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Coal fragmentation in a fluidized bed combustor
Twenty-Second Symposium (International) on Combustion/The Combustion Institute, 1988/pp. 259-26,5
COAL
FRAGMENTATION
IN A FLUIDIZED
BED
COMBUSTOR
KENNETH W. RAGLAND AND FERDINAND A. PECSON Department of Mechanical Engineering University of Wisconsin Madison, WI 53706
The fragmentation behavior of a Colorado bituminous, a Montana sub-bituininous and a Texas lignite coal during combustion in a 0.3 m by 2.1 m atmospheric fluidized bed was investigated. A sampling probe was developed to remove bed samples at different times, enabling the temporal determination of the number, size distribution, density, and surface appearance of the char particles. Fragmentation depends on coal type, weakly on bed particle size and not on fluidization velocity. Microphotographs of the char particles show distinctive surface fissures with a cell size of 0.2 mm for lignite and an order of magnitude greater tot bituminous and sub-bituminons coal. Most of the fissure lines do not lead to fi'actures, but some secondary fragmentation does occur. Particle size distrihutions curves are presented. The coal feed had a narrow size distribution with a surface mean diameter of 5 ram, and the overall char size distribution was broader with the surface mean diameter reduced by a factor of two due to fragmentation. Calculations were made to separate the effects of size reduction due to burning and attrition from fragmentation.
0.3-0.4 mm silica sand 10 cm deep at 1123 K with a velocity of 0.8 m/s. The number of particles increased during devolatilization by factors of 1.5 to 8 depending on coal feed size. As reaction progressed the number of particles decreased as a result of combustion and elutriation. Cammarota et al.1 measured fragmentation of all anthracite, a marine coal, a low swelling bituminous coal and metallurgical coke. The anthracite produced a 160 fold increase in particles during devolatilization, however 25% of the original number of particles did not fragment. Metallurgical coke exhibited no fragmentation. Marine coal gave a 5 fold increase in the number of particles during devolatilization and no fragmentation was observed during charification. The low swelling bituminous coal had negligible fragmentation during devolatilization, but gave a 1.5 fold increase in number of particles 3 min alter injection into the bed. The effect of fragmentation on burning time of a swelling bituminous coal was studied by Sunback et al. 3 Comparison of a shrinking sphere char combustion model that predicts CO2 concentration in the outlet gas and particle size as a function of time with the measured time history, of CO2 enabled determination of the number and size of particles produced during each fragmentation. Kerstein and Niksa 4 investigated fragmentation due to internal burning using percolation theory. This is a mechanism to produce carbon fines without thermal or mechanical stress-induced breakage. From this theory Walsh et al. 5 developed and tested a model for
Introduction Understanding coal particle size change during fluidized bed combustion is important in making a realistic assessment of combustion rate and combustion efficiency. Experiments were carried out to quantify the fragmentation burning coal particles. Coal particles undergo size reduction as a result of attrition, fragmentation and burning. Attrition refers to the loss of mass from particle surfaces as a result of collisions with bed particles. Cammarota et al.1 concluded that attrition is enhanced by combustion. Fragmentation, which refers to the breakage of a coal particle into several pieces, occurs during devolatilization and char burn. Chirone et al2 explained fragmentation as due to stresses arising from the build-up of pressure inside the pores during escape of volatiles, mechanical stresses caused by collisions, and oxygen penetration into the pores which result in combustion of bridges connecting different sections of the particle. Experiments on coal fragmentation have been performed using the removable basket, which allows bed material but not larger coal particles to pass through. Experiments were performed batchwise usually starting with a single particle, and the basket was removed at different times to examine the particles. Chirone et al. z determined the effect of initial coal particle size and oxygen concentration on the number of coal particles in the bed as a function of time. A South African bituminous coal was tested in a 0.04 m i.d. quartz combustor using 259
260
COAL COMBUSTION: FLUIDIZED BED COMBUSTION
carbon conversion in an atmospheric fluidized bed. In summary, fragmentation of a range of coals has been studied in small fluidized beds (0.04 m diam.) with small bed material (0.4 mm) using a few coal particles in a basket, and by interpreting the CO2 emissions from combustion of single particles. The disadvantage of the basket technique is that in order to capture the coal fragments a small mesh size and small bed material must be used, and only a few particles can be studied at a time. The CO2 method 3 requires a number of assumptions and results do not agree with the basket method. The purpose of this study is to investigate fragmentation in a larger fluidized bed as a function of coal rank, bed particle size and fluidization velocity, and to obtain particle size distributions using a wall sampling probe technique.
