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The behaviour of large jetsam particles in fluidised beds
Powder Technology,
52 (1987) 263 - 266
263
Short Communication
The Behaviour of Large Jetsam Particles in Fluidised Beds B. CARTER, M. GHADIRI and R. CLIFT Department of Chemical and Process Engineering, University of Surrey, Guildford, Surrey GU2 6XH (U.K.) and A. W. JURY British Coal Corporation, Stoke Orchard, Cheltenham, Glos. GL52 4RZ (U.K.) (Received May 13,1987)
Introduction Certain applications of gas fluidisation use beds of particles which differ with respect to size or density. The tendency of the particles to segregate is then of concern. Particles which tend to settle to the bottom of the bed are termed “jetsam”, while particles which tend to float are known as “flotsam”; simple rules are available to distinguish between flotsam and jetsam [l]. It is sometimes desirable for the jetsam particles to sink so that they can be removed selectively. An example is fluid&d bed combustion, where large particles of shale or sir&red ash may be selectively removed at the distributor level [ 21. However, the behaviour of segregated jetsam particles and their effect on gas flow is not well understood. This note reports observations made in large two-dimensional and rectangular beds of the effect of a significant quantity of jetsam material, complementing observations by Bemrose et al. [3] of the behaviour of individual jetsam particles and preliminary observations of the effect of piles of jetsam reported by Parkinson et al. [4]. Experimental details The “twodimensional” fluid&d bed, at the University of Surrey, and internal dimensions 1.35 m wide, 38 mm thick, and approximately 2 m high. Front and back faces were made of acrylic sheets 13 mm thick, reinforced by external horizontal cross-bars to reduce distortion. The distributor was a thin 0032-5910/87/$3.50
horizontal steel plate, drilled with 1.5mm orifices on &mm square pitch to give 4 rows of orifices across the thickness of the bed, each with 160 orifices across the width. Air was supplied by a Rootes blower capable of delivering 0.094 m3 SC’ of air against 70 kPa pressure. The fluidising air entered the plenum chamber below the distributor through a manifold of eight pipes, each with a baffle at the point of discharge into the plenum. These measures ensured that, even though the bed was wide, uniform gas distribution was achieved. The bed was illuminated with strong backlighting on to a diffuse screen placed behind the bed, with weaker photoflood illumination on the front face. Phenomena occurring were recorded by filming the whole particle bed on to video tape. The Figures shown here are traced from individual video frames. Quarry sand in the size range 600 850 pm was used as the main bed material, with static bed depth 450 mm; the minimum fluidisation velocity of this material was measured as 0.33 m s- ‘. The jetsam particles used were sintered coal ash in the size range 5 - 15 mm, with bulk density 1120 kg mP3 and mean particle density 2500 kg rne3. A bed of these particles showed channelling for gas velocities above about 2.4 m s-l, but the particles were too large and angular to be fluidised. The rectangular fluidised bed, at the Coal Research Establishment of British Coal, had internal dimensions 1 m X 0.25 m. It had a transparent front wall, through which gross flow patterns could be observed. Initially, the distributor consisted of a flat baseplate fitted with 36 “standpipes”, each comprising a capped vertical tube terminating in three horizontal slots covered with fine wire mesh through which the fluidising air entered the bed. In a later set of experiments, standpipes with 32 holes were used. Sand with mean particle size 850 pm was used as the main bed material, while the jetsam was sintered coal ash similar to that used in the two-dimensional bed. Two sets of experiments were performed, the first with the orifice jet velocity com@ Elsevier Sequoia/Printed in The Netherlands
264
