An experimental study on oxidation rates of coal at low temperature

Twenty-first Symposium (International) on Combustion/The Combustion Institute, 1986/pp. 193-201

AN EXPERIMENTAL

STUDY ON OXIDATION TEMPERATURE

R A T E S OF C O A L A T L O W

L. PETARCA, L. T O G N O T T I , S. ZANELLI axn G. BERTOZZI Dipartimento Ingegneria Chimica Universitfi di Pisa--Via Diotisalvi, 2 56100 PISA, ITALY

An experimental study was made of the oxidation of bituminous coal in the temperature range 80-180~ The weight increase of the sample, and oxygen consumption during oxidation were measured. Two different flow configurations were used to determine the oxygen consumption rate: a continuous flow technique (CF) in which reactor effluent gases were analyzed by gas chromatography; and a pulse feed (PF) technique in which small and known amounts of various N~/O2 mixtures were injected into a carrier gas stream before the gas entered the reactor; the oxygen up-take was measured by G.C. analysis. The PF technique yielded the reaction rate in the first moments of oxidation, during which the oxygen consumption rate was relatively high. The effect of temperature, oxygen partial pressure, particle size, and gas flow rate were evaluated. The rate of oxidation decreased with increasing time of oxidation: the lower the temperature, the faster the rate decreased. At 100~ the rate was independent of particle size, indicating full chemical control while at 160~ rate was affected by internal diffusion, since oxidation of larger particle sizes resulted in a decrease in the rate. The rate was also affected by the time during which previously oxidized samples were kept in an inert atmosphere (restoration time): the restoration of reactivity increased with restoration time. Initial rates measured by the PF technique were used to obtain the activation energy and the order of reaction. Activation energies of 8-9 Kcal/mole and an order of reaction of about 0.9 were found.

Introduction I n this p a p e r we r e p o r t m e a s u r e m e n t s o f weight increase and rates o f o x y g e n absorption by coat at n e a r - a m b i e n t t e m p e r a t u r e s ( 8 0 - 1 8 0 ~ as a function o f t e m p e r a t u r e , particle size, oxygen concentration, and experimental m e t h o d . T h e results o b t a i n e d are relevant in several d i f f e r e n t contexts o f coal use, but we m e n t i o n h e r e two o f p a r t i c u l a r significance to use at this time: the self-heating o f coal; and p r e t r e a t m e n t o f caking coals. T h e p h e n o m e n o n o f self-heating o f coal has b e e n o f f u n d a m e n t a l and practical i m p o r t a n c e for o v e r a century. O f particular interest in recent years has b e e n the d e v e l o p m e n t o f m e t h o d s to predict self-heating.a'2 T h e s e methods r e q u i r e directly o r indirectly a k n o w l e d g e o f the rate o f o x i d a t i o n o f coal, since the p r i m a r y cause o f self-heating o f coal is atmosp h e r i c oxidation at low t e m p e r a t u r e . For the second p r o b l e m , a p r e t r e a t m e n t o f caking coals by m e a n s o f mild oxidation 3 may

be a successful solution to the p r o b l e m of a g g l o m e r a t i o n and r e d u c t i o n o f gas permeability o f most b i t u m i n o u s coals d u r i n g gasification. A l t h o u g h such o x i d a t i o n is k n o w n to decake coal, the p h e n o m e n o n is not sufficiently u n d e r stood to p e r m i t efficient design and optimization o f the p r e t r e a t m e n t process. T h e literature shows that the rate o f oxidation o f coal d e p e n d s on m a n y factors including the rank, m o i s t u r e c o n t e n t , particle size, temp e r a t u r e and e x t e n t o f p r e v i o u s oxidation o f coal as well as the c o m p o s i t i o n of a m b i e n t air (i.e. o x y g e n c o n c e n t r a t i o n a n d humidity). Num e r o u s papers h a v e dealt with the kinetics o f oxidation o f specific coals at t e m p e r a t u r e s < 300 ~ 1'4-I4 but very few h a v e b e e n c o n c e r n e d with the initial stages o f oxidation, d u r i n g which the o x y g e n c o n s u m p t i o n or absorption rate is relatively higher. T h i s p a p e r presents initial rate data for a South African coal, but the e x p e r i m e n t a l m e t h o d s d e s c r i b e d can be easily e m p l o y e d for the d e t e r m i n a t i o n o f initial rates in d i f f e r e n t reacting gas-solid systems.

