Investigation on Hydrodynamics of Triple-Bed Combined Circulating

Article pubs.acs.org/IECR

Investigation on Hydrodynamics of Triple-Bed Combined Circulating Fluidized Bed Using Electrostatic Sensor and Electrical Capacitance Tomography Wenbiao Zhang,†,‡ Yongpan Cheng,† Chao Wang,‡ Wuqiang Yang,§ and Chi-Hwa Wang*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117576, Singapore School of Electrical Engineering and Automation, Tianjin University, Tianjin 300072, China § School of Electrical and Electronic Engineering, The University of Manchester, Sackville Street, Manchester M13 9PL, United Kingdom ‡

ABSTRACT: To investigate the hydrodynamics, a cold model of triple-bed combined circulating fluidized bed (TBCFB) has been built with an electrostatic sensor and a twin-plane electrical capacitance tomography (ECT) sensor. Experimental results show that with the increase in the superficial air velocity, the flow regime in the riser would transit from a dense plug flow to a core-annular flow, and finally to a dilute suspension flow. In the dense plug flow regime, the passage of the solids plugs in the riser can be monitored and the velocity measured using the twin-plane ECT sensor. In the core-annular flow regime, when the solids holdup is moderate, the measured solids velocities by the electrostatic sensor and the ECT sensor are comparable and complementary. With a dilute suspension flow in the riser, a homogeneous flow is observed with a nearly flat velocity profile of solid particles. On the contrary, in the downer the flow becomes inhomogeneous, and the solids velocities in the center of the downer are higher than those near the wall. This study provides a proof-of-concept design to monitor the flow dynamics in the fluidized bed by the combination of electrostatic and ECT sensors over a wide range of flow regimes. charge in fluidized beds. Their results showed that when the air velocity or air pressure was increased, more electrostatic charge was generated because of more frequent particle−wall, particle−particle collisions and the increased bubble rise velocity. With the increase in air temperature, the charge generation was reduced in a limited range of conditions studied. In addition to experimental study, numerical simulation was widely applied to investigate the electrostatics phenomenon. AlAdel et al.6 pointed out that it is necessary to consider the effects of electrostatics for analyzing the separation of solid particles in the radial direction in a riser. Their model regarding the influence of an electric field captured important flow regimes in riser flows. Considering the presence of an electrostatic field, Lim et al.7 numerically investigated the transport of solid particles in a vertical pipe as well as an inclined pipe. The simulation results showed flow regimes similar to those of previous experimental measurements. Rokkam et al.8 developed a numerical model to analyze the effect of electrostatics on polymerization fluidized bed reactors. A Discrete Element Method (DEM) was adopted by Cheng et al.9 to simulate generation and transfer of electrostatic charge occurring in a downer. While electrostatics can cause serious problems in industry, it can be utilized to measure gas−solids flows. Information about the flow dynamics can be obtained from the generated electrostatic signals. Matsusaka et al.10 used two different

1. INTRODUCTION In fluidization processes and granular flow systems, triboelectrification is inevitable due to the collision, friction, and rolling between particle−particle and particle−wall. This phenomenon may result in undesirable particle agglomeration and affects the hydrodynamics of the fluidization process.1 Tribo-electrification may also result in inaccuracy in the measurement and even malfunction of instruments. Excessively charged particles tend to discharge in the form of fire and explosion, which poses safety disasters in industry. Therefore, it is of profound theoretical and practical significance to investigate the electrostatic phenomena in gas−solids fluidized beds systematically, while this task is challenging. In the past few decades, research has been done on the tribo-electrification phenomena and its effects on the flow dynamics in gas−solids fluidized beds. Park et al.2 investigated the effect of fluidization gas humidity on the electrostatic charge accumulation. They found that by increasing the humidity to 40−80%, the conductivity of surface would increase, and the accumulation of electrostatic charge would reduce, and therefore the dissipation of charge would enhance. To understand the influence of fine particles on generation and dissipation of electrostatic charge, Mehrani et al.3 did experiments in a fluidization column. Experimental results showed that the charge polarity on smaller particles was opposite to that on larger particles. The size effect of polyethylene resin on the generation of electrostatic charge and the fouling of reactor wall was studied by Sowinski et al.4 Their results showed that more charge was generated on smaller particles, leading to more fouling on the wall. Moughrabiah et al.5 examined effects of air temperature, air velocity, and pressure on the generation of © 2013 American Chemical Society

