ACERC
Abstracts - 1989 |
ACERC
Abstracts - 1989 |
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Thrust Area 1: Fuel Structure and Reaction Mechanisms |
Morin, F.G. and Grant, D.M.
VCH Publishers, New York, Peter W. Rabideau (Editor), 1989 (In press). Funded
by National Institutes of Health.
Since the original observation or nuclear magnetic resonance (NMR), the technique of solution-state NMR has become a premier method of structural and conformational analysis of organic molecules. With the introduction of Fourier transform methods, C-13-NMR has exceeded H-1-NMR as a tool for conformational analysis and a number of books have become standard reference texts in this field.
The chemical shift experienced by a C-13 nucleus is determined by several electronic factors such as hybridization and inductive effects of substituents. Relatively smaller but highly informative changes in chemical shift result from even subtle changes in conformation, for example, the difference in shift of an equatorial and an axial methyl group in methylcyclohexane. The utility of conformationally dependent C-13 shifts in hydroaromatic compounds is the focus of this chapter.
Hydroaromatic compounds such as terralin (1,2,3,4-tetrahydronaphthalene), while of significant importance in the field of fuel science as reactive components in the liquefaction and gasification of coals, have not received their share of NMR attention over the years compared with the well-studied and understood saturated ring systems such as cyclohexane. These mixed cyclic systems (both aromatic and aliphatic) possess interesting differences compared with either the pure aliphatic or aromatic ring systems. Therefore, a good understanding of the conformational properties and the energetics involved in these compounds would be valuable. An organized and systematic study of the conformationally dependent C-13 chemical shifts of these molecules is aided by the use of a multiparameter regression analysis. To introduce this technique and to provide a comparison of the conformational properties of the cyclohexanes with hydroaromatics, we begin with a brief historical review of the use of C-13 chemical shifts in the conformational analysis of alkanes and cyclohexanes.
Jakab, E.; Yun, Y. and Meuzelaar,
H.L.C.
Chemistry of Coal Weathering, Nelson, C.R., Editor, Chicago, Illinois,
Elsevier Science Publishers, 1989. Funded by ACERC (National Science Foundation
and Associates and Affiliates).
A variety of external oxidative stresses have potential for altering the molecular composition and structure of organic substances. Examples include molecular oxygen, gamma radiation, near-ultraviolet (near-UV) radiation, ozone, and peroxides. The aerial oxidation of organic substances under ambient conditions is a well-known chemical reaction of interest both from an academic and an industrial viewpoint. Numerous types of aerial oxidation processes, e.g., respiration, rancidity, etc., are of profound biological and technical importance. Freshly mined coal is a very air-sensitive material, and atmospheric oxygen is believed to be the principal chemical agent responsible for the progressive coal degradation phenomenon known as weathering. In this article, the types of compositional and structural changes that occur as a result of the reaction of coalified organic matter with molecular oxygen are described in terms of the time evolution of reactant, intermediate, and product structures.
Facelli, J.C. and Grant,
D.M.
Book Chapter to be published in Vol. 9 of Topics in Stereochemistry,
E. L. Eliel and S. H. Wilen (Eds.), 1989. Funded by US Department of Energy,
National Institutes of Health, and National Science Foundation.
One of the most important observations in the early days of NMR spectroscopy was the dependence of the magnetic resonance frequency upon the chemical or electronic environment of the nucleus. These experimental results were followed rather quickly by Ramsey's theoretical formulation (1) of chemical shielding which recognized the tensorial character of the interaction. The physical basis of chemical shielding may be visualized from an idealized experiment in which the transition frequency of a nuclear spin is considered for an isolated molecule at various orientations relative to an external magnetic field. The local magnetic field at the nucleus depends upon the orientation of the molecule with respect to the external magnetic field as a consequence of differential shielding resulting from anisotropy in the electron distribution in the vicinity of the nucleus. Thus, the reorientation of such an idealized molecule would be accompanied by a shift in the NMR frequency of the nucleus. A similar but realizable experiment, to be discussed later, can be performed by reorienting a single crystal in a magnetic field while following the orientational dependence of the resonance frequency. These data determine the C-13 shielding tensors and provide insight into the three dimensional distribution of molecular electrons.
