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An experimental study on the primary fragmentation and attrition of limestones in a fluidized bed
Fuel Processing Technology 91 (2010) 1119–1124
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
An experimental study on the primary fragmentation and attrition of limestones in a fluidized bed Xuan Yao, Hai Zhang, Hairui Yang ⁎, Qing Liu, Jinwei Wang, Guangxi Yue Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China
a r t i c l e
i n f o
Article history: Received 16 November 2009 Received in revised form 19 March 2010 Accepted 28 March 2010 Keywords: Fluidized bed Limestone Fragmentation Attrition Calcination
a b s t r a c t In this paper, an experimental study on the primary fragmentation and attrition of 5 limestones in a fluidized bed was conducted. The intensity of fragmentation and attrition were measured in the same apparatus but at different fluidizing velocities. It was found that the averaged size of the particles decreased by about 10–20% during the fragmentation process. The important factors for particle comminution include limestone types, heating rate, calcination condition and ambient CO2 concentration. Fragmentation mainly occurred in the first a few minutes in the fluidized bed and it was more intense than that in the muffle furnace at the same temperature. The original size effect was ambiguous, depending on the limestone type. The comminution caused by attrition mainly occurred during calcination process rather than sulphation process. The sulphation process was fragmentation and attrition resisted. The attrition rate of sulphate was similar to that of lime in trend, decaying exponentially with time, but was one-magnitude-order smaller than that of lime. Present experimental results indicate that fragmentation mechanism of the limestone is dominated by CO2 release instead of thermal stress. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Limestone is commonly used as the desulphurization sorbent in the fluidized bed (FB) and circulating fluidized bed (CFB) boiler to reduce SO2 emission. After the limestone particles are added into the furnace, together with chemical reactions of calcination and sulphation, a remarkable reduction of particle size called comminution or the size evolution of the limestone, occurs due to the synergetic effect of fragmentation and attrition [1–7]. Based on the previous studies, fragmentation is often classified into the primary and the secondary ones [6]. The primary one refers to the generation of fragments, either coarse or fine, within several minutes after the injection of limestone particles in the hot furnace [4,6,8,9]. This process often occurs in the dense bed or in the splashing zone of the FB or CFB combustors. The secondary fragmentation refers to the generation of fragments mostly from high-velocity collisions against the bed material or reactor walls and internals. It could happen in the air distributor region, the exit region of the riser and the cyclone, producing rather coarse fragments. Attrition refers to the generation of fine particles by abrasion, depending on the resistance of the bed particles to surfaces wear. The size evolution caused by fragmentation and attrition affects not only the properties of limestone and its products, such as density and porous structure that are strongly coupled with the calcination and
⁎ Corresponding author. Tel.: + 86 10 62773384. E-mail address: [email protected] (H. Yang). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.03.025
sulphation process, but also the material balance, e.g., distribution of solid suspension density in the bed. Thus, they could play an important role in desulphurization efficiency, fluid dynamics, thereby heat transfer and combustion in the furnace [10]. Especially, for a CFB boiler, in which material balance is of particular importance against bubbling FB, the influence of the primary fragmentation and attrition of limestone could be a major concern [4,6,10]. Previous studies found that fragmentation and attrition of the limestone are influenced by limestone size and other properties [2,4,7,11–14], bed temperature[7], heating rate[5,14], duration[3–7], fluidizing gas velocity[6,8,9] etc. For a CFB boiler, they are also strongly influenced by the solid circulation flow rate [2,3,8] and the inventory of inert bed material[13–15]. Moreover, the size evolution is coupled with the progress of calcination and sulphation [2,4,8,15]. Usually, fragmentation and attrition are severe in the beginning as limestone particles are added into the furnace, and become weaker and remain at low intensity after a certain period. It was noted that sulphation decreased the attrition rate of sorbents [4], and the phenomenon can be attributed to the harder surface [2] or the fill of mechanical cracks [2,4]. However, the main factors and the associated mechanisms have not been well clarified. In this paper, a set of experiments on the primary fragmentation and attrition of 5 different limestones under different size group, heating rate, calcination and sulphation atmosphere were conducted. The pre-calcined and pre-sulphatized samples were also tested. The experimental results and analyses would provide additional information to understand the size evolution process of limestone particle in a fluidized bed.
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2. Experimental Fig. 1 shows the bench-scaled fluidized bed reactor used in this study. The reactor was a round tube made of silica glass, with inner diameter of 54 mm and height of 800 mm. It was electrically heated and the main section could be maintained at a desired temperature with a deviation of ±5 °C. An air distributor was placed near the middle of the reactor. The section below the distributor was used to preheat the input gases. During the fragmentation experiments, some inert bed material, i.e., quartz sand (90–125 μm) was pre-laid on the air distributor. The limestone was added in a batch (about 20 g) and then mixed with the quartz sand. The static height of the bed material was kept at about 40 mm. After the limestone was added into the reactor for a certain time, all materials including the limestone fragments and the bed material were sucked out and collected by a solid separation system. Based on the causes of the particle comminution discussed in the Introduction section, in this study, the primary fragmentation was proposed to be measured at specific superficial fluidizing velocity Ug. It was found that when a rather low value Ug was set, e.g., Ug = 0.1– 0.2 m/s at the reactor temperature of 850 °C, the attrition and secondary fragmentation could be minimized, and thus all fragments could be regarded as the results of the primary fragmentation. After the FB was operated under such condition for a certain period, the fragments of the tested samples and bed materials were vacuumed into a solid collector. Then, the bed material was separated from the collected mixture by using a sieve shaker, and the size distribution information of the fragments was obtained after the fragments were sieved and weighted. The mass of the fragments that fall into the size range of the inert bed material (90–125 μm) was estimated by calcination loss in the muffle furnace. Similar to the previous study [16], the coefficient of average particle size variation Fd was used to characterize the degree of particle size change, and expressed as: Fd = df = do = ∑ðXi di Þ = do
ð1Þ
df is the final average particle diameter; do is the average size of original sample particles; i is the size interval number of the sample particles; Xi is the mass fraction of the fragments of sample particles in size interval i; and di is the corresponding average particles size of the fragments in size interval i. For easier explanation, another parameter fragmentation intensity coefficient (FIC), which was defined as FIC=1 − Fd, was introduced to describe the intensity of fragmentation.
