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of the liquid products was reduced, but in the process, significant quantities of carbon were deposited on the catalyst. The catalysts did not, however, simply remove the more intractable material from the tar to form coke but also catalyzed the reactions of some of the more reactive functional groups in the tar. The cracking of long-chain aliphatic groups to light hydrocarbon gases and aromatization reactions that produce highly condensed PAH species are two examples of such reactions. These reactions can be explained on the basis of existing knowledge of the cracking reactions of long-chain aliphatics and other model compounds over solid acid catalysts. From the point of view of alternate fuels production, these products have considerable disadvantages. First, a considerable proportion of valuable product has been lost due to carbon formation. Second, valuable aliphatic material has been transformed to hydrocarbon gas and PAH. Besides the considerable problems of toxicology presented by the latter, their use in liquid fuels would entail highseverity hydrocracking, the most expensive refining process. Therefore, it is clear that with present day catalysts the upgrading of flash pyrolysis tars using high-pressure hydrotreatment is more attractive. However, there may be
advantages in pyrolyzing coal in the presence of catalysts and high hydrogen pressures, particularly at lower temperatures where gas formation can be controlled.20 The high temperatures investigated in this study were necessary to avoid tar condensation. In the more general area of coal pyrolysis, the results suggest that the solid acid components of the mineral matter in the coal could significantly influence volatiles yield and composition, particularly of light hydrocarbon gas and aromatic species.
Acknowledgment. We thank A. McCutcheon for hydrocarbon gas analyses, T. D. Gilbert for GC/MS identifications, and K. Riley for elemental analyses. Registry No. Ni, 7440-02-0; Mo, 7439-98-1; methane, 74-82-8; ethane, 74-84-0; ethylene, 74-85-1; propane, 74-98-6; propylene, 115-07-1; toluene, 108-88-3; benzene, 71-43-2; ethylbenzene, 100-41-4;p-xylene, 106-42-3;m-xylene, 108383; o-xylene, 95-47-6; indan, 496-11-7; indene, 95-13-6; naphthalene, 91-20-3; 2methylnaphthalene, 91-57-6; 1-methylnaphthalene, 90-12-0; dimethylnaphthalene, 28804-888; trimethylnaphthalene, 28652-17-9; fluorene, 86-73-7;methylfluorene, 26914-17-0; anthracene, 12012-7; fluoranthrene, 206-44-0; pyrene, 129-00-0;phenanthrene, 85-01-8; benzanthacene, 56-55-3; chrysene, 218-01-9; benzofluoranthene, 56832-73-6; benzo[a]pyrene, 50-32-8.
Modification of Coal by Subcritical Steam: An Examination of Modified Illinois No. 6 Coal Susan D. Brandes and Robert A. Graff* The Clean Fuels Institute, The City University of New York, New York, New York 10031
Martin L. Gorbaty and Michael Siskin Exxon Research and Engineering Company, Clinton Township, Route 22 East, Annandale, New Jersey 08801 Received September 15, 1988. Revised Manuscript Received April 27, 1989
It has been reported that Illinois No. 6 coal is modified by exposure to 50 atm of steam at temperatures between 320 and 360 OC in such a way as to give dramatically improved liquid yields in steam pyrolysis and mild extraction. Here we investigate the character of steam-modified coal. Steam-treated coal swells considerably more than the raw coal in water. This implies that the steam-treated coal is more hydrophilic. It exhibits a lower degree of hydrogen bonding and has an IR spectrum different from that of raw coal. Elemental analysis of treated coal shows a significant decrease in organic oxygen content. A mild 0-alkylation of treated coal with labeled methyl iodide, however, introduces twice the enrichment by 13Cas does the same procedure carried out on raw coal. It may be concluded that steam-modified coal contains twice as many hydroxyl groups as raw coal. It is postulated that steam reacts with the ether linkages in coal, forming hydroxyl groups and thereby substantially reducing the presence of an important covalent cross-link in the coal structure.
Introduction We have previously reported that Illinois No. 6 coal that has first been contacted with subcritical steam (50 and temperatures between 320 and 360 OC) gives dramatically improved yields both in steam pyrolysis and in solvent extraction.' This effect is not produced when coal is contacted with an inert gas at the same pressure and (1) Graff, R.A.; Brandes, S.
D.Energy Fuels
1987,1, 84.
