Energy & Fuels 1989, 3, 522-527
522
general, ESR silent in agreement with conclusions drawn from experimental results."16 Conclusions
1. In situ ESR studies of coal pyrolysis are explained by a conceptual model of coal reactivity. The wide variety of ESR concentration profiles obtained from coals is the result of a complex interplay between mobile and immobile free-radical species that are relatively stable (ESR observable) or reactive (ESR silent). 2. A simple topological model of coal structure indicates that more free radicals must be created during coal pyrolysis in order to release the observed yield of tar than are observed by in situ ESR. The implication is that many
of the free radicals created by pyrolysis must be ESR silent. 3. The weight of evidence from kinetic calculations of the concentration of free-radical intermediates formed during pyrolysis confirm that the concentration of reactive free-radical intermediates is small compared with that from stable product free radicals. The observation of reactive coal free-radical intermediates with in situ ESR during thermal treatment, therefore, appears unlikely and remains to be demonstrated. Acknowledgment. We express our thanks to British Coal (CRE) for the supply of samples and the U.K. Science and Engineering Research Council for support under Research Grant GR/D/03581.
Activation Energy of Air-Oxidized Bituminous Coals Mollie L. E. TeVrucht and Peter R. Griffiths* Department of Chemistry, University o f California, Riverside, California 92521 Received April 6, 1989. Revised Manuscript Received May 22, 1989
The kinetics of air oxidation of a series of bituminous coals were investigated by using diffuse reflectance Fourier transform infrared spectrometry. A low-volatile bituminous coal, a medium-volatile bituminous coal, and two high-volatile A bituminous coals were ground to three particle sizes and oxidized at 150,200, and 250 OC. Activation energies and rate constants for the air oxidation of these coals were determined by tracking the decrease in the intensity of the aliphatic C-H band centered near 2920 cm-'. The air oxidation is found to obey pseudo-first-order reaction kinetics if oxygen is present in unlimited supply. The rate of oxidation was found to depend on particle size, rank, and temperature but to be independent of geographic origin and mineral content for the coals studied. Activation energies were calculated to be between 25.6 and 26.6 kcal/mol.
Introduction Oxidation has a number of important effects upon the chemical and physical properties of coal. It can alter the coal's coking properties, the amount of extractable material available from the coal, and the tar yield of the coal.'P2 The effects of oxidation have been studied by using a variety of analytical techniques including Mossbauer spectro~copy,~ Audibert-Arnu d i l a t ~ m e t r y ,Gieseler ~ plastometry,3i4X-ray photoelectron spectroscopy (XPS),3v4 fluorescence spectroscopy? and infrared ~pectrometry.~-" (1) Berkowitz, N. A n Introduction t o Coal Technology; Academic Press: New York, 1979; pp 95-105. ( 2 ) Gray, R. J.; Rhoades, A. H.; King, D. L. Trans. SOC.Min. Eng. AIME 1976,260, 334. (3) Huffman, G. P.; Huggins, F. E.; Dunmyre, G. R.; Pignocco, A. J.; Lin, M.-C. Fuel 1985, 64, 849. (4) Wu, M. M.; Robbins, G. A.; Winschel, R. A.; Burke, F. P. Energy Fuels 1988, 2, 150. (5) Kister, J.; Guiliano, M.; Mille, G.; Dou, H. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1987,32, 21. (6) Donini, J. C.; LaCour, S. A.; Lynch, B. M.; Simon, A. Coal: Phoenix '80s. h o c . CIC Coal Symp., 64th, 1981, 1981, 1, 132. (7) Kister, J.; Mille, G.; Dou, H. C.R. Acad. Sci., Ser. 2 1986,302,621. (8) Gethner, J. S.Appl. Spectrosc. 1987, 41, 50. (9) Gethner, J. S. Fuel 1987, 66,1091. (10) Rhoads, C. A.;Senftle, J. T.; Coleman, M. M.; Davis, A.; Painter, P. C. Fuel 1983, 62, 1387. (11) Lynch, B. M.; Lancaster, L.-I.;MacPhee, J. A. Energy Fuek, 1988, 2, 13. (12) Fredericks, P. M.; Moxon, N. T. Fuel 1986,65, 1531. (13) Chien, P.-L.;Markuszewski, R.; McClelland, J. F. Prepr. PapA m . Chem. SOC., Diu. Fuel Chem. 1985, 30, 13.
