Attrition of coal ash particles in a fluidized bed

Attrition of coal ash particles in a fluidized bed

Powder Technology, 66 (1991) 4146 Attrition 41 of coal ash particles in a fluidized bed* J. J. Pis, A. B. Fuertes, V. Artos, A. Suarez and F. Rub...

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Powder Technology, 66 (1991) 4146

Attrition

41

of coal ash particles

in a fluidized bed*

J. J. Pis, A. B. Fuertes, V. Artos, A. Suarez and F. Rubiera Institute National

(Received

de1 Carbbn,

November

C.S.I.C.,

Apartado

73, 33080

27, 1989; in revised form September

Oviedo (Spain)

25, 1990)

Abstract In this work, an attrition study has been carried out using a fluidized bed of coal ash. Experiments were carried out in a cold fluidized bed 0.14 m in diameter and 2.20 m in height. The effect on attrition of the following variables has been studied: bed particle size, static height of bed, time of attrition and fluidizing velocity. It was concluded that attrition is mainly due to abrasion, resulting in the removal of fines of a size smaller than 50 pm. The size distribution of fines produced is independent of the original particle sizes. The attrition rate varies linearly with the parameter (U- U,). It was observed that the variation of the static bed height has no effect on the attrition rate under steady-state conditions.

Introduction Recent developments in fluidized bed combustion technology have stimulated research in areas of fluidization which had previously received little attention. The subject of solids attrition in fluidized beds has already been treated in relation to, catalytic cracking [l-3]. However, it is now a topic which is arousing increasing interest owing to the important effect it has on various aspects of coal combustion in fluidized beds. The production of fines from solids in the bed will lead to a loss both of combustible material from char attrition [4, 51 and of the limestone fed for SO2 capture, causing a decrease in combustion efficiency and sulfur retention. Heat transmission in the bed will also be affected due to variation in particle size. The design of systems for collecting particles as they leave the reactor will be determined by this phenomenon. Furthermore, the production of very fine particles may have an adverse effect on devices situated at the outlet of the reactor, as in the case of gas turbines in pressurized fluidized beds. There are a number of factors which cause the breakage of solids in fluidized bed systems: bubbling bed attrition, grid jet attrition, thermal shock, calcination, transfer lines and cyclones, internal gas pressure and ejection of particles from bed surface. *Paper presented at the 1989 International Conference on Fluidized Bed Combustion, San Francisco, April 30-May 3, 1989. Published with the permission of The American Society of Mechanical Engineers.

0032-5910191/$3.50

This work is concerned mainly with bubbling bed attrition. A large proportion of the fines produced in the fluidized bed combustion of coal arise from attrition. The main objective of the present work is to study the attrition of coal ash in a fluidized he’d, in order to contribute to the understanding of this phenomenon. The experimental values for the extent of attrition are manipulated to obtain equations which give an adequate description of attrition rate. The influence of static bed depth and fluidization velocity on attrition rate under steady-state conditions is also considered.

Attrition mechanism

in fluidized

beds

Blinichev et al. [6] have proposed a very simple model to explain the phenomenon of attrition in a fluidized bed. Due to collisions, the particles may break up, giving rise to new particles. In general, two types of mechanism may be observed: (i) Abrasion: Particles of a much smaller size break away from the original particle. The resulting bed particles are slightly smaller than the original one. (ii) Fragmentation: The breaking-away process gives rise to a number of particles of a smaller size than the original one. Figure 1 shows schematically both mechanisms. A number of authors [6-111 have found that, in fluidized beds, attrition takes place by an abrasivetype mechanism. Consequently, between the son

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in The Netherlands

42

ABRASION

INITIAL DISTRIBUTION

SON PARTICLE

DISTRIBUTION AFTER ATTRITION

FINES

c l-

MOTHER PARTICLE

/

,

,/

:

\. FRAGMENTATION

INITIAL

PARTICLE

Fig. 1. Attrition

particles and the fines produced, there will be no intermediate-sized particles. The mass of fines produced will therefore provide a means of measurement of the degree of attrition. The attrition rate R is defined by

where A4 represents the mass of coarse particles. Bearing in mind the above expression, the extent of attrition will be defined as A= 100

s

SIZE

mechanisms.

where a and b are coefficients whose value depends on the characteristics of the material as well as on the operation conditions, and R, is the attrition rate in the steady-state condition. It follows that, in the case of fluidized bed combustors, the attrition rates of solids (ash, sand, limestone) will depend on the solids residence time. Given that this depends on factors such as ash content, sulfur content (Ca/S ratio), bed depth, excess air, fluidization velocity, etc., the rate of attrition can only be estimated if all these factors are taken into account.