Experiments The experiments were performed using a 0.3 m i.d. by 2.1 m high atmospheric fluidized bed combustor with steel walls. Nine bubble caps, one at the center and eight distributed uniformly along the perimeter of the bed, serve as the gas distributor. The bed is heated by a propane-air gas burner in the inlet air plenum. A second gas burner heats a double wall surrounding the bed. Coal is fed from the top through two ball valves in series. A solids sampling probe with a 14 mm diameter rounded inlet and a double wall for cooling and suction was used. Suction is created at the tip when air pressure is applied to the probe. The probe tip was located 0.127 m from the reactor wall and 0.076 m above the bottom of the bed. The estimated residence time in the probe was 0.2 s. Cold flow tests showed that the probe itself did not cause coal particle fracture and that a representative sample of coal particles was obtained. Three different coals were used: Texas lignite A (PSOC 1442), Montana sub-bituminous B coal (PSOC 230) and Colorado high volatile bituminous C coal (PSOC 543). The coal was screened between number 4 and 5 mesh (3.9 by 6.9 mm), and bed materials were 0.4 mm silica sand or 0.8 mm aluminum oxide. The initial bed weight was 31 to 34 kg and the static bed depth was 30 cm. Coal was fed into the combustor when the bed temperature reached 1073 K. Using the sampling probe, bed material was collected at regular intervals in cans with "cushion sand" in the bottom. The samples were immediately quenched by immersing the cans in a water bath and pouring ambient sand on top of the samples. Three sets of experiments were performed. The first set investigated the differences in fragmentation behavior of the three coals using silica sand and a fluidizing velocity of 1.8 m/s. The second set
investigated the effect of lower fluidizing velocity while burning Texas lignite in silica sand at a gas velocity of 0.9 m/s. For the third set, Texas lignite and Montana sub-bituminous coal were burned in a bed of aluminum oxide sand at 1.8 m/s to investigate the effect of changing to a coarser bed material. The collected samples were gently screened to separate the burned coal particles from the bed material. The weight of bed material in the sample, the number and weight of the burned coal particles in the sample, and the size distribution of collected coal particles were measured. The particle size was defined as the width of the smallest parallelopiped that would inscribe the particle, when the particle is lying on its flattest side. The size was measured using a graticule equipped microscope. Few char particles below 0.5 mm were observed because particles less than this size tended to be elutriated from the bed. The size distributions were determined by counting and weighing the particles that fell within seven size ranges from 0.5-6.9 mm. The bed particle population was obtained from the sampling probe data using sample weight and the weight of the bed prior to sampling. Uniform mixing in the bed was assumed. For a particular test run approximately 3800 coal particles (0.2 Kg) in the optically measured size range of 4 to 6 mm were added to the 1073 K bed. For this size of particles and bed temperature the devolatilization time was roughly 30 s. 6 The first sample was taken at 90 s to allow for mixing of the coal particles in the bed as determined by cold flow tests. The particles collected were therefore char particles. Each sampling lasted 15 s and a uniform interval of 45 s was maintained between samplings.
Test Results Figure 1 shows that the number of bituminous particles increased due to fracturing by a factor of 2.1, the sub-bituminous particles by a factor of 3.2, and the lignite particles by a factor of 2.7. In general, the coal particle population increased during the first third of the burn and then decreased. Figure 2 shows the effect of reducing fluidizing velocity and increasing bed size on the population of Texas lignite particles. It was expected that the reduced velocity would induce less mechanical stresses on the coal particles and thus less fragmentation. However, Fig. 2 shows that there was no significant reduction in fragmentation when the fluidizing velocity was reduced from 1.8 m/s to 0.9 m/s, although the apparent burn time increased. This indicates that kinetic forces of the bed are not a dominant factor for fragmentation. Increasing the bed size significantly reduced the residence time of the Texas lignite particles (Fig.
COAL FRAGMENTATION
261
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FIG. ]. N u m b e r of coal particles (greater than 0.5 m e ) vs. burn time. Fluidizing velocity 1.8 m/s, oxygen 13.4%; * lignite, 0 sub-bituminous, + bitu-
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FIG. 3. Effe('t of increased bed size on number of sub-bituminous particles vs. bum time; * silica sand (0.4 ram); 0 aluminum oxide sand (0.8 ram); fluidization velocity 1.8 m/s and oxygen 10.1%.
2) and the Montana sub-bituminous particles (Fig. 3). The measured normalized rate of mass loss per unit external surface area during the first 90 s of burning for Texas lignite increased from 0.484 to 0.682 mg/cm2"s and for Montana sub-bituminous from 0.533 to 0.711 mg/cm2-s when the bed was changed from 0.4 mm silica sand to 0.8 mm aluminum oxide, probably due to increased attrition rates. This is supported by the attrition rates obtained by Arena et al. 7 with a South African bituminous coal. Their results at a bed temperature of 1123 K, coal feed size between 1 to 3 mm and fluidizing velocity of 1.6 m/s, showed that the rate of attrition was increased by a factor of 6 when changing from 0.2-0.4 mm sand to 1-1.4 mm. In our experiments the elutriated particles burned in the freeboard before they could be collected in the cyclone. The peak number of sub-bituminous particles was almost the same as for the 0.4 mm sand,
while the peak number of lignite particles was reduced. Microphotographs of the char particles showed extensive surface fissures especially tor the lignite samples (Fig. 4). After 90 s the fissure cell size was 0.1 to 0.2 mm. Some fissure lines were deeper than others, and after 270 s and 390 s in the bed the finer-scale fissure lines have disappeared while others have widened. Most of the fissure lines apparently do not lead to fractures, but they weaken the structure of the char particles aud contribute to fragmentation during char burn. Char particles taken from the coarser bed indicate a more advanced stage of combustion after 90 s indicating higher attrition rate. The fissure cell size is larger with the 0.8 mm bed material, probably due to the reduced heat transfer rate with the coarser bed material. The bituminous chars exhibited a fissure cell size an order of magnitude larger than the lignite.
o
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!
Zl~_ o
0
Burn time, •
FIG. 2. Effect of changing bed material and flu-
idizing velocity on number of lignite coal particles vs. burn time. 0 silica sand (0.4 mm) fluidized at 1.8 m/s; * silica sand (0.4 mm) fluidized at 0.9 m/s; + aluminum oxide sand (0.8 ram) fluidized at 1.8 m/s; oxygen 10.1%.
F]c. 4. Microphotograph of lignite particle removed 'after 90 s in silica sand bed. The white mark is 100 i~m.