0887-0624/89/2503-0522$01.50/0
Although the effect of oxidation upon coal properties has been well characterized, the kinetics and mechanism of oxidation are not fully understood. It is not known which step in the oxidation is the rate-limiting one. If oxidation is diffusion-controlled, a coal that is finely ground will oxidize more quickly because of its higher surface area to volume ratio. However, if some other reaction step limits the rate of oxidation, all particle sizes should oxidize at an equal rate. The parameter of particle size has been investigated by several researchers, with conflicting results. In a study of oxidized coal using photoacoustic Fourier transform infrared spectrometry, McClelland and co-workers found that coal particles (44-125 pm) appeared to oxidize uniformly rather than to a decreasing degree with depth.13 Their results imply that the rate of oxidation is not diffusion-limited. Carpenter and Sargeant determined that coal crushed to 50-100-pm particles had an oxidation rate independent of particle size.18 Using much larger particles (14) Fuller, M. P.; Hamadeh, I. M.; Griffiths, P. R.; Lowenhaupt, D. E. Fuel 1982, 61, 529. (15) Griffiths, P. R.; Wang, S.-H.;Hamadeh, I. M.; Yang, P. W.; Henry, D. E. Prepr. Pap.-Am. Chem. Sot., Diu. Fuel Chem. 1983,28, 27. (16) Smyrl, N. R.; Fuller, E. L., Jr. In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E. L., Jr., Ed.; ACS Symposium Series 205: American Chemical Society: Washin-n, DC, 1982: p 133. (17) Fuller, E. L., Jr.; Smyrl, N. R. Fuel 1985, 64, 1143. (18) Carpenter, D. L.; Sargeant, G. D. Fuel 1966,45, 311.
0 1989 American Chemical Society
Air-Oxidized Bituminous Coals
4000
3000
Energy & Fuels, Vol. 3, No. 4, 1989 523
2000
1000
Wavenumbers (cm-1)
Figure 1. Diffuse-reflectance infrared spectra of low-volatile bituminous coal (PSOCNo. 616) before oxidation,after 15 min at 250 O C . and after 100 h at 150 O C .
(0.64-2.31 mm), Karsner and Perlmutter reported on fixed-bed reactor oxidation studies of seven different coals, including a lignite and high-volatile C bituminous (hvCb), high-volatile B bituminous (hvBb) and high-volatile A bituminous (hvAb) The hvCb coal was the most porous and most reactive coal, and its oxidation rate was independent of particle size at 150 "C. However, its oxidation at 200 OC proceeded more quickly for smaller particles than for larger ones. Intermediate results were obtained at 175 OC. The lignite and other high-volatile bituminous coals, which were all less porous than the hvCb coal, demonstrated an inverse relationship between oxidation rate and particle size at all three temperatures. Although many analytical methods provide valuable information regarding the chemical and physical changes of the coal, infrared spectrometry is the most valuable for studying oxidation on a molecular level. Despite its relative insensitivity to small degrees of oxidation? it provides information concerning the molecular structure of the whole coal. Oxidation causes changes in this structure, and resulting transformations are seen in the infrared spectrum (Figure 1). The intensity of the carbon-hydrogen aliphatic stretching mode near 2920 cm-' decreases with increasing oxidation. A band attributable to an aromatic C=O moiety grows in near 1690 cm-', and there are differences in the C-0 stretching bands, due to phenolic and ether groups, centered near 1200 cm-'. Studying the evolution of these changes with increasing oxidation of the coal allows the kinetics of the process to be analyzed. Huffman, Huggins,and co-workers introduced the use of an oxidation index to quantify the spectral changes concomitant with coal o x i d a t i ~ n . ~ This l ~ > ~oxidation ~ index is computed by dividing the integrated band area in the carbonyl region (1850-1635 cm-') by that of the C-H stretching region (3194-2746 cm-'). A similar analysis used by Kister et al. employs the integrated band intensity of the C-H stretching modes ratioed to the sum of the integrated band intensities in the C-H stretching region plus the 2000800-cm-' Although substantial changes are seen in the intensities of both the C-H and C=O stretching modes during the air oxidation of coals, those changes are not due to the same oxidation reaction step. The C-H groups may be initially oxidized to a variety of products other than car(19) Karsner, G . G.; Perlmutter, D. D. AZChE J . 1981, 27, 920. (20) Huggins, F. E.;Huffman,G. P.; Dunmyre, G . R.; Nardozzi, M. J.; Lin, M. C. Fuel Process. Technol. 1987, 15, 233.
