binder for pelletization. The cement was supplied by Almatis Inc. as a very fine powder with >80% of the particles <45 lm. The sorbent-to-cement weight ratio in this study was 9:1. The type of binder and the sorbent/binder weight ratio were chosen based on the optimized results of previous binder screening studies [13,14]. For comparison purposes, the original Graymont limestone particles (0.425–1 mm, 95% CaCO3) were also used in the reactivity and attrition tests as a baseline reference. 2.2. Pellet preparation procedure

Fig. 1. Schematic of bubbling fluidized bed.

worth noting that such approaches have still not found general favor for use with FBC boilers. In the CaLC system, although previous research shows that CO2 carrying capacity can be significantly recovered by reactivation of the spent sorbent, the same research has also confirmed that the reactivated sorbents are mechanically fragile and unsuitable for use in fluidized bed conditions [11,12]. Therefore, if reactivation is applied to the CaLC spent sorbent, fragmentation and attrition are critical issues that must be dealt with because the sorbents should be repeatedly cycled as long as possible. To overcome these drawbacks the use of pelletized sorbents is instead proposed. Here, a mechanical pelletizer was used in the process for pelletization of both fresh and/or reactivated sorbent. Calcium aluminate cement was used as a binder for the pelletization process, as this material has shown appropriate properties in previous binder screening studies [13]. The physical and chemical characterization of the modified pellet sorbents were investigated along with the long-term performance of the sorbents. In particular, this study focuses on the reactivity in repeated calcination/carbonation cycles in a thermogravimetric analyzer (TGA) and on attrition behavior produced in a bubbling fluidized bed reactor.

As noted above, two types of pellets were prepared: (i) 100% CaO (QL1 and QL2) or Ca(OH)2 (HL), designated as QL1-Ca100, QL2-Ca100 or HL-Ca100; and (ii) 90% CaO (QL2) or 90% Ca(OH)2 (HL) with 10% cement (designated as QL2-Ca90C10 or HLCa90C10). The pellets were prepared in batch mode using a mechanical pelletizer (Glatt GmbH). The powdered material (300 g in total) was mixed in the desired proportions in the pelletization vessel (1 L). Water was sprayed intermittently, during pelletization, with a nozzle which can produce micron-sized water droplets (<300 lm at about 25 mL water per minute under an excess pressure of 700 kPa) required for the pelletization process. The amount of water varied depending on the starting material used. The water droplet size and the total amount of water added appear to be the most sensitive factors affecting the final size of the pellets being made. The pellet size was also controlled by the speed of a pair of rotor blades attached to the vessel, i.e., one agitator (operated at 500 rpm) located on the bottom and one chopper (operated at 2500 rpm) on the side. Typically, it took 20–30 min to produce one batch of pellets. After pelletization, the pellets were sieved and the size fraction of 0.425–2 mm was collected. The collected pellets were air-dried before storage. Given the particular suitability of pellet size of 0.425–1 mm for fluidized bed systems, most of the reactivity and attrition tests were conducted with this size range. In the case where quick lime was involved in the pellet manufacture, after each run the smaller (<0.425 mm) and larger (>1.4 mm) particles were collected and returned to the pelletization vessel to be re-pelletized with fresh CaO powder in the next run. Namely, it was noted that fresh CaO absorbed considerable amounts of water and released considerable heat during hydration (more water was necessarily sprayed as well), which allowed for the comminution of any excessively large particles from the previous run.

2. Experimental 2.1. Materials Quick lime (CaO, QL) and hydrated lime (Ca(OH)2, HL) powders (<30 lm, originated from Graymont limestone) were used as the starting materials for the pelletization process. Both QL and HL powders were provided by Graymont Limestone Inc. Two different batches of QL powders were used: QL1 was kept in contact with the atmosphere and this material contains only 30% of fresh lime by mass (with the rest being reaction product of lime with atmospheric moisture and CO2) when used in our tests to explore the possibility of employing an aged sorbent to produce pellets; and a fresh product QL2 was also used. The latter produced a much stronger heat-release effect when reacting with water and handling protocols were developed for this material. For these studies, a commercial refractory calcium aluminate cement, CA-14 (71%

Fig. 2. SEM image of pelletized sorbent (HL-Ca100) derived from Ca(OH)2 powder.

