Observed evolution of petrographic and textural features of coal particles, accompanying primary fragmentation during the earlier stages of combustion

Observed evolution of petrographic and textural features of coal particles, accompanying primary fragmentation during the earlier stages of combustion

Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 1223-1230 OBSERVED EVOLUTION OF PETROGRAPHIC AND TEXTURAL FEA...

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Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 1223-1230

OBSERVED EVOLUTION OF PETROGRAPHIC AND TEXTURAL FEATURES OF COAL PARTICLES, ACCOMPANYING PRIMARY FRAGMENTATION DURING THE EARLIER STAGES OF COMBUSTION M. FKYERAT, E. HOSSEINI, L. DELFOSSE, AND A. DELEBARRE* Universit~ des Sciences et Techniques de Lille Feandres-Artois Laboratoire de Cin~tique et Chimie de la Combustion CNRS UA 876 59655 Villeneuve d'Ascq Cedex France

Five coals of different origins have been observed during the first stages of combustion and pyrolysis at 850~ at atmospheric pressure in a mixture of 5% 02-95% N2 or in pure N2. Coals investigated were two french high volatile bituminous ("Flambant de Provence" and "'Freyming"), two gondwanian medium volatile bituminous ("Rietspruit'" and "Ullan") and a french anthracite ("La Mure"). Initial particle size distribution was 0.5-1 mm. Followed parameters are, the maceral composition, the B.E.T. surface area (i.e. according to Brunauer, Emmet and Teller), swelling and primary fragmentation. It is shown that after 15 seconds with the used granulometry, it is no longer possible to recognize macerals. Nevertheless the analysis of fissuration leads to the conclusion that fissures mostly develop in vitrinite. The B.E.T. surface area very sharply increases during the first seconds of reaction especially in the case of "Flambant de Provence" and then decreases. Except for gondwanian coals the process is not affected by the presence of oxygen. The comparison of the swelling behaviour of the coals with the B.E.T. surface area development does not allow to fully attribute the initial jump in surface area to swelling. Fragmentation is very fast during the first seconds of reaction, particularly for "Flambant de Provence" and for "La Mure," but also for "'Rietspruit" though its vitrinite content is relatively low. Oxygen sharply accelerates the fragmentation of only the "'Flambant de Provence.'"

Introduction Many operating parameters and basic processes of coal combustion in fluidized bed combustors depend critically on the particle size distribution of burning particles. 1.z The rate of carbon conversion in such units depends on the rate of size reduction of coal and char particles. This in turn is affected by combustion, particle comminution and particle to particle collisions. On the other hand, combustion kinetics knowledge has to be improved especially as regard the correlation between coals characteristics and their behaviour during fluidized bed combustion. 3 Quite a large amount of work, experimental and theoretical, has been previously reported in this area. The literature shows that one of the most frequent used experimental methods is the so called basket technique. It consists in immersing an inox basket containing a certain amount of coal into the fluidized bed. The sample is extracted periodically to obtain the size distribution

*CERCHAR rue Aim6 Dubost, BP n ~ 19 62670 Mazingarbe France. 1223

of char particles as a function of time. Since the basket mesh is coarse, it is generally not possible to get particles smaller than 800 Ixm. This technique has been used by Sundback et al. 4 to study swelling and fragmentation. They were able to correlate fragmentation with sudden accelerations in the formation of carbon oxides. Usin~ that same experimental method, Chirone et al. conclude that the presence of fines should be ascribed to attrition rather than to fragmentation. Another technique is based on direct in-bed video observations. Thus, Prins et al.6 distinguish between two causes to fragmentation: thermal shock and sudden volatiles accumulation. Nevertheless, these conclusions must be modulated by considerations of the intrinsic mechanical resistance of the coal. As a matter of fact, a rather pronounced propensity to fragmentation is noted to occur with the more fragile high rank coals. Both coal or char reactivities must be studied in such systems to insure a complete description. Char reactivity allows one to account for the so called secondary fragmentation through, for example percolation theory, 7,s or by using statistical functions. 9'10'11 Similarly, Siemons 12 shows that fragmentation alone cannot account for increased