bonyls, such as ethers, hydroperoxides, alcohols, and phenols.6J0J1 Those moieties may in turn be oxidized to aldehydes, ketones, carboxylic acids, and anhydrides.6J1 Ratioing the band intensities from the two regions can provide no clear answer, because different mechanistic and kinetic processes produced the changes in each region. Thus, it is best to analyze one region of the spectrum. The C-0 stretching mode region suffers from severe band overlap, and accurate measurement of band intensities is precluded. Although the C=O stretching region is less complicated, many of the oxidation product features appear as shoulders on the intense 1600-cm-' aromatic band, reducing the accuracy of band measurements; in addition, the absorptivities of C = O stretching bands vary with the chemical nature of the carbonyl group. The absorptivity of the 1600-cm-' band is strongly dependent on the ring substituents and will be expected to change significantly during oxidation if phenols and aromatic aldehydes, ketones, and carboxylic acids are formed. Thus, using this band as an internal standard, as has been done by Rhoads et al.,1° is probably not viable. In this study, the intensity of the C-H aliphatic stretching mode band near 2920 cm-' was selected. Fredericks and Moxon concluded that the C-H aliphatic mode stretching region was the spectral region most sensitive to oxidation by comparing the infrared spectra of a series of oxidized coals to the data from crucible swelling number measurements.12 That spectral region has the additional benefit of being well isolated from other bands in the coal spectrum. Although the broad 0-H stretch of carboxylic acids underlies the C-H stretching band, measurements of the C-H band intensity are easy to obtain. The absorptivity of the aliphatic C-H stretching mode is intermediate in strength, which is advantageous because very strong bands tend to exhibit the greatest sensitivity to slight sample preparation differences and very weak bands are heavily influenced by the noise in the spectrum. For these reasons the region between 3100 and 2800 cm-' is ideal for the study of coal oxidation. Fourier transform infrared (FT-IR) spectra may be acquired of coal by a variety of means. Researchers have utilized coal pressed in alkali-metal halide p e l l e t ~ , ~ ~ ' J ~ J ~ thin sections of ~0al,8,~ and diffuse-refle~tance?,~J"'~~~ and photoacoustic s p e ~ t r o m e t r i e s . ~ J Diffuse ~J~ reflectance spectrometry was chosen as the method of sample presentation for this work because it offers several advantages over other infrared spectrometric techniques. Minimal sample treatment is involved: no further grinding is required after the oxidation step, and high pressures are not needed to pack the sample cups. Also no diluent is necessary, thus eliminating the interference from the OH stretching mode of water and the sloping base line associated with dispersing the sample in an alkali-metal halide matrix. Unlike photoacoustic spectrometry, diffuse reflectance spectrometry is a bulk technique, capable of probing past the surface of a particle. Diffuse reflectance spectra in the midinfrared region are typically presented in Kubelka-Munk units, as the intensity of a peak in a Kubelka-Munk spectrum is proportional to the concentration of the functional group giving rise to that absorption in the spectrum. Since the absorptivities of the CH stretching modes and the scattering coefficient of coal in the midinfrared region are not known, the absolute concentration of that group in the coal at any given time cannot be determined. However, by ratioing the intensity of the C-H band at time t against its initial intensity, a measure of the relative concentration of CH-containing moieties is obtained.