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temperatures. It is likely that the cross-linked structure of Coal is somehow disrupted by steam, perhaps through chemical reaction. Certainly, important changes in the structure of cod are induced by treatment with subcritical and these changes warrant examination* To explore the structure of steam-modified coal, we have used four techniques: solvent swelling, diffuse reflectance infrared spectroscopy (DRIS),0-alkylation, and elemental analysis. Each analytical method was applied to raw coal and coal treated in steam. Most methods were also applied steamy
0 1989 American Chemical Society
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Modification of Coal by Subcritical Steam to coal treated in helium to distinguish thermal effects.
Experimental Section Coal Sample. Tests were conducted on a batch of Illinois No. 6 coal2 ground under nitrogen to pass 200 mesh. A Trost Gem T Mill, used for this grinding, was enclosed in a plastic glovebag to contain the inert atmosphere. A 11/2-in.gate valve was added between the outlet of the mill and the filter bag. When grinding was complete and while the pressure was still slightly above atmospheric, the gate valve was closed. This protected the ground sample while it was sieved, riffled, divided, and placed in sealed containers while inside the glovebag. The sealed containers were stored in a freezer. An elemental analysis for this coal is given in Table IV. Coal Treatment. Coal samples were treated in steam or helium by using one of several fixed-bed reactors of different size according to the amount of sample to be prepared. The reactors were constructed from lengths of stainless-steel tubing either 2.54 or 1.35 cm i.d. The length of the tube was varied to accommodate from 1.5 to 30 g of coal with the reactor partially full. The reactor was operated in the horizontal position so that there is a space above the coal for a continuous sweep of steam and evolution of volatile5 during treatment. After being loaded, the reactor was attached to a source of helium and steam, and a thermocouple was wired to its midpoint. The reactor, together with valves a t both ends, was wrapped in heating tape. A flow of helium at 50 atm was established through the reactor. When the air had been flushed from the treatment reactor and with helium flowing a t about 1 cm3/min, controlled by the outlet needle valve, the reactor was heated to 300 "C. Heating was done slowly, taking about 10 min, to avoid overshooting the desired 300 "C. Since this is above the condensation point of 50 atm of steam (265 "C), helium was then replaced by steam a t 50 atm and the reactor further heated to the desired treatment temperature in the next 2-3 min. Steam flow rate was between 3 and 10 cm3/min. Treatment temperature (340 2 "C) was maintained for 15 min, and then the heat was turned off and the steam flow replaced by helium. The reactor was allowed to cool to room temperature over a period of approximately 20 min. The flow of helium was maintained until there was no detectable moisture in the reactor effluent. Moisture was detected by observing condensation on the shiny surface of a cold metal object or by the use of a piece of indicating Drierite held in the outlet flow. A sample moisture content is given in Table IV. The treatment reactor was then sealed under 50 atm of helium, disconnected from the treatment system, and placed into a nitrogen-filled glovebox. Under this protective atmosphere, the reactor was opened and the sample removed. The steam-treated coal, which was slightly agglomerated, was crushed and sieved to pass 200 mesh. It was then divided and transferred to weighed vials that were sealed before removal from the glovebox. During these operations, the atmosphere of the glovebox was continuously monitored with a mass spectrometer, and the oxygen content was maintained below 2%. Additional details are given in ref 3. Swelling. Swelling experiments were carried out as described in Green et a1.4 Approximately 1g of coal sample was measured into an 8 X 60 mm centrifuge tube and covered with a neoprene stopper. The samples were centrifuged at 1700 rpm for approximately 10 min to settle the dry coal. The heights of the samples were then measured with an external rule. Solvents were added and the tubes capped and shaken until homogeneity was complete. The tubes were left to stand for 18 h and then were centrifuged again until a constant height of the solid material was attained. When pyridine was used as solvent, because of the dark color of the solution, tubes were turned upside down in order to read the heights of the solids. The height of the solid measured after solvent addition divided by the height of the dry solid is the reported swelling ratio. On the basis of repeated measurements, the precision of the swelling ratio is k0.03.
*
(2)We thank R. C. Neavel of Exxon Research and Engineering Co. for providing this coal sample. (3)Brandes, S. D. Modification of Coal by Subcritical Steam. Doctoral Dissertation, The City University of New York, 1986, pp 63-65. (4)Green, T.K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935.