1224

HETEROGENEOUS KINETICS

temperatures measured in the bed. Changes in rectivity and not just in heat transfer coefficients must be taken into consideration. More detailed studies of the evolution of char texture are therefore needed. The aim of present work is to get preliminary data with simultaneous observations of fragmentation, swelling, petrographic composition and texture during the early stages of combustion. Parametric conditions close to those prevailing in fluidized beds are used, but the technique employed is simpler than that involved in direct bed exposure. These data will be compared for coals of different ranks and origins. Experimental Figure 1 is a simplified sketch of the apparatus used. A particle, or a limited number of particles of known granulometry (in our case typically 0.5-1 mm) are placed in a metallic basket of 40 Izm mesh and borne inside a vertical oven maintained at 850~ C under a downward flow of oxidizing gas or nitrogen. When oxidation is performed, the value of oxygen concentration is chosen to be as close as possible to that encountered in a fluidized bed combustor. In the present work, 5% oxygen in nitrogen has been used. A chromel-alumel thermocouple is positioned a few millimeters from the basket ensuring a suitable control of temperature. The

basket is maintained in position by means of an electro-magnet. This ensures a rapid quenching of the reaction, when the current is switched off and as the basket falls into the cool zone of the reaction vessel. To avoid agglomeration, a restricted number of particles is initially introduced in the basket in such a way that no contact is possible between them during reaction. Tile same experiment must therefore be repeated several times (typically 20 times) in order to get significant results. The cumulated number of initial particles supporting the same experimental conditions was of the order of 400 to 500 depending on size. After trapping, the particles were sieved and each obtained fraction was weighed to determine the histogram of sizes. After that two fractions of the whole were made, one for petrographic analysis the other for B.E.T. surface area measurements. Petrographic analysis was performed according to the recommendations of the International Committee for Coal Petrology. 13 After inclusion in epoxy resin the preparation was polished and observed through an immersion objective using a reflexion microscope (type orthoplan Leitz). The counting of macerals was done us!ng a classical reticle, but for the counting of fissures and crevices, the probability of the reticle cross falling exactly on one fissure is very low. The observation technique was therefore modified by defining a square zone in the scope around the reticle center of 1225 Izm2 in area. A fissure was counted each time it fell within the square, and the maceral in which this occured was noted accordingly. Some 500 points were counted on each preparation. The coals investigated were the followings: french "Flambant de Provence" (High volatile bituminous B), french "Freyming" (High volatile bituminous A), Australian "Ullan" and South-African "Rietspruit," both classified as medium volatile bituminous and French "La Mure" (anthracite); Table I shows the main characteristics of these coals, including proximate and ultimate analysis.

THERMOEOUPLE

Results

Petrographic Study of Coals During the Earlier Stages of Combustion:

BASKET_~~LJ F~..~SILICAWOOL OUT"'~_~TRAPPINI3ZONE

6AS

F]c. 1. Scheme of the apparatus.

Macerals do not disappear immediately on heating. Due to heat transfer limitations, 14 they cannot disappear simultaneously throughout the bulk of the particle but rather according to the progression of a devolatilization wave front. It is nevertheless possible to follow their dissapearance by trapping char particles during the first seconds of reaction and exploring cuts of epoxy resin included particles to get the petrographic composition at a given time. Under the present parametric conditions (850~ C 5%

PETROGRAPHIC CHANGES DURING COAL PYROLYSIS

1225

TABLE I Proximate maceral and ultimate analysis of coals used. Proximate analysis Maeeral analysis (% volumic dry basis)

Coal

Vitrinite reflectance

Ultimate analysis (% dry basis)

Exinite %

Vitrinite %

Inertinite %

Ro

%

C

14

70

14

0.80

77.00

1.00 5.00

1.2

7.45 2 9 . 5 3 44.6

51

12

0.46

47.62 3.65 3.63

1.5

5.97 33.05

84

11

4.95

59.29 2.30 1.27

1.2

26

61

0.82

65.1

0.63 3.82

1.9

40

44

0.69

66.2

0.65 4.34

1.9

H20 %

ASH %

4.91

5.27

MV %

S

H

N

Freyming

(Fl~)

35.89 I

Flambant de Provence (FP) La Mure (LAM) Rietspruit

(R~)

6.62 1

3.05

16.91

24.5

7.21

18.03 : 29.62

Ullan

(UL)

