ARTICLE pubs.acs.org/EF
K2CO3/Al2O3 for Capturing CO2 in Flue Gas from Power Plants. Part 4: Abrasion Characteristics of the K2CO3/Al2O3 Sorbent Chuanwen Zhao, Xiaoping Chen,* and Changsui Zhao School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: K2CO3/Al2O3 used for capturing CO2 in flue gas is thoroughly investigated. This paper focuses on the abrasion characteristics of K2CO3/Al2O3 particles. The attrition index for K2CO3/Al2O3 is only 1.54%. In comparison to the K2CO3/ activated carbon particles, the elutriation amount and the rate of the particle size change are much lower for K2CO3/Al2O3 in the same operating conditions, and they are in the range from 0.1 to 2.9% and from 1.5 10 3 to 15.1 10 3 mm/h, respectively, when the abrasion tests for K2CO3/Al2O3 are carried out in various conditions. In comparison to other variables, the temperature change is the most important variable for abrasion. The elutriation amount decreases when the carbonation reaction is carried out. The rate of change in particle size increases with the increase of the fluidization number, temperature, mean particle size, and loading amount but decreases with the increase of the operation time and chemical reaction cycle number. The elutriation amount increases when the aforementioned variables increase to a high value. The sorbent of K2CO3/Al2O3 shows excellent attrition-resistance performance.
1. INTRODUCTION There were many studies on the attrition and fragmentation behaviors of other sorbent particles, such as the impact attrition of MgO, NaCl, and KCl,1,2 the attrition characteristics of limestone in the system of fluidized-bed reactors for CO2 capture,3 5 and the attrition behaviors of activated carbon (AC), activated alumina (Al2O3), molecular sieve 5A, and molecular sieve 13X.6 However, investigations on the abrasion characteristics of alkalimetal-based sorbents were few. Three samples of Na2CO3, NaHCO3, and KHCO3 and two samples of supported potassium carbonate (10 and 40%) were subjected to air jet attrition testings.7,8 In comparison to the commercial fluidized catalytic cracking (FCC) catalysts, the attrition indices were given for many sodium-based sorbents, such as Sorb NX30,9 Research Triangle Institute’s (RTI) sorbents,10 Sorb NX35,11 and Sorb NX,12 and some potassium-based sorbents, such as Sorb KX3513 and Sorb A.14 All of the research pertaining to the abrasion characteristic of the alkali-metal-based sorbents was conducted in accordance with the American Society for Testing and Materials (ASTM) D5757-95 standard test method by air jets. The attrition was only related to mechanical stresses because of rubbing and particle collisions and with the reactor walls. However, the abrasions of the alkali-metal-based sorbents were affected by a lot of factors when they were used in the CO2-capture process with a fluidizedbed reactor system. For example, the attrition rate might be affected by the properties of the sorbents, such as size, surface, porosity, hardness, cracks, density, shape, loading amount, and particle strength. With the temperature increasing, the thermal stresses play an important role as well. The attrition processes may be affected by the progress of chemical reactions (calcination and carbonation). To investigate the attrition-resistance performance of the K2CO3/Al2O3 sorbents thoroughly and to find the key phenomenological features and mechanistic pathways of sorbent r 2011 American Chemical Society
attrition, the attrition behaviors of K2CO3/Al2O3 were studied with a factor decomposition method and then the effects of individual factors, including gas velocity, operation time, temperature, loading amount, particle size, and chemical reaction, were investigated.
