Devolatilization of bituminous coals at medium to high heating rates

COMBUSTION AND FLAME

6 3 : 3 2 9 - 3 3 7 (1986)

329

Devolatilization of Bituminous Coals at Medium to High Heating Rates A . S. J A M A L U D D I N ,

J . S. T R U E L O V E ,

and T. F. WALL

Department of Chemical Engineering, University of Newcastle, NSW. 2308, Australia

A high-volatile and a medium volatile bituminous coal, size-graded between 53 and 63 ~tm, were devolatilized in a laboratory-scale laminar-flow furnace at 800-1400°C at heating rates of 1 x 104-5 × 104 *C/s. The weight loss was determined by both gravimetric and ash-tracer techniques. The experimental results were well correlated by a two-competing-reactions devolatilization model. The model was also evaluated against data from captive-sample experiments at moderate heating rates of 250-1000"C/s. Heating rate was found to affect substantially the devolatilization weight loss.

1 INTRODUCTION

2 EXPERIMENTAL

The i m p o r t a n c e o f volatiles in stabilizing and p r o p a g a t i n g p u l v e r i z e d - c o a l (p.c.) flames has stimulated extensive research on coal devolatilization [1-3]. Studies at high t e m p e r a t u r e s (above 100°C) have suggested that the ultimate yield o f volatiles, V*, is significantly influenced by heating rate [4-7].

2.1 Sample Preparation

The present investigation was undertaken to d e t e r m i n e values o f V* o f two nonswelling bituminous coals at t e m p e r a t u r e s and heating rates t y p i c a l l y e n c o u n t e r e d in p.c. flames. The influence o f heating rate was e x a m i n e d by d e v o l a t i l i z i n g the coals at high heating rates in a l a m i n a r - f l o w furnace and at m o d e r a t e heating rates in a c r u c i b l e heated in an ash-fusion furnace. A simple m o d e l was d e v e l o p e d to calculate the t e m p e r a t u r e - t i m e history o f the coal particles in the l a m i n a r - f l o w furnace and to predict the d e v o l a t i l i z a t i o n weight loss. Copyright © 1986 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017

The coals were crushed, w e t - s i e v e d to a size range o f 5 3 - 6 3 / ~ m , and then d r i e d at 45 °C. The mean d i a m e t e r for the p r e p a r e d s a m p l e s was 68 /~m as m e a s u r e d by a M a l v e r n 2200 particle size analyzer. The p r o x i m a t e a n a l y s e s o f the prep a r e d coals are given in Table 1.

2.2 Devolatilization under Rapid Heating Conditions The d e v o l a t i l i z a t i o n e x p e r i m e n t s under r a p i d heating conditions were c a r r i e d out in an electrically heated muffle-tube furnace ( A S T R O M o d e l 1000A). The furnace p r o d u c e d a unif o r m - t e m p e r a t u r e hot zone 45 m m in d i a m e t e r and 150 m m in length. The coal was s u p p l i e d at a uniform rate o f 4 - 5 g/h by a positived i s p l a c e m e n t type coal f e e d e r using high purity argon as the c a r r i e r gas. The hot m a i n s t r e a m gas

0010-2180/86/$03.50

330

A. S. JAMALUDDIN ET AL. TABLE 1 Coal Analysis (wt %) Coal Type High-Volatile Bituminous

Medium-Volatile Bituminous

Proximate" Moisture Volatile matter Ash Fixed carbon

4.3 35.2 2.4 58.1

1.5 18.4 10.8 69.3

Ultimate b C H O N S

79.37 5.13 8.41 1.71 0.38

76.03 4.03 4.58 1.38 0.35

34.5

35.8

Component

Calorific value, MJ/kg (gross, m.a.f, basis)

" '

= rt-t.e==~<-~ mainstream heatshield---~~

muffletube

observati on7[ port

re#o%°n-"~'___ samplinig probe

a Analysis of the prepared coal used in the experiments. b Analysis of the raw coal (m.a.f. basis).

water out

<----waterin charbottle

collection

was also high purity argon. The carrier gas velocity was 0.655 m/s, and the main gas velocity was in the range 0.04-0.06 m/s. Watercooled probes were used to feed the coal into the furnace and to collect the char. The probes were positioned 100 mm apart and the transit time for the coal particles was 0.7-1.0 s. A schematic of the furnace system is shown in Fig. 1. Further details are available elsewhere [8]. The devolatilization weight loss was determined as [9] xv = 1

fM +fAL +fR +f~

x 100.

