Elucidation of hydrogen transfer mechanisms in coal liquefaction using a tritium tracer method: Effects of solvents on hydrogen exchange reactions of coals with tritiated molecular hydrogen

Abstract

To estimate the hydrogen mobility of coal and solvent quantitatively, reactions of tetralin, decalin and 1-methylnaphthalene with tritiated gaseous hydrogen in the absence and presence of Wandoan coal were performed under conditions generally used for coal liquefaction: 300–400°C, initial H 2 pressure 5.9 MPa. Tritium in the gas phase was transferred to coal and solvent mainly through hydrogen exchange and slight hydrogen addition. The amount of hydrogen exchanged between the gas phase, coal and solvents was estimated from hydrogen and tritium balances. At 400°C and 120 min, coal conversion in the presence of tetralin, decalin and 1-methylnaphthalene was 87, 45 and 54%, respectively. Under these conditions, the amount of hydrogen exchanged between the gas phase and coal decreased in the order 1-methylnaphthalene (1.38 g 30 g −1 of coal) > tetralin (0.85 g 30 g −1 of coal) > decalin (0.72 g 30 g −1 of coal). Compared with the amount of hydrogen exchanged, the amount of hydrogen added from the gas phase to the coal was smaller than 0.07 g in each solvent. Coal hydrogen exchange reactions in the presence of tetralin or decalin were similar to those in the absence of solvent. On the contrary, remarkable hydrogen exchange with coal in the presence of 1-methylnaphthalene occurred with a rise from 350 to 400°C. Further, a significant amount of 1-methylnaphthalene decomposed to naphthalene and methane in this temperature range, while hydrogen addition from 1-methylnaphthalene to coal did not occur. This suggests that hydrogen exchange reactions among the gas phase, coal and 1-methylnaphthalene proceeded by radical chain mechanisms, with the supply of radicals from coal.