Electrostatic characteristics in a large-scale triple-bed circulating fluidized bed system for coal gasification

Chemical Engineering Science 75 (2012) 435–444

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Electrostatic characteristics in a large-scale triple-bed circulating fluidized bed system for coal gasification Yongpan Cheng a, Eldin Wee Chuan Lim a, Chi-Hwa Wang a,n, Guoqing Guan b, Chihiro Fushimi c, Masanori Ishizuka d, Atsushi Tsutsumi d a

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3 Matsubara, Aomori 030-0813, Japan c Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan d Collaborative Research Center for Energy Engineering, Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan b

a r t i c l e i n f o

abstract

Article history: Received 24 August 2011 Received in revised form 21 March 2012 Accepted 24 March 2012 Available online 4 April 2012

Electrostatics charge generation by triboelectrification has significant implications for the proper design and operation of a circulating fluidized bed. In this study, electrostatics in the fully developed regions of both the riser and downer of a large-scale triple-bed combined circulating fluidized bed was characterized in terms of the equivalent currents over the cross section of the developed region. The average equivalent currents and solids holdup were measured under different superficial velocities in the riser, downer and gas-sealing bed. It was found that in the fast fluidization regime in the riser, the negative equivalent currents were comparable with the positive equivalent currents due to the typical core-annulus flow pattern. With increasing superficial velocities in the riser or in the gas-sealing bed, the flow pattern would approach dilute phase transport or dense suspension upflow regime. Thus, the positive equivalent currents became dominant because the backflow of sand particles were greatly suppressed. Some dominant frequencies for the equivalent currents in the riser were almost identical regardless of the magnitude of superficial velocities in the riser and the gas-sealing bed, indicating that they were determined by the inherent characteristics of electrostatics and/or signal noise, and were not affected by gas–solids flow behaviors. The frequencies in the downer were focused on the low value range, and the dominant frequencies were nearly zero. Both the solids mass flux and solids holdup had significant influence on the average equivalent currents. With increasing superficial velocities in the gas-sealing bed, the average equivalent currents first increased and then approached a constant. This led to the same trend in variation of solids mass flux, as well as solids holdup. With increasing air superficial velocities in the riser, the average equivalent currents in the riser increased, while the average equivalent currents decreased in the downer with increasing air superficial velocities in the downer. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Electrostatics Energy Multiphase flow Chaos Fluidization Hydrodynamics

1. Introduction Coal gasification is an effective way to reduce the emission of CO2, which is a major contributor to climate change and global warming, especially for low-rank coals such as lignite and subbituminous coal. In coal gasification, circulating fluidized beds have been widely adopted to improve the energy conversion efficiency. So far the hydrodynamics in the riser and downer reactor in the circulating fluidized bed has been studied extensively, but the influence of electrostatics on hydrodynamics is seldom addressed (Matsusaka et al., 2002, 2010). In addition to

n

Corresponding author. Tel.: þ65 6516 5079; fax: þ 65 6779 1936. E-mail address: [email protected] (C.-H. Wang).

0009-2509/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2012.03.040

being a safety hazard, the buildup of electrostatic charges within the riser and downer may cause deviation in performance of the system from that for which it was originally designed. The generation of electrostatic charges due to continuous and frequent collisions and friction between the granular material being transported and the walls of the conveying pipes, also referred to as triboelectrification, is an unavoidable and usually undesirable phenomenon in most dry particulate systems (Matsusaka et al., 2002, 2010). Joseph and Klinzing (1983) found that the pressure drop at choking conditions and the required gas velocity at minimum pressure drop in vertical pneumatic conveying were higher in the presence of electrostatic forces. These were accompanied by violent pressure fluctuations and increased power requirements for the pneumatic transport operation, hence they suggested that electrostatic effects should be minimized in

