A Modeling Study of Coal Pyrolysis

A model of coal pyrolysis is developed with the elements of (i) structure, (ii) chemical reactions, and (iii) transport. (i) The chemical structure of coal is modeled as aromatic clusters bearing aliphatic side chains, bridges, and phenolic functional groups, consistent with various spectroscopic data and known chemical behavior. (ii) A detailed scheme of thermal free radical reactions leading to the formation of tar, gases and deactivated char is postulated. State variables are defined as concentrations of the participating functional groups to describe the stoichiometry and rates of these reactions. (iii) Macropores and transitional pores partition the coal particle into smaller subunits in which transport of tarry and gaseous products is by means of activated diffusion. The transport model postulates an active surface region of the subunits and a process of renewal of that region. A counting procedure is devised to predict the amount of aromatic fragments that escape from the subunits. The physicochemical processes are translated into a system of ordinary differential equations in the state variables, parametrized by the initial conditions and appropriate rate parameters. Numerical simulation is performed for two case studies where the necessary parameters are deduced from available pyrolysis and structural data and from additional assumptions. The model is applicable within the range of primary pyrolysis ((650°c) and predicts the ultimate weight loss as well as the distribution of products as a function of temperature and coal type. A supplementary model is also formulated for the reaction and transport of volatile materials in the macropores. This model takes into account the effects of the external pressure and the particle size on the evolution of products.