262
COAL COMBUSTION: FLUIDIZED BED COMBUSTION
The size distribution of char particles was investigated for the three coals. In the absence of fragmentation it is expected that the size distribution curve would gradually shift to the left as the particles shrink by burning and attrition, and the total number of particles would remain the same as long as the particles remain above the elutriable size. Figure 5 shows that the size distribution abruptly moved to the left at first and then showed a more gradual shift. The size distribution of char particles under steady state operation was estimated from our batch experiments by averaging the size distributions from each of the different sampling times. The quantity A n / N / A d is plotted against d/do, in Fig. 6, where An is the number of particles in the size interval (d, d + Ad), N is the total number of particles and do is the initial feed size. Although the same nominal coal feed size (5 mm) was used, the size distribution curves differed for the three coals due to differences in burning and fragmentation characteristics. The most reactive coal of the three investigated, Texas lignite, tended to have a large fraction of particles in the smallest size range, and thus a larger total surface area. The surface mean diameter (~, which is inversely proportional to the mean surface area per unit volume, was estimated for Fig. 6 to be 3.0, 2.5 and 2.3 mm for the bituminous, sub-bituminous and lignite, respectively. With the coarser bed (0.8 mm alumina, instead of 0.4 mm silica) the surface mean diameter was 2.0 mm for the sub-bituminous and 2.2 for lignite. The feed coal has a narrow size distribution with a d of 5.0 mm and fragmentation reduces the overall char d by a factor of 1.6 to 2.5. Analysis of Results
In considering the change in size distribution with time a question arises as to the extent of fragmen-
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tation versus combustion and attrition. In the absence of fragmentation the shrinkage rate of a particle of size d is given by,
(dd/dt) = (dd/dt)c + (dd/dt)a
(1)
where c refers to the combustion component of the shrinkage rate and a is the attrition component. Considering both kinetic and diffusional resistances, the combustion component of the shrinkage rate may be expressed as: 6
(dd/dt)c = -(24Co/O~)(kcShDg/(dk~ + ShDg))
(2)
where Pc is the char density, kc is the reaction rate constant, Sh is the Sherwood number, D¢ is the molecular gas diffusivity, and Co is the concentration of oxygen in the particulate phase of the fluidized bed. The density of the char is assumed constant throughout the burn time. Arena et al. 7 measured the rate of generation of attrited fines under different fluidizing and bed conditions and used the following correlation
Umf)W/~l
(dd/dt)a = - k a ( U - U m f ) o
0.0
1.0
(3)
where Ec is proportional to the excess of the superficial gas velocity above the minimum for fluidization and to the total carbon loading W, and inversely proportional to the mean surface diameter of the char particles d. They obtained a value of k equal to 1.86 × 10 7. From Eq. 3 one can obtain the expression for (dd/dt)a by equating Ec to the rate of carbon mass loss (d(pcd3/6)dt),
-o
o
0.8
FIG. 6. Steady-state char size distribution curves obtained from averaging the size data.
Ec = k(U 120 -
0.6
d/do
1.0
2.0
3.0
4.0
5.0
Char particle size. d. mm
6.0
7.'0
FIG. 5. Sub-bituminous particle size distribution: a, time = 0; b, t = 90 s; c, t = 270 s; d, t = 450 s. Silica sand fluidized at 1.8 m/s, 14.4% oxygen.
(4)
in which k a has been substituted for k/3 to give the form of the correlation proposed by D'Amore et al. s Substituting Eq. 2 and 4 into Eq. 1 and integrating from time t~ to t and from di to d yields,
COAL FRAGMENTATION
kc t-
ti = B ( d i - d)
\A + B dJ
(5)
where A = ( 2 4 / p ~ ) k ~ S h D : o + k~(U - Umf)ShD~; B = (kak~)(U - Umf); d is the particle size at t, d| is the initial particle size at t i. Equation 5 was used to compare the measured char size distribution with the size distribution that would be obtained if the particles reduced in size by burning and attrition without fragmentation. The predicted and actual size distributions are shown in Fig. 7a and b for t = 150 s and 210 s respectively for Montana sub-bituminous chars burned in silica sand. In Eq. 5 the Sherwood number of the particles was assumed to be constant for the interval t~ to t and given by: ~ Sh = 2e + 0.69(Re/•)°5Sc °'3~
(6)
where • is the bed voidage which in this experi600o500-
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FIG. 7. Size distribution shift predicted without fragmentation (dashed) and measured (solid) for subbituminous char in silica sand: a) 90 s to 150 s; b) 150 s to 210 s.