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Y. Wu et al. / Fuel 96 (2012) 454–461 Table 1 Pore surface area analysis of calcined pellets/particles. Pellet/particle

Size range (mm)

BET surface area (m2/g)

BJH adsorption average pore dia. (nm)

BJH adsorption total pore volume (cm3/g)

HL-Ca100

<0.425 0.425–1 0.85–1.4

13.4 7.5–11.4 9.6

9.1 7.1–8.5 7.5

0.03 0.014–0.024 0.018

HL-Ca90C10

<0.425 0.425–1 1–1.4 1.4–2

13.1 10.3–12.5 10.6 11.0

12 10.5–10.9 8.9 9.4

0.039 0.027–0.034 0.024 0.026

QL1-Ca100

0.425–1 1–2

13.5 13.2

8 8.2

0.027 0.027

QL2-Ca100

0.425–1

6.6

10.3

0.017

QL2-Ca90C10

0.425–1 1–1.4

8.5–9.6 8.1–8.6

8.1–9.8 8.2–8.5

0.017–0.023 0.017–0.018

Graymont limestone

0.425–1

1.5–1.6

8.0

0.0029–0.0035

Furthermore, this effect was also enhanced by the high speed of the chopper under these conditions. The use of particles from previous runs helped reduce the amount of waste material (too small and too large sized particles) and effectively converted the process to semi-continuous operation which would be beneficial in practice. The external morphologies of the air-dried pellets were examined by scanning electron microscopy (SEM) using a Hitachi S3400 microscope with 20 kV of accelerating voltage under high vacuum. The images obtained by secondary electrons are presented here. Nitrogen adsorption/desorption isotherms used for analyzing Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore size distribution were obtained on a Micromeritics ASAP 2000 apparatus using nitrogen physisorption at 77 K. Before that, samples were calcined in air at 850 °C for 2 h and degassed at 110 °C in vacuum for 12 h. 2.3. Reactivity test A Mettler-Toledo TGA/SDTA851e/LF/1100C TGA was used for the repeated calcination/carbonation cycles. The sample typically weighing 30 mg was placed in a ceramic pan (5 mm inside diameter). The temperature and gas used were controlled by STARe software. The gas flow rate was 0.04 dm3/min. The samples were heated in 100% N2 from room temperature to 800 °C at a heating rate of 50 °C/min and calcined at 800 °C for 15 min to complete the first calcination, and then the carbonation/calcination cycles were started. Data on sample mass during the experiments were monitored and CO2 carrying capacity was calculated on the basis of mass change, assuming that mass change occurs only due to the formation and decomposition of CaCO3. Both carbonation and calcination were performed isothermally at 800 °C with carbonation in 25% CO2 (N2 balance) for 30 min and calcination in 100% N2 for 15 min. It should be noted that 30 min of carbonation implies a prolonged reaction under the diffusion-controlled regime, but no significant conversion is expected to have occurred during that stage. The carbonation/calcination cycles were repeated and typically a minimum of 30 cycles were performed for each sample. 2.4. Attrition test The main goal of the attrition tests was to determine the attrition resistance of the produced pellets. The attrition experiment was carried out in a 316L stainless steel atmospheric bubbling fluidized-bed (BFB) reactor of 50 mm ID and 1.2 m in height (see Fig. 1). The fluidization gas distributor was about 400 mm from the bottom of the reactor, supported by three small tubes on the bottom flange. There were seven mushroom-shaped nozzle tuyeres

Fig. 3. CO2 carrying capacity of pellet sorbents: (a) CaO pellets; (b) Ca(OH)2 pellets.

evenly distributed on the distributor plate. Here air was employed as the fluidization gas for all tests and the gas flow rate was controlled by a digital flow meter. The fluidization column was heated by a 4.5 kW electrical furnace. The wind box between the bottom flange and distributor worked as a pre-heat section. There was another 0.7 kW in-line heater below the bottom flange to help preheat the fluidization gas (to 200 °C) before the wind box. The bed temperature was measured by a K-type thermocouple located closely above the gas distributor. The pressure drop (DP) in the bed between the dense section and freeboard was monitored by a digital manometer with pressure detection tubes inserted into the bed from the top. The pellets (0.425–1 mm) were calcined in a muffle furnace in air for 2 h at 850 °C and their particle size distribution was

