Energy & Fuels 1998, 12, 159-173
159
Nitrogen Transformations in Coal during Pyrolysis S. R. Kelemen,* M. L. Gorbaty, and P. J. Kwiatek Exxon Research and Engineering Company, Annandale, New Jersey 08801
T. H. Fletcher and M. Watt Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602
M. S. Solum and R. J. Pugmire Departments of Chemistry and Fuel Engineering, University of Utah, Salt Lake City, Utah 84112 Received July 21, 1997. Revised Manuscript Received October 7, 1997X
X-ray photoelectron spectroscopy (XPS) was used to identify and quantify the changes in organically bound nitrogen forms present in the tars and chars of coals after pyrolysis. For fresh coal, pyrrolic nitrogen is the most abundant form of organically bound nitrogen, followed by pyridinic, quaternary, and amino types. Some of the quaternary nitrogen species initially present in coal are lost upon mild pyrolysis, prior to hydrocarbon devolatilization. These quaternary species are attributed to pyridinic or basic nitrogen species associated with hydroxyl groups from carboxylic acids or phenols. A portion of the quaternary nitrogen species is lost at the very earliest stage of pyrolysis. Upon devolatilization, the resultant tar and char contain mostly pyrrolic and pyridinic forms; however, a portion of the quaternary nitrogen initially present in the coal appears in the coal char and tar. The relatively strong bonding interactions associated with these quaternary species suggests that there may be other quaternary nitrogen, in addition to protonated pyridines, in low-rank coal. For low-rank coal, amino groups are preferentially released and concentrate in the tar. XPS analysis of chars and tars produced during rapid heat-up (104 deg/s) pyrolysis show similar trends. However, severe pyrolysis of the devolatilized char results in the appearance of an asymmetric carbon (1s) line shape indicative of very large polynuclear “graphiticlike” units. This transformation is accompanied by a rise in the relative number of quaternary nitrogen forms and occurs over a relatively narrow temperature range. Quaternary and pyridinic nitrogen forms become the dominant forms in severely pyrolyzed chars. The relatively low level of quaternary nitrogen in the rapid heat-up chars indicates that very large polynuclear aromatic structures are not fully developed under these pyrolysis conditions.
I. Introduction The development and application of direct analytical tools for quantifying heteroatom functionalities in complex solid and nonvolatile carbonaceous systems has been the focus of much research.1-57 X-ray photoelectron spectroscopy (XPS)13-17,20,27-29,36 and X-ray absorpAbstract published in Advance ACS Abstracts, December 15, 1997. (1) Gorbaty, M. L.; George, G. N.; Kelemen, S. R., Fuel 1990, 69,945. (2) George, G. N.; Gorbaty, M. L.; Kelemen, S. R.; Sansone, M. Energy Fuels 1991, 5, 93. (3) Huffman, G. P.; Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, S.; Lu, F. Energy and Fuels 1991, 5, 574. (4) Taghiei, M. M.; Huggins, F. E.; Shah, N.; Huffman, G. P. Energy and Fuels 1992, 6, 293. (5) Brown, J. R.; Kasrai, M.; Bancroft, M. G.; Tan, K. H.; Chen, J. H. Fuel 1992, 71, 649. (6) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; Cramer, S. P. Fuel 1993, 72, 133. (7) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252. (8) Mullins, O. C.; Mitra-Kirtley, S; Van Elp, J; Cramer, S. P. Appl. Spectrosc. 1993, 8, 1268. (9) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J.; Cramer, S. P. Fuel 1992, 71, 53. X
