Solum, MS
2000
A Global Free-Radical Mechanism for Light Gas Nitrogen Release from Coal during Devolatilization
Perry, S.T.; Fletcher, T.H.; Pugmire, R.J. and Solum, M.S.
Energy & Fuels, 14, 1094-1102 (2000).
Contact: Fletcher
Solid-State C-13 NMR Characterization of Matched Tars and Chars From Rapid Coal Devolatilization
Perry, S.T.; Hambly, E.M.; Fletcher, T.H.; Solum, M.S. and Pugmire, R.J.
Proceedings of the Combustion Institute, 28 (in press, 2000).
Contact: Fletcher
1998
Nitrogen Transformation in Coal During Pyrolysis
Kelemen, S.R.; Gorbaty, M.L.; Kwiatek, P.J.; Fletcher, T.H.; Watt, M.; Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 12:159-73 (1998).
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 (10^14 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 "graphitic-like" 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.
1997
Dynamic Nuclear Polarization of Nitrogen-15 Benzamide
Hu, J.Z.; Zhou, J.; Yang, B.; Li, L.; Qiu, J.; Ye, C.; Solum, M.S.; Wind, R.A.; Pugmire, R.J. and Grant, D.M.
SOLID STATE: Nuclear Magnetic Resonance, 8:129-37(1997). Funded by US Department of Energy/Basic Energy Services and ACERC.
A N-15 dynamic nuclear polarization (DNP) experiment is reported in which a N-15 DNP enhancement factor of approximately 2.6 x 10² is obtained on free radical doped samples of 99% N-15 labeled benazmide. The free radicals BDPA (1:1 complex of alpha, gamma-bisdiphenylene-beta-phenylallyl with benzene) and DDPH (2,2 -Di (4 - tert - octylphenyl) -1-picrylhydrazyl) are used as dopants and the spin relaxation effects of adding these dopants are studied by means of changes in proton and nitrogen T1 values of the samples. The combination is solids of a very low natural abundance, 0.37%, a small gyromagnetic ration, and a log spin - lattice relaxation time for N-15 nuclei create severe sensitivity problems that, in large part, are ameliorated by the signal enhancement observed in the N-15 DNP experiment on samples containing free electrons.
Solid State N-15 and C-13 NMR Study of Several Metal 5-, 10-, 15-, 20-Tetraphenylporphyrin Complexes
Strohmeir, M.; Orendt, A.M.; Facelli, J.C.; Solum, M.S.; Pugmire, R.J.; Pery, R.W. and Grant, D.M.
Journal of the American Chemical Society, 119:7114-120(1997). Funded by National Institutes of Health, US Department of Energy.
The principal values both the C-13 and N-15 chemical shift tensors are reported in the Zn, Ni, and Mg 5, 10, 15, 20- tetraphenylporphyrin (TTP) complexes. The principal values of the 15N chemical shift tensors were obtained from static powder patterns of N-15 enriched samples. Due to overlap between the powder patterns of the different carbons, the C-13 values were obtained using the recently developed magic angle turning (MAT) 2D experiment on unenriched materials. The measured principals values are presented along with theoretical calculations of the chemical shift tensors and a discussion of the effects that the metal bonding has on the chemical shift tensors in these compounds. Both the isotropic chemical shift and the principal values of the N-15 chemical shift tensor are nearly identical for the Mg and Zn complexes. The N-15 isotropic chemical shift for the NiTPP, however, changes by nearly 80 ppm relative to the Mg and Zn values, with large changes observed in each of the tree principal values. Calculations show that the differences between the N-15 chemical shifts are almost entirely determined by the metal-nitrogen separation. In addition, both the experimental data and the calculations show only very minor differences in the 13C chemical shift tensor components as the metal is changed.
Nitrogen Transformations in Coal During Pyrolysis
Kelemen, S.R.; Gorbaty, M.L.; Kwiatek, P.J.; Fletcher, T.H.; Watt, M.; Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 12:159-72(1997). Funded by ACERC, Exxon and Federal Energy Technology Center.
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 coal 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 theses 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, sever pyrolysis of the devolatilized char results in the appearance of asymmetric carbon (1s) line shape indicative of very large polynuclear "graphitic-like" 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.
N-15 CPMAS NMR of the Argonne Premium Coals
Solum, M.S.; Pugmire, R.J; Grant, D.M.; Kelemen, S.R.; Gorbaty, M.L. and Wind, R.A.
Energy & Fuels, 11:491-94(1997). Funded by ACERC and US Department of Energy.
N-15 NMR data are reported for the Argonne Premium Coals. Arguments are presented to explain discrepancies between observations and conclusions obtained from NMR experiments and those obtained by XPS and XANES techniques. Delectability of different types of nitrogens is discussed in terms of cross-polarization dynamics together with effects of the large chemical shift anisotropy that is found in different types of nitrogen functional groups. Data on acid-treated coals confirm the presence of pyridinic type nitrogens that were not observed in a previous N-15 NMR study of coals.
N-15 NMR Spectroscopy of Coals and Pyrolysis Products
Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Fletcher, T.H. and Wind, R.A.
Proceedings of the 9th International Conference on Coal Science, Essen, Germany, September 7-12, 1997. Funded by ACERC, US Department of Energy/University Coal Research and New Energy and Industrial Technology Development Organization.
N-15 NMR spectra are reported for a number of coals from Pacific Rim countries. Arguments are presented to explain discrepancies between observations and conclusions obtained from NMR experiments and those obtained by XPS and XANES techniques. Detection of different types of nitrogen species is discussed in terms of cross polarization dynamics together with the effects of the large chemical shift anisotropy that are found in different types of nitrogen functional groups. Significant differences are observed in the types of nitrogen present in these coals and these variations are associated with coal rank as has been in a previous study of the Argonne Premium coals. We have also begun to examine pyrolysis char and tar samples. The NMR data indicate that significant differences exist between the types of nitrogen structures observed in coal, char and tar samples. These differences suggest that different mechanisms may exist for nitrogen release from tar and char samples.
1993
Chemical Structure of Coal Tar During Devolatilization
Fletcher, T.H.; Watt, M.; Bai, S.; Solum, M.S. and Pugmire, R.J.
Twenty-Sixth Symposium (International) on Combustion, (in press) 1996. (Also presented at the Twenty-Sixth Symposium (International) on Combustion, Naples, Italy, July, 1996.) Funded by ACERC and US Department of Energy/University Coal Research.
Three coals of different rank were pyrolyzed in a drop tube reactor at a maximum temperature of 900 K and residence time of 160 ms. The coal and char were analyzed with solid state C-13 NMR. The tar was dissolved in deuterated methylene chloride. It was found that the tar was only partially soluble in CD2CL2-. The non-soluble tar portion was analyzed using a recently developed high-resolution C-13 NMR technique developed for liquid phases. The tar structure was found to be significantly different from the structure of the char and coal. The number of bridges and loops per cluster in the tar was up to 65% lower than in the char. In addition, the number of aromatic carbons per cluster in the tar was significantly lower than that found in either the coal or the char. Since the molecular weight per cluster in the tar is lower than reported average tar molecular weights, these data imply that tar is made up of a number of multiple clusters (dimers, trimers, etc.) as well as single clusters (i.e., monomers). The mass of nitrogen per cluster in the tar was found to be significantly lower in the tar than in either the coal or the char. These experimental findings suggest that changes may be necessary in current network devolatilization models to accurately describe the changes in chemical structure.