Oz and atmospheric pressure), except for those of the inertinite group which last a bit longer, macerals remain observable during less than 15 seconds for all observed coals. After that, one can only identify semicoke and mineral matter. In Fig. 2 is shown for example, the maceral analysis of "Flambant de Provence" at t = 0; 5 and 10 seconds of reaction. The progressive transformation of maceral composition is dearly visible and especially the rapid disappearance of vitrinite (mostly collinite and telinite) with simultaneous accumulation of semicoke. In the case of the gondwanian coals, one could notice the relative stability of semifusinite and of other macerals of the inertinite group. The same global features were also observed with the other coals tested. Careful observation of the surface of the preparations obtained with all samples allowed the detection of numerous fissures and crevices running through all the macerals but especially in collinite. As an example, Fig. 3 represents four micrographs taken in the case of "Flambant de Provence" (Fig. 3a) and "Freyming" (Fig. 3b, c, d) at various times exposure. Fissures are shown at different stages of development. In their earlier form, they appear very thin, with an increase of the reflecting power at the edge. This is consistant with a beginning of cokefaction. 15 At a more advanced stage the fissures are well defined, and the reflecting power remains stronger on the edges. In the case of "Flambant de Provence," two kinds of fissures are visible: a general system of crevices having the same global orientation and fractures developping perpendicularly to the first direction. This could be the result of two different mechanisms of rupture: for example volatiles departure in the first case, and mechanical relaxation in the second case. Nevertheless, an-

other possible interpretation is that this second kind of fracture may preexist due to geological stress. In order to get an idea of the part played by the different macerals in the rupture mechanism, fissures have been counted according to the method described above. Table II sums up the obtained results for four of the samples and at two different times of reaction. The predominent role of vitrinite appears clearly. An attempt was made to show, for all coals, the dependance of the observed percentage of fissures versus the vitrinite content of the sample at time t - At(At = 5 s being the time interval between two trapping of particles). Only a weak correlation appeared (R = 0.75). But, considering that the mechanical properties of vitrinite may depend on the rank of coal, the representation was altered slightly by using an abscissa unit that was equal to the vitrinite content divided by its reflectance. Though this method of correcting for coal rank may appear to be quite arbitrary, it is nevertheless evident that the correlation in Fig. 4 is substantially improved (R = 0.88). This group of observations allows one to conclude that vitrinite plays an important role in the fragmentation mechanism of coals.

Evolution of Fragmentation and Textural Properties of Coals During the Earlier Stages of Combustion: In Fig. 5 are simultaneously shown as a function of time, in the case of"Flambant de Provence," the mass loss, the B.E.T surface area, the mass fraction of particles whose diameter is less than 0.5 mm (fragmentation), and that of particles with a diameter greater than 1 mm (swelling). This has been

1226

ttETEROGENEOUS

KINETICS

I FLAMBANT DE PROVENCE t=0 sec

80

60

I

r ,p. M') p--

uJ

:S .J

0 40 >

o< 20

co te sp cu re mi ma sf fu sc id mm

MACERALS

80

t=5 sec

80

t=10 sec

60

60 tll

I,u

=E ,_1

0 40

r

-I

0 40'

20'

0 co te sp cu re mima sf fu sc id mmsk MACERALS

co te sp cu re mima sf f u s c i d m m s k MACERALS

FIG. 2. Evolution of macerals of "'Flambant de Provence" as a function of t i m e at 850 ~ C a n d atmospheric p r e s s u r e in a 5% 0 2 - 9 5 % NIj. mixture. Co: colinite, Te: tellinite, Sp: sporinite, Cu: cutinite, Re: resinite, Mi: micrinite, Ma: macrinite, Sf: semifusinite, Fu: fusinile, So: sclerotinite, Id: inertodetrinite, m m : mineral m a t t e r a n d Sk: semicoke.

PETROGRAPHIC CHANGES DURING COAL PYROLYSIS

(a)

(b)

(c)

(d)

1227

FIC. 3. Four aspects of developing fissures in collinite, a) "Flambant de Provence," b), c), d): "Freyming." Exposure times: a) 5s; b) 10s; c) 5s and d) 10s. TABLE II Repartition of fissures in different macerals after 5 and 10 seconds reaction at 850 ~ C and atmospheric pressure in 5% 02-95% N~ mixture for various coals.

Percentage of of fissures Percentage of vitrinite Percentage of exinite Percentage of Inertinite Percentage of fissures

t=5 see

t = 1O see

85.40

t=5

RI

UL

FP

FF

t=10

t=5

t=

10

t=5

t = 10

see

see

see

see

sec

sec

82.00

82.60

76.60

91.20

88.40

86.00

87.60

10.60

13.00

11.80

16.80

5.80

5.60

5.60

3.80

1.20

1.80

1.40

1.00

0.60

0.80

0.60

0.20

2.80

3.20

4.20

5.60

2.40

5.20

7.80

8.40

14.60

18.00

17.40

23.40

8.80

11.60

14.00

12.40

counts out fissures in fissures in fissures in counts in

done for all coals investigated under pure nitrogen, and under 5% 02, 95% N2 atmospheres. It appears clearly that devolatilization is complete in less than 30 sec. The effective amount of volatile matter obtained in nitrogen is in good agreement with the

A.S.T.M. value. W h e n experiments are performed in the presence of oxygen, the combustion of fixed carbon becomes visible after roughly 100 sec for all samples. The other observed features are discussed separately below.

