Coal lithotypes before and after saturation with CO2; insights from micro- and mesoporosity, fluidity, and functional group distribution

Four lithotypes, vitrain, bright clarain, clarain, and fusain, were hand-picked from the core of the Pennsylvanian Springfield Coal Member (Petersburg Formation) in Illinois. These lithotypes were analyzed petrographically and for meso- and micropore characteristics, functional group distribution using FTIR techniques, and fluidity. High-pressure CO ₂ adsorption isotherm analyses of these lithotypes were performed and, subsequently, all samples were reanalyzed in order to investigate the effects of CO ₂. After the high-pressure adsorption isotherm analysis was conducted and the samples were reanalyzed, there was a decrease in BET surface area for vitrain from 31.5m ²/g in the original sample to 28.5m ²/g, as determined by low-pressure nitrogen adsorption. Bright clarain and clarain recorded a minimal decrease in BET surface area, whereas for fusain there was an increase from 6.6m ²/g to 7.9m ²/g. Using low-pressure CO ₂ adsorption techniques, a small decrease in the quantity of the adsorbed CO ₂ is recorded for vitrain and bright clarain, no difference is observed for clarain, and there is an increase in the quantity of the adsorbed CO ₂ for fusain. Comparison of the FTIR spectra before and after CO ₂ injection for all lithotypes showed no differences with respect to functional group distribution, testifying against chemical nature of CO ₂ adsorption. Gieseler plastometry shows that: 1) softening temperature is higher for the post-CO ₂ sample (389.5°C vs. 386°C); 2) solidification temperature is lower for the post-CO ₂ sample (443.5°C vs. 451°C); and 3) the maximum fluidity is significantly lower for the post-CO ₂ sample (4 ddpm vs. 14 ddpm).