Point sources like large coal fired power plants typically account only for about 1/3rd of the total CO2 released to the atmosphere. Post combustion CO2 capture technologies currently focus on large point source emitters. This implies that no technology currently exists that can account for nearly 2/3rd of the total CO2 emissions. Direct capture of CO2 from air, air capture, is an approach that can account for emissions from all sources. However, the economic and technical feasibility of air capture is an intensely debated topic because of the high energy costs associated with absorption based air capture processes.
Recently, amine functionalized mesoporous adsorbents have shown considerable capacity and selectivity for the adsorption of CO2. It seems possible that a temperature swing adsorption process using these highly selective materials can be used to capture CO2 directly from air with reasonable energy inputs. In this work, two novel equilibrium adsorption processes are presented that use the maximum and the minimum temperatures of the day as the driving force for the adsorption cycle. The thermal energy required for the temperature swings is provided by the daily temperature variations, which reduces the external energy requirements to a minimum. Using the actual annual temperature data of six different locations from the United States, our calculations show that product CO2 purity close to 0.15 % and 3 % is obtained for the two processes. On using low pressure steam (at 110 °C) for desorption the purity increases to almost 5 % and 20 % respectively for the two processes. In all the cases, the energy required is significantly smaller than the previously reported air capture processes. Our results identify some of the key issues that must be considered in potential air capture processes and provide guidelines for future development of adsorbents for these applications.