Characterization of coals and their laboratory-prepared black carbon using advanced solid-state 13C nuclear magnetic resonance spectroscopy

Lignite, anthracite, humic acid fraction of lignite, and their laboratory-prepared black carbon (BC) were characterized in detail by advanced solid state 13C NMR spectroscopy to examine how the differences in chemical structures of fuels (lignite and anthracite) would affect the structures of their BC. Anthracite was almost completely dominated by aromatics (96.9%), with very minor CCH3. Although lignite contained mostly aromatics (76.1%), significant aliphatics (21.3%) as well as small amounts of COO/NCO and O-alkyls were also present. In addition, anthracite had more aromatic CC groups than lignite. Their BC samples both exhibited increased oxygenated functional groups such as COO, aromatic CO and O-alkyl groups, and concomitant decrease of aromatic CH and nonpolar alkyls. The 1H13C recoupled long-range dipolar dephasing experiments indicated the growth of aromatic cluster sizes in BC. Although the aromaticity of anthracite was much larger than those of lignite and lignite humic acid, their aromatic cluster sizes were quite similar. The BC samples had much larger aromatic cluster sizes than anthracite despite their smaller aromaticities. Therefore, higher rank or higher aromaticity may not imply larger aromatic cluster size in coal. The structures of two BC samples were similar despite the contrasting structural differences of their original coals.

► Anthracite has larger aromaticity and more aromatic C-C groups than lignite.
► BC structures are similar although their original coals are structurally distinct.
► BC samples have larger aromatic cluster sizes than their source fuels.
► Oxygenated functional groups increase in BC samples.
► Higher rank or aromaticity may not imply larger aromatic cluster size in coal.