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the design of pneumatic conveying systems as these have an adverse effect on the optimal operating conditions. Al-Adel et al. (2002) have also emphasized the importance of considering electrostatic effects in analyzing gas–solid flows in their study of radial segregation of particles in vertical risers. The clustering patterns of such granular materials have been observed to be dependent on the voltage applied as well as the humidity of the environment. A similar phenomenon was observed by Yao et al. (2004) for larger particles with diameter 2.8 mm. They observed formation of three characteristic aggregation patterns referred to as clusters, half-ring and ring during pneumatic conveying through a vertical pipe in the presence of electrostatic effects. Yao and Wang (2006) developed a method to investigate the effects of granule size and shape on electrostatics generation in pneumatic conveying systems and demonstrated that the proposed method is a useful tool for characterization of electrostatic effects in systems where granules have different sizes and geometries. The electrostatic field strength in a pneumatic conveying system is determined by the amount of charges accumulated on the pipe walls at the state of electrostatic equilibrium. Yao et al. (2006) have shown that as the state of electrostatic equilibrium is established, it is possible to evaluate the timeinvariant electrostatic field strength. By using miniature collision ball probe, Yao et al. (2002) measured the local instantaneous electrostatic charges in the freely bubbling fluidized bed as well as differential pressure fluctuations. Their results showed that the amplitude of voltage signals from the ball probe was mainly equivalent by passing bubbles. Giffin and Mehrani (2010) studied the influence of fluidization time on the electrostatic charge characteristics at three different regions of the fluidized bed in the bubbling and slugging flow regimes. Bipolar charging was observed with relatively smaller particles becoming predominately positively charged and the larger particles becoming predominately negatively charged. With regards to the suppression of electrostatic charges, Zhang et al. (1996) reported that the addition of an anti-static agent (Larostat powder) caused cohesive particles belonging to Group C in the Geldart classification scheme to behave similarly to Groups A and B particles. More recently, Wu and Bi (2011) reported that particle size had significant effects on the accumulation of electrostatic charges in bulk powder. The addition of small particles to large bulk powders reduced electrostatic charge accumulation and it was observed that carbon nanotubes was a much more effective antistatic agent than other additives. Park and Fan (2007) extended the electrostatic study to three-phase gas–liquid–solid fluidized bed, and found that electrostatic charging was affected by the superficial gas and liquid velocities due to the variation in the frequency and the intensity of the particle collisions. By adding fine powder and an anti-static agent, the charges inside the fluidized bed could be greatly reduced. In the area of numerical simulations, Matsusaka et al. (2002) pointed out that the electrostatic charge distribution in gas–solids pipe flow was determined by (1) the number of collisions of a particle with the wall, (2) initial particle charge, and (3) the impact electrification factor characterizing the transferred charge. Based on these assumptions, they proposed a model for tribocharging and validated it with experiments using fly ash particles. Park et al. (2002) developed a mechanistic model to distinguish equivalent and transferred charges by applying the method of images in the bubbling fluidized bed. The results showed that the trace of equivalent charge vs. time is insensitive to the thickness of the layer of charged particles at the bubble surface with a uniform charge density distribution. However, the charge transfer during collision of particles surrounding the rising bubble with a probe was greatly affected by the particle velocity profile. This model was improved later by Chen et al. (2003) by considering

the charge buildup on particles remote from the bubble, and the particle charge density distribution in the vicinity of the bubble, thus significant improvement in agreement between the model and experimental results was achieved. Discrete Element Method (DEM) coupled with Computational Fluid Dynamics (CFD) has been used for numerical simulations of pneumatic conveying of granular materials through pipes (Lim et al., 2006a). In particular, it was shown that the eroding dunes regime observed experimentally by previous research workers to occur in pneumatic conveying through an inclined pipe where significant electrostatic effects were present could be reproduced computationally by incorporating an electrostatic field model into the CFD–DEM method (Lim et al., 2006b). A thin layer of particles remained adhered to the pipe walls during the pneumatic conveying process due to the effects of strong electrostatic forces of attraction towards the pipe walls. Based on dynamic analyses of forces acting on individual particles, it was concluded that electrostatic effects played a dominant role in influencing particle behaviors during pneumatic conveying through vertical and horizontal pipes at low flow rates while drag forces became more important at high flow rates (Lim et al., 2012). From the literature review above, it could be seen that although the effects of electrostatics on flow behaviors in fluidized beds have been well-established by numerous studies conducted using laboratory scale fluidized bed systems, studies of such effects in larger scale systems are still lacking in the literature. In this paper, the effects of electrostatics were characterized in a large-scale riser (16.6 m in length and 0.10 m in diameter) and downer (6.5 m in length and 0.10 m in diameter). The influence of several operating parameters, such as superficial velocity in the riser, downer and gas-sealing bed, solids mass flux, solids holdup on the electrostatic charges and the corresponding power spectra were analyzed. Furthermore, different flow regimes may occur in the riser of the circulating fluidized bed depending on the operating solids mass flux, superficial velocity, particle property, etc. The flow regimes are expected to affect the electrostatic effects in the riser and this will also be addressed in this paper.

