Methane generation from coal during open system pyrolysis investigated by isotope specific, Gaussian distributed reaction kinetics

A reaction kinetic model is presented to differentiate individual complexes of reactions of the thermal decomposition of sedimentary organic matter during pyrolysis experiments. This model is applied to methane generation from coal during open system pyrolysis experiments with linear heating. The reaction kinetic model is based on the concept of a certain number of reaction complexes contributing to the generation of the product methane, each reaction complex distinguished by an initial pool of precursor sites with a defined stable carbon isotope signature and a common reaction rate determining step. For each reaction the mathematical model defines Gaussian distributed activation energies, a single pre-exponential factor, an initial isotope ratio of the precursor and a single difference in activation energy between the heavy and the light isotope species. Based on this concept, stable carbon isotopes and the dynamics of thermal methane generation from a Carboniferous coal with 0.72% vitrinite reflectance are explained with a set of four independent reactions. It is suggested that these reactions represent the following pathways (listed in the order of occurrence): (1) reactions involving the cleavage of thermally unstable C-O and C-S bonds, (II) demethylation, (III) release of groups which cross link ring structures and/or secondary cracking of long chain hydrocarbons within the molecular network of the coal, and (IV) polymerisation and condensation reactions. Reactions II and III contribute the major part to total methane potential. However, the addition of the minor reactions I and IV has significant influence on the stable carbon isotope trend of methane. (C) 2004 Elsevier Ltd. All rights reserved.