Received: Revised: Accepted: Published: 11198

March 21, 2013 June 20, 2013 July 23, 2013 July 23, 2013 dx.doi.org/10.1021/ie4009138 | Ind. Eng. Chem. Res. 2013, 52, 11198−11207

Industrial & Engineering Chemistry Research

Article

pipes to simultaneously measure the charge to mass ratio and the solids flow rate. The flow dynamics of gas−solids flow in a pipe was extracted and characterized by Xu et al.11 using power spectrum analysis of the electrostatic fluctuations as well as Hilbert−Huang transform. Portoghese et al.12 developed triboelectric probes to measure the moisture content in fluidized beds. The moisture contents were correlated with tribo-electric signals from the probes. It was found that this method could be used to measure the moisture contents below 100 ppm. As a nonintrusive and high-speed technique, electrical capacitance tomography (ECT) has been widely applied in monitoring fluidized beds.13−19 The particle velocity and solids distribution can be measured using ECT, which are important to the operation of fluidized beds. However, due to the small variation in permittivity and the corresponding small change in capacitance, it is difficult to measure dilute flows (i.e., solids volume fraction below 5%) by ECT. For this study, a cold model of triple-bed combined circulating fluidized bed (TBCFB) has been built to investigate the flow dynamics. An electrostatic sensor has been fabricated to measure the solids velocity profile and electrostatic distribution inside the riser and downer of TBCFB. A twin-plane ECT sensor has been used for the measurement of the average particle velocity and the solids holdup and distribution in the riser. Combining the results from the electrostatic sensor and ECT, the hydrodynamics of TBCFB in a wide range of flow conditions has been revealed.

2. EXPERIMENTAL FACILITY AND SENSOR FABRICATION 2.1. Experimental Facility. Figure 1 shows the cold-model of lab-scale TBCFB, which is made of acrylic. It is consisted of a riser with an inner diameter of 40 mm and length of 4 m, a downer with an inner diameter of 40 mm and length of 2 m, a gas−solids separator, and a bubbling fluidized bed (BFB) (150 mm × 300 mm × 700 mm). An air compressor is used to supply compressed air (about 5% relative humidity) to the bottom of the riser. The bed material is polyethylene (PE) particles with the average diameter of 2.7 mm and density of 941 kg/m3. The relative electrical permittivity of PE particle is 2.25. Under gravity, the PE particles flow down from the BFB to the lower part of the riser, and then compressed air blows up the particles along the riser. After passing through a smooth elbow, particles enter a cyclone for gas−solids separation. In the separation section, air escapes from the outlet of the cyclone, and the solid particles move down to the downer. Gas and solids are separated by a separator at the outlet of downer, and particles return to BFB. At the bottom of downer, by measuring the weight of the collected particles on a piece of gauze netting within a given collection time, the solids mass flux can be measured. The solids mass flux in the dower of TBCFB varies from 13.42 to 30.03 kg/m2·s, and the corresponding superficial air velocity in the riser ranges from 11.97 to 15.9 m/s. 2.2. Alternating Current Electrical Capacitance Tomography (AC-ECT). In this study, an AC-ECT system (from ECT Instruments Ltd., Manchester, UK) is employed to measure the changes in capacitance caused by the variations in material distributions inside a pipe/vessel for online monitoring of fluidized beds. Wang et al.18,19 used this system to study the bubbling and slugging fluidized beds, as well as fluidized bed dyers, and achieved satisfactory results. The AC-ECT system includes an AC-ECT front-end unit, a data acquisition board, and a computer. As compared to the charge/discharge ECT system, the AC-ECT system has much improved sensitivity and

Figure 1. Lab-scale TBCFB as cold model experimental setup for coal gasification. The locations of ECT sensor and electrostatic sensor in the riser are indicated in the diagram as position A. In contrast, the location of electrostatic sensor in the downer is shown as position B.