tion near edge structure (XANES)6-8 studies have established that pyrrolic and pyridinic forms are the (10) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Fuel 1990, 69, 939. (11) Kelemen, S. R.; Gorbaty, M. L.; George, G. N.; Kwiatek, P. J., Sansone, M. Fuel 1991, 70, 396. (12) Kelemen, S. R.; Gorbaty, M. L.; Vaughn, S. N.; George, G. Fuel 1993, 73, 645. (13) Jones, R. B.; McCourt, C. B.; Swift, P. Proc. Int. Conf. Coal Sci., Dusseldorf 1981, 657. (14) Perry, D. L.; Grint, A. Fuel 1983, 62, 1029. (15) Bartle, K. D.; Perry, D. L.; Wallace, S. Fuel Process. Technol. 1987, 15, 351. (16) Wallace, S; Bartle, K. D.; Perry, D. L. Fuel 1989, 68, 1450. (17) Burchill, P; Welch, L. S. Fuel 1989, 68, 100. (18) Wilhelms, A.; Patience, R. L.; Larter, S. R.; Jorgensen, S., Geochim. Acta 1992, 56, 3745. (19) Patience, R. L.; Baxby, M.; Bartle, K. D.; Perry, D. L.; Rees, A. G. W.; Rowland, S. J. Org. Geochem. 1991, 18, 161. (20) Clark, D. T.; Wilson, R. Fuel 1983, 62, 1034. (21) Weitzsacker, C. L.; Gardella, J. A., Jr. Anal. Chem. 1992, 64, 1068. (22) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Int. Conf. Coal Sci. 1993, 2, 270. (23) Fiedler, R.; Bendler, D. Fuel 1992, 71, 381. (24) Kelly, M. D.; Buckley, A. N.; Nelson, P. F. Int. Conf. Coal Sci. 1991, 356.
S0887-0624(97)00124-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/12/1998
160 Energy & Fuels, Vol. 12, No. 1, 1998
most abundant nitrogen forms present in fresh coal. There have been several XPS and XANES studies which have characterized nitrogen in petroleum and coal derived products,25,53-56 kerogen,18,57 and crude oil asphaltenes.7,19 These nondestructive techniques along with 15N NMR37,38 are being developed to yield complementary new information about nitrogen functionalities in carbonaceous materials. Recently, XPS has been used to study the nitrogen transformations during pyrolysis of coal24,26-29,40,41 and model compounds.26,42 The disappearance of quaternary nitrogen species in coal after mild pyrolysis has been associated with the loss of nearby or adjacent hydroxyl groups from carboxylic acids or phenols.28 The appearance of significant levels of quaternary nitrogen following high-temperature pyrolysis26,41 has been associated with nitrogen incorporated into the resulting large polynuclear aromatic carbon structures. Other studies have quantified changes in carbon and oxygen species in coal chars and tars upon pyrolysis.42-48 However, it is desirable to view information about the transformations of nitrogen, carbon, and oxygen together. Understanding nitrogen, carbon and oxygen transformations in both the char and (25) Jimenez Mateos, J. M.; Fierro, J. L. G. Surf. Interface Anal. 1996, 24, 223. (26) Pels, J. R; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641. (27) Nelson, P. F.; Buckley, A. N.; Kelly, M. D. Symp. Int. Combust., [Proc.] 24th 1992, 1259. (28) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896. (29) Pels, J. R.; Wojtowicz, M. A.; Moulijn, Fuel 1993, 72, 373. (30) Blair, D. W.; Wendt, J. O. L.; Bartok, W. Symp. Int. Combust., [Proc.] 16th 1977, 475. (31) Kelemen, S. R.; Kwiatek, P. J. Energy Fuels 1995, 9, 841. (32) Kelemen, S. R.; Freund, H. Energy Fuels 1990, 4, 165. (33) Kelemen, S. R., Freund. H. Energy Fuels 1989, 3, 498. (34) Wu, M. M., Robbins; G. A.; Winschel, R. A.; Burke, F. P. Energy Fuels 1988, 2, 150. (35) Baltrus, J. P.; Diehl, J. R.; D′Este, J. R.; Lander, E. P. Fuel 1990, 69, 117. (36) Kelemen, S. R.,; Gorbaty, M. L.; Vaughn, S. N.; Kwiatek, P. J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, 384. (37) Knicker, H.; Hatcher, P. G.; Scaroni, A. W. Energy