To measure the attrition rate by abrasion, a rather high Ug was used, e.g., Ug = 0.5 m/s, such that fines with diameter less than 80 μm generated by attrition could be elutriated and then collected by the external cyclone. Before the addition of the testing sample, certain amount of bed material of quartz sand (∼ 160 g, 250–300 μm) was pre-loaded in the reactor. The quartz sand was abrasion resisted and remained in the reactor under the preset Ug. The bed was put into operation under preset Ug and bed temperature for a certain period until it was steady. Then, the testing sample in amount of 50 g was put into the bed. By collecting and weighting the collected fine particles in the cyclone every 10 min, the attrition rate Rs was obtained, and thereby attrition rate constant Ka could be calculated. Experiment is conducted until attrition rate became steady value. Based on previous study [1], the Rs is defined as
Rs = −
1 dm m dt
ð2Þ
m is the mass of sample particles within a given size range at a given moment, and dm/dt is the mass variation rate vs time of the sample. The Ka is expressed as:
Ka ¼
Rs Ug −Umf
ð3Þ
Ug is the superficial fluidizing velocity, m/s; and Umf is the minimum fluidization velocity of the sample, m/s. (Ug − Umf) is the excess fluidizing velocity. In this paper, the attrition of limestone calcination product and sulphate were studied. The influence of the chemical reactions, including calcination, sulphation reaction and simultaneous calcination and sulphation was assessed. Calcination experiments were conducted in the FB apparatus as described above and the muffle furnace at temperature 850 °C. For the muffle furnace experiments, the sample particles (limestone) were placed in a ceramic container, as used in the standard volatile content measurement. The sample filled only a small portion of the container thus the top layer was exposed to the air at ambient pressure. Then the container and sample is heated in the muffle for 30 min. Five kinds of limestones (types A–E) were studied in this paper. Their CaCO3 and MgCO3 compositions, derived from calcium and magnesium ones measured by ICP (Inductively Coupled Plasma) are shown in Table 1. Each kind of limestone was tested with two size groups of 200–400 μm and 400–600 μm respectively.
Fig. 1. Schematic of the bench-scale fluidized bed reactor.
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Table 1 The composition of the limestone samples (% in mass). Sample
Origin
Ca
Mg
CaCO3
MgCO3
Others
A B C D E
Jingxi France Turkey Shandong Guizhou
36.79 34.99 37.18 37.28 29.23
0.24 0.19 0.22 0.42 4.76
91.93 87.43 92.95 93.20 73.03
0.94 0.74 0.77 1.64 18.41
7.13 11.83 6.28 5.16 8.55
3. Results and discussion 3.1. Effect of limestone type and heating rate on primary fragmentation Figs. 2 and 3 show the cumulative size density distribution of Limestone A with two different original sizes after calcination at a temperature of 850 °C in a FB and a muffle furnace respectively. During FB calcination, before the limestone was added, CO2 was used as the fluidizing gas to suppress the calcination. It was found that right after the addition of limestone, the reactor temperature dropped in a certain degree and then rose gradually. When the bed temperature returned to the preset value, the fluidizing gas was switched to air, and then the calcination process occurred. At the same time, particles experienced a size change. As shown in Table 2, for all 5 limestones, after calcination the particle size decreases. However, the fragmentation intensity, expressed in FIC, is different in the FB from that in the muffle furnace. In the FB, fragmentation is much more intensive, and the calcined fragments are much finer. As shown in Table 2, in the muffle furnace, FIC = 0.01–0.08, while in the FB, FIC = 0.11–0.22. From Fig. 2, it can be seen for Limestone A, the cumulative size distribution curves of the original sample and products in the muffle furnace are close, obviously differing from those in the FB.
Fig. 3. Cumulative undersize distribution of the particle sizes for Limestone A (400–600 μm).
