Energy Fuels 2010, 24, 1682–1686 Published on Web 02/08/2010
: DOI:10.1021/ef900986a
Experimental Study on Abrasion Characteristics of Rapidly Hydrated Sorbent for Moderate Temperature Dry Flue Gas Desulfurization Yuan Li, Chenxing Song, and Changfu You* Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China Received September 10, 2009. Revised Manuscript Received December 29, 2009
A rapidly hydrated sorbent for moderate temperature (600-800 °C) dry flue gas desulfurization (FGD) was developed with the fine calcium-containing particles formed by hydrating the lime adhering to the fly ash particles to increase the residence time of these fine particles in the circulating fluidized bed (CFB) reactor to increase the desulfurization efficiency. Abrasion of the rapidly hydrated sorbent was investigated by sieving the sorbent for 1 h with a sieving displacement of 1.5 mm. The abrasion ratio, Ra (%), which is the percentage of calcium (Ca) mass below the sieve to the original sorbent mass, was used to represent the sorbent abrasion characteristics. The sorbent desulfurization ability, Rd (mg/g), was represented by the mass of sulfur dioxide (SO2) absorbed per gram of sorbent. The experimental results show that Ra decreased from 7.35% to 3.16% as the fly ash particle size increased from 74 to 182 μm, while Rd of sieved sorbent increased from 124 to 139 mg/g, indicating that sorbent made from larger fly ash particles had a lower abrasion ratio and retained higher desulfurization ability after sieving. The sorbents prepared by firing, alkali, and acid modified fly ash particles had lower abrasion ratios and higher desulfurization abilities than those of sorbent prepared from the original fly ash.
dry FGD processes.6-11 Most studies reported that the sorbent activity increased because of the pozzolanic structures formed during the hydration process. However, the difficult preparation conditions limited the application of the hydrating sorbent. A process has been developed to prepare hydrated sorbent by hydrating lime and fly ash at ambient temperature for 2 h with drying for 1.5 h at 150 °C, which is fast and simple and results in high Ca conversion ratios for the sulfate reaction.12 The thermogravimetric analyzer (TGA) results showed that the Ca conversion ratio exceeded 95% at 700 °C mainly due to the significantly improved particle characteristics including the particle specific surface areas and the specific pore volumes. It was important to point out that highly active hydration products were not produced in significant quantities at the ambient hydration temperature.12,13 However, there were still some problems when the rapidly hydrated sorbent in circulating fluidized bed flue gas desulfurization (CFB-FGD) process was used. Some of the fine calcium-containing particles did not adhere to the fly ash surface during the sorbent preparation process, and some of the fine particles originally adhering to the fly ash surface were abraded during the desulfurization process due to collisions in the CFB reactor. These very active fine particles were difficult to be collected by the cyclone separator for recirculation, which greatly reduced the particle residence time and the Ca conversion ratio in the CFB reactor.4 Thus, further research on the abrasion of the rapidly hydrated sorbent is important to reduce the loss of the fine calcium-containing particles.
