Energy & Fuels 1988,2, 721-722
X-ray Studies of Coal Oxidation
Sir: A reasonable model of the structure of coal consists of ordered and disordered domains. In this model the ordered domains consist of tiny regions composed of planar, condensed, aromatic rings stacked in parallel. The disordered domains are composed of various connecting chains as well as terminal, organic groups. X-ray diffraction can detect the ordered domains in the structure which give rise to a diffraction maximum similar to the spacing of the graphite 002 peak, with d-spacing values of 3.54-3.43 A for coals with 84-94% C.' The diffractions observed are broad because the domains contain very few planes of atoms. The increase in diffraction widths with decrease in crystal size has been described by Warren.2 Ordered domain development and the 002 graphite-like diffraction are considered to increase with coal rank. It has long been generally accepted that coals of low rank are more easily oxidized than those of high rank.3 Experiments on H202oxidation of coals have confirmed this general rule.41~Reaction of anthracite with 30% Hz02at room temperature has been reported to oxidize organic material in the coal only below pH 1.5 unless the coal has been previously demineralized by acid? Studies on NaClO oxidation of c0al7v8indicate that the connection between rank and extent of reaction depends on reaction conditions. The coals used in this study are listed in Table I along with their analyses for carbon, hydrogen, and volatile matter and the relative height of the 002 graphite-like diffraction. The high volatile bituminous coal is a washed and blended coal from Clearfield County, PA, the low volatile bituminous coal is from the People's Republic of China, and the anthracite is from the Mammoth Seam, Schuylkill County, PA. The coal samples were ground and sieved to 250 pm or less. The coal samples were dried at 105 OC for 1.5 h before reaction. A nitrogen atmosphere was used in reactions with bituminous coals. Oxidations were run at 75 OC. Except where noted otherwise, 3% H202(pH 3.3) was reacted for 18 h and 5% NaOCl was reacted for 4 h. Following reaction, the samples were centrifuged, washed, and dried at 105 "C to constant weight. X-ray diffraction patterns were run on a modified GE XRD-5 diffractometer using Cu K a radiation. The height of the 002 graphite-like diffraction was determined by comparison with a diffraction from 5% by weight NaF, which was added to the samples. Table I1 gives the weight changes observed for the reactions of coal samples with 5% NaClO and 3% HzOz. These results are consistent with a model in which the NaClO reacts better with a more ordered coal structure, whereas the HzOzreacts better with a less ordered coal structure. Thus 5% NaOCl reacts more extensively with higher rank samples, and 3% H202reacts more extensively with lower rank samples. The effect of the oxidation reactions on the ordered domains in the samples was tested by (1) Gerstein. 3.C.: MurDhv. P. D.: Rvan. L. M. In Coal Structure:
Meyers, R. A., Ed.; Academi; 'New Yoik, 1982;pp 87-129. (2) Warren. B. E.Phvs. Reu. 1941.59. 693-698. (3j Krmers, W. S. 1n"Progress in Coal'Science; Banghorn, D. H., Ed.; Interscience: New York, 1950. (4) Nalwalk, A. J.; Friedel, R. A.; Queiser, J. A. Energy Sources 1974, I, 179. (5)Ward, C. R. Fuel 1974, 53, 220. (6) Heard, I.; Senftle, F. E. Fuel 1984,63, 220-226. (7)Chakrabartty, S. K. Organic Chemistry of Coal; ACS Symposium Series 71; American Chemical Society: Washington, DC, 1978; pp 131-141. (8) Chakrabartty, S. K. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1981, 26, 10. I~
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Table I. Analytical Results for Coals Studied material %C %H % vol 002 height, high-vol bit. coal 85.4 5.2 31.5 5.6 low-vol bit. coal 87.6 4.0 15.9 6.4 anthracite coal 94.4 1.8 3.7 7.0
A
Table 11. Weight Changes Resulting from the Reactions of the Samples with 5% NaOCl and 3% H20zn % w t change material NaOCl reacn Hz02 reacn high vol -10.4 f 3.2 (2) -7.4 f 3.8 (3) -12.2 f 2.5 (2) -6.7 f 0.1 (2) low vol anthracite -18.7 f 4.8 (5) -4.9 f 0.8 (3) Numbers in parentheses denote the number of runs taken to determine each value.
Table 111. Relative Heights of the 002 Diffractions of Reacted Samples Compared with Unreacted Samples" re1 height material 5% NaOCl 3% H,O, high vol 1.4 f 0.1 (2) 0.64 (1) low vol 0.86 (1) 1.00 f 0.2 (3) anthracite 0.67 f 0.03 (2) 1.13 f 0.02 (2) "Numbers in parentheses denote the number of runs taken to determine each value.
Table IV. Extent of Reaction of Excess 5% NaOCl with Anthracite as a Function of Time reacn mol of NaOCl/g reacn mol of NaOCl/g time, h of coal time, h of coal 0.50 0.030 4.0 0.044 1.0 0.039 12.0 0.044 2.0 0.043 Table V. Weight Losses Based on the Weight of the Original Sample for Alternating Reactions of NaOCl and H20z with Anthraciten reacn reagent w t loss, % original sample 1 NaOCl 12.8 (4) 2 Hz02 16.5 (4) 3 NaOCl 6.8 (4) 4 Hz02 13.9 (3) 5 NaOCl 7.9 (3) "The number of runs for each determination is given in parentheses.
Table VI. Widths at Half-Height and the d Spacings of the Graphite-like Diffraction Maxima for Anthracite Reacted Alternatingly with 5% NaOCl and 3% H202 A(26'), deg d spacing, 8, native coal 6.0 3.520 NaOCl 7.5 3.518 HzO2 6.0 3.525 NaOCl 8.5 3.520 H202 6.0 3.521 NaOCl 7.5 3.522
measuring the heights of the 002 diffractions in the reacted samples and comparing with the unreacted materials. The results shown in Table I11 indicate that for the two higher rank coals the ordered domains were degraded in the NaOCl reactions and removal of disordered material by H202made the ordered material more prominent following reaction by this reagent. In order to determine the limiting extent of reaction of anthracite coal with NaOCl, a measured excess of 5% NaOCl was reacted with anthracite for increasing times. The extent of reaction was determined by iodometric titration of the NaOCl remaining. The results given in Table 0 1988 American Chemical Society
722 Energy & Fuels, Vol. 2, No. 5, 1988
Communications
Table VII. Extent of Reaction as a Function of Particle Size for the Reaction of Anthracite with 5% NaOCl and 3%
HzOz wt loss, %
size, fim 250-149 149-105 105-74
