Network models of coal thermal decomposition

Abstract

Several groups have considered statistical network fragmentation models to describe coal thermal decomposition. In these models, the coal macromolecule is viewed as a collection of fused aromatic rings (monomers) linked by bridges. During thermal decomposition, existing bridges break and new bridges are formed. The parameters of the models are the geometry of the network, which is expressed as the number of attachments per monomer (the coordination number, σ + 1), and the chemistry of bridge breaking and formation. Given σ + 1 and the instantaneous number of unbroken and formed bridges, the molecular weight distribution can be predicted. The different groups have employed both Monte Carlo methods and percolation theory to describe the network statistics. The former approach has advantages in terms of describing both the depolymerization and crosslinking processes in coal decomposition, since it does not require a constant coordination number. The latter method provides closed form solutions and is computationally less demanding. The models differ in the geometry of the network, the chemistry of bridge breaking and bridge formation (crosslinking) and the mass transport assumptions. This paper considers for three such models: the mathematical schemes; the assumed network geometries; the assumed bond breaking and bond formation chemistries; and the mass transport assumptions. The predictions of three models were compared by comparing the oligomer populations as a function of the number of unbroken bridges per ring cluster. This paper also presents results from a new model which combines the geometry, chemistry and mass transport assumptions of the FG-DVC model with the mathematics of a modified percolation theory.