Studies of Coal Liquefaction at Very Short Reaction Times. 2

A previous paper has shown that direct coal liquefaction in the absence of a catalyst consists of two distinct stages which are readily distinguishable with an appropriate short contact time liquefaction reactor and thermogravimetric analysis. The first stage, an extraction of liquefaction solvent-soluble material in the coal, is very rapid and levels off in the first minute or so. The heat of this extraction process for Illinois No. 6 coal in tetralin is about 42 kJ/mol (10 kcal/mol), suggesting this is primarily a physical process. The second stage is a slower breakdown of the coal structure itself. This stage consists of several steps of progressively slower rates, probably representing the cleavage of the various connecting groups between the aromatic clusters which make up the coal structure. Between the first extraction stage and the second stage of coal structure breakdown, there is a transition period during which both processes are taking place simultaneously. A kinetic model for the liquefaction processes based on these observations is introduced. Rate constants and activation energies are determined for the individual liquefaction stages. Experimental results show that the extraction stage is largely determined by the solubility characteristics of the liquefaction solvent while the structure breakdown stage is primarily influenced by the hydrogen donor ability of the solvent.