Maceral controls on porosity characteristics of lithotypes of Pennsylvanian high volatile bituminous coal: Example from the Illinois Basin

Porosity characteristics of vitrain, clarain, durain, and fusain lithotypes of the Springfield Coal Member of the Petersburg Formation and the Danville and Hymera Coal Members of the Dugger Formation from the Illinois Basin were investigated with a special emphasis on the control of coal macerals on pore-size distribution. These Pennsylvanian coals are of high volatile bituminous rank and have vitrinite reflectance ranging from 0.51 to 0.60%. The lithotypes studied show decreasing values of Brunauer-Emmett-Teller surface area, and micro- and mesopore volumes from vitrain through clarain and durain to fusain. Within the mesopore size range, vitrain and clarain are dominated by pore-size widths of 4 to 10 nm, whereas durain and fusain have more volume for pores larger than 20 nm. In contrast to mesopores, micropore sizes are very similar for all lithotypes, averaging 1.37 to 1.39 nm. In addition to differences among lithotypes, there are significant differences in pore characteristics among the three coals studied, with largest surface areas and pore volumes documented for the Hymera, followed by the Danville and the Springfield. A strong relationship exists between surface area, mesoporosity, and microporosity and maceral composition, with vitrinite having a very strong positive correlation, liptinite having a weak positive correlation, and inertinite having a strong negative correlation. Negative correlations of total porosity with vitrinite and liptinite and positive correlations with inertinite suggest that among the maceral groups, pores in inertinite contribute most to the total porosity. The Fourier transform infrared spectrometry technique demonstrates that fusains from the three coals studied have higher aromaticity and a higher degree of aromatic ring condensation and lower hydrocarbon potential than the other lithotypes, whereas chemical differences between vitrain, clarain, and durain are less distinct. In addition, there is a relationship between aromaticity of the lithotypes and surface area and mesopore and micropore volumes.