Thermodynamic and CO2 sorption investigations on improved Li3BO3-based sorbents by NaOH addition

Abstract

The development of new solid sorbents with high carbon capture capacity and good reversible sorption properties is considered a key factor for mitigating CO2 emissions. Tri-lithium borate (Li3BO3) has shown to be a high capacity CO2 sorbent adequate for an intermediate temperature range (500–650 ◦C). However, the performance of this sorbent has been little studied in particular at low CO2 concentrations, requiring the introduction of changes in Li3BO3 to make it competitive. Here, highly efficient NaOH-modified Li3BO3 sorbents were synthesized by a simple two-step mechano-thermal method at low temperature using different amounts of NaOH. For the first time, the influence of increasing amounts of an additive on the standard enthalpy change of Li3BO3 as well as on the relation between CO2 pressure and temperature equilibrium was determined. A progressive rise in the standard enthalpy change and a decrease in the equilibrium CO2 pressure at a fixed temperature were observed with the increase in the amount of additive. Moreover, the NaOH addition to Li3BO3 improves its CO2 capture capacity, CO2 absorption rate and stability during carbonation/regeneration cycles. The CO2 capture capacity ranges between 50 wt% and 35 wt% at pure CO2 and 15 % CO2, respectively, for low amounts of NaOH added. Further studies showed that CO2 capture/desorption efficiency is kept high after 20 cycles at 525 ◦C and 15 % CO2. The improved performance of NaOH-modified Li3BO3 sorbents and the changes in the thermodynamic properties were associated with two factors: the partial substitution of boron by carbon in the Li3BO3 structure, and the formation of Li2CO3-Na2CO3 molten carbonates. Therefore, these NaOH-modified Li3BO3 materials represent attractive CO2 sorbents for moderate temperatures, applicable for post-combustion industrial processes.