Enhancing low severity coal liquefaction reactivity using mild chemical pretreatment

Abstract

We describe results from a study in which mild chemical pretreatment of coal has been used to enhance low severity liquefaction reactivity. We have found that ambient pretreatment of eight Argonne coals using methanol and a trace amount of hydrochloric acid improves tetrahydrofuran (THF)-soluble conversions by 24.5 wt% (daf basis) for Wyodak subbituminous coal and 28.4 wt% for Beulah-Zap lignite with an average increase of 14.9 wt% for liquefaction of the eight coals at 623 K (350 °C) reaction temperature and 30 min reaction time. Similar enhancement results occurred using hexane or acetone in place of methanol. Pretreatment with methanol and HCl separately indicated that both reagents were necessary to achieve maximum liquefaction improvement. Acid concentration was the most important pretreatment variable studied; liquefaction reactivity increased with increasing acid concentration up to 2vol%. No appreciable effect on reactivity was observed at higher acid concentrations. Although vapour phase alcohol/HCl mixtures have been shown to partially alkylate bituminous coals, analysis of Wyodak and Illinois no. 6 coal samples indicated that no organic phase alteration occurred during pretreatment; however, over 90 wt% of the calcium was removed from each coal. Calcium is thought to catalyse retrogressive reactions during coal pyrolysis, and thus calcium removal prior to low severity liquefaction minimizes the rate of THF-insoluble product formation.