(1)

r/c The various terms are defined in the nomenclature. Moisture in the coal was determined by a standard ASTM test. Particle losses were estimated by simulating the experimental gas flow velocities at room temperature and measuring the particle collection efficiency of the probe. The collection efficiency was found to be better than 99% when the probes were 100 mm apart. Therefore, the particle collection efficiency was

Fig. 1. The furnace system.

taken as 100% for the weight-loss determination. The ash lost due to vaporization was estimated by subtracting the weight of ash in the collected char from the weight of ash in the coal fed to the furnace. The fractions of ash in the coal and char were determined by ashing at 750"C. The soot formed due to cracking of the volatiles was estimated by ashing the sooty deposits removed from the walls of the char collection bottle and sampling probe. The weight of char in the deposit was determined from the ash formed, and the residue was presumed to be soot.

2.3 Devolatilization under Medium Heating Conditions The devolatilization experiments at medium heating rates were carried out in a carbolite ashfusion furnace using small samples of coal (1020 mg) spread in a thin layer on a platinum

0

I 10

[ lime, s

5

10

I

time, s

5

I

heating r a l e : 97G ~ / s

Fig. 2. Temperature traces for the platinum crucible heating up in the ash fusion furnace: (a) 1000*C; (b) I200*C; (c) I400*C.

time, s

I

5

I

10

e. heatlng rate : 2 5 5 " C l s

b. h e a l i n g r a t e : 4 6 0 "C/s

c.

20O

40O

60O

g

800 ~

1000

1200

400

0 >.

Z 0

,-d

Z ©

t" N -]

,.-]

< © t-

332

A . S . JAMALUDDIN ET AL. 70 O t~

E

v

6O

50

# . ¢n

=n 4(2 0

proximate VM (maO

.c

._=

~ 3(2 2(3

I

I

T

i

36

"~ 32

J

m

24

roximate .L.y_ap--

.1=

• -

20

16

i 800

I I000

I

f

1200

1400

f u r n a c e teml~erature,

C

Fig. 3. Weight losses at different furnace temperatures: (a) high-volatile bituminous coal; (b) medium-volatile bituminous coal; ©, weight loss using gravimetric method; e , weight loss using ash-tracer method; . . . . , predictions of the model.

crucible. The heating rates at different furnace temperatures were measured using an empty crucible (see Fig. 2). The coal was assumed to heat at the same rate as the empty crucible because the sample was less than 2.5% by weight of the crucible. An inert atmosphere was maintained in the furnace by a continuous flow of high purity argon. The char was waterquenched after a devolatilization time of about 10s. The devolatilization weight loss was determined using a simplified form of Eq. (1):

go = 1 --fM --JR.

(2)

3THEORETICAL MODEL

3.1 Gas Velocity and Temperature The gas velocity and temperature in the laminarflow furnace were calculated from the wellknown equations for conservation of mass, axial momentum, and energy [10]. Axial molecular transport and the small rachal vartanon m the pressure were neglected. The coal loadings used in the experiments were small and consequently the effect of the coal particles on the gas velocity and temperature was negligible. The equations were solved by standard numerical techniques.

DEVOLATILIZATION OF BITUMINOUS COALS

333

TABLE 2 Residual Volatile Matter in the Char and the Q-Factors and R-Factors Coal Type Furnace Temperature (*C)

Component a Measured/ Estimated

High-Volatile Bituminous

Medium-Volatile Bituminous

Ultimate yield, V*

47.54

23.94

Residual volatiles in char, VR Q--factor R--factor b

13.6 1.97 1.25

12.25 2.73 I. 14

Ultimate yield, V*

52.64

25.0

Residual volatiles in char, VR Q--factor R--factor °

12.0 2.05 1.40

Ultimate yield, V*

66.1

1000

1200 6.5 1.72 1.22 27.45

1400 Residual volatiles in char, VR Q--factor R--factor b

5.9 2.08 1.64

6.0 1.83 1.31

a Percentage of original coal, on a m.a.f, basis. b Based on the average of measured weight losses using gravimetric technique.

3.2 Particle Velocity and Temperature

3.3 Devolatilization

The equations of motion and energy for the coal particles are given by

The rates of devolatilization and the ultimate weight loss were estimated using the following two-competing-reactions scheme:

dur, = 1 dt r (Ug- Up),

k. / ~ l V l + ( 1 - - c q ) R l

(3) Coal

mpCpp dTp ~rdp 2

at

- h o ( Tg -

Tp) + ca( Tw4 -- Tp 4)

-RwHw.

"~"~2 (4)

The evaporation of moisture from the coal particle was modeled as a mass-transfer-controlled process; the mass fraction of moisture at the particle surface was calculated from the vapor pressure of water at the temperature of the particle.