For many years, owing to experimental limitations, NMR spectra of organic compounds were studied to any appreciable extent only in the liquid phase, where the rapid molecular tumbling prevented the observation of the tensorial features of chemical shielding. Only average or 'isotropic' shifts can be determined from liquid phase data. Even so, early isotropic C-13 chemical shifts were found to depend significantly upon molecular conformations. The importance of conformational effects found in the cyclic alkanes firmly established C-13 resonance methods as a powerful tool in the field of conformational analysis.
When one records only the isotropic chemical shifts obtained from liquids, a great deal of information on the shielding tensor is left unmeasured, Molecular rotation, rapid on the NMR time scale, averages the components of the shielding tensor to its isotropic value and eliminated the three-dimensional information available in the full tensor. Furthermore, quenching of the molecular motion in the solid state often provides interesting additional information which is absent in both liquids and gases. The immobilization of molecules in a solid is usually associated with a freezing of the conformational structure as observed in a number of solid samples. The rigidity imposed by the crystal lattice may destroy the effective symmetry found in rapidly equilibrating molecules. Such a break in symmetry can give rise to more lines in a magic angle spinning (MASS) spectrum than are found in the corresponding liquid phase spectrum. A brief description of some relevant results, obtained using cross polarization (CP) and MAS methods in organic molecules is given as a background for the results obtained from tensor information. CP/MAS can not only be used to understand conformational changes in solids, but at times becomes the best tool for conformational analysis of materials (e.g. amorphous substances) that are unsuitable for X-ray studies. The CP/MAS method is also useful for relatively insoluble materials (e.g. intractable polymers) or molecules for which conformational information cannot be obtained by NMR in solution. It is apparent that a great deal of information on molecular electronic structure can be obtained if the C-13 chemical shift tensors, or at least their principal values, are measured and properly rationalized. These techniques have been applied to several types of solids including single crystals, polycrystalline and amorphous powders, frozen liquids and gases, and matrix isolated compounds, using rather elaborate instrumental techniques, complicated data reduction schemes, and extensive theoretical methods to correlate the molecular structural information with the experimental results. Isolation of organic compounds at cryogenic temperatures in glasses and matrices deserves special comment because in many cases it is the only accessible method for studying highly reactive compounds using NMR.
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.
Fletcher, T.H.; Kerstein,
A.R.; Pugmire, R.J. and Grant, D.M.
Accepted for publication by Energy & Fuels, 1989. Funded by US Department
of Energy and ACERC (National Science Foundation and Associates and Affiliates).
The CPD model previously developed to describe the devolatilization behavior of rapidly heated coal was based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe generation of finite tar clusters as labile bonds are cleaved in the infinite coal lattice. The model is used here to describe effects of heating rate and temperature on tar and gas release from coal. Coefficients for the net rate of competition between char formation and side chain formation are generated from heated screen data performed at five different heating rates. The model also compares well with heated screen data obtained at 1000 K/s and different hold times at the final temperature as well as with data from entrained flow reactors obtained at higher heating rates (104 K/s) where particle temperatures have been measured. Results indicate that the CPD model predictions yield good agreement with published data for a wide range of coals and particle heating rates.
Hughes, C.D.; Sethi, N.K.;
Baltisberger, J.H. and Grant, D.M.
Macromolecules, 22, 2551, 1989. Funded by US Department of Energy.
A new method for determining the crystallinity of linear polyethylene (LPE) utilizing static, solid state C-13 Nuclear Magnetic Resonance (NMR) spectroscopy and computer lineshape simulation is described. Good agreement between the crystallinity measured by this method and a standard method was found. The computer simulation of the experimental spectrum also reflects the degree of gauche and trans chain conformations in the amorphous region of the sample.
Alderman, D.W.; Sherwood,
M.H. and Grant, D.M.
Accepted for publication in J. Mag. Res., 1989. Funded by US Department
of Energy.
Chemical shift tensors can be determined from a single crystal placed in six or more orientations in a magnetic field. The sensitivity of this determination as a function of the selected crystal orientations is considered with a statistical figure of merit. A variety of configurations are examined, and it is found that the set of six orientations based on the vertices of an icosahedron optimizes the figure of merit and results in the most sensitive determination of the tensor. The relationship of these optimal orientations to those used previously in two-dimensional chemical shift tensor correlation spectroscopy is discussed. It is shown that the high symmetry of the icosahedron simplifies the design on which the construction of a new sample orienting mechanism may be based.
Huffman, G.P.; Huggins,
F.E.; Mitra, S.; Shah, N.; Pugmire, R.J.; Davis, B.H.; Lytle, F.W. and Greegor,
R.B.