2. Experimental set-up Fig. 1 shows the schematic diagram of the cold model of a large-scale triple-bed circulating fluidized bed (TBCFB), which is composed of a riser, a downer, a bubbling fluidized bed (BFB), a multi-tube solid distributor for the downer, a gas–solid separator and a gas-sealing bed (GSB). The dimensions can be seen in Table 1. In order to provide sufficient driving force to create a large solids mass flux from the BFB to the riser, the GSB was installed between the riser and the BFB (Liu et al., 2008; Fushimi et al., 2011). In addition it served as a gas seal between the BFB and the riser. At the top of the riser, the solids passed through a smooth elbow into the primary cyclone for gas–solid separation, and some escaped solids were collected by the secondary and tertiary cyclones and returned to the circulating system. A modular parametric current transformer (MPCT: Bergoz Instrumentation, France) was mounted at the upper region of the riser (at the height of 16.15 m) and the lower region of the downer (3.8 m from the entrance) to characterize the electrostatic effects, as the flow was considered to be fully developed at these two locations from the analysis of pressure variations. As the top of riser and the whole downer were made of acrylic, electrostatic charge would be generated when the sand particles flowed inside them. At the top of the downer, the solids were redistributed by fluidizing air flowing through a solids distributor with 13 vertically positioned brass tubes each with an inner diameter of

Y. Cheng et al. / Chemical Engineering Science 75 (2012) 435–444

19 mm. Downward flowing air was introduced at the entrance of the downer. The co-current flowing air and solids moved along the downer, and the solids were separated from the air at the end of the downer by a quick inertial separator and returned to the BFB. 46 pressure taps were installed along the TBCFB system and differential pressure sensors (Keyence Corp., AP48) were used.

437

The distance between adjacent sensors along the riser was 1 m and that along the downer was 0.5 m. The output signals from the differential pressure sensors and the MPCT were acquired at a sampling frequency of 100 Hz in the riser and 1000 Hz in the downer within 180 s via a data acquisition system (CONTEC, AIO163202FX) and a laptop computer. The equivalent current could be generated due to the motion of a charged particle. Therefore, in the system when the bulk of sand particles with charges were moving, the current would pass through the cross section of riser or downer, and it can be detected by MPCT using a non-invasive DC beam with a resolution of 1 mA, hereafter the current would be called as ‘‘equivalent current’’. Solids mass flux was measured using a butterfly valve by measuring the time taken to accumulate a given amount of particles. A mean value was calculated based on 10 sets of measurements made at the steady state. The details of the apparatus and procedures have been reported in our previous studies (Fushimi et al., 2011, Guan et al., 2011). The size distribution of the particles has also been reported previously (Guan et al., 2011). Due to the frequent particle-particle and particle-wall collisions and friction in the riser and downer, the sand particles tended to acquire electrostatic charges via triboelectrification (Matsusaka et al., 2002, 2010). Matsusaka et al. (2002) suggested that, the charge transferred per unit time, i.e. electric current, is an important evaluation factor in particle charging in gas–solids pipe flow. Since the sand particles had different sizes and velocities, the charges on the sand particles were widely scattered, and the equivalent electric currents by sand particles had different strengths and directions. In this experiment only the equivalent current was measured over the cross-section by MPCT, as seen in Fig. 2. The weak signals for the equivalent current were amplified through the electronics card, and then acquired into the laptop through the data acquisition system. The pressure was determined by the static solid pressure, the gas–wall friction, the sand–wall friction and the solid acceleration (Guan et al., 2010). Based on the pressure gradient method, the apparent solids holdup averaged over the cross section was calculated by assuming that the wall friction and solids

Riser

Particles with charge Electronics card

Current

MPCT

Data acquisition system Laptop computer

Air

Fig. 1. Schematic diagram of the large scale triple-bed combined circulating fluidized bed (TBCFB) cold model and position of MPCT.

Fig. 2. Schematic diagram of measurement on equivalent current with modular parametric current transformer (MPCT).

Table 1 Experimental Parameters. Dimension of riser Dimension of downer Dimension of bubbling fluidized bed Dimension of gas-sealing bed Sand density, rs Mean particle size,dm Superficial velocity in downer, Ugd Superficial velocity in riser, Ugr Superficial velocity in gas-sealing bed, Ugsb

f0.1 m(I.D.)  16.6 m(H) f0.1 m(I.D.)  6.5 m(H) 0.75 m(L)  0.27 m(W)  3.4 m(H) f0.158 m(I.D.)  5.0 m(H) 2600 kg/m3 128 mm 0–4 m/s 6–10 m/s 0–0.085 m/s