change rate of the average particle size is close for particles with different sizes. As pointed out by Scala et al. [8], the micro-structure of the particles could be very different for different limestones and could introduce a significant difference on the fragmentation and attrition. We think the micro-structure effect, combining with other effects associated with particles size such as heating rate, could be the reason that makes the effect of the original size ambiguous. More details will be studied in further research. 3.3. Effect of calcination time on fragmentation
3.2. Effect of original particle size Fig. 4 shows the change of the average size at different conditions of the limestone. It can be seen, for Limestone A, with a larger original particle size, the fragmentation in the muffle furnace becomes more detectable. However, as shown in Table 2, the above result is not always general. For Limestones C and E, the change of the average particle size for larger particles is less than that for smaller particles, contradicting that for Limestone A, while for Limestones B and D, the
It was found in Fig. 2 and Fig. 3 that the size distributions after 10 min, 20 min and 30 min calcination for the Limestone A are similar, indicating that the comminution of limestone (a large portion might be converted into lime) nearly stops after 10 min. Due to the experimental difficulty, no experiment was conducted within the time less than 10 min. Present experimental results are accorded with what was found by Karidio, Scala and Benedetto [1,4,7]. The results imply that in a FBC/CFB boiler, fragmentation is only needed to be considered for the newly-added limestone/lime particles. 3.4. Effect of the calcination atmosphere Fig. 5 shows the results of fragment size distribution after 30 min calcination respectively under air and CO2 atmosphere in the FB reactor for Limestone E with original size of 400–600 μm. The bed temperature Tbed = 850 °C and Ug = 0.15 m/s. Under air atmosphere, Table 2 Fragmentation intensity coefficients of the limestone samples. Sample
Original size (μm) Range
Average
In muffle
In FB
A
200–400 400–600 200–400 400–600 200–400 400–600 200–400 400–600 200–400 400–600
309 519 292 534 300 527 300 535 308 536
303/0.02 478/0.08 288/0.01 520/0.03 275/0.08 515/0.02 288/0.04 518/0.03 287/0.07 529/0.01
244/0.21 417/0.20 257/0.12 457/0.14 231/0.23 458/0.13 245/0.18 473/0.12 253/0.18 475/0.11
B C D E Fig. 2. Cumulative undersize distribution of the particle sizes for Limestone A (200–400 μm).
Fragment Size(μm)/ FIC (30 min)
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Fig. 4. Average size of the fragments under different test conditions for Limestone A.
fragmentation was intense and many fine fragments were generated. Under pure CO2 atmosphere, no obvious size change was found compared with air atmosphere. Generally, three mechanisms are attributed to the limestone fragmentation: thermal stress at high temperature, inner high pressure caused by the organics or water evaporation and the CO2 release during calcination [5]. Calcination is a chemical reaction process which will produce CO2 and lime (CaO). With the existence of high CO2 concentration the environment will suppress the calcination reaction [17]. Previous studies found that at a certain temperature, only if the partial pressure of CO2 in the environment is lower than the CO2 pressure as the calcination reaction is at chemical equilibrium, the calcination reaction occurs [9,17]. As shown in Fig. 5, when CO2 was used as fluidizing gas, the size change of the limestone was insignificant. The results indicate that fragmentation of limestone is dominated by the calcination reaction rather than the thermal stress found in study [18].
Fig. 6. Temporal variation of attrition rate constant of lime and its sulphatized product in a fluidized bed (limestone A, particle size is based on lime).
3.5. Effect of the sulphation on attrition of lime The limestone is calcined into lime (CaO) after it is added into the hot furnace of a FB. Then, lime reacts with SO2 into solid CaSO3 or CaSO4. It is important to know the comminution characteristic of the lime and its sulphatized products in the bed. Based on our discussion in the above section, other than the calcination with rapid gas release from the particles, the fragmentation is trivial. Thus, the comminution of lime and its sulphatized products is mainly attributed to the attrition by abrasion [6]. In our study, we used muffle furnace (850 °C) to calcinate fresh limestone to produce lime. Calcined products were sieved in to different size ranges. We chose particles that fall into the size ranges of 200–400 μm and 400–600 μm as sulphation reactant and lime attrition sample. Sulphatized products are produced in the FB reactor as a fixed-bed under temperature 850 °C and SO2 atmosphere. Then we tested the attrition of the lime and its sulphatized products in the FB with air fluidizing as we described in the Experimental section. Fig. 6 shows the temporal variation of Ka for lime with two origin particle sizes (200–400 μm and 400–600 μm) and its sulphatized products of Limestone A. It can be found that for both samples, Ka decays exponentially with time, and after 90 min it reaches nearly a constant (K∞). In the beginning, the difference of Ka, between lime and sulphatized products is obvious at a given velocity. And K∞ of lime is a few times larger than that of the sulphatized products. In this study, the value of K∞ was defined as the value at time 90 min. Other samples have similar tendency and the K∞s are summarized in Table 3. It can be seen that K∞ varies with the limestone type and original particle size. For the same kind of sample, finer particles have a larger K∞, and the difference in degree depends on the original limestone type. Typically, K∞ is of about 10− 5–10− 6 for lime, and about 10− 6–10− 7 for the sulphatized products. The results show that the sulphate is more attrition resistant, which is consistent with those found by Scala et al. [6,8]. The reason as analyzed by Scala et al. [6,8] is the presence of the layer of CaSO4, on the surface of the sulphatized particles. 3.6. Effect of simultaneous calcination and sulphation on attrition
Fig. 5. Cumulative undersize distributions of the particle sizes for Limestone E (400– 600 μm) under different calcination atmospheres.