263
ment is equal to 0.4. The Reynolds number reduced to 14.4 di and the Schmidt number was 0.826 for our conditions at 1.8 m/s. The binary diffnsivity of oxygen into nitrogen, D¢, was calculated using Fuller's 1° correlation as 154 mm'Z/s. The reaction rate constant kc was interpolated from Table III of Ref. 5 as 0.72 m/s. The attrition rate constant k, was obtained from Fig. 16 of Arena et al. 7 as 2 × 10 -7. This gives a value of ka equal to 0.67 × 10 7. The char densities of particles at different residence times were measured and except for Colorado bituminous, the densities did not show a significant decrease with burn time. From t = 90 s to t = 450 s, the density of Colorado bituminous decreased from 0.83 m g / m m 3 to 0.76 m g / m m 3, and the density of Montana sub-bituminous decreased from 0.94 to 0.90 m g / m m 3. The density of lignite char decreased from 0.73 m g / m m 3 at t = 90 s to 0.7 m g / m m 3 at t = 270 s. For Montana sub-bituminous chars an average char density of 0.92 m g / mm 3 was used. The value of Umf was computed using Wen and Yu's 11 correlation and is equal to 0.043 m/s. After all the above values are substituted, examination of Eq. 5 shows that for a given time interval, the unknowns left in the equation are the predicted particle diameters and the oxygen concentration in the dense phase. The number of particles, n i of size dl are known, and in the absence of fragmentation the number, n, of particles of size d is also known. From the number and size one can obtain the coresponding total mass of particles. The size, number and mass of particles for each sampling time were measured, and the predictions were made for each sampling time. Thus to predict the size distribution of particles at time t, Eq. 5 was applied to each di using a value of Co such that the final values of d's and the known number of particles yielded the same mass of particles as was actually measured for time t. The values of co for the different time intervals were obtained bv trial and error. The measured inlet oxygen concentration was 1.5 mol/mm 3 and dropped to 0.022 tool/ mm 3 at t = 90 s before gradually recovering. The actual and predicted size distribution curves have been plotted such that the area under each curve represents the mass of particles in the bed, and the area under the predicted size distribution curve is the same as the area of the actual size distribution curve. Areas outside the overlapping areas then represent the mass of fragments. Figure 7a indicates that during the time interval 90 to 150 s, fragments originated from particles of size between 3.5 and 3.9 mm and also from 0.75 to 2.75 ram. Figure 7b shows that fragmentation also occurred during the time interval 150 to 210 s, despite what is indicated by Fig. 3 which showed a 3% decrease in the number of particles between 0.5 and 5 into. During this time interval fragments of 2.7 to 3.1
264
COAL COMBUSTION: F L U I D I Z E D BED COMBUSTION
mm and 1.1 to 1.6 mm were produced. Similar observations were obtained at other burn times indicating that fragmentation continues during char burn, as has been observed by Massimilla and coworkers lz from basket experiments, however it is not as extensive as indicated by the analysis of Sunback. 3
Conclusions Coal particles fed into a 0.3 m diam. fluidized bed combustor experienced a doubling or tripling of the number of particles due to fracture during devolatilization and early char burn. The fragmentation process depends significantly on coal type, weakly on bed size, and does not depend on fluidization velocity. Microphotographs of the char particles show extensive surface fissures. Lignite char had a 0.1 to 0.2 mm fissure cell size, while the subbituminous and bituminous coal had an order of magnitude greater fissure cell size. The coarser bed material results in larger fissure cell size. Most fissures do not lead to fractures, but they indicate a potential for fragmentation. The particle size distribution shifted from a coal feed surface mean diameter (d) of 5 m m to an overall char d of 3 mm to 2 mm due to fragmentation depending on coal type and bed size. A model used to separate size reduction effects due to burning and attrition from fragmentation showed that fragmentation continued during char burn.
Acknowledgments This work was enabled by a cooperative agreement between the Ministry of Energy of the Philippines, USAID and the University of Wisconsin. We are grateful to the Kodak Company for donating the fluidized bed pilot plant to UW.
REFERENCES 1. CAMMAROTA,A., CHIRONE, R., D'AMORE, M. AND MASSIMILLA, L.: Eighth International Conference on Fluidized Bed Combustion, p. 43, US D O E / M E T C 85/6021, 1985. 2. CHIRONE, R., CAMMABOTA,A., D'AMORE, M. AND MASSIMILLA, L.: Seventh International Conference on Fluidized Bed Combustion, p. 1023, US D O E / M E T C 83-48, 1983. 3. SUNBACK,C., BEER, J., AND SAROFIM, A.: Twentieth Symposium (International) on Combustion, p. 1495, The Combustion Institute, 1985. 4. KERSTEIN, A. R. AND NIKSA, S.: Twentieth Symposium (International) on Combustion, p. 941, The Combustion Institute, 1985. 5. WALSH, P., DUTrA, A., Cox, R., SAROFIM, A., AND BEER, J.: Fragmentation of a Bituminous Coal Char During Bubbling Atmospheric Pressure Fluidized Bed Combustion: Effects of Bed Carbon Load and Carbon Conversion, paper presented at Ninth International Conference on Fluidized Bed Combustion, Boston MA, 1987. 6. RAGLAND, K., JEHN, T. AND YANG, J.: Eighteenth Symposium (International) on Combustion, p. 1295, The Combustion Institute, 1981. 7. ARENA, U., D'AMORE, M., AND MASSIMILLA, L.:
AICHE J, 29, 40 (1983). 8. D'AMORE, M., DONSI, G., AND MASSIMILLA, L.: Sixth International Conference on Fluidized Bed Combustion, p. 675, CONF-800428, 1980. 9. JUNG, K. AND LANAUZE, R. D.: Fluidization, O. Kunii and R. Toei (eds.), 427, The Engineering Foundation, 1984. 10. FULLER, E. N., SCHETrLER, P. D., AND GIDD1NGS, J. C.: Ind. Eng. Chem. 58, 19 (1966). 11. WEN, C. Y. AND YU, Y. H., AIChE, 12, 610
(1966). 12. CHIRONE, R., D'AMORE, M. D., MASSIMILLA, L., AND MAZZA, A., AIChE J., 31, 812 (1985).
COMMENTS H. A. Becket, Queen's Univ., Canada. Are you down-rating the importance of percolative fragmentation? If so, your conclusions seem to be completely at variance with Walsh, et al. (paper no. 43). Author's Reply. Using the values quoted in the paper attrition and char oxidation contribute approximately equally to char size reduction and they underpredict the burnout time if fragmentation is neglected.
J. F. Stugington, Univ. of NS. Wales, Australia. From your modelled results, what were the relative importances of attrition and char oxidation to particle size reduction. 2
Author's Reply. Fragmentation of coal particles in a fluidized bed is due to the progressive weakening of fracture planes which are formed during devolatil-
COAL FRAGMENTATION ization. This causes the char size distribution to shift to sm'filer sizes. Fines less than 0.5 m m are knocked off of t h e char surface by t h e b e d particles. C o m bustion e n h a n c e s t h e attrition clue to i n c r e a s e d porosity of t h e char surface. F r a g m e n t a t i o n i n c r e a s e s
265
t h e surface area e x p o s e d to attrition. T h e fines blow o u t of t h e b e d and b u r n in t h e free board, This is e o m p a t a b l e with the findings of the p r e v i o u s pre sentation by \Valsh. Pereolative f r a g m e n t a t i o n without attrition is relatively insignificant.