193

194

COAL COMBUSTION

Experimental Three different experimental methods were used to obtain the oxidation kinetics: 1) measurement of the change in weight with time by TGA; 2) m e a s u r e m e n t of the oxygen consumption at the outlet of a packed reactor (illustrated in Fig. l), using a gas chromatograph in a continuous flow (CF) configuration; and 3) repeat of (2) in a pulsed feed (PF) mode using the r e a r r a n g e m e n t shown in Fig. 2. In all experiments, a weighted a m o u n t of fresh coal was heated in nitrogen to 110 ~ (at about 5 ~ for overnight drying. T h e sample was b r o u g h t to the desired operating temperature in N2, a n d the reaction was started by feeding a N2/02 mixture continuously or periodically at a known rate and composition. In the T G A experiments, a sample of a few milligrams was placed on a stainless steel net plate to ensure a good gas contact with the solid surface. The change in weight with time was then recorded for different temperatures and particle sizes. In the CF and PF configurations, cylindrical

F1G. 1. Schematic of experimental apparatus: CF configuration.

reactors of various diameter and length were used. The coal was held by small plugs of Pyrex glass in Pyrex glass reactors. The weight of sample was varied for the different operating temperatures and gas flow rates, in order to obtain small variations in oxygen concentration between the inlet and outlet streams. In the continuous flow (CF) configuration, shown in Fig. l, the reaction was started by feeding air, preheated to the reactor temperature. Samples of the product gas were analyzed every 8 - 1 0 min d u r i n g the first 2 or 3 hr, and at longer intervals thereafter. The gas flow rate was measured at the beginning of the r u n and checked d u r i n g the experiment. Table I lists the range of some of the parameters. To obtain the initial oxidation rate, the alternative PF method was used: a small and known a m o u n t of air, or of different 02/N2 mixtures, was injected into a carrier gas stream (He) before the gas entered the packed bed reactor. T h e oxygen in the product gas was then measured by a gas chromatograph, nitrogen being the internal standard (Fig.2). T h e volume of the pulse, the gas flow, and geometry of reactor were chosen to create a rectangular pulse of reactant gas. The shape of the pulse was checked by injecting the oxidizing gas into the reactor, at room temperature, and then directly into the detector, by passing the G-C column. T h e chromatogram obtained in this way showed that the pulse could be regarded as rectangular. T h e measured pulse width was used to calculate the oxidation rates. Table I lists the experimental conditions. A bituminous South African coal was used in this study. T h e coal analysis is given in Table lI. The coal was drawn from a storage pile of a power station in a commercially crushed form and was split into desired size fractions using a rotating mill. After crushing, each fraction was packed u n d e r an inert atmosphere to preserve the "initial" characteristics of the samples.

Theoretical Consideration

r -

.

. .

-

-.,

, - . - - - ,

There are two extreme methods of treating raw kinetic data to determine rate coefficients. ~ One extreme is to assume a set of "accurate", "elementary" reaction steps or mechanisms that may be combined into a composite equation. z'~5'~6T h e other extreme is to fit the raw rate data to the simplest possible nth-order, semi-empirical relation, usually of the form

R=kCTC ~ FIG. 2. Schematic of experimental apparatus: PF configuration,

(1)

where R is the rate of oxygen consumption, C~ is the concentration of reacting species on the coal

LOW TEMPERATURE OXIDATION OF COAL solid surface, Co is t h e g a s e o u s o x y g e n c o n c e n tration, and n and m are operational orders of reaction. Since t h e e x t r e m e l y c o m p l e x a n d v a r i a b l e n a t u r e o f coal m a k e it difficult "a p r i o r i " to i d e n t i f y specific c o n s t i t u e n t s in coal as t h e reactive species in o x i d a t i o n reactions, a n d t h e n in t h e r e a c t i o n steps, t h e s e c o n d m e t h o d has b e e n u s e d in this work. H o w e v e r , physical c o n s i d e r a t i o n s o n t h e n a t u r e o f the p h e n o m e n a