Energy & Fuels, 3, 200, 1989. Funded by US Department of Energy.
X-ray absorption fine structure (XAFS) spectroscopy has been used to investigate the molecular structure of organic sulfur in a suite of maceral separates and in several biodesulfurized and extracted coal specimens. For most samples, the X-ray absorption near-edge structure (XANES) exhibits sharp peaks just above the absorption edge that are characteristic of s Æ p transitions of compounds containing an aromatically bound sulfur atom and a broad, structured maximum at somewhat higher energies. The latter maximum is believed to arise from resonant backscattering of photoelectrons by carbon atoms 3.5-4.1 Å from the sulfur atom and possibly from s Æ p transitions of sulfur bonded to oxygen. The radial structure functions derived by Fourier analysis of the EXAFS exhibit peaks at distances that are compatible with the first three neighbor shells surrounding an aromatically bound sulfur atom.
Curtis, J.; Grant, D.M.
and Pugmire, R.J.
J. Am. Chem. Soc., 111, 7711, 1989. Funded by National Institutes of
Health.
Distortions in six-membered aliphatic ring systems were examined using deuterium chemical shifts and multiple linear regression analysis. A new set of regression parameters has been developed that successfully interprets not only spectra for those methylcyclohexanes with normal chair conformations but also spectra for cis- and trans-decalin and for three highly distorted trimethylcyclohexanes with skew-boat type distortions to their equilibrium conformations. The origin of deformations in the cyclohexane ring and the effect of distortions on ring hydrogen/deuterium chemical shifts are discussed in terms of gauche interactions between vicinal C-C and C-D bonds. The fits for 115 deuterium shifts in these cyclic alkanes exhibited a multiple R = 0.9816 and s = 0.0623 ppm for an overall shift range of about 1.5 ppm. As the 14 structural parameters obtained in this study are very sensitive to distortions and conformational features, the results indicate that ²H NMR methods should be very powerful in those cyclic paraffins in which highly second-order banding is observed in the corresponding H-1 NMR spectrum.
Nip, M.; DeLeeuw, J.W.;
Schenck, P.A.; Windig, W. and Meuzelaar, H.L.C.
Geochemica et Cosmochimica Acta, 53, 671-683, 1989. Funded by University
of Utah.
Samples obtained from the high-volatile bituminous Indiana cuticle-rich and paper coals (Indiana, U.S.A.) were investigated by white light and blue light microscopy and by Curie point pyrolysis-mass spectrometry in combination with multivariate data analysis, Curie point pyrolysis-gas chromatography and Curie point pyrolysis-gas chromatography-mass spectrometry. Samples obtained from several sites and of different degrees of weathering were analyzed. The maceral cutinite from the same samples was studied as well. The cutinite was concentrated by a standard palynological chemical oxidation technique as well as by density gradient centrifugation. The major constituent of the Indiana paper coal samples appeared to be a recently discovered non-saponifiable polymethylenic biopolymer, present in both recent and fossil plant cuticles. The maceral cutinite is almost exclusively made up of this highly resistant biopolymer. It is shown that upon severe natural weathering this biopolymer, because of its resistance against organic alteration and diagenesis, is relatively enriched and manifests itself in humic coals as the maceral cutinite.
Smith, K.L. and Smoot, L.D.
Accepted for publication in Prog. Energy Combust. Sci., 1989. Funded
by ACERC (National Science Foundation and Associates and Affiliates).
This review summarizes the selection and characterization of a set of coals commonly used in research programs in the United States. These coals have been selected from available U.S. coal data bases. Organizations that provide coal samples and/or coal characterization data include the following: (1) Pennsylvania State University, which has characterized many of the Nation's coal resources, as documented in the Penn State Coal Data Base operated by the Energy and Fuels Research Center, (2) the coal sample suite used in the Direct Utilization-Advanced Research and Technology Development program managed by the Pittsburgh Energy Technology Center, and (3) Argonne National Laboratory's Premium Coal Sample Program. The selection of eleven coals from these national banks provides a standard suite of coals for the Advanced Combustion Engineering Research Center of Brigham Young University and the University of Utah. These standard coals were selected according to the following criteria: (1) representative of a variety of characteristics, ranks and properties, (2) available analyses of chemical and physical properties with wide property variations among coal types and ranks, (3) availability from major producing seams, (4) future production expected, (5) wide geographical distribution within the U.S., (6) used in previous combustion research work, (7) common to existing prominent coal banks, and (8) availability of small, controlled samples. Information about the general aspects of coal characterization is summarized. Experimental data on the physical and chemical properties of these coals are documented, and the selected coals are related to the coal data banks. Major combustion research studies where these coals have been or are being used are referenced. General use of these well-characterized coals will help coordinate and integrate a national research effort in coal combustion and conversion.