When limestone is added into the hot furnace with SO 2 atmosphere, calcination and sulphation reactions occur simultaneously. The lime conversion rates measured in FB reactor (Fig.7) confirm that sulphation is a slow process. In about 20 min, the conversion rate
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Table 3 Attrition rates of calcined and sulphatized limestones. Sample
A B C D E Ref[6] Ref[15]
Original size distribution (μm)
K∞
Mass of quartz/ mass of sample
Fluidizing gas velocity
Calcined product
Sulphatized product
200–400 400–600 200–400 400–600 200–400 400–600 200–400 400–600 200–400 400–600
1.79 × 10− 5 1.14 × 10− 5 5.86 × 10− 6 1.94 × 10− 6 6.47 × 10− 6 3.45 × 10− 7 2.14 × 10− 6 7.02 × 10− 7 6.02 × 10− 6 1.58 × 10− 6
3.17 × 10− 6 1.48 × 10− 6 2.58 × 10− 6 4.76 × 10− 7 2.07 × 10− 6 3.01 × 10− 7 6.50 × 10− 7 3.79 × 10− 7 4.14 × 10− 7 1.56 × 10− 7
3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
425–600 400–600
1.18 × 10− 4 1.40 × 10− 6
4.91 × 10− 6
20.0 9.0
0.75 3.00
reaches a constant for a given SO2 concentration. The final constant is around 10%, much smaller than the unity, confirming that sulphate product covers the particle surface. Fig. 8 further shows that the final constant is a variable depending on the particle size and the ambient SO2 concentration. Since the time for significant attrition and the sulphation is of the same order, it is important to examine the attrition under simultaneous calcination and sulphation. Thus, comparison experiments were conducted on the attrition of fresh limestone fluidized with dry air and a mixture of air and SO2 (2000 ppm) respectively. The experimental condition and procedure were the same as the ones used for the above lime/limestone attrition experiments. Figs. 9 and 10 show the attrition results of the Limestone A with two size ranges. The results of the sulphatized products were also shown in the figure. Though fragmentation occurs in these processes, the amount of the generated and collected fine particles is small compared to the attrition part, and its influence on the attrition experiments is expected to be negligible. It can be seen that in the first 15 min, the attrition rate Rs of the fresh limestone under air and air/ SO2 atmosphere are nearly the same and much higher than that of the fully sulphatized products. The reason is that during the beginning
Fig. 7. Temporal variations of CaO conversion at different SO2 concentrations in FB for Limestone A.
Fig. 8. Variations of maximum CaO conversion with ambient SO2 concentration in FB for Limestone A.
period, the fresh CaSO4 particle produced by the chemical reaction is not tightly adhered on the particle surface. While, after a certain period, e.g., 20 min, the layer of sulphatized products will cover the surface of the sample fluidized by air/SO2 mixture, and thus has a smaller Rs gradually approaching to that of sulphate products.
4. Conclusions Fragmentation and attrition occur as the limestone is added into the FB or CFB boiler, introducing particle comminution and thereby influencing the performance of the boiler. In this paper, a new and rather simple experimental measurement method was proposed and then validated. Experimental results confirmed that fragmentation and attrition could be significantly affected by limestone types, original size, heating rate, calcination condition and ambient CO2 concentration. The average size of the particles decreased about 10–20% during fragmentation process. The
Fig. 9. Temporal variations of attrition rate for Limestone A (200–400 μm) in a fluidized bed.
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References
Fig. 10. Temporal variations of attrition rate for Limestone A (400–600 μm) in a fluidized bed.
primary fragmentation mainly occurred in the first a few minutes after the limestone was added into the hot FB. Fragmentation of limestone in the FB was more intense than that in the muffle furnace at the same temperature. The original size effect is ambiguous, depending on the limestone type. The comminution caused by attrition mainly occurred during calcination rather than sulphation. The sulphate is fragment and attrition resistant. It is further confirmed that attrition rate was abated by the formation of CaSO4 over the limestone/lime particle surface. Given sulphation is a rather slow process, in the practical application where calcination and sulphation simultaneously happen, it plays a minor role on attrition rate of a limestone though the attrition rate of sulphate much smaller than that of lime. In addition, the present experimental data clarified primary fragmentation of limestone is dominated by CO2 release instead of thermal stress. More studies under CFB condition are suggested in the future. Acknowledgements Financial supports of this work by High Technology R&D (863) (2009AA05Z302) and the National 11th-Five Year Plan (No. 2006BAD07A14-4) are gratefully acknowledged.
[1] D. Merrick, J. Highley, Particle size reduction and elutriation in a fluidized bed process, AICHE Symposium Series, 137, 1974, pp. 366–378. [2] R.R. Chandran, J.N. Duqum, Attrition characteristics relevant for fluidized-bed combustion, in: J.R. Grace, L.W. Shemile, M.A. Bergougnou (Eds.), Fluidization VI, Engineering Foundation, New York, 1989, pp. 571–580. [3] M.F. Couturier, I. Karidio, F.R. Steward, Study on the rate of breakage of various Canadian limestones in a circulating transport reactor, in: A.A. Avidan (Ed.), Circulating Fluidized Bed Technology IV, American Institute of Chemical Engineering, New York, 1993, pp. 672–680. [4] I. Karidio, Sulphation and breakage characterization of various Canadian limestones, University of New Brunswick, Canada, PhD Thesis, 1994. [5] N. Hu, A.W. Scaroni, Fragmentation of calcium-based sorbents under high heating rate, short residence time conditions, Fuel 74 (1995) 374–382. [6] F. Scala, A. Cammarota, R. Chirone, et al., Comminution of limestone during batch fluidized-bed calcination and sulfation, AIChE Journal 43 (1997) 363–373. [7] A. Benedetto, P. Salatino, Modelling attrition of limestone during calcination and sulfation in a fluidized bed reactor, Powder Technology 95 (1998) 119–128. [8] F. Scala, P. Bareschino, R. Boerefijn, et al., Attrition of sorbents during fluidized bed calcination and sulphation, Powder Technology 107 (2000) 153–167. [9] F. Scala, F. Montagnaro, P. Salatino, Sulphation of limestones in a fluidized bed combustor: the relationship between particle attrition and microstructure, Canadian Journal of Chemical Engineering 86 (2008) 347–355. [10] G. Yue, J. Lu, H. Zhang, et al., Design theory of circulating fluidized bed boilers. Proceedings of 18th International Conference on Fluidized Bed Combustion, Toronto, 