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Experimental study on lateral mixing of particles in a quasi-slot-rectangular spouted bed
Powder Technology 243 (2013) 1–8
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Experimental study on lateral mixing of particles in a quasi-slot-rectangular spouted bed Lu Gan ⁎, Xiaofeng Lu ⁎⁎, Quanhai Wang, Qing Hu, Ye Chen, Jie Xu Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing, PR China
a r t i c l e
i n f o
Article history: Received 7 October 2012 Received in revised form 3 February 2013 Accepted 11 March 2013 Available online 16 March 2013 Keywords: Fluidized bed ash cooler (FBAC) Slot-rectangular spouted bed (SRSB) Tracer particles Lateral mixing Lateral dispersion coefficient
a b s t r a c t A novel fluidized bed ash cooler called Bi-spouted-bed fluidized bed ash cooler was developed for circulating fluidized bed (CFB) boilers. It consists of chambers that have some characteristics of slot-rectangular spouted bed (SRSB), and the flow structure is similar to a combination bubbling bed and SRSB, thus we call it quasi-slot-rectangular spouted bed (QSRSB). A cold model of quasi-slot-rectangular spouted bed (QSRSB) with a cross-section of 900 mm × 200 mm was built. And a binary mixture of different sized particles was adopted, and the jetsam (bigger) and flotsam (smaller) component were used as tracers and bed material respectively. Experimental investigation on lateral mixing of tracer particles has been conducted. The lateral mixing process of tracer particles was studied. In addition, the effects of particle size of bed material dp, superficial gas velocity u, static bed height h and air inlet section width δ on the effective lateral dispersion coefficient of tracers were investigated. Meanwhile, a comparison of the effective lateral dispersion coefficient of tracers in QSRSBs and the bubbling fluidized bed with the same upper cross-section was conducted. It is found that the tracers disperse from regions of higher tracer concentration to regions of lower tracer concentration. Furthermore, the effective lateral dispersion coefficient of tracers increase with the increase of superficial gas velocity u, static bed height h and air inlet section width δ, but decrease with the increase in size of bed material dp, respectively. Additionally, the effective lateral dispersion coefficient of bubbling fluidized bed appears to be larger than that of QSRSBs when gas velocity at the upper section of bed, U, is greater than 0.3 m/s. However, the minimum U that ensuring the normal operation of QSRSBs is less than that of bubbling fluidized bed in the present work, which means that QSRSB has a better adaptability to the tracer granularity with the same gas flowrate. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Since spouted bed was developed by Mathur and Gishler [1], it has been widely studied and extensively used in various physical and chemical processes such as drying, coating, granulation, combustion, gasification, etc. [2–7]. Spouted bed has been proven to be an appropriate alternative to fluidized bed to dealing with particles of relatively large size, which are too large to be satisfactorily fluidized in fluidized beds. Slot-rectangular spouted bed (SRSB), which was originally called “two-dimensional spouted bed” until the revelation of the three-dimensional effects by Dogan et al. [8] and Freitas et al. [9,10], has been proposed as a modification for conventional spouted bed (CSB) to eliminate its scale-up difficulties [11,12]. The circulating fluidized bed (CFB) boiler technology has become one of the approved clean coal combustion technologies that commercially applied. As a key auxiliary device for CFB boilers, bottom ash cooler (BAC) is used to treat the high temperature bottom ash discharged ⁎ Corresponding author. Tel./fax: +86 23 65102475. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (L. Gan), xfl[email protected] (X. Lu). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.03.021
from the furnace of CFB boiler, and to recover heat in the ash and make the ash easily handled and transported. Without good operational reliability of the BAC, the stable, secure and economic operation of the boiler cannot be guaranteed [13–15]. There are many kinds of BACs available for large-scale CFB boilers, and rotary ash cooler (RAC) and fluidized bed ash cooler (FBAC) are commonly applied in China. FBAC has been proven to be more attractive than RAC, because of its unique technical features such as higher heat-transfer coefficient, greater output capacity, a function of returning partial of fine particles of the bottom ash back into the furnace by fluidizing air from the FBAC, and a better thermal economy of reclaiming heat in the bottom ash [16,17]. And the majority of FBACs are the bubbling fluidized-bed type. Unfortunately, the application and further development of those FBACs are restricted by some disadvantages such as poor flow ability of the bottom ash, defluidization, agglomeration, etc. All in all, the poor performance of those FBACs is attributed to the high limitation for the ash size. Whereas practical bottom ash has a wide range of size distribution, and contains coarse particles that are too large to be satisfactorily fluidized in FBACs [17–19]. It is noteworthy that there is another kind of FBAC, which is called gas-tank spouted bed ash cooler, could handle practical bottom ash
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under the use of a rather high superficial gas velocity. But its large air consumption, which finally returns to the furnace, has a great impact on the combustion condition of the furnace, and a restriction to the output capacity [19,20]. Therefore, it's necessary to develop a novel FBAC that can overcome the problems encountered by the existing FBACs and meet the needs of large-scale CFB boilers. Based on the basic operating principles and technical features of SRSB, which could be successfully used for particles with a very broad particle size distribution, a novel FBAC called Bi-spouted-bed fluidized bed ash cooler has been proposed by Chongqing University, and has obtained an invention patent in China [21]. The patented ash cooler consists of chambers that have similar structure to SRSB. Compare with SRSB, yet the chamber has an air inlet section located below the inclined base, and a porous plate distributor is provided to supply fluidizing air into the chamber instead of a slot. Moreover, to enhance the heat transfer between fluidizing air and particles on the premise of guaranteeing a good flow ability of the bottom ash, a relatively larger ratio of the distributor width to the column width and a relatively low superficial gas velocity range are selected. Thus, the central region of the bed of particles is working in bubbling fluidized or turbulent fluidized state. In the central region of the bed, particles are carried upwards by the bubble wakes. As bubbles burst at the upper surface of the bed, part of these particles that are projected above the bed surface fall back upon the bed surface of the central region of the bed after reaching a certain height, and a part of these particles fall into the surrounding downcomer region where the particles slide downwards due to gravity and then enter the central region again. Considering that the flow structure is similar to a combination of bubbling bed and SRSB, and it contains some characteristics of SRSB, we call the bed quasi-slot-rectangular spouted bed (QSRSB). It has been found that the amount of fluidizing air has a direct influence on the conveying of ash in FBAC [15,18]. Actually, if coarse particles of the bottom ash that characterized of poor flow ability could not be diffused quickly, then they would accumulate, causing defluidization and even agglomeration gradually, resulting in shutdown of FBAC finally. Hence, the operational reliability and the maximum output capacity under a specific working condition are based on the characteristics of lateral dispersion of the coarse particles of bottom ash. Numerous investigations have been carried out for SRSBs. SRSB hydrodynamics has been extensively investigated, and most previous work focus on global flow properties and local flow structure, e.g. minimum spouting velocity, maximum pressure drop, pressure drop, maximum spoutable bed height, flow regimes and profiles of voidage and particle velocity [8–10,22–31]. SRSBs have also been studied for their stability and scale-up [9,25,31–33], and for their applications, e.g. coating and drying [34–41]. To the best of our knowledge, investigation on lateral mixing of particles in SRSB has rarely been reported in literatures. Therefore, in order to provide available guidelines for the design, operation and structure optimization of Bi-spouted-bed fluidized bed ash cooler and valuable reference for the investigation of lateral solids mixing in SRSBs, experimental studies of the lateral mixing of tracer particles in a cold model of QSRSB are carried out in this work. And a binary mixture of different sized particles is adopted, and the jetsam (bigger) and flotsam (small) component are used as tracers and bed material respectively. In this work, the lateral mixing process of tracer particles has been studied. Furthermore, the effects of particle size of bed material dp, superficial gas velocity u, static bed height h and air inlet section width δ on the effective lateral dispersion coefficient of tracer particles, which represent its overall lateral mixing rate, has also been investigated thoroughly. And a comparison of the effective lateral dispersion coefficient of tracers in QSRSBs and the bubbling fluidized bed with the same upper cross-section has been conducted.