parable to that in a typical fluidised bed boiler, and the second with the distributor pressure drop roughly equal to that across the bed, as would be the case in a typical boiler. In the second set, the jet velocity issuing from the standpipes was about four times that in the first set. Static bed depths of 0.25 and 0.4 m above the orifices were used. Hot wire and rotating vane anemometers were used to measure the flow rates through selected individual standpipes. Observations The Figure summarises observations made using the two-dimensional bed. A batch of approximately 50 kg of jetsam was tipped on to one side of the bed of sand, maintained at the point of incipient fluidisation. On increasing the gas velocity to 0.5 m s-i to induce gentle bubbling, the ash gradually sank to the distributor to form heaps of jetsam. A marked change in the bubble pattern resulted, shown in Figure (a). Intense bubble activity occurred above each jetsam layer, with the rest of the bed quiescent and probably unfluidised. This general flow pattern persisted on increasing the average superficial velocity. The enhanced gas flow above each jetsam heap took the form of a spout, breaking down at higher
levels into a stream of bubbles. Some jetsam particles could be observed moving in each spout. This pattern persisted even when the average gas velocity was raised to 1 m SC’, at which velocity bubbling occurred across the whole bed section but with clearly greater activity above each jetsam layer (Figure (b)). At 1.2 m s-l, the gas flow through the jetsam was sufficient to cause visible agitation of the particles at the surface of the layer; thus, the jetsam piles were subject to some horizontal spreading, but still retained their identity (Figure (c)). On increasing the average gas velocity to 1.5 m s-l, the jetsam appeared to acquire greater mobility. The ash now started to move upwards to mix into the sand bed. Figure (d) shows the remaining jetsam after about 2 min of fluidisation at 1.5 m s-l. After a further minute, the jetsam was completely dispersed, and the ash particles distributed throughout the bed. The gas flow was then cut off sharply and restarted gradually. The ash particles segregated rapidly, again forming localised deposits on the distributor with resultant gas maldistribution of the type seen in the Figures. Uniform distribution of ash particles and gas flow could again only be achieved by increasing the average gas velocity to 1.5 m s-l.
(b)
(d) Fig. Instantaneous gas flow patterns in two-dimensional fluidised bed. Bed width 1.35 m. 8, Stagnant jetsam; q, fluidised sand. Average superficial gas velocities: (a), 0.5 m s-‘; (b), 1.0 m s-l; (c), 1.2 m s-‘; (d), 1.5 m s-l.
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Very similar effects were observed in both series of runs using the rectangular bed with standpipe distributor. A local layer of ash caused a vigorous vertical flow path through the bed, even at fluidising velocities as low as 0.5 m s-l (cf. Figure (a)), with increased bubble flow above the jetsam piles. The air flow through standpipes below an ash pile was typically 50% higher than the average, with correspondingly low flow through regions of the bed free from jetsam layers. As in the twodimensional bed, to’ disperse the jetsam required the air flow to be increased substantially, in this case to about 2 m s-l.
Discussion The piles of large jetsam particles present a gas path of lower resistance than the sand bed. This is clearly the reason for the gas maldistribution caused by the jetsam: gas flows preferentially through the parts of the distributor covered by jetsam, to form a local spout or bubble track above each jetsam pile. The jetsam particles used in this work are very much larger than the flotsam. According to Chiba and Nienow [5], the jetsam layer should then be dispersed when the bubbles present become large enough to lift jetsam particles in their wakes, and this would normally occur at a gas velocity well below the minimum fluidisation velocity for the jetsam. However, this mechanism was suggested by observations on individual jetsam particles in a much smaller bed. The criterion of Chiba and Nienow predicts that a 5-mm ash particle can be lifted in the wake of (threedimensional) bubbles of diameter approximately 1 cm or more. The bubbles observed here were larger than this size at all velocities above minimum fluid&&ion, so that the simple criterion suggests that no jetsam layer should have been observed. However, the large jetsam piles observed here were associated with spouting rather than local bubble formation, so that wake transport could not affect the settled jetsam layers. This appears to be the reason why high gas velocities were needed to complete the process of erosion and dispersal. Jetsam particles which are large compared with the primary bed material experience an upward force, loosely analogous to buoyancy, given by the weight of displaced fluidised material [6,7]. In this work, the ash particle density was greater than the density
