CHEMICAL-STRUCTURE OF CHAR IN THE TRANSITION FROM DEVOLATILIZATION TO COMBUSTION

Coal devolatilization experiments are generally conducted separately from char oxidation experiments, and the relationship between the chars generated in the two types of research is often ignored. However, char is one of the most important products of coal devolatilization and must be characterized as a function of temperature and heating rate in a manner similar to that for gaseous devolatilization products. The chemical structure of the parent coal directly affects devolatilization behavior. In this work, the chemical structure of chars from five coals of different rank are examined, and implications on char reactivity are discussed. Chars were obtained as a function of residence time in a devolatilization experiment (1250 K, 2 X 10(4) K/s in nitrogen) and just subsequent to devolatilization in a laminar flame-fired experiment (1500 K, 5 x 10(4) K/s). Quantitative measurements of chemical structure were performed on the coals and chars using C-13 nuclear magnetic resonance (NMR). Results show that the chemical structures of fully-devolatilized chars are very similar, even though a wide diversity is seen in the parent coal structures. For example, the average cluster molecular weights of the chars span a range of only 50 amu, and the side-chain molecular weights of the chars span a range of only 6 amu. The similarity in chemical structure of fully-devolatilized coal chars suggests that differences in measured heterogeneous char reactivities are caused by differences in the physical structure of the char.