2005, pp. 1–12. [11] J. Wang, S. Li, H. Yang, et al., Study of the explosive cracking and wear characteristics of limestone, Journal of Engineering for Thermal Energy & Power 21 (2006) 366–369 (In Chinese). [12] F. Scala, P. Salatino, The influence of sorbent properties and reaction conditions on attrition of limestone by impact load in fluidized beds, The Proceedings of the 20th FBC Conference, Xi'an China, 2009, pp. 486–491. [13] F. Montagnaro, P. Salatino, F. Scala, et al., Sorbent inventory and particle size distribution in air-blown circulating fluidized bed combustors: the influence of particle attrition and fragmentation, The Proceedings of the 20th FBC Conference, Xi'an China, 2009, pp. 966–971. [14] F. Scala, F. Montagnaro, P. Salatino, Attrition of limestone by impact loading in fluidized beds, Energy and Fuels 21 (2007) 2566–2572. [15] P. Bareschino, A. Marzocchella, P. Salatino, et al., Segregation and attrition of polydisperse solids in a circulating fluidized bed, 8th International Conference on Circulating Fluidized Beds, Hangzhou, 2005, pp. 224–230. [16] H. Zhang, K. Cen, J. Yan, et al., The fragmentation of coal particles during the coal combustion in a fluidized bed, Fuel 83 (2002) 1835–1840. [17] G. Hu, Kim Dam-Johansen, Stig Wedel, et al., Review of the direct sulphation reaction of limestone, Progress in Energy and Combustion Science 32 (2006) 386–407. [18] J. Saastamoinen, T. Pikkarainen, A. Tourunen, et al., Model of fragmentation of limestone particles during thermal shock and calcination in fluidised beds, Powder Technology 187 (2008) 244–251.
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
An experimental study on the primary fragmentation and attrition of limestones in a fluidized bed Xuan Yao, Hai Zhang, Hairui Yang ⁎, Qing Liu, Jinwei Wang, Guangxi Yue Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China
a r t i c l e
i n f o
Article history: Received 16 November 2009 Received in revised form 19 March 2010 Accepted 28 March 2010 Keywords: Fluidized bed Limestone Fragmentation Attrition Calcination
a b s t r a c t In this paper, an experimental study on the primary fragmentation and attrition of 5 limestones in a fluidized bed was conducted. The intensity of fragmentation and attrition were measured in the same apparatus but at different fluidizing velocities. It was found that the averaged size of the particles decreased by about 10–20% during the fragmentation process. The important factors for particle comminution include limestone types, heating rate, calcination condition and ambient CO2 concentration. Fragmentation mainly occurred in the first a few minutes in the fluidized bed and it was more intense than that in the muffle furnace at the same temperature. The original size effect was ambiguous, depending on the limestone type. The comminution caused by attrition mainly occurred during calcination process rather than sulphation process. The sulphation process was fragmentation and attrition resisted. The attrition rate of sulphate was similar to that of lime in trend, decaying exponentially with time, but was one-magnitude-order smaller than that of lime. Present experimental results indicate that fragmentation mechanism of the limestone is dominated by CO2 release instead of thermal stress. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Limestone is commonly used as the desulphurization sorbent in the fluidized bed (FB) and circulating fluidized bed (CFB) boiler to reduce SO2 emission. After the limestone particles are added into the furnace, together with chemical reactions of calcination and sulphation, a remarkable reduction of particle size called comminution or the size evolution of the limestone, occurs due to the synergetic effect of fragmentation and attrition [1–7]. Based on the previous studies, fragmentation is often classified into the primary and the secondary ones [6]. The primary one refers to the generation of fragments, either coarse or fine, within several minutes after the injection of limestone particles in the hot furnace [4,6,8,9]. This process often occurs in the dense bed or in the splashing zone of the FB or CFB combustors. The secondary fragmentation refers to the generation of fragments mostly from high-velocity collisions against the bed material or reactor walls and internals. It could happen in the air distributor region, the exit region of the riser and the cyclone, producing rather coarse fragments. Attrition refers to the generation of fine particles by abrasion, depending on the resistance of the bed particles to surfaces wear. The size evolution caused by fragmentation and attrition affects not only the properties of limestone and its products, such as density and porous structure that are strongly coupled with the calcination and
⁎ Corresponding author. Tel.: + 86 10 62773384. E-mail address: [email protected] (H. Yang). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.03.025
sulphation process, but also the material balance, e.g., distribution of solid suspension density in the bed. Thus, they could play an important role in desulphurization efficiency, fluid dynamics, thereby heat transfer and combustion in the furnace [10]. Especially, for a CFB boiler, in which material balance is of particular importance against bubbling FB, the influence of the primary fragmentation and attrition of limestone could be a major concern [4,6,10]. Previous studies found that fragmentation and attrition of the limestone are influenced by limestone size and other properties [2,4,7,11–14], bed temperature[7], heating rate[5,14], duration[3–7], fluidizing gas velocity[6,8,9] etc. For a CFB boiler, they are also strongly influenced by the solid circulation flow rate [2,3,8] and the inventory of inert bed material[13–15]. Moreover, the size evolution is coupled with the progress of calcination and sulphation [2,4,8,15]. Usually, fragmentation and attrition are severe in the beginning as limestone particles are added into the furnace, and become weaker and remain at low intensity after a certain period. It was noted that sulphation decreased the attrition rate of sorbents [4], and the phenomenon can be attributed to the harder surface [2] or the fill of mechanical cracks [2,4]. However, the main factors and the associated mechanisms have not been well clarified. In this paper, a set of experiments on the primary fragmentation and attrition of 5 different limestones under different size group, heating rate, calcination and sulphation atmosphere were conducted. The pre-calcined and pre-sulphatized samples were also tested. The experimental results and analyses would provide additional information to understand the size evolution process of limestone particle in a fluidized bed.