1. Introduction Removal of SO2 from flue gas emitted during fossil fuel combustion has been a worldwide concern. Various flue gas desulfurization (FGD) technologies have been developed to remove SO2 with calcium-based sorbents widely used because of their economic advantages. The wet FGD process1,2 is effective for a variety of fuels and has been widely commercialized to achieve SO2 removal rates in excess of 95%. However, this technology generates a large amount of wet solid waste, requires extensive wastewater treatment, and involves complicated and costly operations.3 A moderate temperature dry FGD process with a calciumbased sorbent operating at 600-800 °C using circulating fluidized bed (CFB) reactors has the advantages of low capital investment, low operating cost, and little water consumption.4,5 Hydrating fly ash and lime to improve the sorbent desulfurization ability is effective for making the sorbent for *To whom correspondence should be addressed. Telephone: þ86-1062781740. Fax: þ86-10-62770209. E-mail:
[email protected]. (1) Klingspor, J. S.; Bresowar, G. E. Am. Soc. Mech. Eng. 1995, 1, 3–9. (2) Trzepierczynska, I.; Gostomczyk, M. A. Environ. Prot. Eng. 1992, 18, 51. (3) Jiang, X.; Huang, L.; Wu, X. W.; Liu, Z. M. Tech. Equip. Environ. Pollut. Control 2003, 4-3, 82–84. (4) Zhang, J.; You, C. F.; Qi, H. Y.; Chen, C. H.; Xu, X. C. Environ. Sci. Technol. 2006, 40, 4300–4305. (5) Hou, B.; Qi, H. Y.; You, C. F. Energy Fuels 2005, 19, 73–78. (6) Jozewicz, W.; Chang, C. S.; Sedman, C. B. Environ. Prog. 1990, 9, 137–142. (7) Tsuchiai, H.; Ishizuka, T.; Nakamura, H. Ind. Eng. Chem. Res. 1996, 35, 2322–2326. (8) Fernandez, J.; Renedo, J.; Garea, A.; Viguri, J. A. I. Powder Technol. 1997, 94 (2), 133. (9) Renedo, M. J.; Fernandez, J. Ind. Eng. Chem. Res. 2002, 41, 2412– 2417. (10) Fernandez, J.; Renedo, M. J. Fuel 2004, 83 (45), 525. (11) Lee, K. T.; Mohamed, A. R.; Bhatia, S.; Chu, K H. Chem. Eng. J. 2005, 114, 171–177. r 2010 American Chemical Society
(12) Zhang, J.; Zhao, S. W.; You, C. F.; Qi, H. Y.; Chen, C. H. Ind. Eng. Chem. Res. 2007, 46, 5340–5345. (13) Zhang, J.; You, C. F.; Zhao, S. W.; Chen, C. H.; Qi, H. Y. Environ. Sci. Technol. 2008, 42, 1705–1710.
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Table 1. Material Compositions (wt %) lime fly ash
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
MnO
P2O5
LOI
2.23 45.87
0.38 39.28
0.15 4.06
92.7 4.43
0.28 0.28
0.05 0.83
0.03 0.12
0.01 0.03
0.14 0.65
3.76 1.05
The particle abrasion in CFB reactors is a complex process,14,15 and many researchers have investigated the particle abrasion.16-19 The direct measurements of the particle abrasion in CFB reactors at moderate temperature are difficult and complex. Yang et al.19,20 suggested sieving after calcination as a simple and useful method to study the particle abrasion at moderate temperature instead of direct measurements in CFB reactors. This investigation used the sieving method and the thermogravimetric analysis (TGA) to study the influence of fly ash particle size and fly ash particle modifications on the abrasion and desulfurization characteristics of the rapidly hydrated sorbent. The results illustrated the reason for the low Ca conversion ratio in actual applications and provided guidelines for improving the abrasion and desulfurization characteristics of the rapidly hydrated sorbent.
Figure 1. Particle size distributions of the lime slurry and the fly ash.