2. EXPERIMENTAL SECTION 2.1. Samples. Because K2CO3/AC was reported to have excellent CO2 capture capacity by many preliminary results,15 17 it was compared to K2CO3/Al2O3. K2CO3/Al2O3 and K2CO3/AC used in this study were prepared by impregnating K2CO3 on Al2O3 and AC, respectively. K2CO3 was provided as an analytical reagent, AC and a special γ-Al2O3 were supplied by the Research Institute of Nanjing Chemical Industry Group. The preparation process of the sorbent consisted of three steps as follows: mixed and impregnated, dried at 105 °C for dehydration, and calcined at 300 °C. More information about the method of sorbent preparation was reported in detail in a previous work.18 The K2CO3 loading amount is in the range of 10 50%. The range of particle sizes for K2CO3/Al2O3 is 0.18 0.63 mm. The physical properties and microscopic structures of the sorbents and the minimum fluidization velocity of the sorbents are shown in Tables 1 and 2, respectively. The attrition index is much lower for K2CO3/Al2O3. However, the minimum fluidization velocity of K2CO3/Al2O3 is much higher than that of K2CO3/AC when the mean particle size is the same, because the apparent density of K2CO3/Al2O3 is high. The gas flow rate is high, and the rubbing and collisions of the particles are high for K2CO3/Al2O3; therefore, it is necessary to compare the abrasion behaviors of these two sorbents. 2.2. Apparatus and Procedure. Figure 1 presents the schematic diagram of the experimental apparatus for sorbent abrasion behavior tests. Received: March 1, 2011 Revised: April 26, 2011 Published: May 03, 2011 1395
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Table 1. Physical Properties and Microscopic Structures of the Sorbents Al2O3
AC
K2CO3/Al2O3
K2CO3/AC
apparent density
2390.55
1296
2714.6
1557.34
(kg/m3) bulk density
723.73
623.73
827
682.66
196.44
734.9
95.77
389.64
0.43
0.58
0.31
0.19
mean pore size
10.68
2.07
17.48
1.84
(nm) attrition index
1.36
6.58
1.54
4.17
0.18 0.63
0.18 0.63
0.18 0.63
0.18 0.63
(kg/m3) surface area (m2/g) pore volume (cm3/g)
(AI)a (%) particle size
Figure 1. Schematic diagram of the experimental apparatus for the abrasion characteristic test.
(mm) a
AI is determined by the ASTM D5757 method.
Table 2. Minimum Fluidization Velocity of the Sorbents minimum fluidization velocity (m/s) mean particle size (mm)
K2CO3/Al2O3
K2CO3/AC
0.20
0.029
0.017
0.31
0.070
0.040
0.40
0.115
0.066
0.51
0.185
0.107
0.60
0.252
0.147
The apparatus consists of three sections: a gas injection system, a bubbling fluidized-bed reactor, and a gas post-treatment system. High-purity CO2 and N2 from cylinders were delivered to the experimental apparatus with rotameters. H2O was fed using a high-precision piston pump and then heated to ensure complete vaporization. A fluidizedbed reactor with an inner diameter of 0.05 m and a height of 1.0 m was made of stainless steel and was set in an electrically heated furnace. More information about this fluidized-bed reactor was reported in previous papers.18,19 A cyclone was used for collecting the particles escaping from the reactor, and a filter was used for catching the ultrafine particles. When the test was carried out, about 0.3 kg of the sorbent was placed into the reactor and the height of the material was about 0.15 0.25 m. Because the abrasion characteristics for those samples were complicated in the CO2-capture process with the fluidized-bed reactor system, they were studied with a factor decomposition method and a single influence factor method. The factor decomposition method was based on a five carbonation/regeneration cycle test, and the base test conditions were as follows: the fluidization number (which is defined as the ratio of the superficial gas velocity to the minimum fluidization velocity) was 3, the total operation time was 10 h, the carbonation temperature was 60 °C, and the regeneration temperature was 350 °C. The abrasion behavior was mainly influenced by the aforementioned factors. The test was carried out in N2 for 1 h at 20 °C with the fluidization number of 3, and then the influence factors were added one by one to determine which factor was most important. For the single influence factor method, only one influence factor was changed, while other factors were kept under the same conditions. The effects of the factors of the fluidization number, operation time, temperature, particle size, loading amount, and chemical reaction were studied on abrasion.
