Energy & Fuels 1989,3, 223-230 peratures and time expended in equilibrating surfaces improved the quality of calibrations. Careful water Calibrations could be done with an accuracy of i 5 % . However, many experiments were quite complex. For example, from the Figure 7 experiment we generated gas-evolution profiles for water and 40 other compounds with the spectrometer operating in both MS and MS/MS modes and recording information every 2 "C at three tuning file
223
settings. Accuracy for water was only about f25%,in this case. However, on the basis of information obtained in the present study, we have recently implemented a convenient standard calibration procedure for water that gives good results (f5% accuracy) even when calibrations must be done hastily. Registry No. H20, 7732-18-5.
Influence of Steam Pretreatment on Coal Composition and Devolatilization M. Rashid Khan* Morgantown Energy Technology Center, U S . Department of Energy, P.O. Box 880, Morgantown, West Virginia 26507-0880
Wei-Yin Chent Gulf South Research Institute, P.O. Box 26518, New Orleans, Louisiana 70186
Eric Suuberg Division of Engineering, Brown University, Providence, Rhode Island 02912 Received July 1, 1988. Revised Manuscript Received November 18, 1988
Previous studies have shown that pretreating coal with steam can enhance the liquid yields during coal pyrolysis. The objective of this research was to characterize steam- and helium-treated coals to better understand the effects of pretreatment on pyrolysis-produced yield and composition. Pretreated samples were pyrolyzed in rapid and slow heating rate reactors. The following characterization techniques were used to analyze the products: elemental analysis, Fourier transform infrared spectroscopy, gas chromatography/mass spectrometry, and cross-polarization/magic-angle-spinning I3C nuclear magnetic resonance spectroscopy. This research demonstrated that steam treatment of a low-rank coal reduces the concentration of methoxy, phenolic, and aliphatic carbonyl and carboxyl groups in the coal. The low-rank coal showed significant reductions in total oxygen concentration after steam treatment, but the high-rank coals showed unchanged or even higher oxygen content. After steam pretreatment, phenols were the major components found in the water used. Pretreatment with steam increased the aromaticity of Wyodak coal. Steam treatment did not enhance total volatile yields from vacuum pyrolysis for any of the coals. Low-rank coals showed increases in tar yields when pyrolyzed at a rapid rate after steam treatment, but these increases seemed to be at the expense of total volatiles yield. When steam-treated coal was devolatilized at a slow heating rate, no increase in tar yield was observed for either a low- or a high-rank coal.
Introduction Graff and Brandes1S2report that pretreating coal with steam enhances the liquid yield from coal pyrolysis. They found that carbon conversion to liquids in steam pyrolysis at 740 "C increased from 23% to over 50% when they pretreated Illinois No. 6 coal with steam. Bienkowski et aL3recently reported that conversion increased 32 % when they pretreated Wyodak coal at 240 "C and subsequently liquified it at 400 "C in the presence of steam. However, none of these workers reported any compositionalchanges in the pyrolysis liquids due to pretreatment. In their reviews of coal conversion mechanisms in nitrogen heterocycles used as solvents, Atherton and Kulik4p5
* To whom correspondence should be addressed. Present address: Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803.
0887-0624/89/2503-0223$01.50/0
suggested a number of chemical models that contributed to the enhancement of liquefaction under mild conditions. The interactions between the nucleophilic solvents and the hydrogen bonds and oxygen-containingcross-links in coal were critical in those mechanisms. Between 200 and 400 "C, a basic nitrogen compound is able to cleave ethers, esters, and amide cross-links and induces tautomerization (1) Graff, R. A.; Brandes, S. D. Energy Fuels 1987, I , 84. (2) (a) Graff, R. A.; Brandes, S. D. Prepr. Pap.-Am. Chem. SOC.,Diu.
Fuel Chem. 1984,29(2), 104. (b)Brandes, S.D.; Graff,R. A. Investigation on the Nature of Steam-Modified Coal. Prepr. Pap-Am. Chem. SOC., Diu.Fuel Chem. 1987, 32(3), 385. (3) Bienkowski, P. R.; Narayan, R.; Greenkom, R. A.; Chao, K. C. Ind. Eng. Chem. Res. 1987,26, 202. (4) Atherton, L. F.; Kulik, C. J. Coal Liquefaction Chemistry. Paper presented at the 1984 AIChE Annual Meeting, Anaheim, CA, 1984. (5) Atherton, L. F.; Kulik, C. J. Advanced Coal Liquefaction. Paper presented at the 1982 AIChE Annual Meeting, Los Angeles, CA, 1982.
0 1989 American Chemical Society
224 E n e r g y & Fuels, Vol. 3, No. 2, 1989 of enols t o relatively weakly bonded ketones. Carbonyl groups were also found to play a significant role during the initial stage of t h e reaction. T h e s e oxygen bonds are critical in the enhancement of liquefaction. It is interesting t o note t h a t water, another nucleophilic agent, could play a similar role. T h a t is, water may hydrolyze t h e cross-links in t h e coal; for example, water m a y hydrolyze esters a n d amides t o alcohol, carboxylic acid, a n d aminesa6 I n addition, because of t h e increased decomposition of water at elevated temperatures, water m a y catalyze tautomerization, which eventually may lead to ether cleavage.' An extensive review of these coal fragmentation reactions are discussed by Atherton a n d K ~ l i k . ~ ~ ~ T h e objective of this study was to better understand the influence of steam pretreatment on coal pyrolysis behavior on the basis of a wider selection of coals a n d under a wider range of pyrolysis conditions t h a n has been studied previously. I n particular, our experiments were performed in a n a t t e m p t to examine how steam pretreatment in-
fluences coal composition, pyrolysis yield, and the c o m position of pyrolysis l i q u i d s . I n addition, little information is available in t h e literat u r e o n how s t e a m influences t h e coal composition at a relatively low temperature (-300 "C). I n this study, we characterized raw a n d pretreated coals a n d performed selected analyses of products of pyrolysis. Results of solid-state nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy ( F T I R ) for t h e raw a n d pretreated low-rank coal a r e also presented. Gas chromatography/mass spectroscopy (GC/MS) analysis of t h e water t h a t was used in t h e pretreatment is also reported. T o obtain independent verification of our results, multilaboratory and multireactor approaches were undertaken in this study. I n a subsequent paper, a special procedure for tracing t h e oxygen pathway in p r e t r e a t m e n t will be discussed.
