The effects of coal reactivities, solvent types, and reaction temperature on the characteristics of direct coal liquefaction were investigated in a set of three microreactors under the 950 psig of initial cold hydrogen or nitrogen atmosphere.
Two kinds of coals, a subbituminous C coal from Indonesia and a high volatile B bituminous coal from Australia, were used as samples.
Both thermal and catalytic liquefactions were performed with varying hydroaromatic/aromatic, hydroaromatic/naphthenic or naphthenic/aromatic solvent blend ratio. Also included in these experimental were liquefaction reactions using only each solvent. Three kinds of model solvents, one hydroaromatic (tetralin), one aromatic (naphthalene), and one naphthenic (decalin) were selected for this study. The catalyst used in this study was Shell 317 (Ni-Mo) extrudate which was crushed to a mesh size of -200 and then sulfided before use in liquefaction.
The results obtained from this study could be summarized as follows: The hydrogen in tetralin solvent which is a hydrogen donor solvent was effectively transferred to the dissolved coal even though the catalyst was not present. This fact implies that catalyst mainly concerns the transfer from gas phase hydrogen. Although the internal hydrogen transfer by aromatic solvent such as naphthalene, so called hydrogen shuttling mechanism, is already widely known, this mechanism had little effect on the efficacy of coal liquefaction yields.
The molecular hydrogen or gas phase hydrogen transfer mechanism could be divided into two types, direct transfer (gas phase hydrogen → coal) and indirect transfer (gas phase hydrogen → solvent → coal). Both hydrogen transfer routes were compatible in the absence of catalyst, but the use of catalyst could enhance the both transfers, especially that of indirect hydrogen transfer.
Temperature increase from 400→430℃ gave a significant effect on the liquefaction yield of Australian bituminous coal, but little improvement in liquefaction yield was observed in Indonesian subbituminous coal liquefaction. This means that liquefaction mechanism of two different types of coals may not be the same and that higher temperature of liquefaction may promote the reactions of larger activation energies such as strong bonds in macromolecules consisting of larger aromatic ring nuclei in bituminous coal than those in subbituminous coal.
The lumped kinetic model showed that the liquefaction scheme combined with parallel, series and reversible reaction paths fitted the data adequately. With Indonesian subbituminous coal, main reaction paths were not changed with increasing reaction temperature. This fact implies that the activation energies of Indonesian subbituminous coal dissolution were so small that the coal dissolution could be very active even under lower temperature (400℃) and thus that liquefaction of Indonesian subbituminous coal could result in higher coal liquefaction yields. Onthe other hand, the reaction paths were shifted in Australian bituminous coal liquefaction, and the series reaction path (preasphaltene → asphaltene → oil) was dominant with the increase in reaction temperature from 400→ 430℃. Finally, the preference of the retrograde or reverse reactions which influences detrimental effects on liquefaction yield was in the following order of lowering of the hydrogen donating capacity ; decalin ＞ naphthalene ＞ tetralin.
