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Stagnant zones in granular moving bed filters for flue gas cleanup
naut Zonesin GranularMovingBed Filters for FlueGasCleanup J. T. Kuo*,
J. Smid*,
*Department
of Mechanical
**Department ***Department
S. S. Hsiau**,
C. Y. Wang**,
and C. S. Chou***
Engineering, National Taiwan University, Taipei, Taiwan 10617, R.O.C. of Mechanical Engineering, National Central University, Chung-Li, Taiwan 32054, R.O.C. of Mechanical Engineering, National Pingtung University for Science and Technology, Ping-Tung, Taiwan 91207, R.O.C.
The flow patterns in three symmetric and two asymmetric louver-walled granular moving bed systems are studied experimentally. The flow pattern histories of granular solids in moving beds were recorded by videocamera throughout the whole test period. The image processing system including a frame grabber was used. Selected discrete images showing the flow pattern development were digitized and stored in files. Individual granular flow frames show a development of quasi-stagnant zones close to the louvered walls, the shear zones and a central flow region with width depending on the louver angle. The vertical shift of one or the other louvered wall in asymmetric configuration considerably influences the history of quasi-stagnant zones.
ranular bed filters may have technical and economical advantages for general purpose gaseous emission control applications such as Pressurized Fluidised Bed Combustion (PFBC) hot gas cleanup, cement and lime kilns, glass and steel furnaces, and incineration processes. Granular bed filtration can be operated in four modes: fixed bed, intermittently moving bed, continuously moving bed and fluidised bed1-5. The possibility of (semi-) continuous regeneration in moving bed systems makes granular bed filtration an attractive option for continuous high temperature gas cleaning purposes. Basically the designs of granular bed filters can be categorized in two groups. One group are screenless filters’, *I ‘j-l’. The other group contains the moving bed filters with a vertical layer of granular material held in place by louver-like walls’s 18-26. Major advantages of a granular bed filter are its potential use in hot gas cleaning operations up to 800°C by using low-cost ceramic granules with high temperature stability and the possibility to operate the filter in a continuous process (at constant pressure drop) using a filter media circulation and regeneration system. Disadvantages of granular moving bed filters are the complexity of the solids flows involved in the filter media handling and cleaning system. A number of granular moving bed filters have used crossflow designs where the filter granules move downwards guided by louvered walls whilst the gas passes the moving granules horizontally, see Figure. 1. Plugging problems were encountered in some of these moving bed filters, at both, low and high temperatures. This paper presents the results of a study of the flow patterns in 2-D cross flow moving beds of non-cohesive granular solids flowing under steady state conditions between two vertical louvered walls with no interstitial fluid flow relative to the solids. It is important to examine this relatively simple case of particulate solids flow, which is of industrial significance, before systems involving solids acceleration and fluid or gas pressure gradient can be effectively analyzed. The objective of this study is to determine a history of quasi-stagnant zones of granular moving bed near the louvers in symmetrical and asymmetrical louver configurations.
G
EXPERIMENTAL
APPARATUS
row layer of spheres of uniform diameter sandwiched between two transparent panels with louver-like side walls. The configuration of the louvers can be changed (louver spacing, louver angle and length). The experimental apparatus is schematically shown in Figure 2. The flow of spherical solids is induced and controlled by a moving belt underneath the exit of the moving bed. Granules are fed into the vertical channel from a top hopper, which has a rectangular discharge slot with the same cross-section as the vertical channel. The height of the channel is fixed (1000 mm) but the width is adjustable (maximum width is 400 mm). The channel width in the experiments mentioned in this work is fixed at the value of 380 mm. For the experiments, 6 mm diameter PE spheres with density of 964 kg/m3 were selected as test granular solids. Packing of the spheres was characterized by the bulk density measurements. Two bulk densities were measured: a poured bulk density of 582 kg/m3 (porosity 0.396) and a tapped bulk density of Granular
Bed
Filter
Granules
Quasi-Stagnant Zone
AND PROCEDURES
For the observation of the flow patterns occurring as the granular bed flows continuously, an experimental technique”, ** with differentially coloured granules has been used. The experimental moving bed is a two-dimensional system. It consists of a nar-
Filter Dust L
Figure 1: The lllustratlon of the granular bed filter.
Granules + Particles
TestParameter Louver angle Louver length Louver spacing Channel width Mass flow rate Time average center line granule velocity (steady state flow) Beginning stage center line granule velocity Granules refreshing rate
‘The
right side
40’ louver
is shifted
vertically
Test1
Test2
Test3
Test4
Test5
(mm/s)
40 200 200 380 3.51 0.91
30 200 200 380 3.53 0.73
15 200 250 380 3.52 0.49
15/40 200/200 2501250 380 3.52 0.63
15/40+ 2001200 200/200 380 3.52 0.62
(mm/s)
0.80
0.63
0.43
0.48
0 54
(!J/s)
0.40
0.63
1.50
0.17
0.37
(“) (mm) (mm) (mm) km
as shown
in Figure 3(e)
600.5 kg/m3 (porosity 0.377). Both densities were measured in a graduated glass cylinder. The mass flow rate measurements were made by continuous collection of the discharged granules in a tared bucket. Weighing of full buckets was made with an electronic balance. Table 1 lists the test conditions of the five experiments. The time-averaged center line granule velocities listed in Table 1 were measured by timing a small group of coloured granules which moved from the top to the bottom of the vertical channel after circulating granules for approximately two hours. At this time it was believed that the moving bed has reached steady state flow conditions (meaning steady state bulk density distribution, flow patterns etc.). The center line granule velocities during the beginning stage of the experiments were measured from the recorded flow images. Figures 3(a) - (e) show the louver geometry of the five tests. Tests 4 and 5 are asymmetric and the right side 40” louvers in Test 5 were shifted vertically as shown in Figure 3(e). The purpose of shifting the louver vertically in Test 5 is to create a gas flow channel configuration that the cross paths of dust laden gas moving from the left inlet to the two corresponding exits are equal, as shown in Figure 3(f). The horizontal lines labeled with letters “a” to “e” show the locations where the velocity profiles were evaluatedzg, 3o
r
During filling of the granular bed, the coloured granules were filled in one louver section as shown in Figures 4-8. Prior to the start of each experiment, the filter channel was filled from a top hopper, which always contained an inventory of filter granules to ensure continuous supply during the experiment. Filter granules were circulated for approximately 2 hours to make certain that the flow has reached steady state conditions. After that a portion of the filter granules was continuously discharged without filling from the top until the level of granules reached the bottom of the first pair of louvers. Then the movement of filter granules was halted momentarily in order to fill the space between the first pair of louvers with coloured granules. Subsequently, the upper hopper was filled again with white granules. The above operation of stopping the bed flow and filling in with additional granules was executed very carefully to avoid any unwanted disturbance of the existing steady state bed structure. A videocamera was used to record the development of the flow of coloured granules until the coloured particles left the granular bed completely. The camera was supported on a sturdy tripod and activated several seconds before the onset of granular flow. A digital image grabber was used to convert flow images from the recorded tape to computer graphic files. TEST RESULTS AND DISCUSSIONS Figure 4 shows the flow history of the coloured granules in eighteen frames under Test 1 conditions. Frame 1 shows the beginning of the experiment. The time interval for frames 1 to 11 is 120 seconds.The frames following frame 13 have progressively longer time intervals until at frame 18 all the coloured granules have moved out of filter channel.