Experimental Section Steam pretreatment experiments on five different coals of various rank were performed. The coals ranged from lignite to high-volatile A bituminous (hvAb). Four of the five coal samples were obtained from the Penn State/DOE Coal Data Bank. These samples were stored and shipped under argon. The Pittsburgh No. 8 coal was obtained directly from the mine mouth and ground and stored under nitrogen. Most subsequent handling of these samples was performed in an inert atmosphere, except as noted below. Coal Pretreatment in a Tubing Bomb Reactor. Pretreatment was performed in a rapidly heated tubing reactor (0.75-in.-o.d. stainless-steel batch tube sealed by two Swagelok caps; total volume 30 cm3). About 0.5 g of coal and 1.0 g of water were used. Pretreatment was performed at 320 "C and 1100 psia of steam with a soak time of 15 min. The tubing bomb experiment provided relatively rapid heating (250 "C/min), and the residence time was well-defined. However, because the tubes weighed much more (320 g) than the coal samples (0.5g) and because the samples could not be quantitatively recovered from the tubes, it was very difficult to obtain accurate weight loss data for the samples after drying. In spite of these limitations, it was noted that some coals gained weight during pretreatment. Additional experiments need to be performed to verify these results. To substantiate the results with a different method, the Parr bomb method was used. The Parr reactor provided relatively more reproducible weight measurement because the containers (test tubes) weighed less. (6) Chung, K. E.; Goldberg, I.; Ratto, J. Influence of Hydrogen Bonding in the Structural Features of Coal on ita Liquefaction. Rockwell International Interim Report No. AP-3889, EPRI Project No. 2147-4, February 1985. (7) Ross, D. Coal Conversion in Carbon Monoxide-Water System. In Coal Science; Gorbaty, M., Larsen, J., Wender, I., Eds.; Academic Press: New York 1984; Vol. 3.
K h a n et al.
After the runs, the tubes were opened in a glovebox flushed by nitrogen. The samples were then dried in a vacuum oven overnight and transferred to sample vials in the glovebox. The sample transport time between the glovebox and the vacuum oven was significantly less than 1 min. Coal Pretreatment in a Parr Reactor. Throughout sample preparation and handling, specific precautions were taken to avoid exposing the samples to air in order to prevent weathering. A 2-g sample of each of the five coals was placed in each of five test tubes, which were plugged by glass wool. The tubes were then placed in a Parr 2-L high-pressure reactor (Parr bomb). About 62 g of water was also added to the reactor to achieve 1300 psia pressure. The Parr bomb temperature was brought to 304 "C (unlessotherwise stated) in exactly 2 h by using a preheated jacket. The reactor was then cooled immediately to 50 "C in circulating air before the samples were removed for vacuum drying at 105 "C. The same sample handling procedure for the tubing bomb samples was used in drying. These samples were subsequently pyrolyzed in a rapidly heated wire-mesh grid reactor. A second series of steam and helium pretreatment work was performed i n a high-pressure Parr reactor in a different laboratory over a wide range of temperatures, pressures, and soak times.
Pyrolysis Reactors. Heated-GridReactor. The five pretreated coals prepared in the Parr bomb reactor were pyrolyzed rapidly under the following conditions in a heated grid reactor described elsewhere? heating rate, 700 "C/s; maximum temperature, -740 "C; pressure, -0.1 atm of He; soak time at temperature, 2 s. In the course of these experiments, short exposures of the samples to air were unavoidable, during loading of the
reactor. T h e duration of exposure was a f e w minutes at most.
Fixed-BedReactor. The second series of samples pretreated in a Parr reactor were pyrolyzed in a fixed-bed reactor (slow heating rate organic devolatilization reactor, SHRODR). The pretreated samples were protected from air exposure by handling the samples i n N 2 under a globe leaf. Detailed descriptions of
the fixed-bed reactor are available in the l i t e r a t ~ r e . ~ JRepro~ ducibility of data and detailed procedures are also described elsewhere.l0 Pressurized TGA System. In order to evaluate the influence of steam pretreatment on pyrolysis weight loss, a pressurized TGA system was used to perform pretreatment/pyrolysis runs. For this study, an Illinois No. 6 coal sample obtained from the Argonne Premium Coal Sample Bank was utilized. The samples were dried overnight at 110 "C in nitrogen. The samples were then cooled under nitrogen and transferred to the TGA basket. Other than this momentary transfer period, the samples were never exposed t o air.
The samples were pelletized under nitrogen to produce a larger diameter material (-2 g) that would not pass through the basket material. (The TGA baskets are constructed of 325 mesh material, and so the coal samples as received were too fine for testing without pelletizing.) All samples were pretreated at 330 "C for 15 min. Both the pretreatment and the devolatilization were (8) Suuberg, E. M.; Peters, W. A,; Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 37. (9) (a) Khan, M. R. Coal Devolatilization at Mild and Severe Conditions: Influence of Lime Additive or Heating Rate on Products Composition. In Proceedings, International Conference on Coal Science; Moulijn, J., Nater, K., Chemin, H., Eds.; Coal Science and Technology 11, Elsevier: Amsterdam, 1987; pp 647-651. (b) Khan, M. R. Fuel Sci. Technol. Int. 1987,5, 185-231. (c) Khan, M. R. Production of a HighQuality Liquid Fuel from Coal by Mild Pyrolysis of Coal-Lime Mixtures. Morgantown Energy Technol. Cent. [Rep.],DOEIMETC (US.Dep. Energy) 1986, DOE/METC-86/4060 (DE86006603). (10) (a) Khan, M. R. Characterization and Mechanisms of Mild Gasification of Coal and Catalytic Fuel Gas Conversion: Low-Temperature Devolatilization Studies. Proceedings, 5th Annual Gasification Contractors' Meeting; METC: Morgantown, WV, 1985;pp 67-69. (b) Khan, M. R.; Kurata, T. The Feasibility of Mild Gasification of Coal: Research Needs. Morgantown Energy Technol. Cent. [Rep.],DOEIMETC (US. Dep. Energy) 1985, DOE/METC-85/4019 (DE85013625),80. (c) Kim, B. C.; et al. Control of Emissions on the Gasifiers Using Coal with Chemically Bound Sulfur Scavenger. Final Report No. BMI-2035, UC-SOC, Contract No. W-7405-ENG-92 with the U S . Department of Energy, April 18,1980. (d) Whitehurst, D. D.; Farcasiu, M.; Mitchell, T. 0. The Nature and Origin of Asphaltenes in Processed Coals. EPRI Report AF-252, Mobil Research Project 401-1, 1976. (e) Khan, M. R. Energy Fuels 1987, I , 366-376.