524 Energy & Fuels, Vol. 3, No. 4, 1989
TeVrucht and Griffiths High Volatile A Bitum!nous Cool
Table I. Data from Ultimate Analyses" PSOC no. rank % C % 0 '70 H seam 616 lvb 87.31 6.50 4.68 Kelly, PA 1139 mvb 87.65 5.30 5.30 Lower Kittanning, PA 1100 hvAb 85.96 6.58 5.85 Pittsburgh, PA 504 hvAb 80.05 12.14 6.33 Upper Sunnyside,
(XI 100)
% ash
(dry) 7.56 6.64 9.52 12.67
UT Parr, dmmf.
Because the effect of particle size on oxidation rate is the subject of some controversy, we chose to study the oxidation of bituminous coals as a function of rank, particle size, and temperature. Coals of three ranks were chosen to determine if lower rank coals oxidize more quickly than higher rank coals, as has been r e p ~ r t e d . To ~ ~test ~ ~ the influence of geographic origin, two hvAb coals, one mined in Pennsylvania and the other in Utah,were investigated. An additional difference between these two coals was the mineral content. The low-temperature oxidation of coal has been shown to be accelerated by pyrite oxidation.20 Since the two hvAb coals have differing mineral content, the effect of mineral content on coal oxidation could also be investigated. The effect of temperature on oxidation rate was explored by reacting the coals at three temperatures: 150,200, and 250 "C. It is commonly believed that three general types of oxidative mechanisms exist for coal. One operates below 100 or 125 "C,* another operates up to around 300 "C, and a third operates above that. In the region chosen for study (150-250 "C), the mechanism of oxidation should be the same at all three temperatures.' In Figure 1,the spectra of two oxidized low-volatile bituminous coals are compared; the spectrum of the untreated coal is also shown. Although one coal was oxidized at 150 "C while the second was reacted at 250 "C, the spectra are essentially identical except for very small changes in relative band intensity. The similarity between the spectra provides strong evidence that the same mechanism operates at both temperatures.
Experimental Section Four bituminous coals were chosen for study: one low-volatile coal, one medium-volatile coal, and two high-volatile A bituminous rank coals. Coals were obtained from the Penn State Coal Bank, and were stored in sealed jars. Data from the ultimate analyses of the coals are listed in Table I. Although the oxygen contents of the low-volatile bituminous and the Utah hvAb coals are somewhat there is no evidence of significant oxidation in the spectrum of the fresh coals. Each coal was ground to three separate particle size ranges before oxidation. By the use of a stainless-steel capsule with a Wig-L-Bug ball and mill grinder, the coals were ground to a small, medium, or large particle size. To minimize sample heating, particles were ground by undergoing cycles of 30 s of grinding followed by 30 s of resting. Although grinding coal has been reported to cause its oxidation: that effect was not observed in this study. As shown in Figure 2, the spectra of small, medium, and large particle size coals are identical. No spectral features attributable to oxidation are seen. The particulate coal was not sieved; size estimates are based on scanning electron micrographs of the ground coals. More than 90% of the particles in each size range fell into the following limits: small, 0.5-1.5 pm; medium, 5-10 pm; large, 10-20 wm. The remaining 10% was composed of minute particles adhering to the outside of the larger particles. Small particles were obtained by grinding (21)Rahman, M.; Hasan, A. R.; Baria, D. N. Proc. Intersoc. Energy Conuers. React. Conf. 1985,20th,1631. (22) Danberger, H. H.; Harvey, R. D.; Ruch, R. R.; Thomas, J., Jr. In The Science and Technology of Coal Utilization;Cooper, B. R., Ellingson, W. A., Eds.; Plenum: New York, 1984; p 9.
