ACERC
Abstracts - 1994 |
ACERC
Abstracts - 1994 |
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Thrust Area 1: Fuel Structure and Reaction Mechanisms |
Pugmire, R.J.
Encyclopedia of NMR, J. Wiley & Sons, Ltd., 1994 (in press). Funded by
ACERC and US Department of Energy/Pittsburgh Energy Technology Center.
Coal has been used as an important source of fuel for thousands of years, but it is a complex, heterogeneous fuel that is difficult to burn or process without serious environmental implications. Coals can vary significantly among different geographic areas in important properties such as rank, ash, sulfur and nitrogen content, mineral impurities, and maceral constituents. Substantial worldwide attention is being focused on more efficient and cleaner methods for utilization of this important energy resource.
The wide heterogeneity of coal has made it difficult to characterize and to correlate its structure. Evidence exists that differences in geologic histories and maceral constituents affect coal chemistry and technological applications of coals since coals of the same apparent rank but of different geologic history can exhibit a variety of physical and chemical properties. Coal structure varies with recognizable geological and geochemical features that affect its reactivity. The variations in the geochemical history and the complexities of the macromolecular structure of coal further confound a clear understanding of the complex nature and interrelationships among coals.
Our understanding of the details of coal structure has improved markedly over the last decade. Coal is no believed to be a heterogeneous mixture composed of a macromolecular network of varying degrees of cross-linking within the macromolecular phase with a smaller molecular phase imbibed or associated with this network. A major portion of the credit fir the present state of knowledge of the general characteristics of coal structure can be traced to studies that employed NMR spectroscopy. It is interesting to observe that NMR was first applied to the study of coals in 1955, only 9 years following the discovery of the NMR phenomenon in bulk matter. These first NMR observations employed broad line H-1 techniques that, in the absence of means to reduce the large proton dipole-dipole interactions, produced very broad bands in fossil fuel related materials. In 1968 Haeberlen and Waugh demonstrated that the hydrogen dipole-dipole interaction could be significantly reduced by averaging in spin space via a multiple-pulse approach rather than attempting to mechanically spin the sample at very high MAS rates in order to achieve the same results. In the mid-1970's Schnable and Gerstein, et.al. demonstrated that if one simultaneously spins the sample about the magic angle then the chemical shift anisotropy (CSA) as well as the inhomogeneous heteronuclear dipolar interactions are simultaneously averaged. Hence, much narrower peaks are obtained with this experiment which Gerstein named CRAMPS. Gerstein first applied the proton CRAMPS experiment to coals in 1981. The CRAMPS experiments have been especially useful as a probe to study the "mobile phase" present in coal structure wherein solvents such as perdeuterated pyridine are imbibed into the parent coal. The reduction in proton NMR line widths is attributed to motional narrowing and to reduction of bulk and molecular susceptibility anisotropies by partial mobilization of certain structural moieties in coal caused by disruption of hydrogen bonds and other non-covalently bonded structural units.
As early as 1966 the first C NMR spectra appeared of materials derived from coal. Broad line C NMR spectroscopy was first applied to whole coals in 1971 to confirm the high aromaticity of anthracite. Significant improvements in resolution began to emerge 5 years later with the application of the cross-polarization technique followed by applications of magic angle spinning to coal. In 1979 Opella introduced the concept of the dipolar dephasing experiment that discriminates between nonprotonated and protonated carbons by means of the effective C-H dipole-dipole interaction. Alemany studied characteristics of the dipolar dephasing experiment on model compounds and then demonstrated the utility of this experiment in a careful study of an Illinois #6 coal. Work by Wilson, Murphy, and Gerstein have verified the value of dipolar dephasing experiments in the study of coal. The data derived from the techniques described, together with others to be described in subsequent sections, have been extremely valuable in probing the general structural features of coals and have greatly enhanced the usable knowledge base for improved coal utilization.
Hu, J.Z.; Wang, W. and Pugmire,
R.J.
Encyclopedia of NMR, J. Wiley & Sons, Ltd., 1994 (in press). Funded by
ACERC and US Department of Energy/Pittsburgh Energy Technology Center.