1994
Magic Angle Turning Experiments for Measuring Chemical-Shift-Tensor Principal Values in Powdered Solids
Hu, J.Z.; Wang, W.; Liu, F.; Solum, M.S.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
J. Magnetic Resonance, 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.
The magic-angle-turning (MAT) technique introduced by Gan employs slow (ca. 30 Hz) rotation of a powdered sample at the magic angle, in concert with pulses synchronized at 1/3 of the rotor period, to obtain isotropic-shift information in one dimension of a 2D spectrum. The other dimension displays a slow-spinning-sideband powder pattern that, at the low rotor frequencies employed, resembles the stationary-sample powder pattern. The MAT method is very effective for measuring chemical-shift principal values in compounds where spectral overlap precludes the use of 1D methods. Previous MAT implementations are reviewed, and it is shown how a new phase correct MAT (PHORMAT) pulse sequence overcomes many of their limitations. This new pulse sequence produces a spinning-sideband-free isotropic-shift spectrum directly as a projection onto the evolution axis with no spectral shearing. Only two purging operations are employed, resulting in a higher signal-to-noise ratio. Pure absorption-absorption phased 2D spectra are produced. Flat 2D base planes result from an echo sequence which delays acquisition until after probe ring-down and receiver recovery. The technique used for synchronizing the pulses to 1/3 the rotor period without relying on absolute rotor-frequency stability is described. The PHORMAT spectrum of methyl a-D-glucopyranoside is presented. The data are analyzed with an emphasis on the quantitative accuracy of the experiment in measuring chemical shift tensor principal values and determining the relative number of spins of each type present. The FID data from the spectrometer acquisition are fitted with numerical simulations that employ a banded-matrix method for calculation spinning sideband amplitudes. The chemical shift principal values, measured in methyl a-D-glucopyranoside with the PHORMAT method, are compared with those from a single-crystal determination of the full chemical shift tensors. The two measurements differ by an rms average distance of only 0.57 ppm.
1993
Chemical Structural Features of Pyridine Extracts and Residues of the Argonne Premium Coals Using Solid State C-13 NMR Spectroscopy
Fletcher, T.H.; Bia, S.; Pugmire, R.J.; Solum, M.S.; Woods, S. and Grant, D.M.
Energy & Fuels, 7 (6):734-742, 1993. (Presented at the Spring Meeting of the Western States Section of the Combustion Institute, Salt Lake City, UT, March 1993.) Funded by ACERC.
Soxhlet extractions were performed on the eight Argonne Premium coals using pyridine purged with argon and followed by a novel washing procedure to remove the pyridine. Mass closure (extracts plus residues) on duplicate experiments accounted for 94-96% of the original coal, repeatable to within 2%. Chemical structural features determined from C-13 NMR analyses of the extracts and residues showed more attachments per aromatic cluster for the residues, indicating a greater degree of covalent bonding in the residue than in the extract. H-1 NMR analysis of the extracts showed a gradual increase in the hydrogen aromaticity with rank, along with a maximum in the percentage of a-hydrogen in the high-volatile bituminous coals. Composite chemical features constructed from weighted averages of the features of the residues and extracts agree with many of the features of the parent coal. Chemical structural features of the extracts determined from H-1 NMR analyses agree with similar data reported previously for early coal tars during devolatilization at rapid heating rates.
Measurement of C-13 Chemical-Shift Anisotropy in Coal
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Carbonaceous Solids, Advances in Chemistry Series #229, 22:419-439, 1993. Funded by US Department of Energy, Basic Energy Services and ACERC.
The methods of available in NMR spectroscopy to obtain the principal values of the chemical-shift tensor are discussed. Applications to coal and to compounds with model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical-shift powder patterns is compared to results obtained by cross-polarization with magic-angle spinning and dipolar dephasing.
Chemical Structural Features of Coal Chars, Tars, and Char Extracts During Rapid Pyrolysis Using C-13 and H-1 NMR Spectroscopy
Fletcher, T.H.; Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Bai, S.; Ma, J. and Woods, S.
7th International Conference on Coal Science, Banff, Alberta, Canada, September 1993 (in press). Funded by ACERC.
Structural characteristics have been determined for parent coals and for chars collected at different stages of pyrolysis. Recent work has focused on trying to understand the relationship between chemical structural features of the unreacted coal and the devolatilization and char oxidation phenomena. Models of coal devolatilization have recently related devolatilization behavior to the structure of the parent coal and the initial amount of pyridine extracts. Fong et al. used pyridine extraction methods to quantify the amount of metaplast formed during pyrolysis of a Pittsburgh #8 coal. These experiments demonstrated that under moderate heating conditions (~500 K/s to 873 K), as much as 80% of the initial coal was transformed into a combination of extractable material and volatiles. The work presented here is an examination of the pyridine extraction procedure of the Argonne Premium coal samples and the detailed study of the carbon skeletal structure of the extracts and the extraction residues from these coals. This is the first stage of an experimental program to examine the yield and chemical features of extracts of coal chars collected as a function of time during pyrolysis.
The Use of Solid State C-13 NMR Spectroscopy to Study Pyridine Extracted and Extraction Residues in the Argonne Premium Coals
Pugmire, R.J.; Solum, M.S.; Bai, S.; Fletcher, T.H.; Woods, S. and Grant, D.M.
Proceedings of the 205th ACS National Meeting, 38, no. 2, Denver, CO, March 1993. Funded by ACERC.
The relationship between coal structure and combustion behavior is a matter of on-going research in our laboratories. A great deal of effort has gone into obtaining data that is used for modeling studies of devolatilization behavior. We have also carefully studied the process of char formation. Our past work has focused on trying to understand the relationship between coal/char/tar formation as they relate to the devolatilization and char oxidation phenomena. The formation of metaplast during pyrolysis was studied by Fong and Howard in terms of extractable material obtained at different stages of the devolatilization process. We have recently turned our attention to metaplast formation in devolatilization and plan to conduct a series of experiments that will help define the formation and chemical structure of metaplast in coals of different rank.
Coal and Char Structural Parameters Derived from Solid State C-13 NMR Studies
Pugmire, R.J.; Solum, M.S.; Fletcher, T.H. and Grant, D.M.
The 5th Australian Coal Science Conference, Melbourne, Australia, March 1993. (Also presented at the 5th Australian Coal Science Conference, Melbourne, Australia, November, 1992.) Funded by ACERC.
In contrast to previous efforts where coal-general devolatilization model input parameters describing chemical structure are adjusted to force agreement between predicted and measured tar and total volatiles yields, coal-dependent chemical structure coefficients for the Chemical Percolation Devolatilization (CPD) model developed by the authors and others are taken directly from C-13 NMR analyses of parent coals. This procedure, outlined in the paper eliminates most adjustable parameters from the model, and predictions of tar and total volatiles yields become true tests of the model and the NMR data, rather than mere results of curve fitting. Resulting model predictions of tar and total volatiles yields as a function of coal type, temperature, heating rate, and pressure comparable with available experimental data, showing the value of both the model and the NMR chemical structure data.
1992
C-13 NMR Techniques for Structural Studies of Coals and Coal Chars
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Chapter 10, Advances in Coal Spectroscopy, (H.L.C. Meuzelaar, ed.), Plenum Publishing Corp., New York, 1992. Funded by Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction, US Department of Energy and ACERC.
Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.
Chemical Structure of Char in the Transition from Devolatilization to Combustion
Fletcher, T.H.; Solum, M.S.; Grant, D.M. and Pugmire, R.J.
Energy & Fuels, 6(5):643-650, 1992. [Also included in the ACS Division of Fuel Chemistry Preprints, 37(2):677-684, 1992 (203rd ACS National Meeting, San Francisco, CA, April 1992)]. Funded by Sandia National Laboratories, Brigham Young University and ACERC.
Coal devolatilization experiments are generally conducted separately from char oxidation experiments, and the relationship between the chars generated in the two types of research is often ignored. However, char is one of the most important products of coal devolatilization and must be characterized as a function of temperature and heating rate in a manner similar to that for gaseous behavior. In this work, the chemical structures of chars from five coals of different rank are examined, and implications on char reactivity are discussed. Chars were obtained as a function of residence time in a devolatilization experiment (1250 K, 2 x 104 J/s in nitrogen) and just subsequent to devolatilization in a laminar flame-fired experiment (1500 K, 5 x 104 K/s). Quantitative measurements of chemical structure were performed on the coals and chars using C-13 nuclear magnetic resonance (NMR). Results show that the chemical structures of fully devolatilized chars are very similar, even though a wide diversity is seen in the parent coal structures. For example, the average cluster molecular weights of the chars span a range of only 50 amu, and the side-chain molecular weights of the chars span a range of only 6 amu. The similarity in chemical structure of fully devolatilized coal chars suggests that differences in measured heterogeneous char reactivities are caused by differences in the physical structure of the char.