characteristics. Three kinds of optimized modification methods were used in this experiment including firing, acid, and alkali modification methods. The firing modification used a mixture of fly ash and Na2CO3 with a mass ratio of 10:17 calcined for 2 h at 850 °C. Then, 3 mol/ L NaOH solution was added to the calcined mixture with a liquid and solid ratio of 10 mL:1 g. The mixture was then exposed to air for 2 h at 55 °C and crystallized for 5 h at 100 °C. Then, the mixture was rinsed 10 times, filtered, dried, and heated in a muffle furnace for 1 h at 500 °C.22 The acid modification used 1 g of fly ash mixed with 15 mL of 1 mol/L NaOH solution. The mixture was stirred for 15 h at 90 °C, then rinsed 10 times, filtered, and dried.23 The alkali modification used 1 g of fly ash mixed with 2 mL of 2 mol/L H2SO4 solution. The mixture was stirred for 1 h at ambient temperature, then rinsed 10 times, filtered, and dried.24 2.3. Sorbent Preparation and Experimental Process. The fly ash particles were mixed with lime and water to produce the rapidly hydrated sorbent.26 The sorbent preparation steps are as follows. First, lime, fly ash, and water were mixed in a hydration mixer with stirring at ambient temperature to produce the sorbent slurry. The water/solid mass ratio was 1:1, and the lime to fly ash mass ratio was 1:4. The hydration time was 2 h. Then, the sorbent slurry was dried in an infrared desiccator to a water content less than 10%. The drying time was 1.5 h with a drying temperature of 150 °C. The sorbent was put on the sieve shaker with a 30 μm sieve and a sieving displacement of 1.5 mm which was determined by the CFB superficial velocity according to Yang et al.19,20 The fine particles collected below the sieve were weighed every few minutes. The Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) method was used to obtain the Ca content of the fine particles below the sieve. The sorbent abrasion, Ra, was represented by the percentage of Ca mass below the sieve to the original sorbent mass, which represented the Ca loss during the sieving experiment. mu Ra ¼ ηCa ð1Þ m0
2. Experimental Section 2.1. Materials. The lime used in the experiment was from the Shougang Building Materials Chemical Plant, and the fly ash was from the Beijing Shijingshan Power Plant. Their compositions measured by an ARL ADVANT XPþ type X-ray fluorescence spectrometer are listed in Table 1. Figure 1 shows the particle size distributions of the lime slurry and the fly ash, measured by a laser diffraction instrument (Malvern Mastersizer 2000). The lime slurry particles were all smaller than 10 μm. Since the sorbent was to be sieved through a 30 μm sieve, the fly ash was presieved through a 30 μm sieve to reduce the influence of fine fly ash particles smaller than 30 μm on the experiments. The average diameter of the presieved fly ash was 83.43 μm. 2.2. Fly Ash Modifications. As shown in Table 1, the main compositions of the fly ash were SiO2 and Al2O3, which contained active Si and Al locations inside the fly ash particles. However, the surfaces of the unmodified fly ash were smooth and compact with few highly active locations exposed on the surfaces. Si and Al can be changed into Si, Al-gelatin, and zeolite molecules by various modification methods which roughen the fly ash surfaces and increase the specific surface areas and specific pore volumes of the fly ash,21-25 then influence the rapidly hydrated sorbent abrasion and desulfurization (14) Chen, Y. C.; Shyh, J. H. Powder Technol. 2002, 127, 185–195. (15) Scala, F; Montagnaro, F.; Salatino, P. Energy Fuels 2007, 21, 2566–2572. (16) Scala, F.; Cammarota, A.; Chirone, R.; Salatino, P. AIChE J. 1997, 43 (2), 363–373. (17) Chen, Y. C.; Kuann, W. H.; Shyh, J. H. J. Hazard. Mater. 2000, B80, 119–133. (18) Scala, F.; Montagnaro, F.; Salatino, P. Can. J. Chem. Eng. 2008, 28, 347–355. (19) Lv, J. F.; Yang, H. R.; Zhang, J. S.; Liu, Q.; Li, D. X.; Yue, G. X.; Yu, L.; Zhang, M.; Jiang, X. G. J. Combust. Sci. Technol. 2003, 5, 387– 390. (20) Wang, J. W.; Zhao, X. M.; Li, S. H.; Yang, H. R.; Lv, J. F.; Yue, G. X. J. Chem. Ind. Eng. (China) 2007, 58, 739–744. (21) Renedo, M. J.; Irabien, J. A.; Fernandez, J.; Garea, A. Chem. Eng. Commun. 2000, 182, 69–80. (22) Li, F. W.; Wei, X. X.; Ma, S. J.; Zhang, D. J.; Zhai, Y. B. Chongqing Environ. Sci. 2003, 25, 25–28. (23) Wang, Y. L.; Li, J.; Ji, H.; Li, Y. L.; Liang, W. J.; Jin, Y. Q. J. China Coal Soc. 2007, 4, 437–440. (24) Yang, L. F.; Zhai, J. P.; Zheng, B.; Sheng, G. H. Fly Ash Compr. Util. 2006, 3, 18–20. (25) Huang, Q.; Ji, W. Y.; Chen, D. W. J. Shanghai Inst. Technol. 2008, 3, 71–75.