Raynie, D.E.; Fields, S.M.;
Djordevic, N.M.; Markides, K.E. and Lee, M.L.
J. High Resoln. Chromatogr., 12, 51, 1989. Funded by Gas Research Institute.
Here, we describe a simple, inexpensive method for the preparation of mixed mobile phase (especially gas + liquid mixtures) for SFC. This method does not require cryogenic freezing of the organic modifier, and vacuum is needed only for initial evacuation of the closed system. Simple calculations allow determination of the exact concentrations of mixtures transferred to the SFC pumping system, accounting for loss of CO2 (or other gaseous primary fluid) during the transfer process.
Davies, I.L.; Xu, B.; Markides,
K.E.; Bartle, K.D. and Lee, M.L.
J. Microcol. Sep., 2, 71, 1989. Funded by Gas Research Institute and
the Utah Centers of Excellence.
A multidimensional system based on capillary supercritical fluid chromatography (SFC) was constructed that utilizes a simple flow-switching interface between two open-tubular 50-mm i.d. columns. A novel solvent-venting injection technique was incorporated in the system that enables single or multiple 0.5-mL volumes to be injected into an uncoated, yet deactivated, length of capillary precolumn without flooding of the analytical column. The effectiveness of multidimensional capillary SFC (SFC-SFC) for complex mixtures is demonstrated by the analysis of polycyclic aromatic hydrocarbons (PAH) in a coal tar extract, the trace determination of a methylcarbonate pesticide and its metabolites in a bird extract, and a group-type separation of hydrocarbons in a high-boiling petroleum distillate. These examples show for the first time that capillary SFC-SFC is a complementary alternative to other multidimensional chromatographic methods involving liquid or gaseous mobile phases.
Skelton, R.J. Jr.; Chang,
H.-C.K.; Farnsworth, P.B.; Markides, K.E. and Lee, M.L.
Anal. Chem., 61, 2292, 1989. Funded by Gas Research Institute and the
Utah Centers of Excellence.
Polycyclic aromatic compounds (PAC) are known to be major constituents in coal and petroleum products. Polycyclic aromatic hydrocarbons (PAH) are by far the most common PAC in these materials. However, nitrogen-, oxygen- and sulfur-containing PAC are also often found in significant quantities. Because of the widespread interest in and use of fossil fuels, detailed study of their compositions has become an important task.
Liu, F.; Mayne, C.L. and
Grant, D.M.
J. Mag. Res., 84, 344, 1989. Funded by National Institutes of Health.
The accuracy with which one can determine spectral densities in a coupled relaxation experiment is strongly dependent on the nature of the non-equilibrium states from which the spin system is allowed to relax. Pulse sequences similar to those used to obtain 2D J-spectra have been used to prepare initial non-equilibrium states of an AX2 spin system. These preparations create a maximum perturbation for one of the magnetization modes which was poorly perturbed by other spin preparations. Inclusion of data obtained using the new preparation significantly reduces the marginal standard deviations of several of the spectral densities and reduces correlations among the spectral densities. Thus, a significant improvement in the structural and dynamical parameters extracted from the spectral densities is achieved.
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.
Jiang, Y.J.; Woolfenden,
W.R.; Orendt, A.M.; Anderson-Altmann, K.L.; Pugmire, R.J. and Grant, D.M.
30th Experimental NMR Conference, Pacific Grove, California, 1989. Funded
by US Department of Energy.
A double tuned probe circuit used in high field solid-state NMR experiments has been evaluated. The idea of improved efficiency in both the decoupling and observe channels has been considered, with emphasis on alleviating the stray capacitance in the proton channel. The results of solid-state NMR experiments are included.
Dunkel, R.; Mayne, C.L.;
Pugmire, R.J. and Grant, D.M.
30th Experimental NMR Conference, Pacific Grove, California, 1989. Funded
by ACERC (National Science Foundation and Associates and Affiliates).
