Adsorptive purification of CO<inf>2</inf>/H<inf>2</inf> gas mixtures of spent disposable wooden chopstick-derived activated carbon: Optimal synthesis condition

Abstract

The production of fossil fuel-based H2 by steam reforming and water–gas shift reactions gives the reformed gas containing both hydrogen (H2) and carbon dioxide (CO2). To get pure H2, the CO2 must be removed. In this work, the CO2 was selectively separated from the gas mixture of CO2 and H2 by adsorption process using the spent disposable wooden chopstick (SWC)-derived activated carbon (AC) synthesized by the chemical activation. An appropriate condition for biochar preparation was first explored and then the chemical activation was carried out. Two types of activation chemicals (ZnCl2 and KOH) were employed to activate the biochar at different temperatures (500 – 800 °C). Effects of activation holding time (60 – 150 min) and activation chemical to biochar weight ratio (0.5 – 2.5) were also investigated. The optimal condition for biochar preparation was found at 500 °C and 15 min which can provide the AC yield of 29.5% and CO2 adsorption capacity of 19.2 mg/g from the gas mixtures with 50 mol% CO2 at 25 °C and 1 atm. The AC synthesized by the KOH activation at 700 °C using a KOH/biochar weight ratio of 1.5 for 90 min exhibited the highest CO2 adsorption of 115.7 mg/g for the first use and dropped approximately 20% after 6 adsorption/desorption cycles. Based on the perspective Grand Canonical Monte Carlo (GCMC) simulation, the best synthesized AC exhibited the selectivity of CO2 adsorption 17.6 times higher than that of H2. The adsorption behavior and kinetic model of CO2 adsorption followed the Freundlich isotherm and the pseudo-2nd order model, respectively and the CO2 adsorption occurred via the combined physical and chemical mechanism.