Molecular Simulation of Carbon Dioxide Adsorption for Carbon Capture and Storage.

Abstract

Capture of CO2 from fossil fuel power plants and sequestration in unmineable coal seams are achievable methods for reducing atmospheric emissions of this greenhouse gas. To aid the development of effective CO2capture and sequestration technologies, a series of molecular simulation studies were conducted to study the adsorption of CO2 and related species onto heterogeneous, solid adsorbents. To investigate the influence of surface heterogeneity upon adsorption behavior in activated carbons and coal, isotherms were generated via grand canonical Monte Carlo (GCMC) simulation for CO2 adsorption in slit-shaped pores with several variations of chemical and structural heterogeneity. Adsorption generally increased with increasing oxygen content and the presence of holes or furrows, which acted as preferred binding sites. To investigate the potential use of the flexible metal organic framework (MOF) Cu(BF4)2(bpy)2 (bpy=bipyridine) for CO2capture, pure- and mixed-gas adsorption was simulated at conditions representative of power plant process streams. This MOF was chosen because it displays a novel behavior in which the crystal structure reversibly transitions from an empty, zero porosity state to a saturated, expanded state at the “gate pressure”. Estimates of CO2 capacity above the gate pressure from GCMC simulations using a rigid MOF model showed good agreement with experiment. The CO2 adsorption capacity and estimated heats of adsorption are comparable to common physi-adsorbents under similar conditions. Mixed-gas simulations predicted CO2/N2and CO2/H2selectivities higher than typical microporous materials. To more closely investigate this gating effect, hybrid Monte-Carlo/molecular-dynamics (MCMD) was used to simulate adsorption using a flexible MOF model. Simulation cell volumes remained relatively constant at low gas pressures before increasing at higher pressure. Mixed-gas simulations predicted CO2/N2 selectivities comparable to other microporous adsorbents. To study the molecular processes relevant to storage of CO2 in unmineable coal seams with enhanced methane recovery, a representative bituminous coal was simulated using MD and a hybrid Gibbs-ensemble-Monte-Carlo/MD method. Simulation predicted a bulk density of 1.24 g/ml for the dry coal, which compares favorably with the experimental value of 1.3 g/ml. Consistent with known coal properties, simulation models showed stacking of macromolecular graphitic regions and preferential adsorption of CO2 relative to methane.