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acceleration were negligible

es ¼ DP=ðrs g DHÞ

14

ð1Þ 12

Ugr = 6 m/s Ugr = 10 m/s

10 Height H, m

This method was validated by Issangya (1998), and he added the terms for gas and solids-to-wall friction to the momentum equation for one-dimensional acceleration suspension flow, which was originally proposed by Louge and Chang (1990), and corrected for the effects of particle acceleration and friction. It was found that the deviation between the apparent and corrected solids holdups was always less than 20%. In a separate test, Issangya (1998) compared the apparent solids holdups with the mean values obtained from integration of local voidage data from an optical fiber probe, and the deviation was found to be about  6 to 10%. These deviations were small enough that the calculated apparent solids holdups were not affected significantly.

8 6 4 2 0

3. Results and discussion

0.00

3.1. Electrostatic characteristics in riser under different superficial velocities (Ugr) 3.1.1. Hydrodynamic behavior Fig. 3 shows the influence of air superficial velocity in the riser (Ugr) on the solids mass flux when air velocity in the gas-sealing bed (Ugsb) was zero. It could be found that the superficial velocity in the riser had little influence on the overall solid mass flux, because the driving force for the solids mass flux came from the static pressure drop of sand particles between the bottom of the gas sealing bed and the riser and was insufficient (Fushimi et al., 2011). As the bed height in the bubbling fluidized bed was kept constant, and the gas velocity in the gas-sealing bed was always zero, the solids mass flux Gs varied in the narrow range of 101– 105 kg/(m2s) even when the air velocity in the riser (Ugr) was changed from 6 to 10 m/s. According to Zhu and Zhu (2008), the riser can be divided into three flow regimes according to the axial solids holdup distributions: a bottom dense region, a transition region and an upper dilute region. Fig. 4 shows the solids holdup variations along the riser at superficial velocity Ugr ¼6 and 10 m/s at Ugsb ¼0. It was found that the high apparent solids holdup appeared at the bottom of the riser, which should be attributed to the particle acceleration and solids entry configuration. Beyond this region the solids holdups decreased sharply, eventually approximately

160

Solids mass flux Gs, kg/(m2s)

140

120

100

80

60 5

6

7 8 9 10 Air superficial velocity in riser Ugr, m/s

11

Fig. 3. Solids mass flux variation over air superficial velocity in riser at Ugsb ¼0.

0.02

0.04 Solids holdup, s

0.06

0.08

Fig. 4. Solids holdup distribution over air superficial velocity in riser at Ugsb ¼ 0.

approaching a constant value in the higher region of the riser. It was also found that the average solids holdup at Ugr ¼6 m/s was higher than that at Ugr ¼ 10 m/s. With the increase in air velocities the upward drag forces exerted on the particles also increased, leading to high upward solid particle velocities. As a result, when the solids mass flux was kept almost constant (as indicated in Fig. 3), the solids holdup decreased with the increase in Ugr.

3.1.2. Equivalent currents The equivalent current was measured with a MPCT to characterize the electrostatics at the top of the riser. Fig. 5 shows the instantaneous fluctuations of equivalent currents under different air superficial velocities in the riser (Ugr) when the air velocity in the gas-sealing bed Ugsb ¼0. The measurements were taken for 3 min, and only data over 2 s are provided here to demonstrate the strong fluctuations present. When the superficial air velocity Ugr o8 m/s, a typical fast fluidization regime (Bi and Grace, 1995; Rabinovich and Kalman, 2011) appeared at the top of the riser. In this regime the macroscopic flow structure consisted of a dilute up-flowing suspension in the central core region, surrounded by a dense annular suspension of solids adjacent to the wall where the particles traveled predominantly downward as streamers. The negative equivalent currents caused by the backflow of sand particles were recorded, as shown in Fig. 5. At Ugr ¼6 m/s, the negative equivalent currents became almost comparable with the positive equivalent currents due to the strong backflow of sand particles in the annular region near the wall. When the superficial air velocity Ugr Z8 m/s, the net backflow of sand particles near the wall disappeared due to high air velocities, and in this case the flow was changed to the dilute phase transport regime from the fast fluidization regime (Bi and Grace, 1995). The axial particle gradient decreased, and the negative equivalent currents were weakened, while the positive equivalent currents were increased, as indicated at Ugr ¼10 m/s in Fig. 5(b). In order to get insights into the strong fluctuations of equivalent currents, Fast Fourier Transform (FFT) was also used to analyze the equivalent currents, as seen in Fig. 5. It was found that some frequencies such as 19.05, 26.98 and 34.90 Hz appeared regardless of air velocity. In the central region of the riser, the upward flowing sand particles flowed in a pulsating fashion instead of the usual steady state flow, leading to fluctuations in the equivalent currents. In the dense phase near the wall,