I
4000
3000
2000
1000
Waren"mbe< ( c m - , )
F i g u r e 2. Diffuse-reflectance infrared spectra of high-volatile A bituminous coal after grinding for 5 min (top), 3 min (middle), and 1 min (bottom). for a total of 5 min; medium and large particles experienced 3 and 1min of grinding, respectively. The gound coals were spread out in trays, and the oxidation reaction was carried out in air at standard pressure in an oven. Samples of coal were removed a t selected time intervals for analysis. Three samples from each particle size at each temperature for each coal were analyzed, and the results averaged. Diffuse-reflectance infrared Fourier transform (DRIFT) spectra were collected of the neat coals packed into 4.5-mm-diameter cups by using a modification of a Parr pellet press.23 Spectra were measured with a Digilab Model FTS-20 Fourier transform infrared spectrometer equipped with diffusereflectance optics of the type described by Fuller and GriffithP and an intermediate-range mercury-cadmium-telluride detector. Typically, 1000 scans at 4 cm-' resolution were coadded, and triangular apodization was employed. Single-beam spectra were ratioed against that of finely ground potassium chloride and then converted to the Kubelka-Munk format for analysis.
Results and Discussion To normalize the spectra taken of the oxidized coals, the intensity of the C-H aliphatic stretching mode (after base-line correction) was ratioed to that of the unoxidized coal of the same rank and particle size. These normalized values for the 200 "C oxidation of the four coals are plotted versus time in Figure 3. The data for all four coals display the same basic trends. The smaller the particle size of the coal, the faster its rate of oxidation. This behavior can be explained on the basis of the particles' surface-area-tovolume ratios if the reaction rate is diffusion-controlled. The smaller the particle, the larger its surface area in proportion to its volume. In order to be oxidized, the coal must be exposed to air (oxygen). Initially, the outer surface is attacked; as the oxygen diffuses into the particle, it can react with the bulk of the particle. The larger the surface area, the higher the effective concentration of coal available for oxidation at any given time and the higher the rate of reaction. Thus, the rate of oxidation appears to be diffusion-limited and is not controlled by some other reaction step even for very small particles. Coal has both a mineral component and an organic component that can undergo oxidation. The organic structural moieties having the greatest probability of reaction are likely to incorporate carbon and hydrogen, probably in a methylene bridge or similar structure. Since oxygen is in unlimited supply at the coal surface during the reaction, pseudo-first- or pseudo-second-order reaction (23) TeVrucht, M. L. E.; Griffiths, P. R. Appl. Spectrosc., in press. (24) Fuller, M. P.; Griffiths, P. R. Anal. Chem. 1978,50,1906.
Energy & Fuels, Vol. 3, No. 4, 1989 525
Air-Oxidized Bituminous Coals
I
1
0'.O h
08
-4
07
1
Low Vol Coal
kr616
4
E
t
4
'6
00 06 04
03 02 n .
