Correlations of composition and structural characteristics between pyrolysis tar and soluble organic species of Baoqing lignite

Abstract

During pyrolysis, the intrinsic organic species in coals play an important role on the formation of tar but the relationship between them has not been elucidated in detail. In this study, the composition and structural characteristics of pyrolysis tars and soluble portions (SPs) via thermal dissolution from Baoqing lignite were analyzed with Fourier transform infrared spectroscopy (FTIR), gas chromatograph/mass spectrometer (GC/MS) and quadrupole/Orbitrap mass spectrometer (Orbitrap MS), and the formation mechanisms of main components in pyrolysis tars were proposed. FTIR analyses showed that both the SPs and tars are rich in -OH, CO, aromatic CC, C-O, aliphatic and aromatic C-H functional groups, suggesting they are partially similar in composition and structure. GC/MS and Orbitrap MS analyses indicated that a part of soluble organic species was directly volatilized into the tars, while others were suffered from complex reactions during pyrolysis, such as the cleavage of bridged bonds, the decomposition of oxygen-containing functional groups and the condensation of aromatics. Compared with the SPs, the higher contents of CH and O1 classes but lower contents of O2-O6 classes in the tars demonstrated that the soluble organic species are partially or completely deoxygenated during pyrolysis.