It is well known that in a solid, the chemical shift of a nucleus is a function of molecular orientation with respect to the external magnetic field. This phenomenon is described as the chemical shift anisotropy (CSA), and it is directly related to the local electronic structure of the nucleus. The principal values of the CSA as well as their orientation in the molecular frame can be obtained through a single crystal study (see "Chemical Shift Measurement by Single Crystal Techniques"). However, for the majority of compounds, the difficulties associated with growing a single crystal of sufficient size limits the application of current single crystal methods. In a powder sample the orientation information is lost because of the random distribution of the crystallites. However, the principal values, which are very useful in characterizing the structure of a molecule, are still available in the powder pattern obtained from a stationary or slowly spinning sample when the molecule has few enough unique nuclei that the spectrum can be interpreted. Unfortunately, overlap of several broad powder patterns often prevents the spectral separation necessary for individual identification and measurement.
In an effort to address this problem of spectral overlap, many 2-D techniques have been developed to obtain a 2D spectrum with an isotropic shift projection along one dimension and a stationary or slow-spinning-sideband powder pattern along the other (see "Chemical Shift Tensors"). One of the first techniques developed was the magic angle hopping (MAH) experiment of Bax et. al. By successively "hopping" the sample 120° about an axis at the magic angle, an isotropic shift dimension is obtained since the average of the resonance frequencies at the three orthogonal positions is the isotropic shift.
An analog of the MAH experiment employing continuous slow rotation of the sample has recently been demonstrated by Gan. Gan's elegant technique uses pulses spaced at 1/3 of the rotor period to obtain the isotropic shift evolution. We call Gan's experimental technique the Magic Angle Turning (MAT) experiment because of the very slow rotation involved. Significant improvements in the experimental details have been made to optimize these two experiments.
In this article, a simple theory is given to describe the MAH experiment and the most recent version of the MAT experiments together with typical experimental results included to show the basic principals and the power of these two related methods.
Smith, L.K.; Smoot, L.D.;
Fletcher, T.H. and Pugmire, R.J.
Plenum Press Co., New York, 1994. Funded by ACERC.
This book characterizes the properties and reaction rates of the eleven U.S. coals selected emphasis by ACERC. Eight of the eleven comprise the Argonne National Laboratory's Premium Coal Sample Bank. The book features the comprehensive measurement of organic and inorganic components of the coal structure by advanced methods (SEM, NMR, GC, porosimetry, Pycnometry, X-ray, and MS). The book features the measurement of coal devolatilization and char oxidation rates by advanced, optical methods and correlative relationships between the structure and reaction processes.
Chen, W.
A Global Reaction Rate for Nitric Oxide Reburning, Ph.D./BYU, December 1994.
Advisors: Smoot, Fletcher and Hill
Maswadeh, W.
Devolatilization Studies of Single Coal Properties at High Heating Rates,
Ph.D./U of U, January 1994. Advisor: Meuzelaar
Bartholomew, C.H. and Hecker,
W.C.
Chemical Engineering, 70-75, June 1994. Funded by Brigham Young University.
Most major processes in the chemical process industries are built around heterogeneous chemical reactions. A solid catalyst is an integral part of almost all these operations. In new-construction or retrofit project for such plants, process engineers must design and specify not only the reactors but also the catalysts. Independent design of the two, without concern for how they will mesh, can mean a more costly design, a low production rate and more-frequent shutdowns. It may even cause the catalyst to fail. Consider, for instance, this debacle at a methanol plant. A carbon-steel pipe had been installed at the entrance to the methanol reactor. High-pressure carbon monoxide in the feed stream reacted with the steel to produce iron carbonyls, which poisoned the catalyst. Remedying the situation cost several million dollars.
With the hope of avoiding such situations, we first summarize the principles of catalyst and reactor design, with emphasis on maintaining interdependence between the two activities. Then we apply the principles to industrial reactors. The focus is solely on heterogeneous catalysis, in which the catalyst (virtually always in solid form) is not the same phase as the process stream. Even with this limitation, the technology is far too detailed for full presentation here. Instead, out aim is to enable readers to keep the big picture in mind whenever getting immersed in the specifics of the project.
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.
Cope, R.F.; Arrington, C.B.
and Hecker, W.C.
Energy & Fuels, 8(5):1095, 1994. Funded by ACERC, National Science Foundation
and US Department of Energy.