where mu represents the mass of the fine particles collected below the sieve, mo represents the original mass of the sorbent, and ηCa is the Ca content in the fine particles collected below the sieve. The desulfurization abilities of various samples were tested in a TGA. The Ca loss during the sieving resulted in the decreased (26) Hou, B.; Qi, H. Y.; You, C. F.; Xu, X. C. J. Tsinghua Univ. (Sci. Technol.) 2004, 44, 1571–1574.
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desulfurization ability. The desulfurization ability of the sieved sorbent collected above the sieve was used to represent the sorbent desulfurization ability in actual applications. The TGA tests used about 10 mg of sorbent in the TGA preheated to 750 °C. After the weight became stable, the reaction gas was introduced into the TGA at a gas flow rate of 100 mL/min for a reaction time of 80 min. The reaction gas consisted of SO2 (2000 ppm), O2 (5%), and N2 (balance gas). CO2 was not added to the reaction gas because CO2 does not obviously affect the desulfurization reaction product at 750 °C.12,26 Hou26 found that almost all the desulfurization reaction product would be CaSO4 for the same conditions. Therefore, the product was assumed to be CaSO4 to calculate the sorbent desulfurization ability. The chemical reaction was considered to be CaOðsolidÞ þ SO2ðgasÞ þ 1=O2ðgasÞ f CaSO4ðsolidÞ The sorbent desulfurization ability, Rd (mg/g), was represented by the mass of SO2 absorbed per gram of sorbent. ðmi -m0 Þ 64 Rd ¼ 1000 ð2Þ 80 m0 where mi represents the sorbent mass at a given time (mg) and m0 represents the original sorbent mass (mg). In addition, a KYKY-2800 type scanning electron microscope (SEM) was used to observe the surface micrographs of the fly ash and the sorbent. The specific surface areas and the specific pore volumes were measured by nitrogen adsorption using an ASAP 2010 type Brunauer-Emmett-Teller (BET) analyzer. Figure 2. Particle size distributions of (a) the lime slurry, fly ash, and rapidly hydrated sorbent and (b) the sorbents before and after the sieving experiment.
3. Results and Discussion 3.1. Particle Size Distributions. The particle size distributions of various samples are shown in Figure 2. It can be seen from Figure 2a that there were two peaks in the particle size distribution for the rapidly hydrated sorbent. The smaller one was consistent for the lime slurry, which indicated that some fine calcium-containing particles did not adhere to the fly ash surface during the sorbent preparation process. The larger peak was located at the same particle size as the fly ash. Figure 2b shows the difference between the sorbents before and after sieving. The largest particles became smaller, and the volume of particles smaller than 10 μm was reduced which indicated that abrasion occurred with fine particles lost through the sieving process. 3.2. Abrasion of the Original Fly Ash Sorbent. The sorbent was calcined for 30 min at 750 °C before abrasion to simulate moderate temperature desulfurization conditions.19 The sieving results for the original fly ash sorbents before and after calcination are shown in Figure 3. The results were similar to existing abrasion results in the fluidized bed,14,18 which indicated that the sieving method was appropriate for investigating the sorbent abrasion. In addition, the sieving experiment results were quite repeatable. Ra increased with the sieving time, which indicated that the loss of fine calciumcontaining particles increased with the sieving time. Ra increased rapidly at first and then slowly. The abrasion rates shown in Figure 4 were obtained by smoothing the curves in Figure 3 to calculate the derivatives. The initial abrasion rates were quite high with the abrasion rates decreasing rapidly with the sieving time. After about 20 min, the abrasion rates decreased more slowly toward a constant rate as the sorbent abrasion approached steady state. Note that the average residence time of particles in a CFB reactor is longer than 2 h.27 Therefore, the experimental time was then
Figure 3. Abrasion of the sorbents before and after calcination.