COAL
FRAGMENTATION
IN A FLUIDIZED
BED
COMBUSTOR
KENNETH W. RAGLAND AND FERDINAND A. PECSON Department of Mechanical Engineering University of Wisconsin Madison, WI 53706
The fragmentation behavior of a Colorado bituminous, a Montana sub-bituininous and a Texas lignite coal during combustion in a 0.3 m by 2.1 m atmospheric fluidized bed was investigated. A sampling probe was developed to remove bed samples at different times, enabling the temporal determination of the number, size distribution, density, and surface appearance of the char particles. Fragmentation depends on coal type, weakly on bed particle size and not on fluidization velocity. Microphotographs of the char particles show distinctive surface fissures with a cell size of 0.2 mm for lignite and an order of magnitude greater tot bituminous and sub-bituminons coal. Most of the fissure lines do not lead to fi'actures, but some secondary fragmentation does occur. Particle size distrihutions curves are presented. The coal feed had a narrow size distribution with a surface mean diameter of 5 ram, and the overall char size distribution was broader with the surface mean diameter reduced by a factor of two due to fragmentation. Calculations were made to separate the effects of size reduction due to burning and attrition from fragmentation.
0.3-0.4 mm silica sand 10 cm deep at 1123 K with a velocity of 0.8 m/s. The number of particles increased during devolatilization by factors of 1.5 to 8 depending on coal feed size. As reaction progressed the number of particles decreased as a result of combustion and elutriation. Cammarota et al.1 measured fragmentation of all anthracite, a marine coal, a low swelling bituminous coal and metallurgical coke. The anthracite produced a 160 fold increase in particles during devolatilization, however 25% of the original number of particles did not fragment. Metallurgical coke exhibited no fragmentation. Marine coal gave a 5 fold increase in the number of particles during devolatilization and no fragmentation was observed during charification. The low swelling bituminous coal had negligible fragmentation during devolatilization, but gave a 1.5 fold increase in number of particles 3 min alter injection into the bed. The effect of fragmentation on burning time of a swelling bituminous coal was studied by Sunback et al. 3 Comparison of a shrinking sphere char combustion model that predicts CO2 concentration in the outlet gas and particle size as a function of time with the measured time history, of CO2 enabled determination of the number and size of particles produced during each fragmentation. Kerstein and Niksa 4 investigated fragmentation due to internal burning using percolation theory. This is a mechanism to produce carbon fines without thermal or mechanical stress-induced breakage. From this theory Walsh et al. 5 developed and tested a model for
Introduction Understanding coal particle size change during fluidized bed combustion is important in making a realistic assessment of combustion rate and combustion efficiency. Experiments were carried out to quantify the fragmentation burning coal particles. Coal particles undergo size reduction as a result of attrition, fragmentation and burning. Attrition refers to the loss of mass from particle surfaces as a result of collisions with bed particles. Cammarota et al.1 concluded that attrition is enhanced by combustion. Fragmentation, which refers to the breakage of a coal particle into several pieces, occurs during devolatilization and char burn. Chirone et al2 explained fragmentation as due to stresses arising from the build-up of pressure inside the pores during escape of volatiles, mechanical stresses caused by collisions, and oxygen penetration into the pores which result in combustion of bridges connecting different sections of the particle. Experiments on coal fragmentation have been performed using the removable basket, which allows bed material but not larger coal particles to pass through. Experiments were performed batchwise usually starting with a single particle, and the basket was removed at different times to examine the particles. Chirone et al. z determined the effect of initial coal particle size and oxygen concentration on the number of coal particles in the bed as a function of time. A South African bituminous coal was tested in a 0.04 m i.d. quartz combustor using 259
260
COAL COMBUSTION: FLUIDIZED BED COMBUSTION
carbon conversion in an atmospheric fluidized bed. In summary, fragmentation of a range of coals has been studied in small fluidized beds (0.04 m diam.) with small bed material (0.4 mm) using a few coal particles in a basket, and by interpreting the CO2 emissions from combustion of single particles. The disadvantage of the basket technique is that in order to capture the coal fragments a small mesh size and small bed material must be used, and only a few particles can be studied at a time. The CO2 method 3 requires a number of assumptions and results do not agree with the basket method. The purpose of this study is to investigate fragmentation in a larger fluidized bed as a function of coal rank, bed particle size and fluidization velocity, and to obtain particle size distributions using a wall sampling probe technique.