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determined by sieving before the attrition tests. If limestone particles were used, they were first calcined and sieved under the same conditions as the pellet samples. The sieved samples were gently and thoroughly mixed again and divided into two or more parts for the attrition tests. One part of the pellets was continuously fluidized for 2 h in the BFB reactor at room temperature (cold attrition). The other part was studied in hot 2 h attrition tests where the sample was continuously fluidized at 800 °C. The bed material was weighed before loading from the top of the reactor. The amount of material used (80–100 g in calcined form) was chosen to create a static bed with an initial height of about 50 mm on the distributor. The minimum fluidization velocity, Umf, was determined experimentally for each batch of sample under cold and hot conditions by pressure drop (DP) vs. superficial gas velocity (U) correlations. In the cold attrition tests, the fluidization number U/Umf was kept at 1.2 which should allow the system to operate in the fluidized state; whereas various superficial gas velocities (U/ Umf = 1.2–3) were tested in the hot attrition tests to ensure vigorous fluidization conditions. The terminal velocity, ut, was estimated using equations recommended by Kunii and Levenspiel [15]. The nominal smallest size of particles (i.e., 0.425 mm) was used to determine ut, which is the upper limit of fluidization gas velocity to avoid carryover of bed materials. According to Vaux [16], two parameters can be used to describe attrition: (1) Attrition rate, R and (2) Extent of attrition, A, expressed in the following equations:

R¼

1 dM M dt

A ¼ 100

Z

further processing. The pore surface area properties of calcined pellets are given in Table 1. As expected, increasing the pellet size resulted in a corresponding decrease in the BET surface area, pore diameter and pore volume. As noted above, the size fraction of 0.425–1 mm was used in most of the reactivity and attrition tests. For this size fraction, when using cement as the binder, the pellets showed slightly higher values of BET surface area, pore diameter and pore volume. These results indicated that a change of internal structure occurred with the addition of cement, which should have an impact on the sorbent reactivity. The different CaO pellets, QL1 and QL2, presented different pore properties, particularly in BET surface area. As mentioned previously, QL1 was stored for an extended time in contact with air, which may result in this pellet having similar properties to Ca(OH)2 pellets (HL). The BET surface area, pore diameter and pore volume of calcined Graymont limestone with original size of 0.425–1 mm were significantly smaller than those of the pellet sorbents. 3.2. Reactivity test Typical results for CO2 carrying capacity of various pellet sorbents in repeated calcination/carbonation cycles are presented in

ð2Þ t

R dt ¼ 100 ln

0

  M0 M

ð3Þ

where M and M0 are the mass of particles in the bed at time t and initial time, respectively. Since it is reasonable to assume that the most significant attrition occurs in the initial calcination/carbonation cycles [12,17,18], the investigation in this study was focused on the extent of attrition in a period of 2 h fluidization and a simplified calculation is given in Eq. (4). The 2-h period was equivalent to about two full cycles of calcination/carbonation.

  M  100% A¼ 1 M0

ð4Þ

When the attrition test was finished, the bed inventory was carefully removed from the bed by vacuum with a 10 mm ID stainless steel tube (1.2 m long) from the top of the reactor. The collected bed material was weighed and the particle size distribution was determined again. A mass balance in the system was not performed because the structure of the experimental apparatus makes this impractical, and elutriated fine particles remaining in the system could not be collected, e.g., in the connecting pipes, etc. However, careful efforts were made to minimize experimental errors due to sample collection. First, after each test, the entire bed was carefully vacuumed, and then before the start of a subsequent test, the bed was again thoroughly vacuumed to ensure there was no significant quantity of particles remaining from the previous runs. 3. Results and discussion 3.1. Pellet characterization The typical morphology of prepared pellets is represented in Fig. 2. Most of the pellets appeared to be quite spherical after pelletization which means they can be used in a fluidized bed without

Fig. 4. CO2 carrying capacity of pellets with different particle size: (a) CaO pellets; (b) Ca(OH)2 pellets.