Table I. Swelling Ratios for Raw and Treated Illinois No. 6 Coalso raw steam treated helium treated solvent in air under N2 in air under N, in air pyridine 2.32 2.11 2.05 1.86 1.86 benzene 1.13 1.06 1.00 1.00 1.06 water 0.98 1.83 1.84 1.56 1.41 "Treatments in steam or helium for 15 min at 50 atm. Steam-treated samples are not wetted by water. Xanthum gum, a polymeric thickening agent, was added to distilled water to promote wetting. Concentrations were about 0.1-0.3 w t %. Experiments were carried out in a glovebag filled with nitrogen unless the sample was to be deliberately exposed to air. In that case, no precautions were taken to exclude air during handling in any step of the above procedure. DRIS. Fourier transform infrared spectra were taken on a Nicolet 7199 instrument with a diffuse reflectance cell. Diffuse reflectance infrared spectroscopy (DRIS) was used to analyze both raw and pretreated coal samples. DRIS is a technique ideally suited to solid samples that are ground to uniform, although not necessarily very small size. Samples after treatment were again ground and sieved, under a nitrogen atmosphere, to pass 200 mesh. For analysis, the samples were handled in a nitrogen-filled glovebag directly attached to the sample chamber of the machine. A constant purge of nitrogen was also maintained through the instrument. Data were acquired over approximately 13 min, the number of data points being 8192 over 1000 s c a m 6 0-Alkylation. 0-Alkylations of raw and treated coals were performed by following procedures described by Liotta e t al.8 Treated coal samples were handled exclusively in a nitrogenflushed glovebag in which the oxygen concentration was approximately 5%. Typically, 1g of coal was weighed into a 250-mL round-bottom flask immersed in an ice bath and 2.9 mmol of tetra-n-propylammonium hydroxide (TnPAH) were added. This was in slight excess of the 2.5 mmol of acidic sites/g of coal found to be in the raw coal by using the method of titration as reported by Liotta et ala6For treated coal, the amount of TnPAH added was approximately 2.5 times this amount. Freshly distilled tetrahydrofuran (50-60 mL) was added while the contents were stirred with a Teflon-covered magnetic stirring bar. Labeled methyl iodide (13CH31),in 20% excess of the amount of TnPAH that was used, was added. Stirring was continued for 72 h. Workup (for which inert-gas protection was not used) entailed removing the T H F on a rotovaporizer and washing the product with 250 mL of a hot methanol and water solution (1:l by volume). The contents of the flask were then vacuum filtered on an 80-rm filter and washed with 4 L of hot water. The coal was dried in a stream of nitrogen, transferred to a Soxhlet extractor, and washed with distilled water for 5 days. The coal was then dried in a stream of nitrogen and then in a vacuum oven a t 90 "C for 4 h. Stable carbon isotope analyses were performed on these samples by Coastal Science Laboratories of Austin Texas using combustion techniques. Elemental Analyses a n d Determination of Organic Oxygen. Analyses were performed by outside laboratories. Treated coals were shipped sealed under nitrogen and their analyses conducted under an inert atmosphere. Total oxygen content was determined by neutron activation analysis (NAA). From this, organic oxygen was calculated by subtracting inorganic oxygen content obtained as follows. Oxygen as carbonates was determined by an HC1 wash of the coal and measurement of the carbon dioxide produced. Oxygen associated with Ca, Mg, Na, and K in the ash were determined as the oxides. Si, Al, Ti, and P were assumed to exist in the ash as their oxides. Although this may not be strictly correct, the error in calculation will be in the same direction and of the same order of magnitude for both raw and treated samples. In the ashing process the sulfur was oxidized and incorporated into the ash as sulfate. This oxygen value was an artifact of the ashing (5) We thank E. W. Sheppard of Mobil Research and Development for his assistance in obtaining and interpreting the DRIS spectra. (6) Liotta, R.;Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277-283.
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Brandes et al.
Table 11. Difference in Swelling Ratio of Pyridine- and Benzene-Swollen Samples sample pyridine - benzene swelling ratio raw in air 1.19 steam treated under Nz 1.05 in air 1.05 helium treated under N2 0.86 in air 0.80 process and was corrected by analyzing for SO3on ash. Oxygen associatedwith iron content in the ash was calculated by correcting for pyritic iron and then assuming that the remaining iron was hematite. Any bias in total oxygen introduced by this assumption will be in the same direct for both raw and treated samples.
