Quantitative analyses of chemical structural change and gas generation profile of coal upon heating toward gaining new insights for coal pyrolysis chemistry

Chemical structure of coal is evolutionary changed during pyrolysis that accompanies gas release. The chemical structural change and gas formation profiles play important roles in determining caking property and physical properties such as strength and size of the resultant coke. However, analyses of volatile components and structural analysis of solid char have been mostly performed individually, and it is difficult to combine both and to obtain quantitative understanding on the thermal decomposition of coal at mechanistic level. In this study, simultaneous analyses of solid chemical structures of the heat treated coals and gas formation profiles were conducted for two kinds of coals that were pyrolyzed at an identical condition. On-line gas analysis with a quadrupole mass spectrometer and spectroscopic methods (NMR and FT-IR) were employed for quantitative evaluation of gas formation characteristics and solid chemical structure, respectively. The information obtained were then integrated to acquire new insight for coal pyrolysis mechanism. Here an approach to quantify the transferable hydrogen that contributes to stabilize radicals formed in pyrolyzing coal was proposed. It includes the quantitative assessment of aromatic cluster growth, decomposition of hydroxyls, and releases of hydrogen and pyrolytic water into gas phase. The proposed approach suggested that a bituminous coal that exhibits plasticity during pyrolysis had 3.5 mol/kg-coal transferable hydrogen, whereas the amount of transferable hydrogen of the sub-bituminous coal, a non-caking coal, was 1.3 mol/kg-coal, during pyrolysis up to 500°C.