Separation of carbon dioxide for biogas upgrading to biomethane

The present paper deals with carbon dioxide capture from dry CO2/CH4 mixtures by means of zeolites produced from spent power plant fly ash. Commercial sorbents such as activated carbon, silica gel and zeolite 13X were also tested. At an operating pressure of 2 bar, the best zeolite synthesized in this work gave rise to recoveries of CH4 and CO2 of 95.2% and 98.1% respectively, at purities of 98 vol% and 99.6 vol% respectively. Among the commercial sorbents tested at 2 bar pressure, the best one was silica gel, with recoveries of CH4 and CO2 of 85.4% and 97.6% respectively at purities of 97.3 vol% and 94.9 vol% respectively. At this pressure, the CO2 adsorption rate was 0.402 moL/kg silica sorbent; at 6 bar this increased to 1.076 mol CO2/kg silica sorbent but at greatly reduced levels of both CH4recovery and CO2 purity. Three zeolites produced from fly ash also underwent PSA (Pressure Swing Adsorption) tests: after five adsorption-desorption cycles no loss in adsorption capacity of CO2 was observed, both activity and selectivity recovering completely after regeneration.