Figure 4. Abrasion rate of the sorbents before and after calcination.
set to be 60 min to further investigate the abrasion characteristics of the rapidly hydrated sorbent. The Ra of the calcined sorbent was higher than that of the uncalcined sorbent due to about 3% water content in the
(27) Yang, H. R.; Xiao, X. B.; Wirsum, M.; Yue, G. X.; Fett, F. N. Coal Convers. 2002, 25 (3), 59–64.
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Figure 7. Desulfurization abilities of the sorbents made from fly ash with various particle sizes.
Figure 5. Desulfurization abilities of various samples after calcination.
Figure 8. Abrasion of sorbents made from the original and modified fly ashes.
Figure 6. Abrasion of sorbents made from fly ash with various particle sizes.
uncalcined sorbent.12 When the sorbent was calcined, cracks appeared between the fly ash and the fine calcium-containing particles due to the different thermal expansion rates and the water loss, which resulted in the fine calcium-containing particles falling off the fly ash surfaces. Thus, for the sieving experiments, the unsieved sorbent was calcined at 750 °C for 30 min and then sieved for 60 min. The desulfurization ability of the calcined sorbent shown in Figure 5 was 178 mg/g before sieving and 127 mg/g after sieving, a reduction of 28%. The desulfurization ability of the fine particles collected under the sieve was 539 mg/g, which was three times that of the calcined sorbent before sieving. This high desulfurization ability was due to the fine calcium-containing particles that did not adhere to the fly ash or fell off the fly ash surface due to collisions which then passed through the sieve for collection. The loss of these high calcium-containing particles greatly reduced the Ca content of the sorbent which resulted in the reduced sorbent desulfurization ability. Thus, it is important to reduce the loss of the high calcium-containing particles. 3.3. Influence of Particle Size. The fly ash was sieved into three parts with average particle sizes of 74, 118, and 182 μm. Figure 6 shows the experimental abrasion results for the rapidly hydrated sorbent made from the fly ash with various particle sizes. Figure 7 shows the desulfurization abilities for the sorbents made from the fly ash with various particle sizes. Ra decreased from 7.35% to 3.16% as the fly ash particle size increased from 74 to 182 μm, indicating that the sorbent abrasion decreased with increasing particle size. The main reason may lie in that for the spherical fly ash particles, larger particles had smaller surface curvatures which improved the
Figure 9. Desulfurization abilities of sorbents made from the original and modified fly ashes.
adhesion of the fine calcium-containing particles which were in the shape of a flat sheet to the fly ash particle surfaces.14 Rd of the unsieved sorbents decreased with increasing fly ash particle diameter, while Rd of the sieved sorbents increased from 124 to 139 mg/g as the fly ash particle size increased from 74 to 182 μm. Although the unsieved sorbent made from the largest fly ash particles had lower desulfurization ability, those particles had less loss of the fine calcium-containing particles in the abrasion process. Therefore, the sieved sorbent made from the largest fly ash particles had a higher Ca content and the lowest loss of desulfurization ability. Therefore, in actual CFB-FGD processes, larger fly ash particles would provide better adhesion for the fine calciumcontaining particles which would reduce the loss of the fine calcium-containing particles, increase the residence time of the fine calcium-containing particles, and improve the sorbent sulfur removal ability. 1685
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Figure 10. Typical micrographs of various fly ash particles (a) Original fly ash particles (b) Firing modified particles (c) Acid modified particles.
surface areas and specific pore volumes had higher desulfurization abilities. Table 2 shows that the specific surface areas and specific pore volumes of the sorbents made from modified fly ashes were higher than those of the sorbent made from unmodified fly ash. In addition, the X-ray analyzer measurements showed that the Na content was increased on the surfaces of the fly ash particles modified by firing or alkali modification. The increased Na on the fly ash surfaces would also increase the desulfurization abilities of the modified fly ash sorbents because the Na compounds react with SO2.
