Enhancement of Low-Severity Coal Liquefaction by Mild Acidic Pretreatment and Cyclic Olefins

Abstract

Three coals, Wyodak and Black Thunder mine subbituminous and North Dakota Beulah-Zap lignite, were pretreated with mildly acidic solutions of hydrochloric or sulfurous acid in methanol, water, or hexane. These pretreated coals were reacted in the presence of one of three hydrogen donors: 1,4,5,8-tetrahydronaphthalene, a cyclic olefin commonly known as isotetralin; 1,2,3,4-tetrahydronaphthalene, isotetralin's hydroaromatic analog commonly known as tetralin; or 1,4,5,8,9,10-hexahydroanthracene, a three-ring cyclic olefin. Coal conversions obtained from combinations of selected pretreatments and hydrogen donors were examined in a factorial experiment. Supplemental reactions were performed evaluating the effect of other parameters. Reaction conditions were 350 °C for 30 min under 3.45 MPa of hydrogen introduced at ambient temperature. Both isotetralin and hexahydroanthracene yielded higher coal conversions than tetralin regardless of the pretreatment condition. Each coal responded differently to the pretreatments. The highest conversions for the three coals were achieved with isotetralin and different acid pretreatments. Black Thunder mine coal yielded 41.2% coal conversion after reacting with isotetralin following pretreatment with hydrochloric acid in methanol compared to 27.7% for the same system without HCl. Wyodak yielded 39.0% and Beulah-Zap yielded 35.0% coal conversion after reacting with isotetralin following pretreatment with hydrochloric acid in water compared to 24.9 and 23.2%, respectively, without HCl. Hexahydroanthracene was less effective than isotetralin with hydrochloric acid pretreatments but more effective than isotetralin with the sulfurous acid pretreatments. Conversions with hexane as a pretreatment solvent were similar but slightly lower than those with water. Combinations of some factors exhibited interactions.