R dt

Experimental

0

MO

=lOO s( M =lOOln

LiM M 1 2

(

)

(2)

where MO represents the initial mass of coarse particles. The attrition process shows non-steady-state behaviour in the initial stages. According to some authors [12], the transient period may be as long as 80 h. Vaux and Fellers [13] have proposed the following empirical equation to express the variation of the attrition rate with time: R=R,(l+ae-“)

(3)

The experimental set-up used consists essentially of a transparent Plexiglas column 0.14 m in diameter and 2.2 m in height. The distributor was made of transparent Plexiglas with orifices 1 mm in diameter, 5 mm apart and arranged in squares. The fines elutriated were collected using a cyclone and ,a packing filter. Figure 2 contains a diagram of the experimental equipment. The ash used comes from the combustion of coal washery rejects (-70% ash) in a fluidized bed [14]. Combustion was carried out at 1 123 K, with a fluidizing velocity of 1 m/s and air in excess of 30%. Chemical analysis of the ash used is as follows: SiOl = 54.8%, A1203 = 28.9%, Fez03 = 5.9%, K@ = CaO=2.5%, MgO=lS%, NazO-0.4%, 3.7%, others=2.3%. Three size ranges were used in the

43

distinguishing between the fines produced by abrasion and the son particles. The extent of attrition A was determined for each experiment according to eqn. (2). The attrition rate R was calculated from the values of A by numerical differentiation. The particle size distributions of the fines produced by attrition were determined using a photosedimentograph LUMOSED (RETSCH).

Results

Fig. 2. Diagram of the experimental set-up. 0, Air supply; 0, flowmeter; 0, valve; @, distributor; 0, fluidized bed; 8, cyclone; 0, filter.

PARTICLE

Fig. 3. Cumulative material used.

DIAMETER

particle size distribution

curves of the

experiments carried out: 0.200-0.315 mm, 0.315-0.500 mm and 0.500-l mm. Figure 3 shows the cumulative particle size distribution curves of the three fractions used. In each experiment, the bed was charged with approximately 1 kg of material and fluidized for predetermined periods of time. At the end of the experiment, the bed material was screened in order to eliminate the particles which had a size smaller than the initial lower size limit. In the particles collected in the cyclone, the fraction under 100 pm was separated by using wet sieving. Finally, the total elutriated material was determined by means of a material balance. 100 pm was taken as the limit for

and discussion

The following experiment was carried out in order to test the hypothesis that the attrition process proceeds by an abrasion mechanism. Independent analyses of size were performed for the particles from the bed and from the cyclone produced over an attrition period between 8 and 12 h. In the case of the bed, only those particles with a size smaller than the initial lower limit size were analyzed. The experiment was repeated with each one of the three studied fractions. In the three cases, the experiment was initiated from a similar static bed depth and the same value for the parameter (V- VJ was used, 0.31 m/s. Figure 4 shows the cumulative particle size distributions that correspond to the three studied fractions. Two important conclusions can be deduced from this figure. Firstly, it can be observed that there are no particles in the Xl-120 pm range, supporting the abrasive mechanism hypothesis and justifying the use of the 100 pm limit to distinguish between fines produced by abrasion and the son particles. Secondly, in the light of the particle size distribution for the fines produced by abrasion (cyclone), it can be seen that this is independent of the size of the mother particles, the result being identical for the three fractions. This latter finding is in agreement with the observations of Ray et aE. [lo], who point out that “the abrasion mechanism tends to produce fines of size characterizing the structure of the material”. The attrition process was studied over long periods of time (up to 48 h). Special emphasis was laid on its rate during the initial phase (non-steady-state period). Figure 5 shows the extent of attrition for different periods of time. Two sets of curves have been shown: I and II. In curves of type I, the extent of attrition (Ar) has been obtained from the fines produced by abrasion (< 100 pm). Au-curves were obtained following the method proposed by Vaux and Fellers [13] taking into account all particles with a size below the initial lower limit (fines plus son particles). In spite of the fact that this work concerns abrasion, and therefore the AI-curve is the only one representing this phenomenon, the AII-curve is also shown in Fig. 5, in order to compare both proposals.