An Efficient Large Sample Volume System for Solid State NMR
Pugmire, R.J.; Jiang, Y.J.; Solum, M.S. and Grant, D.M.
Bulletin of Magnetic Resonance, 14:144, 1992. (Presented at ISMAR 92, Vancouver, B.C., July 1992.) Funded by ACERC.
An efficient large sample volume system has been developed to carry out MAS solid-state NMR experiments. The system components are primarily zirconia and macor and no background C-13 is observed. The stator design employs separate air bearing and drive systems and is run using dry air at a bearing pressure of about 32 psi. the rotor can be spun in a stable manner from less than one hundred Hz (with a driving pressure of 5 psi) to 4.3 KHz (24 psi driving pressure). This low gas pressure feature makes the system easy to operate. The volume of the rotor is 1.8 cm³ and it can hold 1.1 g of HMB. The S/N ratio obtained is a factor of 4.6 better than the rotor previously designed and used in our laboratory (volume 0.6 cm³ : 0.28g HMB). This increased sample size allows us to obtain the same S/N ratio in a MAS spectrum with a factor of 21 saving in spectrometer time. The time saving achieved with this rotor system is extremely useful in obtaining data on biological samples and polymers, and is especially useful when experiments on fossil fuels require the use of the Bloch delay technique. Examples of relevant applications will be discussed.
Coal and Char Structural Parameters Derived from Solid-State C-13 NMR Studies
Pugmire, R.J.; Solum, M.S.; Fletcher, T.H. and Grant, D.M.
Fifth Australian Coal Science Conference, Melbourne, Australia, November 1992. Funded by ACERC.
In contrast to previous efforts where coal-general devolatilization model input parameters describing chemical structure are adjusted to force agreement between predicted and measured tar and total volatiles yields, coal-dependent chemical structure coefficients for the Chemical Percolation Devolatilization (CPD) model developed by the authors and others are taken directly from C-13 NMR analyses of parent coals. This procedure, outlined in the paper eliminates most adjustable parameters from the model, and predictions of tar and total volatiles yields become true tests of the model and the NMR data, rather than mere results of curve fitting. Resulting model predictions of tar and total volatiles yields as a function of coal type, temperature, heating rate, and pressure comparable with available experimental data, showing the value of both the model and the NMR chemical structure data.
The Use of C-13 NMR Structural Information for Predicting Coal Devolatilization Behavior
Pugmire, R.J.; Solum, M.S.; Grant, D.M. and Fletcher, T.H.
Second International Conference on Coal Structure, Krakow, Poland, September 1992. Funded by ACERC.
Recent coal devolatilization models have attempted to incorporate coal structural information to predict product yields and composition. The chemical structure of the parent coal directly affects devolatilization behavior. The parent coal structure also has implications for pyrolysis char structure and reactivity. C-13 NMR data have been used to obtain coal-dependent chemical structural data that is used as input parameters to the chemical percolation devolatilization (CPD) model. The CPD model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the parent coal. Data will be presented on 20 different coals and the value of NMR data in predicting gas and tar yields will be discussed. The use of C-13 NMR data will also be used to describe the relationship between chemical structure features of the parent coals and the resultant pyrolysis chars.
1991
C-13 NMR Techniques for Structural Studies of Coals and Coal Chars
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Advances in Coal Spectroscopy, (H.L.C. Meuzelaar, ed.), Plenum Publishing Corp., New York, 1991 (in press). Funded by Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction, US Department of Energy and ACERC.
Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.
Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments
Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 70:414-423, 1991. Funded by Pittsburgh Energy Technology Center and ACERC.
Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.
Measurement of C-13 Chemical Shielding Anisotropy in Coal
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Solid Carbonaceous Fuels, ACS Symposium Series, (R.E. Botto and Y. Sanada, eds.), 1991 (in press). Funded by US Department of Energy and ACERC.
The methods available in nuclear magnetic resonance to obtain the principal values of the shielding tensor are discussed. Applications to coal and to compounds that model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical shielding powder patterns is compared to results obtained by cross polarization with magic angle spinning (CP/MAS) and dipolar dephasing (DD).
Network Changes During Coal Pyrolysis: Experiment and Theory
Solomon, P.R.; Charpenay, S.; Yu, Z.-Z.; Serio, M.A.; Kroo, E.; Solum, M.S. and Pugmire, R.J.
8th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, October 1991. Funded by US Department of Energy and ACERC.
Coal pyrolysis is a complicated combination of chemical and physical processes in which coal is transformed at elevated temperatures to produce gases, tar, and char. These processes are described in the Functional Group - Depolymerization, Vaporization, and Crosslinking (FG-DVC) model of coal pyrolysis. An important aspect of this model is that crosslinking is rank dependent. This is based on solvent swelling experiments on chars made from coals of different rank. Low rank coals start to loose their solvent swelling ability prior to significant depolymerization at temperatures as low as 200ºC. Including such crosslinking in the FG-DVC model leads to predictions for low rank coals of a highly crosslinked network (exhibited by low solubility and low fluidity in chars) and low tar amounts.
While the model is in good agreement with a variety of data, it is difficult to find experiments to validate the predicted behavior of the network. In this paper we have used CP-MAS, NMR with dipolar dephasing and other techniques to examine the chars changing functional group and network characteristics. The changes in the char composition have been modeled using the FG-DVC model and the results compared with the data for Pittsburgh Seam bituminous coal and Zap lignite.
1990
C-13 NMR Techniques for Structural Studies of Coals and Coal Chars
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Advances in Coal Spectroscopy, Meuzelaar, H.L.C., Editor, Plenum Publishing Corp., New York, 1990 (In press). Funded by ACERC and US Department of Energy.
Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.
Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments
Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 1990 (In press). Funded by ACERC.
Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.
A Chemical Percolation Model of Coal Devolatilization: 3. Direct Use of C-13 NMR Data to Predict Effects of Coal Type
Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J.; Solum, M.S. and Grant, D.M.
Polycyclic Aromatic Compounds, 1:251-264, 1990. Funded by Gas Research Institute and ACERC.
The chemical percolation devolatilization (CPD) model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe the generation of tar precursors of finite size based on the number of cleaved labile bonds in the infinite coal lattice. The chemical percolation devolatilization model described here includes treatment of vapor-liquid equilibrium and a cross-linking mechanism. The cross-linking mechanism permits reattachment of metaplast to the infinite char matrix. A generalized vapor pressure correlation for high molecular weight hydrocarbons, such as coal tar, is proposed based on data from coal liquids. Coal-independent kinetic parameters are employed. Coal-dependent chemical structure coefficients for the CPD model are taken directly from C-13 NMR measurements, with the exception of one empirical parameter representing the population of char bridges in the parent coal. This is in contrast to the previous and common practice of adjusting input coefficients to precisely match measured tar and total volatiles yields. The CPD model successfully predicts the effects of pressure on tar and total volatiles yields observed in heated grid experiments for both bituminous coal and for lignite. Predicted tar molecular weights are consistent with size-exclusion chromatography (SEC) data and field ionization mass spectrometry (FIMS) data. Predictions of average molecular weights of aromatic clusters as a function of coal type agree with corresponding data from NMR analyses of parent coals. The direct use of chemical structure data as a function of coal type helps justify the model on a mechanistic rather than an empirical basis.
Measurement of C-13 Chemical Shielding Anisotropy in Coal
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Solid Carbonaceous Fuels, ACS Symposium Series, Botto, R.E. and Sanada, Y., Editors, 1990 (In press). Funded by ACERC and US Department of Energy.
The methods available in nuclear magnetic resonance to obtain the principal values of the shielding tensor are discussed. Applications to coal and to compounds that model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical shielding powder patterns is compared to results obtained by cross polarization with magic angle spinning (CP/MAS) and dipolar dephasing (DD).
