Generation of biochars for gas separation upon cellulose pyrolysis: A reactive molecular dynamics study

Abstract

Abstract The pyrolysis of cellulose via reactive molecular dynamic simulation to obtain an adsorbent for CO2 separation is the main goal of the present study. Biochars with six different densities ranging from 0.160-0.987 g/cm3 were produced and adsorption of pure CO2, CH4, N2 and mixtures of CO2 (0.05)/CH4 (0.95) and CO2 (0.2)/N2 (0.8) at 300 K was investigated as a function of pressure by Grand Canonical Monte Carlo simulations. Dual-site Langmuir (DSL) model was adjusted to the isothermal adsorption data with very good accuracy. Thermodynamic quantities were calculated based on the DSL parameters which showed that the amount of adsorbed CH4 and N2 decreases as the density of biochar increases. Adsorption isotherms, selectivity, and isosteric heat were calculated, which revealed that the biochar with 0.351 g/cm3 density had the highest selective adsorption of CO2. The higher Gibbs free energy and surface potential of pure CO2 indicated that CO2 adsorption on biochars is more favorable and spontaneous than CH4 and N2. Henry's constant for CH4 and N2 was smaller than that of CO2, which indicates that their affinity for biochar surfaces was weaker than CO2. The higher entropy changes of CO2 adsorption of pure gases and binary gas mixtures showed that CO2 molecules constitute a much more stable rearrangement than CH4 and N2. We may conclude that pyrolysis of cellulose, as a cheap and available material, can be used to make an economic structure with high ability for CO2 separation.