Fuels 1995, 9, 999. (38) Solum, M. S.; Pugmire, R. J.; Grant, D. M.; Kelemen, S. R.; Gorbaty, M. L.; Wind, R. A. Energy Fuels 1997, 11, 491. (39) Watt, M.; Allen, W.; Fletcher, T. Int. Conf. Coal Sci. 1995, 1685. (40) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K., Energy Fuels 1993, 7, 1013. (41) Wojtowicz, M. A.; Pels, J. R.; Moulijn, J. A., Fuel 1995, 74, 507. (42) Stanczyk, K.; Dziembaj, R.; Piwowarska, Z.; Witkowski, S. Carbon 1995, 33, 1383. (43) Watt, M.; Fletcher, T. H.; Bai, S.; Solum, M. S.; Pugmire, R. J. Symp. Int. Combust., [Proc.] 26th 1993. (44) Fletcher, T. H.; Solum, M. S.; Grant, D. M.; Critchfield, S.; Pugmire, R. J. Symp. Int. Combust. [Proc.] 23rd 1990, 1231. (45) Pugmire, R. J.; Solum, M. S.; Grant, D. M.; Critchfield, S.; Fletcher, T. H. Fuel 1991, 70, 414. (46) Fletcher, T. H.; Solum, M. S.; Grant, D. M.; Pugmire, R. J. Energy Fuels 1992, 6, 643. (47) Minknis, F. P.; Turner, T. F.; Ennen, L. W.; Netzel, D. A. Fuel 1988, 67, 1568. (48) Vasallo, A. M.; Wilson, M. A.; Edwards, J. H. Fuel 1987, 66, 622. (49) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405. (50) Niksa, S. Energy Fuels 1991, 5, 673. (51) Fletcher, T. H.; Kerstein, A. R.; Pugmire R. J.; Grant, D. M. Energy Fuels 1992, 6, 414. (52) Solomon, P. R.; Fletcher, T. H. Symp. Int. Combust., [Proc.] 25th 1994. (53) Buckley, A. N. Fuel Process. Technol. 1994, 38, 165. (54) Jones, C.; Sammann, E. Carbon 1990, 28, 509. (55) Stohr, B.; Boehm, H. P.; Schlogl, R. Carbon 1991, 29, 707. (56) Jansen, R. J. J.; van Bekkum, H. Carbon 1995, 33, 1021. (57) Mitra-Kirtley, S; Mullins, O. C.; Branthaver, J. F.; Cramer, S. P. Energy Fuels 1993, 7, 1128.
Kelemen et al.
in the volatilized tar would help to complete the picture for nitrogen pyrolysis chemistry in carbonaceous materials. Kinetic information about these transformations is fundamental for developing accurate models of combustion processes49-51 and for predicting nitrogen emissions during combustion of coal.52 The development of direct analytical probes for nitrogen will help advance the understanding of nitrogen in petroleum. Much of the nitrogen in petroleum is associated with the heavy fraction which makes chromatographic and mass spectrometric characterization difficult. The present work quantifies the thermal transformations of nitrogen forms in coals using XPS under kinetically well-defined pyrolysis conditions. II. Experimental Section The XPS spectra were obtained with a Vacuum Generators (VG) ESCA Lab system using either Mg KR or Al KR nonmonochromatic radiation and a five-channel detection arrangement. Data acquisition and analysis were done using the VG supplied VGS 5000 software package. The 5 °C/ s) back to room temperature. Under these conditions, roughly an 85% of the expected hydrocarbon evolution (as represented by the m/e ) 41 peak) had occurred.61 The pyrolysates from the TPD experiments were collected as thin films deposited directly onto the XPS stainless steel nub surfaces. Each sample nub was positioned line of sight, 2 cm away from the outlet of the TPD pyrolysis reactor. The nub served as a lowtemperature cold trap (T 50 °C) for pyrolysis products. TPD (58) Vorres, K. S., Ed. The Users Handbook for the Argonne Premium Coal Sample Program; Argonne National Laboratory: Argonne, Il, 1989; ANL-PCSP-89-1; Energy Fuels 1990, 4, 420. (59) Freihaut, J. D.; Proscia, W. M.; Seery, D. J. Energy Fuels 1989, 3, 692. (60) Freund, H.; Kelemen, S. R. Am. Ass. Pet. Geo. Bull. 1989, 73, 1011. (61) Kelemen, S. R.; Vaughn, S. N.; Gorbaty, M. L.; Kwiatek, P. J. Fuel 1993, 72, 645.