195

i n v o l v e d h a v e b e e n i n c l u d e d in a n a l y z i n g t h e e x p e r i m e n t a l data. As we shall s h o w in p r e s e n t i n g t h e results, t h e v a r i a t i o n o f R with time, t, c a n b e r e p r e s e n t e d by the e m p i r i c a l r e l a t i o n log R = l o g A * - B log t

(2)

T o p r e s e n t this in a n o n - l o g a r i t h m i c f o r m , we m u s t i n c l u d e a small t i m e c o n s t a n t , to, to give finite r a t e values at t = 0; t h u s

TABLE I Experimental Conditions

R = A * / ( t o + t) B

(3a)

= (A*/toB)/(1 + t/to) B CONTINUOUS F L O W Temperature Reactor pressure Gas flow rate

80-160~ Atmospheric 10-100 (cm3/min at 25~ and 1 Arm) 180-355 Ixm 355-500 .... 710-1000 ....

Particle size

Sample weight Inlet gas Reactor diameter Reactor length

PULSED FEED Temperature Reactor pressure Gas flow rate Inlet gas (% 02) Pulse width Pulse volume Particle size Reactor diameter Reactor length

t h e e r r o r in e v e n for t h e 10 m i n u t e s . = 0 is t h e n

Ro=A*/t ~

(4)

I f t h e A r r h e n i u s f o r m o f t h e rate c o n s t a n t is a s s u m e d we can i d e n t i f y

100-180~ Atmospheric 100 cm3/min 21, 50, 90 7.2 sec 12 cm 3 180-355 ~*m 5 - 1 2 mm 5 - 1 5 cm

so t h a t activation e n e r g i e s ( a n d r e a c t i o n o r d e r s ) c a n be o b t a i n e d e v e n f r o m e m p i r i c a l t r e a t m e n t o f t h e rates.

(Weight percent) 58.0 26.4 12.7 2.9 (Weight percent--dry coal) 72 3.6 1.7 0.55 9.1 13.05

Surface areas Nz--Surface area CO2--Surface area

o r less, t h a n 1% time of Ro, at t

10-35 g Air 5-12 mm 10-50 cm

TABLE II Coal Analysis

Proximate Analysis Fixed Carbon V.M. Ash Moisture Ultimate Analysis C H N S 0 Ash

so t h a t if to is 10 s e c o n d s n e g l e c t i n g it in Eq. 2 is less s h o r t e s t CF m e a s u r e m e n t T h e initial r e a c t i o n rate,

(3b)

A* = K e x p ( - E / R T )

(5)

Results

General Characteristics of Behaviour F i g u r e s 3 a n d 4 s h o w typical results for t h e T G A a n d t h e CF ( c o n t i n u o u s flow) t e c h n i q u e s , respectively.

__ ....

total Oaconsumpt,on weqght ncrease

J

160~

SA coat 180 355 y r n

% o4

-

05

50

100 TIME , nq,r,

0.55 m2/gr 4.9 m~/gr

FIG. 3. Weight increase and total oxygen consumption at 120~ and 160~

196

COAL COMBUSTION

Below 300 ~ T G A experiments generally show an increase in weight of the samples with time. Such a weight increase, however, does not represent the total oxygen used up by the coal. The total used is result of a combination of multiple reactions that produce either gaseous products (CO2, H 2 0 etc.), or create coal-oxygen complex c o m p o u n d s on the solid surface (dashed lines in Fig. 3). For interpretation, if a reaction mechanism is not assumed, weight variation data can only lead to qualitative considerations or conclusions about the a m o u n t of oxygen taken up by the solid surface. On the other hand, the continuous flow experiments yield the total oxygen consumption. T h e CF procedure averages the reaction over the whole sample, however, and care must be taken in evaluating reaction rates, particularly if the oxygen concentration varies substantially along the reactor. As seen in Fig. 4, the rate of oxidation of the coal decreased with increasing time o f oxidation. Several longer runs of up to 6 hours duration showed a further decrease of rate although Kam et al. 3 and Karsner and Perlmutter 14 have r e p o r t e d an approach to a non-zero asymptotic limit. Because of the falling reaction rate with time, the pulse technique was used to investigate the initial stages of the oxidation d u r i n g which