Table 2 Umf results in cold and hot attrition tests, m/s. Sorbent

Cold attrition

Hot attrition

QL1-Ca100 QL2-Ca100 QL2-Ca90C10 HL-Ca100 HL-Ca90C10 Graymont limestone

0.15 ± 0.01 No test 0.16 ± 0.01 0.13 ± 0.01 0.14 ± 0.01 0.12

No test 0.08 ± 0.01 0.12 0.08 0.11 ± 0.02 0.1 ± 0.02

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Fig. 3. As a comparison, the CO2 carrying capacity of the original limestone particles with the same size range (0.425–1 mm), tested under the same condition, are also given in the figure. All pellet sorbents performed better in long-term cycles than the limestone particles, as expected given the greater BET surface area, etc., for the CaO pellets (see Fig. 3a), QL1-Ca100 and QL2-Ca100 exhibited the same CO2 carrying capacity although QL1 was stored in contact with air. The carrying capacity of cement-bound pellets (QL2Ca90C10) was lower than that of non-cement pellets in the early cycles (<10). However, the cement-bound pellet maintained a higher and more stable carrying capacity for longer-term cycles (>20). This result agrees with the literature [19,20] on the use of CaO/Al2O3 sorbent for CO2 capture. The reason for the more stable carrying capacity was that Ca12Al14O33 (mayenite) was formed when Al2O3 was present, resulting in enlarged pore size which helps to maintain CO2 sorption performance. Other studies [21–

23] also suggested CaO/Ca12Al14O33 sorbents give excellent performance in cyclic CO2 capture. Pellets prepared without a cement binder, e.g. Ca(OH)2 pellets, HL-Ca100, had a higher carrying capacity than pellets produced with a cement binder for the first 30 cycles. However, pellets prepared with cement binders (HL-Ca90C10) appeared to have a more stable carrying capacity (as shown for CaO pellets) in extended cycles (up to 120), although the carrying capacity in the first 5–10 cycles was lower than that of the limestone particles. The CaO pellets prepared with cement showed higher carrying capacity than for Ca(OH)2 pellets prepared with cement, 27 and 18 mg/100 mg sorbent, respectively, after 90 cycles. It is interesting to note that in the TGA tests the pellet’s particle size had limited impact on the carrying capacity, as shown in Fig. 4, for both CaO and Ca(OH)2 pellets. This finding is potentially advantageous in situations where larger pellets are required for

Fig. 5. Particle size distribution before attrition and after 2-h hot attrition in bubbling fluidized bed. The initial size range was 0.425–1 mm. The fluidization gas was air. The temperature of the fluidized bed was 800 ± 10 °C.

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operation at higher fluidization velocity, since it would suggest that this can be done without significant negative impact on the sorbent reactivity. 3.3. Attrition test It is arguable whether constant gas velocity (U) or constant fluidization number (U/Umf) is more important in attrition tests [17,24]. Umf depends on the temperature and properties of the fluidizing gas; therefore, it is easier to maintain constant fluidization gas velocity rather than Umf. However, as the fluidizing gas was air (i.e., no evolution of CO2 or other gas from the calcined sorbent is expected to occur during the test) and the temperature was constant, the volumetric gas flow rate did not vary. In addition, Umf was determined experimentally at the beginning of each attrition test for every batch of samples, which made it possible to maintain constant fluidization number, U/Umf, throughout the duration of each attrition test. Umf results are presented in Table 2. To ensure sufficient and vigorous fluidization conditions, larger fluidization number (U/ Umf) of 2 or 3 was used for some samples in hot attrition tests, although most of the attrition tests were carried out with U/ Umf = 1.2. Terminal velocity (ut) for the nominal smallest particle, i.e., 0.425 mm, was 1.8 and 2.0 m/s at cold and hot conditions, respectively. These values were well above the actual operating gas velocity so there should be minimal bed material carryover issues, except for attrited fine particles elutriated from the reactor during the attrition test. It should be noted that the chemical and thermal stresses occurring in the pellets during repeated calcination/carbonation cycles can be an important cause of sorbent attrition. However, because air was the fluidizing gas in this study, and the attrition tests were performed isothermally, only mechanical related attrition (e.g. fragmentation and abrasion caused by inter-particle or wall-particle collisions) is mainly involved. Fig. 5 shows how the particle size distribution changed in the hot attrition tests with a low fluidization number of 1.2. The HLCa100 and Graymont PSD show the mass distribution shifted from both the 0.6–0.85 mm range and the 0.85–1 mm range to smaller size ranges, mainly 0.25–0.425 mm and 0.425–0.6 mm. The other three pellets show that there was a shift in mass distribution only from the largest size range of 0.85–1 mm to smaller size ranges, which indicates these pellets may have better resistance to attrition. However, the QL2-Ca100 and QL2-Ca90C10 present the greatest shift (>10 wt.%) in mass distribution from 0.85 to 1 mm range than do any other samples. QL2-Ca100, HL-Ca90C10 and Graymont limestone were tested with larger fluidization numbers (U/Umf = 2 and 3) in hot attrition. These results are shown in Fig. 6, representing the attrition behavior under more intensive fluidization conditions. It is not surprising that Graymont limestone showed a greater degree of fragmentation with continuous shift in mass distribution from both the 0.6–0.85 mm range and the 0.85–1 mm range to smaller size ranges, as U/Umf increased from 2 to 3. The mass distribution in the 0.425–0.6 mm range changed more severely at U/Umf = 3. By comparison, the other two pellet samples, QL2-Ca100 and HLCa90C10, presented greater attrition resistance. The mass distribution change occurred only from 0.85–1 mm range to smaller size ranges, while the mass of the 0.6–0.85 mm particle range was steady. Although the mass distribution in 0.425–0.6 mm range increased with the fluidization number, the degree of the increase was less significant for the two pellet samples than for Graymont limestone. When the samples were fluidized, the pellet/particle size (diameter) was reduced and to study this effect, the equivalent average diameter, d50, obtained from the cumulative PSD graph (not shown

