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

Abstract

• CO 2 adsorption mechanism of porous K 2 CO 3 -doped Li 4 SiO 4 pellets was investigated. • Adsorption rate increase with time highlighted a nucleation phase of solid products. • Rapid decrease in adsorption rate explained as CO 2 diffusivity exponential decay. • New SCM-based model was developed describing both nucleation and diffusion steps. • Fitting results confirmed model accuracy in depicting adsorption by doped Li 4 SiO 4 . High-temperature CO 2 removal by Li 4 SiO 4 -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 CO 2 concentration, kinetic limitations can occur due to diffusion resistance through carbonation reaction solid products. Doping Li 4 SiO 4 with alkali carbonates, such as K 2 CO 3 , can address this issue enhancing CO 2 diffusion thanks to the formation of a molten eutectic layer. In this work, macro-porous pellets of Li 4 SiO 4 doped with K 2 CO 3 were produced and tested for CO 2 capture at different adsorption temperatures and low CO 2 concentrations (0.5–4 vol%). The pellets exhibited a maximum adsorption capacity of 137 mg CO 2 /g sorbent at 595 °C and 4 vol% CO 2 , 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 Li 4 SiO 4 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 CO 2 diffusion takes control due to the presence of the product layer that covers the unreacted core of Li 4 SiO 4 . A kinetic analysis was then performed, aimed at developing a comprehensive phenomenological model for simulating the physical–chemical processes that occur during CO 2 adsorption by K 2 CO 3 -doped Li 4 SiO 4 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 CO 2 adsorption process of K 2 CO 3 -doped Li 4 SiO 4 sorbents and for predicting the adsorption parameters, as kinetic constants and CO 2 diffusivity, in view of industrial-scale application of the sorbent in packed bed reactors.

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