S.A. Coal o

3

9

1%O.-3S ~ m o

OO

dl

Oo

2

oo

O 9@

0000000

~

". .

ld! 9

o

[3 9

gs

~ 1 7 6 1 7 6 1 o7 6 1160~ 76 ell II O O O llll llll l l l l l l llll I I I

~

[] o[]

9

II

u[]~Dc ) 9

O3 Z

o2

[]

3

[]

[]

120

Quantitative Behaviour: CF Technique

II &

II

Figures 5 and 6 illustrate, respectively, the effects of t e m p e r a t u r e (Fig. 5), and of particle size and gas flow rate (Fig. 6) using the CF technique. Both plots support the validity of the empirical equation.

I

& klJ
140 II

9 mIm

9 9

&

9

100

zx AA A

reaction rates are higher than the rates measured using the CF technique. T h e pulse technique uses oxidant contact times of less than 10 see, followed by 10 minute recovery periods, and it uses smaller samples than the CF technique. This results in much smaller variations of oxygen concentration through the sample in the reactor. In the pulse technique, the recovery time o f about 10 minutes between one pulse and the next allows the carrier gas to "restore" the reactivity of coal sample. As might be expected, the oxidation rates measured by the CF and PF techniques thus follow different trends, as described in the following sections. The trends shown by Figs. 3 and 4 can be explained on the assumption that different numbers of fresh sorption sites are created by the evolution of the gaseous oxidation products, at different temperatures. T h e comparison in Fig.3 between total oxygen uptake calculated from the CF results, and the weight increase obtained by T G A at two different temperatures, shows that the ratio between weight increase and total oxygen consumption is greater at the lower temperature. Analogous 9 1.3 results were r e p o r t e d by Polat and Harris, and by Swann, n who showed that the weight increase of coal at 35~ represented the major fraction of the total oxygen taken up by the coal; and by Kam et al. 3 who found that the CO and CO2 evolved (in terms of percentages of overall oxygen reacted) increased with temperature. It can be assumed that, d u r i n g isothermal oxidation, a changing n u m b e r of active sites in the coal are saturated by the formation of a surface-oxygen complex, and that the amount of complex can be quantitatively represented by the weight increase. It is also assumed that the sites are then reactivated by thermal decomposition of the complexes into gaseous products. Site reactivation is faster at higher temperatures, resulting in an overall slower decrease of their concentration C,.

A~

zx~ 80 I '~zxAd~ 100 TIME, rain

I 200

I

FIG. 4. Oxygen consumption rates at different temperatures; air flow: 15-20 cm3/min.

a) Effect of Temperature The log-log plot o f R versus time in Fig. 5 shows that a family of straight lines, with different slopes, is obtained in the range 8 0 160~ these are the data in Fig. 4. In Fig. 5

LOW TEMPERATURE OXIDATION OF COAL some of Kam's ~ results at 200~ in the same range of oxidation time are also reported. It should be noted that the slopes of the lines increase as the temperature decrease. The values of the slopes are listed in Table III for comparison with the results described later. T h e interpretation of the trends is partly discussed in the previous section.

b) Effect of particle size and gas flow rate T h e results using three particle sizes and three gas flow rates at two temperatures, are shown in Fig.6, again using the log-log plot. As is well known, reactions of a porous solid with a gas may be divided into three kinetic regimes, according to whether the overall process is controlled by:(i) the rate of external mass transfer from the bulk gas to the solid surface; (ii) the rate of gaseous diffusion within the porous solid, or (iii) the intrinsic rate of chemical reaction. In the investigation of the operating regimes, Fig. 6 shows no effect of gas flow rate, indicating that external mass transfer is not the controlling mechanism. If it were, a dependence of rate on velocity to a power in the range 0.5-0.8 could be expected. There is an effect of particle size, however, that appears as the temperature increases. At 100 ~ Fig.6 shows that there is no effect, indicating that internal diffusion is rapid enough for this coal to allow overall chemical control. At 160~ there is a clear effect, suggesting that internal diffusional resistance 1