This work examines the effect of char burnout level and calcium content on the intrinsic char oxidation rates and physical properties of three series of chars. Three starting chars were prepared by devolatilizing a 63-74 mm fraction of North Dakota Zap lignite in a flat-flame burner; then, a portion of this char was washed in HCl to remove mineral matter; finally, a portion of the acid-washed char was reloaded with calcium. The three starting chars (ND (untreated), NDW (acid-washed), and NCa (Ca reloaded)) were then oxidized to various levels of conversion (6-92%) in a heated-wall drop-tube reactor (DTR) at high temperature (1523 K) in a 5-7% O2 environment. Low-temperature intrinsic oxidation rates were determined for each resulting sample using isothermal TGA (648-748 K, 10% O2). Other measured properties include burnout, N2 BET and CO2 DP surface areas, and CaO surface area. The latter was determined using a selective CO2 chemisorption technique. Intrinsic oxidation rate decreased as burnout increased for the calcium containing chars (ND and Nca). For the NDW char (69% Ca removed), the intrinsic rate was independent of burnout. Increased burnout produced general decreases in N2 BET and CO2 DP surface areas for all three chars, but they did not correlate well with rate. Increased burnout also produced decreases in C1) surface areas for the ND and Nca chars, but not for the NDW. The decreases in CaO surface area paralleled the decreases in intrinsic rates. This led to good correlation of CaO surface area with rate. Furthermore, the normalized values of rate per CaO surface area were essentially independent of burnout for the Ca-containing char series. This result suggests that catalysis by Ca is very significant during low-temperature oxidation. These results also indicate that the reason for decreasing rate with burnout is due to sintering of CaO with increased time of exposure to the high-temperature environment. Activation energies for the chars in the three series were found to be independent of burnout level and calcium content. Average values were 32.9 ± 1.4, 34.7 ± 3.3, and 33.6 ± 1.3 kcal/mol (uncertainties expressed as 95% confidence intervals) for ND, NDW, and Nca, respectively.
Blackham, A.U.; Smoot, L.D.
and Yousefi, P.
Fuel, 73:602-612, 1994. Funded by US Department of Energy/Morgantown
Energy Technology Center through Advanced Fuel Research Co. and ACERC.
Rates of oxidation of 5-10 mm particles of chars from six coals at various temperatures were measured in air at ambient pressure in simple devices: a muffle furnace, a Meker burner, and a heated ceramic tube. The chars were first prepared from the coals in the Meker burner at comparable temperatures. As well as coal type and oxidation temperature, initial char particle steps of several minutes for periods up to 1 h. The cube root of particles mass decreased linearly with increasing time in all tests. Ash layers formed and usually remained in place around the particle. Average mass reactivities increased with decreasing initial particle mass. With decreasing furnace temperature, char reactivity decreased at the lower temperatures. Two or four closely spaced char particles burned much more slowly than single particles of the same size. Correlative equations are consistent with the data, elucidating the roles of kinetic reaction and oxygen diffusion.
Anderson-Altmann, K.L.;
Phung, C.G.; Mavromoustakos, S.; Zheng, Z.; Facelli, J.C.; Poulter, C.D. and
Grant, D.M.
Journal of Physical Chemistry, 1994 (in press). Funded by National Science
Foundation.
The N-15 chemical shift tensors of uracil are detemined by N-15 powder pattern techniques. The principal values of the N-15 uracil tensors are obtained from the spectra of [1-N-15]uracil and [3N-15]uracil, and the tensor orientations are determined from the spectrum of [1,3-N-152, 2-C-13]uracil with the inclusion of dipolar interactions. Ambiguities in the orientational assignments are resolved using molecular symmetry considerations and results of ab initio calculations of the N-15 chemical shielding tensors. The N1 nitrogen has principal values of 196 ppm, 114 ppm, and 30 ppm and the N3 nitrogen 200 ppm, 131 ppm, and 79 ppm with respect to N-15H4NO3. The components with the largest chemical shifts lie approximately along the N-H bonds. Including the effect of intermolecular hydrogen bonds on the theoretical calculations improves in a significant way the agreement between the calculated and experimental chemical.
Bateman, K.J.; Smoot, L.D.;
Germane, G.J.; Blackham, A.U. and Eatough, C.N.
Fuel, 1994 (in press). Funded by US Department of Energy/Morgantown Energy
Technology Center and ACERC.
