Energy & Fuels 1987,1, 349-351
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Oxidative Weathering of Powder River Basin Coal James J. Isaacs and Ronald Liotta* Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received September 12, 1986. Revised Manuscript Received February 2, 1987 The oxidative weathering of a subbituminous coal has been studied, and the chemistry is compared to that of a higher rank bituminous coal. The Powder River Basin coal undergoes significant decarboxylation when it is exposed to the atmosphere at ambient temperature. This is not a factor in the chemistry of an Illinois bituminous coal, since this higher rank coal is nearly void of carboxylic functionality. There is oxidatively added oxygen to both coals that is not 0-alkylatable, which rules out the possibility it is one of the various hydroxyls or carboxylic groups. By monitoring the oxygen functional group distribution of a coal undergoing the weathering process, we correlated the changes that occur in its properties to the extent of oxidation.
Introduction The oxidative weathering of a bituminous ranked coal, Illinois No. 6, has been previously reported.' Weathering significantly affecta many of the properties of such coals. For example, the plastic properties of bituminous coal are destroyed upon weathering. Thus, a thermoplastic coal converts to a thermosetting coal.24 Another example would be that oxidatively weathered coals give substantially lower liquid conversions in standard autoclave hydroliquefaction runs.6 In both these cases, the proposed explanation for the altered properties of the coal involved the formation of cross-links.41~Free-radical processes likely account for both ether and carbon-carbon bond formations, resulting in a higher density material.'Ve Corresponding changes during oxidative weathering of lower-ranked coals had not been reported since these coals are thermosetting. Weathering does not lower their conversion properties quite as much as bituminous coals.' However, researchers have realized that certain fundamental aspects of lower ranked coals changed when they were exposed to the atmosphere? We chose to undertake a study to determine what chemical reactions were occurring during the weathering of Powder River Basin subbituminous coal, Smith seam. Furthermore, we correlated the chemical modifications with the properties associated with the weathered coal. Experimental Section A fresh consignment of 30 X 60 mesh coal was received from a mine near Gdette, WY. A trained geologist collected the coal, ground and sieved the sample under nitrogen, and placed the consignment into a polyethylene bag with excess water. The sample was purged with nitrogen and then was sealed in a can suitable for shipment. Upon receipt, the consignment was vacuum-oven-dried at 90 "C for 2 days and transferred to a drybox, and samples were put in crystallizing dishes with porous paper caps. If the coal was not dried first, the oxidation process is very slow. The water both in the pores and the bulk water present (1)Liotta, R.; Brons, G.; Isaacs, J. Fuel 1983,62, 781 and references cited therein. (2) Painter, P. C.; Snyder, R. W.; Pearson, D. E.; Kwong J. Fuel 1980, 59, p 282.
( 3 ) Maloney, D. J.; Jenkins, R. G.; Walker, Jr., P. L. Fuel 1982,61,175. (4)Painter, P. C.; Coleman, M. M.; Snyder, R. W.; Mahajan, 0.; Komatau, M.; Walker, Jr., P. L. Appl. Spectrosc. 1981, 35, 106. ( 5 ) Chang, C. Y.;Guin, J. A.; Tamer, A. R. J. Chinese Chem. SOC.1981,
28, p 155.
(6) Wachowska, H. M.; Nandi, B. N., Montgomery, D. S. Fuel 1974,
53, 212.
(7) Swann, P. D.; Evans, D. G. Fuel 1979,58, 276. (8) Dack, S. W.; Hobday, M. D.; Smith, T. D.; Pilbrow, J. R. Fuel 1983, 62, 1510.