Influence of S t e a m Pretreatment carried out at 50 atm of pressure. Samples were not removed from the TGA instrument between pretreatment and devolatilization to eliminate the possibility of exposure to air. The appropriate devolatilization environment was chosen, and the samples were again lowered into the heated zone of the TGA reactor. Samples were lowered through a water-cooled zone immediately above the TGA furnace. This minimized the transitional period from cool sample to sample a t reaction temperature, resulting in a rapid sample heatup rate (about 1600 "C/min). Details of this pressurized TGA system can be found elsewhere.'& The devolatilization was essentially complete within the first 2 min. Weight losses reported were measured by weighing the dried samples before pretreatment and then after the devolatilization. Reproducibility of this weight loss is good within h1.5 wt % (absolute). Tar Separation Technique. The fixed-bed pyrolysis liquids were fractionated by using sequential elution solvent chromatography (SESC),'Od more details of which are The liquid chromatographic separation was achieved by gravity flow on a silica gel column by sequential elution of the liquids with solvents of increasing polarity according to a procedure described elsewhere." In this work, six fractions were collected and 9.5100% of the sample was recovered. The silica gel (Baker Analyzed 3405-5,74-250 pm particle size) was washed twice with absolute methanol and dried overnight a t 130 "C. The gel's water level, after it was cooled to room temperature, was adjusted to 4 w t %, and it was then stored in a closed container. A 100-mL aliquot of this material was placed in a glass column and conditioned with hexane. Then, about 1.5 g of pyrolysis liquid was added to 10 mL of silica gel and 5 mL of sand. The mixture was stirred well and placed on top of the column. The column was eluted with 200 mL of each of the following previously dried, HPLC grade solvents at a flow rate of 4 mL/min: (a) hexane; (b) hexane/l5% Wluene; (c) chloroform; (d) chloroform/4% diethyl ether; (e) diethyl ether/3% ethanol; (fj methanol. The solute and the solvent were collected in a flask. After collection, the solvent was removed by rotary evaporation at 40 "C under partial vacuum. The residue was transferred to a tared vial, the flask washed with methylene chloride, and the excess wash solvent removed in a stream of nitrogen a t ambient temperature to constant weight. Efficient separation was achieved by methods used in an earlier experiment by Seshadri et al." Infrared and NMR spectra verified that a good separation of fractions was achieved. S t r u c t u r a l Analysis of a Low-RankCoal. Raw and treated Wyodak coals were chosen for FTIR and the cross-polarization (CP) 13C NMR analyses. In the FTIR analyses, two KBr pellets were made of approximately 1 mg of dry sample and 300 mg of KBr. The NMR spectra were obtained on a Varian XL200 NMR spectrometer by using a Doty Scientific, Inc. solids probe. A cross-polarization/magic-angle-spinning technique was used with high-power proton decoupling to observe the 13C nuclei at 50.3 MHz. Side bands were suppressed by using the TOSS technique (total suppression of spinning sidebands). Characterization of Water a f t e r Coal Pretreatment. The water remaining in the Parr bomb after the pretreatment experiment was extracted with methylene chloride. Although methylene chloride is a commonly used solvent in the analysis of coal tar, the carbon content of the water after extraction was not analyzed. Prior to GC/MS analysis, the solutions were fortified with six internal standards a t 50 pg/mL of each. The internal standards provided a retention time reference and a basis for semiquantitative determination of the unknown materials present. The internal standards used in this study were deuterium-labeled analogues of common analytes (such as polynuclear aromatics): IS-1, p-dichlorobenzene-d4; IS-2, naphthalene-d8; IS-3,
M. Fuel 1977, 56, 9. (12) (a) Solomon, P.; Carangelo, R. M. Fuel 1982,61, 663. (b) Solomon, P.; Hamblen, D.; Carangelo, R. ACS Symp. Ser. 1982, No. 205,77. (c) Solomon, P.; Carangelo, R. M. Fuel, in press. (13) Farnum. S.: Messick. D. D.: Farnum. B. W. h e m . PaD.-Am. Chem. Soc. Diu: Fuel Chem.' 1986,31(1),60164. (14) Zhou, P.; Dermer, 0. C.; Crynes, B. L. Oxygen in Coals and Coal-Derived Liquids. In Coal Science; Gorbaty, M. L., et al., Eds.; Academic Press: New York, 1984; Vol. 3, p 253.