Mass loss and burnout ties of large (five and eight millimeter diameter) char particles at pressures between 101 to 760 kPa have been measured in a newly designed and constructed high-pressure reactor. A cantilever balance attachment was fitted to the reactor to measure instantaneous particle mass while an optical pyrometer measured particle temperature continuously. The process was also videotaped at 1/30 s frame speed. Sixty-two combustion experiments produced burning and oxidation times for two sizes of Utah bituminous (HVBB) coal and North Dakota Lignite (L) at 101, 507, 760 kPa total pressure. The reactor air temperatures were about 900 or 1200 K while the airflow Reynolds Number was varied by a factor of two. Coal particles were placed in a platinum-wire basket inside the reactor at the end of the balance beam. The oxidation process was recorded by computer and on videotape, while continuous char oxidation rates were measured to burnout. An ash layer accumulated around the particles, and receded as the char was consumed. In all of the tests, including the elevated pressure tests, a linear decrease in the cube root of char mass with time was observed during char oxidation until near the end of burnout. Changes in air velocity had little effect on oxidation times while either increasing gas temperature or increasing pressure from 101 kPa to 507 kPa reduced oxidation times by about one-quarter. Further increase in pressure caused no further reduction in burn time. Pairs of nearly equally sized particles of coal had oxidation times similar to single particles that had a mass equal to the sum of the pairs.
Facelli, J.C.; Hu, J.Z.;
Orendt, A.M.; Arif, A.M.; Pugmire, R.J. and Grant, D.M.
Journal of Physical Chemistry, 1994 (in press). Funded by US Department
of Energy and ACERC.
This paper presents a detailed study of the principal components of the C-13 chemical shift tensors in p-tolyl ether. The tensor components of a relative large number of carbon atoms are measured by using the two-dimensional magic angle turning (MAT) technique that allows for the determination of the principal components of the chemical shift tensors in powders. Theoretical calculations of the C-13 chemical shieldings, using the X-ray molecular geometry, are used to assign the NMR resonances to individual carbon nuclei. The principal values of the chemical shift tensors permit assignments that would be unreliable if only the isotropic shift information is used. The chemical shift tensors of the carbons directly attached to the oxygen atom are very sensitive to the structural and electronic properties of the ether linkage. The combination of the C-13 MAT experiment and theoretical chemical shieldings proves to be important in the study of electronic properties and molecular structure.
Gale, T.K.; Bartholomew,
C.H. and Fletcher, T.H.
Combustion and Flame, 1994 (in press). (Also presented at the 25th
International Symposium on Combustion, Irvine, CA, August 1994). Funded
by ACERC.
Concern about comparability and validity of different methods for producing coal chars for reactivity experiments has led to research on the effect of devolatilization conditions on the char physical and chemical structure. Particle diameter and porosity changes during devolatilization significantly affect char oxidation rates. In particular, physical properties of chars prepared in drop tube reactors differ greatly from chars prepared in flat flame burner experiments. Recent data indicate that the presence of oxygen in the gas atmosphere has no effect on swelling until char oxidation has begun. The present research concentrates on the effects of heating rate, particle temperature and residence time on the swelling and porosity of a plastic coal, and compares these results with a non-plastic coal. The heating rate at which the transition from increasing swelling to decreasing swelling occurs in approximately 5 x 10³ K/s for swelling coals. Swelling coals also reach a maximum porosity near this heating rate. At low particle heating rates swelling gradually increases versus heating rate in contrast to a decline in the swelling at high heating rates in a narrow heating rate region of 2 x 10^4 to 7 x 10^4 K/s. Non swelling bituminous and lignite coals continue to increase in porosity beyond the heating rate of 2 x 10^4 K/s.
Gale, T.K.; Fletcher, T.H.
and Bartholomew, C.H.
Energy & Fuels, 1994 (in press). Funded by ACERC.