Table I. Compositional Analyses of Fresh and Weathered Powder River Basin Coals davs of weathering 0 7 17 30 45 156 %C 68.4 67.2 67.9 66.9 67.9 68.3 4.6 4.4 4.3 %H 4.8 4.8 4.5 % N 0.8 0.9 0.9 0.9 0.8 0.8 0.5 0.6 0.6 0.4 0.6 % ST 0.4 6.9 7.2 7.3 6.4 % ash 7.1 6.5 Table 11. Compositional Analyses of Fresh and 156-Days-Weathered Powder River Basin Coals davs of weathering 0 156 (a) After 0-Perdeuteriomethylation % C 70.7 69.7 %H 4.6 4.9 %D 2.6 2.3 (b) After Acid Hydrolysis 70.9 % H 5.0 %D 1.4 %C
70.5 4.8 1.5
helps prevent the oxidation process. In order to accelerate the chemistry, other researchers have performed the oxidation at elevated temperatures. We, instead, chose to merely dry the coal and oxidize it at low temperatures. These dishes were exposed to the atmosphere for varying lengths of time prior to analysis. The bulk of the fresh coal was maintained in a sealed bottle in the drybox until the end of the weathering experiment, at which time its full compositional analysis was also obtained. Samples were analyzed at 0,7,17,30,45 and 156 days of weathering (Table I). Each one of the samples was exhaustively extracted under Soxhlet conditions with tetrahydrofuran (THF). The weight percent extractable for the fresh coal was 7%. By the time the coal was extensively weathered at 156 days, this value had dropped to 2.5%. Prior to the 0-alkylation of a low-rank coal, citric acid pretreatment is required to remove any ion-exchanged carboxylic acids and reprotonate them. For the subbituminous coal used in this study, we found 0.7 (per 100 carbons) carboxylic acid in the metal salt form. The fresh and the extensively weathered coals were 0-perdeuteriomethylated, and the resulting derivative was then further derivatized with an acid hydrolysis. This double derivatization technique was employed in order to ascertain both the acidic hydroxylic and carboxylic acid functionalities in each material studied? Analyses were performed according to ASTM procedures, and the average of duplicate runs are reported. It is very difficult if not impossible to ascertain a specific trend in the gross composition of this weathering coal on the basis of the inspections presented in Table I. Thus, selective derivatization (9) Gouker, T.; Liotta, R. Fuel 1985, 64, 200.
0887-0624/87/2501-0349$01.50/00 1987 American Chemical Society
Isaacs and Liotta
350 Energy & Fuels, Vol. 1, No. 4, 1987 12700~
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Figure 1. Comparison of the O/C and H/C ratios for weathering of Illinois No. 6 coal. procedures are required to elucidate the chemistry of oxidation. Citric Acid Pretreatment. A 1.0 M aqueous citric acid solution (50 mL) was mixed with 1.0 g of coal and heated to reflux under nitrogen atmosphere for 3 h. The reacton vessel was cooled, the solid coal product was filtered and washed with distilled water (1 L). The reaction was also performed in similar fashion using citric acid-d,. The compositional analysis of this latter product revealed that 0.7 deuterium/100 carbons were incorporated. Perdeuteriomethylation Procedure. The reaction conditions, workup procedure, and methods of analysis (Table IIa) were the same for all samples. The reactions were performed entirely under a nitrogen atmosphere. The following reagents were mixed and allowed t o stand overnight: 5.0 g ofkoal, 25.0 mL of 1.0 M aqueous t e t r a - n - b u t y h o n i u m hydroxide (polarographic grade), 100 mL of freshly distilled tetrahydrofuran (THF) and 7.25 g (50 mmol) of perdeuteriomethyl iodide (99%). At this point, 1.0 M HC1 was added to lower the pH to 7, with mixing continuing for 1 h. The volatile materials were distilled under vacuum and a 50/50 mixture of methanol/water was then added to redissolve the quaternary ammonium salts. The product was washed with hot methanol/water. When the filtrate gave a negative test with silver nitrate, the alkylated coal was determined to be clean. The solid product was dried first under a stream of nitrogen and then in a vacuum oven at 90-100 OC for 48 h. Hydrolysis of 0-Perdeuteriomethylated Coal. The same procedure was used for all the samples (Table IIb). The alkylated coal (2.0 g) was placed in a 250-mL round-bottom flask with concentrated HC1 (>30%) and heated to reflux overnight. At this point, the slurry was filtered (medium-porosity glass frit) and washed with 1 L of water. The product was first dried under a stream of nitrogen and then in a vacuum oven (90-100 O C ) for 48 h.
