Chemical structure of semifusinite and fusinite of steam and coking coal from the Upper Silesian Coal Basin (Poland) and its changes during heating as inferred from micro-FTIR analysis

The objective of this study was to characterize internal structure of semifusinite and fusinite in inertinite concentrates (I content — 80–81%) before and after heat-treatment (400–1200 °C), under inert conditions. Maceral concentrates were prepared using gravity separation from two samples of steam coals (Rr = 0.98 and 0.99%) and two samples of coking coals (Rr = 1.01 and 1.42%) collected from the same seam. Micro-FTIR and reflectance (Rr) measurements were carried out. Mass loss of the concentrates upon heating was also determined. Semifusinite in concentrates before heating is characterized by lower aromaticity and condensation of the structure (lower values of CHar/CH2 + CH3 and CHar/C═Car ratios) than fusinite. The difference in aromaticity between semifusinite and fusinite is similar despite the rank of the parent coal. Semifusinite from the coking coal is characterized by lower C═O/C═Car ratio as well as higher aromaticity than semifusinite in the steam coal. Statistical analysis showed occurrence of two types of semifusinite which differed in aromaticity and condensation of the structure. Fusinite from the higher rank coal has higher aromaticity in comparison to fusinite from the lower rank coal. Under heating, semifusinite of both types and fusinite experience an increase in aromaticity and condensation of the structure, which results in significant reflectance increase. Strong correlations between the values of both CHar/CH2 + CH3 and CHar/C═Car ratio and reflectance of semifusinite and fusinite in concentrates before and after heating have been found. The most significant alteration of the chemical structure of semifusinite occurs at 600 °C and 800 °C (coking coal) as well as 800 °C and 1000 °C (steam coal) and for fusinite at 600 °C and 800 °C. After heating at 1200 °C, semifusinite in the steam coal concentrates has lower C═O/C═Car ratio and much higher aromaticity than semifusinite in the coking coal concentrates. Fusinite in the coking coals after heating the concentrates at 1200 °C has lower C═O/C═Car ratio and condensation of the structure but higher aromaticity than semifusinite. Rate of aromaticity increase in 400–1200 °C range calculated for semifusinite is inversely proportional to aromaticity of this maceral in concentrate before heating.

Research highlights
► Under heating semifusinite and fusinite are subjected to increase in aromaticity and condensation of the structure, which results in significant reflectance increase.
► The most significant alteration of the chemical structure of semifusinite occurs at 600 and 800 °C (coking coal) as well as 800 and 1000 °C (steam coal), and for fusinite at 600 and 800 °C.
► Rate of both aromaticity and condensation increases in the whole studied temperature range and it is much higher for semifusinite than fusinite.