A phase sensitive two-dimensional INADEQUATE spectrum contains carbon-carbon connectivity information; and, hence, allows one to trace molecular carbon skeletons. When a bond is present between two carbons, there are antiphase doublets centered at the chemical shift of each of the carbons. This AB pattern is displaced in F1 by the double quantum frequency that is the sum of the chemical shifts. For each pair of lines in the 1D spectrum two small regions may be defined which must contain the pattern if a bond is present. Using eight parameters the AB pattern can be modeled as a sum of four two dimensional Lorentzians. A nonlinear least squares fit of this pattern to the experimental data from the two small regions is then performed for each pair of resonances in the 1D spectrum. The best-fit values of the parameters and the covariance matrix are used to decide whether the two carbons are bonded.
Pugmire, R.J.; Dunkel, R.;
Mayne, C.L. and Grant, D.M.
1989 International Conference on Coal Science, Vol. 2,Tokyo,1989. Funded
by ACERC (National Science Foundation and Associates and Affiliates).
NMR spectroscopy is a widely used analytical tool for probing molecular structure and dynamics. The growth in the use of this tool for studying coal-derived liquids has been quite dramatic. Spectral analysis of these complex hydrocarbon mixtures, however, has been hampered by the vast amount of data present in any given spectrum. We have embarked on a program to use multiple-pulse, two-dimensional (2D) NMR spectroscopy (1, 2) coupled with computer assisted analysis techniques to extract structural information from these materials. Described herein is a program called CCBOND for the computer aided analysis of 2D-INADEQUATE spectra.(3)
Fletcher, T.H.; Kerstein,
A.R.; Pugmire, R.J. and Grant, D.M.
Fuel Div. Preprint, 34 (4), 1272-1279, 198th ACS National Meeting, Miami,
1989. Funded by US Department of Energy and ACERC (National Science Foundation
and Associates and Affiliates).
It is well known that the yield of volatile matter obtained from a pulverized coal is dependent upon the temperature history of the particle. However, the effect of heating rate on volatiles yield is difficult to study independently of final temperature. For example, the volatile yields obtained in an entrained flow reactor study by Kobayashi, et al. increase with both temperature and heating rate, but the independent contribution of heating rate could not be assessed. Heated screen experiments were developed to study devolatilization behavior at different heating rates independently from the final particle temperature. The data of Anthony and Howard show little increase in volatiles yield when particles are heated to the same final temperature on a heated screen at different heating rates. In a more recent study, Gibbins-Matham and Kandiyoti show evidence for small increases in the volatiles yield from a Pittsburgh #8 coal as the heating rate is increased from 1 K/s to 1000 K/s on a heated screen. Coal samples were heated at 5 different heating rates to a final temperature of 700ºC and held for 30 s. Experiments were repeated several times in order to ensure accuracy of the data. The total volatiles yield increases from 41.5% at 1 K/s to 46.8% at 1000 K/s, a relative increase in yield of 13%. This increase in yield with increase in heating rate is small, but is larger than associated experimental errors.
The chemical percolation devolatilization (CPD) model was developed as a means to describe coal devolatilization behavior based upon the chemical structure of the patent coal. Some of the input parameters for this model are obtained from NMR characterizations of the parent coal. Percolation statistics are used to describe the probability of generating finite tar fragments from the infinite coal matrix. Pyrolysis yields of tar, gas, and char for three different types of coal are described using a single set of kinetic parameters: only chemical structure parameters are changed for the different coals. The initial description of the CPD model allowed for a temperature dependence of the competition between side chain formation and char formation. However, this option was not exercised in the initial study in order to demonstrate general utility of the model for one set of devolatilization data on three coals collected over a narrow range of temperatures and heating rates. In the present work, the Gibbins-Matham and Kandiyoti data are used to determine additional coefficients for the CPD model that accurately predict the changes in char and tar yield as a function of heating rate.
Yun, Y.; Maswadeh, W.; Meuzelaar,
H.L.C.; Simmleit, N. and Schulten, H.R.
ACS Preprint, Div. of Fuel Chemistry, Miami Beach, Florida, 34 (4), 1308-1316,
1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Nowadays it is widely recognized that the initial pyrolysis step in coal conversion processes has a profound effect on the yield and distribution of end products such as coal-derived liquids, gases, coke, or pollutant emissions. Two general approaches for modeling coal pyrolysis reactions can be distinguished, namely: (a) phenomenological modeling and (b) chemical modeling. The phenomenological modeling approach is useful in conversion processes such as high temperature gasification where detailed chemical information may be advantageous but is probably not indispensable. Other conversion processes, however, e.g., liquefaction and hydropyrolysis, may require more detailed chemical information to predict the distribution of final products. Whether pyrolysis ("devolatilization") models for pulverized coal combustion processes require detailed information on coal structure and reactivity or can be based primarily on a phenomenological approach is still a matter of considerable debate.