2. Experiments 2.1. Experimental apparatus A schematic diagram of the experimental apparatus is shown in Fig. 1. It consists of a gas supply system, a test rig body, a dedusting system, etc. The fluidizing gas flows into the distribution header after flowing though a ball valve installed at the gas supply line, and then the fluidizing gas is divided into three equal fluxes by three control valves connected to their individual calibrated rotameters before flowing into the wind boxes. The ball valve is installed to cut off the fluidizing gas immediately. The test rig body consists of a rectangular column with a cross section of 900 mm × 200 mm and a height of 800 mm, and an inclined base located at the bottom of the column with inclination angle θ = 45°. The front side and back side of the column are made of transparent Plexiglas, and six sampling windows of 100 mm × 200 mm are made on the back side. An air inlet section of 40 mm height is installed under the inclined base. Three air inlet sections with widths, δ, of 44 mm, 52 mm and 60 mm, and their individual corresponding inclined bases are used in this study to investigate the effect of the δ on the effective lateral dispersion coefficient of the tracer particles. A porous plate distributor, with two hundred and forty evenly distributed orifices of 3 mm in diameter, is used. And a layer of canvas is set on the distributor to prevent the materials leaking into wind boxes and to reinforce the uniformity of gas distribution across the bed. A conventional bubbling fluidized bed is available, if the inclined base and air inlet section are removed and the porous plate distributor and wind boxes are replaced with appropriate one respectively. 2.2. Materials The bottom ash from a CFB boiler is used as the materials in this study after sieving into four narrow particle size ranges, and the properties of these particles are listed in Table 1. The bottom ash with larger particle size of 0.71–1.00 mm are used as tracers, while the others with smaller particle size are chosen as bed materials. 2.3. Experimental procedure The bed collapse method [42–44] and a set of partitions are developed and adopted here to measure the lateral concentration distribution of tracer particles in this study. As shown in Figs. 1 and 2, it consists of a single partition and a partition-box made of thin stainless steel plates so as to separate the whole bed into parts conveniently. The single partition can be used individually or be accompanied with the use of partition-box. If the single partition and partition-box are inserted into the column simultaneously, the column would be divided into six sampling regions of equal volume along the lateral direction: A, B, C, D, E and F. Before each run, the single partition was inserted into the column to separate the region A from the others. Then a batch of mixture of tracers and bed material was poured into region A to a specific static bed height, and the others were charged with bed material to the same height. All experiments were carried out with the mass of tracer particles accounted for 12% of the total particles mass. After the above preparation, the ball valve was turned on and the gas flow rates were adjusted according to the desired operation condition. When the stable state of fluidization was established, the single partition was quickly removed. After a specific time interval that depending on the particle size of bed material dp, the superficial gas velocity u, static bed height h and the air inlet section width δ [43], the single partition and partition-box located on the top of the column were instantaneously inserted to divide the bed into six lateral sampling regions, as shown in Fig. 2. At the same time, the gas supply was terminated by immediately closing the ball valve and opening the bypass valve
L. Gan et al. / Powder Technology 243 (2013) 1–8
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Fig. 1. Schematic diagram of experimental apparatus.
simultaneously, and then the bed collapsed. The sampling windows on the back side were opened, and the mixture of particles in each sampling region were sucked by electric vacuum cleaner, separated by screening, and weighed. The local tracer concentration η i (i = A, B, C, D, E, F.) of each sampling region is defined as the ratio of the mass of local tracer particles over the total mass of particle mixture in the sampling region, and the mass of bed material and tracers in a given sampling region are marked as mbi and mti, respectively. Experiments showed that, in different tests, the mass of particles carried away by the fluidizing gas and then captured by the cyclone account for less than 1% of the total mass. Consequently, the influence of the tiny variance of the mass of particles before and after each run was ignored. To investigate the reliability and repeatability of the
Table 1 Physical properties of material. dp (mm)
ρp (kg/m3)
umf a (m/s)
0.25–0.35 0.35–0.50 0.50–0.71 0.71–1.00
2344 2344 2344 2344
0.09 0.12 0.34 0.56
a
umf was determined by experiments in a bubbling fluidized bed.
measured tracer concentrations, five run were repeated for three test conditions. Fig. 3 presents the means with error bars for the five repeats of the three different cases. Obviously, the measurement is repeatable and the maximal error is below 3%, which means that the method is feasible and accurate with good repeatability. The experimental conditions studied in this work are listed in Table 2. 3. Mechanism of lateral mixing and treatment of data 3.1. Mechanism of lateral mixing It is commonly recognized that solids mixing in a fluidized bed is mainly caused by the movement of bubbles [45,46]. The lateral mixing of solids in fluidized bed is attributed to the following reasons: (a) As a bubble rises, which carries particles up through the bed in its wake, it pushes emulsion aside. However, the solids passing close enough to the bubble are eventually swept into the wake, mixed with the solids already there completely and then leave the wake from random positions back into the emulsion eventually, thereby giving rise to lateral mixing of solids. (b) As a bubble bursts at the upper surface of the bed, part of these particles projected above the bed surface fall back upon the surrounding bed surface, thus inducing lateral mixing of solids eventually. (c) The gross particle circulation or
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Fig. 2. Schematic diagram of the sampling regions.
so-called “Gulf Stream”, caused by the lateral non-uniform distribution of bubbles, could also contribute significantly to the lateral mixing of solids [42,43,46–48]. 3.2. Treatment of data A number of works concerning lateral solids mixing in gas fluidized bed reactors focused on the overall lateral solids mixing rate and fitted the lateral mixing by a Fickian-type diffusion equation. Actually, Lateral solids mixing in gas fluidized beds are attributed to the contributions from lateral convection and lateral dispersion. However, to simplify the treatment, the contribution of lateral convection to the lateral solids
mixing was included into lateral dispersion, which means that convection term and dispersion term were combined into dispersion term. And then dispersion terms with an effective lateral solids dispersion coefficient were adopted, and there were no longer a convective term in the equations, and the effective lateral dispersion coefficient was generally considered to be constant over the bed and a representation of the overall lateral solids mixing rate [43–45]. As mentioned in Section 1, the central region of QSRSB is working in bubbling fluidized or turbulent fluidized state, thus the Fickian-type diffusion equation for lateral solids mixing in gas–solid fluidized beds is used to characterize lateral mixing of tracer particles in the present work. Considering that experiments are carried out on the lateral mixing of tracer particles at the macroscopic level, the tracers are assumed to be evenly distributed over the vertical cross section normal to the lateral direction, thus the one-dimensional dispersion model is used here. Additionally, there is no source term in this study. Consequently, the governing differential equation of the model is ∂C ∂2 C ¼ Dsr 2 ∂t ∂x
ð1Þ
The appropriate initial and boundary conditions are C¼
mt 0:15
C¼0
for t ¼ 0;
for t ¼ 0;
∂C ¼ 0 for ∂x
ti0;
ð2Þ
0≤x≤0:15
ð3Þ
0:15≤x≤0:9 x¼0
and x ¼ 0:9
ð4Þ
Table 2 Summary of the experimental conditions.
