Ronald J. Pugmire

Progress in coal pyrolysis

Abstract The heterogeneous nature of coal and the complexity of the pyrolysis process have made it very difficult to perform unambiguous experiments to determine the rates and mechanisms in coal pyrolysis. However, recent years have seen a number of new experimental and theoretical approaches which shed new light on the subject. This paper considers the recent progress on kinetics, the formation of volatile products, network models, cross-linking, rank effects, and the ‘two-component’ model of coal structure. Recent experiments which measured coal particle temperatures at high heating rates provide reasonable agreement on kinetic rate constants. These rates also agree with those derived from experiments at low heating rates. In tar formation and transport, a consensus is being reached on the central role of the volatility of tar molecules in explaining the variation with operating conditions (pressure, heating rate, particle size, etc.) of the amounts and molecular weight distributions of tars. Progress in the quantitative prediction of tar and char yields is being made through recently developed models for the fragmentation of the macromolecular coal network. These models, which provide quantitative descriptions of the relations between the chemical structure of the coal and the physical and chemical properties of the pyrolysis products (gas, tar, soot, and char), are an exciting advance in the understanding of the pyrolysis process. Such models are linking the occurrence of the plastic phase of the coal with the ‘liquid’ fragments formed during pyrolysis. On the subject of retrogressive cross-linking reactions, both solvent swelling and n.m.r. measurements confirm important rank-dependent differences in reaction rates; these appear to be related to the oxygen functionalities. Reasonable agreement is also seen for variations with coal rank of kinetic rates derived from measurements at low heating rates. Experiments suggest that the recently revived ‘two-component’ hypothesis of coal structure has application to low-rank coals, which are mixtures of two distinct components: polymethylenes and a more aromatic network. Bituminous coals, however, appear far more homogeneous. Although experiments can distinguish loosely and tightly bound fractions these fractions appear to consist of similar materials and are differentiated primarily in their molecular weight and degree of connection to the network. These coals appear to behave in a manner that is described by the network decomposition models.

A sensitive, high resolution magic angle turning experiment for measuring chemical shift tensor principal values

A sensitive, high-resolution ‘FIREMAT’ two-dimensional (2D) magic-angle-turning experiment is described that measures chemical shift tensor principal values in powdered solids. The spectra display spinning-sideband patterns separated by their isotropic shifts. The new methods sensitivity and high resolution in the isotropic-shift dimension result from combining the 5π magic-angle-turning pulse sequence, an extension of the pseudo-2D sideband-suppression data rearrangement, and the TIGER protocol for processing 2D data. TPPM decoupling is used to enhance resolution. The method requires precise synchronization of the pulses and sampling to the rotor position. It is shown that the technique obtains 35 natural-abundance 13C tensors from erythromycin in 19 hours, and high quality natural-abundance 15N tensors from eight sites in potassium penicillin V in three days on a 400 MHz spectrometer.

Carbon-13 CP/MAS spectroscopy of coal macerals

Abstract 13 C n.m.r. spectra of kerogen concentrates isolated from several different subbituminous to high-volatile bituminous coal macerals have been obtained by a combined cross polarization/magic-angle spinning technique. The samples comprise three vitrinites, two sporinites, two alginites and one fusinite, all of Upper Paleozoic age. It is shown that this technique can be used to differentiate the maceral types by providing characteristic spectral fingerprints. Aromaticities decrease in the order fusinite vitrinite sporinite alginite, as expected with the rank range studied. Furthermore, fine spectral details provide general information on the nature and distribution of discrete structural moieties and their variations with both type and rank.

