Hydrothermal pretreatment of coal. [Heat and hydrothermal treatment at 350 degrees C

Abstract

We recently examined Argonne supplied Wyodak coal under both thermal (no added water, under N{sub 2}) and hydrothermal (liquid water present, under N{sub 2}) conditions at 350{degrees}C for periods of 30 min. and 5 hr. We found that the coal produces a tar that is deposited on the reactor insert walls solely at hydrothermal conditions. The shift from 30 min. to 5 hr. yields a tar that is more volatile and has a slightly increased molecular weight. The coals recovered from thermal and hydrothermal treatments are different by pyrolysis-field ionization mass spectrometry (py-FIMS). Specifically, the hydrothermal condition yields py-FIMS volatiles with a higher weight average molecular weight and greater volatility. They are thus less polar, a conclusion consistent with other py-FIMS data showing that the volatiles from the hydrothermally treated coal are lower in phenolics. Our results show that the phenols and catechols in the coal behave very differently. Our data are consistent with a scheme in which the catechol units in the coal engage in condensation at thermal conditions, probably through a catalyzed process related to acidic sites on the mineral matter. The phenols in contrast are unreactive. At hydrothermal conditions, on the other hand, both are released hydrolytically. Thus it appears that the presence of added water decreases or eliminates thermally promoted crosslinking tied to catechol condensation. Unexpectedly, we see acetone and other simple ketones in the Wyodak pyrolysate from both the thermal and hydrothermal treatment. Acetone in some cases is the single most prominent product. These ketones are not seen, however, in the unconfined py-FIMS heating. The difference between confined and unconfined heating suggest that water evolved from the coal itself in confined heating acts in some hydrolytic fashion to liberate the ketones.