44 CYCLONE

BED

i

P

PARTICLE

DIAMETER,

IN MICRONS

Fig. 4. Cumulative particle size distributions of fines collected in the cyclone and of fines produced in the bed for different size ranges. 0, 0.2bO-0.315; A, 0.315-0.500; 0, 0,500-l mm.

The Table contains the values of the parameters and the standard deviation. The standard deviation shows that eqn. (5) offers a better fit than the equation suggested by Vaux and Fellers [13]. Bearing in mind that

I

I

two expressions for the attrition rate as a function of time are obtained from eqns. (4) and (5). Thus, from eqn. (4), we get R=R1,(l

+KIedC1’)

where R1, = lo-‘bi From eqn. (5) R-,2-[

Fig. 5. Variation of the extent of attrition 21s.the fluidization time.

In agreement with what has been said above, At will represent the real extent of attrition. In the case ofAt, an initial rapid growth can be observed, reaching values of between 3 and 4% for times of 24 h. The values of the extent of attrition have been fitted to a function of time (in minutes) according to the equation suggested by Vaux and Fellers [13]: A=al+blt-ale-cL’ Also the following been used:

equation,

proposed

112 1 +C$

and KI =aIcIlb~.

(8)

$51

where RZm = 10e2b2 and K2 =a2czlb2. With a view to modelling, it is important to have equations which describe attrition rate adequately+ as a function of time. In Fig. 6, the experimental TABLE.

Fitting parameters

Size range (mm>

Equation

Standard deviation

Parameters a

bX103

cx10*

(4)

0.200-0.315

;:;

2.37 2.71

1.26 1.02

3.03 3.76

0.180 0.114

here, has

0.315-0.500

$;

1.76 2.01

1.07 0.90

3.05 3.74

0.120 0.069

1.66 1.94

1.78 1.59

2.61 3.06

0.154 0.107

0.500-l A=az+b2t-

1+

(7)

(5)

45 and Highley [7] and Ray et al. [lo]. We therefore consider the equation suggested by Merrick and Highley [7] to be valid. According to their model, the attrition rate in the steady state is proportional to the excess gas velocity over the minimum fluidization value according to the equation R, =K(U-

200

400

600

TIME,

Fig.

800

4000

min

6. Variation of the attrition rate vs. time.

.

.

6

/



1

8

STATIC

10

BED

DEPTH

.

I I2

” 14

,cm

Fig. 7. Variation of attrition rate DS.the static bed depth. values of attrition rate (obtained by numerical differentiation from the values of A) are compared with those obtained from eqn. (8). Firstly, it can be seen that the variation in attrition rate is independent of the range of studied sizes. On the other hand, the curves R=f(t) obtained from eqn. (8) afford a good description of the variation in attrition rate 2~. time. There is some disagreement as to the influence which the static bed depth (Ho) has over the attrition rate in steady state (R,). Kono [15] considers that R, aH,0~78andUlerichetal. [8] propose thatR, aH,. On the other hand, Ray et al. [lo] and Merrick and Highley [7] conclude that R, is independent of Ho. In order to determine the influence of this parameter, experiments were carried out at different static bed depths. Figure 7 shows the steady-state attrition rates evaluated at t=48 h as a function of static bed depth, for the intermediate-sized material. It can be seen that the attrition rate hardly changes with bed depth. This is in agreement with Merrick

U,)

(9)

where K is the attrition constant (m-l). The value of K represents the proportion of the bed weight which is converted to fines in 1 s, with (V-U,) of 1 m/s. A study of the variation of attrition rate at steady state with the parameter (U-U,) has been carried out. In this study, particles with a size between 0.200 and 0.315 mm have been used. The attrition rate was evaluated at t = 60 h, and a value of 2.5 X 10e6 min-’ was obtained for this time. However, as can be seen in Fig. 6, a value of about 6~ 1O-6 min-’ was obtained for 18 h. This difference suggests that, in the second case, steady state conditions have not been reached. Figure 8 shows the variation of attrition rate at steady state with (V-U,). A linear relation can be observed in accordance with eqn. (9). A value of K=1.6XlO-’ m-l for the attrition constant was obtained by fitting the experimental data to eqn. (9). The results of this work agree with those of the literature [7, 131. However, it should be noted that some authors [lo, 151 have indicated that, at low values of (V- V,,), the attrition rate approaches the origin asymptotically.