7
i
14
!ki2e (b)
(a)
40’
15’
40.
a b c d e
Louver Length
Figure 2: The schematic drawing of the experimental apparatus.
Cd)
1.5
a b
90 mm
(e)
Figure 3: The louver geometry for the five test conditions.
Figure 4: Flow pattems of granules in the louvered channel (Test 1). Frames l-11, time interval 2 min; frames 12-13, time interval 4 min; frame 14, time 49 min; frame 15, time 89 min; frame 16, time 129 min; frame 17, time 169 min; frame 18, time 209 min.
I
I
I
Figure 6: Flow patterns of granules in the louvered channel (Test 3). Frames l-10, time interval 1 min; frames 11-15, time interval 2 ndn; frame 16, time 29 min; frame 17, time 49 min; frame 18, time 69 min.
,
I
Figure 5: Flow pattems of granules in tfm louvered channel (lest 2). Frames l-13, tune Merval2 ndn; frame 14, tii 29 mln; frame 15, time 49 min; frame 16, time 89 mln; frame 17, time 129 min; frame 18, tllne 149 min.
I
Figure 7: Flow patterns of granules in the louvered chaneel (rest 4). Frames l-13, time Interval 2 mlr; frames 14-15, time Interval 10 min; frame 16, time 104 mln; frame 17, time 224 mln; frame 18, time 534 min.
Four different flow regions were found: 1. A quasi-stagnant zone adjacent to the louvered wall. When the particulate solids in the filter channel are sufficiently circulated it is possible to register displacements of filter granules in this zone. It was shown experimentally (Figure 4, frames 4-15) that the quasi-stagnant zone diminishes gradually when the filter bed is operated for a long time. As the frames 17 and 18 show, the last granules adjacent to the walls left the quasi-stagnant zone in about 3 hours. A new bed structure and porosity is formed in this zone as filter granules flow out from the upper pair of louvers and fill the underneath louver sections. 2. A transition region between the quasi-stagnant zone and a central flowing core. This is a shear zone with movement of filter granules one layer next to another and with significant velocity changes from point to point over a small horizontal distance. The friction forces between granules cause the boundary between the quasi-stagnant zone and this region slowly to move toward the walls. The influence of the granules moving in this transition region will eventually reach the zone of the granules right next to walls. The development of a transition region (shear zone) after frame 4 is manifested by the thinning of the quasi-stagnant zone. The movements of the granules shown in frames 5 to 13 indicate that the granules are moving predominantly in a vertical direction with relatively small horizontal dispersion. It is shown clearly in these frames that there is a cascading granular transport in the transition regions at different louver sections of the granular bed from top to the bottom. Granules flow from the upper transition region into the underneath transition region. Significant dispersion of granules occurred only when the quasi-stagnant zone became very thin (see frame 14 and after). 3. A central plug flow core region. There are small velocity fluctuations about the average plug velocityzg, 30. The boundary between the central plug flow core region of filter granules and the transition region is slightly curved outwards. 4. Left and right free surface regions. These free surfaces are not characterized by a unique angle of repose but by a minimum angle of repose and an angle of maximum free surface stability. The angle of free surface of filter granules oscillates between these angles. It leads to avalanches of filter granules over the free surfaces. When the angle of free surface drops below the minimum angle of repose, the new filter granules will roll down the slope of free surface until the angle of maximum stability is restored. Figure 5 shows the flow history of the coloured granules in eighteen frames under Test 2 conditions. In Test 2 two of the louvered sections were filled with coloured granules. Flow patterns observed in Test 2 are like those in Test 1. With two coloured louvered sections in Test 2, the cascading of granules in transition regions is clearly shown in Figure 5, frames 10 and after. Test 2 results show that as the louver angle becomes less steep, the average center line granule velocity is slower - see Table 1. On the other hand the percentage of granules in motion is higher. In general the granules refreshing rate is higher in the filter bed with steeper louvers. Figure 6 shows the flow history of the filter granules in 18 frames under Test 3 conditions. Since the narrowest passage of Test 3 louver geometry is approximately twice that of Test 1, for the same mass flow rate the central core granule velocity is also a half of that of Test 1. The time interval for frames 1 to 10 is a half of the time interval of the corresponding frames in Figure 4. In Test 3 conditions there are only three louver sections in comparison with Test 1 and Test 2 which have four louver sections. The largest flow region is the central plug flow core in Figure 6. The core flow dominates the motion of granules in the filter bed2g, 30. Free surface regions in Figure 6 are narrower than those in Figure 4. The granules refreshing rate is high in these regions so there is no danger of dust plugging interstices between filter granules and causing excessive pressure drop. The granules refreshing rate can be determined by the time required to completely replace the whole granular bed with a new batch of circulated granules. A contrary situation exists in Figure 4. At the beginning stage of the flow a relatively wide quasi-stagnant zone makes possible a
dust plug to build up across the narrow transition region. This could lead to a stable arch of dust with high inter-particle cohesion and wall adhesion formed over the whole free surface region on the dirty gas side of the filter bed. In Test 4, the asymmetric louver configuration causes the asymmetric quasi-stagnant zones in the moving bed filter. Figure 7 shows the flow history of the filter granules in 18 frames under Test 4 conditions. The time interval for frames 1 to 13 is the same as in Test 1 (Figure 4) for frames 1 to 11. In Test 4 conditions there are only three louver sections. Despite the asymmetric quasi-stagnant zones, the central flowing core is symmetric. The granules refreshing rate is very high in the region close to the 15” louver and the quasi-stagnant zone is very thin. This is similar to the Test 3 (Figure 6) situation. On the other hand, the quasi-stagnant zones close to the right 40” louver in Test 4 are similar to that in Test 1, but the area of the quasi-stagnant zone seems to be larger in Test 4. Comparing Figures 4 and 7, the time for the coloured particles leaving the granular bed in Test 4 is about 2.5 times longer than that in Test 1. It indicates that the granules refreshing rate is decreased in a moving granular bed with asymmetric louver wall configuration. Figure 8 shows the flow history of filter granules in 18 frames under Test 5 conditions where the right 40” louvers are shifted vertically. In Test 5 geometry, there are only three louver sections. As indicated in Table 1, the center line granule velocities achieved at steady state flow conditions for the two different louver configurations in Test 4 and Test 5 are almost the same. Comparing Figure 7 and Figure 8, the times for all the coloured granules leaving the moving bed are quite different (534 min for Test 4 and 289 min for Test 5). This shows that the vertical shift of the louvered wall could have a considerable influence on the granules refreshing rate in the moving bed. The beneficial effect of the higher granules refreshing rate can be that the pressure drop across the filter bed may remain at a rel-
Figure 8: Flow patterns of granules in the louvered channel (Test 5). Frames l-13, time interval 2 rin; frame 14, time 34 min; frame 15, time 44 min; frame 16, time 109 min; frame 17, time 189 min; frame 18, time 289 min.