of the fluidised sand so that the “buoyancy” alone was insufficient to support them in the bed. However, once the particles were exposed to the pressure gradient in the surrounding bed, their “immersed weight” was reduced. The jetsam piles then became more liable to spreading (as shown by the observed change in angle of repose observable in the two-dimensional bed), to erosion of particles from the surface, and finally to dispersion when the agitation of the surrounding fluidised sand became sufficiently vigorous. Bemrose et al. [3] have suggested that jetsam dispersal may be assisted by gas jets issuing from the distributor. However, the observations reported here give no clear indication that this is the case. In the two-dimensional bed, the jetsam heaps were eroded by entrainment of particles from their surfaces into the vigorously moving fluidised bed, rather than by the action of gas passing through the distributor. It is possible that the jets-issuing from the standpipes in the rectangular bed aided dispersion of the jetsam, but the precise mechanism could not be observed clearly. In a fluidised bed boiler, the standpipe gas velocity would generally be less than in the cold model, so that jetsam dispersal would be even less effective. As a result, a preferential flow path, once established, could persist. Bemrose et al. [3 ] and Whitehead et al. [8] have also observed that regions of high gas velocity and hence high transport of flotsam tend to draw in particles from other, less active parts of the distributor, and also that the jetsam layer disperses less easily with distributors of the type used in the twodimensional bed than with tuyere distributors. When a sloping distributor is used in a relatively shallow bed, the reduced pressure drop through the bed above the upper end leads to increased gas flow in that region. As a result, large jetsam particles tend to move up the distributor [ 31. The present work shows that a very similar process causes the instability which leads to gas maldistribution in beds with larger quantities of jetsam. A local concentration of settled jetsam causes higher local gas flow, which in turn causes more jetsam particles to be drawn into this area, amplifying the non-uniformity. Thus it appears that a sloping distributor might accumulate jetsam at its upper end and so
266
cause serious gas maldistribution, unless particular design measures are taken to ensure correct gas distribution and to promote appropriate movement of jetsam across the distributor. This type of maldistribution is clearly harmful. In addition to promoting gas bypassing and hence reducing combustion efficiency, high local gas flow rates will also lead to increased metal wastage from heat transfer tubes in these regions [4].
Acknowledgements The authors are grateful to BCURA Ltd. for support of this work, and to the staff of the Coal Research Establishment (particularly Messrs. M. J. Cooke, M. J. Parkinson and E. A. Rogers) for constructive discussions. However, the views expressed are those of the authors, and do not necessarily reflect those of the Coal Research Establishment.
References A. W. Nienow and T. Chiba, in J. F. Davidson, R. CIift and D. Harrison (eds.), Fluidization, Academic Press, London, 1985, pp. 357 - 382. M. J. Cooke, E. A. Rogers, R. L. Dando and D. W. Gauld, Proc. 3rd Znt. Fluidized Corn bustion Confce., Inst. Energy, London, 1984, p. DISC/ 271232. C. R. Bemrose, J. S. M. BotteriII, J. Bridgwater and A. W. Nienow, in K. Ostergaard and A. Sorensen (eds.), Fluidization V, Engineering Foundation, New York, 1986, p. 201. M. J. Parkinson, K. A. G. Jones and A. W. Jury, Cold Model Studies of AFBC Erosion, presented at EPRI Workshop on Materials Issues in Fluidised Bed Combustion, Port Hawkesbury, N.S. (1985). T. Chiba and A. W. Nienow, in D. Kunii and R. Toei (eds.), Fluidization ZV, Engineering Foundation, New York, 1984, p. 195. J. A. BickneiI and R. L. Whitmore, in A. A. H. Drinkenburg (ed.), Proc. Znt. Symp. on Fluidization, Netherlands Univ. Press, Amsterdam, 1967, pp. 31- 37. R. CIiit, J. P. K. Seville, S. C. Moore and C. Chavarie, Chem. Eng. Sci., 42 (1987) 191. A. B. Whitehead, D. C. Dent and R. Close, in D. Kunii and R. Toei (eds.), Fluidization ZV, Engineering Foundation, New York, 1984, p. 515.
