Effects of porous carbon additives on the CO2 absorption performance of lithium orthosilicate

Abstract

Lithium orthosilicate (Li4SiO4) is an attractive high-temperature CO2 sorbent (>650 °C) because of its large theoretical absorption capacity of up to 36.7 wt%. However, slow kinetics and partial reactions with CO2 hinder its proper operation as a sorbent under practical conditions. To allow the use of this sorbent at lower operation temperatures, the present studies explored the way to improve the CO2 absorption kinetics and increase the degree of reaction of Li4SiO4. Porous carbon materials such as CMK-3 were introduced into the sorbent to provide an internal gas pathway. Upon calcination conditions, the carbon amount was controlled in the composites (Li4SiO4@CMK-X%, where X represents the amounts of CMK-3). In Li4SiO4@CMK-1.8%, CMK-3 is distributed over the whole solid; in contrast, the additive in Li4SiO4@CMK-0.5% is mainly observed near the surface of the solid. CO2 gas sorption study of the composites showed that pores of CMK-3 in Li4SiO4 aid the diffusion of CO2. In addition, we found that the incorporation of porous carbon provides more active sites for interactions with CO2 through the formation of cavities between Li4SiO4 and CMK-3. Li4SiO4@CMK-1.8% had an increased CO2 absorption capacity (35.4 wt%) and rate (15.2 wt% for the first 5 min) at 600 °C, compared to the CO2 absorption capacity (16.3 wt%) and rate (5.1 wt% for the first 5 min) of pristine Li4SiO4 (p-Li4SiO4). To confirm the influence of porous carbon on the CO2 absorption properties, multi-walled carbon nanotube (MWCNT) was also examined as an additive instead of CMK-3. Li4SiO4@CNT showed similar trends with Li4SiO4@CMK sorbents.