Energy & Fuels, Vol. 3, No. 2, 1989 225 Table I. Pretreatment Experimental Conditions and Resu1tsa tubing bomb Parr bomb reactor nominal temp profile isothermal nonisothermal heatup time, min rapid 120 from room temp highest temp, OC 320 304 holding time, min 15 0 pressure, psia 1100 1350 w t loss upon treatment, % (daf) PSOC-1406P (lignite) b 16.84 PSOC-1520 (subbit C) b 9.79 PSOC-1109 (hvC) b 4.74 PSOC-1323 (hvB) b 7.63 Pittsburgh No. 8 (hvA) b 0.60 Pretreatment conditions for the parr reactor: nonisothermal heating to 304 "C; heatup time, 120 min; holding at temperature, 0 min; pressure, 1350 psi. Pretreatment conditions for the tubing bomb: isothermal heating at 320 "C; holding at temperature, 15 min; pressure, 1000 psi; rapid heatup of sample to the isothermal temperature. *Because of the heavy weight of the tubing reactor (320 g) compared to weight of the coal (0.5 g), the weight loss data from this reactor are subject to large uncertainties. acenaphthene-dlo; IS-4, phenanthrene-d,,; IS-5, chrysene-d,,; IS-6, benzo[a]pyrene-d,,. The fractions were analyzed by GC/MS on a Finnigan 5100 GC/MS/DS instrument. The following chromatographic conditions were used column, 0.32 mm X 30 m DB-5 fused silica capillary with 0.25-pm film thickness; temperature program, inject 45 "C and hold 4 min and then program deg/min to 300 "C and hold 12 min; injector, 280 "C modified splitless mode with grab time of 0.6 min; carrier, He a t 28 psi head pressure (100 cm/s). The mass spectral data were acquired in the continuous-scanning electron-impact mode. The MS was scanned from 45 to 600 amu with a 1 scan/s time. Unknown (noninternal standard) compounds were identified by the following means: (a) comparison with the U.S. National Bureau of Standards library spectrum (over 35000 compounds) and (b) manual recognition of the spectrum as that of a common material or as a member of a generic chemical class (e.g., phenol, alkylbenzene). Quantification was accomplished by comparing the peak height of each unknown peak with that of the nearest internal standard.
Results and Discussion Characterization of Pretreated Samples. Table I lists the weight loss that each coal experienced d u r i n g pretreatment in the Parr and tubing bomb reactors. The treatment temperature of 304 "C i n the Parr reactor is higher than that used by Bienkowski et aL3but lower than that used by Graff and Brandes.'s2 Both of these groups claimed that steam pretreatment enhanced the liquid yield during subsequent heat treatment. It is expected that the pretreatment temperature for optimal tar yield depends on coal nature and other operating variables used i n the first and second stages. In this research, five coals and an intermediate treatment temperature were selected on the basis of these literature evidences. T h i s s t u d y resulted i n a number of interesting observations. (a) The two low-rank coals (PSOC-1406P and PSOC-1520) lost relatively more weight during pretreatment. It will be shown later i n this paper that the loss of oxygen is an i m p o r t a n t factor i n t h i s weight loss d u r i n g H 2 0 pretreatment. (b) The two high-rank coals (PSOC1323 and P i t t s b u r g h No. 8) agglomerated i n t o lumps during treatment. The Utah coal sample (PSOC-1109) became semiagglomerated; that is, the forms of original coal particles were still visible. No agglomeration was observed for the two low-rank coals. These observations are consistent in both sets of experiments. Light agglomeration of Illinois No. 6 coal after s t e a m t r e a t m e n t was previously reported b y Graff and Brandes., (c) The two
226 Energy & Fuels, Vol. 3, No. 2, 1989
Khan et al.
Table 11. Proximate and Ultimate Analyses of Untreated and Steam-Treated Sampleso steam treated tubing untreated Parr bomb bomb A. Lignite (PSOC-1406P) 78.36 72.67 % C (daf) 67.72 4.48 4.25 % H (daf) 4.97 1.40 1.35 90 N (daf) 1.40 1.18 1.33 1.12 % S (daf) 24.58 14.64 20.55 % 0 (daf, by diff) % ash (as received) 8.23 16.85 13.90 % moisture 29.64 1.27 5.56 H/C atomic ratio (daf) 0.88 0.69 0.70 O/C atomic ratio 0.272 0.14 0.21 wt loss, % (daf) 16.8 9.4
Table 111. Yields of Dried Coals and Treated Coals from Vacuum Pyrolysisn soak wt tar tar temp time, loss, yield,b yield/wt sample "C s % % 1osse PSOC-1406P (ND lignite) 738 2 58.6 2.1 0.04 PSOC-1406P in bomb 740 2 31.8 4.2 0.13 PSOC-1520(Wyodak 772 0 51.9 3.9 0.08 subbituminous) PSOC-1520 in bomb 745 2 29.1 7.0 0.24 PSOC-1109 (Rocky 730 2 59.2 17.8 0.30 Mountain coal) PSOC-1109 in bomb 745 2 45.7 10.9 0.24 PSOC-1323 (Illinois No. 6) 740 2 48.4 24.9 0.51 PSOC-1323 in bomb 735 2 34.0 21.3 0.62 Pittsburgh No. 8 740 2 44.3 23.5 0.53 2 37.9 21.5 0.57 Pittsburgh No. 8 in bomb 738
B. Subbituminous C Coal (PSOC-1520) 73.78 77.16 % H (daf) 4.62 4.74 1.33 % N (daf) 1.11 1.38 1.11 % S (daf) 15.66 % 0 (daf, by diff) 19.11 % ash (as received) 9.08 11.89 % moisture 26.69 0.11 H/C atomic ratio (daf) 0.75 0.74 O/C atomic ratio 0.19 0.15 wt loss, % (daf) 9.8
78.15 5.01 1.20 1.36 14.28 12.59 5.64 0.77 0.14
OTemperatures are accurate to 2 "C. Weight loss measurement error is fl% (absolute). Tar yield measurement error is f2% (absolute). *Does not include water. Water and tar formed were dissolved in THF. Subsequently, water and the THF were separated by evaporation a t appropriate temperatures in an inert atmosphere. The amount of water formed was relatively small. CThiscorresponds to percent by weight of tar in total volatiles.