Y. Cheng et al. / Chemical Engineering Science 75 (2012) 435–444

1.6x10-5

1E-10 1E-11

Gs = 102 kg/(m2s)

1.2x10-5

εs = 0.011

19.05Hz

26.98Hz 34.90Hz

1E-12 1E-13

8.0x10-6 Power

Equivalent current I, A

439

4.0x10-6

1E-14 1E-15 1E-16

0.0 1E-17 1E-18

-4.0x10-6

1E-19 0.0

0.5

1.0 Time t, s

1.5

1.6x10-5

20 30 Frequency f, Hz

40

50

40

50

1E-9 Gs = 104 kg/(m2s) εs = 0.0057

1.2x10-5

1E-10

19.05Hz

1E-11

26.98Hz 34.90Hz

1E-12 8.0x10-6

1E-13 Power

Equivalent current I, A

10

0

2.0

4.0x10-6

1E-14 1E-15 1E-16

0.0

1E-17 1E-18

-4.0x10-6

1E-19 0.0

0.5

1.0 Time t, s

1.5

2.0

0

10

20 30 Frequency f, Hz

Fig. 5. Instantaneous fluctuations and power spectra of equivalent currents in the riser at Ugsb ¼ 0. (a) Ugr¼6 m/s and (b) Ugr¼10 m/s.

Here Q stands for the charge passing through the MPCT over a period of time. Fig. 6 shows the variation of charge through the MPCT under different superficial air velocities. As the charge increased linearly with time, the time interval for integration was chosen as 60 s. The total charge was divided by the time interval, as shown below Im ¼ Q =Dt

ð3Þ

Fig. 7 shows the variation of average equivalent currents in the riser with air superficial velocity. At low air superficial velocity Ugr ¼6 m/s, although the downward solids velocity near the wall was low, the equivalent negative currents could still counterbalance the positive current due to the high solids holdup there. Therefore, the average current was quite low. However, at high air

-4

4.0x10 Accumulated Charge Q,C

the sand particles would tend to become streamers or strands due to the influence of electrostatic and cohesive forces. These streamers or strands did not flow along the wall with constant velocity, but moved in a reciprocating way instead, thus affecting the equivalent currents. The different frequencies may reflect the different flow patterns of solids flow inside the riser. In order to eliminate the fluctuations caused by positive and negative values in Fig. 5, the equivalent current was integrated with respect to time to get the charge, as follows: Z Q¼ Idt ð2Þ

Ugr = 6 m/s Ugr = 10 m/s

-4

3.0x10

-4

2.0x10

-4

1.0x10

0.0 0

10

20

30

40

50

60

Time t,s Fig. 6. The variation of induced charges through MPCT in the riser at Ugsb ¼ 0.

superficial velocities Ugr Z8 m/s, almost all the sand particles flowed upwards and there was little counterbalance among the individual currents. Hence the equivalent currents were quite high, more than 4 times of the equivalent current at Ugr ¼6 m/s.

Y. Cheng et al. / Chemical Engineering Science 75 (2012) 435–444

5.0x10

-6

0.015

4.0x10

-6

3.0x10

-6

2.0x10

-6

1.0x10

-6

350

0.020

0.010

0.005

0.0 6

7

8

9

10

0.000

Ugr, m/s Fig. 7. Variations of average current and solids holdup in the riser over superficial velocity in riser at Ugsb ¼ 0.

