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Evaluation of Graphite Nanofilters for Biogas Purification

Ashleigh Barnes
Department of Chemistry, University of Tennessee

Biogas is a renewable energy source composed primarily of CH4 and CO2, with some small concentrations of H2S and other impurities. In order to be used as an efficient fuel source, CO2 and H2S must be removed or at least have their concentrations significantly reduced. This is because CO2 reduces the heating value of the gas and H2S can corrode engine components if the biogas is used as a vehicle fuel[1]. Therefore efficient means of purifying the biogas are necessary. Previous theoretical studies have investigated doped multilayer graphene nanostructures for this purpose with some success using GFMC simulations[2]. The simulations presented here invesitgate the use of undoped graphite nanofilters for biogas purification. Nanofilters are constructed by periodically replicated graphite slabs (consisting of n = 5, 9, or 11 graphene sheets with lattice parameters a = 2.4612 Å and c = 6.7079 Å) separated by channel distances of D = 10-70 Å. Each graphene sheet was oriented parallel to the y,z-plane with dimensions 52 Å x 168 Å. A gas mixture of 500 molecules consisting of CH4, CO2, and H2S in a 60:39:1 ratio was allowed to equilibrate in the nanofilter at 300 K in the NVT ensemble for 25 ps, and then run in the NVE for another 25 ps in the absense of external flow.

From these simulations it was determined that nanofilters with n = 9, D = 20 Å result in the greatest increase in the CH4/CO2 ratio. External flow was then applied to the system by adding velocity in the z direction through the center of the channel (see visual below), and the composition of gas flowing past a detector at a fixed vertical position was measured over time (25 ps). The greatest improvement in the CH4 mole fraction was obtained using a flow velocity of 0.5 Å/fs, which resulted in an average mole fraction of 0.73, 0.13 higher than the initial composition. While this is an improvement, less than half of the CO2 was removed from the system. In addition, the simulations were inconclusive regarding the separation of H2S from the mixture due to the low initial concentration. From this we can conclude that undoped graphite nanofilters can be used to increase the purity of methane in biogas, however, under the studied conditions, it cannot remove the majority of the minor components from the mixture.

Interactive Structure


Color Legend:

  • Grey = Carbon (CO2)
  • Red = Oxygen
  • White = Hydrogen (H2S)
  • Blue = Carbon (graphite)
  • Yellow = Sulfur
  • Tan = Methane

References

  1. Q. Zhao, E. Leonhardt, C. MacConnell, C. Frear, and S. Chen, "Purification Technologies for Biogas Generated by Anaerobic Digestion," Climate Friendly Farming, CSANR Research Report 2010-001.
  2. J.-J. Chen, W.-W. Li, X.-L. Li, and H.-Q. Yu, "Improving Biogas Separation and Methane Storage with Multilayer Graphene Nanostructure via Layer Spacing Optimization and Lithium Doping: A Molecular Simulation Investigation," Environmental Science & Technology 46 10341 (2012).

    3. posted: March 2016.
    updated: March 2016.