Nitrogen Transformations in Coal during Pyrolysis
Figure 1. Example TPD spectra from selected Argonne Premium Coals for the tar related hydrocarbon peak (m/e ) 41), CO2 (m/e ) 44), H2S (m/e ) 34), and CH4 (m/e ) 16). data are expressed as relative abundance of certain m/e peaks as a function of temperature, such as m/e ) 41 for light hydrocarbons, m/e ) 44 for CO2, and m/e ) 16 for methane. An example of TPD scans is shown in Figure 1. Higher temperature (T g 630 °C) coal chars were also produced in the TPD apparatus. The linear heat-up rate was 0.23 °C/s. Upon reaching the desired temperature the sample was rapidly cooled (>1 °C/s) back to room temperature. The time spent at the targeted temperature was 1 s. The coal char samples were made into fine powders by grinding in a nitrogen drybox using a wig-l-bug. The char samples were mounted in the same way as the coal samples. Slow heat-up pyrolysis experiments below 400 °C were done in a quartz lined reactor in helium at 1 atm. The reactor temperature was raised to 400 °C at approximately 0.2 °C/s. and held at 400 °C for 5 min. Under these conditions, little of the ultimate amount of hydrocarbons expected from the volatile matter determination is released while much of the oxygen as CO2, H2O, and CO evolves.31 The pyrolysis of five different rank coals was performed under rapid heat-up conditions. Samples of tar and char pyrolysis products were generated at atmospheric pressure in a high-pressure controlled-profile (HPCP) drop tube reactor,43,62 which is a laminar flow furnace with a computercontrolled wall temperature profile. The five coal samples were pyrolyzed in nitrogen with a sample heat-up rate of ∼10,000 °C/s. The maximum particle temperatures were 777 °C with a residence time of 210 ms. A permeable liner inside the collection probe tube allows quench gas injection radially along the length of the probe to reduce particle and tar deposition. A virtual impactor and cyclone follow the collection probe to aerodynamically separate char particles from tars and aerosols.43,63 The tars and aerosols are collected on polycarbonate filters and tar samples are scraped from the filters. Wyodak coal pyrolysis was studied as a function of mass release in the HPCP reactor. A sample heat-up rate of ∼104 °C/s was used and the residence time was varied between 110 and 130 ms at maximum particle temperatures from 577 to 647 °C to achieve different percentages of mass release. Additional samples of Wyodak coal chars were prepared by injecting entrained pulverized coal particles through a methane/hydrogen/ air flat flame burner operating under fuel-rich conditions. This allowed for char samples to be produced at ∼105 °C/s at a gas (62) Monson, C. R., Germane, G. J. Energy Fuels 1993, 7, 928. (63) Fletcher, T. H. Combust. Flame 1989, 78, 223.
