Novel insights into the molecular structure of lignite and its oxidation characteristics: A comprehensive characterization from single molecule of intermediates to complex coal

Abstract

The structure of lignite is commonly believed to consist of fused aromatic rings (FARs) with 1 and 2 rings and short methylene chains, with few FARs with 3 or more rings and long aliphatic hydrocarbon (LAH) chains. This work presents a controlled stepwise alkali‑oxygen oxidation (AOO) of Xiaolongtan lignite to produce intermediates with characteristic structures. The humic acids and oxidative residues obtained were rich in aromatics and aliphatics, respectively. Based on complementary information of FARs with 3 or more rings and LAHs in the intermediates, a new structural model of lignite (C3661H3435O1036N81S7) from single molecules of intermediates to complex particles has been developed in conjunction with material characterization and computation. There are 43.7% one fused aromatic ring (1FAR), 39.3% 2FAR, 14.7% 3FAR, and 2.4% 4FAR, amount which there 15.9% pyrrole- and 2.4% pyridines- based aromatic rings. The controlled oxidation could prevent the ring-opening reaction of FARs and excessive oxidation of LAHs, thus leading to an in-depth understanding of the AOO process and lignite structure.