Meso- And Micropore Characteristics Of Coal Lithotypes: Implications For Co_2 Adsorption

摘要

Lithotypes (vitrain, clarain, and fusain) of high volatile bituminous Pennsylvanian coals (R_o of 0.56-0.62%) from Indiana (the Illinois Basin) have been studied with regard to meso- and micropore characteristics using low-pressure nitrogen and carbon dioxide adsorption techniques, respectively. High-pressure CO_2 adsorption isotherms were obtained from lithotypes of the Lower Block Coal Member (the Brazil Formation) and the Springfield Coal Member (the Petersburg Formation), and after evacuation of CO_2, the lithotypes were reanalyzed for meso- and micropore characteristics to investigate changes related to high-pressure CO_2 adsorption. Coal lithotypes have differing Brunauer-Emmett-Teller (BET) surface areas and mesopore volumes, with significantly lower values in fusains than in vitrains or clarains. Fusains have very limited pore volume in the pore size width of 4-10 nm, and the volume increases with an increase in pore size, in contrast to vitrain, for which a 4-10 nm range is the dominant pore width. For clarain, both pores of 4-10 nm and pores larger than 20 nm contribute substantially to the mesoporosity. Micropore surface areas are the smallest for fusain (from 72.8 to 98.2 m~2/g), largest for vitrain (from 125.0 to 158.4 m~2/g), and intermediate for clarain (from 110.5 to 124.4 m~2/g). Similar relationships are noted for micropore volumes, and the lower values of these parameters in fusains are related to smaller volumes of all incremental micropore sizes. In the Springfield and the Lower Block Coal Members, among lithotypes studied, fusain has the lowest adsorption capacity. For the Lower Block, vitrain has significantly higher adsorption capacity than fusain and clarain, whereas for the Springfield, vitrain and clarain have comparable but still significantly higher adsorption capacities than fusain. The Lower Block vitrain and fusain have much higher adsorption capacities than those in the Springfield, whereas the clarains of the two coals are comparable. After exposure of coal to CO_2 at high pressure, vitrains experienced the largest porosity changes among all lithotypes studied. These changes are dominantly manifested in the mesoporosity (decrease in mesopore volume) range, whereas little to no change occurred in the micropore size range. In other lithotypes (clarains, the dominant lithology in the coals studied, and sporadic fusains), the changes were minimal.