Argonne Coal Structure Rearrangement Caused by Sorption of CO_2

摘要

The exposure of powdered unconfined coals to CO_2 results in changes in the coals' physical structures. The presence of water changes the behavior of the coals on exposure to CO_2. The sorption of CO_2 on seven Argonne premium coals was measured by using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy as a function of time at constant CO_2 pressure (～0.62 MPa) and temperature (55℃). The depth sampled is at least 1 μm, and this ensures that both the bulk and surface of the sample were interrogated. Concentrations of CO_2 in the top 1—7 μm of the coal were measured by using the peak area of the CO_2 ATR-FTIR band near 2333 cm~(-1). Diffusion rates were measured by using the time dependence of the area of the 2333 cm~(-1) band. Surface adsorption is effectively instantaneous. The coals were either extensively or briefly dried. The coals were exposed to CO_2, evacuated, and then exposed to CO_2 a second time. For the extensively dried coals, removal of the CO_2 under vacuum was much faster than CO_2 sorption, indicating a coal structure change caused by CO_2 sorption. The diffusion rate of the CO_2 into the coal was much faster for the second exposure, confirming that the coal underwent a physical structure rearrangement. Structure rearrangement was observed for all seven extensively dried coals and for the Pittsburgh No. 8 briefly dried coal. The presence of residual moisture in the briefly dried coal samples appeared to inhibit or block CO_2 uptake as equilibrium was reached within minutes. The second exposure of the briefly dried coals universally resulted in greater CO_2 uptake that was again instantaneous on our experimental time scale.