Chemical Structure Changes of Coal, Char, and Tar during Devolatilization

Enormous progress has been made in coal pyrolysis research during the last decade. Models of coal devolatilization have progressed from simple rate expressions based on total mass release's to empirical relationships based on the elemental composition of the parent coal3 to models that attempt to describe the macromolecular network of the ~ o a l . ~ ~ Measurements of particle temperature during devolatilization have eliminated much of the controversy regarding overall rates of devolatili~ation.~-'~ In the last several years, advancements in chemical analysis techniques have allowed quantitative investigations of the chemical structure of both coal and its pyrolysis products, including the nature of the resulting char. A prominent research goal is to accurately predict the rates, yields, and products of devolatilization from measurements of the parent coal structure. This goal necessitates modeling the reaction processes on the molecular scale, with activation energies that relate to chemical bond breaking rather than release of products from the coal. 13C and 'H NMR spectroscopy have proven particularly useful in obtaining average values of chemical structure features of coal, char, and tar.l1-I4 This paper reviews experimental data regarding chemical structure features of coal, char, and tar during rapid devolatilization, and how these data have impacted the development and input parameters for devolatilization models. In particular, the relationship between pyridine extract yields and extract yields predicted purely from NMR chemical structure data is discussed.