atively lower level. It is important to recognize that the granules refreshing rate is a more proper index of the filter bed performance rather than the granules circulation rate. Granules refreshing rate should be an important design parameter in the granular bed filter operation, The granules refreshing rate depends strongly on the flow pattern of the moving granular bed for a given granules circulation rate (mass flow rate of filter granules). Knowing the flow pattern of the granules in the filter bed is important for selecting a proper granules circulation rate. On the other hand a very high granules refreshing rate may lower the overall dust collection efficiency of the granular bed filter. The reason is that a batch of newly refreshed granules usually is not very efficient in capturing dust particles. Another implication of this negative effect of the high granules refreshing rate is that the energy used in circulating filter bed material is not utilized very effectively. When the granules refreshing rate is high the filter media being transported may contain a significant portion of not fully utilized granules. Ideally the energy should only be used to transport fully spent dirty filter granules for cleaning and reuse. Transporting relatively clean filter granules wastes a considerable amount of energy. When operating the granular bed filter in too low granules refreshing rate the average granules residence time in the filter bed will be longer. Filter granules will have enough time to reach their full capability of capturing dust particles but at this time the pressure drop of filter bed could be excessive. This will increase the energy demand to move the gas through the filter bed. Another negative effect of too low a granules refreshing rate is that the filter granules may be covered with excessive amount of dust and this may burden the granules cleaning system to cause the deterioration of the cleaning system performance. This will lead to accumulation of dust in the filter bed. The granules refreshing rates of the five tests can be estimated according to Figures 4-8. In Figure 4, by the time of frame 18, the whole filter bed was completely replaced by new granules. In this case, the granules refreshing rate is about 0.4 g/set which is obtained by dividing the filter bed mass with the time of frame 18 (12540 seconds). Similarly, the granules refreshing rates for Test 2 and Test 3 conditions shown in Figure 5 and Figure 6 are 0.63 g/set and 1.5 g/set. The granules circulating rates (mass flow rates) for five tests are almost the same, see Table 1. These estimations show that the granules circulating rate can be several times higher than the granules refreshing rate. The five tests have completely different louver geometry, but the same granules circulating rate. This indicates that the granules refreshing rates are strongly influenced by louver design. CONCLUSIONS Granular bed flow in filter channels is influenced by the angle of louvers and by the vertical shift of louvers. When the angle of louvers is small (steep louvered wall) the central flowing core is the dominant flow region in the filter channel. This is close to mass flow conditions observed in the flow of bulk solids in hoppers. However, in this case the free surface regions of the filter bed are narrow and the flue gas velocity is high, dust particles coming with flue gases can plug the narrow gap between louvers. If the transport of the filter granules along the louvered wall is sufficient, then the danger of plugging can be avoided. Thus filter granules moving in a mass-flow like pattern is a situation less prone to plugging. When the angle of louvers is large, the internal flow pattern of the filter bed is more complicated especially during the beginning stage of filter operation. Four different flow regions were observed: a) a quasi-stagnant zone adjacent to the louvered wall; b) a transition region: a shear zone in which significant velocity changes occurred at each horizontal level; c) a central plug flow core of nearly uniform velocity distribution; d) the left and right free surface regions where gas flows in and out. Louver geometry influences the granules refreshing rate of filter media in the filter channel. A steeper louver is favourable for a higher granules refreshing rate. When the granules refreshing rate is high, the pressure drop across the filter bed can be maintained
at a relatively low level. On the other hand a very high granules refreshing rate may lower the overall dust collection efficiency of the granular bed filter. The selection of an optimum granules refreshing rate is a balance between filter bed efficiency and operating energy demands. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support from the National Science Council of the R.O.C. for this work through project NSC 87-2211 -E-008-01 6. Special financial support from National Science Council of the R.O.C. for one of the author (J. Smid, Czech Republic) is also appreciated. REFERENCES 1. Saxena, SC., Henry, RF and Podolski, W.F.: “Particulate Removal from High-Temperature, High-Pressure Combustion Gases”, frog. Energy Con~bu.st. Sci.. 1985, 11(3), pp. 193-251. 2. Zevenhoven, C.A.P.: “Particle charging and granular bed filtration for high temperature applications”, Ph.D. Dissertation, Delft University of Technology, The Netherlands, 1992. 3. Seville, J.P.K. and Clift, IX: “Granular bed filters”, in: Gas Cleaning in Demanding Applications, ed. J.P.K. Seville, Chapter 9, pp. 170-l 92, Blackie Academic & Professional, London, 1997. 4. Tardos, G.I.: “Granular Bed Filters, Part 1. The Theory”, in: Handbook of Powder Science and Technology, ed. M.E. Fayed and L. Otten, 2nd Ed., Chapter 17, pp. 771-780, Chapman and Hall, New York, 1997. 5.‘Zenz, F.A.: “Granular Bed Filters, Part 2. Application and Design”, in: Handbook of Powder Science and Technology, ed. M.E. Fayed and L. Otten, 2nd Ed., Chapter 17, pp. 781602, Chapman and Hall, New York, 1997. 6. Klingspor, J.S. and Vernon, J.L.: “Particulate ccntrol for coal combustion”, IEA Coal Research Fleporf, IEACR/OS,‘IEA Coal Research, The Clean Coal Center,‘London, 1988. 7. Rubow, L.N., Borden, M., Buchanan, T.L., Cramp, J.A.C., Fischer, W.H., Klett, M.G., Maruvada, S.M., Nelson, E.T., Weinstein, R.E., and Zacharchuk, R.: “Technical and economic evaluation of ten high temperature high pressure particulate cleanup systems for pressurized fluidized bed combustion”, DOE Report DOE/MC/79196-7654, Department of Energy, Washington, D.C., 1984. 8. Wilson, J.S.: “Status of pressurized fluidized bed research projects sponsored by the U.S. Department of Energy”, IEA PFBC Basic Res. Workshop, Goeteborg, Sweden, June 1988. 9. Zakkay, V., Gbordzoe, E.A.M., Radhakrishnan, R., Sellakumar, K.M., Patel, J., Kasinathan, R., Haas, W.J. and Eckels, D.E.: “Particulate and Alkali Capture from PFBC Flue Gas Utilizing Granular Bed Filter (GBF)“, Cornbust. Sci. and Tech., 1989, 66(4-6), pp. 113-130. 10. Zakkay, V. and Gbordzoe, E.A.M.: “A review of hot-gas cleanup devices for PFBC”, Combustion en lechos fluidizados, pp. 216-275, Programa Comett Comunidad Economica Europea, Zaragoza, Spain, May 31-June 2,1989. ,I 1. Moresco, L.L. and Cooper, J.L.: “High Temperature Continuous Granular Bed Filtration of Fine Combustion Particulate”, AlChE
I’
Summer 198 1 National Meeting, Technology, Part II, Paper No.
