Thermodynamic and kinetic properties of Li3BO3 modified by the alkali fluorides addition in the CO2 capture process

Abstract

Tri-lithium borate (Li3BO3) is a high-capacity CO2 sorbent recently proposed for the intermediate temperature range of 500 °C to 650 °C. Up to date, only a study of CO2 sorption kinetic behavior of Li3BO3 modified by nitrite salts has been carried out. The equilibrium properties that relate the dependence of the partial pressure of CO2 and temperature are also unknown. In this work, a novel two-step mechano-thermal method was optimized to produce both pure and modified Li3BO3 by addition of 0.1 mol of Na- and K-fluorides. Fast reaction by milling of Li2CO3 and H3BO3 forms the intermediate LiB5O8·5H2O compound at room temperature, which transforms to Li3BO3 as single phase by heating at 600 °C. Pressure-composition isotherms allow to determine the existence of two plateaus, each of them ascribed to different phases: Li6B4O9 at low CO2 partial pressure (ΔH = 58 ± 5 kJ/mol) and LiBO2 at high CO2 partial pressure (ΔH = 106 ± 6 kJ/mol). The mathematical expression that relates the equilibrium CO2 absorption partial pressure and the temperature of modified and unmodified Li3BO3 was reported. Na- and K-modified Li3BO3 show remarkably fast kinetics with high CO2 capture capacity (e.g. greater than 45 wt% at 500 °C) and excellent regenerability, with minor deterioration in performance over 30 cycles of CO2 sorption. The kinetic behavior of modified Li3BO3 by alkali fluorides is enhanced by the formation of an eutectic melt phase during carbonation, as confirmed by differential scanning calorimetry and the simultaneous detection of LiF. In addition, alkali fluorides could generate oxygen vacancies and modify the surface composition by enrichment of Li-O sites, also favoring the CO2 sorption kinetics.