Heated screen pyrolysis techniques have been widely used to provide modeling parameters for phenomenological models based on the thermal behavior of light gas components detected by gas chromatography, mass spectrometry (MS) and other spectroscopic techniques. Due to limitations of the analytical techniques used, tar components are generally lumped into a single component.
Since time-resolved mass spectrometry (TR-MS) data can be used to analyze single mass profiles or mass spectra as a function of temperature, TR-MS data. Our estimations will be based on the chemical assignment of tar components observed in soft ionization mass spectra in combination with kinetic evaluation or temperature-resolved intensity profiles of single mass peaks and measured or simulated thermogravimetric weight loss curves.
Meuzelaar, H.L.C.; Yun,
Y.; Simmleit, N. and Schulten, H.R.
ACS Preprint, Div. of Fuel Chemistry, Miami Beach, Florida, 34 (3), 693-701,
1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
In recent years few topics have generated a more spirited discussion among coal scientists than the issue of the putative binary (mobile + network) phase nature of coals. Initially based on NMR observations, the concept of a "mobile phase" in coal soon came to encompass a broad range of more or less readily extractable and/or distillable lower molecular weight (MW) components, variously referred to as "guest molecules", "clathrates" or simply, and perhaps most succinctly, "bitumen". None of these terms appears to be completely satisfactory. The concept of "mobility" in NMR spectroscopy is quite different from that in the field of separation science, where mobility generally requires a measurable degree of solubility and/or distillability in liquid or gaseous media, respectively. For example, polymethylene-like moieties, such as found in some coal components, are highly "mobile" in NMR terms, without necessarily being extractable by solvents or distillable by nondestructive heating. The term "guest molecules", originally introduced to indicate specifically labeled marker molecules used in NMR studies of coal, is equally unsatisfactory for mobile phase components indigenous to the coal itself. Also, there appears to be insufficient evidence for the presence of sizeable quantities of true "clathrates" to rule out other possibilities, e.g., strong noncovalent bonding rather than physical entrapment. Finally, completely equating the "mobile phase" with solvent (e.g. pyridine) extractable "bitumen" in coal ignores the potential presence of colloidal particulate matter in the pyridine extracts as well as possible solvent-induced scission of weak chemical bonds. Furthermore, the solvent-extractable fraction may well include macromolecular components, such as resinites.
Mass spectrometric observations have thus far played a rather limited role in the "mobile phase" discussions but are starting to shed some light on the key question: is there conclusive evidence for the presence of a chemically and/or physically distinct "mobile phase", as opposed to a continuum of possible molecular sizes and structures? In the context of the present discussion, the term "mobile phase" will be used to describe those components which can be thermally extracted ("distilled" "desorbed") under vacuum at temperatures below the thermal degradation range of the coal. The residue, designated as the nonmobile ("network") phase, is thermally degraded in the pyrolysis temperature range. Of course, the onset of pyrolysis may vary considerably, depending on heating rate, rank and coal type.
Maswadeh, W.; Roberts, K.A.;
McClennen, W.H.; Meuzelaar, H.L.C. and Arnold, N.S.
37th ASMS Conference on Mass Spectrometry and Allied Topics, Miami Beach,
Florida, 304-305, 1989. Funded by ACERC (National Science Foundation and Associates
and Affiliates), US Department of Energy, and the State of Utah.
A laser pyrolysis mass spectrometry experiment was designed to study the devolatilization behavior of individual coal particles at high heating rates (104-106 K/s), characteristic of pulverized coal combustion reactors. The experimental set-up consists of an electrostatic particle levitation cell, also known as an "electrodynamic balance", a 50 W cw CO2 laser and a Finnigan-MAT ITMS system. The particle levitation cell was constructed by modifying a regular ion trap electrode assembly in such a way as to provide line-of-sight access to the center of the cell for the CO2 laser beam as well as for visual observation by means of a stereo microscope. Typical cell operating parameters for levitating a 120 mm dia. Spherocarb particle are: ring electrode 3000 V (60 Hz ac), upper end-cap +100 V dc, lower end-cap -100 V dc. The CO2 laser (Apollo 3050 OEM) is capable of electronic pulsed beam operation. The 8 mm dia. beam is split equally into 2 opposing beams focused at the center of the levitation cell (beam waist ca. 400 um, power density ca. 4-10 MW/m2), as a co-linear parfocal HeNe laser beam permits positioning the levitated particle in the optical and electrical center of the cell. Two IR detectors measure the integrated pulse and time-resolved pulse energy.