Fig. 3. Means with error bars of the tracer concentrations for the five repeats of three different cases.
dp (mm)
δ (mm)
h (mm)
u (m/s)
0.25–0.35 0.35–0.50 0.50–0.71
44, 52, 60 60 60
150 100, 125, 150, 175, 200 150
0.70–3.67 0.90–2.56 1.29–2.56
L. Gan et al. / Powder Technology 243 (2013) 1–8
where C is tracer mass concentration (kg/m), a function of coordinate x (m) and time t (s), corresponding to the mass of tracer particles per unit length, mt is the total mass of the tracer particles (kg) loaded in the bed for each run, and Dsr is the effective lateral solids dispersion coefficient (m 2/s) lumped together all lateral transport processes, which is considered to be a constant across the bed. To avoid doing numerical calculation repeatedly and simplify the solution procedure, Eqs. (1)–(4) were rewritten into dimensionless form and the effective lateral solids dispersion coefficient was removed as a parameter [45]. The corresponding governing equation, initial and boundary conditions are:
mt 0:15
C¼0
∂C ¼0 ∂xn
1 ≤x ≤1 6 n
ð7Þ
4.2. Effects of superficial gas velocity and particle size of bed material on the effective lateral dispersion coefficient
xn ¼ 0
ð8Þ
for τ ¼ 0;
for τ〉0;
4.1. Mixing process with time
ð6Þ
ð5Þ
for τ ¼ 0;
4. Results and discussion
Fig. 4 also illustrates experimental measured transient tracer concentration profiles of the lateral mixing process for a typical test condition (dp = 0.25–0.35 mm, δ = 60 mm, h = 150 mm and u = 0.9 m/s). To a specific profile, it can be seen that the longer the distance away from region A, the smaller the local tracer concentration. Moreover, the differences of local tracer concentrations among regions are gradually leveled off with time, i.e., the tracer concentration becomes more even as time proceeds, indicating the tracers are gradually dispersed away from the region A. To investigate the effects of parameters on the lateral mixing process of tracer particles more intuitively and conveniently, it is appropriate to calculate the effective lateral dispersion coefficient which characterizes the rate of lateral mixing process based on the experimental measured tracer concentration profile.
∂C ∂2 C ¼ ∂τ ∂xn 2
C¼
5
0≤xn ≤
1 6
and xn ¼ 1
where xn is the dimensionless coordinate, expressed as xn = x/Lx, τ is the dimensionless time and τ ¼ Dsr t=Lx 2 , and Lx is the length of the column along the lateral direction, Lx = 0.9 m. The solution of Eq. (5) subjected to these initial and boundary conditions is a series of profiles of tracer mass concentration, C(xn,τ), as a function of dimensionless coordinate and dimensionless time. It was firstly converted to a series of profiles of tracer concentration, i.e., η i(τ), where i = A, B, C, D, E, F. Then found out the appropriate transient tracer concentration profile that has the minimum of the sum of the squares of the errors with the experimental measured lateral distribution of tracer concentration and its corresponding dimensionless time. Finally, the effective lateral dispersion coefficient Dsr could be determined by inserting numerical values of the time interval that the mixing process lasted, dimensionless time and Lx, corresponding to the run, into Dsr ¼ τLx 2 =t. The numerical calculated tracer concentration profiles at t = 90 s, 150 s and 240 s for a typical test condition (dp = 0.25–0.35 mm, δ = 60 mm, h = 150 mm and u = 0.9 m/s) are shown in Fig. 4. And the experimental measured tracer concentrations, expressed as scattered points, are compared with the predicted results. It is obvious from Fig. 4 that the agreement between the experimental measured and predicted concentrations appears to be reasonably good, i.e. a proper value of Dsr is acceptable to match the predicted and measured tracer concentrations satisfactorily.
Fig. 5 is a plot of the effecitive lateral tracer dispersion coefficient Dsr against superficial gas velocity u for three different bed materials at a bed height h of 150 mm and an air inlet section width δ of 60 mm. From the figure, it can be seen that for all bed materials, the effective lateral dispersion coefficient Dsr increases with the increase of superficial gas velocity u. It's mainly because increasing gas velocity results in the increase of bubble size. Correspondently, bubbles become faster and more vigorous, which leads to more intense projection of solids by the burst of bubbles at the bed surface, thus promoting lateral dispersion of tracer particles. Additionally, similar to the results of literature [44], the effective lateral dispersion coefficient Dsr increases with decreasing particle size of bed material at a given gas velocity. And this finding is useful in practice: for a FBAC, while the flowrate of fluidizing air is restricted, it is feasible to improve lateral solids dispersion by selecting the location of the ash discharge chute appropriately, since the location of the chute has been proven to be able to adjust the bed composition [46]. 4.3. Effect of static bed height on the effective lateral dispersion coefficient The effect of static bed height h on the effective lateral tracer dispersion coefficient Dsr is illustrated in Fig. 6. Obviously, for all gas velocities, the dispersion coefficient Dsr increases with increasing static bed height h. This can be attributed to the facts that the observed bubble flow increases with height in the bed [46], and bubbles become larger and faster caused by the coalescence as they rise up through the bed. Therefore, the lateral tracer dispersion, induced by the bursting of bubbles at the bed surface, would become more intense as the static bed height rises, resulting in the increase of Dsr.
Fig. 4. Comparisons of experimental tracer concentration profiles and predicted results with Dsr = 6.77 × 10−4 m2/s at dp = 0.25–0.35 mm, δ = 60 mm, h = 150 mm and u = 0.9 m/s.
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Fig. 5. Effect of u on the effective lateral dispersion coefficient Dsr under different dp at δ = 60 mm and h = 150 mm.