Characterization of Fine Particulate Matter Produced by Combustion of Residual Fuel Oil

ABSTRACT Combustion experiments were carried out on four different residual fuel oils in a 732-kW boiler. PM emission samples were separated aerodynamically by a cyclone into fractions that were nominally less than and greater than 2.5 |j.m in diameter. However, examination of several of the samples by computer-controlled scanning electron microscopy (CCSEM) revealed that part of the PM2.5 fraction consists of carbonaceous cenospheres and vesicular particles that range up to 10 |j.m in diameter. X-ray absorption fine structure (XAFS) spectroscopy data were obtained at the S, V, Ni, Fe, Cu, Zn, and As K-edges and at the Pb L-edge. Deconvolution of the X-ray absorption near edge structure (XANES) region of the S spectra established that the dominant molecular forms of S present were sulfate (26-84% of total S) and thiophene (13-39% of total S). Sulfate was greater in the PM2.5 samples than in the PM25+ samples. Inorganic sulfides and elemental sulfur were present in lower percentages. The Ni XANES spectra from all of the samples agreed fairly well with that of NiSO4, while most of the V spectra closely resembled that of vanadyl sulfate (VO•SO4•xH2O). The other metals investigated (i.e., Fe, Cu, Zn, and Pb) also were present predominantly as sulfates. Arsenic was present as an arsen-ate (As+5). X-ray diffraction patterns of the PM2.5 fraction exhibit sharp lines due to sulfate compounds (Zn, V, Ni, Ca, etc.) superimposed on broad peaks due to amorphous carbons. All of the samples contain a significant organic component, with the loss on ignition (LOI) ranging from 64 to 87% for the PM2.5 fraction and from 88 to 97% for the PM2.5+ fraction. Based on 13C nuclear magnetic resonance (NMR) analysis, the carbon is predominantly condensed in graphitic structures. Aliphatic structure was detected in only one of seven samples examined.

Carbon-13 magnetic resonance of coal-derived liquids

Abstract Carbon-13 Nuclear Magnetic Resonance Spectroscopy has been applied to coal-derived liquids in order to obtain additional information regarding molecular structure and composition. The data have demonstrated that, although the chemical structure is extremely complex, a significant amount of material is present in the form of normal paraffinic material both as free paraffins and alkyl substituents on aromatic and hydroaromatic materials. Semiquantitative estimates are made of the alkyl content of the liquids and the atom percentage of this material that must exist in the unprocessed coal.

The fate of cellulose and lignin in peats: an exploratory study of the input to coalification

Abstract The plant organs that give rise to preserved tissue in peats are often composed chiefly of cells with cellulosic (unlignified) walls, which frequently retain the birefringence characteristic of cellulose. Yet unaltered polymeric cellulose is present in very low concentration and, if inserted into peat as cotton, is rapidly destroyed. Our objective is to resolve this paradox and also identigy chemical changes undergone by any lignin that escapes depolymerization. Cores of peat from four environments in the Florida Everglades and one in Okefenokee Swamp (Georgia) are being studied. Size separation of peat from various depths by slurrying in water and passing through an 80 mesh sieve is an effective method of segregating rootlets and other plant tissue from fine-grained humic matter. In most cases, the various plant tissue fragments seem to consist chiefly of the principal plant polymers (cellulose and lignin), though these are somewhat oxidized and cellulose tends to decrease with depth. The fine-grained humic matter contains more or less the same structural features as the plant polymers, but shows bands of different shapes and relative intensities. In the case of the Cypress peat (only), the spectra of coarse and fine materials are quite similar. Thus there is evidence that cellulose does survive in some peats and that the polymers can be somewhat altered without destruction of cellular morphology.

Nuclear magnetic resonance spectroscopy of soils and related materials. Relaxation of 13C nuclei in cross polarization nuclear magnetic resonance experiments

Abstract Relaxation of 13 C nuclei in a peat, a soil, and three soil fractions have been investigated in order to improve structural resolution and to investigate quantification of various carbon types. Rotating frame spin lattice relaxation times ( T 1 ϱ s) and transverse relaxation times ( T 2 ′s) are similar to those observed for coals. T 2 ′s of carbons in different magnetic environments differ sufficiently that spectra can be obtained containing only nonprotonated carbon and methyl substituents if a 40 μsec delay without decoupling is inserted into the pulse programme before data acquisition (dipolar dephasing). Provided quantitative data is obtained in simple cross polarization experiments and allowance is made for loss in signal intensity of nonprotonated carbon during dipolar dephasing, then the fraction of aromatic carbon which is protonated in the samples can be determined.

