Natural gas storage properties of adsorbents synthesised from three different coal waste in South Africa

Abstract

Adsorbed natural gas (ANG) storage has been identified as the most promising low-pressure alternative to storing natural gas. However, in addition to the thermal management of the system, successful implementation of this technology is largely constrained by the cost of adsorbents. The usefulness and cost of suitable adsorbent such as activated carbon are essential to ANG technology's productivity. In this study, activated carbons were prepared from three South Africa coal waste samples (run-of-mine fines, discard and flotation slurry) by KOH activation. The adsorption isotherm of methane on the synthesized activated carbons revealed that the activated carbon synthesized from run of mine fines (ACR) has the highest adsorbed amount compared to activated carbons synthesized from coal discard (ACD) and slurry (ACS). The maximum adsorbed amount of methane was attained at an average pressure of 37 bar with ACR having the largest volume of methane adsorbed at 162.71 cm3/g followed by ACD (157.58 cm3/g) and ACS (106.69 cm3/g) respectively. This sequence is consistent with the BET surface areas of the activated carbons; ACR (1925.34 m2/g) > ACD (1826.41 m2/g) > ACS (1484.96 m2/g). The methane adsorption data were validated with Langmuir, Toth, and D-A isotherm models for the three activated carbons. It was found that, for all three activated carbons, the D-A model was the best fit. Based on the requirements of suitable porous materials for ANG application, the study indicated that the produced activated carbons from South Africa coal wastes could well be used for natural gas storage.