4.4. Effect of air inlet section width on the effective lateral dispersion coefficient Fig. 7 shows the effect of superficial gas velocity u on the effective lateral dispersion coefficient Dsr with the width of air inlet section δ as a parameter at a given static bed height of 150 mm and bed material of dp = 0.35–0.50 mm. It can be seen that for all air inlet section widths, the effective lateral dispersion coefficient Dsr increases with the increase of gas velocity u. Moreover, the effective lateral dispersion coefficient Dsr increases as the air inlet section width δ increases. Probably, this is attributed to the fact that the increase of air inlet section width δ results in the increase of observed bubble flow, and the weakening of the wall effects, thus leading bubbles to become larger and faster, and causing Dsr increase finally. The relationships between U and effective lateral tracer dispersion coefficient Dsr for QSRSBs of different δ and bubbling fluidized bed are shown in Fig. 8. Here, U is the gas velocity at the upper section with a cross section of 900 mm × 200 mm that represent gas flowrate, whereas superficial gas velocity u is the gas velocity at the air inlet section. It is shown for four cases that as U increases gradually, the Dsr increases monotonically. However, unlike the relationships between u and Dsr for QSRSBs of different air inlet section widths revealed in Fig. 7, the lateral dispersion coefficient of QSRSB is virtually independent of air inlet section width δ for a certain U. This is explainable that at the same U, the decrease of δ results in the increase of u and thus counteracting the
Fig. 6. Effect of h on the effective lateral dispersion coefficient Dsr under different u at δ = 60 mm and dp = 0.35–0.50 mm.
Fig. 7. Effect of u on the effective lateral dispersion coefficient Dsr under different δ at h = 150 mm and dp = 0.25–0.35 mm.
influence of δ, which make the Dsr remain unchanged. Moreover, when U is greater than 0.3 m/s, the Dsr of bubbling fluidized bed appears to be larger than that of QSRSBs. This phenomena may be attributed to the following reasons: 1) in QSRSB, parts of particles that are projected when bubbles burst at the upper surface of the bed fall back into the downcomer, where the particles just slide downwards and then reenter the central region, and no lateral movement take place during this period for these particles; 2) wall effects retard the rise of bubbles more seriously in QSRSBs; 3) the gross particle circulation or so-called “Gulf Steam” contribute significantly to the lateral dispersion of solids in bubbling fluidized bed. However, it is interesting to note that the minimum U, Um, ensuring the normal operation of QSRSBs are less than that of bubbling fluidized bed in this study, and tracer particles would be found to rest on the gas distributor of region A if U is less than Um. In other words, it demonstrates that the QSRSB could handle coarser tracers that are too large for bubbling fluidized bed with the same U or gas flowrate. It also indicates that the Bi-spouted-bed fluidized bed ash cooler based on the QSRSB has a better adaptability to the bottom ash granularity than FBACs of bubbling fluidized-bed type. 5. Conclusions Based on the well-designed experiments, the lateral mixing of tracer particles has been studied. The bed collapse method and a
Fig. 8. Effect of U on the effective lateral dispersion coefficient Dsr under different δ at h = 150 mm and dp = 0.25–0.35 mm.
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set of partitions were successfully applied to obtain the lateral tracer concentrations. Furthermore, the lateral dispersion coefficient of tracer particles was measured, and the effects of particle size of bed material dp, superficial gas velocity u, static bed height h and air inlet section width δ on the effective lateral dispersion coefficient of tracers were investigated. And a comparison of the effective lateral dispersion coefficient of tracers in the QSRSBs and the bubbling fluidized bed with the same upper cross-section was conducted. The following conclusions can be drawn from this study: (1) The tracers disperse from regions of higher tracer concentration to regions of lower tracer concentration, and the differences of local tracer concentrations among regions are gradually leveled off as time proceeds. (2) The effective lateral dispersion coefficient of tracer particles Dsr increases with the increase of gas velocity u, static bed height h and width of air inlet section δ, respectively. However, increase in the size of bed material dp causes a decrease in the effective lateral dispersion coefficient Dsr. (3) Air inlet section width δ has no influence on the effective lateral dispersion coefficient of tracers at given U for QSRSBs. Dsr of bubbling fluidized bed appears to be larger than that of QSRSBs when U is greater than 0.3 m/s, but the Um of QSRSBs are less than that of bubbling fluidized bed, indicating that the QSRSB could handle coarser tracers that are too large for bubbling bed with the same U or gas flowrate. And this also indicates that the Bi-spouted-bed fluidized bed ash cooler based on the QSRSB has a better adaptability to the bottom ash granularity than FBACs of bubbling fluidized-bed type.
Nomenclature BAC bottom ash cooler CFB circulating fluidized bed CSB conventional spouted bed FBAC fluidized bed ash cooler QSRSB quasi-slot-rectangular spouted bed RAC rotary ash cooler SRSB slot-rectangular spouted bed C, C(xn,τ) tracer mass concentration, i.e. mass of tracers per unit length (kg/m) Dsr effective lateral dispersion coefficient of tracers (m 2/s) Lx length of the column along the lateral direction, Lx = 0.9 m U gas velocity at the upper section with a cross section of 900 mm × 200 mm (m/s) Um minimum U that ensuring the normal operation of QSRSBs (m/s) dp particle size of bed material (mm) h static bed height (mm) i labels for the sampling regions, i = A, B, C, D, E, F mt total mass of the tracers loaded in the bed for each run (kg) mbi mass of bed material in sampling region (kg) mti mass of tracers in sampling region (kg) t time (s) u superficial gas velocity at the air inlet section (m/s) x lateral coordinate (m) xn dimensionless coordinate (−) δ width of the air inlet section (mm) η i, η i(τ) tracer concentration of sampling region, ηi(τ) = mti/(mti + mbi) (−) θ inclination angle of the inclined base located at the bottom of the bed, θ = 45° τ dimensionless time (−)
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Dogan, et al., Identification of flow regimes in slot-rectangular spouted beds using pressure fluctuations, Canadian Journal of Chemical Engineering 82 (2004) 60–73. [24] G.Q. Liu, S.Q. Li, et al., Experimental studies of particle flow dynamics in a two-dimensional spouted bed, Chemical Engineering Science 63 (2008) 1131–1141. [25] X.L. Zhao, S.Q. Li, et al., DEM simulation of the particle dynamics in two-dimensional spouted beds, Powder Technology 184 (2008) 205–213. [26] M.L. Passos, A.S. Mujudar, et al., Prediction of the maximum spoutable bed height in two-dimensional spouted beds, Powder Technology 74 (2) (1993) 97–105. [27] Z. Chen, C.J. Lim, et al., Hydrodynamics of slot-rectangular spouted beds: effect of slot configuration on the local flow structure, Canadian Journal of Chemical Engineering 86 (2008) 598–604. [28] F. Zanoelo, C.S. Rocha, et al., Influence of operating parameters on the average spout width in two-dimensional spouted beds, Canadian Journal of Chemical Engineering 82 (2004) 89–93. [29] B.Q. Wang, B.L. Luo, Minimum spouting velocity of multi-component particle mixture in a slot-rectangular spouted bed, The Chinese Journal of Process Engineering 1 (2) (2001) 113–116. [30] B.Q. Wang, B.L. Luo, et al., Maximum spouted pressure drop of multi-component particle mixture in a slot-rectangular spouted bed, Chemical Reaction Engineering and Technology 17 (2) (2001) 107–111. [31] Z. W. Chen, Hydrodynamics, stability and scale-up of slot-rectangular spouted bed, Ph.D Thesis, The University of British Columbia, Canada, 2008. [32] Z. Chen, C.J. Lim, et al., Stability of slot-rectangular spouted beds: effect of slot configuration, Canadian Journal of Chemical Engineering 89 (2011) 1401–1407. [33] O.P. Taranto, et al., Scale-up and spouting of two-dimensional beds, Canadian Journal of Chemical Engineering 81 (2003) 264–267. [34] O.P. Taranto, S.C.S. Rocha, et al., Convective heat transfer during coating of particles in two-dimensional spouted beds, Drying Technology 15 (6–8) (1997) 1909–1918. [35] O.P. Taranto, S.C.S. Rocha, et al., Coating of urea with an aqueous polymeric suspension in a two-dimensional spouted bed, Drying Technology 20 (3) (2002) 685–704.