From Fig. 4, it was found that at the upper region of the riser, there was little variation of solids holdup because the solid flow had reached the fully developed state. Hence the average solids holdup above H¼10 m was calculated under different air superficial velocities in the riser. As indicated in Fig. 7, the solids holdup at the lowest velocity Ugr ¼6 m/s was the highest because there was more backflow of sand particles near the wall. With increasing air velocity the solids holdup decreased, but there was little variation of solids holdup at Ugr Z8 m/s where almost all solid particles flowed up throughout the cross section of the riser. As a result, with the same solids mass fluxes and solid holdup at the top of the riser at Ugr Z8 m/s the equivalent currents were almost kept unchanged as well. 3.2. Electrostatic characteristics in riser under different superficial velocities Ugsb in GSB 3.2.1. Hydrodynamic behavior In order to increase the aeration of sand particles between the bottom of the riser and the bubbling fluidized bed, a gas-sealing bed was installed in between the two (Liu et al., 2008; Fushimi et al., 2011). By changing the superficial velocity in the gassealing bed, the flow behaviors of sand particles, which were fed into the riser from the bubbling fluidized bed, as well as the circulating solids mass flux in the system, were found to be changed. Fig. 8 shows the variation of solids mass flux with superficial velocities Ugsb in the gas-sealing bed. Because the top of the riser was connected to the atmosphere through the cyclones, pressure at the top of the riser varied little, thus the pressure at the bottom of the riser increased with increasing air superficial velocity. As the pressure in the gas-sealing bed was almost constant (Fushimi et al., 2011), the pressure drop for the sand particles decreased between the gas-sealing bed and the bottom of the riser. Thus the increasing rate of solids mass flux would decrease and hence when Ugsb Z0.051 m/s the solids mass flux became saturated in the present study. It is possible that air superficial velocity in the gas-sealing bed could not be used to increase the feeding rate of sand particles between the gassealing bed and the bottom of the riser. Thus, the solids mass flux became almost constant. 3.2.2. Equivalent currents Fig. 9 shows the instantaneous fluctuation of equivalent current at the top of the riser with a Gs ¼250 kg/(m2s) at air velocity Ugsb ¼0.085 m/s and Ugr ¼6 m/s. From Fig. 5(a) it was seen that there were comparable negative and positive values in the equivalent current at low Gs conditions (Gs o105 kg/(m2s))

300

2

-6

Solids mass flux Gs, kg/(m s)

6.0x10

Average solids holdup, s

Average current Im, A

440

250 200 150 100 50 0 0.00 0.02 0.04 0.06 0.08 0.10 Air superficial velocity in gas-sealing bed Ugsb, m/s

Fig. 8. Solids mass flux variation over air superficial velocity in gas-sealing bed at Ugr ¼ 6 m/s.

when the air superficial velocity in the gas-sealing bed Ugsb ¼0 due to the strong backflow of sand particles near the wall. When Ugsb as well as the solids mass flux in the riser increased, the sand particles traveled downward in a wall layer with increasing thickness. Then the central region was occupied more and more by particles. This was caused by the fact that the wall layer grew thicker and more sand particles must be transported through the central region to provide for the increased solids flux. Thus, higher shear stress was imposed on the downward wall layer by the upward rising suspension in the central region. This flow could be approaching the regime of Dense Suspension Upflow (DSU) (Grace et al., 1999). As indicated in Fig. 9, it should be noted that the negative equivalent currents were greatly suppressed, and positive values became dominant under this regime. FFT was also used to analyze the equivalent currents at the top of the riser at Ugsb ¼0.085 m/s to obtain the power spectrum, as seen in Fig. 9. Interestingly, similar to the results shown in Fig. 5(a) at Ugsb ¼0, some frequency peaks were also found at Ugsb ¼ 0.085 m/s. The values of these dominant frequencies were almost the same for the two different air superficial velocities. This result shows once again that the dominant frequencies were determined by the inherent characteristics of electrostatics and/ or signal noises, and were not affected by the air superficial velocities or solids mass flux in the riser. With the same method introduced in the previous section, the average equivalent currents over the air superficial velocity in the gas-sealing bed Ugsb are presented in Fig. 10. It was found that at Ugsb ¼0, the average current was only about 1 mA due to the counterbalance between negative and positive equivalent currents. When the air superficial velocity Ugsb in the gas-sealing bed was increased, the average current increased sharply. As the flow was approaching the dense suspension upflow, backflow near the wall was greatly suppressed. Henceforth there was little counterbalance of the positive currents by the negative currents, and the equivalent currents were increased greatly. For example, the average current at Ugsb ¼0.034 m/s was about five times higher than that at Ugsb ¼ 0. It is possible that the upflow of sand particles was dominant and minimally affected by the negative equivalent current. When the air superficial velocities were increased further, there was little variation in the equivalent currents. This was largely because there was little variation on the solids mass flux. As a result, the flow patterns at the top of riser also varied little under the same superficial velocity in the riser. Fig. 10 shows the variation of average solids

