IMPROVEMENT OF COAL DIRECT LIQUEFACTION BY STEAM PRETREATMENT

Pretreatment of coal by reaction with subcritical steam enhances its p erformance in direct liquefaction. Illinois No. 6 coal, first reacted with 51 atm of steam for 15 min at 340 degrees C, was liquefied in; a coal injection autoclave to provide rapid heating. Liquefactions were carried out with raw and pretreated coal at high-severity (400 degrees C, 30 min) and low-severity (385 degrees C, 15 min) conditions under 1500 psia of hydrogen with tetralin as the donor solvent. Substantial improvement in product liquid quality is realized provided the pretrea ted coal is protected from oxygen and heated rapidly to liquefaction t emperature. Under low-severity conditions, the oil yield is more than doubled, going from 12.5 to 29 wt %. Since previous work pointed to th e destruction of ether cross-links by water as the dominant depolymeri zation mechanism during pretreatment, tests were conducted with severa l aromatic ethers as model compounds. These were exposed to steam and inert gas at pretreatment conditions and in some cases to liquid water at 315 degrees C. alpha-Benzylnaphthyl ether and alpha-naphthylmethyl phenyl ether show little difference in conversion and product distrib ution when the thermolysis atmosphere is changed from inert gas to ste am. Hence, these compounds are poor models for coal in steam pretreatm ent. The otherwise thermally stable 9-phenoxyphenanthrene, on the othe r hand, is completely converted in 1 h by liquid water at 315 degrees C. At pretreatment conditions, however, mostly rearranged starting mat erial is obtained. Therefore, 9-phenoxyphenanthrene, though less react ive, is a model for ether linkages in coal.