Solid-State C-13 and H-1 NMR Studies of the Evolution of the Chemical Structure of Coal Char and Tar During Devolatilization
Fletcher, T.H.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Pugmire, R.J.
Twenty-third Symposium (International) on Combustion, The Combustion Institute, France, 1990 (In press). Funded by US Department of Energy and ACERC.
Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the chemical structure of char particles and condensed tar vapors produced as pyrolysis products from an Illinois #6 coal at rapid heating conditions (~104 K/s) at two gas conditions (maximum gas temperatures of 1250 K and 1050 K). The temperature history of particles in the flow reactor is determined using a unique infrared sizing-pyrometry system. The C-13 NMR analyses of the coal chars indicate that significant amounts of aliphatic material are released from the coal during devolatilization, with little change to the aromatic cluster size or number of attachments per cluster. At long residence times, and at higher temperatures, small increases in the cluster size in the char are observed. The H-1 NMR analyses indicate that thermal decomposition of tar vapor occurs at the 1250 K gas condition, as evidenced by increases in the aromaticity and decreases in the peripheral aliphatic groups, such as methyl groups and aliphatic bridge material.
1989-1987
C-13 Solid-State NMR of Argonne Premium Coals
Solum, M.S.; Pugmire, R.J. and Grant, D.M.
Energy & Fuels, 3, 187, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Eight Argonne Premium Coal samples and three other oxidized coals have been investigated by the C-13 CP/MAS NMR technique. Spin-lattice relaxation, variable contact time, and dipolar-dephasing experiments were performed on each coal. The proton spin-lattice relaxation time, T1H, the proton spin-lattice relaxation in the rotating frame time, T1pH, and the cross-polarization time, TCH, are reported along with the dipolar-dephasing time constants, TG, and TL, for both the aromatic and aliphatic regions of the 11 coals. These data, together with normal CP/MAS integrations over selected chemical shift ranges, have been used to derive 12 parameters relating to the carbon skeletal structure, including the aromaticity. From the mole fraction of bridgehead or inner carbons as measured by NMR, the aromatic cluster size may be estimated for each coal.
Solid-State C-13 NMR Studies of Coal Char Structure Evolution
Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Fletcher, T.H. and Solomon, P.R.
Fuel Fiv. Preprints, 34 (4), 1337-1346, 198th ACS National Meeting, Miami, 1989. (Also presented at the Western States Section, The Combustion Institute Spring Meeting, Pullman, Washington, 1989.) Funded by ACERC (National Science Foundation and Associates and Affiliates).
Solid state C-13 NMR techniques have been used to study the evolution of char structure during pyrolysis processes. The effects of residence time, heating rate, and final char temperature are observed. The NMR data demonstrates that extensive loss of aromatic ring bridge material precedes significant change in aromatic cluster size.
C-13 Dipolar Spectroscopy of Nitromethane
Solum, M.S.; Facelli, J.C.; Gan, Z.H. and Grant, D.M.
Submitted to Molecular Physics, 1988. Funded by US Department of Energy.
The C-13 low temperature (25K) dipolar spectrum of C-13H3N-14O2 is reported. The spectrum was fit and the principal values of the C-13 shielding tensor are given. The C-13 shielding tensor principal values are similar to those found in other C-13H3 groups. Also, the N-14 quadrupolar parameters and the direct dipolar couplings were obtained from the fit. Because of librational motions an effective trace term in the dipolar interaction has to be included in the fit.
The Use of Variable Angle Sampling Spinning C-13 NMR Spectroscopy to Assess Aromatic Cluster Size in Coals, Coal Chars, and Carbonaceous Materials
Pugmire, R.J.; Sethi, N.K.; Solum, M.S.; Facelli, J.C. and Grant, D.M.
Proc. Carbon Conference, 1988. 3 pgs. Funded by US Department of Energy and Basic Energy Sciences.
Magic angle spinning experiments produce narrow lines in solids but valuable chemical shift anisotropy (CSA) information is lost as the chemical shift tensors are averaged to their isotropic values. CSA is a second rank tensor and is characterized by three unique resonance frequencies s11, s22 and s33 called the principal values of the tensor elements [1]. Knowledge of all three frequencies with the isotropic value is a source of information that is indispensable for molecular characterization. Measurement of the tensor elements is carried out by recording the C-13 NMR spectrum of a finely powdered sample to obtain the solid-state line shape which can then be analyzed to determine tensor elements of chemically and structurally different types of carbons. We have analyzed both static and variable angle sample spinning line shapes [2, 3] to obtain the CSA tensor values and population factors for the aromatic carbons of coals, macerals and carbonaceous materials from which aromatic cluster size is estimated.
Solid State Nuclear Magnetic Resonance of Fossil Fuels
Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 448, 1987. 1 pg. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Axelson has written a valuable book for those interested in the field of NMR spectroscopy of fossil fuels and polymers. This book is best suited for those who have had some experience with solid-state NMR but not perhaps with application to fossil fuels. The Introduction and Appendix A give a brief description of some of the more common fossil fuel terminology and standard analysis techniques, which was quite helpful. In Chapter 1 a very brief description of line broadening mechanisms in solids is given and cross-polarization and magic angle spinning are introduced. The different carbon and proton relaxation parameters and the pulse sequenced used to measure them are discussed in Chapter 2. This chapter is excellent as a quick reference for help in setting up these experiments. Examples of relaxation times of model organic compounds, polymers, and fossil fuels are given, and their relationship to quantitative analysis of fossil fuels is discussed in Chapter 3. Chapter 4 deals with the removal of instrumental artifacts from the NMR spectrum including spinning sideband suppression methods. Also discussed in this chapter are the causes of many instrumental problems. This chapter is recommended to those who are not experienced in solid-state NMR but are initiating these types of experiments. Line-broadening mechanisms in solid-state NMR arising from both natural and instrumental factors are discussed in Chapter 5. Chapter 6, dealing with resolution enhancement, contains a good section on the dipolar-dephasing experiment and the structural parameters that may be obtained from this experiment. Some less common and more selectively useful NMR pulse sequences are also treated. Chapter 7 provides the fa value for a variety of fossil fuels.
While the treatment of many topics in this book is at an overview level, there is a large selection of references at the end of each chapter where a more in-depth discussion of the topics may be found. This book is recommended as a reference for those interested in solid-state NMR in general and, in particular, in application to coals. The book provides an excellent overview of the work published prior to the appearance of this treatise. For the beginner, the fundamentals of solid NMR techniques are not well developed and one should consult other works in order to obtain the necessary basic foundation. In the main, Axelson has provided a very useful reference for those who work in the fossil energy field.
C-13 NMR Structural Determination of the Coals in the Premium Coal Sample Bank
Solum, M.S.; Pugmire, R.J. and Grant, D.M.
ACS division of Fuels, Preprint, 32, (4), 273-279, 1987. 8 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Solid state C-13 cross polarization (CP) magic angle spinning (MAS) NMR experiments have become very useful for the study of the structure of coal and other fossil fuels (1, 2). Solid-state NMR techniques have the advantage over liquid NMR methods because the whole solid coal can be analyzed in a nondestructive manner and no assumption need be made that the soluble portion represents the whole organic portion of the coal. From CP/MAS experiments twelve NMR structural parameters may be determined including the aromaticity, fa, that give information on the carbon skeletal backbone. The variation of the NMR structural parameters for coals of different rank from lignite to anthracite has been demonstrated (3). The coals in the Premium Coal Sample Bank (PCSB) have been included in the set of coals studied at the Advanced Combustion Engineering Research Center. The NMR data obtained on these coals will be used to provide chemical structural parameters for refinement of the devolatilization sub-model of the PCGC-2 combustion model.
This paper reports the twelve structural parameters for two of the PCSB coals, Pittsburgh #8 (HVB) and Blind Canyon (HVB). These data were derived from integrated intensities in a normal CP/MAS experiment along with a complete set of dipolar dephasing experiments. In addition, the proton spin lattice relaxation times for these coals have been determined. From relaxation experiments on the Pittsburgh #8 coal there is some evidence of heterogeneity between different sample vials of the same standard coal.