high resolution (up to 0.01 fF). The data acquisition rate is around 300 frames/s with an 8-electrode sensor and 140 frames/s with a 12-electrode sensor. Another improved feature of the AC-ECT system is that it is not affected by electrostatics; that is, electrostatics of a gas−solids flow would not affect the operation of the AC-ECT system. The design of ECT sensor is important to the performance of the ECT system. The sensor diameter, the number and length of electrodes, as well as the earthed screen size are the main factors in designing ECT sensors.20 As shown in Figure 2, the ECT sensor used in this study is composed of twin planes with earthed copper screen in between and both ends. Each plane has eight measurement electrodes, which are fabricated on a flexible printed circuit board (PCB). There are earthed axial guards between the adjacent electrodes in each plane. The dimensions of the ECT sensor are given in Table 1. The ECT electrodes are wrapped on an acrylic pipe with an earthed copper sheet around it. The ECT sensor is calibrated by taking measurements when the sensor is empty (air only) and is full of PE particles. 11199

dx.doi.org/10.1021/ie4009138 | Ind. Eng. Chem. Res. 2013, 52, 11198−11207


Article

Figure 2. Twin-plane ECT sensor.

Landweber algorithm under different numbers of iterations and different relaxation factors are given in Figure 3. The images and the calculated solids concentration using different algorithms are qualitatively similar. Because the LBP algorithm is simpler and faster than the Landweber algorithm, it is used to reflect fast changing flow characteristics in this study. The solids concentration can be obtained by:

Table 1. Dimensions of ECT Sensor length of sensor outer diameter of pipe dimensions of each electrode distance between planes distance between electrodes and earthed screens distance between electrodes and axial guard

300 mm 50 mm 100 mm ×15.6 mm 120 mm 1 mm 1 mm

α(x , y , z , t ) = min{max(ST λ , 0), 1}

During the experiments, the AC-ECT system was used to record the time series of capacitance values and reconstruct images using linear back projection (LBP) and/or Landweber algorithms. The reconstructed images from LBP algorithm and

(1)

where S is the sensitivity matrix, and λ is the normalized capacitance value. The flow information can be obtained at two cross sections from the twin-plane ECT sensor, based on which

Figure 3. Image reconstruction using the LBP and Landweber algorithms under different numbers of iterations and relaxation factors. 11200



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Figure 4. Electrostatic sensor with the ring-shape electrodes and arc-shape electrodes.

where ρ is charge density, ε0 is free space permittivity, and φ is electric potential in the pipe. The charge density σ on the electrode surface and electric field strength E is calculated by:

the solids velocity can be obtained. In this study, the average particle concentration at two planes, α̅s(z1, t) and α̅s(z2, t), is correlated: αs̅ (z , t ) =

1 A

∬ α (x , y , z , t ) d x d y

σ = ε0E = −ε0∇φ (2)

The induced charge Q on the electrode surface can be calculated by the surface integral of the charge density σ:

where A is the cross-sectional area. Time averaged value of α̅s(z, t) is given by 1 α ̅ (z ) = T

∫0

Q=

T

αs̅ (z , t ) dt

∫0

Se = Q /Q p

[αs̅ (z1 , t ) − α̅ (z1)][αs̅ (z 2 , t + d) (4)

where z1 and z2 represent planes in the upstream and downstream, and d denotes the time delay. The average particle velocity, Uap, is calculated from Uap = L/D, where D is the value of d, at which C(d )is the maximum and L is the axial distance between the two planes of ECT sensor. 2.3. Electrostatic Sensor. Electrostatic charge is inevitable in gas−solids flow due to frequent particles collisions, separation, and friction. The resulted electrostatic signal actually contains information of the flow dynamics, by which the fluidized bed performance can be monitored. The electrostatic sensor used in our experiment includes two ringshape electrodes and eight arc-shape electrodes, as shown in Figure 4. The 10 electrodes are classified into five groups, one ring-shape group (A) and four arc-shape groups (B−E). The distance between the upstream electrode and downstream electrode is 50 mm. The axial width and the thickness of each electrode are 6 and 2 mm, respectively. The electrodes are made of copper and are tightly wrapped around the outside surface of an acrylic pipe. The signal conditioning circuits together with the electrodes are shielded in a metal box to eliminate external noise. To analyze the sensor’s characteristics, the sensitivity distribution of the electrostatic sensor needs to be obtained. On the basis of the charge distribution, the electrostatic field inside the pipe is calculated by Poisson’s equation: ∇2 φ = −

