Week #8 Dynamics of Particulate systems (Part 3)

1. Week #8 Dynamics of Particulate systems (Part 3) Chi-Hwa Wang Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, 117585, Singapore. E-mail: chewch@nus.edu.sg.

2. Investigation on Electrostatics in Fluidized Bed Yongpan Cheng, Eldin Wee Chuan Lim, Chi-Hwa Wang Department of Chemical and Biomolecular Engineering National University of Singapore, 4 Engineering Drive 4, Singapore Jun Yao School of Energy Research, Xiamen University, Xiamen, China Guoqing Guan North Japan Research Institute for Sustainable Energy, Hirosaki University, Matsubara, Aomori, Japan Chihiro Fushimi, Atsushi Tsutsumi Collaborative Research Center for Energy Engineering, Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153- 8505, Japan

3. Outlines • Background for study of electrostatics. • Hydrodynamics in triple-bed combined circulating fluidized bed. • Equivalent current in electrostatics in the riser and downer. • Induced current in electrostatics below the cyclone. • Conclusions.

4. Background Electrostatics are generated through tribocharging due to the collisions and frictions between particle-particle or particle-wall Hazards • Alter hydrodynamics. • Cause agglomeration. • Interfere with instrumentation. • Generate nuisance discharge. • Create the danger of explosions. ……… Reduction of electrostatics. • Adding fine particles. • Adding anti-static agents. • Being well-grounded. • Increasing humidity. • Improving wall smoothness. • Altering pipe wall materials. …….. Ruins after explosions in Mill City Museum in Minneapolis

5. Triple-Bed combined Circulating Fluidized Bed (TBCFB) Experimental set-up : Air velocity in downer Riser: Ф0.1m ×15.7m Downer: Ф 0.1m × 6m Moving bed: Ф 0.15 m × 5 m BFB: 0.75m × 0.27m × 3.4 m : Air velocity in riser : Air velocity in gas-sealing bed

6. Silica sand particles with mean particle size are circulating in the Circulating fluidized bed. Particle size distribution Strong electrostatics can be observed. Sand particles are adhered to acrylic wall due to strong electrostatics.

7. Modular Parametric Current Transformer (MPCT) When particles with charges pass through the tube, the equivalent current will be formed due to the electromagnetic waves. This current can be detected by MPCT, with the resolution of Two locations for measurements: Top of riser, Middle of downer. The flow is fully developed there.

8. Variation along riser at air velocity in gas-sealing bed • Pressures decrease with the increasing height along the riser. • Solids holdups at the bottom are the highest, and almost approach constant at the top region of riser. Solids holdup is calculated with pressure gradient method

9. Solid mass flux variations under different air superficial velocities in riser at There is little variation on the solids mass flux due to the constant driving force between bottoms of riser and gas-sealing bed.

10. Instantaneous fluctuations of equivalent current in riser at Fast fluidization regime: Dilute up-flowing suspension in the central core region; Dense down-flowing annular suspension of solids adjacent to the wall Equivalent current may become negative due to strong down-flowing.

11. (Continued) When , flow is transited to dilute phase transport regime, there will be all up-flowing suspensions, hence less negative equivalent currents.

12. Power spectra of equivalent currents in riser Fast Fourier Transform (FFT) is used to analyze the equivalent currents. Although air velocities are different, the dominant frequencies are fixed.

13. In order to eliminate the influence of positive and negative currents At , solids holdup is high, but average current is low due to strong counterbalance by negative equivalent currents. Average current

14. Solids mass flux variation over air superficial velocity in gas-sealing bed at With increasing air velocity in the gas-sealing bed, the solids mass flux increased rapidly first, then approached a constant. This is determined by the driving force between bottoms of riser and gas-sealing bed.

15. Instantaneous fluctuations of equivalent current in riser at With increasing Gs, shear force on the particles of the down flowing region near the wall is increased, then all particles start to flow upward (known as the Dense Suspension Upflow), thus negative equivalent currents are decreased. Increase Gs Fast Fluidization Dense Suspension Upflow

16. Power spectra of equivalent currents in riser The dominant frequencies are almost the same.

17. Average currents in riser under different superficial velocities in gas-sealing bed Solids holdups and average currents increase first, then approach a constant. They are in the same trend with solids mass flux.