V2 + (1

- ot2)R 2

The kinetic parameters were taken from the works of Badzioch and Hawksley [11] and Ubhayakar et al. [6]. The stoiehiometric factor c~1 was taken as the mass fraction of volatile matter in the coal as determined by proximate analysis (Vm), and o~2 was determined from the measured devolatitization data (see Section 4.1). In the present work, the factor cz2 has been

334

A. S. JAMALUDDIN ET AL. 2.0

/

1.8

° go Ooj .d

1.6

T

1.4

'[.2 -

1,0-I. . . . . .

0.8 6OO

800

!,

t 000

1"200

t 1400

.... 1600

peak temperature, C

Fig, 4. Comparison of measured weight losses with those of other investigators for various bituminous coals: O , Kimber and Gray [4]; @, Coates et al, [15]; O , Mentzer et al. [16]; Ax, Goldberg and Essenhigh [7]; [], Anthony et al. [17]; m , Desypris et al. I18]; &, • , present study,

termed the "maximum possible" devolatilization weight loss.

4 RESULTS AND DISCUSSION 4.1 Devolatilization under Rapid Heating Conditions The measured weight losses from the two coals at different furnace temperatures are shown in Fig. 3. The weight loss from both coals exceeded that obtained by proximate analysis and increased substantially with increasing furnace temperature. The calculated heating rate of the particles (defined by the time taken for the particle temperature to rise through 63 % of the initial temperature difference) increased with furnace temperature from 1 x 104 °C/s at 800°C to 5 x 104 °C/s at 1400°C. Consequently, the increased weight loss at higher furnace temperatures is a combined effect o f temperature and heating rate. The value of the model parameter or2 is estimated as [ 11-13] o~2= Q Vm,

where

(5)

Q=

V*

Vm- VR

(6)

The measured values of VR and calculated values of Q are given in Table 2. Similar values for the Q-factor for high-volatile bituminous coals were obtained by Kimber and Gray [4] and Goldberg and Essenhigh [7]. The Q-factors for the theoretical model were taken to be 2.0 for the high-volatile coal and 1.8 for the mediumvolatile coal. The predicted weight losses for the two coals are shown in Fig. 3 along with the measured data. For the high-volatile coal the predicted and measured weight losses are in very good agreement. For the medium-volatile coal the weight losses are overpredicted by up to 5%. The discrepancy may be due to deposition of soot and/or tar on the char particles. Sooty deposits have been found to account for up to 15 % of the original weight for some coals [14]. The measured enhancements in volatiles yield (the R-factor, defined by Kimber and Gray [4] as the ratio V*/Vm) from the present work (see Table 2) are compared with those of other

DEVOLATILIZATION OF BITUMINOUS COALS

335

45[ 43

ta .ta

° E

t

4"1

3 c.

o

roximate

- -

a 35

~

i

25

23

E

¢M (mar) 21 .....

ae e~ _o

19

"

17

"~

15

t

b

I 1000

I 1200

furnace temperature,

I 1400

"C

Fig. 5. Weight losses in crucible experiments: (a) high-volatile bituminous coal; (b) medium-volatile bituminous coal; ©, experimental data; - - - , predictions of the model. investigators in Fig. 4. Good agreement is achieved despite the wide scatter in the data.

4.2 Devolatilization under Medium Heating Conditions The weight losses from the two coals devolatilized at medium heating rates are shown in Fig. 5 with error bars to indicate the spread of the data and circles to indicate the average values. At the lowest furnace temperature (1000*C) the weight loss from both coals was below that obtained by proximate analysis. With increasing furnace temperature the weight loss from both coals increased but did not significantly exceed the proximate-analysis weight loss even at the highest temperature (1400 oC). The predictions of the devolatilization model,

using the measured heating rates, are also shown in Fig. 5. The predicted and measured weight losses are generally in good agreement. This is considered to be a severe test of the devolatilization model, as none of the parameters was changed.