Concern about comparability and validity of different methods for producing coal chars for research has motivated this investigation of the effects of devolatilization conditions on the physical properties of coal chars. It is evident from the findings of this study that care must be taken to prepare chars under conditions similar to those of full-scale coal combustion boilers prior to performing char oxidation studies. Two different entrained flow reactors were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures and heating rates between 840 to 1627 K and 10^4 to 2 x 10^5 K/s respectively. Under these conditions micro-pore (CO2) surface area generally increases with residence time and mass release for lignite and bituminous coals, as does true density. Micro-pore surface area also increases somewhat with increasing maximum particle temperature and heating rate. Meso-pore (N2) surface area is most affected by reactive gas atmospheres (carbon activation). The presence of steam in the post flame gases of methane/air flat flame burners is a significant factor in increasing meso-pore surface are of chars prepared in such burners, even though the mass conversion by steam gasification is small. Partial char oxidation with O2 significantly affects char N2 and CO2 surface area at these heating rates and residence times (50 to 100 ms), sometimes increasing and sometimes decreasing internal surface area. Low rank lignite and sub-bituminous coals have higher potentials for forming chars with increased N2 surface are than bituminous coals. The moisture content of low rank coals may be more important than rank. Lignite with high moisture content yields char with a significantly higher N2 surface area than char prepared from lower moisture content lignite. However, initial coal moisture has less effect on CO2 surface area.
Hu, J.Z.; Orendt, A.M.;
Alderman, D.W.; Pugmire, R.J.; Ye, C. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 5:181, 1994. Funded by US Department
of Energy and ACERC.
The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of C-13 chemical shift tensors in powdered solids. A large-volume MAT prove with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively, can enhance measurements of the principal values of chemical shift tensors in complex compounds. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the C-13 principal values in 2,3-dimethlnaphthalene and Pocohontas coal are reported at typical examples.
Bateman, K.J.; Germane,
G.J.; Smoot, L.D. and Eatough, C.N.
Energy & Fuels, 1994 (in press). Funded by US Department of Energy/Morgantown
Energy Technology Center and ACERC.
A study was undertaken to design, construct, characterize, and demonstrate a new facility for determination of reaction rates of large coal particles at elevated pressures. A cantilever balance attachment (CBA) was designed, fabricated, and utilized in conjunction with the existing High Pressure Controlled Profile (HPCP) reactor. Large particle (8mm diameter) combustion experiments of Utah HVBB coal at both atmospheric and elevated pressures were performed to demonstrate the facility's capabilities. Measurements were obtained of particle mass loss rate and surface temperature coupled with a video record for visual observation.
Maswadeh, W.; Tripathi,
A.; Arnold, N.S.; DuBow, J. and Meuzelaar, H.L.C.
Journal of Analytical and Appl. Pyrolysis, 28:55-70, 1994. Funded by
ACERC.
A high speed, two-wavelength radiation thermometer that is capable of monitoring the surface temperature of 50-150 µm diameter particles in the 600-2000 K range at heating rates of up to 106 K/s, characteristic of pulverized coal combustion, was designed and constructed. To meet the above characteristics, special attention was paid to detector wavelength range and speed, detection electronics and optical system alignment. The thermometer was calibrated using an in-house constructed, black cavity radiation source. Spherocarb model particles, which have a more uniform size; physical properties and emissivity than coal particles, were used to demonstrate the level of short-term reproducibility attainable. Consistent, reproducible temperature-time profiles obtained for particles from different coals indicate that non-grey effects do not dominate these measurements.
Keogh, R.A.; Hardy, R.H.;
Taghizadeh, K.; Meuzelaar, H.L.C. and Davis, B.H.
Fuel Processing Technology, 37:33-52, 1994. Funded by US Department of
Energy.
The mobile component of western Kentucky coals were extracted and analyzed by conventional methods and Curie-point mass spectroscopy. The liquefaction of the parent coals, extracted coals, and blends of the extracted coals plus mobile components indicated that the absence of the mobile component generally decreases the observed conversions obtained. The results also show that, in general, blending the mobile component and extracted coal also produces lower conversions than those obtained from the parent coal. These data suggest that the location of the mobile component in the pore structure of the coal is as important as the presence of the mobile component in coal conversion.
Fletcher, T.H. and Pugmire,
R.J.
Proceedings of the ACS Spring Meeting, San Diego, CA, March 1994, ACS
Preprints, Division of Fuel Chemistry, 39:108-112. Funded by ACERC, Brigham
Young University and University of Utah.