ρ ε0

(7)

(8)

where Qp is the charge on the particles. The sensitivity distributions of the ring-shape electrodes and arc-shape electrodes can be obtained using a model built in COMSOL as shown in Figure 5, with only one arc-shape electrode. The COMSOL model in this study is similar to the model used in our previous study,21 where detailed descriptions

T

− α̅ (z 2)] dt

∫s σ ds

The sensitivity of an electrode is defined as the ratio of induced charge and particle charge:

(3)

where the averaging period T is 10 s. The cross-correlation coefficient C(d) can then be computed as 1 C(d) = T

(6)

(5)

Figure 5. COMSOL model of the arc-shape electrode. 11201



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where a is the proportion coefficient. The voltage signals are sampled using a data acquisition device NI USB 6353 using LabVIEW 8.5. The signals from the 10 electrodes in the sensor are sampled simultaneously. The cross-correlation coefficient Ce(d) of electrostatic signals between the upstream and downstream electrodes can be computed:

of the model can be found, and hence not reproduced here. The sensitivity distributions of the arc-shape electrode and ringshape electrode are given in Figure 6. The color definition of

Ce(d) =

1 T

∫0

T

Ve(z1 , t )Ve(z 2 , t + d) dt

(10)

The sampling rate is important to the accuracy of correlation measurements. According to the author’s previous investigation,21 the sampling rate should be 20 kHz, and sampling time under each air flow rate should be 60 s. The typical sampled signals from the upstream and downstream ring-shape electrodes are shown in Figure 7. There is an obvious peak in the corresponding correlation function (see Figure 7b). In this study, the signals from the five groups of electrodes are cross-correlated to obtain the velocity. The number of samples

Figure 6. Sensitivity distribution.

these two figures is the same to facilitate comparison. It is found that the sensitivity zone of the arc-shape electrode is localized, indicating that the arc-shape electrode is only sensitive to the charged particles near the electrode. For comparison purpose, the sensitivity distribution of the ringshape electrode is also given in Figure 6. Although the sensitivity is a little higher near the wall, the ring-shape electrodes provide the average information of the flow dynamics in the entire cross-section. It means that using the new electrostatic sensor, both local and cross-sectional average information can be obtained. When charged particles move near the electrostatic electrodes, the charge is induced on the electrodes. Because of the virtual earth on the signal conditioning circuit, there is an induced current on the electrodes. It can be integrated, filtered, and amplified to generate a voltage signal Ve proportional to the instantaneous induced charge Q on the electrodes: Ve = aQ

Figure 7. (a) Sampled electrostatic signals Ve of the upstream and downstream ring-shape electrodes when superficial air velocity is 14.63 m/s and (b) the correlation function.

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Figure 8. Reconstructed images of two-plane ECT in the riser when a plug passes through the sensor: (a) upstream plane at t, (b) downstream plane at t, (c) upstream plane at t + 0.2 s, (d) downstream plane at t + 0.2 s, (e) upstream plane at t + 0.4 s, (f) downstream plane at t + 0.4 s, (g) upstream plane at t + 0.6 s, and (h) downstream plane at t + 0.6 s. The percentage value in the color bar shows the relative solids holdup α in the cross-section calculated from the reconstructed image (100% represents the random closed solids packing volume fraction value 0.64).

