The search for cheaper and cleaner automotive fuels has driven the study of carbon based adsorbed natural gas (ANG) tanks, for next-generation clean vehicles [1-6]. Storage of methane in such a tank is fully reversible under the standard fill/discharge protocol: pressurization to 35 bar or higher at ambient temperature to fill the tank (adsorption); depressurization to 2 bar or higher at ambient temperature to empty the tank (desorption) [5]. But storage of natural gas, consisting of approximately 90% methane and 10% non-methane components, is reversible only in part under this protocol. Co-adsorbed non-methane components, depending on their binding energy on the adsorbent and kinetic factors, are released only in part under the stated discharge conditions and therefore will accumulate in the tank during successive fill/discharge cycles, reducing the amount of useable fuel stored in the tank over time. A typical value for retention of non-methane components is 2 mass% per fill/discharge cycle [6, 7]. Normally, to restore the full fuel storage capacity of the tank, the tank is heated off-board under vacuum for a prolonged period of time (regeneration, forced desorption of non-methane components) [8]. One proposed way to eliminate the need for off-board regeneration is to add an on-board fuel extraction system [9], in which a compressor provides suction on the tank, upstream of the compressor, and enhanced fuel flow to the engine, downstream of the compressor. We tested the effectiveness of such a system, for extraction of non-methane components, on the 40-liter flat-panel ANG tank at the University of Missouri [5]. We ran 20 fill/discharge cycles and monitored the mass of the tank and composition of the discharged gas as a function of time over the 20 cycles. The mass of the full tank (35 bar) and mass of the empty tank (2 bar) both increased with increasing cycle number, n, but the fuel delivered in cycle n, mass(full, n) – mass(empty, n), decreased with increasing n. In as few as five cycles, n = 5, the tank capacity mass(full, n) – mass(empty, n) had dropped by over 2.5% from the “pristine capacity” mass(full, 1) – mass(empty, 1). The increase in empty-tank mass, mass(empty, n) – mass(empty, 1), gives the mass of non-methane components that remain in the tank after n cycles, and the ratio [mass(empty, n) – mass(empty, 1)]/”pristine capacity” gives the fraction of non-methane components in the tank after n cycles. This ratio was 0.9 kg/3.4 kg for n = 5. Thus, already after 5 cycles, the tank holds 26% non-methane components. This rapid decrease in fuel delivered and rapid increase of non-methane components suggests that the compressor does not eliminate the need for periodic regeneration of the tank. However, the two metrics considered here for successful forced fuel extraction, do not grow linearly with n but approach constant values for large n. We will discuss to what extent the approach to such constant values can be treated as reversible storage, but with reduced capacity compared to storage of pure methane.
[1] P. Pfeifer et al., “High-surface-area biocarbons for reversible on-board storage of natural gas and hydrogen.” Mater. Res. Soc. Symp. Proc. 1041, 1041-R02-02 (2008).
[2] J. Romanos, S. Sweany, T. Rash, L. Firlej, B. Kuchta, J.C. Idrobo, P. Pfeifer, “Engineered porous carbon for high volumetric methane storage.” Adsorpt. Sci. & Technol. 32, 981-991 (2014).
[3] P. Pfeifer, G.J. Suppes, P. Shah, J.W. Burress, “High surface area carbon and process for its production.” U.S. Patent No. 8,691,177 (2014) and No. 8,926,932 (2015).
[4] J.A. Mason, M. Veenstra, J.R. Long, “Evaluating metal–organic frameworks for natural gas storage.” Chem. Sci. 5, 32-51(2014).
[5] P. Pfeifer, R. Little, T. Rash, J. Romanos, B. Maland, “Advanced Natural Gas Fuel Tank Project.” California Energy Commission, Energy Research and Development Division, Publication No. CEC-500-2016-038 (Sacramento, CA, 2016).
http://www.energy.ca.gov/publications/displayOneReport.php?pubNum=CEC-500-2016-038
[6] T.A. Rash, “A comprehensive report on preparing and analyzing adsorptive materials suitable for hydrogen and methane Storage.” Ph.D. Dissertation, University of Missouri (2014).
[7] This value can be reduced by introduction of a filtering system, often referred to as a guard bed, which removes non-methane components from the gas entering the tank [5, 6]. With a guard bed, ideally the gas entering the tank is pure methane and retention of non-methane components is zero. A high-performance, but at the same time high-maintenance, guard bed was developed and demonstrated in: Atlanta Gas Light Adsorbent Research Group, “Low Pressure Adsorbent Natural Gas Vehicle Demonstration—Final Report.” Report to Brookhaven National Laboratory (1997).
[8] R.B. Rios, M. Bastos-Neto, M.R. Amora, A.E.B. Torres, D.C.S. Azevedo, C.L. Cavalcante, “Experimental analysis of the efficiency on charge/discharge cycles in natural gas storage by adsorption.” Fuel 90, 113-119 (2010).
[9] R. Bonelli, “Fuel Extraction Pressurization System (FEPSTM) for sorbent-based low-pressure gaseous delivery.” U.S. Patent and Trademark Office, Notice of Allowance, Oct. 12, 2016.