After the test was finished, all of the samples from the reactor and the filter were collected and the change of the particle size was determined by a sieving method. The elutriation amount (EA) and the rate of the particle size change (RP) were used to represent the abrasion behavior. EA was defined as the amount of the sorbent particles with a free-falling velocity of less than the superficial gas velocity for the operation condition. RP was defined as the change of the mean particle size with the time. Experiments were repeated at least 3 times to obtain the average and standard deviation, expressed as error bars.
3. RESULTS AND DISCUSSION 3.1. Abrasion Behaviors Studied with the Factor Decomposition Method. 3.1.1. Samples with a Narrow Particle Size Range of 0.22 0.35 mm. The abrasion behaviors of K2CO3/AC
and K2CO3/Al2O3 with a narrow particle size range, whose mean particle size is 0.31 mm, are shown in Figure 2. As shown in Figure 2, the EA and RP of K2CO3/AC are higher than those of K2CO3/Al2O3 for all of the conditions. EA for K2CO3/AC is in the range of 1.2 14.4%, while EA for K2CO3/ Al2O3 is in the range of 0.3 2.9%. When the influence factors of the fluidization number, operation time, temperature change, and chemical reaction were added one by one, the increase of EA for K2CO3/AC was 1.2, 4, 9.2, and 5.4%, respectively, and the increase of EA for K2CO3/Al2O3 was 0.3, 0.6, 2.0, and 0.2%, respectively. The temperature is the most important factor for the increase of the elutriation amount. The second most important factor is the operation time. When the factor of the chemical reaction is added, the abrasion is decreased. RP is 30 10 3 and 15 10 3 mm/h for K2CO3/AC and K2CO3/Al2O3, respectively, when the operation time is 1 h. It decreases to 6 10 3 and 3 10 3 mm/h, respectively, as the operation time increases to 10 h. The operation time plays an important role. RP increases to 19 10 3 and 4 10 3 mm/h for K2CO3/AC and K2CO3/Al2O3, respectively, when the factor of the temperature change is added and decreases as the factor of the chemical reaction is added. It can be concluded that the abrasion-resistance capacity is higher for K2CO3/Al2O3 than for K2CO3/AC. The temperature change is the most important factor for abrasion. When the factor of the chemical reaction is added, the abrasion is decreased. 3.1.2. Samples with a Wide Particle Size Range of 0.18 0.63 mm. The results for the abrasion behaviors of K2CO3/AC and 1396
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Figure 2. Effect of factors added one by one on the (a) elutriation amount and (b) rate of the particle size change for the factors of the (1) fluidization number, (2) operation time, (3) temperature change, and (4) chemical reaction.
K2CO3/Al2O3 with a wide particle size range, whose mean particle size is the same as the samples in section 3.1.1, are shown in Figure 3. The abrasion behavior for the samples with a wide particle size range is similar to those with a narrow particle size range. The operation time and the temperature change play important roles in the abrasion, because the particle size changes of K2CO3/AC and K2CO3/Al2O3 are higher for conditions 2 and 3 than other conditions. The particle size change is much higher for K2CO3/ AC than for K2CO3/Al2O3. The particle of K2CO3/Al2O3 shows a good abrasion-resistance capacity. 3.2. Abrasion Behaviors Studied with the Single Influence Factor Method. To study the effect of all of the factors on abrasion more thoroughly, the abrasion behaviors of K2CO3/AC and K2CO3/Al2O3 were investigated with the single influence factor method. 3.2.1. Effect of the Fluidization Number. The effect of the fluidization number on the abrasion behaviors of K2CO3/AC and K2CO3/Al2O3 is shown in Figure 4. EA and RP increase linearly for those two samples as the fluidization number increases. EA increases from 2.4 to 4.2% for K2CO3/AC and increases from 0.3 to 0.7% for K2CO3/Al2O3 when the fluidization number increases from 1 to 5, while RP increases from 10 10 3 to 33 10 3 mm/h for K2CO3/AC and increases from 4 10 3 to 8 10 3 mm/h for K2CO3/ Al2O3. Because the minimum fluidization velocity is constant, the superficial gas velocity increases with the fluidization number increasing. The reason for the effect of the fluidization number on abrasion is that the rubbing and collisions of particles are increased with the change of the superficial gas velocity.
