In situ analytical pyrolysis of coal macerals and solid bitumens by laser micropyrolysis GC–MS

Abstract

Specific macerals in coals and distinct solid organic bitumens within limestones from northern China were separately analysed in situ by laser micropyrolysis gas chromatography–mass spectrometry. The major pyrolysates from the resinite and cutinite macerals included aromatic hydrocarbons such as alkylnaphthalenes, alkylbenzenes, alkyltetralins (in the resinite) and alkylphenols (in the cutinite), and aliphatic hydrocarbons such as n-alk-1-enes, n-alkanes, pristane and phytane. The same classes of hydrocarbons were also detected in co-occurring vitrinites, including very similar relative distributions of alkylnaphthalenes in the case of the resin-rich coal. Several observed quantitative differences are consistent with previous molecular data from maceral concentrates. The qualitative similarity of the maceral data suggest (1) extensive assimilation of free hydrocarbons by all macerals within the coals; or (2) inefficient targeting of the macerals by the laser, although microscopic observation showed that the laser craters were generally constrained within surface boundaries of the maceral. The organic composition of a solid bitumen within a Carboniferous limestone was characterised by a distinctive distribution of C 0–C 4 alkyldibenzofurans, as well as alkylbenzenes, alkylnaphthalenes and a low molecular weight distribution of n-alk-1-enes and n-alkanes. These data suggest the solid bitumen comprises a strongly aromatic matrix with terrestrially-derived oxygen functionality and alkyl linkages. The same aromatic and aliphatic hydrocarbon products were also detected in the morphologically homogenous carbonates hosting the solid bitumens, indicating the presence of sub-microscopic organic moieties throughout the limestone.