I

I

I

I

i I III

I

g~o-

180-355 Air

flow

L

I

197

affects the overall rate for larger particles at these higher temperatures. The location of the reaction on the internal surface can be verified by a simple calculation. The weight-increase data were converted to an equivalent monolayer coverage by assuming that in the oxygen uptake, the average surface area coverage was 16.6 ~2/oxygen molecule. ]4 The weight increase of the sample at 160 ~ (and after 1 hour of oxidation) corresponded to a monolayer surface coverage of the same order of magnitude as the CO2 -surface area in Table II, while the superficial areas calculated for spherical particles represented less than 0,5% of the CO2 area. Thus, coal oxidation is not limited to the outer surface, and most of the reaction takes place within the coal particle.

Quantitative Behaviour." PF Technique a) Parametric Behaviour The PF technique was used to determine the reaction rate in the first stages of oxidation; and the results for the influence of temperature are shown in Fig. 7. As before, straight lines are obtained on the log-log plots, with slopes (B) listed in Table III. Fig. 11 and Tab. III show

-.

i~rttd e 9 o A si Ze(Jim) D

-

1~0"355 355"500 710-1000

I

J~m 15~20 cq/min ~

<

.~

,

150%

2 16 %

W [--

<~ n_

~I0 -E

03 Z 0 q~

2 (2) (D

Z LU

Z W 0

9x ~7

~G 7

r162

0 I

I

I

I

I

l

I I Ilq ~\

10

100 TIME

Fro.

9 90 OA o IS20

5.

I

I

I

I

I

I

I

J

I i I rl

tO

iiii

i

100 TIM-~

, rnHq

, rain

Log R versus log t plot.

Fro. 6. Effect of particle size and gas flow rate on oxygen consumption rate.

198

COAL COMBUSTION

the initial rate d a t a and the slopes, respectively, at d i f f e r e n t 02 concentrations.

b) Comparison of techniques T h e d i f f e r e n t results o b t a i n e d f r o m the CF and PF techniques are s h o w n in Fig. 8, for the same particle sizes a n d t e m p e r a t u r e s . T h e s e results show that the initial values o f the reaction rate o b t a i n e d by the PF t e c h n i q u e lie on the e x t r a p o l a t i o n line o f CF data for each t e m p e r a t u r e , while the s u b s e q u e n t values o f the P F - d e t e r m i n e d rates are h i g h e r than the values extrapolated f r o m the CF straight lines. T h e reason for this, o f course, is the time interval between two pulses o f the oxidizing gas, d u r i n g which the reactivity o f the coal is partially restored. As would be e x p e c t e d , the extent o f the

L~J Y_d

L.q~

- ~

o- 160

[O. U) Z

~ '

8

~ m ~ .

D

~---{3

I20

Z UJ

~-

~7'IU

I

I

i

9 9 I i l lLI

0.1 7 15/l~ - ,

1 rain

I

100 I i

2

3 L,

FIG. 7. Oxygen consumption rates at different temperature in air; PF technique results. l

f

J.c ~ " ~ > oa.,, i - PULSE

LA

restoration d e p e n d s on the time interval between pulses. T h i s is clearly shown in Fig, 9, which c o m p a r e s d a t a o b t a i n e d in the CF m o d e , with PF runs u s i n g 10 min. and 30 min. between pulses. T h i s shows that the l o n g e r the restoration time, the smaller the slope o f the log-log plot, i.e. the h i g h e r the rate. T h e values o f these slopes are also i n c l u d e d in T a b l e III. T h e reactivation process was verified experimentally in s o m e CF runs, T h e p r o c e d u r e was TABLE III Slope of Straight Lines in LogR-Logt Plots, B. TEMP. ~

TECHNIQUE CF PFl0

Particle Size: 180-355 txm; air, 180 160 .43 140 .45 120 .49 100 ,52 80 .65 Different Particle Size; air. 160 .41 (355-500txm) 160 .41 (710-1000) 100 .53 (710-1000)

.24 .28 .30 .37 .41

PF30

.18

.34

Particle Size 180-355 Ixm; (% 02) 160 (50) .33 160 (90) .32 140 (50) .40 120 (50) .36 120 (90) .34 100 (50) ,41 CF: restoration time t, = 0 PFl0: restoration time t, = 10 rain PF~0: restoration time t, = 30 rain

t

S A Coal 180 365 # m

4

-

9 PF tr=3Omin: o PE tr=10 ,,

CF

~2

CONTINU0OS

...,.~0 ~ 160

i

167 0.1

.