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[42] Q. Xiang, G.Q. Huang, et al., Lateral dispersion of large coal particles in an industrial-scale fluidised bed combustor, Journal of Zhejiang University 21 (6) (1987) 64–72. [43] Y.F. Shi, L.T. Fan, Lateral mixing of solids in batch gas–solids fluidized beds, Industrial and Engineering Chemistry Process Design and Development 23 (2) (1984) 337–341. [44] D.Y. Liu, X.P. Chen, Experimental profiles of lateral mixing of feed particles in a three-dimensional fluidized bed, AICHE Journal 57 (6) (2011) 1459–1469. [45] F. Niklasson, H. Thunman, et al., Estimation of solids mixing in a fluidized-bed combustor, Industrial and Engineering Chemistry Research 41 (18) (2002) 4663–4673. [46] O. Daizo Kunii, Levenspiel, Fluidization Engineering, 2nd ed. Butterworth-Heinemann, Boston, 1991. [47] L.H. Shen, M.Y. Zhang, et al., Solids mixing in fluidized beds, Powder Technology 84 (1995) 207–215. [48] X. Xu, Y. Chi, et al., Experimental study on particle motion characteristic in the internal circulating fluidized bed, Proceeding of the CSEE 21 (11) (2001) 9–13.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Experimental study on lateral mixing of particles in a quasi-slot-rectangular spouted bed Lu Gan ⁎, Xiaofeng Lu ⁎⁎, Quanhai Wang, Qing Hu, Ye Chen, Jie Xu Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing, PR China
a r t i c l e
i n f o
Article history: Received 7 October 2012 Received in revised form 3 February 2013 Accepted 11 March 2013 Available online 16 March 2013 Keywords: Fluidized bed ash cooler (FBAC) Slot-rectangular spouted bed (SRSB) Tracer particles Lateral mixing Lateral dispersion coefficient
a b s t r a c t A novel fluidized bed ash cooler called Bi-spouted-bed fluidized bed ash cooler was developed for circulating fluidized bed (CFB) boilers. It consists of chambers that have some characteristics of slot-rectangular spouted bed (SRSB), and the flow structure is similar to a combination bubbling bed and SRSB, thus we call it quasi-slot-rectangular spouted bed (QSRSB). A cold model of quasi-slot-rectangular spouted bed (QSRSB) with a cross-section of 900 mm × 200 mm was built. And a binary mixture of different sized particles was adopted, and the jetsam (bigger) and flotsam (smaller) component were used as tracers and bed material respectively. Experimental investigation on lateral mixing of tracer particles has been conducted. The lateral mixing process of tracer particles was studied. In addition, the effects of particle size of bed material dp, superficial gas velocity u, static bed height h and air inlet section width δ on the effective lateral dispersion coefficient of tracers were investigated. Meanwhile, a comparison of the effective lateral dispersion coefficient of tracers in QSRSBs and the bubbling fluidized bed with the same upper cross-section was conducted. It is found that the tracers disperse from regions of higher tracer concentration to regions of lower tracer concentration. Furthermore, the effective lateral dispersion coefficient of tracers increase with the increase of superficial gas velocity u, static bed height h and air inlet section width δ, but decrease with the increase in size of bed material dp, respectively. Additionally, the effective lateral dispersion coefficient of bubbling fluidized bed appears to be larger than that of QSRSBs when gas velocity at the upper section of bed, U, is greater than 0.3 m/s. However, the minimum U that ensuring the normal operation of QSRSBs is less than that of bubbling fluidized bed in the present work, which means that QSRSB has a better adaptability to the tracer granularity with the same gas flowrate. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Since spouted bed was developed by Mathur and Gishler [1], it has been widely studied and extensively used in various physical and chemical processes such as drying, coating, granulation, combustion, gasification, etc. [2–7]. Spouted bed has been proven to be an appropriate alternative to fluidized bed to dealing with particles of relatively large size, which are too large to be satisfactorily fluidized in fluidized beds. Slot-rectangular spouted bed (SRSB), which was originally called “two-dimensional spouted bed” until the revelation of the three-dimensional effects by Dogan et al. [8] and Freitas et al. [9,10], has been proposed as a modification for conventional spouted bed (CSB) to eliminate its scale-up difficulties [11,12]. The circulating fluidized bed (CFB) boiler technology has become one of the approved clean coal combustion technologies that commercially applied. As a key auxiliary device for CFB boilers, bottom ash cooler (BAC) is used to treat the high temperature bottom ash discharged ⁎ Corresponding author. Tel./fax: +86 23 65102475. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (L. Gan), xfl[email protected] (X. Lu). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.03.021
from the furnace of CFB boiler, and to recover heat in the ash and make the ash easily handled and transported. Without good operational reliability of the BAC, the stable, secure and economic operation of the boiler cannot be guaranteed [13–15]. There are many kinds of BACs available for large-scale CFB boilers, and rotary ash cooler (RAC) and fluidized bed ash cooler (FBAC) are commonly applied in China. FBAC has been proven to be more attractive than RAC, because of its unique technical features such as higher heat-transfer coefficient, greater output capacity, a function of returning partial of fine particles of the bottom ash back into the furnace by fluidizing air from the FBAC, and a better thermal economy of reclaiming heat in the bottom ash [16,17]. And the majority of FBACs are the bubbling fluidized-bed type. Unfortunately, the application and further development of those FBACs are restricted by some disadvantages such as poor flow ability of the bottom ash, defluidization, agglomeration, etc. All in all, the poor performance of those FBACs is attributed to the high limitation for the ash size. Whereas practical bottom ash has a wide range of size distribution, and contains coarse particles that are too large to be satisfactorily fluidized in FBACs [17–19]. It is noteworthy that there is another kind of FBAC, which is called gas-tank spouted bed ash cooler, could handle practical bottom ash
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under the use of a rather high superficial gas velocity. But its large air consumption, which finally returns to the furnace, has a great impact on the combustion condition of the furnace, and a restriction to the output capacity [19,20]. Therefore, it's necessary to develop a novel FBAC that can overcome the problems encountered by the existing FBACs and meet the needs of large-scale CFB boilers. Based on the basic operating principles and technical features of SRSB, which could be successfully used for particles with a very broad particle size distribution, a novel FBAC called Bi-spouted-bed fluidized bed ash cooler has been proposed by Chongqing University, and has obtained an invention patent in China [21]. The patented ash cooler consists of chambers that have similar structure to SRSB. Compare with SRSB, yet the chamber has an air inlet section located below the inclined base, and a porous plate distributor is provided to supply fluidizing air into the chamber instead of a slot. Moreover, to enhance the heat transfer between fluidizing air and particles on the premise of guaranteeing a good flow ability of the bottom ash, a relatively larger ratio of the distributor width to the column width and a relatively low superficial gas velocity range are selected. Thus, the central region of the bed of particles is working in bubbling fluidized or turbulent fluidized state. In the central region of the bed, particles are carried