The assessment of maturation changes of humic coal organic matter — Insights from closed-system pyrolysis experiments

• The derived liquid products correlate well with the changes in petrographic characteristics.
• The contents of all vitrinite macerals increase at higher maturities.
• Liptinites decompose at higher maturities and they almost disappear at Rr ~ 1.8%.
• The most sensitive parameters in a wide range of the humic OM maturity are %Rr, PhNR, MCI 2 and (2 − + 3-MF)/(1 − + 4-MF).
• The parameters based on the isomerisation of alkylaromatics are optimal for the maturity assessment of the humic OM.

The artificial maturation changes of the humic coal organic matter (OM) from late diagenesis to the semianthracite stage of coalification were investigated using the closed-system pyrolysis. Two extracted coal samples, a subbituminous coal 26C (with a random vitrinite reflectance Rr = 0.42%) and a high volatile bituminous coal 30C (Rr = 0.56%), were pyrolysed during 4 h at two different temperatures, 250 °C and 400 °C.The applied petrographic and organic geochemical analyses showed that the humic coal OM is sensitive to the artificial thermal stress. A good correlation between the changes in petrographic characteristics and the derived liquid products was observed.The contents of all vitrinite macerals, as well as total vitrinite, increased during the thermal maturation. The abundance of liptinite macerals decreased during the maturation and they practically disappeared after 4 h of the pyrolysis at 400 °C. The total inertinite contents increased after the pyrolysis at 250 °C, yet to be decreased after the pyrolysis at 400 °C. The content of organic carbon increased from coals to the solid residues at 400 °C, whereas the contents of total hydrogen and sulphur decreased. The yields of gases, liquid products and solid residues at respective temperatures were very similar for both coals, regardless of the rank. As expected, the pyrolysis at 400 °C gave higher gas and liquid yields than at 250 °C.Regarding the liquid pyrolysates, more pronounced changes were observed in the distribution of aromatic compounds than in saturated biomarkers. The distributions of the aromatics indicate that apart from cracking and isomerisation (the formation of thermodynamically more stable isomers), the reactions of dehydrocyclization, condensation (the formation of more stable conjugated systems) and aromatization, as well as reactions involving H2S, elemental sulphur and reactive phenyl radicals released during the pyrolysis, also play an important role. It was shown that bentonitic clay plays a more important role in the subsequent aromatization of the OM than the initial maturity of the coal OM.n-Alkane parameters were useful at lower maturities (up to ~ 0.75% Rr), whereas sterane, naphthalene and phenanthrene maturity ratios are applicable at higher maturities (> 0.75% Rr). Hopane parameters showed variable sensitivities over the investigated maturity range (0.5–1.8% Rr). The most sensitive maturity parameters in the wide range of the humic OM maturity (0.5–1.8% Rr) are vitrinite reflectance, phenylnaphthalene ratio, methylchrysene index 2 and a parameter proposed in this study, based on the distribution of methylfluorenes (MF): (2- + 3-MF) / (1- + 4-MF). Generally, maturity parameters based solely on the isomerisation reactions of alkylaromatics, which do not include alkylation/dealkylation processes, are more appropriate for the humic OM, particularly at higher maturities.