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2. Experimental Fig. 1 shows the bench-scaled fluidized bed reactor used in this study. The reactor was a round tube made of silica glass, with inner diameter of 54 mm and height of 800 mm. It was electrically heated and the main section could be maintained at a desired temperature with a deviation of ±5 °C. An air distributor was placed near the middle of the reactor. The section below the distributor was used to preheat the input gases. During the fragmentation experiments, some inert bed material, i.e., quartz sand (90–125 μm) was pre-laid on the air distributor. The limestone was added in a batch (about 20 g) and then mixed with the quartz sand. The static height of the bed material was kept at about 40 mm. After the limestone was added into the reactor for a certain time, all materials including the limestone fragments and the bed material were sucked out and collected by a solid separation system. Based on the causes of the particle comminution discussed in the Introduction section, in this study, the primary fragmentation was proposed to be measured at specific superficial fluidizing velocity Ug. It was found that when a rather low value Ug was set, e.g., Ug = 0.1– 0.2 m/s at the reactor temperature of 850 °C, the attrition and secondary fragmentation could be minimized, and thus all fragments could be regarded as the results of the primary fragmentation. After the FB was operated under such condition for a certain period, the fragments of the tested samples and bed materials were vacuumed into a solid collector. Then, the bed material was separated from the collected mixture by using a sieve shaker, and the size distribution information of the fragments was obtained after the fragments were sieved and weighted. The mass of the fragments that fall into the size range of the inert bed material (90–125 μm) was estimated by calcination loss in the muffle furnace. Similar to the previous study [16], the coefficient of average particle size variation Fd was used to characterize the degree of particle size change, and expressed as: Fd = df = do = ∑ðXi di Þ = do
ð1Þ
df is the final average particle diameter; do is the average size of original sample particles; i is the size interval number of the sample particles; Xi is the mass fraction of the fragments of sample particles in size interval i; and di is the corresponding average particles size of the fragments in size interval i. For easier explanation, another parameter fragmentation intensity coefficient (FIC), which was defined as FIC=1 − Fd, was introduced to describe the intensity of fragmentation.
To measure the attrition rate by abrasion, a rather high Ug was used, e.g., Ug = 0.5 m/s, such that fines with diameter less than 80 μm generated by attrition could be elutriated and then collected by the external cyclone. Before the addition of the testing sample, certain amount of bed material of quartz sand (∼ 160 g, 250–300 μm) was pre-loaded in the reactor. The quartz sand was abrasion resisted and remained in the reactor under the preset Ug. The bed was put into operation under preset Ug and bed temperature for a certain period until it was steady. Then, the testing sample in amount of 50 g was put into the bed. By collecting and weighting the collected fine particles in the cyclone every 10 min, the attrition rate Rs was obtained, and thereby attrition rate constant Ka could be calculated. Experiment is conducted until attrition rate became steady value. Based on previous study [1], the Rs is defined as
Rs = −
1 dm m dt
ð2Þ
m is the mass of sample particles within a given size range at a given moment, and dm/dt is the mass variation rate vs time of the sample. The Ka is expressed as:
Ka ¼
Rs Ug −Umf
ð3Þ
Ug is the superficial fluidizing velocity, m/s; and Umf is the minimum fluidization velocity of the sample, m/s. (Ug − Umf) is the excess fluidizing velocity. In this paper, the attrition of limestone calcination product and sulphate were studied. The influence of the chemical reactions, including calcination, sulphation reaction and simultaneous calcination and sulphation was assessed. Calcination experiments were conducted in the FB apparatus as described above and the muffle furnace at temperature 850 °C. For the muffle furnace experiments, the sample particles (limestone) were placed in a ceramic container, as used in the standard volatile content measurement. The sample filled only a small portion of the container thus the top layer was exposed to the air at ambient pressure. Then the container and sample is heated in the muffle for 30 min. Five kinds of limestones (types A–E) were studied in this paper. Their CaCO3 and MgCO3 compositions, derived from calcium and magnesium ones measured by ICP (Inductively Coupled Plasma) are shown in Table 1. Each kind of limestone was tested with two size groups of 200–400 μm and 400–600 μm respectively.
Fig. 1. Schematic of the bench-scale fluidized bed reactor.