Energy & Fuels, Vol. 12, No. 1, 1998 161 temperature of 1377 °C. The residence time varied between 15 and 30 ms. The flat flame burner experiments used a collection system similar to that used in the drop tube experiments. Elemental analysis of char samples were carried out with XPS. Elemental concentrations are reported relative to carbon, calculated from the areas of the XPS peaks after correcting for differences for atomic sensitivity. The amount of organic oxygen was derived from the total oxygen (1s) signal by taking into account inorganic contributions. The amount of inorganic oxygen associated with silicon and aluminum, calcium, and iron were taken as SiO2, AlO1.5, CaO, and FeO1.5. A detailed analysis was not performed of the XPS carbon (1s) line shape to quantify the kinds of organic oxygen species in coal tars and chars. The relative amount of aromatic carbon, in lower temperature (T < 690 °C) chars, was determined by the XPS method of π to π* shake-up signal intensity.64 For higher temperature chars the XPS carbon (1s) signal gradually changed from a symmetric peak to one with a pronounced asymmetry toward higher binding energies. This marks the transition from small to much larger polynuclear aromatic structures.65,66 The method π to π* shake-up signal intensity could not be used to evaluate the percentage of aromatic carbon for high-temperature chars (T g 690 °C). Additional elemental analysis on starting materials and on char samples were obtained from Galbraith Analytical Labs, Knoxville, TN. Under XPS data acquisition conditions described earlier in the Experimental Section, the nitrogen (1s) spectra from model compounds containing a single nitrogen species could be curveresolved using a single peak. The full width at half-maximum (fwhm) of 1.7 ((0.05) eV and a mixed 70-30% GaussianLorentzian line shape provided the best fit to the model compound data. The XPS nitrogen (1s) signals for the fresh Argonne premium coal samples have already been reported28 and were based on curve resolution analysis of the nitrogen (1s) spectra. Peaks at 398.8, 400.2, and 401.4 ((0.1)(eV) were used for curve resolution. These peaks correspond to the energy positions found for pyridinic, pyrrolic, and quaternary type nitrogen functionalities, respectively. In each case the nitrogen (1s) signal was curve-resolved using a 70% Gaussian 30% Lorentzian line shape and a peak fwhm of 1.7 (eV) for each coal. The peak shape and peak energy positions were fixed in the curve resolution process. Only the amplitudes of these peaks were varied to obtain the best fit to the experimental XPS data. The same curve resolution procedure was used in the present study of nitrogen in the low-temperature pyrolysis chars and tars (prior to developing a pronounced asymmetric carbon (1s) line shape). It should be pointed out that the binding energy of amino species (399.4 eV) in model compounds appear close to pyridinic forms. The special quantification problems that this situation presents have already been extensively discussed.28 Briefly, the lower sensitivity limit for amino groups in fresh coal is 5 mol %. The presence of primary amines at these low levels cannot be unequivocally distinguished from pyridinic forms. A peak at 399.4 eV was needed to obtain a satisfactory fit to the XPS spectra of some coal tar samples. This indicates that the relative percentage of these species in coal tars exceed 5 mol %. The structural transition of the carbon matrix from mostly three to four ring aromatic units to much larger polynuclear aromatic (PNA) units has implications for the XPS nitrogen (1s) curve resolution procedure described above. For hightemperature pyrolysis chars (T g 690 °C) where the transition to large PNA units is favored, the energy position of the carbon (1s) energy moves closer to the energy position of graphitic carbon (284.5 ( 0.2 eV).25 The energy position of graphite (64) Kelemen, S. R.; Rose, K. D.; Kwiatek, P. J. Appl. Surf. Sci. 1993, 64, 167. (65) Cheung, T. T. P. J. Appl. Phys. 1982, 53, 6857. (66) Cheung, T. T. P., J. Appl. Phys. 1984, 55, 1388.
162 Energy & Fuels, Vol. 12, No. 1, 1998 depends on its crystalline perfection.87 The significant change in the electronic properties of the carbon matrix upon hightemperature pyrolysis opened up the possibility for changes in both the relaxation and chemical shift contributions to the nitrogen (1s) binding energy for a given species. It was necessary to modify the curve resolution procedures for the nitrogen (1s) spectrum of high-temperature pyrolysis chars (T g 690 °C) in the following way. The position of the carbon (1s) energy reference was measured at 284.6 eV. Peaks at 398.5, 400.1, and 401.1 ((0.1) eV were used in the curve resolution process. These peaks correspond to the energy positions found for pyridinic, pyrrolic and quaternary type nitrogen functionalities, respectively.