. . . . . . . . . . . . . . . . . . . . . 0
XI
40
80
Bo
100
120
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1Bo
XI0
0
20
40
BO
80
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100
180
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Time (minutes)
Time (minutes) 09
: 0
20
40
BO
80
:
100
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IW
O
0
20
40
Bo
Bo
100
120
140
200
Time (minutes)
I
0
Time (minutes)
0
large 0 medium -/ small Figure 3. Variation of normalized C-H stretching intensities with time for 200 O C oxidation of four bituminous coals. kinetics should be observed. Thus, the change in concentration of a group with time will be proportional to its concentration raised to some power n, where n is probably equal to 1 or 2. -dC - kC" dt -dC - k dt C"
--
--
-&rC-"
dC = k i o cdt
(3)
Rearranging and integrating causes two ca s to become apparent. for n = 1 In ( C / C o ) = -kt (4) for n = 2
1 / c = kt
+ l/C,
-CO-- Cokt + 1
C For a first-order reaction, a plot of the natural log of the relative concentration ( C / C o )versus time will be linear; for a second-order reaction, Co/C will show a linear increase with time. To determine whether coal oxidation is (pseudo) first or second order, C / C owas approximated as I/Io,where I is the Kubelka-Munk intensity of the C-H stretching mode centered at 2920 cm-'; both In (C/C,) and
Co/C were plotted versus time. Sample plots for the 250 "C oxidation of the hvAb and the mvb coals are shown in Figure 4. The logarithmic function of band intensity clearly provides a more linear relationship with time than does Co/C,although some deviation from linearity is seen in the data from the small particle size hvAb coal. In fact, the plot of Co/C versus t is so nonlinear that it was impossible to construct a reasonable best-fit line; accordingly, those data are plotted without one. This result indicates that the air oxidation of bituminous coals obeys pseudofirst-order kinetics. The nonlinearity of the data from the oxidation of the small particle size fraction of the hvAb coal has several causes. For the first 90 min or so of the oxidation, any small temporal imprecision in removing samples from the oven will result in a large error in the measured degree of oxidation of the coal. Also, the oxidation is occurring so rapidly that the reaction is not instantaneously quenched when the sample is taken from the heated oven. Toward the latter stages of oxidation, the coal is essentially completely oxidized; thus, measuring the very small residual C-H stretching bands introduces another source of error. Rates of reaction can be compared quantitatively by calculating the rate constants. Relative rate constants for each coal are presented in Table 11. Both high-volatile coals oxidize more rapidly than does the medium-volatile coal, which is in turn more reactive than the low-volatile coal. That is, as the rank of bituminous coals increases, their susceptibility to oxidation decreases. One can rationalize this result in terms of the porosity of coal. As we progress from hvA through to low-volatile bituminous
526 Energy & Fuels, Vol. 3, No. 4, 1989
Te Vrucht and Griffiths
hvA Bituminous Coal
mv Bituminous Coal
small
\j\\\jl 20
0
40
60
80
100
120
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,
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/ I
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Time (minutes)
Figure 4. First- (A,C) and second-order (B,D) fib for the 250 "C oxidation of hvAb coal (PSOC No. 1100) (A, B) and mvb coal (PSOC No. 1139) (C, D). HVA Bituminous Coal
Table 11. Relative Rates of Oxidation
PSOC no.
T, OC
small
particle size medium
616
150 200 250 150 200 250 150 200 250 150 200 250
2.99 114 1277 4.42 260 1745 6.73 287 2110 6.62 265 1972
1.83 67 717 2.87 124 1000 4.07 132 1406 4.00 122 1361
1139 1100 504
Arrhenius Plot
- 1 small mediumo\
large 1.00 37 443 1.38 62.5 552 1.83 76.5 636 1.68 68.4 585 0 0018
coals, the porosity decreases.25 This means that, for a given particle size, the surface area to volume ratio will be smaller for a low-volatile coal than a high-volatile coal. If the same argument used to explain the differences in rate for different particle sizes is applied, this implies the lower rank coal should react more quickly. It has been reported that there is a correlation between the percent oxygen in a coal and its oxidative rate. Lynch et al. measured the degree of oxidation of bituminous coals using the intensity of the 1690-cm-' band, and they found that coals with a higher percentage of oxygen oxidized more quickly." No such correlation is seen for the four coals in this study. For example, as shown in Table I, one hvAb coal used in this study (PSOC504) has nearly twice the oxygen content (25) King, J. G.; Wilkins, E. T. Proceedings of the Conference on Ultrafine Structure of Coals and Cokes; BCURA London, 1944; p 46.
0 002
0 0022
0 0024
l/Temperature (K)
Figure 5. Arrhenius plot for hvAb coal (PSOC No. 1100).