Fig. 6. Particle size distribution before attrition and after 2-h hot attrition in bubbling fluidized bed. The initial size range was 0.425–1 mm. The fluidization gas was air. The temperature of the fluidized bed was 800 ± 10 °C. U/Umf = 2 or 3.

Fig. 7. Sorbent size reduction after attrition test in bubbling fluidized bed. The initial pellet/particle size range was 0.425–1 mm. The fluidization gas was air. Test duration was 2 h. The temperatures of cold attrition and hot attrition tests were 24 °C and 800 ± 10 °C, respectively. U/Umf = 1.2, 2 or 3.

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here), was investigated based on cold and hot attrition with the results shown in Fig. 7. In cold attrition tests, the reduction of d50 was in the range of 1.8–3.8% for all the samples (U/Umf = 1.2). Tests under hot conditions resulted in more intensive attrition for limestone particles than for the other pellet samples, especially at higher fluidization number of U/Umf = 3 where the d50 was reduced by about 8% for the limestone sample. In comparison, d50 results were steady for pellet sorbents on the whole. There is further evidence indicating that serious attrition occurred for limestone particles; thus in Fig. 8a, for most pellet samples (excluding QL2-Ca90C10) and the limestone sample, a significant mass loss of sorbent was observed even at low fluidization number of 1.2 during the hot attrition test when compared with the cold attrition results. As also shown in Fig. 8a, the limestone sample exhibited the worst attrition resistance characteristics of the samples being examined. This is not an unexpected result as natural limestones showed severe attrition problem in previous studies, unless some degree of sulphation occurs to form a hard sulfate shell on the surface of the particles that mitigates attrition [17,25]. It should be noted that as U/Umf increased, the extent of attrition for the limestone sample increased markedly (Fig. 8b), whereas for the two pellet sorbents, the extent of attrition showed apparently steady behavior with less than 2 wt.% mass loss at U/Umf = 3 compared to 23 wt.% mass loss for the limestone sample at the same fluidization number. It can be concluded that the mechanical strength of the pelletized sorbent was improved compared to the natural limestone sample. More importantly, within the limits of accuracy the results suggest that attrition performance of cement-containing Ca(OH)2 pellets (HL-Ca90C10) did not change as U/Umf increased. Similarly, the reduction of d50 for this sample was at a lower level (Fig. 7) compared with HLCa100 which had no addition of cement. This suggests that the cement can protect sorbents against attrition. Wu et al. [20] con-

Fig. 8. Extent of attrition in bubbling fluidized bed. The initial pellet/particle size range was 0.425–1 mm. The fluidization gas was air. Test duration was 2 h. The temperatures of cold attrition and hot attrition tests were 24 °C and 800 ± 10 °C, respectively.