Table 2. Particles Specific Surface Areas and Specific Pore Volumes unmodified fly ash sorbent
modified fly ash sorbents acid
specific surface area, m2/g specific pore volume, cm3/g
5.60 0.03911
alkali
firing
5.85 8.66 5.60 0.04536 0.05859 0.03911
3.4. Influence of Fly Ash Modification. The abrasion results for the sorbents made from various kinds of modified fly ashes are shown in Figure 8. Figure 9 shows the desulfurization abilities of the unsieved sorbents and sieved sorbents made from various modified fly ashes. Figure 10 shows typical micrographs of the fly ash particles modified by firing and by acid. The specific surface areas and specific pore volumes of the sorbents are listed in Table 2. The results in Figure 8 show that Ra of the sorbents made from the modified fly ashes were lower than that of the sorbent made from unmodified fly ash, indicating that the modifications enhanced the adhesion of the fine particles to the fly ash surfaces. Figure 10 shows that the surface of the unmodified fly ash was smooth and compact but the fly ash surface modified by acid had many large holes due to the acid corrosion, as was observed by Wang et al,24 enhancing the adhesion of the acid modified fly ash. Because the zeolite structure including Si and Al was produced during firing modification process,23 the surfaces of the firing modified fly ash particles were rough, loosen, and porous, but the holes were relatively small. This surface structure did not effectively enhance the fine particle adhesion on the fly ash, so the abrasion of sorbent made from firing modified fly ash particles was greater than that of sorbent made from acid modified particles. As shown in Figure 9, the desulfurization abilities of the sorbents made from the modified fly ashes were higher than those of the sorbent made from the unmodified fly ash. For example, the desulfurization abilities of the sorbents made from the firing modified fly ash before and after sieving were 26.3% and 27.7% higher than those of the sorbent made from the unmodified fly ash. Al-Shawabkeh28 and Fernandez29 indicated that sorbent with larger specific
4. Conclusions The abrasion and desulfurization characteristics of rapidly hydrated sorbent were studied for various particle sizes and modification methods. The main conclusions include the following. (1) Some of the fine calcium-containing particles did not adhere to the fly ash surface during the sorbent preparation process while some of the fine calcium-containing particles fell off the fly ash surfaces during the sieving process, which greatly reduced the sorbent Ca content. The Ca lost because of collisions between sorbent particles and between the particles and the sieve reduced the desulfurization ability of the rapidly hydrated sorbent. (2) The rapidly hydrated sorbent made from larger fly ash particles had lower abrasion ratios and retained higher desulfurization ability after abrasion experiment. Thus, in actual CFB-FGD processes, larger fly ash particles should be used to prepare the rapidly hydrated sorbent to increase the residence time of the fine calciumcontaining particles and the Ca conversion ratio. (3) The surfaces of the modified fly ashes were rough and porous, providing larger specific surface areas and specific pore volumes for the sorbents than the unmodified fly ash. These characteristics reduced the abrasion of the rapidly hydrated sorbent and increased the sorbent desulfurization ability. Therefore, rough and porous fly ash surfaces are appropriate and larger specific surface areas and specific pore volumes could reduce the abrasion and enhance the desulfurization ability of the rapidly hydrated sorbent. Acknowledgment. This research was supported by the Special Funds for Major State Basic Research Projects (No. 2006CB200305).
(28) Al-Shawabkeh, A.; Matsuda, H. Can. J. Chem. Eng. 1995, 73, 678–685. (29) Fernandez, J.; Renedo, M. J. Energy Fuels 2003, 17, 1330–1337.
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