Results and Discussion During t h e study of oxidative weathering of Illinois No. 6 coal, we found t h a t t h e chemical composition of the coal varied u p to about 2 months, a n d t h e n no further change was observed for a n additional 2 months. As shown in Figure 1, t h e H / C ratio decreased a n d t h e n leveled off. T h e O / C ratio rose dramatically at first a n d t h e n , after 2 months, t h e coal appeared t o be fully oxidized. These observations were i n full agreement with those of other researchers who studied caking coals under similar conditions. N o t unexpectedly, t h e calorific value of t h e weathered Illinois No. 6 coal, as determined by t h e method of NeavellO showed a predictable decrease in heat content as extent of oxidation increased (Figure 2). Chemical (10) CV - 151.31 X C + 549.74 X H(org) + 58.96 X (5(py) + 47.58 X S(org) - 400.24. This formula requires the compositional analysis to calculate the calorific value, CV. The calculated values for CV correspond very closely with the direct CV measurements: Neavel, R. C., private communication.
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Figure 4. Calorific values for weathered Powder River Basin coal. analysis of these weathered Illinois No. 6 coals revealed t h a t t h e oxygen content steadily increased, while t h e carbon content declined, thus producing this significant drop a n d heat content of t h e coal. The results found for t h e weathered Powder River Basin coal uncover a different chemistry from t h a t of t h e previously studied Illinois No. 6 coal. T h e change in t h e compositional analysis did not follow a steady trend; this can be most easily seen in Figure 3. T h e H / C ratio starts o u t relatively high at about 0.84 a n d then, after a slight rise, drops sharply over t h e first 45 days of weathering t o a value of 0.76. However, at long exposures, 156 days, t h e H / C value recovers t o nearly 0.79. The O/C ratio rises during t h e first part of t h e weathering a n d then drops off, a n d at long reaction time, t h e final O/C ratio is about midway between its value at t h e point of highest oxygen incorporation a n d that for fresh coal. T h e r e appears t o be more t h a n one mechanism a t work here t h a t would
Weathering of Powder River Basin Coal Table 111. Physical Properties Change during Weathering % THF solvent % vol (dmmQ extract swelling Illinois No. 6 coal 42.8 21.3 3.54 severely weathered 40.1 14.5 2.95 Powder River Basin 43.5 7.0 2.7 severely weathered 39.6 2.5 2.4 Table IV. Comparison of the Oxygen Functional Group Distributions for Illinois No. 6 and Powder River Basin Coals effect of weathering no. of groups per 100 carbon atoms nonalkycoal 0-H C0,H latable 0 tot. Illinois No. 6 coal 4.8 0.3 4.6 10.0 severely weathered 4.9 0.3 7.1 12.6 Powder River Basin 4.0 3.9 7.6 19.4 4.3 2.6 11.0 20.5 severely weathered
account for such behavior. The calorific values of weathered Powder River Basin coal are shown in Figure 4. Again, by use of the procedure of Neave1,'O it is observed that the long exposure time actually causes the surprising result in that severely weathered Powder River Basin coal has a higher heat content than the coal that was exposed to the atmosphere for only about 17 days. Since the dollar value associated with this is determined by the calorific content, these findings have economic implications. In order to help elucidate some of the chemistry responsible for these findings, we decided to probe the cross-linked density of the weathered Powder River Basin samples. As shown in our previous work, as the cross-link density of the coal increases, its volatility decreases and the coal bitumen becomes part of the macromolecular network so the percent extractable also decreases. The propensity for a coal to undergo solvent swelling is the classic measure of its cross-link density. As shown in Table 111, we compare these three properties for both Powder River Basin and Illinois No. 6 coals before and after oxidative weathering. The volatility of all weathered samples in each study steadily declined as a function of each exposure and so did the tetrahydrofuran extractable component. The extent of swelling for the coals was, in each case, found to be less after it was air oxidized (only initial and most severely weathered values are shown). So this information is wholly consistent with the cross-linking mechanism that had been previously