Enormous progress has been made in coal pyrolysis research during the last decade. Models of coal devolatilization have progressed from simple rate expressions based on total mass release to empirical relationships based on the elemental composition of the parent coal to models that attempt to describe the macromolecular network of the coal. Measurements of particle temperature during devolatilization have eliminated much of the controversy regarding overall rates of devolatilization. In the last several years, advancements in chemical analysis techniques have allowed quantitative investigations of the chemical structure of both coal and its pyrolysis products, including the nature of the resulting char. A prominent research goal is to accurately predict the rates, yields, and products of devolatilization from measurements of the parent coal structure. This goal necessitates modeling the reaction processes on the molecular scale, with activation energies that relate to chemical bond breaking rather than release of products from the coal. C-13 and H-1 NMR spectroscopy have proven particularly useful in obtaining average values of chemical structure features of coal, char and tar. This paper reviews experimental data regarding chemical structure features of coal, char, and tar during rapid devolatilization, and how these data have impacted the development and input parameters for devolatilization models. In particular, the relationship between pyridine extract yields and extract yields predicted purely from NMR chemical structure data discussed.
Meuzelaar, H.L.C.
Proceedings of the Specialists Workshop on Applications of Free-Jet Molecular
Beam Mass Spectrometric Sampling, Estes Park Center, CO, October 1994, and
the 25th International Symposium on Combustion, Irvine, CA, Jul-Aug,
1994. Funded by ACERC and the Consortium for Fossil Fuels Liquefaction Science.
Using mass spectrometry as an on-line analytical method for studying the mechanisms and kinetics of reaction processes requires detailed awareness of various reaction/interaction/transport zones between the point of reaction and the point of ionization. The following zones (regimes, regions) can be more or less clearly recognized in most on-line MS studies of reactions in complex solids: (1) intramolecular; (2) intermolecular; (3) intraparticle; (4) interparticle (bed); (5) reactor (headspace); (6) pressure reduction; and (7) ionization. In an ideal, on-line system all products, intermediates and precursors of interest should reach the ionization region without selective losses, chemical degradation or unwanted background contributions. In practice, various secondary reactions occur well past zone 2, often resulting in chemical degradation of target analytes. Furthermore, marked losses of low volatile and/or highly polar compounds are likely to occur between reaction and ionization regions unless potential coal spots and/or active surfaces are carefully eliminated. Finally, unwanted background contributions for residues of previous runs and/or ubiquitous contaminants may originate anywhere between zones 5 and 7.
Liu, K.; Jakab, E.; Zmierczak,
W.; Shabtai, J.S. and Meuzelaar, H.L.C.
ACS Preprints, Division of Fuel Chemistry, 39(2):576-580, 1994. (Proceedings
of the ACS National Meeting, and the 17th Annual Symposium of the
Rocky Mountain Fuels, Golden, CO, March 1994 and the ACS Meeting, Division
of Fuel Chemistry, San Diego, CA, March 1994.) Funded by ACERC and the Consortium
for Fossil Fuels Liquefaction Science.
A recently developed on-line high-pressure themogravimetry (TG)/gas chromatography (GC)/mass spectrometry (MS) system provides certain advantages over other on-line analysis techniques for high-pressure reactors reported previously. The high pressure TG/GC/MS system enables the simulation of solvent-free thermal and catalytic reactions for polymers and coal. During the reactions the total weight change is monitored and the volatile intermediate products are identified. It requires only very small amounts (10-100 mg) of sample and can be operated at high pressure under different atmospheres (N2, He, H2, etc.). Current efforts to recycle lower grade post consumer polymers such as colored polyethylene and polystyrene or used rubber tires, are concentrated on co-processing with coal. Purely thermal degradation processes involve both decomposition and condensation (recombination) reactions and the resulting product is highly olefinic and often aromatic. In order to improve the yield and selectivity of the process, a great deal of effort has been spent on finding the proper catalysts. Catalysts selected for the present studies include ZrO2/SO4, (NH4)2MoS4 and carbon black. Carbon black present in waste rubber tires has been reported to be very selective for the cleavage of specific alkylaryl bonds. (NH4)2MoS4 has been shown to improve the liquid yields in coal liquefaction. The superacid catalyst Zr2O2/SO4 possesses markedly higher hydrogenolytic activity compared to that of conventional SiO2-supported soluble Fe salts.
Nie, X.; Liu, K.; Maswadeh,
W.; Tripathi, A. and Meuzelaar, H.L.C.
ACS Preprints, Division of Fuel Chemistry, 39(2):558-563, 1994. (Also
presented at the 17th Annual Symposium of the Rocky Mountain Fuels Society,
Golden, CO, March 1994, and at the ACS Meeting, San Diego, CA, March
1994.) Funded by ACERC and the Consortium for Fossil Fuel Liquefaction Science.