Experiments The experiments were performed using a 0.3 m i.d. by 2.1 m high atmospheric fluidized bed combustor with steel walls. Nine bubble caps, one at the center and eight distributed uniformly along the perimeter of the bed, serve as the gas distributor. The bed is heated by a propane-air gas burner in the inlet air plenum. A second gas burner heats a double wall surrounding the bed. Coal is fed from the top through two ball valves in series. A solids sampling probe with a 14 mm diameter rounded inlet and a double wall for cooling and suction was used. Suction is created at the tip when air pressure is applied to the probe. The probe tip was located 0.127 m from the reactor wall and 0.076 m above the bottom of the bed. The estimated residence time in the probe was 0.2 s. Cold flow tests showed that the probe itself did not cause coal particle fracture and that a representative sample of coal particles was obtained. Three different coals were used: Texas lignite A (PSOC 1442), Montana sub-bituminous B coal (PSOC 230) and Colorado high volatile bituminous C coal (PSOC 543). The coal was screened between number 4 and 5 mesh (3.9 by 6.9 mm), and bed materials were 0.4 mm silica sand or 0.8 mm aluminum oxide. The initial bed weight was 31 to 34 kg and the static bed depth was 30 cm. Coal was fed into the combustor when the bed temperature reached 1073 K. Using the sampling probe, bed material was collected at regular intervals in cans with "cushion sand" in the bottom. The samples were immediately quenched by immersing the cans in a water bath and pouring ambient sand on top of the samples. Three sets of experiments were performed. The first set investigated the differences in fragmentation behavior of the three coals using silica sand and a fluidizing velocity of 1.8 m/s. The second set
investigated the effect of lower fluidizing velocity while burning Texas lignite in silica sand at a gas velocity of 0.9 m/s. For the third set, Texas lignite and Montana sub-bituminous coal were burned in a bed of aluminum oxide sand at 1.8 m/s to investigate the effect of changing to a coarser bed material. The collected samples were gently screened to separate the burned coal particles from the bed material. The weight of bed material in the sample, the number and weight of the burned coal particles in the sample, and the size distribution of collected coal particles were measured. The particle size was defined as the width of the smallest parallelopiped that would inscribe the particle, when the particle is lying on its flattest side. The size was measured using a graticule equipped microscope. Few char particles below 0.5 mm were observed because particles less than this size tended to be elutriated from the bed. The size distributions were determined by counting and weighing the particles that fell within seven size ranges from 0.5-6.9 mm. The bed particle population was obtained from the sampling probe data using sample weight and the weight of the bed prior to sampling. Uniform mixing in the bed was assumed. For a particular test run approximately 3800 coal particles (0.2 Kg) in the optically measured size range of 4 to 6 mm were added to the 1073 K bed. For this size of particles and bed temperature the devolatilization time was roughly 30 s. 6 The first sample was taken at 90 s to allow for mixing of the coal particles in the bed as determined by cold flow tests. The particles collected were therefore char particles. Each sampling lasted 15 s and a uniform interval of 45 s was maintained between samplings.
Test Results Figure 1 shows that the number of bituminous particles increased due to fracturing by a factor of 2.1, the sub-bituminous particles by a factor of 3.2, and the lignite particles by a factor of 2.7. In general, the coal particle population increased during the first third of the burn and then decreased. Figure 2 shows the effect of reducing fluidizing velocity and increasing bed size on the population of Texas lignite particles. It was expected that the reduced velocity would induce less mechanical stresses on the coal particles and thus less fragmentation. However, Fig. 2 shows that there was no significant reduction in fragmentation when the fluidizing velocity was reduced from 1.8 m/s to 0.9 m/s, although the apparent burn time increased. This indicates that kinetic forces of the bed are not a dominant factor for fragmentation. Increasing the bed size significantly reduced the residence time of the Texas lignite particles (Fig.
COAL FRAGMENTATION
261
400-
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FIG. ]. N u m b e r of coal particles (greater than 0.5 m e ) vs. burn time. Fluidizing velocity 1.8 m/s, oxygen 13.4%; * lignite, 0 sub-bituminous, + bitu-
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minous coal.
FIG. 3. Effe('t of increased bed size on number of sub-bituminous particles vs. bum time; * silica sand (0.4 ram); 0 aluminum oxide sand (0.8 ram); fluidization velocity 1.8 m/s and oxygen 10.1%.
2) and the Montana sub-bituminous particles (Fig. 3). The measured normalized rate of mass loss per unit external surface area during the first 90 s of burning for Texas lignite increased from 0.484 to 0.682 mg/cm2"s and for Montana sub-bituminous from 0.533 to 0.711 mg/cm2-s when the bed was changed from 0.4 mm silica sand to 0.8 mm aluminum oxide, probably due to increased attrition rates. This is supported by the attrition rates obtained by Arena et al. 7 with a South African bituminous coal. Their results at a bed temperature of 1123 K, coal feed size between 1 to 3 mm and fluidizing velocity of 1.6 m/s, showed that the rate of attrition was increased by a factor of 6 when changing from 0.2-0.4 mm sand to 1-1.4 mm. In our experiments the elutriated particles burned in the freeboard before they could be collected in the cyclone. The peak number of sub-bituminous particles was almost the same as for the 0.4 mm sand,
while the peak number of lignite particles was reduced. Microphotographs of the char particles showed extensive surface fissures especially tor the lignite samples (Fig. 4). After 90 s the fissure cell size was 0.1 to 0.2 mm. Some fissure lines were deeper than others, and after 270 s and 390 s in the bed the finer-scale fissure lines have disappeared while others have widened. Most of the fissure lines apparently do not lead to fractures, but they weaken the structure of the char particles aud contribute to fragmentation during char burn. Char particles taken from the coarser bed indicate a more advanced stage of combustion after 90 s indicating higher attrition rate. The fissure cell size is larger with the 0.8 mm bed material, probably due to the reduced heat transfer rate with the coarser bed material. The bituminous chars exhibited a fissure cell size an order of magnitude larger than the lignite.
o
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FIG. 2. Effect of changing bed material and flu-
idizing velocity on number of lignite coal particles vs. burn time. 0 silica sand (0.4 mm) fluidized at 1.8 m/s; * silica sand (0.4 mm) fluidized at 0.9 m/s; + aluminum oxide sand (0.8 ram) fluidized at 1.8 m/s; oxygen 10.1%.
F]c. 4. Microphotograph of lignite particle removed 'after 90 s in silica sand bed. The white mark is 100 i~m.