cluded that the formation of Ca12Al14O33 from Al2O3 present in the sorbent improved the strength of the particles, i.e., producing increased resistance to attrition. 4. Conclusions The modified lime-based pellet sorbents for high-temperature CO2 capture were produced using a mechanical pelletizer. Regardless of whether cement binder was added or not, the pellet sorbents always performed better than the natural limestone samples both in terms of CO2 uptake capacity during repeated calcination/carbonation cycles and in resistance to attrition during fluidization in a bubbling fluidized bed. The cement-bound pellets showed more stable CO2 carrying capacity over long-term cycle tests. The attrition tests indicated that the addition of cement in the pelletization process may increase resistance to attrition, at least for the Ca(OH)2-derived samples. This study showed that the lime-based pellet sorbents are effective for high-temperature CO2 capture as a regenerable sorbent, and that the pelletization process has considerable potential to produce appropriate types of pellet sorbents for use in applications of calcium looping combustion. Acknowledgement The authors would like to thank Mr. Rose Kennedy of Graymont Limestone Inc. for supplying quick lime and hydrated lime samples used in this work. References [1] Intergovernmental panel on climate change (IPCC): Climate change 2007. Synthesis report, Geneva, Switzerland; 2007. [2] IEA report: capturing CO2, Gloucester, UK; 2007. [3] MacKenzie A, Granatstein DL, Anthony EJ, Abanades JC. Economics of CO2 capture using the calcium cycle with a pressurized fluidized bed combustor. Energy Fuels 2007;21:920–6. [4] Abanades JC, Grasa G, Alonso M, Rodriguez N, Anthony EJ, Romeo LM. Cost structure of a postcombustion CO2 capture system using CaO. Environ Sci Technol 2007;41:5523–7. [5] Shimizu T, Hirama T, Hosoda H, Kitano K, Inagaki M, Tejima K. A twin fluid-bed reactor for removal of CO2 from combustion processes. Trans IChemE 1999;77(Part A):62–8. [6] Manovic V, Anthony EJ. Thermal activation of CaO-based sorbent and selfreactivation during CO2 capture looping cycles. Environ Sci Technol 2008;42:4170–4. [7] Manovic V, Anthony EJ. Steam reactivation of spent CaO-based sorbent for multiple CO2 capture cycles. Environ Sci Technol 2007;41:1420–5. [8] Fennell PS, Davidson JF, Dennis JS, Hayhurst AN. Regeneration of sintered limestone sorbents for the sequestration of CO2 from combustion and other systems. J Energy Inst 2007;80:116–9. [9] Wu Y, Anthony EJ, Jia L. Experimental studies on hydration of partially sulphated CFBC ash. Canadian J Chem Eng 2003;81:1200–14. [10] Wu Y, Anthony EJ, Jia L. Steam hydration of CFBC ash and the effect of hydration conditions on reactivation. Fuel 2004;83:1357–70. [11] Wu Y, Blamey J, Anthony EJ, Fennell PS. Morphological changes of limestone sorbent particles during carbonation/calcination looping cycles in a thermogravimetric analyzer (TGA) and reactivation with steam. Energy Fuels 2010;24:2768–76. [12] Blamey J, Paterson NPM, Dugwell DR, Fennell PS. Mechanism of particle breakage during reactivation of CaO-based sorbents for CO2 capture. Energy Fuels 2010;24:4605–16. [13] Manovic V, Anthony EJ. Screening of binders for pelletization of CaO-based sorbents for CO2 capture. Energy Fuels 2009;23:4797–804. [14] Manovic V, Anthony EJ. Long-term behavior of CaO-based pellets supported by calcium aluminate cements in a long series of CO2 capture cycles. Ind Eng Chem Res 2009;48:8906–12. [15] Kunii D, Levenspiel O. Fluidization engineering. 2nd ed. Newton (MA): Butterworth-Heinemann; 1991. [16] Vaux WG. Attrition of particles in the bubbling zone of a fluidized bed. Am Power Conf 1978;40:793–802. [17] Jia L, Hughes R, Lu D, Anthony EJ, Lau I. Attrition of calcining limestones in circulating fluidized-bed systems. Ind Eng Chem Res 2007;46:5199–209. [18] González B, Alonso M, Abanades JC. Sorbent attrition in a carbonation/ calcination pilot plant for capturing CO2 from flue gases. Fuel 2010;89:2918–24. [19] Wu SF, Li QH, Kim JN, Yi KB. Properties of a nano CaO/Al2O3 CO2 sorbent. Ind Eng Chem Res 2008;47:180–4.

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