CROSS-LINKING REACTIONS DURING COAL CONVERSION

During coal conversion, the breakup of the coal macromolecular network and resulting product formation are controlled by the relative rates of bond breaking, cross-linking, and mass transport. The objective of this work was to systematically study the variations in cross-linking with several parameters (rank, temperature, heating rate, pretreatment, etc.), to identify the factors that control cross-linking, to try to identify the reactions responsible for cross-linking, and to determine the cross-linking rates. This paper describes a study of cross-linking behavior in which chars of a number of coals (including the Argonne premium coal samples) have been pyrolyzed under a variety of temperature histories and analyzed at intermediate extents of pyrolysis for solvent swelling behavior and functional group compositions. The variations in these properties were correlated with the tar molecular weight distribution measured by field ionization mass spectrometry and with the gas evolution. The study of cross-linking as a function of coal rank and pyrolysis temperature shows that there are at least two distinct cross-linking events: one occurs at low temperature prior to tar evolution (in low-rank coals only) and the second occurs at moderate temperatures slightly above that for tar evolution. The low-temperature cross-linking process results in low tar yields, low fluidity (e.g., measured by Geissler plastometer), low extract yields, and low molecular weight tar. Low-temperature cross-linking is increased by oxidation of the coal and reduced by methylation. Studies that compare char solvent swelling behavior to gas evolution have shown that low-temperature cross-linking occurs simultaneously with CO2 and H2O evolution. Moderate-temperature cross-linking appears to correlate best with methane formation. Studies that compare char swelling behavior to changes in char functional group concentrations have shown that cross-linking reactions occur with the loss of carboxyl groups present in the coal. A clear role for hydroxyl groups in low-temperature cross-linking could not be established, nor could it be ruled out. © 1990, American Chemical Society. All rights reserved.