Solum, MS
2000
A Global Free-Radical Mechanism for Light Gas Nitrogen Release from Coal during Devolatilization
Perry, S.T.; Fletcher, T.H.; Pugmire, R.J. and Solum, M.S.
Energy & Fuels, 14, 1094-1102 (2000).
Contact: Fletcher
Solid-State C-13 NMR Characterization of Matched Tars and Chars From Rapid Coal Devolatilization
Perry, S.T.; Hambly, E.M.; Fletcher, T.H.; Solum, M.S. and Pugmire, R.J.
Proceedings of the Combustion Institute, 28 (in press, 2000).
Contact: Fletcher
1998
Nitrogen Transformation in Coal During Pyrolysis
Kelemen, S.R.; Gorbaty, M.L.; Kwiatek, P.J.; Fletcher, T.H.; Watt, M.; Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 12:159-73 (1998).
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 (10^14 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 "graphitic-like" 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.
1997
Dynamic Nuclear Polarization of Nitrogen-15 Benzamide
Hu, J.Z.; Zhou, J.; Yang, B.; Li, L.; Qiu, J.; Ye, C.; Solum, M.S.; Wind, R.A.; Pugmire, R.J. and Grant, D.M.
SOLID STATE: Nuclear Magnetic Resonance, 8:129-37(1997). Funded by US Department of Energy/Basic Energy Services and ACERC.
A N-15 dynamic nuclear polarization (DNP) experiment is reported in which a N-15 DNP enhancement factor of approximately 2.6 x 10² is obtained on free radical doped samples of 99% N-15 labeled benazmide. The free radicals BDPA (1:1 complex of alpha, gamma-bisdiphenylene-beta-phenylallyl with benzene) and DDPH (2,2 -Di (4 - tert - octylphenyl) -1-picrylhydrazyl) are used as dopants and the spin relaxation effects of adding these dopants are studied by means of changes in proton and nitrogen T1 values of the samples. The combination is solids of a very low natural abundance, 0.37%, a small gyromagnetic ration, and a log spin - lattice relaxation time for N-15 nuclei create severe sensitivity problems that, in large part, are ameliorated by the signal enhancement observed in the N-15 DNP experiment on samples containing free electrons.
Solid State N-15 and C-13 NMR Study of Several Metal 5-, 10-, 15-, 20-Tetraphenylporphyrin Complexes
Strohmeir, M.; Orendt, A.M.; Facelli, J.C.; Solum, M.S.; Pugmire, R.J.; Pery, R.W. and Grant, D.M.
Journal of the American Chemical Society, 119:7114-120(1997). Funded by National Institutes of Health, US Department of Energy.
The principal values both the C-13 and N-15 chemical shift tensors are reported in the Zn, Ni, and Mg 5, 10, 15, 20- tetraphenylporphyrin (TTP) complexes. The principal values of the 15N chemical shift tensors were obtained from static powder patterns of N-15 enriched samples. Due to overlap between the powder patterns of the different carbons, the C-13 values were obtained using the recently developed magic angle turning (MAT) 2D experiment on unenriched materials. The measured principals values are presented along with theoretical calculations of the chemical shift tensors and a discussion of the effects that the metal bonding has on the chemical shift tensors in these compounds. Both the isotropic chemical shift and the principal values of the N-15 chemical shift tensor are nearly identical for the Mg and Zn complexes. The N-15 isotropic chemical shift for the NiTPP, however, changes by nearly 80 ppm relative to the Mg and Zn values, with large changes observed in each of the tree principal values. Calculations show that the differences between the N-15 chemical shifts are almost entirely determined by the metal-nitrogen separation. In addition, both the experimental data and the calculations show only very minor differences in the 13C chemical shift tensor components as the metal is changed.
Nitrogen Transformations in Coal During Pyrolysis
Kelemen, S.R.; Gorbaty, M.L.; Kwiatek, P.J.; Fletcher, T.H.; Watt, M.; Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 12:159-72(1997). Funded by ACERC, Exxon and Federal Energy Technology Center.
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 coal 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 theses 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, sever pyrolysis of the devolatilized char results in the appearance of asymmetric carbon (1s) line shape indicative of very large polynuclear "graphitic-like" 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.
N-15 CPMAS NMR of the Argonne Premium Coals
Solum, M.S.; Pugmire, R.J; Grant, D.M.; Kelemen, S.R.; Gorbaty, M.L. and Wind, R.A.
Energy & Fuels, 11:491-94(1997). Funded by ACERC and US Department of Energy.
N-15 NMR data are reported for the Argonne Premium Coals. Arguments are presented to explain discrepancies between observations and conclusions obtained from NMR experiments and those obtained by XPS and XANES techniques. Delectability of different types of nitrogens is discussed in terms of cross-polarization dynamics together with effects of the large chemical shift anisotropy that is found in different types of nitrogen functional groups. Data on acid-treated coals confirm the presence of pyridinic type nitrogens that were not observed in a previous N-15 NMR study of coals.
N-15 NMR Spectroscopy of Coals and Pyrolysis Products
Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Fletcher, T.H. and Wind, R.A.
Proceedings of the 9th International Conference on Coal Science, Essen, Germany, September 7-12, 1997. Funded by ACERC, US Department of Energy/University Coal Research and New Energy and Industrial Technology Development Organization.
N-15 NMR spectra are reported for a number of coals from Pacific Rim countries. Arguments are presented to explain discrepancies between observations and conclusions obtained from NMR experiments and those obtained by XPS and XANES techniques. Detection of different types of nitrogen species is discussed in terms of cross polarization dynamics together with the effects of the large chemical shift anisotropy that are found in different types of nitrogen functional groups. Significant differences are observed in the types of nitrogen present in these coals and these variations are associated with coal rank as has been in a previous study of the Argonne Premium coals. We have also begun to examine pyrolysis char and tar samples. The NMR data indicate that significant differences exist between the types of nitrogen structures observed in coal, char and tar samples. These differences suggest that different mechanisms may exist for nitrogen release from tar and char samples.
1993
Chemical Structure of Coal Tar During Devolatilization
Fletcher, T.H.; Watt, M.; Bai, S.; Solum, M.S. and Pugmire, R.J.
Twenty-Sixth Symposium (International) on Combustion, (in press) 1996. (Also presented at the Twenty-Sixth Symposium (International) on Combustion, Naples, Italy, July, 1996.) Funded by ACERC and US Department of Energy/University Coal Research.
Three coals of different rank were pyrolyzed in a drop tube reactor at a maximum temperature of 900 K and residence time of 160 ms. The coal and char were analyzed with solid state C-13 NMR. The tar was dissolved in deuterated methylene chloride. It was found that the tar was only partially soluble in CD2CL2-. The non-soluble tar portion was analyzed using a recently developed high-resolution C-13 NMR technique developed for liquid phases. The tar structure was found to be significantly different from the structure of the char and coal. The number of bridges and loops per cluster in the tar was up to 65% lower than in the char. In addition, the number of aromatic carbons per cluster in the tar was significantly lower than that found in either the coal or the char. Since the molecular weight per cluster in the tar is lower than reported average tar molecular weights, these data imply that tar is made up of a number of multiple clusters (dimers, trimers, etc.) as well as single clusters (i.e., monomers). The mass of nitrogen per cluster in the tar was found to be significantly lower in the tar than in either the coal or the char. These experimental findings suggest that changes may be necessary in current network devolatilization models to accurately describe the changes in chemical structure.