C. High-Volatile C Coal (PSOC-1109) 69.9 68.14 7.1 6.67 1.8 1.75 2.4 1.84 18.8 21.60 % ash (as received) 21.5 14.77 % moisture 8.1 co.01 H/C atomic ratio (daf) 1.22 1.18 O/C atomic ratio 0.20 0.24 wt loss, % (daf) 4.7
70.28 7.18 1.64 1.71 19.19 18.45 1.01 1.23 0.20
D. HvB Coal (PSOC-1323) 79.86 82.05 % H (daf) 5.60 5.10 % N (daf) 1.59 1.47 % S (daf) 4.60 4.07 % 0 (daf, by diff) 8.36 7.31 % ash (as received) 10.27 10.43 % moisture 5.30 0.20 H/C atomic ratio (daf) 0.84 0.75 O/C atomic ratio 0.079 0.067 wt loss, % (daf) 7.6
77.54 5.44 1.34 4.20 11.48 7.80 0.32 0.84 0.11
90 C (daf)
90 C (daf) % H (daf) % N (daf) % S (daf) % 0 (daf, by diff)
70 C (daf)
E. Pittsburgh No. 8 Coal (HvA) 83.74 b C (daf) H (daf) 5.46 N (daf) 1.56 S (daf) 2.15 0 (daf by diff) 7.09 % ash (as received) 7.27 % moisture 0.57 H/C atomic ratio (dan 0.78 O/C atomic ratio 0.064 wt loss, % (daf) 0.60 % % % % %
81.35 5.34 1.60 2.11
9.6 5.44 0.73 0.79 0.08
Pretreatment conditions for the Parr reactor: nonisothermal heating to 304 "C; heatup time, 120 min; holding at temperature, 0 min; pressure, 1350 psi. Pretreatment conditions for the tubing bomb: isothermal heating at 320 OC; holding at temperature, 15 min; pressure 1000 psi; rapid heatup of sample to the isothermal temperature. Not measured.
high-rank coals also produced some tar that condensed on the glass wool, whereas the three others produced notably less tar. (d) The water used for pretreatment had a strong sulfide and phenolic odor after treatment; it had a light greenish color that changed to dark brown overnight. The
*
water sample was not protected from contact of air before GC/MS analysis. Ultimate and proximate analyses were performed on untreated and pretreated samples. Assuming that the quantity of ash remains the same in a sample during steam pretreatment, the weight loss is caused by the conversion of organics to volatiles. (Due to the variability observed in the measured ash content of the pretreated samples, it is difficult to make any strong conclusion regarding the effect of steam pretreatment on the ash content of the samples. The researchers at Texas A&M15 have reported ash dissolution in the aqueous stream.15) The weight loss data and the elemental compositions of the raw and treated coal (on a dry, ash-free basis) can provide an index of the conversion of each element to volatiles. Table I1 shows the analysis and the weight loss results for these coals. Except for the Pittsburgh No. 8 coal, the coals that were steam treated in the Parr reactor have slightly lower H/C and O/C ratios than the original coals. The oxygen removal is particularly high for the two low-rank coals, PSOC-1520and PSOC-1406P.The decreases in H/C and O/C were also reported by Rozgonyi et Rapid-Rate Pyrolysis of Pretreated Coals. Table I11 shows the tar yields and percent tar in the total volatiles that result from vacuum pyrolysis of steam-treated and raw coals. We chose a pyrolysis temperature of 740 "C to compare the yields from vacuum pyrolysis with those reported by Graff and Brandes2 for steam pyrolysis a t the same temperature. I t must be emphasized that, in this case, the comparison of results with those of Graff and Brandes must be cautiously performed. It was previously noted by those that air exposure significantly reduces the effect of pretreatment, even under the short contact conditions employed here (a few minutes). Still, it was felt worthwhile, from the perspective of establishing behavior on a variety of coals under conditions less stringent than those examined earlier, to see if any effect of steam pretreatment could be discerned. The following trends are noted. (a) Steam pretreatment generally results in lower total volatiles. (b) The tar yields from the two low-rank coals benefitted from pretreatment, but not (15) Rozgonyi, T. G.; Mohan, M. S.; Zingaro, R. A.; Zoeller, J. H. Hydrothermal Treatment of Coals for the Removal of Inorganic Constituents and Forms of Sulfur. In Processing and Utilization of High Sulfur Coals II; Yoginder, P., et al., Eds.; Elsevier: Amsterdam, 1987.
Influence of Steam Pretreatment -
Energy & Fuels, Vol. 3, No. 2, 1989 227
1. Pretreated in HelDevolatilized in He 2. Pretreated in SteamlDevolatilized in He 3. Pretreated in SteamlDevolatilized in Steam 4. Pretreated in H2lDevolatilized in H 2
2
3
Table IV. Influences of Steam and Helium Pretreatment on the ComDosition of Evolved Gases (Cold)" yield of evolved gas, vol % evolved gas helium pretreatment steam pretreatment A. Pittsburgh No. 8 Coal (Pretreatment Temperature 250 "C) eo 16.3 6.34 co2 62.48 77.56 H2S 0.253 1.36 cos 0.00054 0.027 so2 0.0009 0.0047 CH4 5.42 3.06 C2H4 0.11 CzH6 0.547 14.11 4
Figure 1. Total weight loss during pretreatment/devolatilization steps (ArgonnePremium Illinois No. 6 coal). Pretreatments were performed at 330 "C for 15 min at 50 atm in a TGA. Devolatilization of the pretreated sample was performed at 740 "C and 50 atm of pressure in the same reactor. Heating rate during 1600 deg/min. devolatilization
-
significantly. The yield of Wyodak coal tar increased from 3.9 to 7.0% after pretreatment, while the yield of lignite tar increased from 2.1 to 4.2%. Pretreatment has adverse effects on the tar yield from all the high-rank coals. Several pretreatment/ pyrolysis experiments were also performed under conditions similar to those reported by Graff and Brandes, in a pressurized TGA instrument. Illinois No. 6 coal (Argonne Premium) was used for this study. Figure 1demonstrates the overall weight loss during pretreatment and pyrolysis processes in various atmospheres. The results show that steam pretreatment of coal results in a very small increase in total weight loss when compared with the sample pretreated and pyrolyzed in He. Pretreatment and pyrolysis were performed in the same reactor, and the samples were never exposed to air after they were pretreated. This small increase in weight loss, however, is close to the experimental error of measurements (f1.5%, absolute). The largest increase in weight loss, not surprisingly, was observed for the case where the sample was pretreated/devolatilized in H2. The small increase in overall volatile yield during steam pretreatment is of too small of a magnitude to justify capitalization in a commerical process. The fixed-bed and heated-grid data present in this study are consistent with those reported by Serio et al.16 Serio et al. steam treated three different coal samples (Wellmore No. 8 [hvAb]; Illinois No. 6 [hvCb]; and Montana Rosebud [subbituminous]) at 700 psi and 350 "C. The pretreated coal samples were pyrolyzed in a heated-grid reactor at 650 "C under vacuum. The pretreated coals showed significantly lower (up to 43%) tar yield. For example, for the Montana Rosebud coal, the tar yield was 16.3% for the steam-treated coal compared to 28.5% for the untreated coal. These samples were adequately protected from exposure to air after pretreatment. Results of this study are also consistent with the reported data by Graff et al." for the Wellmore No. 8 coal. Graffet a1.l' noted little or no increase in liquid yield when this coal was subjected to pretreatment conditions similar to that described pre(16) Serio, M.; Solomon, p.; Krou, E.; Deshpande, G. Expansion of High-Temperature, High-pressure Data Set for Coal Gasification. Morgantown Energy Technol. Cent. [Rep.],DOEIMETC (U.S.Dep. Energy) 1987, DOE / MC/21004-2605 (DE88010268). (17) Graff, R. A.; et al. Conditioning Techniques for Mild Gasification to Produce Coproducts. In Proceedings of Sixth Annual Gasification Contractors Meeting; DOEIMETC-8616043 (DE86006617); METC: Morgantown, WV, 1586; p 148.
