Molecular Simulation of Carbon Dioxide Adsorption for Carbon Capture and Storage.
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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.Ph.D.Applied Physics and Environmental EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/62442/1/ctenney_1.pd