3. EXPERIMENTAL SECTION 3.1. Flow Dynamics in the Riser. Experiments were first conducted in the riser of TBCFB with the electrostatic sensor and twin-plane ECT sensor. Both sensors were mounted at the same position, that is, 250 mm from the inlet of riser. Under various superficial air velocities, the hydrodynamics in the riser would change. When the superficial air velocity was increased, the main flow regimes encountered near the inlet of riser were a dense plug flow, a core-annular flow, and a dilute suspension flow, respectively. At a low superficial air velocity, a dense solids plug flow occurred in the riser. As seen in Figure 8, the solids plugs were monitored using the twin-plane ECT sensor. Figure 8 shows passage of a solids plug in the sensor zone. At first, the whole

used for correlation is 10 000, which corresponds to the integration time of 0.5 s. In total, 120 correlation velocities are obtained from one group of electrodes under each air flow rate. The maximum correlation coefficient shows the similarity between the upstream and downstream signals. However, the calculated result does not give an accurate particle velocity with a small correlation coefficient. Therefore, the correlation velocity is discarded if the maximum correlation coefficient is less than 0.6, as proposed by Nieuwland et al.22 The standard deviations (SD) of electrostatic signals from the arc-shape electrodes are computed, which reflect the magnitude of charge fluctuations of different electrodes and can be applied to monitor the electrostatic distribution of solid particles. 11203



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the dense plug flow regime, the high solids concentration in the cross-section resulted in high electrostatic charge. The electrostatic signals from the signal conditioning circuits became saturated, resulting in less similarity of signals between the upstream and downstream electrodes. As a result, the electrostatic sensor could not work in this regime. As the superficial air velocity increased to 13.3 m/s, the dense plug flow regime transited to the core-annular regime. Figure 11 shows the reconstructed images of the core-annular flow regime. The reconstructed images from the upstream and downstream planes are similar to each other in this regime, and hence only the results from the upstream plane are given. The time averaged solids holdup profile is calculated from the reconstructed images, as seen in Figure 11e. Although the average solids concentration fluctuates (see Figure 11a−d), the solids distribution remains the same, with lower solids holdup in the central area than in the annular region. In this flow regime, the average solids velocity is calculated by both the electrostatic sensor and the ECT sensor. The results from the ECT sensor and the ring-shape electrostatic electrodes are compared in Figure 10. In an integrated ECT/electrostatic sensor system, an ECT sensor can provide the permittivity distribution, and an electrostatic sensor can provide the electrostatic charge distribution. Although they are based on different measurement principles, the ECT sensor and the electrostatic sensor give comparable and complementary measurement results, which prove the satisfactory performance of both systems. With the superficial air velocity increased further, the flow regime changed to a dilute suspension flow in the riser. Because of the low solids concentration and the corresponding small change in capacitance, it was difficult to measure a dilute phase flow using the ECT sensor. In contrast, the electrostatic sensor could monitor the flow dynamics in this regime. The measured velocities from different groups of electrodes under different air velocities are given in Figure 12a. Four groups of arc-shape electrodes are used to measure the local solids velocity near the pipe wall, and one group of ring-shape electrodes is used to measure the cross-sectional average solids velocity. It is found that with the increase in the air velocity, the average solids particle velocity and local solids particle velocities near the wall increase. As shown by the standard deviations of the particle velocities from the ring-shape and arc-shape electrodes, the velocities from the ring-shape electrodes and those from four groups of arc-shape electrodes are quite close to each other, representing a nearly flat velocity profile. Table 2 gives the standard deviations of electrostatic signals from the upstream arc-shape electrodes in the dilute suspension flow, where SD_B, SD_C, SD_D, and SD_E represent the standard deviations of electrostatic signals from the groups B, C, D, and E of the upstream arc-shape electrodes, respectively. With the increase in the air velocity, the standard deviations of electrostatic signals increase. The reason for this is that solids mass flux increases with the increase in the air velocity, and there would be a higher possibility of tribo-electrification, resulting in the increase in charge accumulated on the particles. Besides, the standard deviations of different electrodes given in Table 2 are similar to each other, showing a homogeneous solids distribution in this regime. 3.2. Flow Dynamics in the Downer. Experiments were also conducted in the downer at 1.65 m from the inlet. When the air velocity in the riser increased, the solids mass flux in the downer would increase, and the solids velocity flowing into the

cross-section of the pipe was occupied by the plug at the upstream plane (see Figure 8a), and later the front of plug arrived at the downstream plane (see Figure 8b). After 0.2 s, the solids concentration in the downstream plane increased (see Figure 8d), which showed the upward movement of the plug. After 0.4 s, the downstream plane was full of the plug (see Figure 8f), and the tail of the plug remained in the upstream plane (see Figure 8e). Finally, after 0.6 s, the plug left the sensor domain (see Figure 8g and h). The average solids concentration in the upstream and downstream planes can be calculated from the reconstructed images at an air velocity of 11.97 m/s, as seen in Figure 9. A