B. Wyodak Subbituminous Coal (Pretreatment Temperature 350 "C) H2 1.81 1.57 eo 1.12 0.95 co2 82.84 86.25 H2S 0.31 0.59 cos 0.01 0.023 so2 0.0012 0.039 CH4 8.7 6.41 C2H4 0.04 0.14 C2H6 1.84 1.53 c1-c8 12.25 10.41
" Pretreatment conditions: Parr reactor; pressure, 500 psi; holding time at temperature, 1 h. vious1y.l This lack of steam pretreatment effect on liquid yield for the Wellmore No. 8 coal was explained by Graff et a1.l' in terms of preoxidation of the parent coal, although no direct evidence of preoxidation was provided. However, invoking the preoxidation argument alone does not explain these latest results. Graff et a1.2bpostulated that steam pretreatment cleaves oxygen linkages. It is known1&that preoxidation can introduce oxygen linkages in the coal structures, the type of cross-links that steam is proposed to cleave. If the proposed mechanism is correct, one would expect to observe enhanced liquid yield by pyrolysis of steam-treated preoxidized coal compared to the untreated preoxidized coal. However, to our knowledge, no such experimental results are available. Composition of the Evolved Gases Generated during Pretreatment. Table IV presents the influences of helium and steam pretreatment on the composition of evolved gases for the Wyodak and Pittsburgh No. 8 coals. The Wyodak coal (PSOC-1520) was pretreated at 350 "C under 500 psi of steam pressure, and the Pittsburgh seam coal was pretreated at 250 "C at the same pressure. The Pittsburgh coal is highly swelling (free swelling index [FSI] = 8.0), whereas the Wyodak coal does not soften. In our studies, we wanted to work with a coal sample before it softened and agglomerated. The softening temperature of the Pittsburgh No. 8 coal was determined to be close to -300 "C. We note that compared to pretreatment in He, steam pretreatment increases significantly the yields of gaseous H2S and COS. The yields of C02 increase slightly during steam pretreatment. These results suggest that steam may play a chemical role by selectively removing certain functional groups present on the coal surface. Because these groups are not present on the steam-treated coal during subsequent pyrolysis, the chemistry of pyrolysis is expected to be different depending on whether the coal pretreatment is performed in steam or He. Influence of Steam Pretreatment on the Yields and Composition of Pyrolysis Products. As stated in the Experimental Section, additional pretreatment/pyrolysis experiments were performed to elaborate the influence of
Khan et al.
228 Energy & Fuels, Vol. 3, No. 2, 1989 Table V. Influence of Pretreatment Conditions on the Yield and Composition of Pyrolysis Productsn pretreatment helium steam tot. gas yield,b L/100 g of dry coal 6.0 7.4 11.9 9.7 tar field,b wt % '(dry coal)
Gas Composition (vol % ) 5.36 10.08 16.27 3.13 0.19 0.22 0.0044 40.39 2.01 9.00 64.59
9.96 8.94 13.27 2.77 0.088 0.225 0.0046 39.57 1.72 8.41 64.49
Tar Composition (wt 70) C 80.95 H 10.11 S 0.67 N 0.46 H / C (atomic ratio) 1.50 Btu/lb 16 849
81.98 10.14 0.70 0.51 1.48 16727
Char Composition (wt %, As Received) 70.54 2.51 1.16 1.43 16.88 15.78 0.84 0.43 But/lb 11357
73.93 2.72 1.20 1.23 15.51 15.47 0.05 0.44 11897
HZ
co coz H2S cos HZ0
SO2
CH, CZH, CZH6
CrCs
C H N S ash VM HZO H/C
I
Aromalic
c.c
,CH3,CH2
Mineral Matter Corrected Speclrum steam Treated Coal
h 4000
3600
3200
2800
2400
2000
1600
1200
800
400
Wavenumbers
Figure 3. FTIR spectra of dried raw and treated Wyodak Coals
nPretreatment conditions: 350 O C ; 500 psi; soak time, 1 h; Wyodak subbituminous coal (PSOC-1520); pyrolysis temperature, 500 "C. bTheseyields are based on 100 g of dry pretreated samples.
(PSOC-1520).