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Table 1 The composition of the limestone samples (% in mass). Sample
Origin
Ca
Mg
CaCO3
MgCO3
Others
A B C D E
Jingxi France Turkey Shandong Guizhou
36.79 34.99 37.18 37.28 29.23
0.24 0.19 0.22 0.42 4.76
91.93 87.43 92.95 93.20 73.03
0.94 0.74 0.77 1.64 18.41
7.13 11.83 6.28 5.16 8.55
3. Results and discussion 3.1. Effect of limestone type and heating rate on primary fragmentation Figs. 2 and 3 show the cumulative size density distribution of Limestone A with two different original sizes after calcination at a temperature of 850 °C in a FB and a muffle furnace respectively. During FB calcination, before the limestone was added, CO2 was used as the fluidizing gas to suppress the calcination. It was found that right after the addition of limestone, the reactor temperature dropped in a certain degree and then rose gradually. When the bed temperature returned to the preset value, the fluidizing gas was switched to air, and then the calcination process occurred. At the same time, particles experienced a size change. As shown in Table 2, for all 5 limestones, after calcination the particle size decreases. However, the fragmentation intensity, expressed in FIC, is different in the FB from that in the muffle furnace. In the FB, fragmentation is much more intensive, and the calcined fragments are much finer. As shown in Table 2, in the muffle furnace, FIC = 0.01–0.08, while in the FB, FIC = 0.11–0.22. From Fig. 2, it can be seen for Limestone A, the cumulative size distribution curves of the original sample and products in the muffle furnace are close, obviously differing from those in the FB.
Fig. 3. Cumulative undersize distribution of the particle sizes for Limestone A (400–600 μm).
change rate of the average particle size is close for particles with different sizes. As pointed out by Scala et al. [8], the micro-structure of the particles could be very different for different limestones and could introduce a significant difference on the fragmentation and attrition. We think the micro-structure effect, combining with other effects associated with particles size such as heating rate, could be the reason that makes the effect of the original size ambiguous. More details will be studied in further research. 3.3. Effect of calcination time on fragmentation
3.2. Effect of original particle size Fig. 4 shows the change of the average size at different conditions of the limestone. It can be seen, for Limestone A, with a larger original particle size, the fragmentation in the muffle furnace becomes more detectable. However, as shown in Table 2, the above result is not always general. For Limestones C and E, the change of the average particle size for larger particles is less than that for smaller particles, contradicting that for Limestone A, while for Limestones B and D, the
It was found in Fig. 2 and Fig. 3 that the size distributions after 10 min, 20 min and 30 min calcination for the Limestone A are similar, indicating that the comminution of limestone (a large portion might be converted into lime) nearly stops after 10 min. Due to the experimental difficulty, no experiment was conducted within the time less than 10 min. Present experimental results are accorded with what was found by Karidio, Scala and Benedetto [1,4,7]. The results imply that in a FBC/CFB boiler, fragmentation is only needed to be considered for the newly-added limestone/lime particles. 3.4. Effect of the calcination atmosphere Fig. 5 shows the results of fragment size distribution after 30 min calcination respectively under air and CO2 atmosphere in the FB reactor for Limestone E with original size of 400–600 μm. The bed temperature Tbed = 850 °C and Ug = 0.15 m/s. Under air atmosphere, Table 2 Fragmentation intensity coefficients of the limestone samples. Sample
Original size (μm) Range
Average
In muffle
In FB
A
200–400 400–600 200–400 400–600 200–400 400–600 200–400 400–600 200–400 400–600
309 519 292 534 300 527 300 535 308 536
303/0.02 478/0.08 288/0.01 520/0.03 275/0.08 515/0.02 288/0.04 518/0.03 287/0.07 529/0.01
244/0.21 417/0.20 257/0.12 457/0.14 231/0.23 458/0.13 245/0.18 473/0.12 253/0.18 475/0.11
B C D E Fig. 2. Cumulative undersize distribution of the particle sizes for Limestone A (200–400 μm).
Fragment Size(μm)/ FIC (30 min)
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Fig. 4. Average size of the fragments under different test conditions for Limestone A.
fragmentation was intense and many fine fragments were generated. Under pure CO2 atmosphere, no obvious size change was found compared with air atmosphere. Generally, three mechanisms are attributed to the limestone fragmentation: thermal stress at high temperature, inner high pressure caused by the organics or water evaporation and the CO2 release during calcination [5]. Calcination is a chemical reaction process which will produce CO2 and lime (CaO). With the existence of high CO2 concentration the environment will suppress the calcination reaction [17]. Previous studies found that at a certain temperature, only if the partial pressure of CO2 in the environment is lower than the CO2 pressure as the calcination reaction is at chemical equilibrium, the calcination reaction occurs [9,17]. As shown in Fig. 5, when CO2 was used as fluidizing gas, the size change of the limestone was insignificant. The results indicate that fragmentation of limestone is dominated by the calcination reaction rather than the thermal stress found in study [18].
Fig. 6. Temporal variation of attrition rate constant of lime and its sulphatized product in a fluidized bed (limestone A, particle size is based on lime).
3.5. Effect of the sulphation on attrition of lime The limestone is calcined into lime (CaO) after it is added into the hot furnace of a FB. Then, lime reacts with SO2 into solid CaSO3 or CaSO4. It is important to know the comminution characteristic of the lime and its sulphatized products in the bed. Based on our discussion in the above section, other than the calcination with rapid gas release from the particles, the fragmentation is trivial. Thus, the comminution of lime and its sulphatized products is mainly attributed to the attrition by abrasion [6]. In our study, we used muffle furnace (850 °C) to calcinate fresh limestone to produce lime. Calcined products were sieved in to different size ranges. We chose particles that fall into the size ranges of 200–400 μm and 400–600 μm as sulphation reactant and lime attrition sample. Sulphatized products are produced in the FB reactor as a fixed-bed under temperature 850 °C and SO2 atmosphere. Then we tested the attrition of the lime and its sulphatized products in the FB with air fluidizing as we described in the Experimental section. Fig. 6 shows the temporal variation of Ka for lime with two origin particle sizes (200–400 μm and 400–600 μm) and its sulphatized products of Limestone A. It can be found that for both samples, Ka decays exponentially with time, and after 90 min it reaches nearly a constant (K∞). In the beginning, the difference of Ka, between lime and sulphatized products is obvious at a given velocity. And K∞ of lime is a few times larger than that of the sulphatized products. In this study, the value of K∞ was defined as the value at time 90 min. Other samples have similar tendency and the K∞s are summarized in Table 3. It can be seen that K∞ varies with the limestone type and original particle size. For the same kind of sample, finer particles have a larger K∞, and the difference in degree depends on the original limestone type. Typically, K∞ is of about 10− 5–10− 6 for lime, and about 10− 6–10− 7 for the sulphatized products. The results show that the sulphate is more attrition resistant, which is consistent with those found by Scala et al. [6,8]. The reason as analyzed by Scala et al. [6,8] is the presence of the layer of CaSO4, on the surface of the sulphatized particles. 3.6. Effect of simultaneous calcination and sulphation on attrition
Fig. 5. Cumulative undersize distributions of the particle sizes for Limestone E (400– 600 μm) under different calcination atmospheres.
