Insights into adsorption mechanism and kinetic modeling of K2CO3-doped Li4SiO4 pellets for CO2 capture at high temperature and low concentration

High-temperature CO2 removal by Li4SiO4-based sorbents is considered a promising carbon capture strategy to mitigate fossil-fueled process emissions owing to its high adsorption capacity and excellent stability. However, for application under low CO2 concentration, kinetic limitations can occur due to diffusion resistance through carbonation reaction solid products. Doping Li4SiO4 with alkali carbonates, such as K2CO3, can address this issue enhancing CO2 diffusion thanks to the formation of a molten eutectic layer. In this work, macro-porous pellets of Li4SiO4 doped with K2CO3 were produced and tested for CO2 capture at different adsorption temperatures and low CO2 concentrations (0.5-4 vol%). The pellets exhibited a maximum adsorption capacity of 137 mg CO2/g sorbent at 595 degrees C and 4 vol% CO2, and excellent stability for over 50 adsorption/desorption cycles. The reaction equilibrium and the adsorption profiles, in terms of both conversion and reaction rate evolution through time, have been analyzed to clarify the reaction mechanism. The results showed that the reaction rate increases with increasing Li4SiO4 conversion during the earliest stages of adsorption, where the process is chemical-controlled, highlighting the presence of a nucleation phase. Whereas, the reaction rate rapidly decreases with the conversion as CO2 diffusion takes control due to the presence of the product layer that covers the unreacted core of Li4SiO4. A kinetic analysis was then performed, aimed at developing a comprehensive phenomenological model for simulating the physical-chemical processes that occur during CO2 adsorption by K2CO3-doped Li4SiO4 sorbents in view of their industrial application. A new shrinking core-based model was developed, which describes the carbonation reaction with a unique kinetic equation capable of representing both the nucleation and diffusion regimes observed. The proposed model demonstrated excellent fitting to the experimental data obtained by adsorption tests, thus making it suitable for describing the CO2 adsorption process of K2CO3-doped Li4SiO4 sorbents and for predicting the adsorption parameters, as kinetic constants and CO2 diffusivity, in view of industrial-scale application of the sorbent in packed bed reactors.