Figure 9. Average solids concentration α̅ s of the upstream and downstream ECT planes at superficial air velocity of 11.97 m/s; 100% represents the random closed solids packing volume fraction value 0.64.

low solids concentration zone exists between two adjacent plugs. Furthermore, the profiles of the solids concentrations are quite similar between two planes in the upstream and downstream positions. The average solids holdup is applied to estimate the plug velocity with the cross-correlation method, as shown in Figure 10. When the air velocity is increased, the upward movement of the plug would accelerate. However, in

Figure 10. Average particle velocities obtained from electrostatic sensor and ECT sensor. The velocity results with three flow regimes, dense plug flow, core-annular flow, and dilute suspension flow. The standard deviations of the velocities from the ring-shape electrodes and ECT are shown as error bars. 11204



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Figure 11. Reconstructed images of core-annular flow regime: (a) upstream plane at t, (b) upstream plane at t + 0.2 s, (c) upstream plane at t + 0.4 s, (d) upstream plane at t + 0.6 s. The percentage value in the color bar shows the relative solids holdup α (100% represents the random closed solids packing volume fraction value 0.64). (e) Average solids holdup profile calculated from the reconstructed images.

Table 3 shows the standard deviation of the electrostatic signals from the upstream arc-shape electrodes in the downer. With the increase in the solids mass flux in the downer, the standard deviations of the signals from the arc-shape electrodes increase, indicating the increase in electrostatic charges on different sides of the wall. Besides, the electrostatic distribution is inhomogeneous due to the inhomogeneous solids distribution in the downer.

downer would also increase. Because of the large falling velocity of particles in the downer, the solids volume fraction in the cross-section of the downer was lower than 5%, and hence the ECT sensor could not work under this condition. Fortunately, the electrostatic sensor could still be applied to monitor the gas−solids flow. Figure 12b shows the measured solids velocities from different groups of electrodes in the downer under various air velocities in the riser. With the increase in the air velocity in the riser, the initial particle velocity at inlet of the downer increases, resulting in the increase in velocities measured from the electrostatic sensor. The difference in the velocities from different groups of electrodes can be clearly observed in Figure 12b, which shows inhomogeneous velocity distribution in the downer. The average velocity from the ringshape electrodes is higher than the local velocity from the arcshape electrodes, which means that the particle velocities in the center of downer are higher than those near the wall, because near the wall, the air velocity is lower than that in the central region. Besides, the solid particles near the wall have more interactions with the wall, such as collisions and frictions, which decelerate the particle movement. It is noted that a pipe bend located between the top of the riser and the cyclone immediately above the downer generates inhomogeneous solids distribution after the bend. The differences in the local velocities from different groups of arc-shape electrodes may also come from the inhomogeneous solids distribution in the downer.

4. CONCLUSIONS In this study, the electrostatic sensor and ECT sensor are used to investigate the hydrodynamics in TBCFB. Different electrostatic electrodes are proposed. The ring-shape electrodes can provide overall information of solid particles in a crosssection, and the arc-shape electrodes can provide local information on solids flow dynamics near the wall. By a combination of the electrostatic sensor and ECT sensor, the hydrodynamics of the riser has been investigated in a wider range of flow regimes. In the dense plug flow regime, the passage of the solids plug and its velocity can be measured using the twin-plane ECT. In the core-annular regime, the average solids velocities can be measured by both the ECT sensor and the electrostatic sensor, and comparable measurement results from these two types of sensors are observed. In the dilute suspension flow, the results from electrostatic sensor reveal homogeneous solids distribution and nearly flat velocity profile of solid particles at the inlet of riser. In the downer, due 11205



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Table 3. Standard Deviation of the Electrostatic Signals from the Upstream Arc-Shape Electrodes in the Downer solids mass flux in downer (kg/m2·s)

SD_B (V)

SD_C (V)

SD_D (V)

SD_E (V)