III. Results The pyrolysis of coal has been conceptually divided into three general stages by the Rock Eval method of pyrolysis.67-69 The first stage is characterized by the volatilization of existing hydrocarbons present in the sample and occurs below about 200 °C. This hydrocarbon evolution is called the Rock Eval S1 peak.69 During and after this period, but generally prior to the pyrolytic evolution of more hydrocarbons, there is a loss of organic oxygen and sulfur functionalities. The loss is predominantly a result of CO2, H2O, CO, and H2S evolution due to pyrolytic reactions. In Rock Eval the CO2 is trapped and measured and is called the Rock Eval S3 peak.69 The main pyrolytic evolution of hydrocarbon as tar generally occurs between 400 and 550 °C and is called the Rock Eval S2 peak. CH4 evolves both during and after the S2 peak.67 For coal, a distinction in pyrolytic volatile matter between the lower temperature evolution of CO2, H2O, CO, and H2S31,61,67,70,71 and the evolution of tar and higher temperature CH436,67,70,71 is usually not made. In our study of the pyrolysis of coal it is convenient to define the different pyrolysis stages simply as pyrolysis that occurs either before, during, or after the tar or S2 hydrocarbon peak. Since the tar or (67) Burnaham, A. K.; Oh, M. S.; Crawford, R. W. Energy Fuels 1989, 3, 42. (68) Espitalie, J.; Laporte, J. L.; Madec, M.; Marquis, F.; Leplat, P.; Paulet, J.; Boutefeu, A. Rev. Inst. Fr. Pet. 1977, 32, 23. (69) Tissot, B. P.; Welte Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: Berlin, 1984. (70) Solomon, P. R.; Colket, M. B. Fuel 1978, 5, 749. (71) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Bassilakis, R.; Gravel, D.; Baillargeon, M.; Baudais, F.; Vail, G. Energy Fuels 1990, 4, 320. (72) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels, 1993, 7, 710. (73) Daniels, E. J.; Altaner, S. P. Am. Mineral. 1990, 75, 835. (74) Juster, T. C.; Brown, P. E.; Bailey, S. W. Am. Mineral. 1987, 72, 555. (75) Daniels, E. J.; Altaner, S. P. Int. J. Coal Geol. 1993, 22, 21. (76) Buckley, A. N.; Kelly, M. D.; Nelson, P. F.; Riley, K. W. Fuel Process. Technol. 1995, 43, 47. (77) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (78) Clark, D. T., Dilks, A. J. Polym. Sci., Polym. Chem. Ed. 1976, 14, 533. (79) Clark, D. T.; Dilks, A.; Peeling, J.; Thomas, H. R. Faraday Discuss. Chem. Soc. 1975, 60, 183. (80) Clark, D. T.; Adams, D. B.; Dilks, A.; Peeling, J.; Thomas, H. R. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 51. (81) Clark, D. T.; Dilks, A. J. Polym. Sci., Chem. Ed. 1977, 14, 15. (82) Dwight, D. W.; McGrath, J. E.; Wightman, J. P., J. Appl. Polym. Sci. 1978, 34, 35. (83) Salaneck, W. R.; Thomas, H. R. Solid State Commun. 1978, 27, 685. (84) Fabish, T. J.; Thomas, H. R. Macromolecules 1980, 13, 1487. (85) Daniels J.; Festenberg, C. V.; Raether, H.; Zeppenfeled, R. Springer Tracts Mod. Phys. 1970, 54, 78. (86) Kelemen, S. R.; Freund, H. Energy Fuels 1988, 2, 111. (87) Schlogl, R.; Boehm, H. P. Carbon 1983, 21, 345.