18. Instantaneous fluctuations and power spectra of equivalent currents in the downer at At higher downer velocity , the equivalent current becomes lower. The dominant frequency is focused on the low frequency regime.

19. Variation of average current in downer The average current decreases with increasing air velocity in the downer because fewer particle-particle and particle-wall collisions are observed with the smaller solids holdup.

20. Instantaneous fluctuations and power spectra of equivalent currents in the downer at Dominant frequencies are focused on the low frequency regime.

21. Variations of average current and solids holdup in downer The solids holdup and average current increase rapidly first, then increase mildly, consistent with the trend of solids mass flux.

22. Induced current measurements below cyclone • Acrylic tube wall; 2. Copper sheet. 3. Polymer film. 4. Aluminum film Induced current Charge is transferred from particles to the wall, then to the ground, thus forms the induced current

23. Instantaneous fluctuations of induced current at • With increasing solids mass flux, the induced current is increased greatly. • At high solids mass flux, there is a peak, which means the sparks happen at this point.

24. Accumulated charge on the wall • Accumulated charges on the wall is significant at high solids mass fluxes.

25. Summary • With increasing air velocity in the gas-sealing bed, solids mass flux increased first, then approached a constant. The solids holdups and average currents in both riser and downer have the same trend. • The flow pattern in the riser has great influences on the instantaneous equivalent currents. • The dominant frequencies in the riser are almost fixed, the dominant frequencies in the downer are focused on the low frequency regime. • Induced current on the wall increases with increasing solids mass flux.

26. Studies of Solid-Solid Mixing Behaviors in a Downer Rector Yongpan Cheng, Eldin Wee Chuan Lim, Chi-Hwa Wang Department of Chemical and Biomolecular Engineering National University of Singapore, 4 Engineering Drive 4, Singapore Guoqing Guan North Japan Research Institute for Sustainable Energy, Hirosaki University, Matsubara, Aomori, Japan Chihiro Fushimi, Atsushi Tsutsumi Collaborative Research Center for Energy Engineering, Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153- 8505, Japan

27. Co-production flowchart for gasification Adapted from C. Fushimi, A. Tsutsumi, “Advanced-Integrated Gasification Combined Cycle with Exergy Recuperation,“, The University of Tokyo-Imperial College London Joint Symposium on Innovation in Energy Systemsat Imperial College London,, 1 February 2008.

28. Heat carried particles CO CO2 Pyrolyzer Purification Coal Combustor air Pyrolysis gases Gas turbine Waste heat recovery boiler Power generator CO､H2 Gasifier Steam Steam turbine Char+ particles Char Cooling water Heat carried particles O2 Steam Next generation high-efficiency A-IGCC (advanced Integrated coal Gasification Combined Cycle) system Power generator Power efficiency can be increased by 9%

29. Outlines • Numerical simulation on mixing of sand and coal with Discrete Element Method in downer. • Experimental study on mixing of cold and hot sand in downer. • Conclusions.

30. Lab-scale triple-bed circulating fluidized bed Sand particles Coal Tangential Normal Good mixing between sand and coal particles is needed to guarantee significant heat transfer between them Riser: Ф40mm ×3,600mm Downer: Ф 40mm × 2150mm BFB: 300mm × 300mm × 500mm

31. Governing equations Continuity Momentum Drag coefficient Drag force

32. Numerical method and parameters Discrete Element Method is used with coupling between EDEM and FLUENT Ergun, Wen & Yu model is used to calculate the drag force exerted on the particles by air

33. Mixing of coal and sand particles at Normal arrangement Tangential arrangement

34. Mixing of coal and sand particle at Normal arrangement Tangential arrangement

35. Solids holdup distribution along downer at Normal Tangential Sand Sand Coal Coal 0.022 0.042 Z=0.1 0 0 Low holdup High holdup Z=0.2 Z=0.5

