Sol-gel derived, Na/K-doped Li4SiO4-based CO2 sorbents with fast kinetics at high temperature

Abstract

Li4SiO4 is considered as one of the most potential high-temperature sorbent materials for CO2 capture from the flue gas of fossil fuel-fired power plants. However, it usually suffers from slow kinetics at low CO2 concentrations. In this work, a series of Na- and K-doped Li4SiO4-based sorbents with Na/Si (or K/Si) atomic ratios of 0.05, 0.1 and 0.2 were prepared by a citrate sol-gel method. The prepared materials were characterized by various techniques, which revealed the presence of Li3NaSiO4 (for Na-doped sorbents) and LiKCO3 (for K-doped sorbents) on the surface of Li4SiO4 particles. All Na- and K-doped Li4SiO4 showed greatly enhanced CO2 sorption rates and capacities in 15% CO2 as compared to the heteroatom-free Li4SiO4, and in particular, the Na-doped sorbents had the fastest rates. The improved performance of K-doped Li4SiO4 resulted from the formation of eutectic Li2CO3-K2CO3 molten carbonates during CO2 sorption, while for Na-doped Li4SiO4, both the carbonation of Li3NaSiO4 and the presence of eutectic Li2CO3-Na2CO3 molten salts contributed to the improvement. In addition, both Na- and K-doped Li4SiO4 exhibited incomplete regeneration during cyclic operation, but the latter had better stability. In this context, we deliberately prepared a Na,K-codoped Li4SiO4-based sorbent with a Na:K:Si atomic ratio of 0.1:0.1:1, which showed fast kinetics and good cyclic stability. During 30 consecutive sorption-regeneration cycles (10 min of sorption in 15% CO2 at 650 °C, 5 min of regeneration in pure N2 at 750 °C), the Na,K-codoped Li4SiO4 presented a stable CO2 capture capacity of 0.22 gCO2/gsorbent after the first 10 cycles.