HETEROGENEOUS KINETICS

1228

onds, followed by a gradual decrease as the reaction proceeds. The same observation is made for "Ullan" and "'Rietspruit." While this behaviour is not affected by the presence of oxygen in the ease of "Flambant de Provence" it is quite modified for the two gondwanian coals, for which a continuous increase in surface area is observed v,fth no marked initial peak. No valid interpretation of this can be made without any further investigation. On the other hand, one may suggest that the strong initial peak observed with "Flambant de Provence" could be due to rapid decarbonation. The anthracite "La Mure" has practically no porosity and its surface area is hardly increased using a 5% oxygen atmosphere. Figure 6 shows the evolution of the surface areas in the presence of 5% oxygen.

R = 0,88 24

FP . /

22 2O 03 1,1,,I Q:: 18

FF"

03 16 14i

Swelling: 12 J

10 10

20

30

40

50

60

% VITRINITE/RV

AT

FIG. 4. Dependence of the percentage on the ratio of vitrinite content at t flectance (At = 5 sec).

100-

Swelling is assessed by a rapid initial jump in the mass fraction of char particles whose diameter is greater than 1 mm (Fig. 7). The proportion of swollen particles remains roughly constant after twenty 70 80 seconds of reaction either in pure nitrogen or in the presence of oxygen, except in the case of "'UIt-At lan" for which a second step of swelling is observed with oxygen a~er 100 seconds. The observed initial of fissures jump in the swollen particle number at the begenAt to re-

5% 02

5% 0 2

200

9 MASS LOSS ~" "SURFACEAREA ~ =ca xMASSFRACTIONd<0,5mm 150 oJ / =. oMASSFRACTIONd>lmm Cn 50

100

:i

tU

~ * * * o , ' x - - ~ a--~:---~'::-:- ..... 0

-

200

~ 150 Cq E

D

<~ 100

~

LU

0

TIME (see) FIG. 5. Dependance of mass loss, B.E.T. surface area, fragmentation and swelling upon time in the ease of "Flamhant de Provence'" at 850~ and atmospheric pressure in 5% 02-95% Nz mixture.

9

The variations versus time, of the N2-B.E.T surface area, have been investigated under pure nitrogen or in the presence of 5% oxygen. Under nitrogen, "Flambant de Provence" shows a strong increase in surface area during the first twenty sec-

:

"~-__:___

n-u" 03

50

~:..:.; ..--:---" . . . . - - - - - _

0

B.E.T Surface Area:

F.DE PROVENCE RIETSPRUIT ULLAN LAMURE

0

50

100 150 T I M E (sec)

"200

FIe. 6. Comparison of the evolution of B.E.T. surface area for different coals as a function of time at 850~ C and atmospheric pressure in 5% 02-95% N2 mixture.

1229

PETROGRAPHIC CHANGES DURING COAL PYROLYSIS

F. DE PROVENCE RIETSPRUIT ULLAN

5% 02

/

80

F. DE PROVENCE RIETSPRUIT ULLAN LAMURE

5% 0 2 30

o~

o~ 60

20!

or) L~ 40 Z

/

10

.-I .-I

LU

m

/

J

20.

oJ

f :..

/f

N

0

0

50

100

150

200

TIME (sec)

0

0

"

"~..~..

50

100

9

150

-

200

TIME (sec)

FIG. 7. Comparison of the evolution of swelling for different coals as a function of time at 850~ C and atmospheric pressure in 5% 02-95% Nz mixture.

FIG. 8. Comparison of the evolution of primary fragmentation for different coals as a function of time at 850~ C and atmospheric pressure in 5% O2-95% N2 mixture.

niug of reaction could be related to the increase in surface area, but in that case, "Ullan" would show the highest surface area followed by "Rietspruit" and "'Flambant de Provence.'" This is obviously not the case, and swelling is therefore not the only cause of the increase in initial surface area.

that case, primary fragmentation jumps up to 22% during the first seconds, passes through a maximum, then begins to slowly increase again. It is likely that this second increase is related to the beginning of secondary, fragmentation. With other coals, things evolve slower, but with also a first maximum followed by secondary fragmentation. One has, up to now, no valid explanation for the particular role of oxygen in the primary fragmentation of "'Flambant de Provence."