During the past decade marked progress has been made with regard to our understanding of the chemical processes occurring during the thermal degradation ("devolatilization," "desorption + pyrolysis") of coal and several advanced mechanistic models offering a qualitative and quantitative description of these processes, e.g., FG-DVC and CPD models, are now available. By contrast, there appears to be a comparative lack of progress in the description and understanding of the physical processes involved. It is becoming increasingly clear that the frequent lack of interlaboratory reproducibility almost invariably originates within the physical parameters of the experiment. Although heating rate, particle size and reactor pressure have long been recognized as the dominant physical parameters influencing the rates and product yields of coal devolatilization processes, current models pay little or no attention to heat and mass transport limitations. In fact, particle size is not an input parameter in these models. Furthermore, although most industrial scale coal devolatilization processes occur at near ambient pressures, current renewed interest in high pressure coal conversion processes would seem to dictate a more detailed look at the effects of pressure.
The objective of the research reported here is to exploit the capabilities of two novel experimental techniques, based on the on-line coupling of microscale, TG-type reactors to mass spectrometry and combined gas chromatography/mass spectrometry systems. The TG/GC/MS technique has high-pressure TG capabilities and will be described separately at this meeting. The direct TG/MS instrument is characterized by a heated, all quartz interface and will be discussed here. The complementary nature of both systems enables us to investigate the nature and extent of physical control mechanisms over a broad range of experimental conditions.
Pugmire, R.J.; Hu, J.Z.;
Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
ACS Preprints, Division of Fuel Chemistry, 39:8-112, 1994. Funded by
Pittsburg Energy Technology Center, US Department of Energy and ACERC.
The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high-resolution spectra in complex solids such as coal. MAS narrows the chemical shift anisotropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values. In and effort to address this problem of spectral overlap, many 2D techniques have been developed to simultaneously obtain the dispersion by isotropic shift, such as produced by MAS, in one dimension and the tensorial information as a separate powder patters in the second dimension. A very successful technique is the slow spinning modification of the magic angle hopping experiment recently proposed by Gan, which we call the Magic Angle Turning (MAT) experiment. This experiment has a number of advantages over earlier 2D methods. The use of very slow spinning speeds (<50 Hz) favors the quantitative polarization of all carbons and allows the use of a large volume sample rotor resulting in a typical 2D spectrum acquisition requiring less than 24 hours. The mechanical device for slow spinning is very stable and high resolution in the isotropic chemical shift dimension can be easily obtained. The MAT experiment could be done on a suitably stable MAS probe. The only disadvantages of the original MAT experiment is that data acquisition starts right after the last pulse, causing distortion in the evolution dimension (the second dimension) even if a delay as short as 20 ms is used.
In this paper, a triple-echo MAT sequence, previously described, is employed which improves the 2D baseline. Two additional experiments, using short contact times and dipolar dephasing techniques, are also employed to further separate the powder patterns of protonated and nonprotonated carbons in complex compounds. Experimental results on representative model compounds as well as coals are presented in this paper.
Meuzelaar, H.L.C.
ACS Preprints, Div. Of Fuel Chemistry, 39(2):36-41, 1994. (Also presented
at the ACS Symposium: Division of Fuel Chemistry, San Diego, CA, March
1994.) Funded by ACERC and the Consortium for Fossil Fuel Liquefaction (US Department
of Energy).
Less than two decades ago a typical mass spectrometer was an extremely expensive and delicate instrument that would completely take up a moderately sized laboratory room. Few coal scientists or engineers had access to such an instrument and even fewer mass spectrometrists were willing to "contaminate" their instrument with something as complex and dirty as coal and its tar. Against this historic background it is nothing less than amazing that as early as 1966 Vastola et al. at Penn State University, using a finely focused ruby laser and a time of flight (TOF) mass spectrometer, already carried out laser pyrolysis experiments on coal samples inside the ion source. Joy et al soon followed their example. However, since Vastola's experiment was too far ahead of the state-of-the-art in signal processing electronics it would take more than 15 years before his group was able to obtain reproducible pyrolysis mass spectrometry (Py-MS) patterns from a series of PSOC coal samples.