Y. Cheng et al. / Chemical Engineering Science 75 (2012) 435–444

1E-9

1.6x10-5 Gs = 250 kg/(m2s) εs = 0.051

1.2x10-5

1E-10

19.07Hz

26.97Hz

1E-11 34.88Hz

1E-12 -6

8.0x10

Power

Equivalent current I, A

441

4.0x10-6

1E-13 1E-14 1E-15 1E-16

0.0

1E-17 1E-18

-4.0x10-6

1E-19 0.0

0.5

1.0

1.5

2.0

0

10

20

30

40

50

Frequency f, Hz

Time t, s

1.0x10

-5

8.0x10

-6

6.0x10

-6

4.0x10

-6

2.0x10

-6

0.15

0.10

0.05

0.0 0.00 0.02 0.04 0.06 0.08 Air superficial velocity in gas-sealing bed Ugsb, m/s

Average solids holdup, s

Average current Im, A

Fig. 9. Instantaneous variation and power spectra of equivalent current in the riser at Ugsb ¼ 0.085 m/s and Ugr ¼ 6 m/s.

0.00

Fig. 10. Variations of average current and solids holdup in the riser over air superficial velocity in gas-sealing bed at Ugr ¼ 6 m/s.

holdup in the upper region of the riser, which had the same trend as that of solids mass flux. With increasing air superficial velocity in the gas-sealing bed Ugsb, the average solids holdup first increased and approached a constant. Due to the higher solids holdup at higher air superficial velocities in the gas-sealing bed, the average equivalent currents were also higher. 3.3. Electrostatic characteristics in downer under different superficial velocities (Ugd) The counter-current flow in the riser and co-current flow in the downer may have different influence on electrostatic charging. Here the air superficial velocity in the riser Ugr ¼6 m/s and in the gas-sealing bed Ugsb ¼0.085 m/s, corresponded to the solids mass flux Gs ¼250 kg/(m2s). Fig. 11 shows the instantaneous fluctuations of equivalent currents in the downer at Ugd ¼1 and 4 m/s. It was found that the equivalent currents were all positive because there was no backflow in the downer. The equivalent current at Ugd ¼4 m/s was weaker than that at Ugd ¼1 m/s, because with the increased superficial velocity in the downer, the solids holdup decreased so that particle-particle and particlewall collisions became less frequent. As seen in Fig. 11, it should be noted that the power spectra of equivalent currents in the downer were also completely different from those in the riser. In the riser, the frequencies of equivalent currents were widely distributed, while in the downer frequencies were predominantly

found in the low frequency range. There was no significant power of equivalent currents beyond a frequency of 10. Under different superficial velocity in the downer, the dominant frequencies were as low value as 0.017 Hz. Perhaps this was caused by the low frequency variation of solids mass flux and solids holdup, as well as the co-current flow pattern in the downer. The average equivalent currents in the downer are shown in Fig. 12 under different superficial velocities in the downer (Ugd). With increasing air velocity in the downer (Ugd), equivalent currents decreased. This should be because the solids holdup in the downer decreased with increasing air velocity Ugd. It should be noted that this result was quite different from the variation of equivalent currents in the riser due to different flow patterns. As shown in Fig. 7, the average equivalent currents varied little at high air superficial velocity in the riser (Ugr) due to the constant solids holdup at the top of the riser. 3.4. Electrostatic characteristics in downer under different superficial velocities in gsb (Ugsb) Fig. 13 shows the instantaneous fluctuations of equivalent currents in the downer under different air superficial velocity in the gassealing bed, which caused the variation of solids mass flux through the downer, as seen in Fig. 8. Due to the co-current flow characteristics in the downer, the equivalent currents were all positive. It was also found that the frequencies were mainly in the low value range, and the dominant frequencies were fixed at 0.017 Hz. The influence of air superficial velocity in the gas-sealing bed on the average currents and solids holdups is shown in Fig. 14. According to Guan et al. (2011), the sand particle flow became fully developed after a distance of 2 m from the entrance of the downer. Thus, there would be little variation of solids holdup in the downstream of the downer. In Fig. 14, the variation of solids holdup in the downer, in fact, showed the same trend as that of solids mass flux, i.e. with increasing air superficial velocity Ugsb, solids holdup first increased and then approach a constant. As the equivalent current was equal to the product of solids mass flux and charge density of solid particles, the variation of average equivalent current followed the same trend as that of solids holdup. 4. Conclusions The electrostatics in a riser and downer of a triple-bed combined circulating fluidized bed were characterized experimentally in

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8.0x10-6

4.0x10-14 εs = 0.015

7.0x10-6 6.0x10-6 5.0x10-6

2.0x10-14

1.0x10-14

4.0x10-6 3.0x10-6

0.0 0

3.0x10-6

2

1

3 Time t, s

5

4

0

10 20 30 Frequency f, Hz

40

50

0

10

40

50

2.0x10-14

Gs = 250 kg/(m2s) εs = 0.013

1.5x10-14 2.0x10-6 Power

Equivalent current I, A

3.0x10-14 Power

Equivalent current I, A

Gs = 250

kg/(m2s)

1.0x10-14

1.0x10-6 5.0x10-15

0.0 0.0

0.0 0.5

1.0

1.5

2.0

20

30

Frequency f, Hz

Time t, s

Fig. 11. Instantaneous fluctuations and power spectra of equivalent currents in the downer at Ugr ¼6 m/s Ugsb ¼ 0.085 m/s. (a) Ugd ¼1 m/s and (b) Ugd ¼4 m/s.

