Measuring heat capacity of activated carbons for CO₂ capture

Capturing CO₂ with solid adsorbents can be often limited by heat transfer during the regeneration process rather than mass transfer during the adsorption process. The use of a low-heat capacity adsorbent would result in a lower energy penalty for the regeneration step. Specific heat capacity (Cp) data knowledge of solid adsorbents is still scarce in the literature. To gather information in this regard, activated carbons (ACs) prepared from different precursor materials, synthesis conditions, activating agents and surface modifications have been experimentally tested. The specific heat capacity has been evaluated at temperatures ranging from 50 to 190 °C, relevant for temperature swing adsorption (TSA) based CO₂ capture processes. A thermogravimetric analyser/differential scanning calorimeter (TGA/DSC) has been used for the experimental testing. Cp values of the evaluated adsorbents evidenced significant dependency on temperature: adsorbents either showed a linear upward trend with temperature -phenolic resin-derived ACs- or they peaked at a determined temperature range -biomass based ACs-. This peak has been ascribed to oxidation reactions of biomass based carbons during the experiment. It is noteworthy that adsorbent surface modification plays a key role in the specific heat capacity of the resultant carbon. Likewise, the acid and basic character of the carbon surface has been identified as key parameter for effective regeneration in TSA processes. AC adsorbents with an acidic character are undesirable owing to their higher specific heat capacities.