Aromatic Cluster Size Research Articles

Vacuum pyrolysis of maceral concentrates in a wire-mesh reactor

The pyrolytic behaviour of maceral concentrates from three bituminous coals was investigated in a wire-mesh reactor under vacuum and at atmospheric pressure. A new set of traps, designed to overcome difficulties encountered in tar capture and recovery during vacuum pyrolysis, has been developed. Experiments reported were carried out at 1000 K s −1 to 700 °C with a 5 s hold; tar characterization was carried out by size exclusion chromatography, u.v. absorption and u.v. fluorescence spectroscopy. During pyrolysis experiments with whole coals, vitrinite, liptinite and inertinite concentrates, it was systematically found that about 5 wt% more gas was formed at atmospheric pressure than under vacuum. No major synergistic effects were observed between the petrographie components of coals during pyrolysis. Liptinite concentrate tars were found to have larger molecular mass distributions than tars from other maceral concentrates, and were found to undergo greater change from vacuum to atmospheric pressure. U.v. absorption and u.v. fluorescence spectra suggested that tars produced at atmospheric pressure were richer in aromatic content compared to vacuum tars. Liptinite concentrate tars appear to be more aliphatic/hydroaromatic in character and to have shown greater sensitivity to intraparticle secondary reactions. Spectra of tars from all three maceral concentrates show similar structural features suggesting that aromatic cluster size distributions of tars from different macerals may not be significantly different even though their aromaticities may differ.

Characterization of tars from variable heating rate pyrolysis of maceral concentrates

Product yields and structures of tars from the pyrolysis of a set of maceral concentrates derived from Linby coal were determined and compared as a function of heating rate (1 and 1000 K s −1) and temperature (400–900 °C). A new trap was developed for the collection and quantitative recovery of tars evolved during atmospheric pressure wire-mesh pyrolysis experiments, allowing the capture and recovery of tars expected to reflect closely the characteristics of evolved primary tars. Yields from all three maceral concentrates were found to increase with increasing heating rate, but the liptinite concentrate showed the smallest sensitivity. Tar characterization was undertaken with size exclusion chromatography and u.v. fluorescence spectroscopy. Comparison of molecular mass distributions suggested that liptinite concentrate tars contain greater fractions of large molecular mass units than corresponding vitrinite and inertinite concentrate tars. Molecular mass distributions of liptinite concentrate tars from experiments at 400 and 500 °C, obtained using an evaporative analyser as mass detector, showed the presence of substantial quantities of material not detected by a u.v. detector, set at 254 nm and used in tandem, suggesting that the material identified in this region is predominantly aliphatic/hydroaromatic in character. In fast heating experiments (1000 K s −1), this material tended to diminish rapidly with increasing pyrolysis temperature from 400–500 °C to 600–900 °C, suggesting intraparticle destruction. Results from u.v. fluorescence spectroscopy suggest the presence of substantially similar aromatic cluster size distributions in vitrinite and liptinite tars ; greater intensities of spectra from vitrinite concentrate tars suggest, however, the presence of greater concentrations of aromatic clusters and/or polars within vitrinite tars.

FLASHCHAIN theory for rapid coal devolatilization kinetics. 1. Formulation

This theory invokes a new model of coal's chemical constitution, a four-step reaction mechanism, chain statistics, and the flash distillation analogy to explain the devolatilization of various coal types. It is called FLASHCHAIN. The constitution submodel segregates the elements into only four pseudocomponents using the ultimate analysis, the carbon aromaticity and aromatic cluster size from 13 C NMR analysis, the proton aromaticity, and the extract yield in pyridine

Structural evolution of matched tar-char pairs in rapid pyrolysis experiments

Solid-state 13C and 1H nuclear magnetic resonance (n.m.r.) spectroscopy techniques were used to investigate the relationship between the chemical structure of coal and the char particles and condensed tar vapours produced from coals of various ranks at rapid heating conditions. The 13C n.m.r. analysis of the coal chars indicated that significant amounts of aliphatic material were released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross-links per cluster. The evolution of the char structure following tar release was a function of the temperature history of the char. The structure of the primary tars were compared to the parent coal, and the gas phase evolution of the tar structure was followed with time.

Solid state 13C and 1H NMR studies of the evolution of the chemical structure of coal char and tar during devolatilization

Solid-state 13 C and 1 H nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the chemical structure of char particles and condensed tar vapors produced as pyrolysis products from an Illinois #6 coal at rapid heating conditions (≈10 4 K/s) at two gas conditions (maximum gas temperatures of 1250 K and 1050 K). The temperature history of particles in the flow reactor is determined using a unique infrared sizing-pyrometry system. The 13 C NMR analyses of the coal chars indicate that significant amounts of aliphatic material are released from the coal during devolatilization, with little change to the aromatic cluster size or number of attachments per cluster. At long residence times, and at higher temperatures, small increases in the cluster size in the char are observed. The 1 H NMR analyses indicate that thermal decomposition of tar vapor occurs at the 1250 K gas conditions, as evidenced by increases in the aromaticity and decreases in the peripheral aliphatic groups, such as methyl groups and aliphatic bridge material.

High-resolution solid-state 13C NMR studies of Australian coals

A detailed solid-state 13C NMR study on 29 Australian coals, varying in rank from semianthracite to brown coals, are reported. The experiments have been carried out at a high magnetic-field of 7.05T by using the standard techniques of CP/MAS/HPPD in conjunction with a TOSS pulse sequence. The results include the study of cross-polarization dynamics in the colas and dipolar-dephasing analyses of the samples. With the high-field spectrometer and the techniques used, various carbon types have been identified and the distribution of these carbon functionalities has been determined. Additional parameters related to aromatic structures, namely, the degree of substitution of aromatic rings and the average aromatic cluster size have been estimated. The variations in the structural parameters are discussed in view of the aromatic and aliphatic structures of the coals.