upwards by the bubble wakes. As bubbles burst at the upper surface of the bed, part of these particles that are projected above the bed surface fall back upon the bed surface of the central region of the bed after reaching a certain height, and a part of these particles fall into the surrounding downcomer region where the particles slide downwards due to gravity and then enter the central region again. Considering that the flow structure is similar to a combination of bubbling bed and SRSB, and it contains some characteristics of SRSB, we call the bed quasi-slot-rectangular spouted bed (QSRSB). It has been found that the amount of fluidizing air has a direct influence on the conveying of ash in FBAC [15,18]. Actually, if coarse particles of the bottom ash that characterized of poor flow ability could not be diffused quickly, then they would accumulate, causing defluidization and even agglomeration gradually, resulting in shutdown of FBAC finally. Hence, the operational reliability and the maximum output capacity under a specific working condition are based on the characteristics of lateral dispersion of the coarse particles of bottom ash. Numerous investigations have been carried out for SRSBs. SRSB hydrodynamics has been extensively investigated, and most previous work focus on global flow properties and local flow structure, e.g. minimum spouting velocity, maximum pressure drop, pressure drop, maximum spoutable bed height, flow regimes and profiles of voidage and particle velocity [8–10,22–31]. SRSBs have also been studied for their stability and scale-up [9,25,31–33], and for their applications, e.g. coating and drying [34–41]. To the best of our knowledge, investigation on lateral mixing of particles in SRSB has rarely been reported in literatures. Therefore, in order to provide available guidelines for the design, operation and structure optimization of Bi-spouted-bed fluidized bed ash cooler and valuable reference for the investigation of lateral solids mixing in SRSBs, experimental studies of the lateral mixing of tracer particles in a cold model of QSRSB are carried out in this work. And a binary mixture of different sized particles is adopted, and the jetsam (bigger) and flotsam (small) component are used as tracers and bed material respectively. In this work, the lateral mixing process of tracer particles has been studied. Furthermore, the effects of particle size of bed material dp, superficial gas velocity u, static bed height h and air inlet section width δ on the effective lateral dispersion coefficient of tracer particles, which represent its overall lateral mixing rate, has also been investigated thoroughly. And a comparison of the effective lateral dispersion coefficient of tracers in QSRSBs and the bubbling fluidized bed with the same upper cross-section has been conducted.
2. Experiments 2.1. Experimental apparatus A schematic diagram of the experimental apparatus is shown in Fig. 1. It consists of a gas supply system, a test rig body, a dedusting system, etc. The fluidizing gas flows into the distribution header after flowing though a ball valve installed at the gas supply line, and then the fluidizing gas is divided into three equal fluxes by three control valves connected to their individual calibrated rotameters before flowing into the wind boxes. The ball valve is installed to cut off the fluidizing gas immediately. The test rig body consists of a rectangular column with a cross section of 900 mm × 200 mm and a height of 800 mm, and an inclined base located at the bottom of the column with inclination angle θ = 45°. The front side and back side of the column are made of transparent Plexiglas, and six sampling windows of 100 mm × 200 mm are made on the back side. An air inlet section of 40 mm height is installed under the inclined base. Three air inlet sections with widths, δ, of 44 mm, 52 mm and 60 mm, and their individual corresponding inclined bases are used in this study to investigate the effect of the δ on the effective lateral dispersion coefficient of the tracer particles. A porous plate distributor, with two hundred and forty evenly distributed orifices of 3 mm in diameter, is used. And a layer of canvas is set on the distributor to prevent the materials leaking into wind boxes and to reinforce the uniformity of gas distribution across the bed. A conventional bubbling fluidized bed is available, if the inclined base and air inlet section are removed and the porous plate distributor and wind boxes are replaced with appropriate one respectively. 2.2. Materials The bottom ash from a CFB boiler is used as the materials in this study after sieving into four narrow particle size ranges, and the properties of these particles are listed in Table 1. The bottom ash with larger particle size of 0.71–1.00 mm are used as tracers, while the others with smaller particle size are chosen as bed materials. 2.3. Experimental procedure The bed collapse method [42–44] and a set of partitions are developed and adopted here to measure the lateral concentration distribution of tracer particles in this study. As shown in Figs. 1 and 2, it consists of a single partition and a partition-box made of thin stainless steel plates so as to separate the whole bed into parts conveniently. The single partition can be used individually or be accompanied with the use of partition-box. If the single partition and partition-box are inserted into the column simultaneously, the column would be divided into six sampling regions of equal volume along the lateral direction: A, B, C, D, E and F. Before each run, the single partition was inserted into the column to separate the region A from the others. Then a batch of mixture of tracers and bed material was poured into region A to a specific static bed height, and the others were charged with bed material to the same height. All experiments were carried out with the mass of tracer particles accounted for 12% of the total particles mass. After the above preparation, the ball valve was turned on and the gas flow rates were adjusted according to the desired operation condition. When the stable state of fluidization was established, the single partition was quickly removed. After a specific time interval that depending on the particle size of bed material dp, the superficial gas velocity u, static bed height h and the air inlet section width δ [43], the single partition and partition-box located on the top of the column were instantaneously inserted to divide the bed into six lateral sampling regions, as shown in Fig. 2. At the same time, the gas supply was terminated by immediately closing the ball valve and opening the bypass valve
L. Gan et al. / Powder Technology 243 (2013) 1–8
3
Fig. 1. Schematic diagram of experimental apparatus.
simultaneously, and then the bed collapsed. The sampling windows on the back side were opened, and the mixture of particles in each sampling region were sucked by electric vacuum cleaner, separated by screening, and weighed. The local tracer concentration η i (i = A, B, C, D, E, F.) of each sampling region is defined as the ratio of the mass of local tracer particles over the total mass of particle mixture in the sampling region, and the mass of bed material and tracers in a given sampling region are marked as mbi and mti, respectively. Experiments showed that, in different tests, the mass of particles carried away by the fluidizing gas and then captured by the cyclone account for less than 1% of the total mass. Consequently, the influence of the tiny variance of the mass of particles before and after each run was ignored. To investigate the reliability and repeatability of the
Table 1 Physical properties of material. dp (mm)
ρp (kg/m3)
umf a (m/s)
0.25–0.35 0.35–0.50 0.50–0.71 0.71–1.00
2344 2344 2344 2344
0.09 0.12 0.34 0.56
a
umf was determined by experiments in a bubbling fluidized bed.
