C.Özgen Karacan； Gareth D.Mitchell
The behavior and effect of different coal lithotypes during sequestration of carbon dioxide (CO2) into coal seams are important knowledge gaps when modeling sequestration processes and planning for field application. This paper presents the results of a laboratory study for sequestration of CO2 in a 2.5-cm diameter Pittsburgh coal sample. During the test, the sample was kept under a constant effective stress during gas uptake and CO2 storage was observed by qualitative and quantitative X-ray computerized tomography (CT) scanning. Petrographic analysis was also performed on the sample after the sequestration test to identify the microlithotypes showing different adsorption behavior.
Qualitative X-ray computerized tomography (CT) and petrographic analysis showed that different microlithotypes behave differently during gas adsorption. Vitrite was observed to swell and to decrease the initial density about 5%, when CO2 diffused into the micropores. A density increase was observed in other microlithotypes present in the sample due to gas adsorption.
The CT images were quantified to map the temporal and spatial variation of mass change of coal due to CO2 adsorption. These data were used to study the equilibrium isotherm parameters and to investigate the kinetics of the adsorption process for calculating the diffusivity of CO2 in different microlithotypes. Quantitative analysis of X-ray CT images indicated that the regions that were rich in inertite and clay stored higher quantities of gas in the adsorbed phase compared to other organic microlithotypes in the sample. Calculations for adsorption kinetics in different microlithotypes indicated that pore diffusion is the prime mechanism. However, only an order of magnitude difference between pore and surface diffusion coefficients suggested that both phenomena might be important for CO2 sequestration process. The calculated pore diffusion coefficients were higher in clay and inertinite regions. But, surface diffusion started to increase in vitrinite- and liptinite-rich regions, where pore diffusion was limited.
Carbon dioxide sequestration；Coal；Lithotypes；Adsorption kinetics；X-ray CT；Vitrinite；Liptinite