36. (Continued) Normal Tangential Coal Coal Sand Sand Z=1.0m Z=1.5m Z=1.9m

37. Solids holdup distribution along downer at Normal Tangential Sand Sand Coal Coal 0.015 0.045 Z=0.1 0 0 Z=0.2 Z=0.5

38. (Continued) Normal Tangential Coal Coal Sand Sand Z=1.0m Z=1.5m Z=1.9m

39. Variation of solids holdup and velocity along downer at • Solids holdups for both coal and sand decrease along downer due to increasing velocity at a fixed solids mass flux. • Sand holdups are higher than coal holdups due to their higher solids mass flux. • Sand velocity is higher than coal velocity due to larger density.

40. Variation of solids holdup and velocity along downer at Similar results can be obtained at high air velocities.

41. Variation of solids holdup and velocity along downer in normal arrangement Although air velocity is increased by four times, the coal and sand holdups decrease a little bit because drag force is insignificant as compared with gravity.

42. Radial distributions of solids holdup at Near inlet, sand holdup decreases from the center to the wall. Near outlet the solids holdup distribution in the central region is quite uniform, the solids holdup near the wall is the lowest. Normal Tangential

43. Radial distributions of solids holdup at In the central region, solids holdups are almost constant, and solids holdups are lowest near the wall. Normal Tangential

44. Calculation of mixing index Divide a certain section (length is 0.1m, located at z=0.2, 0.5, 1.0, 1.5, 1.9 in a downer into equal-sized 100 bins. To minimize the influence of transient fluctuations, the numbers of sand and coal particle passing through each bin are calculated within 1 s. z=0.2 z z=1.0 z=1.9

45. In each bin, mass ratio of coal over sand Average mass ratio Mixing index Weighting factor: mass fraction of particle in a certain sample over the all the samples. Mixing index when it is completely segregated. Dimensionless mixing index 0 : Completely segregated 1: Completely mixing

46. Variation of mixing indices along downer • Along downer, mixing indices increase first, then approach constant • Tangential arrangement has higher mixing indices than normal arrangement. • Higher air velocity corresponds to higher mixing indices.

47. Experimental study on mixing in pilot plant Experimental set-up Riser: Ф0.1m ×15.7m Downer: Ф 0.1m × 6m Moving bed: Ф 0.15m × 5m BFB: 0.75m × 0.27m × 3.4m

48. Hot sand particles are injected from the nozzles, cold sand particles flow into downer through the distributor C. Fushimi, G. Guan, Y. Nakamura , M. Ishizuka, A. Tsutsumi, Y. Suzuki , Y. Cheng, E.W.C Lim, C.H. Wang “Mixing behaviors of cold–hot particles in the downer of a triple-bed combined circulating fluidized bed”, Powder Technology, 221, 70-79 (2012). sand (dp=128 μm) from riser (0.86-0.98 kg/s) Downer: 0.10 m i.d. Nozzle: 0.018 m i.d. soliddistributor Cold sand from distributor 13 brass tubes hot sand hopper hot sand hopper TC TC nozzle nozzle air (20 m/s) air (20 m/s) valve Hot sand (dp=128 μm ) 0.055 kg/s 17 thermocouples (TC) downer

49. Normal arrangement Tangential arrangement downer nozzle downer top view nozzle hot sand + air hot sand + air hot sand + air hot sand + air side view 150 mm ⑦ ⑧ ③ ⑨ ⑩ ⑦ ⑧ ③ ⑨ ⑩ 100 mm thermocouples thermocouples C. Fushimi, G. Guan, Y. Nakamura , M. Ishizuka, A. Tsutsumi, Y. Suzuki , Y. Cheng, E.W.C Lim, C.H. Wang “Mixing behaviors of cold–hot particles in the downer of a triple-bed combinedcirculating fluidized bed”, Powder Technology, 221, 70-79 (2012).

50. Dimensionless temperature Non-dimensional mixing index : Complete separation : Complete mixing Local temperature measured by thermocouples Hot sand temperature at injection Ambient temperature Average temperature at the cross section Arrangement of thermocouples in the cross section.