Figure 3. Effect of factors added one by one on abrasion for (a) K2CO3/ AC and (b) K2CO3/Al2O3 for the factors of the (1) fluidization number, (2) operation time, (3) temperature change, and (4) chemical reaction.
3.2.2. Effect of the Operation Time. The effect of the operation time on the abrasion behaviors of K2CO3/AC and K2CO3/ Al2O3 is shown in Figure 5. Figure 5 shows that EA rapidly increases from 1.2 to 5.2% at the first 10 h and then gradually reaches a stable value of 6.2% after 20 h for K2CO3/AC. It only increases from 0.2 to 1.1% for K2CO3/ Al2O3 when the operation time increases from 1 to 20 h. On the contrary, RP decreases from 30 10 3 to 3.3 10 3 mm/h for K2CO3/AC and decreases from 15 10 3 to 1.5 10 3 mm/h for K2CO3/Al2O3 when the operation time increases from 1 to 20 h. In combination with the result in section 3.2.1, it is deduced that the rubbing and collisions of particles are mainly caused by the change of the superficial gas velocity. When the gas velocity is kept constant, the abrasion of particles is minimal. 3.2.3. Effect of the Temperature. The effect of the temperature on the abrasion behaviors of K2CO3/AC and K2CO3/Al2O3 is shown in Figure 6. EA and RP increase linearly for those two samples as the temperature increases, except for RP of K2CO3/Al2O3. EA increases from 2.8 to 14.4% for K2CO3/AC and increases from 0.4 to 2.9% for K2CO3/Al2O3 when the temperature increases from 20 to 350 °C, while RP increases from 16 10 3 to 66 10 3 mm/h for K2CO3/AC and increases from 5.3 10 3 to 7.5 10 3 mm/h for K2CO3/Al2O3. The effect of the temperature on abrasion is more important than other factors. It means that, with the temperature increasing, the thermal stress plays an important role. Because a small amount of water had been 1397
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Figure 4. Effect of the fluidization number on the (a) elutriation amount and (b) rate of the particle size change. Figure 6. Effect of the temperature on the (a) elutriation amount and (b) rate of the particle size change.
Figure 5. Effect of the operation time on the (a) elutriation amount and (b) rate of the particle size change.
adsorbed on K2CO3/AC and K2CO3/Al2O3 (the water contents of the sorbents were determined with a F4100 type online platetype solid moisture meter, and they were 7.31 and 5.42% for K2CO3/AC and K2CO3/Al2O3, respectively), the processes of evaporation play important roles also.
3.2.4. Effect of the Particle Size. The effect of the particle size on the abrasion behaviors of K2CO3/AC and K2CO3/Al2O3 is shown in Figure 7. EA and RP increase rapidly as the mean particle size increases for those two samples. EA increases from 2.4 to 6.8% for K2CO3/ AC and increases from 0.1 to 10.8% for K2CO3/Al2O3 when the mean particle size increases from 0.2 to 0.6 mm, while RP increases from 9.2 10 3 to 39 10 3 mm/h for K2CO3/ AC and increases from 1.8 10 3 to 56 10 3 mm/h for K2CO3/Al2O3. Because the fluidization number is kept the same and the minimum fluidization velocity is much higher for the same sorbent with a higher particle size, the superficial gas velocity increases with the particle size increasing and the particle size for elutriation increases as well. EA and RP are affected by the change of the superficial gas velocity. Because the superficial gas velocity is much higher for K2CO3/Al2O3 than for K2CO3/AC with the same particle size, the increase of EA and RP for K2CO3/ Al2O3 is higher. 3.2.5. Effect of the Loading Amount. The effect of the loading amount on the abrasion behaviors of K2CO3/AC and K2CO3/ Al2O3 is shown in Figure 8. For K2CO3/AC, EA rapidly increases from 2.2 to 5.4% and RP rapidly increases from 11 10 3 to 29 10 3 mm/h when the loading amount increases from 10 to 50%. For K2CO3/Al2O3, EA and RP remain stable at about 0.5% and 4.3 10 3 mm/h, respectively, when the loading amount is lower than 40%. They rapidly increase to 1.26% and 8.0 10 3 mm/h, respectively, when the loading amount increases to 50%. The main reason is the different active component distribution behaviors for those two sorbents. As reported before,16 for K2CO3/AC, K2CO3 would be held in macroporous spaces without blocking micropores. The highest loading amount inside the sorbent was 23%, 1398
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Figure 9. Effect of the chemical reaction on the (a) elutriation amount and (b) rate of the particle size change: (open symbols) carbonation reaction and (solid symbols) regeneration reaction. Figure 7. Effect of the particle size on the (a) elutriation amount and (b) rate of the particle size change.
