Pretreatment of coal by reaction with subcritical steam enhances its performance in direct liquefaction. Illinois No. 6 coal, first reacted with 51 atm of steam for 15 min at 340 °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 °C, 30 min) and low-severity (385 °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 pretreated coal is protected from oxygen and heated rapidly to liquefaction temperature. Under low-severity conditions, the oil yield is more than doubled, going from 12.5 to 29 wt %. Since previous work pointed to the destruction of ether cross-links by water as the dominant depolymerization mechanism during pretreatment, tests were conducted with several 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 °C. α-Benzylnaphthyl ether and α-naphthylmethyl phenyl ether show little difference in conversion and product distribution when the thermolysis atmosphere is changed from inert gas to steam. Hence, these compounds are poor models for coal in steam pretreatment. The otherwise thermally stable 9-phenoxyphenanthrene, on the other hand, is completely converted in 1 h by liquid water at 315 °C. At pretreatment conditions, however, mostly rearranged starting material is obtained. Therefore, 9-phenoxyphenanthrene, though less reactive, is a model for ether linkages in coal.

The model compounds a-benzylnaphthyl ether and a-naphthylmethyl phenyl ether were reacted in the presence of 5A zeolite. Zeolite proved effective in suppressing isomerization of the starting material and improving the yield of thermolysis products, especially for the former. These results suggested that zeolites might be beneficial in steam pretreatment of coal and in coal liquefaction. Pretreatment and liquefaction of mixtures of coal and zeolites increases yields of asphaltenes and preasphaltenes. V CONCLUSIONS The following conclusions are drawn from this work: 1. Steam pretreatment substantially improves the quality of the liquid product in coal slurry liquefaction. Under low severity conditions, the oil yield is more than doubled, going from 12.5 to 29 wt 76. 2. The benefits of steam pretreatment can be realized only if the pretreated coal is rapidly heated to liquefaction temperature. This is necessary in order that the pretreated coal pass quickly through the region of retrogressive reactions. This region, in which retrogressive reactions can mitigate or even destroy the effects of pretreatment, extends downward fkom360°C to perhaps as low as 320°C. The upper limit on heating time can not yet be specified, but appears to be about one minute (which was the recovery time for the low severity b series of tests). (Tubing bombs, in which improved yields after pretreatment are not obtained, require about 3 minutes to reach reaction temperature.) 3. Pretreated coal must be protected from oxygen to preserve the benefits of pretreatment. 3. The compounds a-benzylnaphthyl ether and a-naphthylmethyl phenyl ether show little difference in conversion and product distribution when the thermolysis atmosphere is changed from inert gas to steam. Hence, these compounds are poor models for coal in steam pretreatment. The otherwise thermally stable 9-phenoxyphenanthrene, on the other hand, is completely converted in one hour by liquid water at 315°C. At pretreatment conditions, however, mostly rearranged starting material is obtained. 9-Phenoxyphenanthrene, therefore, is a model for ether linkages in coal although it is less reactive. The improvement over the abenzylnaphthyl and a-naphthylmethyl phenyl ethers is a result, we speculate, of the presence of the larger number of condensed aromatic rings. 5. 5A Zeolite is effective in suppressing the formation of isomerized starting material in the thermolysis of a-benzylnaphthyl ether and a-naphthylmethyl phenyl ether. 6. The presence of 4A and 5A zeolites during pretreatment and liquefaction increases yields of asphaltenes and preasphaltenes.