measured tracer concentrations, five run were repeated for three test conditions. Fig. 3 presents the means with error bars for the five repeats of the three different cases. Obviously, the measurement is repeatable and the maximal error is below 3%, which means that the method is feasible and accurate with good repeatability. The experimental conditions studied in this work are listed in Table 2. 3. Mechanism of lateral mixing and treatment of data 3.1. Mechanism of lateral mixing It is commonly recognized that solids mixing in a fluidized bed is mainly caused by the movement of bubbles [45,46]. The lateral mixing of solids in fluidized bed is attributed to the following reasons: (a) As a bubble rises, which carries particles up through the bed in its wake, it pushes emulsion aside. However, the solids passing close enough to the bubble are eventually swept into the wake, mixed with the solids already there completely and then leave the wake from random positions back into the emulsion eventually, thereby giving rise to lateral mixing of solids. (b) As a bubble bursts at the upper surface of the bed, part of these particles projected above the bed surface fall back upon the surrounding bed surface, thus inducing lateral mixing of solids eventually. (c) The gross particle circulation or
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Fig. 2. Schematic diagram of the sampling regions.
so-called “Gulf Stream”, caused by the lateral non-uniform distribution of bubbles, could also contribute significantly to the lateral mixing of solids [42,43,46–48]. 3.2. Treatment of data A number of works concerning lateral solids mixing in gas fluidized bed reactors focused on the overall lateral solids mixing rate and fitted the lateral mixing by a Fickian-type diffusion equation. Actually, Lateral solids mixing in gas fluidized beds are attributed to the contributions from lateral convection and lateral dispersion. However, to simplify the treatment, the contribution of lateral convection to the lateral solids
mixing was included into lateral dispersion, which means that convection term and dispersion term were combined into dispersion term. And then dispersion terms with an effective lateral solids dispersion coefficient were adopted, and there were no longer a convective term in the equations, and the effective lateral dispersion coefficient was generally considered to be constant over the bed and a representation of the overall lateral solids mixing rate [43–45]. As mentioned in Section 1, the central region of QSRSB is working in bubbling fluidized or turbulent fluidized state, thus the Fickian-type diffusion equation for lateral solids mixing in gas–solid fluidized beds is used to characterize lateral mixing of tracer particles in the present work. Considering that experiments are carried out on the lateral mixing of tracer particles at the macroscopic level, the tracers are assumed to be evenly distributed over the vertical cross section normal to the lateral direction, thus the one-dimensional dispersion model is used here. Additionally, there is no source term in this study. Consequently, the governing differential equation of the model is ∂C ∂2 C ¼ Dsr 2 ∂t ∂x
ð1Þ
The appropriate initial and boundary conditions are C¼
mt 0:15
C¼0
for t ¼ 0;
for t ¼ 0;
∂C ¼ 0 for ∂x
ti0;
ð2Þ
0≤x≤0:15
ð3Þ
0:15≤x≤0:9 x¼0
and x ¼ 0:9
ð4Þ
Table 2 Summary of the experimental conditions.
Fig. 3. Means with error bars of the tracer concentrations for the five repeats of three different cases.
dp (mm)
δ (mm)
h (mm)
u (m/s)
0.25–0.35 0.35–0.50 0.50–0.71
44, 52, 60 60 60
150 100, 125, 150, 175, 200 150
0.70–3.67 0.90–2.56 1.29–2.56
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where C is tracer mass concentration (kg/m), a function of coordinate x (m) and time t (s), corresponding to the mass of tracer particles per unit length, mt is the total mass of the tracer particles (kg) loaded in the bed for each run, and Dsr is the effective lateral solids dispersion coefficient (m 2/s) lumped together all lateral transport processes, which is considered to be a constant across the bed. To avoid doing numerical calculation repeatedly and simplify the solution procedure, Eqs. (1)–(4) were rewritten into dimensionless form and the effective lateral solids dispersion coefficient was removed as a parameter [45]. The corresponding governing equation, initial and boundary conditions are:
mt 0:15
C¼0
∂C ¼0 ∂xn
1 ≤x ≤1 6 n
ð7Þ
4.2. Effects of superficial gas velocity and particle size of bed material on the effective lateral dispersion coefficient
xn ¼ 0
ð8Þ
for τ ¼ 0;
for τ〉0;
4.1. Mixing process with time
ð6Þ
ð5Þ
for τ ¼ 0;
4. Results and discussion
Fig. 4 also illustrates experimental measured transient tracer concentration profiles of the lateral mixing process for a typical test condition (dp = 0.25–0.35 mm, δ = 60 mm, h = 150 mm and u = 0.9 m/s). To a specific profile, it can be seen that the longer the distance away from region A, the smaller the local tracer concentration. Moreover, the differences of local tracer concentrations among regions are gradually leveled off with time, i.e., the tracer concentration becomes more even as time proceeds, indicating the tracers are gradually dispersed away from the region A. To investigate the effects of parameters on the lateral mixing process of tracer particles more intuitively and conveniently, it is appropriate to calculate the effective lateral dispersion coefficient which characterizes the rate of lateral mixing process based on the experimental measured tracer concentration profile.
∂C ∂2 C ¼ ∂τ ∂xn 2
C¼
5
0≤xn ≤
1 6
and xn ¼ 1
where xn is the dimensionless coordinate, expressed as xn = x/Lx, τ is the dimensionless time and τ ¼ Dsr t=Lx 2 , and Lx is the length of the column along the lateral direction, Lx = 0.9 m. The solution of Eq. (5) subjected to these initial and boundary conditions is a series of profiles of tracer mass concentration, C(xn,τ), as a function of dimensionless coordinate and dimensionless time. It was firstly converted to a series of profiles of tracer concentration, i.e., η i(τ), where i = A, B, C, D, E, F. Then found out the appropriate transient tracer concentration profile that has the minimum of the sum of the squares of the errors with the experimental measured lateral distribution of tracer concentration and its corresponding dimensionless time. Finally, the effective lateral dispersion coefficient Dsr could be determined by inserting numerical values of the time interval that the mixing process lasted, dimensionless time and Lx, corresponding to the run, into Dsr ¼ τLx 2 =t. The numerical calculated tracer concentration profiles at t = 90 s, 150 s and 240 s for a typical test condition (dp = 0.25–0.35 mm, δ = 60 mm, h = 150 mm and u = 0.9 m/s) are shown in Fig. 4. And the experimental measured tracer concentrations, expressed as scattered points, are compared with the predicted results. It is obvious from Fig. 4 that the agreement between the experimental measured and predicted concentrations appears to be reasonably good, i.e. a proper value of Dsr is acceptable to match the predicted and measured tracer concentrations satisfactorily.