Symposium
on Industrial
Aerosol
32h, Detroit, Michigan, U.S.A., 16-19
August, 1981. 12. Cicero, D.C., Dennis. R.A., Geiling, D.W. and Schmidt, D.K.: “Hotgas cleanup for coal-based gas turbines”, Mechanical Engineering, 1994, 116(g), pp. 70-75. 13. Nykanen, J., Zevenhoven, C.A.P., Andries, J. and Hein, K.R.G.: “Moving Granular Bed Filter Research at Delft University of Technology”, Proc. gh Finnish National Aerosol Symposium, pp. 119124, Finland, 1-3 June, 1993. 14. Zevenhoven, CAP., Droppert, PJ. and van de Leur R.H.M.: “Electrical characterization of granular materials”, Adv. Powder Technol., 1991, 2(3), pp. 225235. 15. Zevenhoven, C.A.P., Scarlett, B. and Andries, J.: “The Filtration of PFBC Combustion Gas in a Granular Bed Filter”, Filtration & Separation, 1992,29(3), pp. 239-244. 16. Zevenhoven, C.A.P., Hein, K.R.G. and Scarlett, 8.: “Moving Granular Bed Filtration with Electrostatic Enhancement for High-
Temperature
Gas Clean-up”,
Filtration
& Separation,
1993,30(6),
pp.
550-553.
17. Zevenhoven, C.A.P., Andries, J., Hein, K.R.G. and Scarlett, 6.: “High temperature gas cleaning for PFBC using a moving granular bed filter”, in: Gas Cleaning at High Temperatures, ed. R. Clift and J.P.K. Seville, pp. 400-418, Blackie Academic & Professionals, London, 1993. 18. Squires, A.M.: “Air Pollution: The Control of SO, From Power Stacks, Part II -The Removal of SO, From Stack Gases”, Chemical Engineering, 1967, 74(24), pp. 133-l 40. 19. Squires, A.M. and Pfeffer, R.: “Panel Bed Filters for Simultaneous Removal of Fly Ash and Sulphur Dioxide: I. Introduction”, J. Air Pollut. Control Assoc., 1970, 20(8), pp. 534-538. 20. Dorfan, Mt.: “Method and apparatus for suppressing steam and dust rising from coke being quenched”, US. Patent No. 2,604,187 (July 22, 1952). 21. Perry’s Chemical Engineers’ Handbook: “Dust-Collection Equipment: Granular Bed Filters”, ed. D.W. Green and J.O. Maloney, 6th Ed., International Student Edition, Section 20, So/ids Drying and Gas-So/id Sysfems, pp. 20-104, 20-105, McGraw-Hill, New York, 1984. 22. Saxena, SC., Henry, R.F. and Podolski W.F.: “Technology Status of Particulate Removal from High Temperature and Pressure Combustion Gases”, A/C/-/E Summer 1987 National Meeting, Symposium on lndusfrial Aerosol Technology, Part II, Paper No. 32e, Detroit, Michigan, U.S.A., 16-19 August, 1981. 23. Reese, R.G.: “The application of a dry scrubber to exhaust gas
cleanup”, Tappi (Journal of the Technical Association of the Pulp and Paper Industry), 1977,6(3), pp. 109-l 11. _ 24. Jordan, S. and Lindner, W.: “Betriebserfahrungen mit einem Querstrom-Schuettschichtfilter” (Operating experiences with a crossflow granular bed filter), 1988, Chem.-/ng.-Tech. 60(l), pp.34-35. 25. Ishikawa, K., Kawamata, N. and Kamei, K.: “Development of a Simultaneous Sulfur and Dust Removal Process for IGCC Power Generation System”, in: Gas Cleaning at High Temperatures, ed. R. Clift and J.P.K. Seville, pp. 419-435, Blackie Academic & Professionals, London, 1993. I 26. Guillory, J.L., Placer, F.M. and Grace, D.S.: “Electrostatic enhancement of moving bed granular filtration”, Environmenf International, 1981, 6, pp. 387-395. , 27. Martin, J.B. and Richards, J.C.: “The determination of the dynamic zone within a free-flowing granular mass”, Journal of Science and Technology, 1965, 11(l), pp. 31-34. 28. Gardner, G.C.: “The region of flow when discharging granular materials from bin-hopper systems”, Chemical Engineering Science, 1966, 21(3), pp. 261-273. 29. Wang, C.Y.: “Experimental Analysis of Granular Moving Bed Filters for Flue Gas Cleanup”, Master Thesis, National Central University, Chung-Li, Taiwan, R.O.C., 1997. c 30. Hsiau, S.S., Smid, J., Wang, C.Y., Kuo, J.T. and Chou, C.S.: “Velocity profiles of granules in moving bed filters for flue gas cleanup”, Proceedings 23rd International Technical Conference on Coal Utilization & Fuel Systems, pp. 783-794, Clearwater, Florida, , U.S.A, March 9-13, 1998. /
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ELSEVIER ‘ADVANCED rb(‘HNOl.OGY
J. Smid*,
*Department
of Mechanical
**Department ***Department
S. S. Hsiau**,
C. Y. Wang**,
and C. S. Chou***
Engineering, National Taiwan University, Taipei, Taiwan 10617, R.O.C. of Mechanical Engineering, National Central University, Chung-Li, Taiwan 32054, R.O.C. of Mechanical Engineering, National Pingtung University for Science and Technology, Ping-Tung, Taiwan 91207, R.O.C.
