Finding order in coal pyrolysis kinetics

Abstract

A number of experiments have suggested that the rate constants for the release of tar and for the thermal decomposition of the various functional groups in coal pyrolysis depend on the nature of the bridging bond or of the functional group, but appear relatively insensitive to coal rank for lignites, subbituminous and bituminous coals. The principal variation of pyrolysis behavior with rank is due to variations in the concentrations of functional groups and hence, the amount of each pyrolysis product. If the insensitivity of coal pyrolysis kinetics to coal rank can be generally demonstrated, it represents an important simplifying assumption in any general theory of coal pyrolysis. But the rank insensitivity of rate constants is controversial. There are two major questions. What species exhibit rank insensitive kinetics? Quantitatively, what does insensitivity mean, variations less than factors of two, ten, hundred, etc.? This paper considers whether pyrolysis data in the literature support the hypothesis of rank insensitive kinetic rate constants. The experiments considered vary in duration from 1.4 msec to 12 hr and in temperature from 350°C to 1800°C. Considering the available data, it appears that the decomposition of aliphatic, methyl and aromatic functional groups and the evolution of tar and hydrocarbon species have rates which are relatively insensitive to rank variation. The rate varies by at most a factor of five between lignite and bituminous coals. Oxygen species are somewhat more rank sensitive. The factor of five variation in rate due to coal rank is substantially less than the factors of 100–10,000 in variation typical of reported rates. Rank variation appears therefore to be a minor cause for these differences which consequently must be attributed to the effects of heat and mass transfer and to the assumptions used in deriving a kinetic rate. The observation that pyrolysis rates are insensitive to rank over such a wide range of conditions suggests that using this approximation in a pyrolysis theory can have wide applicability.