52 (1987) 263 - 266
263
Short Communication
The Behaviour of Large Jetsam Particles in Fluidised Beds B. CARTER, M. GHADIRI and R. CLIFT Department of Chemical and Process Engineering, University of Surrey, Guildford, Surrey GU2 6XH (U.K.) and A. W. JURY British Coal Corporation, Stoke Orchard, Cheltenham, Glos. GL52 4RZ (U.K.) (Received May 13,1987)
Introduction Certain applications of gas fluidisation use beds of particles which differ with respect to size or density. The tendency of the particles to segregate is then of concern. Particles which tend to settle to the bottom of the bed are termed “jetsam”, while particles which tend to float are known as “flotsam”; simple rules are available to distinguish between flotsam and jetsam [l]. It is sometimes desirable for the jetsam particles to sink so that they can be removed selectively. An example is fluid&d bed combustion, where large particles of shale or sir&red ash may be selectively removed at the distributor level [ 21. However, the behaviour of segregated jetsam particles and their effect on gas flow is not well understood. This note reports observations made in large two-dimensional and rectangular beds of the effect of a significant quantity of jetsam material, complementing observations by Bemrose et al. [3] of the behaviour of individual jetsam particles and preliminary observations of the effect of piles of jetsam reported by Parkinson et al. [4]. Experimental details The “twodimensional” fluid&d bed, at the University of Surrey, and internal dimensions 1.35 m wide, 38 mm thick, and approximately 2 m high. Front and back faces were made of acrylic sheets 13 mm thick, reinforced by external horizontal cross-bars to reduce distortion. The distributor was a thin 0032-5910/87/$3.50
horizontal steel plate, drilled with 1.5mm orifices on &mm square pitch to give 4 rows of orifices across the thickness of the bed, each with 160 orifices across the width. Air was supplied by a Rootes blower capable of delivering 0.094 m3 SC’ of air against 70 kPa pressure. The fluidising air entered the plenum chamber below the distributor through a manifold of eight pipes, each with a baffle at the point of discharge into the plenum. These measures ensured that, even though the bed was wide, uniform gas distribution was achieved. The bed was illuminated with strong backlighting on to a diffuse screen placed behind the bed, with weaker photoflood illumination on the front face. Phenomena occurring were recorded by filming the whole particle bed on to video tape. The Figures shown here are traced from individual video frames. Quarry sand in the size range 600 850 pm was used as the main bed material, with static bed depth 450 mm; the minimum fluidisation velocity of this material was measured as 0.33 m s- ‘. The jetsam particles used were sintered coal ash in the size range 5 - 15 mm, with bulk density 1120 kg mP3 and mean particle density 2500 kg rne3. A bed of these particles showed channelling for gas velocities above about 2.4 m s-l, but the particles were too large and angular to be fluidised. The rectangular fluidised bed, at the Coal Research Establishment of British Coal, had internal dimensions 1 m X 0.25 m. It had a transparent front wall, through which gross flow patterns could be observed. Initially, the distributor consisted of a flat baseplate fitted with 36 “standpipes”, each comprising a capped vertical tube terminating in three horizontal slots covered with fine wire mesh through which the fluidising air entered the bed. In a later set of experiments, standpipes with 32 holes were used. Sand with mean particle size 850 pm was used as the main bed material, while the jetsam was sintered coal ash similar to that used in the two-dimensional bed. Two sets of experiments were performed, the first with the orifice jet velocity com@ Elsevier Sequoia/Printed in The Netherlands
264