~

i 1

10 TIME ~ mm

100

Fro. 8. Comparison between PF technique and CF technique results.

~C

160-355~.m 10 T I M E , rnin

FIG. 9. Effect of different restoration times between pulses (PF technique); comparison with CF results at 160~

LOW TEMPERATURE OXIDATION OF COAL to stop air flow, to feed inert gas for 10 hours, and then feed air again. T h e resulting behaviour is shown in Fig. 10. After recovery, R is initially much higher than the values before reactivation, but lower than the initial value. It then decays back to the original trend line, and with substantially the same slope.

Data Evaluation: Kinetic Constants a) Activation Energy T h e initial rate data obtained from the PF data at almost zero time were used, combining Eqs. 4 and 5, to determine the activation energies of the process. Figure 11 shows the Arrhenius plot for the reaction rates in air, 50% and 90% 02 atmospheres. T h e activation energies are in the range 8.3-9.0 Kcal/mole. These results give added support to the conclusion that the oxidation rates are influenced by

~4

_a~

<2 6~

restorc-,tlon time10hr

~

~176

c~ -6

199

diffusional resistance, but not by external mass transfer. In the latter case, the rates would have shown activation energies of about one Kcal/ mole, or significantly less than 8.3 Kcal/mole.

b) Reaction Order These determinations were also made using the initial value rate data, and using the empirical n-th order equation (Eq.1) as the theoretical basis. T h e raw data values have already been given in Fig. 11. T h e orders of reaction vary over the narrow range of 0.83-0.95 at the different temperatures, and tend to decrease as 02% tends to i00. T h e values of n are well within the range of reported literature data: for comparison Van Krevelen 2~ reported that the rate was first order with respect to oxygen concentration; Karsner and Perlmutter 14 f o u n d n = 0.7-0.74, and Sondreal and Ellman I reported n = 1 for lignite u n d e r 100 ~ These kinetic values for the orders of reaction and the activation energies both provide a good support for previous experimental findings, b with adsorption activation energies, in particular, in the range from 3-15 kcal/mole, accompanied by a first-order reaction.

"

"10

~

IGO"C

355-500~.m I

f

10

I l I ' I'L 100 TIME , mm

I

Discussion

I

~ I I II

FIG. 10. Effect of 10 hours reactivation time on oxygen consumption rate.

180

1r

I

140 f

r

-q.

100 coc ) t

t

" ~ -'~. ~

b~

"/~ o 9

50

n

90

21

CJ~

o

_

I

22

I

1

2~ 10(70/T

I

26 (K

L

I

2.8 )

FIB. 11. Arrhenius plot for initial oxygen consumption rates.

These results indicate the occurrence of different p h e n o m e n a d u r i n g oxidation. Firstly, there is a deactivation or saturation of active sites by the formation of surface oxygen complexes, and whose decomposition can explain the general trend of the results: During oxidation both reactions are present, but during the restoration or recovery times in inert atmospheres, only film decomposition can take place, with a consequent relative increase of C,, the active sites concentration. In addition, there may be some reaction-inhibiting mechanism other than the decrease in the concentration of the active sites, that can affect the reaction rate. As reported by Polat and Harris, 13 the most likely reaction inhibitors in this system are water and carbon dioxide, which are both products of the reaction. In a study of the chemistry of the low-temperature oxidation of Victorian coal, Swann and Evans ~v found that 98% of the material desorbing from the oxidized coal was water. O n account of their polar nature, the water molecules attach themselves to polar sites 18A9 (hydroaromatic functional groups) with hydrogen bonds. When

200

COAL COMBUSTION

sufficient oxidation occurs, the product water may inhibit the newly formed and/or already existing reactive sites from further reaction with oxygen. Furthermore, values of the slope B shown in Table III suggest an approach to a non-zero asymptotic limit for long recovery times in which desorption of desorbable inhibiting products is probably complete. This decrease of reactivity seems to indicate the existence of some stable oxygen-coal complexes which do not decompose to give gaseous products. Fig.10 supports this hypothesis since after ten hours of restoration, R was still less than the initial value.