A heated transfer line column (2m x .18 mm DB5) equipped with a special air sampling inlet enables intermittent sampling of volatiles from the center of the levitation cell into the ITMS vacuum system. Feasibility studies were performed on 120-150 mm Spherocarb particles impregnated with an alkylnaphthalenes mixture and heated with a single 10 ms CO2 laser pulse. Ample signal intensities were obtained with the first laser pulse to permit the recording of "transfer line" GC/MS profiles. By contrast, the second laser pulse produced <10% of the volatiles observed from the first pulse, thereby demonstrating nearly complete devolatilization of the impregnated particle by a single laser pulse.
Finally, a series of experiments was performed with actual coal particles in the 100-130 mm size range, prepared by careful sieving of two coals from the Argonne National Laboratory Premium Coal Sample bank, namely a Pittsburgh #8 seam (Illinois) coal and a Blind Canyon seam (Utah) coal of comparable rank. From previous studies of Pitt. #8 coal using time-resolved Curie-point pyrolysis, pyrolysis temperatures necessary to produce these phenolic building blocks at heating rates of 105 K/sec are estimated to be ca. 900 K. Of course, kinetic parameters obtained at the much lower heating rates of the Curie-point pyrolysis MS experiment (approx. 102 K/s) may only be extrapolated to the laser pyrolysis experiment if the underlying reaction mechanisms are comparable. Evidence supporting this assumption is presented showing similar C1 and C2-alkyl phenol profiles for the Pitt. #8 coal obtained by Curie-point pyrolysis GC/MS using a 15 m fused silica capillary with temperature programming.
Hyde, W.D.; Hecker, W.C.;
Cope, R.F.; Painter, M.M.; McDonald, K.M. and Bartholomew, C.H.
Western States Section/The Combustion Institute, Livermore, California,
1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Coal char reactivity has been found to vary greatly depending on the rank and type of the parent coal. Also, the conditions under which a given coal is devolatilized to produce char can significantly effect the reactivity of the resulting char. Preparation conditions such as gas environment, heating rate, peak temperature, residence time, and particle size are very important in determining the resulting char reactivity in that they effect its chemical and physical structure.
The general objectives of this work are to (1) understand the effect of the rank of the parent coal on the oxidation rate (reactivity) of its derived char, (2) understand the effect of devolatilization conditions on the char oxidation rate, and (3) determine any correlations which may exist between char oxidation rates and the chemical and physical properties of the chars. Specifically, oxidation rates for char from 5 coals of various ranks were measured and compared. The differences in char reactivity of chars produced in three different char preparation apparatus: a muffle furnace, a flat-flame methane burner, and a high temperature inert-atmosphere reactor, were studied. The effects of peak temperature, residence time, and particle size were also studied. Finally, correlations of oxidation rate with hydrogen content, cluster size, and surface area were attempted.
Samples of chars from Beulah Zap (Lignite), Dietz (Subbituminous A), Utah Blind Canyon (hvC Bituminous), Pittsburgh #8 (hvA Bituminous), and Pocahontas #3 (lv Bituminous) were prepared at different residence times in the Flat-Flame Char Preparation Apparatus; samples of Pittsburgh #8, Beulah Zap, and Dietz were also prepared in the muffle furnace and the high temperature inert-atmosphere reactor. The low temperature reactivity of all the coal char samples was determined in a TGA using Tcrit as the reactivity indicator. Tcrit is defined at the temperature at which the mass loss of the sample reaches 11 percent per minute.
Hecker, W.C.; Hyde, W.D.
and McDonald, K.M.