Fig. 5 is a plot of the effecitive lateral tracer dispersion coefficient Dsr against superficial gas velocity u for three different bed materials at a bed height h of 150 mm and an air inlet section width δ of 60 mm. From the figure, it can be seen that for all bed materials, the effective lateral dispersion coefficient Dsr increases with the increase of superficial gas velocity u. It's mainly because increasing gas velocity results in the increase of bubble size. Correspondently, bubbles become faster and more vigorous, which leads to more intense projection of solids by the burst of bubbles at the bed surface, thus promoting lateral dispersion of tracer particles. Additionally, similar to the results of literature [44], the effective lateral dispersion coefficient Dsr increases with decreasing particle size of bed material at a given gas velocity. And this finding is useful in practice: for a FBAC, while the flowrate of fluidizing air is restricted, it is feasible to improve lateral solids dispersion by selecting the location of the ash discharge chute appropriately, since the location of the chute has been proven to be able to adjust the bed composition [46]. 4.3. Effect of static bed height on the effective lateral dispersion coefficient The effect of static bed height h on the effective lateral tracer dispersion coefficient Dsr is illustrated in Fig. 6. Obviously, for all gas velocities, the dispersion coefficient Dsr increases with increasing static bed height h. This can be attributed to the facts that the observed bubble flow increases with height in the bed [46], and bubbles become larger and faster caused by the coalescence as they rise up through the bed. Therefore, the lateral tracer dispersion, induced by the bursting of bubbles at the bed surface, would become more intense as the static bed height rises, resulting in the increase of Dsr.
Fig. 4. Comparisons of experimental tracer concentration profiles and predicted results with Dsr = 6.77 × 10−4 m2/s at dp = 0.25–0.35 mm, δ = 60 mm, h = 150 mm and u = 0.9 m/s.
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Fig. 5. Effect of u on the effective lateral dispersion coefficient Dsr under different dp at δ = 60 mm and h = 150 mm.
4.4. Effect of air inlet section width on the effective lateral dispersion coefficient Fig. 7 shows the effect of superficial gas velocity u on the effective lateral dispersion coefficient Dsr with the width of air inlet section δ as a parameter at a given static bed height of 150 mm and bed material of dp = 0.35–0.50 mm. It can be seen that for all air inlet section widths, the effective lateral dispersion coefficient Dsr increases with the increase of gas velocity u. Moreover, the effective lateral dispersion coefficient Dsr increases as the air inlet section width δ increases. Probably, this is attributed to the fact that the increase of air inlet section width δ results in the increase of observed bubble flow, and the weakening of the wall effects, thus leading bubbles to become larger and faster, and causing Dsr increase finally. The relationships between U and effective lateral tracer dispersion coefficient Dsr for QSRSBs of different δ and bubbling fluidized bed are shown in Fig. 8. Here, U is the gas velocity at the upper section with a cross section of 900 mm × 200 mm that represent gas flowrate, whereas superficial gas velocity u is the gas velocity at the air inlet section. It is shown for four cases that as U increases gradually, the Dsr increases monotonically. However, unlike the relationships between u and Dsr for QSRSBs of different air inlet section widths revealed in Fig. 7, the lateral dispersion coefficient of QSRSB is virtually independent of air inlet section width δ for a certain U. This is explainable that at the same U, the decrease of δ results in the increase of u and thus counteracting the
Fig. 6. Effect of h on the effective lateral dispersion coefficient Dsr under different u at δ = 60 mm and dp = 0.35–0.50 mm.
Fig. 7. Effect of u on the effective lateral dispersion coefficient Dsr under different δ at h = 150 mm and dp = 0.25–0.35 mm.
influence of δ, which make the Dsr remain unchanged. Moreover, when U is greater than 0.3 m/s, the Dsr of bubbling fluidized bed appears to be larger than that of QSRSBs. This phenomena may be attributed to the following reasons: 1) in QSRSB, parts of particles that are projected when bubbles burst at the upper surface of the bed fall back into the downcomer, where the particles just slide downwards and then reenter the central region, and no lateral movement take place during this period for these particles; 2) wall effects retard the rise of bubbles more seriously in QSRSBs; 3) the gross particle circulation or so-called “Gulf Steam” contribute significantly to the lateral dispersion of solids in bubbling fluidized bed. However, it is interesting to note that the minimum U, Um, ensuring the normal operation of QSRSBs are less than that of bubbling fluidized bed in this study, and tracer particles would be found to rest on the gas distributor of region A if U is less than Um. In other words, it demonstrates that the QSRSB could handle coarser tracers that are too large for bubbling fluidized bed with the same U or gas flowrate. It also indicates that the Bi-spouted-bed fluidized bed ash cooler based on the QSRSB has a better adaptability to the bottom ash granularity than FBACs of bubbling fluidized-bed type. 5. Conclusions Based on the well-designed experiments, the lateral mixing of tracer particles has been studied. The bed collapse method and a
Fig. 8. Effect of U on the effective lateral dispersion coefficient Dsr under different δ at h = 150 mm and dp = 0.25–0.35 mm.
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set of partitions were successfully applied to obtain the lateral tracer concentrations. Furthermore, the lateral dispersion coefficient of tracer particles was measured, and the effects of particle size of bed material dp, superficial gas velocity u, static bed height h and air inlet section width δ on the effective lateral dispersion coefficient of tracers were investigated. And a comparison of the effective lateral dispersion coefficient of tracers in the QSRSBs and the bubbling fluidized bed with the same upper cross-section was conducted. The following conclusions can be drawn from this study: (1) The tracers disperse from regions of higher tracer concentration to regions of lower tracer concentration, and the differences of local tracer concentrations among regions are gradually leveled off as time proceeds. (2) The effective lateral dispersion coefficient of tracer particles Dsr increases with the increase of gas velocity u, static bed height h and width of air inlet section δ, respectively. However, increase in the size of bed material dp causes a decrease in the effective lateral dispersion coefficient Dsr. (3) Air inlet section width δ has no influence on the effective lateral dispersion coefficient of tracers at given U for QSRSBs. Dsr of bubbling fluidized bed appears to be larger than that of QSRSBs when U is greater than 0.3 m/s, but the Um of QSRSBs are less than that of bubbling fluidized bed, indicating that the QSRSB could handle coarser tracers that are too large for bubbling bed with the same U or gas flowrate. And this also indicates that the Bi-spouted-bed fluidized bed ash cooler based on the QSRSB has a better adaptability to the bottom ash granularity than FBACs of bubbling fluidized-bed type.
Nomenclature BAC bottom ash cooler CFB circulating fluidized bed CSB conventional spouted bed FBAC fluidized bed ash cooler QSRSB quasi-slot-rectangular spouted bed RAC rotary ash cooler SRSB slot-rectangular spouted bed C, C(xn,τ) tracer mass concentration, i.e. mass of tracers per unit length (kg/m) Dsr effective lateral dispersion coefficient of tracers (m 2/s) Lx length of the column along the lateral direction, Lx = 0.9 m U gas velocity at the upper section with a cross section of 900 mm × 200 mm (m/s) Um minimum U that ensuring the normal operation of QSRSBs (m/s) dp particle size of bed material (mm) h static bed height (mm) i labels for the sampling regions, i = A, B, C, D, E, F mt total mass of the tracers loaded in the bed for each run (kg) mbi mass of bed material in sampling region (kg) mti mass of tracers in sampling region (kg) t time (s) u superficial gas velocity at the air inlet section (m/s) x lateral coordinate (m) xn dimensionless coordinate (−) δ width of the air inlet section (mm) η i, η i(τ) tracer concentration of sampling region, ηi(τ) = mti/(mti + mbi) (−) θ inclination angle of the inclined base located at the bottom of the bed, θ = 45° τ dimensionless time (−)
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