1994
Magic Angle Turning Experiments for Measuring Chemical-Shift-Tensor Principal Values in Powdered Solids
Hu, J.Z.; Wang, W.; Liu, F.; Solum, M.S.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
J. Magnetic Resonance, 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.
The magic-angle-turning (MAT) technique introduced by Gan employs slow (ca. 30 Hz) rotation of a powdered sample at the magic angle, in concert with pulses synchronized at 1/3 of the rotor period, to obtain isotropic-shift information in one dimension of a 2D spectrum. The other dimension displays a slow-spinning-sideband powder pattern that, at the low rotor frequencies employed, resembles the stationary-sample powder pattern. The MAT method is very effective for measuring chemical-shift principal values in compounds where spectral overlap precludes the use of 1D methods. Previous MAT implementations are reviewed, and it is shown how a new phase correct MAT (PHORMAT) pulse sequence overcomes many of their limitations. This new pulse sequence produces a spinning-sideband-free isotropic-shift spectrum directly as a projection onto the evolution axis with no spectral shearing. Only two purging operations are employed, resulting in a higher signal-to-noise ratio. Pure absorption-absorption phased 2D spectra are produced. Flat 2D base planes result from an echo sequence which delays acquisition until after probe ring-down and receiver recovery. The technique used for synchronizing the pulses to 1/3 the rotor period without relying on absolute rotor-frequency stability is described. The PHORMAT spectrum of methyl a-D-glucopyranoside is presented. The data are analyzed with an emphasis on the quantitative accuracy of the experiment in measuring chemical shift tensor principal values and determining the relative number of spins of each type present. The FID data from the spectrometer acquisition are fitted with numerical simulations that employ a banded-matrix method for calculation spinning sideband amplitudes. The chemical shift principal values, measured in methyl a-D-glucopyranoside with the PHORMAT method, are compared with those from a single-crystal determination of the full chemical shift tensors. The two measurements differ by an rms average distance of only 0.57 ppm.
1993
Chemical Structural Features of Pyridine Extracts and Residues of the Argonne Premium Coals Using Solid State C-13 NMR Spectroscopy
Fletcher, T.H.; Bia, S.; Pugmire, R.J.; Solum, M.S.; Woods, S. and Grant, D.M.
Energy & Fuels, 7 (6):734-742, 1993. (Presented at the Spring Meeting of the Western States Section of the Combustion Institute, Salt Lake City, UT, March 1993.) Funded by ACERC.
Soxhlet extractions were performed on the eight Argonne Premium coals using pyridine purged with argon and followed by a novel washing procedure to remove the pyridine. Mass closure (extracts plus residues) on duplicate experiments accounted for 94-96% of the original coal, repeatable to within 2%. Chemical structural features determined from C-13 NMR analyses of the extracts and residues showed more attachments per aromatic cluster for the residues, indicating a greater degree of covalent bonding in the residue than in the extract. H-1 NMR analysis of the extracts showed a gradual increase in the hydrogen aromaticity with rank, along with a maximum in the percentage of a-hydrogen in the high-volatile bituminous coals. Composite chemical features constructed from weighted averages of the features of the residues and extracts agree with many of the features of the parent coal. Chemical structural features of the extracts determined from H-1 NMR analyses agree with similar data reported previously for early coal tars during devolatilization at rapid heating rates.
Measurement of C-13 Chemical-Shift Anisotropy in Coal
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Carbonaceous Solids, Advances in Chemistry Series #229, 22:419-439, 1993. Funded by US Department of Energy, Basic Energy Services and ACERC.
The methods of available in NMR spectroscopy to obtain the principal values of the chemical-shift tensor are discussed. Applications to coal and to compounds with model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical-shift powder patterns is compared to results obtained by cross-polarization with magic-angle spinning and dipolar dephasing.
Chemical Structural Features of Coal Chars, Tars, and Char Extracts During Rapid Pyrolysis Using C-13 and H-1 NMR Spectroscopy
Fletcher, T.H.; Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Bai, S.; Ma, J. and Woods, S.
7th International Conference on Coal Science, Banff, Alberta, Canada, September 1993 (in press). Funded by ACERC.
Structural characteristics have been determined for parent coals and for chars collected at different stages of pyrolysis. Recent work has focused on trying to understand the relationship between chemical structural features of the unreacted coal and the devolatilization and char oxidation phenomena. Models of coal devolatilization have recently related devolatilization behavior to the structure of the parent coal and the initial amount of pyridine extracts. Fong et al. used pyridine extraction methods to quantify the amount of metaplast formed during pyrolysis of a Pittsburgh #8 coal. These experiments demonstrated that under moderate heating conditions (~500 K/s to 873 K), as much as 80% of the initial coal was transformed into a combination of extractable material and volatiles. The work presented here is an examination of the pyridine extraction procedure of the Argonne Premium coal samples and the detailed study of the carbon skeletal structure of the extracts and the extraction residues from these coals. This is the first stage of an experimental program to examine the yield and chemical features of extracts of coal chars collected as a function of time during pyrolysis.
The Use of Solid State C-13 NMR Spectroscopy to Study Pyridine Extracted and Extraction Residues in the Argonne Premium Coals
Pugmire, R.J.; Solum, M.S.; Bai, S.; Fletcher, T.H.; Woods, S. and Grant, D.M.
Proceedings of the 205th ACS National Meeting, 38, no. 2, Denver, CO, March 1993. Funded by ACERC.
The relationship between coal structure and combustion behavior is a matter of on-going research in our laboratories. A great deal of effort has gone into obtaining data that is used for modeling studies of devolatilization behavior. We have also carefully studied the process of char formation. Our past work has focused on trying to understand the relationship between coal/char/tar formation as they relate to the devolatilization and char oxidation phenomena. The formation of metaplast during pyrolysis was studied by Fong and Howard in terms of extractable material obtained at different stages of the devolatilization process. We have recently turned our attention to metaplast formation in devolatilization and plan to conduct a series of experiments that will help define the formation and chemical structure of metaplast in coals of different rank.
Coal and Char Structural Parameters Derived from Solid State C-13 NMR Studies
Pugmire, R.J.; Solum, M.S.; Fletcher, T.H. and Grant, D.M.
The 5th Australian Coal Science Conference, Melbourne, Australia, March 1993. (Also presented at the 5th Australian Coal Science Conference, Melbourne, Australia, November, 1992.) Funded by ACERC.
In contrast to previous efforts where coal-general devolatilization model input parameters describing chemical structure are adjusted to force agreement between predicted and measured tar and total volatiles yields, coal-dependent chemical structure coefficients for the Chemical Percolation Devolatilization (CPD) model developed by the authors and others are taken directly from C-13 NMR analyses of parent coals. This procedure, outlined in the paper eliminates most adjustable parameters from the model, and predictions of tar and total volatiles yields become true tests of the model and the NMR data, rather than mere results of curve fitting. Resulting model predictions of tar and total volatiles yields as a function of coal type, temperature, heating rate, and pressure comparable with available experimental data, showing the value of both the model and the NMR chemical structure data.
1992
C-13 NMR Techniques for Structural Studies of Coals and Coal Chars
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Chapter 10, Advances in Coal Spectroscopy, (H.L.C. Meuzelaar, ed.), Plenum Publishing Corp., New York, 1992. Funded by Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction, US Department of Energy and ACERC.
Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.
Chemical Structure of Char in the Transition from Devolatilization to Combustion
Fletcher, T.H.; Solum, M.S.; Grant, D.M. and Pugmire, R.J.
Energy & Fuels, 6(5):643-650, 1992. [Also included in the ACS Division of Fuel Chemistry Preprints, 37(2):677-684, 1992 (203rd ACS National Meeting, San Francisco, CA, April 1992)]. Funded by Sandia National Laboratories, Brigham Young University and ACERC.
Coal devolatilization experiments are generally conducted separately from char oxidation experiments, and the relationship between the chars generated in the two types of research is often ignored. However, char is one of the most important products of coal devolatilization and must be characterized as a function of temperature and heating rate in a manner similar to that for gaseous behavior. In this work, the chemical structures of chars from five coals of different rank are examined, and implications on char reactivity are discussed. Chars were obtained as a function of residence time in a devolatilization experiment (1250 K, 2 x 104 J/s in nitrogen) and just subsequent to devolatilization in a laminar flame-fired experiment (1500 K, 5 x 104 K/s). Quantitative measurements of chemical structure were performed on the coals and chars using C-13 nuclear magnetic resonance (NMR). Results show that the chemical structures of fully devolatilized chars are very similar, even though a wide diversity is seen in the parent coal structures. For example, the average cluster molecular weights of the chars span a range of only 50 amu, and the side-chain molecular weights of the chars span a range of only 6 amu. The similarity in chemical structure of fully devolatilized coal chars suggests that differences in measured heterogeneous char reactivities are caused by differences in the physical structure of the char.
