(288f) Activated Carbons from Biomass for Methane and Hydrogen Storage

An investigation was carried out on the technical feasibility of adopting activated carbons from biomass, e.g., sorghum and wheat, for the adsorbed natural gas (ANG) storage in the fuel tanks of motor vehicles. The grain-based activated carbons were prepared by chemical activation; the conditions were varied to identify the optimum condition. The porosity of the resultant activated carbons was evaluated through nitrogen adsorption; and the storage capacity, through methane adsorption. A comparative study was also carried out with commercial activated carbons from coal. The bulk densities of activated carbons were varied by combining activated carbon particles of different sizes as well as by mechanical compactions. Most of the activated carbons examined had BET surface areas ranging between 900 and 2000 m²/g, and the ratio of micropore volume to total pore volume ranging between 0.26 and 0.65. In addition, the pore-size distributions (PSD's) of activated carbons were narrow and peaked around 1.1 to 1.2 nm, implying the PSD's of these activated carbons were suitable for ANG storage. The highest storage factor (volume of methane adsorbed at STP/total volume of the tank) attained was 89 for compacted activated carbons from grain sorghum with a bulk density of 0.65 g/cm³, and the highest storage factor attained was 106 for compacted commercial activated carbons with a bulk density of 0.70 g/cm³. Thus, both the sorghum-based and commercial activated carbons would be effective for ANG storage in the fuel tanks of motor vehicles at 25^oC and 500 psig: The reported ANG break-even points are 78 for light-duty vehicles and 120 for heavy-duty vehicles. The storage factor was found to increase approximately linearly with increasing bulk density. Additional experiments were carried out to modify the activated carbons from biomass to generate molecular-sieve like carbon adsorbents. The preliminary estimation indicates that such carbon adsorbents can be highly effective for hydrogen storage.