0

02

06 ( U-Lb),

m/s

‘*

Fig. 8. Variation of attrition rate 2rs. (V-U,).

46

Conclusions A study of coal ash attrition in a fluidized bed has been carried out. It was found that the process takes place according to an abrasive-type mechanism. The size of the fines produced by abrasion is smaller than 50 pm and their distribution is independent of the size of the original particles. The attrition process takes place in a non-steady state and a marked decrease in the attrition rate is observed during the first 200 min. This variation was fitted by means of an empirical equation that offers a satisfactory description of the attrition rate as a function of time. The influence which the bed depth and the fluidization velocity have on the attrition rate in a steady state was also studied. It was found that: (i) the attrition rate is independent of the bed depth, (ii) the attrition rate varies linearly with the parameter (U- U,). The attrition constant calculated was 1.6~10~’ m-l.

Cl

ii0 K

Kl K2 M MCI

R

R, t u uo

References W. L. Forsyth and W. R. Hertinig, Ind. Eng. Chem.,

Acknowledgements The authors are grateful to Dr. E. J. Anthony (CANMET, ERL, Canada) for helpful suggestions and critical reading of the manuscript. The authors thank the Comisi6n Interministerial de Ciencia y Tecnologia (CICYT) (Proj. N.PB0368) and Fundaci6n para el Foment0 en Asturias de la Investigaci6n Cientifica Aplicada y la Tecnologia (FICYT) for financing this work.

9 10 11

List of symbols A a1 a2

bl bz

extent of attrition, % coefficient, eqn. (4) coefficient, eqn. (5) coefficient, eqn. (4) coefficient, eqn. (5)

coefficient, eqn. (4) coefficient, eqn. (5) static bed depth, m attrition constant, m-l coefficient, eqn. (7) coefficient, eqn. (8) mass of coarse particles in bed, kg initial mass of coarse particles in bed, kg attrition rate, min-’ attrition rate in steady state, min-’ time, min fluidization velocity, m/s minimum fluidization velocity, m/s

12 13

14

15

6 (1949) 1200. F. A. Zenz, Hydrocarbon Process., 51 (1972) 103. J. E. Gwyn, AIChE J., 15 (1969) 35. G. Donsi, L. Massimilla and M. Miccio, Cornbust. Flume, 41 (1981) 57. V. Arena, M. D’Amore and L. Massimilla, AIChE J., 29 (1983) 40. V. N. Blinichev, V. V. Streltsov and W. S. Lebedeva, Znt. J. Chem. Eng., 8 (1968) 615. D. Merrick and J. Highley, AI&E Symp. Ser., 70 (1972) 366. N. H. Ulerich, W. G. Vaux, R. A. Newby and L. Keairns, Experimental/Engineering Support for EPA’s FBCprogram: Final Repoti, Vol. 1, Sulphur oxide control, EPA-600/7-80-015a, January 1980. T. A. Kutyavina and A. P. Baskakov, Chem. Technol. Fuel Oil, 8 (1972) 210. Y. Ray, T. Jiang and C. Y. Wen, Powder Technoi., 49 (1987) 193. K. Patel, A. W. Nienow and I. P. Milne, Powder Techno/., 47 (1986) 257. W. G. Vaux, Proc. Am. Pow. Co& 40 (1978) 793. W. G. Vaux and A. W. Fellers, A Model for Particle Attrition by Abrasion in the Upper Zone of a Fluidized Bed, Research Report 80-9E3-FBCOM-R2 (1980) Aug. J. J. Pis, A. B. Fuertes, V. Artos, A. Sulrez, J. J. Jul, F. J. Alvarez and J. G. Cafiibano, Energia, 1.5 (1989) 94. H. Kono, AZChE Symp. Ser., 77 (1981) 96.