Carbon capture from low CO2 concentration stream through ternary alkali metal nitrates promotion of MgO sorbents at intermediate temperature and low space velocity

Abstract

Many MgO sorbents exhibit high CO₂ uptake in the presence of high concentration CO₂ and aided by high space velocity, which unfortunately leads to low CO₂ capture efficiency from the gas stream. This study investigates the effect of Li/Na/K ratio on sorption performance in dilute CO₂ streams (15% CO₂) using alkali metal nitrates-promoted MgO sorbents at a low space velocity of 20 ml/g·min in fixed bed reactor. Study shows that the ratio of Li/Na/K in ternary alkali nitrate salt mixture significantly affects the induction periods for the second phase CO₂ sorption, which is a key contributor to high CO₂ uptake. High LiNO₃ content in ternary mixture leads to long induction period, and therefore lower CO₂ capacity during the 4 h period. Conversely, high ratios of NaNO₃ and KNO₃ leads to shorter induction periods. Among the samples, MgO with 9 mol% (Li_0.18Na_0.52K_0.3) exhibits the highest CO₂ sorption capacity of 5.56 mmol/g at 240°C in 4 h (20.6% CO₂ capture efficiency) and 3.18 mmol/g after 5 cycles (10.8% CO₂ capture efficiency). Comparative studies show that optimal temperatures for CO₂ sorption are higher when using pure CO₂ (280 to 320°C) than when using 15% CO₂ (240°C) due to the thermodynamic equilibrium. CO₂-TPD results indicate that CO₂ desorption proceeds in two phases similar to CO₂ sorption, and the presence of alkali salt in MgO also promotes the decomposition of bulk carbonate to CO₂. It is shown that molten salt migration and agglomeration of MgO particles are likely the factors that lead to decline in CO₂ uptake over the sorption-desorption cycles.