parable to that in a typical fluidised bed boiler, and the second with the distributor pressure drop roughly equal to that across the bed, as would be the case in a typical boiler. In the second set, the jet velocity issuing from the standpipes was about four times that in the first set. Static bed depths of 0.25 and 0.4 m above the orifices were used. Hot wire and rotating vane anemometers were used to measure the flow rates through selected individual standpipes. Observations The Figure summarises observations made using the two-dimensional bed. A batch of approximately 50 kg of jetsam was tipped on to one side of the bed of sand, maintained at the point of incipient fluidisation. On increasing the gas velocity to 0.5 m s-i to induce gentle bubbling, the ash gradually sank to the distributor to form heaps of jetsam. A marked change in the bubble pattern resulted, shown in Figure (a). Intense bubble activity occurred above each jetsam layer, with the rest of the bed quiescent and probably unfluidised. This general flow pattern persisted on increasing the average superficial velocity. The enhanced gas flow above each jetsam heap took the form of a spout, breaking down at higher
levels into a stream of bubbles. Some jetsam particles could be observed moving in each spout. This pattern persisted even when the average gas velocity was raised to 1 m SC’, at which velocity bubbling occurred across the whole bed section but with clearly greater activity above each jetsam layer (Figure (b)). At 1.2 m s-l, the gas flow through the jetsam was sufficient to cause visible agitation of the particles at the surface of the layer; thus, the jetsam piles were subject to some horizontal spreading, but still retained their identity (Figure (c)). On increasing the average gas velocity to 1.5 m s-l, the jetsam appeared to acquire greater mobility. The ash now started to move upwards to mix into the sand bed. Figure (d) shows the remaining jetsam after about 2 min of fluidisation at 1.5 m s-l. After a further minute, the jetsam was completely dispersed, and the ash particles distributed throughout the bed. The gas flow was then cut off sharply and restarted gradually. The ash particles segregated rapidly, again forming localised deposits on the distributor with resultant gas maldistribution of the type seen in the Figures. Uniform distribution of ash particles and gas flow could again only be achieved by increasing the average gas velocity to 1.5 m s-l.
(b)
(d) Fig. Instantaneous gas flow patterns in two-dimensional fluidised bed. Bed width 1.35 m. 8, Stagnant jetsam; q, fluidised sand. Average superficial gas velocities: (a), 0.5 m s-‘; (b), 1.0 m s-l; (c), 1.2 m s-‘; (d), 1.5 m s-l.
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Very similar effects were observed in both series of runs using the rectangular bed with standpipe distributor. A local layer of ash caused a vigorous vertical flow path through the bed, even at fluidising velocities as low as 0.5 m s-l (cf. Figure (a)), with increased bubble flow above the jetsam piles. The air flow through standpipes below an ash pile was typically 50% higher than the average, with correspondingly low flow through regions of the bed free from jetsam layers. As in the twodimensional bed, to’ disperse the jetsam required the air flow to be increased substantially, in this case to about 2 m s-l.
Discussion The piles of large jetsam particles present a gas path of lower resistance than the sand bed. This is clearly the reason for the gas maldistribution caused by the jetsam: gas flows preferentially through the parts of the distributor covered by jetsam, to form a local spout or bubble track above each jetsam pile. The jetsam particles used in this work are very much larger than the flotsam. According to Chiba and Nienow [5], the jetsam layer should then be dispersed when the bubbles present become large enough to lift jetsam particles in their wakes, and this would normally occur at a gas velocity well below the minimum fluidisation velocity for the jetsam. However, this mechanism was suggested by observations on individual jetsam particles in a much smaller bed. The criterion of Chiba and Nienow predicts that a 5-mm ash particle can be lifted in the wake of (threedimensional) bubbles of diameter approximately 1 cm or more. The bubbles observed here were larger than this size at all velocities above minimum fluid&&ion, so that the simple criterion suggests that no jetsam layer should have been observed. However, the large jetsam piles observed here were associated with spouting rather than local bubble formation, so that wake transport could not affect the settled jetsam layers. This appears to be the reason why high gas velocities were needed to complete the process of erosion and dispersal. Jetsam particles which are large compared with the primary bed material experience an upward force, loosely analogous to buoyancy, given by the weight of displaced fluidised material [6,7]. In this work, the ash particle density was greater than the density