In the meantime, the same coal samples had already been studied by Curie-point pyrolysis mass spectrometry (Py-MS) in our own laboratory as part of a series of 104 Rocky Mountain Province coals. The latter study demonstrated the reproducibility of carefully designed, dedicated Py-MS instruments, as well as the power of multivariate statistical analysis techniques, for reducing the voluminous MS data and bringing out the most significant chemical components and trends.
Already during the late seventies and early eighties several organic geochemistry groups, e.g., at the Technical University Delft at Chevron and at the University of Bartlesville had started to use pyrolysis-gas chromatography/mass spectrometry to characterize a broad range of different coals and coal macerals. Yet another promising instrumental approach, namely thermogravimetry (TG) in direct combination with MS was being developed further by Szekely's laboratory in Budapest followed by the development of a vacuum TG/MS system in our own laboratory. In the mid eighties further advances in TG/MS techniques were reported by Ohrbach and Kettrup using a commercially available molecular beam type interface. Most recently, a homebuilt TG/MS system based on similar principles was successfully tested in our own laboratory. Finally, the promising results of the various TG/MS combinations prompted us to pursue more sophisticated analytical configurations such as TG/IR/MS and TG/GC/MS, with the latter method eventually being adapted to on-line analysis of high pressure reactions, as reported by Kui et al.
In the mid eighties, Schulten's laboratory in West Germany started pursuing an entirely different approach involving direct probe type pyrolysis of coal directly in the ion source of a high resolution magnetic sector MS system with field ionization (FI) and field desorption (FD) capabilities. Related FIMS work was reported at SRI by Malhotra et al. That a wealth of information on coal conversion processes and reaction products could also be obtained by high resolution MS in combination with other ionization methods, such as low voltage electron ionization (LVEI) and fast atom bombardment (FAB) was elegantly demonstrated by Winans et al. at Argonne National Laboratory. Last year, a collaborative comparison between different desorption/ionization methods capable of producing ion signals up to several thousand Dalton was performed by two different research groups. Barely was their report submitted or one of the authors published several articles raising the upper mas limit of detected ion species to 4,000 and 270,000 Dalton for FAB and matrix assisted laser desorption/ionization, respectively.
Obviously, high mass MS techniques are presently a hot topic in coal science. However, in order to keep the scope of this article within the limitations posed by the ACS Fuel Chemistry Division preprint format, only techniques and applications of MS methods involving direct coupling to micro-scale or upscale coal conversion reactors will be discussed.
Solomon, P.R. and Fletcher,
T.H.
25th Symposium International on Combustion, 1994 (in press). (Also presented
at the 25th Symposium (International) on Combustion, Irvine, CA, Jul-Aug,
1994.) Funded by ACERC.
The pyrolysis process has impacts throughout coal combustion. The roles of pyrolysis in various aspects of the coal combustion process are described including the devolatilization yield, nitrogen release, softening and swelling, soot formation, and char reactivity. These processes can be understood and quantitatively predicted using recently developed network pyrolysis models that describe the transformation of the coal's chemical structure. The models are described and examples of their predictive ability for important coal combustion phenomena are presented.
Reade, W.C. and Hecker,
W.C.
Proceedings of the 17th Sympsium of the Rocky Mountain Fuels Society,
Golden, CO, March 1994; Western States Section/The Combustion Institute,
Davis, CA, March 1994; and AIChE Annual Meeting, San Francisco, CA, November
1994. Funded by ACERC.
In this work a char oxidation model that predicts changes in high-temperature reactivity with particle burnout is described. This model accounts for changes due to pore structure evolution and reactivity distributions, although there are many factors that can affect high-temperature reactivity (e.g., densification, mineral matter effects, etc.). Some factors, such as reactivity changes due to gasification-induced densification (Hurt, 1988), are implicitly included in the reactivity distribution effects. The only input parameters needed for this model are low-temperature kinetics (as measured by isothermal thermogravimetric analysis) and an approximate pore structure. This model is unique in that the pore structure evolution/reactivity distribution effects are lumped together in a preexponential that is a function of char conversion. The model predictions of high-temperature rates for two chars (Dietz and Pitt. #8) are compared with experimental data (Mitchell, et al., 1992) and show excellent agreement.
Gale, T.K.
Effects of Pyrolysis on Coal Char Properties, M.S./BYU, August 1994,
Advisor: Bartholomew
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