262
COAL COMBUSTION: FLUIDIZED BED COMBUSTION
The size distribution of char particles was investigated for the three coals. In the absence of fragmentation it is expected that the size distribution curve would gradually shift to the left as the particles shrink by burning and attrition, and the total number of particles would remain the same as long as the particles remain above the elutriable size. Figure 5 shows that the size distribution abruptly moved to the left at first and then showed a more gradual shift. The size distribution of char particles under steady state operation was estimated from our batch experiments by averaging the size distributions from each of the different sampling times. The quantity A n / N / A d is plotted against d/do, in Fig. 6, where An is the number of particles in the size interval (d, d + Ad), N is the total number of particles and do is the initial feed size. Although the same nominal coal feed size (5 mm) was used, the size distribution curves differed for the three coals due to differences in burning and fragmentation characteristics. The most reactive coal of the three investigated, Texas lignite, tended to have a large fraction of particles in the smallest size range, and thus a larger total surface area. The surface mean diameter (~, which is inversely proportional to the mean surface area per unit volume, was estimated for Fig. 6 to be 3.0, 2.5 and 2.3 mm for the bituminous, sub-bituminous and lignite, respectively. With the coarser bed (0.8 mm alumina, instead of 0.4 mm silica) the surface mean diameter was 2.0 mm for the sub-bituminous and 2.2 for lignite. The feed coal has a narrow size distribution with a d of 5.0 mm and fragmentation reduces the overall char d by a factor of 1.6 to 2.5. Analysis of Results
In considering the change in size distribution with time a question arises as to the extent of fragmen-
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tation versus combustion and attrition. In the absence of fragmentation the shrinkage rate of a particle of size d is given by,
(dd/dt) = (dd/dt)c + (dd/dt)a
(1)
where c refers to the combustion component of the shrinkage rate and a is the attrition component. Considering both kinetic and diffusional resistances, the combustion component of the shrinkage rate may be expressed as: 6
(dd/dt)c = -(24Co/O~)(kcShDg/(dk~ + ShDg))
(2)
where Pc is the char density, kc is the reaction rate constant, Sh is the Sherwood number, D¢ is the molecular gas diffusivity, and Co is the concentration of oxygen in the particulate phase of the fluidized bed. The density of the char is assumed constant throughout the burn time. Arena et al. 7 measured the rate of generation of attrited fines under different fluidizing and bed conditions and used the following correlation
Umf)W/~l
(dd/dt)a = - k a ( U - U m f ) o
0.0
1.0
(3)
where Ec is proportional to the excess of the superficial gas velocity above the minimum for fluidization and to the total carbon loading W, and inversely proportional to the mean surface diameter of the char particles d. They obtained a value of k equal to 1.86 × 10 7. From Eq. 3 one can obtain the expression for (dd/dt)a by equating Ec to the rate of carbon mass loss (d(pcd3/6)dt),
-o
o
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FIG. 6. Steady-state char size distribution curves obtained from averaging the size data.
Ec = k(U 120 -
0.6
d/do
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5.0
Char particle size. d. mm
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FIG. 5. Sub-bituminous particle size distribution: a, time = 0; b, t = 90 s; c, t = 270 s; d, t = 450 s. Silica sand fluidized at 1.8 m/s, 14.4% oxygen.
(4)
in which k a has been substituted for k/3 to give the form of the correlation proposed by D'Amore et al. s Substituting Eq. 2 and 4 into Eq. 1 and integrating from time t~ to t and from di to d yields,
COAL FRAGMENTATION
kc t-
ti = B ( d i - d)
\A + B dJ
(5)
where A = ( 2 4 / p ~ ) k ~ S h D : o + k~(U - Umf)ShD~; B = (kak~)(U - Umf); d is the particle size at t, d| is the initial particle size at t i. Equation 5 was used to compare the measured char size distribution with the size distribution that would be obtained if the particles reduced in size by burning and attrition without fragmentation. The predicted and actual size distributions are shown in Fig. 7a and b for t = 150 s and 210 s respectively for Montana sub-bituminous chars burned in silica sand. In Eq. 5 the Sherwood number of the particles was assumed to be constant for the interval t~ to t and given by: ~ Sh = 2e + 0.69(Re/•)°5Sc °'3~
(6)
where • is the bed voidage which in this experi600o500-
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FIG. 7. Size distribution shift predicted without fragmentation (dashed) and measured (solid) for subbituminous char in silica sand: a) 90 s to 150 s; b) 150 s to 210 s.
263
ment is equal to 0.4. The Reynolds number reduced to 14.4 di and the Schmidt number was 0.826 for our conditions at 1.8 m/s. The binary diffnsivity of oxygen into nitrogen, D¢, was calculated using Fuller's 1° correlation as 154 mm'Z/s. The reaction rate constant kc was interpolated from Table III of Ref. 5 as 0.72 m/s. The attrition rate constant k, was obtained from Fig. 16 of Arena et al. 7 as 2 × 10 -7. This gives a value of ka equal to 0.67 × 10 7. The char densities of particles at different residence times were measured and except for Colorado bituminous, the densities did not show a significant decrease with burn time. From t = 90 s to t = 450 s, the density of Colorado bituminous decreased from 0.83 m g / m m 3 to 0.76 m g / m m 3, and the density of Montana sub-bituminous decreased from 0.94 to 0.90 m g / m m 3. The density of lignite char decreased from 0.73 m g / m m 3 at t = 90 s to 0.7 m g / m m 3 at t = 270 s. For Montana sub-bituminous chars an average char density of 0.92 m g / mm 3 was used. The value of Umf was computed using Wen and Yu's 11 correlation and is equal to 0.043 m/s. After all the above values are substituted, examination of Eq. 5 shows that for a given time interval, the unknowns left in the equation are the predicted particle diameters and the