The flow patterns in three symmetric and two asymmetric louver-walled granular moving bed systems are studied experimentally. The flow pattern histories of granular solids in moving beds were recorded by videocamera throughout the whole test period. The image processing system including a frame grabber was used. Selected discrete images showing the flow pattern development were digitized and stored in files. Individual granular flow frames show a development of quasi-stagnant zones close to the louvered walls, the shear zones and a central flow region with width depending on the louver angle. The vertical shift of one or the other louvered wall in asymmetric configuration considerably influences the history of quasi-stagnant zones.
ranular bed filters may have technical and economical advantages for general purpose gaseous emission control applications such as Pressurized Fluidised Bed Combustion (PFBC) hot gas cleanup, cement and lime kilns, glass and steel furnaces, and incineration processes. Granular bed filtration can be operated in four modes: fixed bed, intermittently moving bed, continuously moving bed and fluidised bed1-5. The possibility of (semi-) continuous regeneration in moving bed systems makes granular bed filtration an attractive option for continuous high temperature gas cleaning purposes. Basically the designs of granular bed filters can be categorized in two groups. One group are screenless filters’, *I ‘j-l’. The other group contains the moving bed filters with a vertical layer of granular material held in place by louver-like walls’s 18-26. Major advantages of a granular bed filter are its potential use in hot gas cleaning operations up to 800°C by using low-cost ceramic granules with high temperature stability and the possibility to operate the filter in a continuous process (at constant pressure drop) using a filter media circulation and regeneration system. Disadvantages of granular moving bed filters are the complexity of the solids flows involved in the filter media handling and cleaning system. A number of granular moving bed filters have used crossflow designs where the filter granules move downwards guided by louvered walls whilst the gas passes the moving granules horizontally, see Figure. 1. Plugging problems were encountered in some of these moving bed filters, at both, low and high temperatures. This paper presents the results of a study of the flow patterns in 2-D cross flow moving beds of non-cohesive granular solids flowing under steady state conditions between two vertical louvered walls with no interstitial fluid flow relative to the solids. It is important to examine this relatively simple case of particulate solids flow, which is of industrial significance, before systems involving solids acceleration and fluid or gas pressure gradient can be effectively analyzed. The objective of this study is to determine a history of quasi-stagnant zones of granular moving bed near the louvers in symmetrical and asymmetrical louver configurations.
G
EXPERIMENTAL
APPARATUS
row layer of spheres of uniform diameter sandwiched between two transparent panels with louver-like side walls. The configuration of the louvers can be changed (louver spacing, louver angle and length). The experimental apparatus is schematically shown in Figure 2. The flow of spherical solids is induced and controlled by a moving belt underneath the exit of the moving bed. Granules are fed into the vertical channel from a top hopper, which has a rectangular discharge slot with the same cross-section as the vertical channel. The height of the channel is fixed (1000 mm) but the width is adjustable (maximum width is 400 mm). The channel width in the experiments mentioned in this work is fixed at the value of 380 mm. For the experiments, 6 mm diameter PE spheres with density of 964 kg/m3 were selected as test granular solids. Packing of the spheres was characterized by the bulk density measurements. Two bulk densities were measured: a poured bulk density of 582 kg/m3 (porosity 0.396) and a tapped bulk density of Granular
Bed
Filter
Granules
Quasi-Stagnant Zone
AND PROCEDURES
For the observation of the flow patterns occurring as the granular bed flows continuously, an experimental technique”, ** with differentially coloured granules has been used. The experimental moving bed is a two-dimensional system. It consists of a nar-
Filter Dust L
Figure 1: The lllustratlon of the granular bed filter.
Granules + Particles
TestParameter Louver angle Louver length Louver spacing Channel width Mass flow rate Time average center line granule velocity (steady state flow) Beginning stage center line granule velocity Granules refreshing rate
‘The
right side
40’ louver
is shifted
vertically
Test1
Test2
Test3
Test4
Test5
(mm/s)
40 200 200 380 3.51 0.91
30 200 200 380 3.53 0.73
15 200 250 380 3.52 0.49
15/40 200/200 2501250 380 3.52 0.63
15/40+ 2001200 200/200 380 3.52 0.62
(mm/s)
0.80
0.63
0.43
0.48
0 54
(!J/s)
0.40
0.63
1.50
0.17
0.37
(“) (mm) (mm) (mm) km
as shown
in Figure 3(e)
600.5 kg/m3 (porosity 0.377). Both densities were measured in a graduated glass cylinder. The mass flow rate measurements were made by continuous collection of the discharged granules in a tared bucket. Weighing of full buckets was made with an electronic balance. Table 1 lists the test conditions of the five experiments. The time-averaged center line granule velocities listed in Table 1 were measured by timing a small group of coloured granules which moved from the top to the bottom of the vertical channel after circulating granules for approximately two hours. At this time it was believed that the moving bed has reached steady state flow conditions (meaning steady state bulk density distribution, flow patterns etc.). The center line granule velocities during the beginning stage of the experiments were measured from the recorded flow images. Figures 3(a) - (e) show the louver geometry of the five tests. Tests 4 and 5 are asymmetric and the right side 40” louvers in Test 5 were shifted vertically as shown in Figure 3(e). The purpose of shifting the louver vertically in Test 5 is to create a gas flow channel configuration that the cross paths of dust laden gas moving from the left inlet to the two corresponding exits are equal, as shown in Figure 3(f). The horizontal lines labeled with letters “a” to “e” show the locations where the velocity profiles were evaluatedzg, 3o
r
During filling of the granular bed, the coloured granules were filled in one louver section as shown in Figures 4-8. Prior to the start of each experiment, the filter channel was filled from a top hopper, which always contained an inventory of filter granules to ensure continuous supply during the experiment. Filter granules were circulated for approximately 2 hours to make certain that the flow has reached steady state conditions. After that a portion of the filter granules was continuously discharged without filling from the top until the level of granules reached the bottom of the first pair of louvers. Then the movement of filter granules was halted momentarily in order to fill the space between the first pair of louvers with coloured granules. Subsequently, the upper hopper was filled again with white granules. The above operation of stopping the bed flow and filling in with additional granules was executed very carefully to avoid any unwanted disturbance of the existing steady state bed structure. A videocamera was used to record the development of the flow of coloured granules until the coloured particles left the granular bed completely. The camera was supported on a sturdy tripod and activated several seconds before the onset of granular flow. A digital image grabber was used to convert flow images from the recorded tape to computer graphic files. TEST RESULTS AND DISCUSSIONS Figure 4 shows the flow history of the coloured granules in eighteen frames under Test 1 conditions. Frame 1 shows the beginning of the experiment. The time interval for frames 1 to 11 is 120 seconds.The frames following frame 13 have progressively longer time intervals until at frame 18 all the coloured granules have moved out of filter channel.
