Characterization of Tar Products from Rapid Pyrolysis of Bituminous Coals. Sixth Quarterly Progress Report, January-March 1983.

摘要

Differences between the molecular weight distributions of tars and coal extracts leads to the conclusion that mass transfer limitations exist under all pyrolysis conditions studied. Only at the limit of vacuum and high temperature does the tar which is evolved begin to be representative of the extractable material left behind. This is evidence of an evaporation rate controlled escape of tar. Tar-precursor formation apparently continues until late during the pyrolysis process. The role of mass transfer limitations becomes more pronounced at higher temperatures, where rates of tar escape most likely become comparable to rates of tar precursor repolymerization. The implication in terms of modeling pyrolysis is that at least three rate processes are important in tar evolution-metaplast formation, metaplast repolymerization and tar evaporation. These conclusions are consistent with an earlier model of softening coal pyrolysis. What remains poorly understood is the possible role of internal mass transfer limitations. It has been previously shown that there is remarkably little temperature dependence exhibited by the molecular weight distributions of the evolved tars, and it was noted that such behavior is similar to that exhibited by evaporating, non-reacting multicomponent liquid droplets, in situations where liquid phase diffusion controls. This analogy is based on the fact that a softened coal particle is physically quite similar to a very low volatility liquid droplet. Simple order of magnitude analyses also suggest that internal transport limitations should be significant under vacuum pyrolysis conditions. (ERA citation 08:039931)