reported. However, it does not explain the correlation between time of exposure and oxygen content of the weathered Powder River Basin coal. Since oxygen functionality appears to be playing an important role in the chemistry, we performed an oxygen functional group analysis to determine the distribution of the various carbon oxides in the samples. All of the alkylatable hydroxylic and carboxylic functionalities were derivatized by standard procedures using perdeuteriomethyl iodide. The breakdown between hydroxylic and carboxylic groups was made by the acid hydrolysis of labeled ester groups. By difference, the nonalkylatable oxygen content was then determined. These results for Powder River Basin coal are presented in Table IV, along with the comparison of the corresponding data for Illinois No. 6 coal. For each coal, the 0-H content remains about constant. The carboxylic acid level of Illinois No. 6 coal is quite low and appears not to change. However, there is a one-third decrease in the carboxylic acid concentration
Energy & Fuels, Vol. 1, No. 4,1987 351 of the severely weathered Powder River Basin coal relative to the fresh coal. The nonalkylatable oxygen content of both the weathered coals is substantially higher than that of each of the corresponding fresh samples. The difference in the chemical behavior of the two coals apparently is in the carboxylic acid chemistry. These groups are especially susceptible to the presence of oxygen. Conclusion The carboxylic acid content of fresh bituminous coal starts out very low and, to within our ability to measure it, remains constant. However, the experimental uncertainty in the measurement (f0.3) obviously makes it impossible to rule out, e.g., even a factor of 2 change in the acid content during weathering. This was not the case for the lower ranked subbituminous coal. One-third of the carboxylic acid groups in the fresh Powder River Basin coal chemically decarboxylated during the weathering process. This loss of acid groups resulted in a 13.4% decrease in oxygen. Concurrently, there was a gain in nonalkylatable oxygen (ethers, ketones, etc.) that amounted to 3.4 0 atom 100 carbons or about 17.5% increase in oxygen, based on the fresh Powder River Basin coal. This substantial oxidation was largely offset by the decarboxylation so that, as shown by the data in Table IV, the overall increase in oxygen content was actually quite small. These findings explain the unexpected trend in the calorific value for the weathered Powder River Basin coal samples. Decarboxylation has the effect of increasing the heat content, and the carbon oxidation lowers it. These two competing reactions compensate one another, resulting in the nonlinear trend shown in Figure 4. Research is under way to identify the precise nature of the oxygen functionality that was incorporated in Powder River Basin coal as a result of weathering. Carboxylic acids can be ruled out since the derivatization techniques used would have accounted for these groups. Preliminary spectal analysis (both solid-state 13CNMR and FT-IR) are not conclusive, but aldehydes do not appear to be present in any of the samples. Ethers are difficult to detect spectroscopically, and usually they are quantified by difference. Aromatic and conjugated ketones exhibit infrared stretching frequency in the same region as the broad band (1600-1660 cm-l) associated with the aromatic system that is native to the coal itself. Such ketones give rise to NMR signals in the region between 190 and 205 ppm. This is a sufficiently wide range that it makes detection of these carbonyls difficult, since only a slight structural modification can shift the C=O frequency a few ppm. However, longer data acquisition times may give a sufficient signal-to-noise ratio that quantification of this region of the NMR spectrum could be obtained. Furthermore, derivatization experiments are planned to help determine what other carbon oxides may have been formed during the weathering process. Since the solvent extractability and swellability of Powder River Basin coal decline after weathering, oxidative cross-linking reactions are likely. These reactions could result in either C-C or ether bond formation. Acknowledgment. The authors express their appreciation to Pam Sharp (Carter Mining Co., Gillette, WY), who supplied the Powder River Basin coal used in the study. Registry No. H02CCH2C(OH)(C02H)CH2C02H, 77-92-9; DSCI, 865-50-9.