240 220 200 180 160 140 120
roo so so
-io
40 20
PPM
Steam Treated
A
/ \
J,,U 240 220 200 180 160 140 120 100 80
60
40
20
-0
-20
PPM
Figure 4. CP 13C NMR spectra of dried and treated PSOC-1520.
P4.Y".
Figure 2. Influence of steam pretreatment on the composition of the fractionated pyrolysis liquid from Pittsburgh No. 8 coal.
steam pretreatment conditions (performed over a wide range of temperature, pressure, and soak time) on pyrolysis liquid yield. For this purpose, multiple laboratories and reactors were used to better understand the effects of steam pretreatment on the yield and composition of pyrolysis products. In one laboratory, a slowly heated fixed-bed reactor was used to pyrolyze the raw and steam-treated coal samples. Table V summarizes the yields and product compositions from the fixed-bed pyrolysis of Wyodak coal. The gas and tar compositions are remarkably similar in the two cases. The only notable difference is in the H2S and COS yields. The steamtreated samples appear to evolve slightly less sulfur during pyrolysis.
Table VI presents the influence of steam pretreatment on tar yield and on the composition of pyrolysis products for the Pittsburgh No. 8 coal. The results for this coal are qualitatively similar to those for the Wyodak coal, presented in Table V. Regardless of the pretreatment conditions, the tar yield did not increase when the coals were steam treated. Figure 2 shows the influence of steam pretreatment on the composition of the pyrolysis liquid as determined by sequential elution solvent chromatography. The results show that the lighter portion of the pyrolysis liquid decreased slightly for the steam-treated sample. This effect is attributable to the fact that, during steam pretreatment, there was some loss of hydrocarbon gases. Loss of this hydrocarbon may reduce the inventory of donatable hydrogen which plays an important role in the formation of the higher aliphatic fraction that would otherwise be present in the lightest fraction. Structural Changes in a Low-RankCoal Resulting from Pretreatment. Figure 3 presents the FTIR spectra of the raw and Parr-bomb steam-treated Wyodak coals. The samples were also analyzed by CP 13C NMR (Figure 4). Table VI1 shows the major functional groups of raw and pretreated PSOC-1520 coal and their relative concentrations.12 The following observations are important from a characterization standpoint: pretreatment reduces aliphatic hydrogen but increases aromatic hydrogen; pretreatment also reduces aliphatic carbon; pretreatment reduces phenolic oxygen and hydrogen; ethers decrease slightly after treatment.
Influence of Steam Pretreatment
Energy & Fuels, Vol. 3, No. 2, 1989 229
Table VI. Influence of Varying Pretreatment Conditions on the Yield and Composition of Pyrolysis Products for Pittsburgh No. 8 Coal Where Pretreatment Was Performed in a Parr Reactor
temp, "C pressure, psi OC/atmosphere soak time pyrolysis temp, OC tot. gas yield," L/100g of dry coal char yield," % (dry coal) tar yield: % (dry coal)
Pretreatment Conditions 395 250 1500/He 1500/steam 25 min l h 500 500 9.6 7.4 71.6 73.7 17.6 14.5 9.76 4.51 6.78 4.44 0.126 0.098 0.0046 47.89 2.02 11.06 74.27
HZ
co COP HZS
cos
Gas Composition (vol % ) 10.71 b 3.46 4.8
4.84 0.32 0.48 0.002 51.54 1.26 12.04 75.39
Tar Composition (wt % ) 82.90 8.8 b 0.62 1.38 1.49 1.34 1.27 13798 15549
C H
80.51 9.00
S N H/C Btu/lb
321 1500/steam 2.5 h 500 6.6 79.2 16.7
79.74 8.98 0.75 1.49 1.35 b
Char Composition (wt %, As Received) b 78.65 78.20 2.81 3.23 1.73 1.75 1.60 1.41 10.64 9.46 10.90 12.99 0.54 0.73 12 961 13252 13 157
C H N
S Ash VM HZO Btu/lb
250 500/steam l h 500 9.0 74.8 17.2
250 6OO/steam l h 500 8.8 73.8 17.6
9.48 15.91 4.78 3.98 0.14 0.05 0.009 51.0 2.03 12.08 77.23
11.37 4.27 5.73 4.17 0.099 0.096 0.012 46.20 2.09 11.16 73.55
81.54 8.88 0.85 1.64 1.31 15567
79.21 9.10 b 1.35 1.37 14029
78.15 2.73 1.66 1.63
78.10 2.80 1.71 1.61 11.04 11.38 0.50 12 726
10.91
11.91 0.83 12 859
'Yields are based on 100 g of dry pretreated samples. *Not available. Table VII. FTIR Analysis of Raw and Treated PSOC-1520 Subbituminous Coal Samples'
relative concentrations aromatic hvdrogen 2 adj H,/Hbt 1 adj 3 or more adj 0.33 0.51 0.73 0.30 0.41 0.62 0.79 0.50
oxveen "_ OoH Osther 18.47 5.00 4.98 17.27 3.00 4.19 'Key: a1 = aliphatic; OH = hydroxyl; ar = aromatic; adj = adjacent. Data provided by Advanced Fuel Research (see ref 12 for details of the procedure). The Parr bomb treatment conditions are listed in Table I.