7
i
14
!ki2e (b)
(a)
40’
15’
40.
a b c d e
Louver Length
Figure 2: The schematic drawing of the experimental apparatus.
Cd)
1.5
a b
90 mm
(e)
Figure 3: The louver geometry for the five test conditions.
Figure 4: Flow pattems of granules in the louvered channel (Test 1). Frames l-11, time interval 2 min; frames 12-13, time interval 4 min; frame 14, time 49 min; frame 15, time 89 min; frame 16, time 129 min; frame 17, time 169 min; frame 18, time 209 min.
I
I
I
Figure 6: Flow patterns of granules in the louvered channel (Test 3). Frames l-10, time interval 1 min; frames 11-15, time interval 2 ndn; frame 16, time 29 min; frame 17, time 49 min; frame 18, time 69 min.
,
I
Figure 5: Flow pattems of granules in tfm louvered channel (lest 2). Frames l-13, tune Merval2 ndn; frame 14, tii 29 mln; frame 15, time 49 min; frame 16, time 89 mln; frame 17, time 129 min; frame 18, tllne 149 min.
I
Figure 7: Flow patterns of granules in the louvered chaneel (rest 4). Frames l-13, time Interval 2 mlr; frames 14-15, time Interval 10 min; frame 16, time 104 mln; frame 17, time 224 mln; frame 18, time 534 min.
Four different flow regions were found: 1. A quasi-stagnant zone adjacent to the louvered wall. When the particulate solids in the filter channel are sufficiently circulated it is possible to register displacements of filter granules in this zone. It was shown experimentally (Figure 4, frames 4-15) that the quasi-stagnant zone diminishes gradually when the filter bed is operated for a long time. As the frames 17 and 18 show, the last granules adjacent to the walls left the quasi-stagnant zone in about 3 hours. A new bed structure and porosity is formed in this zone as filter granules flow out from the upper pair of louvers and fill the underneath louver sections. 2. A transition region between the quasi-stagnant zone and a central flowing core. This is a shear zone with movement of filter granules one layer next to another and with significant velocity changes from point to point over a small horizontal distance. The friction forces between granules cause the boundary between the quasi-stagnant zone and this region slowly to move toward the walls. The influence of the granules moving in this transition region will eventually reach the zone of the granules right next to walls. The development of a transition region (shear zone) after frame 4 is manifested by the thinning of the quasi-stagnant zone. The movements of the granules shown in frames 5 to 13 indicate that the granules are moving predominantly in a vertical direction with relatively small horizontal dispersion. It is shown clearly in these frames that there is a cascading granular transport in the transition regions at different louver sections of the granular bed from top to the bottom. Granules flow from the upper transition region into the underneath transition region. Significant dispersion of granules occurred only when the quasi-stagnant zone became very thin (see frame 14 and after). 3. A central plug flow core region. There are small velocity fluctuations about the average plug velocityzg, 30. The boundary between the central plug flow core region of filter granules and the transition region is slightly curved outwards. 4. Left and right free surface regions. These free surfaces are not characterized by a unique angle of repose but by a minimum angle of repose and an angle of maximum free surface stability. The angle of free surface of filter granules oscillates between these angles. It leads to avalanches of filter granules over the free surfaces. When the angle of free surface drops below the minimum angle of repose, the new filter granules will roll down the slope of free surface until the angle of maximum stability is restored. Figure 5 shows the flow history of the coloured granules in eighteen frames under Test 2 conditions. In Test 2 two of the louvered sections were filled with coloured granules. Flow patterns observed in Test 2 are like those in Test 1. With two coloured louvered sections in Test 2, the cascading of granules in transition regions is clearly shown in Figure 5, frames 10 and after. Test 2 results show that as the louver angle becomes less steep, the average center line granule velocity is slower - see Table 1. On the other hand the percentage of granules in motion is higher. In general the granules refreshing rate is higher in the filter bed with steeper louvers. Figure 6 shows the flow history of the filter granules in 18 frames under Test 3 conditions. Since the narrowest passage of Test 3 louver geometry is approximately twice that of Test 1, for the same mass flow rate the central core granule velocity is also a half of that of Test 1. The time interval for frames 1 to 10 is a half of the time interval of the corresponding frames in Figure 4. In Test 3 conditions there are only three louver sections in comparison with Test 1 and Test 2 which have four louver sections. The largest flow region is the central plug flow core in Figure 6. The core flow dominates the motion of granules in the filter bed2g, 30. Free surface regions in Figure 6 are narrower than those in Figure 4. The granules refreshing rate is high in these regions so there is no danger of dust plugging interstices between filter granules and causing excessive pressure drop. The granules refreshing rate can be determined by the time required to completely replace the whole granular bed with a new batch of circulated granules. A contrary situation exists in Figure 4. At the beginning stage of the flow a relatively wide quasi-stagnant zone makes possible a
dust plug to build up across the narrow transition region. This could lead to a stable arch of dust with high inter-particle cohesion and wall adhesion formed over the whole free surface region on the dirty gas side of the filter bed. In Test 4, the asymmetric louver configuration causes the asymmetric quasi-stagnant zones in the moving bed filter. Figure 7 shows the flow history of the filter granules in 18 frames under Test 4 conditions. The time interval for frames 1 to 13 is the same as in Test 1 (Figure 4) for frames 1 to 11. In Test 4 conditions there are only three louver sections. Despite the asymmetric quasi-stagnant zones, the central flowing core is symmetric. The granules refreshing rate is very high in the region close to the 15” louver and the quasi-stagnant zone is very thin. This is similar to the Test 3 (Figure 6) situation. On the other hand, the quasi-stagnant zones close to the right 40” louver in Test 4 are similar to that in Test 1, but the area of the quasi-stagnant zone seems to be larger in Test 4. Comparing Figures 4 and 7, the time for the coloured particles leaving the granular bed in Test 4 is about 2.5 times longer than that in Test 1. It indicates that the granules refreshing rate is decreased in a moving granular bed with asymmetric louver wall configuration. Figure 8 shows the flow history of filter granules in 18 frames under Test 5 conditions where the right 40” louvers are shifted vertically. In Test 5 geometry, there are only three louver sections. As indicated in Table 1, the center line granule velocities achieved at steady state flow conditions for the two different louver configurations in Test 4 and Test 5 are almost the same. Comparing Figure 7 and Figure 8, the times for all the coloured granules leaving the moving bed are quite different (534 min for Test 4 and 289 min for Test 5). This shows that the vertical shift of the louvered wall could have a considerable influence on the granules refreshing rate in the moving bed. The beneficial effect of the higher granules refreshing rate can be that the pressure drop across the filter bed may remain at a rel-
Figure 8: Flow patterns of granules in the louvered channel (Test 5). Frames l-13, time interval 2 rin; frame 14, time 34 min; frame 15, time 44 min; frame 16, time 109 min; frame 17, time 189 min; frame 18, time 289 min.
