Energy & Fuels 1990,4, 165-171 ic-angle spinning with the CP process. Other solutions include (a) avoiding the use of CP and obtaining the spectra of carbon under a single pulse excitation using extra large rotors to obtain desired signal-to-noise ratios, (b) the use of magic-angle hoppingm in which the CP step is carried out with thesampie static, and (c) performing the CP while-.the sample is spinning at other than the magic anglea21
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Acknowledgment. We are grateful for discussions with Dr. R. Wind regarding the measurement of HartmannHahn matching curves. R e g i s t r y No. 13C, 14762-74-4; coronene, 191-07-1. (20) Bax, A.; Szeverenyi, N.; Maciel, G. E. J.Magn. Reson. 1983,52, 147. (21) Sardashti, M.; Maciel, G . E. J. Magn. Reson. 1987, 72, 467.
Oxidation Kinetics of Wyoming Powder River Basin Coal in O2 between 295 and 398 K S. R. Kelemen* and H. Freund Corporate Research Science Laboratory, Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801 Received September 21, 1989. Revised Manuscript Received December 11, 1989 We have quantified the reaction of O2with Powder River Basin subbituminous coal between 295 and 398 K. Initial oxidation produced mainly gaseous C 0 2 and H 2 0 as oxidation products. The rate of increase of C 0 2 can be fit to a single kinetic expression. The apparent activation energy is 14.6 kcal/mol with an A factor of 4.6 X lo6 (O/C) (day)-'. Measurements on the rate of COPformation at 351 K a t different partial pressures of O2 in helium shows nearly a first-order dependence in O2 below 100 mmHg but a dependence approaching zero order at high pressures. The total amount of surface and bulk organic oxygen remains almost constant during the initial oxidation period. The oxidation rate parameters are discussed and compared with previous results from Illinois No. 6 coal.
Introduction The self-heating tendency of coal is especially acute for freshly mined subbituminous and lower rank coals. Despite this awareness there is a general lack of quantitative kinetic information about the low-temperature-oxidation processes. Our previous study' of Illinois No. 6 bituminous coal showed that oxidation produced mainly solid oxidation products and that the surface oxidation reflected bulk oxidation behavior. The rate of increase of the surface oxidation products could be fit to a single kinetic expression between 295 and 398 K. It is known that oxidation of subbituminous coal favors the production of gaseous p r o d ~ c t s .Quantitative ~~~ information about the time evolution of both solid and gaseous oxidation products is therefore essential for an accurate description of the total oxidation kinetics. We have quantified several aspects of the oxidation kinetics of Powder River Basin subbituminous coal between 295 and 398 K. X-ray photoelectron spectroscopy (XPS) was used to determine the changes in surface organic oxygen content and provide a description of the oxygen functional group distribution. GC analysis of gas-phase products provided kinetic information, and thermal gravimetric analysis (TGA) was used to place the XPS results in the context of bulk oxidation. Experimental Section Wyoming Powder River Basin coal was supplied by Dr. R. P. Guerre and Dr. D. R. Neskora from the Baytown Coal charac(1) Kelemen, S. R.; Freund, H. Energy Fuels 1989, 3, 498. (2) Swam, P. D.; Evans, D. G. Fuel 1979,58, 276. (3) Isaacs, J. J.; Liotta, R. J. Energy Fuels 1987, 1, 349.
Table I. Comparison of XPS-Derived Composition a n d Composition from S t a n d a r d B u l k Methods of Analysis for t h e Starting F r e s h Powder River Basin Coal atom ratio re1 to carbon X 100 XPS bulk analysis organic sulfur 0.42 0.47 sulfate sulfur 0.03 0.027 pyritic sulfur 0.00 0.005 organic oxygen 19.3 20.1 nitrogen 1.1 1.0 terization Library! The samples arrived sealed in an amber glass container under a nitrogen atmosphere. Upon receipt the consignment was transferred to a nitrogen drybox. Cuts of -100 to +200 mesh were prepared and stored in the drybox for later use. X P S spectra were obtained with an ion-pumped Vacuum Generators ESCALAB instrument using nonmonochromatic Mg K a radiation. Initial fresh samples for X P S analysis were prepared in a N2 drybox, transferred within 10-s exposure to lab air to the VG fast-entry air lock and evacuated prior to measurement. Further details of the sample handling, data acquisition, and data analysis can be found e1sewhere.l~~ The organic composition was derived from X P S results by subtraction of the inorganic contribution as determined with XPS.' A favorable comparison between the XPS-derived composition and a standard bulk determination is shown in Table I for Powder River Basin coal. The close correspondence shows that the starting surface composition is the same as in the bulk. From Table I, we see there is very little pyrite present in the coal, and we therefore expect little inorganic contributions to the sulfur 2p spectrum. We deconvoluted the sulfur 2p spectrum using a mixed Gaussian Lorentzian line shape and a peak width a t half-maximum of 2.60 eV for each peak to represent the unresolved (4) Neavel, R. C.; Smith, S. E.; Hippo, E. J.; Miller, R. N. Fuel 1986, 65, 312. ( 5 ) Keleman, S. R.; George, G. N.; Gorbaty, M. L. P r e p . Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1989, 34, 729.
0887-0624/90/2504-0l65$02.50/0 0 1990 American Chemical Society
166 Energy & Fuels, Vol. 4 , No. 2, 1990
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Figure 1. Change in the organic oxygen 1s to carbon 1s atom ratio as a function of exposure time at different temperatures in air for Powder River Basin coal.
2p spin doublet. The organic sulfur 2p spectrum showed two main peaks that occur at 164.0 and 168.3 eV. Contributions to the 164.0-eVpeak come from a variety of unoxidized organic species which include thiophenes, sulfides, and mercaptans. While oxidation to produce sulfoxides (166.0 eV) and sulfonic acid (169.0
eV) will contribute to the development of the main 168.3-eV 2p peak, the dominant oxidation products appear as sulfones (168.2 eV) . Oxidation experiments with gas-phase analysis were done in a quartz reactor contained within a furnace. The outlet of the dead-ended reactor was connected to a bellows pump and a re-
circulation loop. The volumes of the reactor and recirculation loop were 60 and 40 cm3,respectively. A gas sample could be taken from the recirculation loop for GC analysis via an evacuated 2-cm3 gas-sample valve. The gas chromatograph used for product analysis was a Hewlett Packard 5840 equipped with a thermal conductivity and a flame ionization detector. Separations were achieved with a Porapack Q column. The sample size was 100 mg. The closed recirculation reactor arrangement enabled the buildup of products over long times. The reactor could maintain a 200-kPa pressure of helium over a period of weeks. The loss of the portion of product gas due to gas sampling itself was corrected in the product analysis. Prior to oxidation a fresh coal sample was introduced into the quartz reactor vessel and evacuated to 1 x Torr. The reactor was then pressurized to 200 kPa with helium and the sample heated at the appropriate reaction temperature in He for 18 h. The sample was again Torr before being pressurized to 200 kPa evacuated to 1 X with a reactant gas mixture consisting of various amounts of oxygen in helium. Oxidation experiments for XPS studies were done in 1 atm air with