Figure 8. Effect of the loading amount on the (a) elutriation amount and (b) rate of the particle size change.
and the excess K2CO3 was distributed on the surface of AC in the form of large aggregates.16 The amount increases on the surface
of AC as the loading amount increases and the abrasion-resistance capacity decreases. For K2CO3/Al2O3,20 when the loading amount is lower than 40%, most K2CO3 is distributed inside the support; therefore, the abrasion-resistance capacity is excellent. When the loading amount is higher than 40%, more than a quarter of the total amount of K2CO3 is distributed on the surface of Al2O3 and the abrasion-resistance capacity becomes poor. 3.2.6. Effect of the Chemical Reaction. The effects of carbonation and regeneration reaction cycles on the abrasion behaviors of K2CO3/AC and K2CO3/Al2O3 are shown in Figure 9. EA increases as the reaction cycle number increases, and it is higher for regeneration conditions than for carbonation conditions for those two samples. For the carbonation conditions, EA increases from 1.5 to 5.6% for K2CO3/AC and increases from 0.3 to 0.9% for K2CO3/Al2O3 when the reaction cycle number increases from 1 to 5. For the regeneration conditions, EA increases from 7.0 to 9.0% for K2CO3/AC and increases from 0.7 to 2.7% for K2CO3/Al2O3. RP decreases as the reaction cycle number increases. For the carbonation conditions, it decreases from 33 10 3 to 4.4 10 3 mm/h for K2CO3/AC and decreases from 8.3 10 3 to 2.8 10 3 mm/h for K2CO3/Al2O3 when the reaction cycle number increases from 1 to 5. For the regeneration conditions, it decreases from 29 10 3 to 11 10 3 mm/h for K2CO3/AC and decreases from 13 10 3 to 3.7 10 3 mm/h for K2CO3/ Al2O3. The effect of reaction cycles on the abrasion is complicated. The operation time increases as the cycle number increases and the temperature is changed. The gas composition is changed also. For the carbonation process, K2CO3 reacts with CO2 and H2O to produce KHCO3. For the regeneration process, KHCO3 is decomposed to K2CO3, CO2, and H2O. The abrasion behavior is affected by all of the previous factors. 1399
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4. CONCLUSION The abrasion characteristics of K2CO3/Al2O3 particles were thoroughly investigated with a factor decomposition method and a single influence factor method. In comparison to the K2CO3/ AC particles, the elutriation amount and the change rate of the particle size are much lower for K2CO3/Al2O3 in the same operating conditions. In comparison to other variables, the temperature change is the most important variable for abrasion. The elutriation amount decreases when the carbonation reaction is carried out. The rate of change of the particle size increases with the increase of the fluidization number, temperature, mean particle size, and loading amount but decreases with the increase of the operation time and chemical reaction cycle number. The elutriation amount increases when the aforementioned variables increase to a high value. The sorbent of K2CO3/Al2O3 shows excellent attrition-resistance performance. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: +86-25-83793453. Fax: +86-25-83793453. E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support from the National High Technology Research and Development Program of China (2009AA05Z311), the National Natural Science Foundation (50876021), the Foundation of State Key Laboratory of Coal Combustion (FSKLCC1006), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1001) is sincerely acknowledged.
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