(12.14%) of the other (PSOC1100; 6.58%), yet their rates of oxidation are very similar. As would be expected, the reaction rates increase with increasing temperature for each coal in each particle size. The temperature dependence of the rate constant can be evaluated by using an Arrhenius plot. The Arrhenius equation relates the rate constant, k, to a function of the preexponential factor A and the activation energy E,: k = A exp(-E,/RT) (7) A plot of In k versus 112' should be linear, with a slope equal to -E,/R and an intercept proportional to the preexponential factor. An Arrhenius-type plot for one hvAb coal is shown in Figure 5. For each coal, the acti-
Air-Oxidized Bituminous Coals
Energy & Fuels, Vol. 3, No. 4, 1989 527
mvb
-D 1 2
h
5
$
4
I
263
262 261 26 258 256 257 256 255 254
253
c.-
hvAb
hvAb
.
1
-
252 r 25;5
t
i
T
l -
-
14
12
i
, 18
16
Wt. % c/ Wt. % H
Figure 6. Activation energies determined for the four coals. The range for each coal encompasses values calculated for each of the three particle sizes. Table 111. Activation Enerdes for Each Coal E,, kcal/mol PSOC no. small medium laree av ~
616 1139 1100 504
26.8 26.5 25.4 25.2
26.2 25.9 25.8 25.7
26.9 26.5 25.9 25.9
~~~~
26.6 26.3 25.7 25.6
vation energy for the oxidation was determined from the slope of the Arrhenius plot, and the activation energies for the four coals are shown in Figure 6. Values of E, were obtained for each particle size; see Table 111. Although previous studies have indicated that the activation energy of oxidation depends upon the particle s i ~ e , ~no ~ ,such ~' relationship is seen from our data. The spread in activation energy values for each coal illustrates the magnitude of the error involved in the determination. Despite the relatively large error limits, the data obviously indicate that the high-volatile coals have the lowest activation energy and the low-volatile coal has the highest. Average acti-
vation energies for each coal are included in Table 111. In comparison, activation energies for the production of carbonic gases have been reported in the range 16.2-25 kcal/mol for three high-volatile bituminous coals oxidized in a fixed-bed reactor.% Using XPS, Kelemen and Freund reported an oxidation activation energy of 11.4 kcal/mol for an Illinois No. 6 bituminous coal,29which is in obvious disagreement with our result. Since they used surface oxygenf carbon ratios to determine the activation energy, they were measuring kinetics of more than one reaction. Thus, the lack of agreement with our results is not surprising. Other workers calculated activation energies for the oxidation of a hvAb coal from data from several methods, including Gieseler maximum fluidity, heating value, and oxygen and carbon ont tent.^ Their results (14 f 4, 12 f 3, 13 f 1, and 11.7 f 0.3 kcal/mol, respectively) are much lower than the values obtained from our analysis. The spread of activation energies may be due to differences between the porosity of the coals, or it may be indicative of variations between the methods of determination. Conclusions In summary, the kinetics of coal oxidation can be determined from a study of the diffuse-reflectance infrared spectra of oxidized coals. Analyzing the attenuation of the carbon-hydrogen stretching mode intensity with increasing oxidation allows rate constants and activation energies for the oxidation reaction to be found. The smaller the particle of coal, the faster it undergoes oxidation, indicating that oxidation of the organic component of coal is a diffusion-limited process. Ease of oxidation shows an inverse correlation with rank, but the amount of mineral matter in the coal and the geographic origin have no apparent effect for the coals investigated in this study.
Acknowledgment. We gratefully acknowledge support from the Department of Energy under Grant No. DEFG22-87PC79907. ~
(26)Carpenter, D.L.; Sergeant, G . D. Fuel 1966,45,311. (27)Carpenter, D.L.; Sergeant, G . D. Fuel 1966,45,429.
(28)Karsner, G.G.;Perlmutter, G. G . Fuel 1982,61, 35. (29)Kelemen, S. R.;Freund, H. Presented at the 196th National Meeting of the American Chemical Society, Loss Angeles, CA, 1988,paper FUEL 57.