1650 1330 1010
Results Swelling. Values of swelling ratios obtained in pyridine, benzene, and water are given in Table I. Swelling ratios in each solvent were measured on raw, steam-treated, and helium-treated coals. Since pyrolysis and extraction yields from treated samples had been found to be strongly affected by exposure to air,' swelling of these materials was also measured under a nitrogen atmosphere. The action of different solvents on the swelling behavior of coal has been shown by Larsen et al.' to depend on the solubility parameter of the solvent. It is also a function of the ability of the solvent to disrupt hydrogen bonds.s In pyridine, raw coal swells more than steam-treated coal while helium-treated coal swells less. In water, however, steam-treated coal swells more than raw coal or heliumtreated coal by a considerable amount. In benzene, raw coal swells insignificantly more than helium- or steamtreated coals. Pyridine is a strong hydrogen-bond-breaking solvent. Benzene does not break hydrogen bonds. The difference between the degree of swelling in pyridine and in benzene is a measure of the amount of hydrogen-bond cross-linking. The smaller the difference the fewer the number of hydrogen-bond cross-links. In Table I1 the difference values are listed. The amount of hydrogen-bond association is highest for raw coal, less for steam-treated coal, and least for helium-treated coal. The values are not significantly affected by exposure to air. When swelling measurements were conducted in water, the increased swelling of the treated sample compared to raw coal was startling. There is a 2-fold increase in the swelling ratio of steam-treated coal over that of raw coal. Helium-treated coal also swells more than raw coal in water, but the effect is not quite as large. DRIS. Diffuse reflectance infrared spectroscopy has been used to analyze the pretreated coal. Samples of coal treated under various conditions, raw coal, and raw coal dried under vacuum at 90 "C were analyzed. There are no apparent differences among the samples in the region attributed to carbonyl bonds (1600-1900 cm-l) (Figure 1). A shift in peak height ratio, however, does occur in the 3200-3700-cm-' region of the infrared spectrum attributed to OH species (Figure 2). In the raw coal sample the broad peak at 3300 cm-' predominates and mostly occludes the one at 3550 cm-'. Vacuum-drying of the coal sample at 90 "C does not alter this ratio. In all treated coal samples the peak at 3550 cm-' becomes very pronounced. It was established that exposure of steam-treated samples to air for times as short as 2 min diminished both the (7) h e n , J. W.; Green, T. K.; Kovac, J. J.Org. Chem. 1986,50,4729. (8)Green, T. K.; Larsen, J. W. Fuel 1984, 63, 1538.
690
WAVE NUMB E RS ( c m-' ) Figure 1. DRIS spectra in the 1600-1900-cm-' region for steam-treated, helium-treated, and raw Illinois No. 6 coal.
I
,
2650 2230 WAVENUMBERS (cm-')
3490
'
3070
'
Figure 2. DRIS spectra in the 3200-3700-cm-l region for steam-treated, helium-treated, and raw Illinois No. 6 coal. Table 111. Carbon-13 Enrichment by Mild 0-Alkylation sample groups/100 C" raw Illinois No. 6 coal 0.03b raw coal, 0-alkylated 5.50 (3.85) 9.03* (7.38) steam-treated coal, 0-alkylated 4.26 (2.62) helium-treated coal, 0-alkylated a Measured relative to the international standard PDB. Values in parentheses calculated assuming 70% of alkylated sites are oxygen. *Average of two values.
liquid yields in flash pyrolysis and the amount of material extractable in pyridine. Since there is no difference in the IR spectra for preserved and air-exposed samples, it is apparent that exposure to air reduces yields by oxygen (possibly ether) bond formation and is not connected to the effect that produces a difference in the OH region of the ir spectrum. The magnitude of the inclusion of oxygen must, however, be small because there is no evidence of it in the ether or carbonyl region; the instrument detection limit is better than 1%. 0-Alkylation. Relative enrichments in I3C resulting from 0-alkylation are given in Table 111. The value for raw coal is given for comparison. Steam-treated O-alkylated coal is almost doubly enriched compared to raw 0-alkylated coal. It is clear that the number of deriva-
Modification of Coal by Subcritical S t e a m
Energy & Fuels, Vol. 3, No. 4 , 1989 497
Table IV. Analyses of Raw and Steam-Treated Illinois No. 6 Coal raw steam treated Whole Coal dry basis 7 0 c (tot.) 61.81 68.14 5.22 4.92 %H 1.23 1.32 %N % s (tot.) 4.04 4.44 70 0 (org, calcd) 12.84 9.20 H/C atomic ratio 0.92 0.86 90 moisture (as received) 0.15