oxygen concentration in the dense phase. The number of particles, n i of size dl are known, and in the absence of fragmentation the number, n, of particles of size d is also known. From the number and size one can obtain the coresponding total mass of particles. The size, number and mass of particles for each sampling time were measured, and the predictions were made for each sampling time. Thus to predict the size distribution of particles at time t, Eq. 5 was applied to each di using a value of Co such that the final values of d's and the known number of particles yielded the same mass of particles as was actually measured for time t. The values of co for the different time intervals were obtained bv trial and error. The measured inlet oxygen concentration was 1.5 mol/mm 3 and dropped to 0.022 tool/ mm 3 at t = 90 s before gradually recovering. The actual and predicted size distribution curves have been plotted such that the area under each curve represents the mass of particles in the bed, and the area under the predicted size distribution curve is the same as the area of the actual size distribution curve. Areas outside the overlapping areas then represent the mass of fragments. Figure 7a indicates that during the time interval 90 to 150 s, fragments originated from particles of size between 3.5 and 3.9 mm and also from 0.75 to 2.75 ram. Figure 7b shows that fragmentation also occurred during the time interval 150 to 210 s, despite what is indicated by Fig. 3 which showed a 3% decrease in the number of particles between 0.5 and 5 into. During this time interval fragments of 2.7 to 3.1
264
COAL COMBUSTION: F L U I D I Z E D BED COMBUSTION
mm and 1.1 to 1.6 mm were produced. Similar observations were obtained at other burn times indicating that fragmentation continues during char burn, as has been observed by Massimilla and coworkers lz from basket experiments, however it is not as extensive as indicated by the analysis of Sunback. 3
Conclusions Coal particles fed into a 0.3 m diam. fluidized bed combustor experienced a doubling or tripling of the number of particles due to fracture during devolatilization and early char burn. The fragmentation process depends significantly on coal type, weakly on bed size, and does not depend on fluidization velocity. Microphotographs of the char particles show extensive surface fissures. Lignite char had a 0.1 to 0.2 mm fissure cell size, while the subbituminous and bituminous coal had an order of magnitude greater fissure cell size. The coarser bed material results in larger fissure cell size. Most fissures do not lead to fractures, but they indicate a potential for fragmentation. The particle size distribution shifted from a coal feed surface mean diameter (d) of 5 m m to an overall char d of 3 mm to 2 mm due to fragmentation depending on coal type and bed size. A model used to separate size reduction effects due to burning and attrition from fragmentation showed that fragmentation continued during char burn.
Acknowledgments This work was enabled by a cooperative agreement between the Ministry of Energy of the Philippines, USAID and the University of Wisconsin. We are grateful to the Kodak Company for donating the fluidized bed pilot plant to UW.
REFERENCES 1. CAMMAROTA,A., CHIRONE, R., D'AMORE, M. AND MASSIMILLA, L.: Eighth International Conference on Fluidized Bed Combustion, p. 43, US D O E / M E T C 85/6021, 1985. 2. CHIRONE, R., CAMMABOTA,A., D'AMORE, M. AND MASSIMILLA, L.: Seventh International Conference on Fluidized Bed Combustion, p. 1023, US D O E / M E T C 83-48, 1983. 3. SUNBACK,C., BEER, J., AND SAROFIM, A.: Twentieth Symposium (International) on Combustion, p. 1495, The Combustion Institute, 1985. 4. KERSTEIN, A. R. AND NIKSA, S.: Twentieth Symposium (International) on Combustion, p. 941, The Combustion Institute, 1985. 5. WALSH, P., DUTrA, A., Cox, R., SAROFIM, A., AND BEER, J.: Fragmentation of a Bituminous Coal Char During Bubbling Atmospheric Pressure Fluidized Bed Combustion: Effects of Bed Carbon Load and Carbon Conversion, paper presented at Ninth International Conference on Fluidized Bed Combustion, Boston MA, 1987. 6. RAGLAND, K., JEHN, T. AND YANG, J.: Eighteenth Symposium (International) on Combustion, p. 1295, The Combustion Institute, 1981. 7. ARENA, U., D'AMORE, M., AND MASSIMILLA, L.:
AICHE J, 29, 40 (1983). 8. D'AMORE, M., DONSI, G., AND MASSIMILLA, L.: Sixth International Conference on Fluidized Bed Combustion, p. 675, CONF-800428, 1980. 9. JUNG, K. AND LANAUZE, R. D.: Fluidization, O. Kunii and R. Toei (eds.), 427, The Engineering Foundation, 1984. 10. FULLER, E. N., SCHETrLER, P. D., AND GIDD1NGS, J. C.: Ind. Eng. Chem. 58, 19 (1966). 11. WEN, C. Y. AND YU, Y. H., AIChE, 12, 610
(1966). 12. CHIRONE, R., D'AMORE, M. D., MASSIMILLA, L., AND MAZZA, A., AIChE J., 31, 812 (1985).
COMMENTS H. A. Becket, Queen's Univ., Canada. Are you down-rating the importance of percolative fragmentation? If so, your conclusions seem to be completely at variance with Walsh, et al. (paper no. 43). Author's Reply. Using the values quoted in the paper attrition and char oxidation contribute approximately equally to char size reduction and they underpredict the burnout time if fragmentation is neglected.
J. F. Stugington, Univ. of NS. Wales, Australia. From your modelled results, what were the relative importances of attrition and char oxidation to particle size reduction. 2
Author's Reply. Fragmentation of coal particles in a fluidized bed is due to the progressive weakening of fracture planes which are formed during devolatil-
COAL FRAGMENTATION ization. This causes the char size distribution to shift to sm'filer sizes. Fines less than 0.5 m m are knocked off of t h e char surface by t h e b e d particles. C o m bustion e n h a n c e s t h e attrition clue to i n c r e a s e d porosity of t h e char surface. F r a g m e n t a t i o n i n c r e a s e s
265
t h e surface area e x p o s e d to attrition. T h e fines blow o u t of t h e b e d and b u r n in t h e free board, This is e o m p a t a b l e with the findings of the p r e v i o u s pre sentation by \Valsh. Pereolative f r a g m e n t a t i o n without attrition is relatively insignificant.