atively lower level. It is important to recognize that the granules refreshing rate is a more proper index of the filter bed performance rather than the granules circulation rate. Granules refreshing rate should be an important design parameter in the granular bed filter operation, The granules refreshing rate depends strongly on the flow pattern of the moving granular bed for a given granules circulation rate (mass flow rate of filter granules). Knowing the flow pattern of the granules in the filter bed is important for selecting a proper granules circulation rate. On the other hand a very high granules refreshing rate may lower the overall dust collection efficiency of the granular bed filter. The reason is that a batch of newly refreshed granules usually is not very efficient in capturing dust particles. Another implication of this negative effect of the high granules refreshing rate is that the energy used in circulating filter bed material is not utilized very effectively. When the granules refreshing rate is high the filter media being transported may contain a significant portion of not fully utilized granules. Ideally the energy should only be used to transport fully spent dirty filter granules for cleaning and reuse. Transporting relatively clean filter granules wastes a considerable amount of energy. When operating the granular bed filter in too low granules refreshing rate the average granules residence time in the filter bed will be longer. Filter granules will have enough time to reach their full capability of capturing dust particles but at this time the pressure drop of filter bed could be excessive. This will increase the energy demand to move the gas through the filter bed. Another negative effect of too low a granules refreshing rate is that the filter granules may be covered with excessive amount of dust and this may burden the granules cleaning system to cause the deterioration of the cleaning system performance. This will lead to accumulation of dust in the filter bed. The granules refreshing rates of the five tests can be estimated according to Figures 4-8. In Figure 4, by the time of frame 18, the whole filter bed was completely replaced by new granules. In this case, the granules refreshing rate is about 0.4 g/set which is obtained by dividing the filter bed mass with the time of frame 18 (12540 seconds). Similarly, the granules refreshing rates for Test 2 and Test 3 conditions shown in Figure 5 and Figure 6 are 0.63 g/set and 1.5 g/set. The granules circulating rates (mass flow rates) for five tests are almost the same, see Table 1. These estimations show that the granules circulating rate can be several times higher than the granules refreshing rate. The five tests have completely different louver geometry, but the same granules circulating rate. This indicates that the granules refreshing rates are strongly influenced by louver design. CONCLUSIONS Granular bed flow in filter channels is influenced by the angle of louvers and by the vertical shift of louvers. When the angle of louvers is small (steep louvered wall) the central flowing core is the dominant flow region in the filter channel. This is close to mass flow conditions observed in the flow of bulk solids in hoppers. However, in this case the free surface regions of the filter bed are narrow and the flue gas velocity is high, dust particles coming with flue gases can plug the narrow gap between louvers. If the transport of the filter granules along the louvered wall is sufficient, then the danger of plugging can be avoided. Thus filter granules moving in a mass-flow like pattern is a situation less prone to plugging. When the angle of louvers is large, the internal flow pattern of the filter bed is more complicated especially during the beginning stage of filter operation. Four different flow regions were observed: a) a quasi-stagnant zone adjacent to the louvered wall; b) a transition region: a shear zone in which significant velocity changes occurred at each horizontal level; c) a central plug flow core of nearly uniform velocity distribution; d) the left and right free surface regions where gas flows in and out. Louver geometry influences the granules refreshing rate of filter media in the filter channel. A steeper louver is favourable for a higher granules refreshing rate. When the granules refreshing rate is high, the pressure drop across the filter bed can be maintained
at a relatively low level. On the other hand a very high granules refreshing rate may lower the overall dust collection efficiency of the granular bed filter. The selection of an optimum granules refreshing rate is a balance between filter bed efficiency and operating energy demands. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support from the National Science Council of the R.O.C. for this work through project NSC 87-2211 -E-008-01 6. Special financial support from National Science Council of the R.O.C. for one of the author (J. Smid, Czech Republic) is also appreciated. REFERENCES 1. Saxena, SC., Henry, RF and Podolski, W.F.: “Particulate Removal from High-Temperature, High-Pressure Combustion Gases”, frog. Energy Con~bu.st. Sci.. 1985, 11(3), pp. 193-251. 2. Zevenhoven, C.A.P.: “Particle charging and granular bed filtration for high temperature applications”, Ph.D. Dissertation, Delft University of Technology, The Netherlands, 1992. 3. Seville, J.P.K. and Clift, IX: “Granular bed filters”, in: Gas Cleaning in Demanding Applications, ed. J.P.K. Seville, Chapter 9, pp. 170-l 92, Blackie Academic & Professional, London, 1997. 4. Tardos, G.I.: “Granular Bed Filters, Part 1. The Theory”, in: Handbook of Powder Science and Technology, ed. M.E. Fayed and L. Otten, 2nd Ed., Chapter 17, pp. 771-780, Chapman and Hall, New York, 1997. 5.‘Zenz, F.A.: “Granular Bed Filters, Part 2. Application and Design”, in: Handbook of Powder Science and Technology, ed. M.E. Fayed and L. Otten, 2nd Ed., Chapter 17, pp. 781602, Chapman and Hall, New York, 1997. 6. Klingspor, J.S. and Vernon, J.L.: “Particulate ccntrol for coal combustion”, IEA Coal Research Fleporf, IEACR/OS,‘IEA Coal Research, The Clean Coal Center,‘London, 1988. 7. Rubow, L.N., Borden, M., Buchanan, T.L., Cramp, J.A.C., Fischer, W.H., Klett, M.G., Maruvada, S.M., Nelson, E.T., Weinstein, R.E., and Zacharchuk, R.: “Technical and economic evaluation of ten high temperature high pressure particulate cleanup systems for pressurized fluidized bed combustion”, DOE Report DOE/MC/79196-7654, Department of Energy, Washington, D.C., 1984. 8. Wilson, J.S.: “Status of pressurized fluidized bed research projects sponsored by the U.S. Department of Energy”, IEA PFBC Basic Res. Workshop, Goeteborg, Sweden, June 1988. 9. Zakkay, V., Gbordzoe, E.A.M., Radhakrishnan, R., Sellakumar, K.M., Patel, J., Kasinathan, R., Haas, W.J. and Eckels, D.E.: “Particulate and Alkali Capture from PFBC Flue Gas Utilizing Granular Bed Filter (GBF)“, Cornbust. Sci. and Tech., 1989, 66(4-6), pp. 113-130. 10. Zakkay, V. and Gbordzoe, E.A.M.: “A review of hot-gas cleanup devices for PFBC”, Combustion en lechos fluidizados, pp. 216-275, Programa Comett Comunidad Economica Europea, Zaragoza, Spain, May 31-June 2,1989. ,I 1. Moresco, L.L. and Cooper, J.L.: “High Temperature Continuous Granular Bed Filtration of Fine Combustion Particulate”, AlChE
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Summer 198 1 National Meeting, Technology, Part II, Paper No.
