Impact evaluation of coexisting gas CO on CO2 adsorption on biochar derived from softwood shavings

Biochar capability in CO2 adsorption has already been confirmed by multiple research works. However, CO2 is never emitted as a pure gas; therefore, studying the impact of other components in the gas mixture on CO2 adsorption on biochar is necessary. One of the components that is always co-released with CO2 is CO. In this work, the affinity of biochar for CO and CO2 was first predicted using molecular modeling binding energy calculations. The results suggested that CO2 would be favored over CO for adsorption on biochar because of its stronger adsorption (higher released adsorption energy). The biochar adsorption of pure CO and CO2 were then studied using batch and dynamic experiments, where the isotherm and kinetic models of the pure gas adsorptions were derived. Dual-site Langmuir isotherm fitted both of the gases equilibrium adsorption data the best, confirming the non-homogeneous surface of biochar. The Avrami kinetic model was the best-fitted model for the adsorption rate of the gases, which showed that both chemical and physical interactions were involved in the adsorption processes.The results from molecular modeling matched the results from pure CO and CO2 adsorption on biochar, as the capacity of pure CO2 was more than three times greater than that of pure CO. The dynamic binary CO/CO2 adsorption experiments with 10-90 vol.% CO2 (balanced with CO), carried out at 20 °C, showed that although both of the gases were adsorbed on biochar when the adsorption bed was fresh, some of the CO molecules were later removed by the more favored CO2 molecules to the extent that for up to 50 vol.% CO, all of the CO molecules were removed by the CO2 molecules, and the CO overall capacity was 0 mmol/g.For examining the impact of temperature on CO2 and CO adsorption on biochar, the pure CO and CO2 along with three selected binary CO/CO2 adsorption tests were repeated at 100 °C. The pure adsorption tests of the two gases on biochar at 100 °C suggested that although CO2 adsorption capacity was negatively impacted by the temperature increase, CO adsorption capacity was not altered by the temperature. Results from binary adsorption tests at 100 °C confirmed that at 100 °C, more CO would adsorb on biochar at the same input levels of the two gases compared to 20 °C due to CO2 decreased adsorption at higher temperatures.