Fragmentation: The variations of the mass fraction of char particles whose diameter is less than 0.5 mm have been followed either in pure nitrogen or in the presence of 5% oxygen. The results obtained for oxygen are shown in Fig. 8. With pure nitrogen, the curves would show a rapid increase during the very beginning of reaction. This is obviously due to primary fragmentation. "Flambant de Provence" and anthracite "La Mure" are more prone to primary fragmentation. As regard their vitrinite content, this observation is consistent with the above conclusions. Following this, "Ullan" would fragment more than "Rietspruit." The reverse is observed indicating that one cannot establish a theory of fragmentation on the sole basis of vitrinite content. When oxygen is present, the same observations can be made with a particular emphasis on "Flambant de Provence" behaviour (see also Fig. 5). In

Conclusion The evolutions of maceral composition, specific surface area, swelling, and primary fragmentation have been followed as a function of time for a number of coals of different origin. It has been shown that at 850~ C either in pure nitrogen or in an atmosphere containing 5% oxygen in nitrogen, macerals were rc~ognizible during a maximum of 15 seconds9 During this time interval, vitrinite and exinite rapidly disappear while semicoke builds up. Fissures and crevices have been counted. The most important finding appears to be that they predominently develop in the macerals of the vitrinite group.

1230

H E T E R O G E N E O U S KINETICS

Surface area develops rapidly, particularly in the case of "Flambant de Provence." For all coals it crosses a maximum as the reaction time is increased. The presence of oxygen only alters the behaviour of gondwanian coals leading to a continuous increase of the surface area. Swelling occurs also in the first stages of the reaction. It is not likely that it could be made wholly responsible of the observed increase of the surface area. Primary fragmentation has been observed and compared for all the coals. "Flambant de Provence" and "La Mure," relatively rich in vitrinite, are rather prone to primary fragmentation. But the different behaviour of South-African and Australian coals suggest that the vitrinite content alone is insuflL cient to get an exact idea of the coal propensity to primary fragmentation.

4.

5.

6.

7.

8.

Acknowledgments This work has been funded by the E.P.R. from Nord/Pas-de-Calais (FRANCE). The a u t h o r s also wish to thank C . N . R . S . (Groupement Scientifique Charbon) for its financial support.

9. 10. 11. 12.

REFERENCES 1. HEREBERTZH., LIENttARDH., BARNERH. E. AND HANSEN P. L.: 10th International Conference on Fluidized Bed Combustion, (A. M. Manaker, Ed.), vol. 1, p. 1., 1989. 2. WALSH P. M.: 10th International Conference on Fluidized Bed Combustion, (A. M. Manaker, Ed.), vol. 2, p. 765, 1989. 3. MASI S., MICCIO M., SALATINOP. AND MASSI-

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14.

15.

MILLA L.: 10th International Conference on Fluidized Bed Combustion, (A. M. Manaker, Ed.), vol. 2, p. 775, 1989. SUNBACKC. A., BEI~RJ. M. AND SAROFIMA. F.: Twentieth Symposium (International) on Combustion, p. 1495, The Combustion Institute, 1984. CHIRONE R., CAMMAROTAA., D'AMORE M. AND MASSIMILLA L.: International Conference on Fluidized Bed Combustors, vol. 7 p. 1023, 1982. PRINSW., SIEMONSa. V., RADOVANOVICM. AND VAN SWAAIJW. P. M.: International Conference on Coal Science, (Moulijn et al Ed.) p. 818, Elsevier Science Amsterdam, 1987. KERSTEIN A., NIKSA S. Twentieth Symposium (International) on Combustion, p. 941, The Combustion Institute, 1985. 8ALATINOP. AND MASSIMILLAL. Twenty-Second Symposium (International) on Combustion, p. 29, The Combustion Institute, 1989. CHIRONE R., SALATINOP. AND MASSIMILLAL.: Comb. Flame 77, 79 (1989), DUNN-RANK1N D.: Combustion Science and Technology 58, 297 (1988). PETERSON T. W. AND ScoTro M. V.: Powder Technology 45, 87 (1985). SmMONSR. V.: Combustion and Flame 70, 191 (1987). International Committee for Coal Petrology, (CNRS Paris Ed.) 1963, Supplements to second edition, 1971 and 1975. MALONEYD. J. AND JENKINS R. G.: Twentieth Symposium (International) on Combustion, p. 1435, the Combustion Institute, 1984. STACHE., MACKOWSKYM. Th., TEICHMULLERM., TAYLOR G. a . , CHANDRAD. AND TEICHMIJLLER R.: Coal Petrology Gebrfider Borntraeger-Berlin, Ed, Chap. 5, 1982.