13.42 16.53 18.47 24.03 30.03

0.137 0.153 0.156 0.162 0.166

0.146 0.161 0.166 0.17 0.176

0.142 0.154 0.159 0.167 0.175

0.128 0.14 0.145 0.154 0.155

solids distribution in both the riser and the downer provides guidance to optimize the coal gasification process in TBCFB in the future.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 65-6516-5079. Fax: 65-6779-1936. E-mail: chewch@nus. edu.sg. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was supported by the Economic Development Board (EDB) of Singapore under grant number R-261-501003-414 through the Minerals, Metals and Materials Technology Center (M3TC), National University of Singapore (NUS). This work was also supported in part by the following three grants: National Natural Science Foundation of China (Grant number 61072101), Program for New Century Excellent Talents in University (NCET-10-0621), and Independent Innovation Foundation of Tianjin University. W. B. Zhang would like to thank the Chinese Scholarship Council (CSC) State-Sponsored Graduate Scholarship Program for Building High-Level Universities for supporting his Ph.D. program during the exchange in NUS.

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(1) Mehrani, P.; Bi, H. T.; Grace, J. R. Electrostatic charge generation in gas-solid fluidized beds. J. Electrost. 2005, 63, 165−173. (2) Park, A. H.; Bi, H. T.; Grace, J. R. Reduction of electrostatic charges in gas-solid fluidized beds. Chem. Eng. Sci. 2002, 57, 153−162. (3) Mehrani, P.; Bi, H. T.; Grace, J. R. Electrostatic behavior of different fines added to a Faraday cup fluidized bed. J. Electrost. 2007, 65, 1−10. (4) Sowinski, A.; Mayne, A.; Mehrani, P. Effect of fluidizing particle size on electrostatic charge generation and reactor wall fouling in gassolid fluidized beds. Chem. Eng. Sci. 2012, 71, 552−563. (5) Moughrabiah, W. O.; Grace, J. R.; Bi, X. T. Effects of pressure, temperature, and gas velocity on electrostatics in gas-solid fluidized beds. Ind. Eng. Chem. Res. 2009, 48, 320−325. (6) Al-Adel, M. F.; Saville, D. A.; Sundaresan, S. The effect of static electrification on gas-solid flows in vertical risers. Ind. Eng. Chem. Res. 2002, 41, 6224−6234. (7) Lim, E. W. C.; Zhang, Y.; Wang, C. H. Effects of an electrostatic field in pneumatic conveying of granular materials through inclined and vertical pipes. Chem. Eng. Sci. 2006, 61, 7889−7908. (8) Rokkam, R. G.; Fox, R. O.; Muhle, M. E. Computational fluid dynamics and electrostatic modeling of polymerization fluidized-bed reactors. Powder Technol. 2010, 203, 109−124. (9) Cheng, Y. P.; Lau, Y. J.; Guan, G. Q.; Fushimi, C.; Tsutsumi; Wang, C. H. Experimental and numerical investigations on the electrostatics generation and transport in the downer reactor of a triple-bed combined circulating fluidized bed. Ind. Eng. Chem. Res. 2012, 51, 14258−14267.

Figure 12. Solids particle velocities from different groups of electrodes. The standard deviations of the particle velocities from the ring-shape and arc-shape electrodes are shown as error bars.

Table 2. Standard Deviation of Electrostatic Signals from the Upstream Arc-Shape Electrodes in the Dilute Suspension Flow air velocity (m/s)

SD_B (V)

SD_C (V)

SD_D (V)

SD_E (V)

15.9 15.3 14.6

0.145 0.142 0.130

0.144 0.141 0.131

0.143 0.139 0.130

0.144 0.139 0.129

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to low solids concentration, only the electrostatic sensor is used. The average cross-sectional velocity is higher than local velocities near the wall, and the solids distribution is inhomogeneous near the outlet of the downer. From the above results, it can be seen that the combination of electrostatic sensors and ECT can expand the detection limits for studying the various hydrodynamics regimes in both risers and downers. This study can be extended further to dense fluidized beds. The knowledge of particle velocity profile and 11206



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Investigation on Hydrodynamics of Triple-Bed Combined Circulating

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