Conclusion These kinetic studies of a South African coal in the range 8 0 - 1 8 0 ~ show that the rate of oxidation decreased with increasing time of oxidation. A logR--logt plot produced a family of straight lines with different slopes. T h e value of the slopes increased as the temperature decreased, indicating that the creation of fresh sorption sites by means of evolution of gaseous products probably became faster as the temperature increased. T h e slopes of those plots was also affected by recovery time in inert gas, during which desorption of inhibiting products and decomposition of oxygen complexes represented the most likely p h e n o m e n a involved in the recovery. At 160 ~ the rate was affected by internal diffusion since oxidation of a sample of larger particle sizes resulted in decreased rates. At 100 ~ the rate was i n d e p e n d e n t of particle size, and thus would be chemically controlled. Calculations using cumulative oxygen adsorption during oxidation showed that the adsorbed oxygen covered an equivalent monolayer surface area; this area was an order of magnitude greater than the superficial surface area, thus indicating that oxidation was not limited to the outer surface and that most of the reaction took place within the coal particles. External mass transfer was not a controlling mechanism since no effect of gas flow rate was found. Finally, the experimental values of reaction order and activation energy were in good agreement with values of other investigators.

REFERENCES 1. SONDREAL, E.A. AND ELLMAN, R.C.: Laboratory

Determination of Factors Affecting Storage of

2. 3.

4.

5.

North Dakota Lignite: Computer Simulation of Spontaneous Heating, BuMines RI 7887, 1974. NORDON,P.: Fuel 58, 456 (1979). KAM, A.Y., HXXON, A.N. AND PERLMUTTER, D.D.: Chem.Eng.Sci. 31,815 (1976). DRYDEN,I.G.C., in Chemistry of Coal Utilization, Suppl. Vol. (Ed. H.H.Lowry), John Wiley, 1963, Ch. 6, pp 232-295. SCHMIDT,L.D. AND ELDER,J.L.: Ind.Eng.Chem. 32, 249 (1940).

6. RADSPINNER,J.A. AND HOWARD, N.CA Ind. Eng.

Chem. (Analyt.) 15,566 (1943). 7. BANARJEE,S.C., BAt~ARJEE,B.D. ANDCHAkravORTV, R.N.: Fuel 49, 324 (1970). 8. CARPENTER,D.L. AND GIDDINGS, D.G.: Fuel 43, 247 (1964). 9. CARPENTER,D.L. AND GIDDINGS, D.G.: Fuel 43, 375 (1964). 10. CARPENTER, D.L. AND SERGEANT, C.D.: Fuel 45, 311 (1966). 11. SEVENSTER, P.G.: Fuel 40, 18 (1961). 12. NORDON, P., YOUNG, B.C. AND BAINBRIDGE, N.W.: Fuel 58,443 (1979). 13. POLAT, S. AND HARRIS, I.J.: Fuel 63,669 (1981). 14. KARSNER,G.C. AND PERLMUTTER, D.D.: AIChEJ.

27, 920 (1981). 15. ESSENHIGH, R.H.: "Fundamentals of Coal Con>

bust." Ch 19 in Chem. Coal Util. (2nd Suppl. Vol.), M.A.Elliott, Ed., John Wiley, N.Y. 1981. 16. NEWMAN,J.O.H., STANLEY,L. EVANS,P.L., COLDRICK, A.J. AND KEMPTON, T.J.: Nature 21, 280

(1967). 17. SWArN, P.D. AND EVANS, D.G.: Fuel 58, 276

(1978). 18. MARINOV,V.N.: Fuel 56, 153 (1977). 19. MAZUMDAR, B.K., BANERJEE, A. AND NANDI,

H.C.: Fuel Sc.Techn.2, 93 (1983). 20. VAN KREVELEN,D.W.: Coal, Elsevier Amsterdam,

1961.