Kelemen et al. Table 1. XPS Analyses of Fresh Argonne Premium Coal and Residues after Pyrolysis at 400 °C for 300 s
coal Beulah Zap
organic nitrogen aromatic oxygen per per 100 carbon per sample 100 carbons carbons 100 carbons
fresh 400 °C Wyodak fresh 400 °C Illinois No. 6 fresh 400 °C Blind Canyon fresh 400 °C Pittsburgh No. 8 fresh 400 °C Lewiston fresh 400 °C Upper Freeport fresh 400 °C Pocahontas fresh 400 °C
18.8 10.7 16.9 10.8 10.9 8.2 10.0 8.5 7.8 6.2 8.0 6.5 4.5 3.8 3.2 3.0
1.5 1.3 1.3 1.3 1.2 1.2 1.6 1.4 1.2 1.5 1.0 1.1 1.5 1.0 1.2 1.0
53 71 52 66 64 71 59 67 65 72 73 75 71 72 80 82
S2 evolution process follows well-defined kinetics61,67,71 at TPD and Rock Eval-like heating rates, we have chosen to characterize the transformations of nitrogen forms in coal that occur just prior to tar evolution (predevolatilized char) and just after tar evolution (devolatilized char). The tar was also collected and analyzed. Additionally, the nitrogen transformations that occurred in chars at higher temperatures after tar evolution (pyrolyzed char) have also been analyzed. Representative TPD spectra for several of the Argonne premium coals are shown in Figure 1 for the tar related hydrocarbon m/e ) 41 peak, CO2, H2S, and CH4. III. A. Predevolatilized Chars (Pyrolysis Prior to Hydrocarbon Devolatilization). Fresh Argonne Premium coal samples were heated to 400 °C and held at that temperature for 300 s. Under these conditions, little of the ultimate amount of hydrocarbons expected from the volatile matter analysis evolved. The XPS analyses for total nitrogen in predevolatilized chars are shown in Table 1. Relative to the initial coal, the amount of nitrogen in the predevolatilized chars is similar for all coals. In contrast to nitrogen, there is a significant loss in the amount of organic oxygen upon pyrolysis, especially with lower rank coals. The loss of organic oxygen functionalities is predominantly via evolution of CO2, H2O, and CO.67,71 The loss of nitrogen as small gaseous molecules (i.e., NH3, HCN, etc.) does not occur at low-temperature (T < 450 °C)30,70-72 prior to the devolatilization of hydrocarbons. Since some carbon is lost with CO2 and CO evolution as well as through formation of small quantities of methane and other light hydrocarbon gases, a slight increase in the relative level of nitrogen-to-carbon ratio might be expected if nearly all of the initial nitrogen is retained in the 400 °C pyrolysis chars; however, the relative amount of nitrogen stays the same in each coal following pyrolysis at 400 °C. The level of aromatic carbon increases upon mild pyrolysis of coal. The relative increase in the level aromatic carbon is greatest for lower rank coals. It is interesting to note that the range in the level of aromatic carbon is much smaller for the predevolatilized chars than for the parent coals. Figure 2 shows the XPS nitrogen (1s) spectra with the curve-resolved individual peaks for the 400 °C coal pyrolysis samples. The relative amounts of nitrogen species present in the prepyrolysis coal chars were
Nitrogen Transformations in Coal during Pyrolysis
Energy & Fuels, Vol. 12, No. 1, 1998 163 Table 3. XPS Results for Argonne Premium Coal Chars and Tars Produced by Pyrolysis at 510 °C for 30 s
coal Beulah Zap Wyodak Illinois No. 6 Blind Canyon Pittsburgh No. 8 Lewiston Upper Freeport Pocahontas
Figure 2. XPS nitrogen (1s) spectra of Argonne Premium Coal chars produced by pyrolysis at 400 °C for 300 s. Included in the figure are the XPS curve resolution results. Table 2. XPS Nitrogen (1s) Curve Resolution Results for Fresh Argonne Premium Coals and Residues after Pyrolysis at 400 °C for 300 s coal Beulah Zap Wyodak Illinois No. 6 Blind Canyon Pittsburgh No. 8 Lewiston Upper Freeport Pocahontas
sample
mol % pyridinic
mol % pyrrolic
mol % quaternary
fresh 400 °C fresh 400 °C fresh 400 °C fresh 400 °C fresh 400 °C fresh 400 °C fresh 400 °C fresh 400 °C
26 29 25 30 26 30 31 39 32 35 31 34 28 35 33 34