Solid state magnetic resonance spectra of Illinois No. 6 coal and some reductive alkylation products

Abstract Illinois No. 6 coal and its reductive methylation and butylation products have been studied by magnetic resonance techniques. Conventional CP-MAS13Cn.m.r. spectroscopy indicates that 62% of the carbon atoms in the coal are aromatic and that about 6% of the carbon atoms are carbonyl. Esters are more abundant than carboxylic acids. The resonances of methoxy groups and other novel etheric carbon atoms are apparent in the high field region. Dipolar dephasing experiments suggest that methyl carbon atoms constitute no more than 16% of the carbon, methylene and methine carbon atoms about 14% and quaternary aliphatic carbon atoms about 2%. The dipolar dephasing experiments in conjunction with previous work also permit estimates of the hydrogen atom distribution. The THF-insoluble products obtained in the reductive alkylation reactions with13C-enriched alkylating agents contain paramagnetic and ferromagnetic substances that adversely influence the solid state n.m.r. spectra. However, good 13C n.m.r. spectra were obtained after these substances were extracted with aqueous hydrochloric acid. The O:C alkylation ratios are 1.2 and 1.0 for methylation and butylation, respectively. Dipolar dephasing experiments establish that the decay constants of functional groups in the whole coal and of C- and O-methyl-13C and C- and O-butyl-1 -13C nuclei in the labelled coal molecules are very similar to those of reference compounds. These findings suggest that the decay constants measured for the 13C nuclei in coals and coal-derived solids provide reliable information about their degree of substitution.

Methyl Libration in Propane Measured with Neutron Inelastic Scattering

The A2 and B2 torsional vibrational frequencies in propane gas have been directly measured at 217 ± 8 and 265 ± 8 cm−1, respectively, with inelastically scattered neutrons using the “small κ” method of neutron molecular spectroscopy. The theoretical basis of the scattering experiment is discussed, and factors affecting resolution and intensity are enumerated. Not subject to the same selection rules as optical spectroscopy, neutron molecular spectroscopy is capable of observing vibrational transitions which are otherwise forbidden by molecular symmetry. The vibrational frequencies are interpreted by numerically solving the two‐dimensional wave equation giving librational motion with a two‐dimensional Fourier expansion. In this approach, the potential energy is conveniently expanded also in Fourier form and we obtain a V3 = 3680 ± 190 cal/mole for individual methyl‐group librations and a V33 or V6 = − 280 ± 130 cal/mole term which couples the two methyl tops. Evaluation of additional Fourier constants in th...

Solid-state 13C NMR characterization of matched tars and chars from rapid coal devolatilization

Matehed tar/char sets were prepared by pyrolysis of a lignite and a bituminous coal in two entrained flow reactors at temperatures between 900 K and 1650K and heating rates of 10 4 –10 5 K/s. Detailed chemical structural characterization of these tars and chars was performed using elemental analysis and solid-state 13 C NMR. This is the first set of detailed solid-state 13 C nuclear magnetic resonance (NMR) data on coal tar samples. The average aromatic cluster sizes of the primary tars from these experiments are quite similar to those of their parent coals, confirming an assumption often made in network devolatilization models. Carbon aromaticities increase in char and tar samples until about 1250 K, after which line broadening in the NMR signal is observed. This line broadening is interpreted as formation of large aromatic radicals. Increases in bridges and loops per cluster are evidence for increased crosslinking above 1250 K. The measured molecular weights per cluster of the primary tars are lower than expected, indicating that some multiple cluster molecules (i.e., dimers) may exist in the tar. Tar and char nitrogen chemical structure is shown to correlate with changes in the carbon aromaticity, which may have implications for nitrogne release models that treat secondary reactions in the tar.