When limestone is added into the hot furnace with SO 2 atmosphere, calcination and sulphation reactions occur simultaneously. The lime conversion rates measured in FB reactor (Fig.7) confirm that sulphation is a slow process. In about 20 min, the conversion rate
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Table 3 Attrition rates of calcined and sulphatized limestones. Sample
A B C D E Ref[6] Ref[15]
Original size distribution (μm)
K∞
Mass of quartz/ mass of sample
Fluidizing gas velocity
Calcined product
Sulphatized product
200–400 400–600 200–400 400–600 200–400 400–600 200–400 400–600 200–400 400–600
1.79 × 10− 5 1.14 × 10− 5 5.86 × 10− 6 1.94 × 10− 6 6.47 × 10− 6 3.45 × 10− 7 2.14 × 10− 6 7.02 × 10− 7 6.02 × 10− 6 1.58 × 10− 6
3.17 × 10− 6 1.48 × 10− 6 2.58 × 10− 6 4.76 × 10− 7 2.07 × 10− 6 3.01 × 10− 7 6.50 × 10− 7 3.79 × 10− 7 4.14 × 10− 7 1.56 × 10− 7
3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
425–600 400–600
1.18 × 10− 4 1.40 × 10− 6
4.91 × 10− 6
20.0 9.0
0.75 3.00
reaches a constant for a given SO2 concentration. The final constant is around 10%, much smaller than the unity, confirming that sulphate product covers the particle surface. Fig. 8 further shows that the final constant is a variable depending on the particle size and the ambient SO2 concentration. Since the time for significant attrition and the sulphation is of the same order, it is important to examine the attrition under simultaneous calcination and sulphation. Thus, comparison experiments were conducted on the attrition of fresh limestone fluidized with dry air and a mixture of air and SO2 (2000 ppm) respectively. The experimental condition and procedure were the same as the ones used for the above lime/limestone attrition experiments. Figs. 9 and 10 show the attrition results of the Limestone A with two size ranges. The results of the sulphatized products were also shown in the figure. Though fragmentation occurs in these processes, the amount of the generated and collected fine particles is small compared to the attrition part, and its influence on the attrition experiments is expected to be negligible. It can be seen that in the first 15 min, the attrition rate Rs of the fresh limestone under air and air/ SO2 atmosphere are nearly the same and much higher than that of the fully sulphatized products. The reason is that during the beginning
Fig. 7. Temporal variations of CaO conversion at different SO2 concentrations in FB for Limestone A.
Fig. 8. Variations of maximum CaO conversion with ambient SO2 concentration in FB for Limestone A.
period, the fresh CaSO4 particle produced by the chemical reaction is not tightly adhered on the particle surface. While, after a certain period, e.g., 20 min, the layer of sulphatized products will cover the surface of the sample fluidized by air/SO2 mixture, and thus has a smaller Rs gradually approaching to that of sulphate products.
4. Conclusions Fragmentation and attrition occur as the limestone is added into the FB or CFB boiler, introducing particle comminution and thereby influencing the performance of the boiler. In this paper, a new and rather simple experimental measurement method was proposed and then validated. Experimental results confirmed that fragmentation and attrition could be significantly affected by limestone types, original size, heating rate, calcination condition and ambient CO2 concentration. The average size of the particles decreased about 10–20% during fragmentation process. The
Fig. 9. Temporal variations of attrition rate for Limestone A (200–400 μm) in a fluidized bed.
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References
Fig. 10. Temporal variations of attrition rate for Limestone A (400–600 μm) in a fluidized bed.
primary fragmentation mainly occurred in the first a few minutes after the limestone was added into the hot FB. Fragmentation of limestone in the FB was more intense than that in the muffle furnace at the same temperature. The original size effect is ambiguous, depending on the limestone type. The comminution caused by attrition mainly occurred during calcination rather than sulphation. The sulphate is fragment and attrition resistant. It is further confirmed that attrition rate was abated by the formation of CaSO4 over the limestone/lime particle surface. Given sulphation is a rather slow process, in the practical application where calcination and sulphation simultaneously happen, it plays a minor role on attrition rate of a limestone though the attrition rate of sulphate much smaller than that of lime. In addition, the present experimental data clarified primary fragmentation of limestone is dominated by CO2 release instead of thermal stress. More studies under CFB condition are suggested in the future. Acknowledgements Financial supports of this work by High Technology R&D (863) (2009AA05Z302) and the National 11th-Five Year Plan (No. 2006BAD07A14-4) are gratefully acknowledged.
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