Acknowledgment This work was supported by the Consiglio Nazionale delle Richerche, Italy (P.F.E. 2). Nomenclature A*

,B Co

constant in Eqns. (2),(3),(4),(5) oxygen concentration Cs concentration of reacting species on the coal E activation energy k reaction rate constant Ko pre-exponential factor m,n orders of reaction R reaction rate ~t gas constant T temperature, K

L O W T E M P E R A T U R E O X I D A T I O N OF COAL

201

COMMENTS Brian G. Wick, General Motors Research, USA. You observe sinificant net 02 a d s o r p t i o n - - t h a t is, more 02 loss than can be attributed to p r o d u c t production in the gas phase. How much Oz, as a fraction of coal present, have you m e a s u r e d as adsorbed, perhaps 1 5%? At General Motors Research, we have observed very similar results o f O atom adsoption on soot at 25~ (See paper p r e s e n t e d at this Symposium.) Authors' Reply. As r e p o r t e d in the paper and during the presentation, if a reaction mechanism is not assumed, weight variation data can only lead to qualitative consideration about the amount o f oxygen taken up by the solid surface. As the matter o f fact, Fig. 3 shows the comparison between weight increase o f the sample with time and total oxygen consumption calculated from the CF results, at two different temperatures. We observe a weight increase in the range 1-5% after 2 to 3 hours o f oxidation, at 100120~ Now, since at these lower t e m p e r a t u r e s the ratio between weight increase and total oxygen consumption is greater (it is practically one at room temperature), the above r e p o r t e d values can be considered close to the actual Oe absorbed by the solid surface. O f course the different physico-chemical structure o f our coal with respect to your material (soot) and the different oxidizing a t m o s p h e r e s (molecular and atomic oxygen respectively) make it difficult to c o m p a r e quantitatively the results.

E. Suuberg, Brown University, USA. Just a brief c o m m e n t to the effect that results such as shown in this study are apparently starting material dependent; on "young" chars o f tignites, we do not see any restoration o f reactivity by heat treatment o f chars in inert gas (at the t e m p e r a t u r e o f previous oxygen chemisorption). Also, we and others have noted the o r d e r o f low t e m p e r a t u r e oxidation to be closer to 89 o r d e r than first order, and the activation energy to be somewhat higher (30 kcal/mol). Clearly the authors o f this p a p e r are c o n c e r n e d with coals and not chars o f

coals, but this comparison o f work on u n h e a t e d coals and pyrolyzed coals suggests that careful attention nmst be paid to the previous t e m p e r a t u r e history o f such materials before drawing general conclusions.

Author's Reply. We agree with your comment; in particular it is i m p o r t a n t to make a general distinction between coals and chars in relation to their behaviour during low t e m p e r a t u r e oxidation (1.t.o.); further classification may be done on the rank o f coals. As the matter o f fact, i n f r a r e d studies on 1.t.o. o f coals 1-2 indicate that initial sites of attack are the aliphatic C-H with p r o d u c t i o n o f hydroperoxides, which then d e c o m p o s e in water and stable groups. On the other hand, recent works on spontaneous combustion of coals 3-4 r e p o r t that lower ranks abound in hydroaromatic structures with a consequent high susceptibility o f these coals to 1.t.o. Many factors, both chemical and physical, make low rank coals more vulnerable than high rank coals (and chars) to l.t.o. In particular, chars generally present a lower fraction o f hydroaromatic constituents than parent coals; so the 1.t.o. o f chars (and high rank coals) takes place mainly by means o f o t h e r mechanisms than oxidation o f atiphatic C-H. T h e lack of restoration o f reactivity by heat treatment o f chars in inert gas at the t e m p e r a t u r e of previous chemisorption that you mention, could be due to the relatively low amount o f inhibiting gaseous products, with no evident restoration o f reactivity. REFERENCES 1. LIOTTA, R., BROr,'S, G. ANn ISAAC,J.: Fuel 62,781 (1983). 2. RHOADS, C.A., SENFTLE, J.T., COLEMAN, M.M., DAVIS, A. AND PAINTER, P.C.: Fuel 62, 1387 (1983). 3. CHOUDHURY, S.S., SANYAL, P.K. AND BANERJEE, A.: Fuel Science & Techn. 1, 99 (1982). 4. MAZUMDAR, B.K., BANERJEE, A. AND NANDI, H.C.: Fuel Science & T e c b n . 2, 93 (1983).