An Efficient Large Sample Volume System for Solid State NMR
Pugmire, R.J.; Jiang, Y.J.; Solum, M.S. and Grant, D.M.
Bulletin of Magnetic Resonance, 14:144, 1992. (Presented at ISMAR 92, Vancouver, B.C., July 1992.) Funded by ACERC.
An efficient large sample volume system has been developed to carry out MAS solid-state NMR experiments. The system components are primarily zirconia and macor and no background C-13 is observed. The stator design employs separate air bearing and drive systems and is run using dry air at a bearing pressure of about 32 psi. the rotor can be spun in a stable manner from less than one hundred Hz (with a driving pressure of 5 psi) to 4.3 KHz (24 psi driving pressure). This low gas pressure feature makes the system easy to operate. The volume of the rotor is 1.8 cm³ and it can hold 1.1 g of HMB. The S/N ratio obtained is a factor of 4.6 better than the rotor previously designed and used in our laboratory (volume 0.6 cm³ : 0.28g HMB). This increased sample size allows us to obtain the same S/N ratio in a MAS spectrum with a factor of 21 saving in spectrometer time. The time saving achieved with this rotor system is extremely useful in obtaining data on biological samples and polymers, and is especially useful when experiments on fossil fuels require the use of the Bloch delay technique. Examples of relevant applications will be discussed.
Coal and Char Structural Parameters Derived from Solid-State C-13 NMR Studies
Pugmire, R.J.; Solum, M.S.; Fletcher, T.H. and Grant, D.M.
Fifth Australian Coal Science Conference, Melbourne, Australia, November 1992. Funded by ACERC.
In contrast to previous efforts where coal-general devolatilization model input parameters describing chemical structure are adjusted to force agreement between predicted and measured tar and total volatiles yields, coal-dependent chemical structure coefficients for the Chemical Percolation Devolatilization (CPD) model developed by the authors and others are taken directly from C-13 NMR analyses of parent coals. This procedure, outlined in the paper eliminates most adjustable parameters from the model, and predictions of tar and total volatiles yields become true tests of the model and the NMR data, rather than mere results of curve fitting. Resulting model predictions of tar and total volatiles yields as a function of coal type, temperature, heating rate, and pressure comparable with available experimental data, showing the value of both the model and the NMR chemical structure data.
The Use of C-13 NMR Structural Information for Predicting Coal Devolatilization Behavior
Pugmire, R.J.; Solum, M.S.; Grant, D.M. and Fletcher, T.H.
Second International Conference on Coal Structure, Krakow, Poland, September 1992. Funded by ACERC.
Recent coal devolatilization models have attempted to incorporate coal structural information to predict product yields and composition. The chemical structure of the parent coal directly affects devolatilization behavior. The parent coal structure also has implications for pyrolysis char structure and reactivity. C-13 NMR data have been used to obtain coal-dependent chemical structural data that is used as input parameters to the chemical percolation devolatilization (CPD) model. The CPD model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the parent coal. Data will be presented on 20 different coals and the value of NMR data in predicting gas and tar yields will be discussed. The use of C-13 NMR data will also be used to describe the relationship between chemical structure features of the parent coals and the resultant pyrolysis chars.
1991
C-13 NMR Techniques for Structural Studies of Coals and Coal Chars
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Advances in Coal Spectroscopy, (H.L.C. Meuzelaar, ed.), Plenum Publishing Corp., New York, 1991 (in press). Funded by Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction, US Department of Energy and ACERC.
Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.
Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments
Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 70:414-423, 1991. Funded by Pittsburgh Energy Technology Center and ACERC.
Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.
Measurement of C-13 Chemical Shielding Anisotropy in Coal
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Solid Carbonaceous Fuels, ACS Symposium Series, (R.E. Botto and Y. Sanada, eds.), 1991 (in press). Funded by US Department of Energy and ACERC.
The methods available in nuclear magnetic resonance to obtain the principal values of the shielding tensor are discussed. Applications to coal and to compounds that model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical shielding powder patterns is compared to results obtained by cross polarization with magic angle spinning (CP/MAS) and dipolar dephasing (DD).
Network Changes During Coal Pyrolysis: Experiment and Theory
Solomon, P.R.; Charpenay, S.; Yu, Z.-Z.; Serio, M.A.; Kroo, E.; Solum, M.S. and Pugmire, R.J.
8th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, October 1991. Funded by US Department of Energy and ACERC.
Coal pyrolysis is a complicated combination of chemical and physical processes in which coal is transformed at elevated temperatures to produce gases, tar, and char. These processes are described in the Functional Group - Depolymerization, Vaporization, and Crosslinking (FG-DVC) model of coal pyrolysis. An important aspect of this model is that crosslinking is rank dependent. This is based on solvent swelling experiments on chars made from coals of different rank. Low rank coals start to loose their solvent swelling ability prior to significant depolymerization at temperatures as low as 200ºC. Including such crosslinking in the FG-DVC model leads to predictions for low rank coals of a highly crosslinked network (exhibited by low solubility and low fluidity in chars) and low tar amounts.
While the model is in good agreement with a variety of data, it is difficult to find experiments to validate the predicted behavior of the network. In this paper we have used CP-MAS, NMR with dipolar dephasing and other techniques to examine the chars changing functional group and network characteristics. The changes in the char composition have been modeled using the FG-DVC model and the results compared with the data for Pittsburgh Seam bituminous coal and Zap lignite.
1990
C-13 NMR Techniques for Structural Studies of Coals and Coal Chars
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Advances in Coal Spectroscopy, Meuzelaar, H.L.C., Editor, Plenum Publishing Corp., New York, 1990 (In press). Funded by ACERC and US Department of Energy.
Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.
Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments
Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 1990 (In press). Funded by ACERC.
Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.
A Chemical Percolation Model of Coal Devolatilization: 3. Direct Use of C-13 NMR Data to Predict Effects of Coal Type
Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J.; Solum, M.S. and Grant, D.M.
Polycyclic Aromatic Compounds, 1:251-264, 1990. Funded by Gas Research Institute and ACERC.
The chemical percolation devolatilization (CPD) model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe the generation of tar precursors of finite size based on the number of cleaved labile bonds in the infinite coal lattice. The chemical percolation devolatilization model described here includes treatment of vapor-liquid equilibrium and a cross-linking mechanism. The cross-linking mechanism permits reattachment of metaplast to the infinite char matrix. A generalized vapor pressure correlation for high molecular weight hydrocarbons, such as coal tar, is proposed based on data from coal liquids. Coal-independent kinetic parameters are employed. Coal-dependent chemical structure coefficients for the CPD model are taken directly from C-13 NMR measurements, with the exception of one empirical parameter representing the population of char bridges in the parent coal. This is in contrast to the previous and common practice of adjusting input coefficients to precisely match measured tar and total volatiles yields. The CPD model successfully predicts the effects of pressure on tar and total volatiles yields observed in heated grid experiments for both bituminous coal and for lignite. Predicted tar molecular weights are consistent with size-exclusion chromatography (SEC) data and field ionization mass spectrometry (FIMS) data. Predictions of average molecular weights of aromatic clusters as a function of coal type agree with corresponding data from NMR analyses of parent coals. The direct use of chemical structure data as a function of coal type helps justify the model on a mechanistic rather than an empirical basis.
Measurement of C-13 Chemical Shielding Anisotropy in Coal
Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Solid Carbonaceous Fuels, ACS Symposium Series, Botto, R.E. and Sanada, Y., Editors, 1990 (In press). Funded by ACERC and US Department of Energy.
The methods available in nuclear magnetic resonance to obtain the principal values of the shielding tensor are discussed. Applications to coal and to compounds that model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical shielding powder patterns is compared to results obtained by cross polarization with magic angle spinning (CP/MAS) and dipolar dephasing (DD).