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August, 1981. 12. Cicero, D.C., Dennis. R.A., Geiling, D.W. and Schmidt, D.K.: “Hotgas cleanup for coal-based gas turbines”, Mechanical Engineering, 1994, 116(g), pp. 70-75. 13. Nykanen, J., Zevenhoven, C.A.P., Andries, J. and Hein, K.R.G.: “Moving Granular Bed Filter Research at Delft University of Technology”, Proc. gh Finnish National Aerosol Symposium, pp. 119124, Finland, 1-3 June, 1993. 14. Zevenhoven, CAP., Droppert, PJ. and van de Leur R.H.M.: “Electrical characterization of granular materials”, Adv. Powder Technol., 1991, 2(3), pp. 225235. 15. Zevenhoven, C.A.P., Scarlett, B. and Andries, J.: “The Filtration of PFBC Combustion Gas in a Granular Bed Filter”, Filtration & Separation, 1992,29(3), pp. 239-244. 16. Zevenhoven, C.A.P., Hein, K.R.G. and Scarlett, 8.: “Moving Granular Bed Filtration with Electrostatic Enhancement for High-
Temperature
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550-553.
17. Zevenhoven, C.A.P., Andries, J., Hein, K.R.G. and Scarlett, 6.: “High temperature gas cleaning for PFBC using a moving granular bed filter”, in: Gas Cleaning at High Temperatures, ed. R. Clift and J.P.K. Seville, pp. 400-418, Blackie Academic & Professionals, London, 1993. 18. Squires, A.M.: “Air Pollution: The Control of SO, From Power Stacks, Part II -The Removal of SO, From Stack Gases”, Chemical Engineering, 1967, 74(24), pp. 133-l 40. 19. Squires, A.M. and Pfeffer, R.: “Panel Bed Filters for Simultaneous Removal of Fly Ash and Sulphur Dioxide: I. Introduction”, J. Air Pollut. Control Assoc., 1970, 20(8), pp. 534-538. 20. Dorfan, Mt.: “Method and apparatus for suppressing steam and dust rising from coke being quenched”, US. Patent No. 2,604,187 (July 22, 1952). 21. Perry’s Chemical Engineers’ Handbook: “Dust-Collection Equipment: Granular Bed Filters”, ed. D.W. Green and J.O. Maloney, 6th Ed., International Student Edition, Section 20, So/ids Drying and Gas-So/id Sysfems, pp. 20-104, 20-105, McGraw-Hill, New York, 1984. 22. Saxena, SC., Henry, R.F. and Podolski W.F.: “Technology Status of Particulate Removal from High Temperature and Pressure Combustion Gases”, A/C/-/E Summer 1987 National Meeting, Symposium on lndusfrial Aerosol Technology, Part II, Paper No. 32e, Detroit, Michigan, U.S.A., 16-19 August, 1981. 23. Reese, R.G.: “The application of a dry scrubber to exhaust gas
cleanup”, Tappi (Journal of the Technical Association of the Pulp and Paper Industry), 1977,6(3), pp. 109-l 11. _ 24. Jordan, S. and Lindner, W.: “Betriebserfahrungen mit einem Querstrom-Schuettschichtfilter” (Operating experiences with a crossflow granular bed filter), 1988, Chem.-/ng.-Tech. 60(l), pp.34-35. 25. Ishikawa, K., Kawamata, N. and Kamei, K.: “Development of a Simultaneous Sulfur and Dust Removal Process for IGCC Power Generation System”, in: Gas Cleaning at High Temperatures, ed. R. Clift and J.P.K. Seville, pp. 419-435, Blackie Academic & Professionals, London, 1993. I 26. Guillory, J.L., Placer, F.M. and Grace, D.S.: “Electrostatic enhancement of moving bed granular filtration”, Environmenf International, 1981, 6, pp. 387-395. , 27. Martin, J.B. and Richards, J.C.: “The determination of the dynamic zone within a free-flowing granular mass”, Journal of Science and Technology, 1965, 11(l), pp. 31-34. 28. Gardner, G.C.: “The region of flow when discharging granular materials from bin-hopper systems”, Chemical Engineering Science, 1966, 21(3), pp. 261-273. 29. Wang, C.Y.: “Experimental Analysis of Granular Moving Bed Filters for Flue Gas Cleanup”, Master Thesis, National Central University, Chung-Li, Taiwan, R.O.C., 1997. c 30. Hsiau, S.S., Smid, J., Wang, C.Y., Kuo, J.T. and Chou, C.S.: “Velocity profiles of granules in moving bed filters for flue gas cleanup”, Proceedings 23rd International Technical Conference on Coal Utilization & Fuel Systems, pp. 783-794, Clearwater, Florida, , U.S.A, March 9-13, 1998. /
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