of the fluidised sand so that the “buoyancy” alone was insufficient to support them in the bed. However, once the particles were exposed to the pressure gradient in the surrounding bed, their “immersed weight” was reduced. The jetsam piles then became more liable to spreading (as shown by the observed change in angle of repose observable in the two-dimensional bed), to erosion of particles from the surface, and finally to dispersion when the agitation of the surrounding fluidised sand became sufficiently vigorous. Bemrose et al. [3] have suggested that jetsam dispersal may be assisted by gas jets issuing from the distributor. However, the observations reported here give no clear indication that this is the case. In the two-dimensional bed, the jetsam heaps were eroded by entrainment of particles from their surfaces into the vigorously moving fluidised bed, rather than by the action of gas passing through the distributor. It is possible that the jets-issuing from the standpipes in the rectangular bed aided dispersion of the jetsam, but the precise mechanism could not be observed clearly. In a fluidised bed boiler, the standpipe gas velocity would generally be less than in the cold model, so that jetsam dispersal would be even less effective. As a result, a preferential flow path, once established, could persist. Bemrose et al. [3 ] and Whitehead et al. [8] have also observed that regions of high gas velocity and hence high transport of flotsam tend to draw in particles from other, less active parts of the distributor, and also that the jetsam layer disperses less easily with distributors of the type used in the twodimensional bed than with tuyere distributors. When a sloping distributor is used in a relatively shallow bed, the reduced pressure drop through the bed above the upper end leads to increased gas flow in that region. As a result, large jetsam particles tend to move up the distributor [ 31. The present work shows that a very similar process causes the instability which leads to gas maldistribution in beds with larger quantities of jetsam. A local concentration of settled jetsam causes higher local gas flow, which in turn causes more jetsam particles to be drawn into this area, amplifying the non-uniformity. Thus it appears that a sloping distributor might accumulate jetsam at its upper end and so
266
cause serious gas maldistribution, unless particular design measures are taken to ensure correct gas distribution and to promote appropriate movement of jetsam across the distributor. This type of maldistribution is clearly harmful. In addition to promoting gas bypassing and hence reducing combustion efficiency, high local gas flow rates will also lead to increased metal wastage from heat transfer tubes in these regions [4].
Acknowledgements The authors are grateful to BCURA Ltd. for support of this work, and to the staff of the Coal Research Establishment (particularly Messrs. M. J. Cooke, M. J. Parkinson and E. A. Rogers) for constructive discussions. However, the views expressed are those of the authors, and do not necessarily reflect those of the Coal Research Establishment.
References A. W. Nienow and T. Chiba, in J. F. Davidson, R. CIift and D. Harrison (eds.), Fluidization, Academic Press, London, 1985, pp. 357 - 382. M. J. Cooke, E. A. Rogers, R. L. Dando and D. W. Gauld, Proc. 3rd Znt. Fluidized Corn bustion Confce., Inst. Energy, London, 1984, p. DISC/ 271232. C. R. Bemrose, J. S. M. BotteriII, J. Bridgwater and A. W. Nienow, in K. Ostergaard and A. Sorensen (eds.), Fluidization V, Engineering Foundation, New York, 1986, p. 201. M. J. Parkinson, K. A. G. Jones and A. W. Jury, Cold Model Studies of AFBC Erosion, presented at EPRI Workshop on Materials Issues in Fluidised Bed Combustion, Port Hawkesbury, N.S. (1985). T. Chiba and A. W. Nienow, in D. Kunii and R. Toei (eds.), Fluidization ZV, Engineering Foundation, New York, 1984, p. 195. J. A. BickneiI and R. L. Whitmore, in A. A. H. Drinkenburg (ed.), Proc. Znt. Symp. on Fluidization, Netherlands Univ. Press, Amsterdam, 1967, pp. 31- 37. R. CIiit, J. P. K. Seville, S. C. Moore and C. Chavarie, Chem. Eng. Sci., 42 (1987) 191. A. B. Whitehead, D. C. Dent and R. Close, in D. Kunii and R. Toei (eds.), Fluidization ZV, Engineering Foundation, New York, 1984, p. 515.