4.3 Comparison of Devolatilization Data Comparison of the devolatilization weight loss data from the laminar-flow and platinum crucible experiments at the same furnace temperatures clearly demonstrates that heating rate has a significant role in enhancing the yield of volatiles. For example, at a furnace temperature of 1400°C, the weight loss from the high-volatile coal was 62% under rapid heating conditions and only 42 % under medium heating conditions. The difference in yields is unlikely to be due to

336

A. S. J A M A L U D D I N ET AL.

s e c o n d a r y reactions in the bed of particles contained in the c r u c i b l e , as these are m i n i m i z e d by the use o f a thin l a y e r o f coal and a continuous p u r g e o f inert gas.

inal coal, char

Xv

Greek Letters ct2

5 CONCLUSIONS The principal conclusions investigation are

from

the present

a. the Q - f a c t o r for rapid d e v o l a t i l i z a t i o n o f m e d i u m and high volatile bituminous coals is about 2; b. the ultimate yield o f volatiles is a p p r e c i a b l y influenced by heating rate; c. the d e v o l a t i l i z a t i o n weight loss at m o d e r a t e to high heating rates is adequately d e s c r i b e d by the t w o - c o m p e t i n g - r e a c t i o n s model,

NOMENCLATURE

Cpp

heat c a p a c i t y o f particle, kJ/(kmol K)

dp

particle d i a m e t e r , m

fkL, fM,

weight fractions o f ash lost, moisture

JR, fs

v a p o r i z e d , char p r o d u c e d , soot formed convective heat transfer coefficient, kJ/(m 2 s K)

Hw k l , k2

latent heat o f v a p o r i z a t i o n of water, kJ/kg rate constants, s - 1

mp

mass o f single coal p a r t i c l e , kg

Q Rw

m u l t i p l y i n g factor rate o f v a p o r i z a t i o n o f water, k g / s

t time, s Tg, Tp, Tw t e m p e r a t u r e o f particle, furnace Ug, Up axial velocity o f particle, m/s V* ultimate yield o f Vm, VR

e

particle collection efficiency o f the sampling p r o b e , percent

o

S t e f a n - B o l t z m a n n constant, kJ/(m 2 s K 4)

r

particle relaxation time, s

REFERENCES 1. Anthony, D. B., and Howard, J. B., A1ChE J. 22:625-656 (1976). 2. Howard, J. B., in Chemistry o f Coal Utilization (M. A. Elliot, Ed.), 2nd Suppl. Vol., Chap. 12, Wiley, New York, 1981. 3. Gavalas, G. R., Coal Pyrolysis, Elsevier, Amsterdam, 1982. 4. Kimber, G. M., and Gray, M. D., Combust. Flame 11:360-362 (1967). 5. Kobayashi, H., Howard, J. B., and Sarofim, A. F., 16th Symposium (International) on Combustion,

6.

8. 9. 10. 11. 12.

gas at centerline, wall, K or °C gas at central line, volatiles

volatile matter ( m . a . f . basis) in orig-

"maximum possible" fractional weight loss due to d e v o l a t i l i z a t i o n e m i s s i v i t y o f coal p a r t i c l e s

r/c

7.

hc

fractional weight loss as volatiles

13. 14. 15.

The Combustion Institute, 1977, pp. 411-425. Ubhayakar, S. K., Stickler, D. B., Rosenberg, C. W. V., and Gannon, R. E., 16th Symposium (International) on Combustion, The Combustion Institute, 1977, pp. 427-436. Goldberg, P. M., and Essenhigh, R. H., 17th Symposium (International) on Combustion, The Combustion Institute, 1979, pp. 145-154. Jamaluddin, A. S., Ph.D. Thesis, University of Newcastle, Australia, 1985. Kobayashi, H., Ph.D. Thesis, Massachusetts Institute of Technology, 1976. Bird, R. B., Stewart, W. E., and Lightfoot, E. N., Transport Phenomena, Wiley, New York, 1960. Badzioch, S., and Hawksley, P. G. W., Ind. Eng. Chem. Process Des. Dev. 9:521-530 (1970). Nsakala, N. Y., Essenhigh, R. H., and Walker, P. L., Jr., Combust. Sci. Technol. 16:153-163. Scaroni, A. W., Walker, P. L., Jr., and Essenhigh, R. H., Fuel 60:71-76 (1981). Eddinger, R. T., Friedman, L. D,, and Rau, E., Fuel 45:245-252 (1966). Coates, R. L., Chen, C. L., and Pope, B. J., in Coal Gasification, Advances in Chemistry Series No. 131, American Chemical Institute, 1974, pp. 92-107.

DEVOLATILIZATION

OF BITUMINOUS

COALS

Mentzer. M., O'Donnell, H. J., Ergun, S., and Friedel, R. A., in Coal Gasification, Advances in Chemistry Series No. 131, American Chemical Institute, 1974, pp. 1-8. 17. Anthony, D. B., Howard, J. B., Hottel, H. C., and Meissner, H. P., 15th Symposium (International)

337

16.

18.

on Combustion, The Combustion lnstitute, 1977, pp. 1303-1317. Desypris, J., Murdoch, P., and Williams, A., Fuel 61:807-816 (1982).

Received 28 January 1985; revised 25 May 1985