sample PSOC-1520 treated PSOC-1520
Hd 2.77 2.59
HOH 0.31 0.19
hydrogen . H, Hbt 1.54 4.62 1.91 4.69
Table VI11 lists the major functional groups of the raw and treated PSOC-1520samples identified by CP 13C NMR. (See ref 13 for details of the procedure.) The following observations can be made: pretreatment increases aromatic carbon while decreasing aliphatic carbon; pretreatment drastically decreases methoxy group content; pretreatment decreases aliphatic carboxyl and carbonyl groups while the concentrations of aromatic carboxyl and carbonyl groups remain the same. Both CP NMR and FTIR analyses indicate an increase in aromaticity after pretreatment. The two analyses also indicate that pretreatment results in lower amounts of methoxy groups. However, further study is required to verify that these oxygen functional groups contribute to the increase in extraction yield in a strong hydrogenbonding solvent, such as water. Atherton and Kulik5 critically reviewed the chemistry of nucleophilic agents (such as tetrahydroquinoline) in coal liquefaction below 400 "C. Oxygen cross-links and carbonyl groups were considered important determinants of yield. Further re-
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I
Cd
search is also required to determine if the rearrangement of oxygen functional groups in steam treatment can contribute to a lower volatile yield in vacuum pyrolysis. The reduction in phenolic groups in our FTIR analysis contrasts with the results reported by Graff and Brandes.' This is attributable to the following reasons. (a) The low-rank coal used in this FTIR analysis has a higher concentration of hydroxy groups than the Illinois No. 6 coal that was used by Graff and Brandes. Further, the rearrangements of oxygen functional groups in low-rank coals may be very complex and differ from those of highrank coals. (b) This research used a relatively slow heating rate and longer residence time during pretreatment. These treatment conditions favor the conversion of phenols to ethers and ar0mati~s.I~ Analysis of Water Used for Coal Pretreatment. Table IX lists the GC/MS identifications of the major components in the pretreatment water that was derived from Parr bomb experiments. Many of the peaks correspond to phenol and alkylphenols. The appearance of
230 Energy & Fuels, Vol. 3, No. 2, 1989
Khan et al.
Table VIII. Normalized Carbon Distribution for the Raw and Steam-Treated Parr Bomb SamDles' carbon distribn, % NMR treated region, PSOCPSOCDDm 1520 1520 240-187 aliphatic carbonyl and 4.6 3.2 carboxyl aromatic carbonyl and 8.6 8.6 187-160 carboxyl 160-149 phenolic and aryl ether 11.5 9.5 aromatic 149-95 56.0 61.2 95-50 6.3 2.2 methoxy (-0-CHJ aliphatic 1.7 1.3 -CHP50-36 -CHI-, -C36-27 3.2 2.7 a-C 27-17 3.9 3.7 -CHB 17-0 5.5 6.1 tot. 99.9 99.9 carbonyl/carboxyl aromatic methoxy aliphatic
Summary 240-160 160-95 95-50 5 0
13.2 65.5 6.3 14.9
11.8
72.7 2.2
13.2
"See ref 13 for the details of the analytical procedure utilized. Treatment conditions are listed in Table I. Table IX. GC/MS Analysis of Water Used to Pretreat Coal compound name relative concentration phenol 64000 2-methylphenol 31000 4-methylphenol 46000 trimethylcyclopent-2-en-l-one 870 2,4-dimethylphenol 18000 dimethylphenol 1200 ethylphenol 3700 dimethylphenol 17000 dimethylphenol 1600 (l-methylethy1)phenol 5500 (l-methylethy1)phenol 3400 ethylmethylphenol 4400 trimethylphenol 1700 ethylmethylphenol 850 diethylphenol 1400 dimethyl(l-methylethy1)benzene 2700 alkylphenol 870 a1k y lphenol 740 alkylphenol 1400 dimethyl-IH-indene-l,2(3Zf)-dione 1100 alkylphenol 410 methylnaphthalen-1-01 410 unknown polyether 540
phenols in the pretreated coal, determined by using diffuse reflectance infrared spectroscopy, was recently reported by Brandes and Graff.' In their field-ionization MS analysis of steam extract that was derived from Wyodak
coal at 430 "C, Bienkowski et al.3 found that dihydroxy aromatics were the major species derived from the twostage operation. Summary and Conclusions
This work demonstrated that steam treatment of coal reduces the concentration of methoxy, phenolic, and aliphatic carbonyl and carboxyl groups. The low-rank coals showed significant reductions in total oxygen concentrations after treatment, but the high-rank coals showed unchanged, or even higher, oxygen content. Phenols were the major components in the pretreatment water. Pretreatment seemed to increase the aromaticity of Wyodak coal. Total volatile yields in vacuum pyrolysis were not benefitted by steam treatment for any of the five coals. But, it must be noted that this conclusion is drawn based on samples that were exposed briefly to air, prior to testing. Another sample, without air exposure, did appear to confirm the conclusion, however. Only low-rank coals showed small increases in tar yields when pyrolyzed rapidly, but these increases seemed to be at the much larger expense of total volatiles. The steam-treated coals pyrolyzed in a fixed-bed reactor did not show increases in liquid yield. (The pretreated samples were protected from air exposure.) The quality of pyrolysis liquids as defined by the hydrogen-to-carbon(atomic) ratio or heating value was essentially unchanged due to steam pretreatment. A key conclusion of this study is that the effects of steam pretreatment of coal on subsequent conversion appear to be a function of feed coal type. Additional work is clearly needed to better understand the fundamentals involved during steam pretreatment/pyrolysis before a pretreatment-based scheme could be recommended as a process of commercial significance. Acknowledgment. We are grateful to Art Ruud of the University of North Dakota Energy Research Center for his kind assistance on the CP 13CNMR analysis. Dr. Peter Solomon of Advanced Fuel Research provided the FTIR results. M. Paisley provided the pressurized TGA data. Registry No. Hydrogen, 1333-74-0;carbon monoxide, 630-08-0; carbon dioxide, 124-38-9; hydrogen sulfide, 7183-06-4; carbonyl sulfide, 463-58-1; sulfur dioxide, 1446-09-5; methane, 74-82-8; ethylene, 74-85-1; ethane, 74-84-0; phenol, 108-95-2; 2-methylphenol, 95-48-7; 4-methylphenol, 106-44-5; trimethylchloropen2-en-l-one, 82000-05-3; 2,4-dimethylphenol, 105-67-9; dimethylphenol, 1300-71-6;ethylphenol, 25429-37-2; (l-methylethyl)phenol, 25168-06-3; ethylmethylphenol, 30230-52-5; trimethylphenol, 26998-80-1; diethylphenol, 26967-65-7; dimethyl(1-methylethyl)benzene, 25321-29-3; dimethyl-lH-indene-l,2(3H)dione, 118357-21-4; methylnaphthalen-1-01, 59534-35-9.