Solid-State C-13 and H-1 NMR Studies of the Evolution of the Chemical Structure of Coal Char and Tar During Devolatilization
Fletcher, T.H.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Pugmire, R.J.
Twenty-third Symposium (International) on Combustion, The Combustion Institute, France, 1990 (In press). Funded by US Department of Energy and ACERC.
Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the chemical structure of char particles and condensed tar vapors produced as pyrolysis products from an Illinois #6 coal at rapid heating conditions (~104 K/s) at two gas conditions (maximum gas temperatures of 1250 K and 1050 K). The temperature history of particles in the flow reactor is determined using a unique infrared sizing-pyrometry system. The C-13 NMR analyses of the coal chars indicate that significant amounts of aliphatic material are released from the coal during devolatilization, with little change to the aromatic cluster size or number of attachments per cluster. At long residence times, and at higher temperatures, small increases in the cluster size in the char are observed. The H-1 NMR analyses indicate that thermal decomposition of tar vapor occurs at the 1250 K gas condition, as evidenced by increases in the aromaticity and decreases in the peripheral aliphatic groups, such as methyl groups and aliphatic bridge material.
1989-1987
C-13 Solid-State NMR of Argonne Premium Coals
Solum, M.S.; Pugmire, R.J. and Grant, D.M.
Energy & Fuels, 3, 187, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Eight Argonne Premium Coal samples and three other oxidized coals have been investigated by the C-13 CP/MAS NMR technique. Spin-lattice relaxation, variable contact time, and dipolar-dephasing experiments were performed on each coal. The proton spin-lattice relaxation time, T1H, the proton spin-lattice relaxation in the rotating frame time, T1pH, and the cross-polarization time, TCH, are reported along with the dipolar-dephasing time constants, TG, and TL, for both the aromatic and aliphatic regions of the 11 coals. These data, together with normal CP/MAS integrations over selected chemical shift ranges, have been used to derive 12 parameters relating to the carbon skeletal structure, including the aromaticity. From the mole fraction of bridgehead or inner carbons as measured by NMR, the aromatic cluster size may be estimated for each coal.
Solid-State C-13 NMR Studies of Coal Char Structure Evolution
Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Fletcher, T.H. and Solomon, P.R.
Fuel Fiv. Preprints, 34 (4), 1337-1346, 198th ACS National Meeting, Miami, 1989. (Also presented at the Western States Section, The Combustion Institute Spring Meeting, Pullman, Washington, 1989.) Funded by ACERC (National Science Foundation and Associates and Affiliates).
Solid state C-13 NMR techniques have been used to study the evolution of char structure during pyrolysis processes. The effects of residence time, heating rate, and final char temperature are observed. The NMR data demonstrates that extensive loss of aromatic ring bridge material precedes significant change in aromatic cluster size.
C-13 Dipolar Spectroscopy of Nitromethane
Solum, M.S.; Facelli, J.C.; Gan, Z.H. and Grant, D.M.
Submitted to Molecular Physics, 1988. Funded by US Department of Energy.
The C-13 low temperature (25K) dipolar spectrum of C-13H3N-14O2 is reported. The spectrum was fit and the principal values of the C-13 shielding tensor are given. The C-13 shielding tensor principal values are similar to those found in other C-13H3 groups. Also, the N-14 quadrupolar parameters and the direct dipolar couplings were obtained from the fit. Because of librational motions an effective trace term in the dipolar interaction has to be included in the fit.
The Use of Variable Angle Sampling Spinning C-13 NMR Spectroscopy to Assess Aromatic Cluster Size in Coals, Coal Chars, and Carbonaceous Materials
Pugmire, R.J.; Sethi, N.K.; Solum, M.S.; Facelli, J.C. and Grant, D.M.
Proc. Carbon Conference, 1988. 3 pgs. Funded by US Department of Energy and Basic Energy Sciences.
Magic angle spinning experiments produce narrow lines in solids but valuable chemical shift anisotropy (CSA) information is lost as the chemical shift tensors are averaged to their isotropic values. CSA is a second rank tensor and is characterized by three unique resonance frequencies s11, s22 and s33 called the principal values of the tensor elements [1]. Knowledge of all three frequencies with the isotropic value is a source of information that is indispensable for molecular characterization. Measurement of the tensor elements is carried out by recording the C-13 NMR spectrum of a finely powdered sample to obtain the solid-state line shape which can then be analyzed to determine tensor elements of chemically and structurally different types of carbons. We have analyzed both static and variable angle sample spinning line shapes [2, 3] to obtain the CSA tensor values and population factors for the aromatic carbons of coals, macerals and carbonaceous materials from which aromatic cluster size is estimated.
Solid State Nuclear Magnetic Resonance of Fossil Fuels
Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 448, 1987. 1 pg. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Axelson has written a valuable book for those interested in the field of NMR spectroscopy of fossil fuels and polymers. This book is best suited for those who have had some experience with solid-state NMR but not perhaps with application to fossil fuels. The Introduction and Appendix A give a brief description of some of the more common fossil fuel terminology and standard analysis techniques, which was quite helpful. In Chapter 1 a very brief description of line broadening mechanisms in solids is given and cross-polarization and magic angle spinning are introduced. The different carbon and proton relaxation parameters and the pulse sequenced used to measure them are discussed in Chapter 2. This chapter is excellent as a quick reference for help in setting up these experiments. Examples of relaxation times of model organic compounds, polymers, and fossil fuels are given, and their relationship to quantitative analysis of fossil fuels is discussed in Chapter 3. Chapter 4 deals with the removal of instrumental artifacts from the NMR spectrum including spinning sideband suppression methods. Also discussed in this chapter are the causes of many instrumental problems. This chapter is recommended to those who are not experienced in solid-state NMR but are initiating these types of experiments. Line-broadening mechanisms in solid-state NMR arising from both natural and instrumental factors are discussed in Chapter 5. Chapter 6, dealing with resolution enhancement, contains a good section on the dipolar-dephasing experiment and the structural parameters that may be obtained from this experiment. Some less common and more selectively useful NMR pulse sequences are also treated. Chapter 7 provides the fa value for a variety of fossil fuels.
While the treatment of many topics in this book is at an overview level, there is a large selection of references at the end of each chapter where a more in-depth discussion of the topics may be found. This book is recommended as a reference for those interested in solid-state NMR in general and, in particular, in application to coals. The book provides an excellent overview of the work published prior to the appearance of this treatise. For the beginner, the fundamentals of solid NMR techniques are not well developed and one should consult other works in order to obtain the necessary basic foundation. In the main, Axelson has provided a very useful reference for those who work in the fossil energy field.
C-13 NMR Structural Determination of the Coals in the Premium Coal Sample Bank
Solum, M.S.; Pugmire, R.J. and Grant, D.M.
ACS division of Fuels, Preprint, 32, (4), 273-279, 1987. 8 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Solid state C-13 cross polarization (CP) magic angle spinning (MAS) NMR experiments have become very useful for the study of the structure of coal and other fossil fuels (1, 2). Solid-state NMR techniques have the advantage over liquid NMR methods because the whole solid coal can be analyzed in a nondestructive manner and no assumption need be made that the soluble portion represents the whole organic portion of the coal. From CP/MAS experiments twelve NMR structural parameters may be determined including the aromaticity, fa, that give information on the carbon skeletal backbone. The variation of the NMR structural parameters for coals of different rank from lignite to anthracite has been demonstrated (3). The coals in the Premium Coal Sample Bank (PCSB) have been included in the set of coals studied at the Advanced Combustion Engineering Research Center. The NMR data obtained on these coals will be used to provide chemical structural parameters for refinement of the devolatilization sub-model of the PCGC-2 combustion model.
This paper reports the twelve structural parameters for two of the PCSB coals, Pittsburgh #8 (HVB) and Blind Canyon (HVB). These data were derived from integrated intensities in a normal CP/MAS experiment along with a complete set of dipolar dephasing experiments. In addition, the proton spin lattice relaxation times for these coals have been determined. From relaxation experiments on the Pittsburgh #8 coal there is some evidence of heterogeneity between different sample vials of the same standard coal.