58 60 60 61 62 63 55 57 61 63 60 63 65 63 64 65
16 11 15 9 12 7 14 4 7 2 9 3 7 2 3 1
obtained from an analysis of the XPS nitrogen (1s) line shape and are shown in Table 2. Included in Table 2 are the results for the fresh coals for comparison. The prepyrolysis chars could be curve-resolved into three peaks corresponding to the energy positions of pyridinic, pyrrolic, and quaternary nitrogen. Under these relatively mild pyrolysis conditions the level of quaternary nitrogen decreases in all coals. There is a corresponding increase in the level of pyridinic nitrogen forms. The level of pyrrolic nitrogen remains almost constant. This pattern for nitrogen has been noted before for Wyodak and Illinois No. 628 as well as for other coals.26 Since the amount of nitrogen in the parent coal and in the
pyrolysis 510 °C sample
organic oxygen per 100 carbons
nitrogen per 100 carbons
aromatic carbon per 100 carbons
char tar char tar char tar char tar char tar char tar char tar char tar
7.5 5.5 10.0 3.7 7.9 7.1 7.3 7.5 6.6 5.1 7.6 6.2 5.5 3.8 3.0 2.2
1.2 1.1 1.5 1.4 1.3 1.7 1.3 1.7 1.4 1.5 1.1 1.2 1.3 1.8 1.1 1.0
69 49 70 43 70 66 66 73 72 73 71 73 82 76 82 76
predevolatilized chars is nearly constant, the pattern can be explained if the quaternary nitrogen species are pyridinic forms associated with hydroxyls and that the association is broken as a result of thermal reaction involving the loss of hydroxyl oxygen species.28 III. B. Slow Heatup Rate Devolatilized Chars and Product Tars. We analyzed chars produced following evolution of most of the tar. These samples were heated to 510 °C for 30 s. Table 3 shows that the relative amount of nitrogen present in the char and tar are similar. These levels are also similar to the amount of nitrogen in the fresh coal samples (Table 1). For Illinois No. 6 and Blind Canyon, the organic oxygen level is similar in the tars and in the chars. Most coal tars have a slightly lower level of organic oxygen than in the corresponding chars with the notable exception of Wyodak coal where there is much less oxygen in the tar. When compared to the starting coals, there is much less oxygen in both the tar and char of Beulah Zap and Wyodak coal. This is easily explained by the evolution of oxygen species, predominantly as CO2, H2O and CO.31,67,71 The level of aromatic carbon is similar in most of the corresponding tar and char samples. The two exceptions are Beulah Zap and Wyodak coal where there is significantly less aromatic carbon in the tar. It is interesting to point out that the aromatic carbon level in all of the chars fall inside a fairly narrow range (69-82 per 100 carbons). This is similar to the result found for predevolatilization chars. In general, the amounts of aromatic carbon are increased in the char samples relative to those in the parent coal. The largest relative increases are observed for the lowest rank coals (Table 3). The relative amounts of nitrogen species present in the coal char and tar samples were obtained from analysis of the XPS nitrogen (1s) line shapes. Figures 3 and 4 show the XPS nitrogen (1s) spectra with the curve-resolved individual peaks for the 510 °C char and tar samples, respectively. Table 4 contains the curve resolution results for the chars and tars. XPS nitrogen (1s) spectra of the chars could be curve-resolved into three peaks corresponding to the energy positions of pyridinic, pyrrolic, and quaternary nitrogen. For the lower rank coal tars it was necessary to include a small peak at the position expected for amino nitrogen in order
164 Energy & Fuels, Vol. 12, No. 1, 1998
Figure 3. XPS nitrogen (1s) spectra of Argonne Premium Coal chars produced by pyrolysis at 510 °C for 30 s. Included in the figure are the XPS curve resolution results.
to obtain a good fit to those spectra. For the higher rank coals tars this was not necessary and indicates that if amino groups are present in the coal tar they are at a concentration of
