Structural evolution characteristics of lignite during pyrolysis based on alkaline-oxygen oxidation, NMR and FTIR

Abstract

Pyrolysis is a crucial conversion process for lignite to improve its quality and produce chemicals. Here, the evolution characteristics and pathways of lignite during pyrolysis were investigated by the combination of oxidation product distribution and physical structural parameters, which reflected the correlations between the size of aromatic clusters and pyrolysis temperature. The results showed that with increasing the pyrolysis temperature up to 500 ℃, the aromatic structures were enriched, and the average size of aromatic clusters increased with pyrolysis temperature. A little variation of interlayer spacing (d002) was observed, while the stacking height (Lc) and the average lateral size (La) increased gradually. Before 400 ℃, double-aromatic clusters were dominant in chars, and the corresponding chars were abundant with precursor structures of the alkaline-oxygen oxidation products benzene tricarboxylic acid (BCA3), benzene tetracarboxylic acid (BCA4) and benzene pentacarboxylic acid (BCA5). Above 400 ℃, more triple- or quadruple-ring aromatic clusters were present, and the precursor structures of benzene dicarboxylic acid (BCA2), benzene tricarboxylic acid (BCA3), and benzene tetracarboxylic acid (BCA4) increased considerably. The results indicate that the benzene carboxylic acids (BCAs) from oxidation are an effective evaluation method to reflect the abundance of aromatic rings in the parent structure of coals.