-6

6.0x10

-6

Average current Im, A

5.0x10

-6

4.0x10

-6

3.0x10

-6

2.0x10

-6

1.0x10

0.0 0

1

2 Ugd, m/s

3

4

approached a constant. This was consistent with the variation of solids mass flux, as well as the average solids holdup. With increasing air superficial velocities in the riser, the average equivalent currents in the riser increased. On the other hand, average equivalent currents decreased in the downer with the increasing air superficial velocities. Instantaneous equivalent currents measured in the riser were consistent with the flow patterns under fast fluidization regime, dilute phase transport regime and near dense suspension upflow regime. Some frequency peaks for the equivalent currents in the riser were almost the same regardless of the superficial velocities in the riser, and the gas-sealing bed. This may be caused by the inherent characteristics of electrostatics and/or inherent signal noises in the riser. The frequencies for the equivalent current in the downer were focused on the low frequency range, and the dominant frequency was nearly zero Hz. In comparison with the electrostatic characteristics in the fully-developed region of the riser, a different phenomenon was observed in the downer due to the different flow patterns involved.

Fig. 12. Variations of average current over superficial velocity in the downer at Ugr ¼6 m/s and Ugsb ¼ 0.085 m/s.

Nomenclature this study. The average solids holdup and equivalent currents in the developed region of the riser and downer were measured under different superficial velocities in the riser, downer and gas-sealing bed, respectively. With increasing superficial velocities in the gassealing bed, the average equivalent currents first increased and then

dm g Gs I

Mean sand particle size, m Gravitational acceleration, m/s2 Solids mass flux, kg/(m2s) Equivalent current, A

9.0x10-6

2.0x10-14

8.0x10-6

1.5x10-14 Power

Equivalent current I, A

Y. Cheng et al. / Chemical Engineering Science 75 (2012) 435–444

7.0x10-6

6.0x10-6

0.0 0.5

1.0 Time t, s

1.5

2.0

10

0

10

20 30 Frequency f, Hz

40

50

40

50

1.5x10-14

9.0x10-6 8.0x10-6

1.0x10-14

5.0x10-15

7.0x10-6 6.0x10-6

0

2.0x10-14

Gs = 248 kg/(m2s) εs = 0.020

Power

Equivalent current I, A

1.1x10-5 1.0x10-5

1.0x10-14

5.0x10-15

Gs = 139 kg/(m2s) εs = 0.011

5.0x10-6 0.0

443

0.0 0

1

2 3 Time t, s

4

5

20 30 Frequency f, Hz

Fig. 13. Instantaneous fluctuations and power spectra of equivalent currents in the downer at Ugr ¼6 m/s Ugd ¼0. (a) Ugsb ¼ 0.017 m/s and (b) Ugsb ¼0.068 m/s.

Average current Im, A

-6

8.0x10

0.06

Acknowledgements

0.05

This study was supported by the Economic Development Board (EDB) of Singapore under grant number R-261-501-003414 through the Minerals, Metals and Materials Technology Center (M3TC) of the National University of Singapore (NUS) and the New Energy and Industrial Technology Development Organization (NEDO), Japan.

0.04 -6

6.0x10

0.03 -6

4.0x10

0.02 -6

2.0x10

Average solids holdup,s

-5

1.0x10

0.0 0.00

0.02

0.04 Ugsb, m/s

0.06

0.00 0.08

Fig. 14. Variations of average current and solids holdup in the downer over air superficial velocity in gas-sealing bed at Ugr ¼ 6 m/s and Ugd ¼ 0.

Im Q t Ugd Ugr Ugsb DH DP

es rs

References

0.01

Mean equivalent current, A Electrostatic charge, C Time, s Superficial velocity in downer, m/s Superficial velocity in riser, m/s Superficial velocity in gas-sealing bed, m/s Interval, m Pressure drop, Pa Apparent solids holdup Solid density, kg/m3

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