International Chemical Congress of Pacific Basin Societies, Honolulu,
1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The objective of this study is to understand the effect of devolatilization conditions on the oxidation rate (reactivity) and surface properties of the resultant char. Chars were produced from 3 ANL coals (Pittsburgh No. 8, Beulah Zap, and Pocahontas No. 3) and Dietz subbituminous (PETC) by three different methods: (1) a flat flame methane burner, (2) a rapid heating-rate inert atmosphere reactor, and (3) a slow heating-rate muffle furnace. Thermogravimetric analysis (TGA) was used to determine the low-temperature reactivity of these chars as a function of residence time and peak temperature. CO2 surface areas pore size distributions were also determined for some samples. Trends of increased reactivity with decreasing rank and decreasing residence time were observed. Preliminary results indicate that surface area goes through a maximum with increasing devolatilization time for Pittsburgh No. 8 while it is essentially unaffected for the Zap lignite char. Char reactivity appears to be correlated with hydrogen content.
Hyde, W.D.; McDonald, K.M.;
Cope, R.F.; Bartholomew, C.H. and Hecker, W.C.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989.
(Also presented at the Rocky Mountain Regional AIChE Meeting, Salt Lake
City, 1989). Funded by ACERC (National Science Foundation and Associates and
Affiliates).
The combustion of coal consists of devolatilization and heterogenous char oxidation. The objective of this study is to understand the effect of devolatilization conditions on the oxidation rate (reactivity) and surface properties of the resultant char. Chars were produced by devolatilizing Pittsburgh No. 8 bituminous coal and North Dakota lignite at varying residence times and peak temperatures in a flat flame methane burner. Thermogravimetric analysis was used to determine the low-temperature reactivity of these chars. Trends of increased reactivity with decreased preparation temperature and residence time are observed. N2 and CO2 surface areas and pore size distributions were determined. Preliminary results indicate that surface areas increase with increased devolatilization. Char reactivity is also correlated with hydrogen content and aromatic cluster size.
Wells, W.F.; Hyde, W.D.;
Cope, R.F.; Smoot, L.D.; Hecker, W.C. and Bartholomew, C.H.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989.
Funded by ACERC (National Science Foundation and Associates and Affiliates).
To aid in the development and provide validation data to the char oxidation submodel, measurements of oxidation rates at high and low temperatures are being collected for chars prepared from five select ACERC coals. Chars are prepared using a flat flame burner and a recently constructed inert atmosphere drop-tube reactor heated using an inductively coupled plasma. The drop-tube reactor is also used to obtain reaction rates at high temperatures; low temperature rates were measured in a TGA. Chemical and physical properties of the fuel were measured. Multivariate statistics are used to correlate fuel properties to reaction rates.
White, W.E.; Bartholomew,
C.H.; Thornock, D.; Wells, W.F.; Hecker, W.C. and Smoot, L.D.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989.
Funded by ACERC (National Science Foundation and Associates and Affiliates).
Results of an ongoing collaborative study of the surface properties and pore structure of a suite of 11 coals selected for comprehensive study by ACERC are reported. The principal objective is to correlate the surface, pore, and chemical properties of coals and chars with their rates of combustion. Surface areas, pore volumes, pore size distributions, and solid densities were measured for Pittsburgh No. 8, Wyodak, Beulah Zap Lignite, Lower Wilcox, Dietz and a Utah Scofield coal and for chars derived from these coals. Surface areas, pore volumes and pore size distributions were measured using nitrogen and carbon dioxide adsorptions, mercury porosimetry and NMR spin-lattice relaxation measurements for samples saturated with water vapor. Solid densities were obtained using helium displacement. The results indicate that chars have larger surface areas and pores volumes relative to coals. New mesopores are created and micropore volume increases during devolatilization. Large fractions of the internal pore volume of coals are not penetrated by nitrogen molecules during adsorption, but are penetrated by carbon dioxide, suggesting that a fraction of the pore volume is microporous, or involves blocked pores. By using several techniques for measuring surface properties (e.g. N2 and CO2 adsorption isotherms, NMR,etc.), the pore structure of coals and chars can be defined more accurately, and char oxidation models can be evaluated with more understanding.
Davis, P.J.; Smith, D.M.;
Bartholomew, C.H.; White, W.E. and Hecker, W.C.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989.
Funded by ACERC (National Science Foundation and Associates and Affiliates).
Due to the wide pore size range and complexity of coals and chars, it is difficult to study the pore structure. Multiple techniques such as gas adsorption, mercury porosimetry, and density measurements are often used. These techniques suffer from limited pore size range, pore shape assumption, network/percolation effects, and sample changes during analysis. To obtain a more complete description of coals and chars, NMR spin-lattice relaxation measurements of water saturated coals and chars have been performed. The NMR technique does not suffer from network/percolation effects, sample changes, pore shape assumption (rp>5nm). Three coals and their chars were compared.
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