Investigation of High-Temperature CO₂ Capture by CaO in CaCl₂ Molten Salt

Abstract

Modifications of CaO-based sorbents with various inorganic salts to overcome the degradation in the reactivity between the active material and CO2 in a carbon capture process have previously been evaluated. The present paper focuses on the performance of a novel CO2 capture technology, where CaCl2 is applied as the solvent for the dissolution/dispersion of CaO and CaCO3. CO2 capture by CaO was carried out with carbonation temperatures in the range of 770–830 °C by bubbling simulated flue gas through the melt, using a fully automated flow-through atmospheric pressure reactor. Subsequently, decomposition of the formed CaCO3 to CaO and CO2 was conducted at 910–950 °C using pure N2. Online gas analysis was performed using a Fourier transform infrared (FTIR) gas detector and gravimetric analysis. The results indicate that the CaO carbonation efficiency decreases at temperatures higher than 800 °C. Increasing the concentration of CaO enhances the carbonation reaction. The amount of CO2 uptake for 15 wt % CaO in calcium dichloride was 0.541 g of CO2/g of sorbent. Moreover, investigation of CO2 absorption/desorption by 5.32 wt % CaO in CaCl2 at 787 °C showed an increase in CaO reactivity, defined as the ratio between the real and theoretical CO2 sorption per unit of CaO, from 55.4 to 64.2% after 10 cycles. In all cases, the decarbonation process proceeded rapidly, reaching 100% efficiency.

Reactivity of CaO with CO2 in molten CaF2–NaF: formation and decomposition of carbonates

“…The total conversion efficiency of CaO to carbonates at 824°C was around 94 wt%, but complete decomposition of the formed carbonates was not reached (65 wt% of CO 2 released). The high carbonation efficiency may be enabled by the ion exchange reaction described by equations and : …”

“…The total conversion efficiency of CaO to carbonates at 824°C was around 94 wt%, but complete decomposition of the formed carbonates was not reached (65 wt% of CO 2 released). The high carbonation efficiency may be enabled by the ion exchange reaction described by equations 2 and 3: [12] (1) At temperatures lower than 335°C, formation of shortite (Na 2 Ca 2 (CO 3 ) 3 ) may take place (eq. 4), but above this temperature shortite decomposes to nyerereite (Na 2 Ca(CO 3 ) 2 ) which melts at 817°C (eq.…”

“…The results indicate that the highest carrying capacity (0.722–0.743 g CO 2 /g CaO) may be achieved with the CaO–CaF 2 /NaF system due to thermodynamically stable Na 2 CO 3 being formed through an ion exchange reaction . Studies of cyclic CO 2 absorption/desorption by 5.32 wt% CaO in molten CaCl 2 showed a stable carrying capacity of 0.504 g CO 2 /g CaO . In this case, the decomposition of the formed carbonates proceeded rapidly and completely, but when using alkali and alkaline‐earth metal halide salts, full decomposition of the carbonates formed in the melt could not be achieved .…”

“…The results indicate that the highest carrying capacity (0.722-0.743 g CO 2 /g CaO) may be achieved with the CaO-CaF 2 /NaF system due to thermodynamically stable Na 2 CO 3 being formed through an ion exchange reaction [11,14]. Studies of cyclic CO 2 absorption/desorption by 5.32 wt% CaO in molten CaCl 2 showed a stable carrying capacity of 0.504 g CO 2 /g CaO [12]. In this case, the…”

“…The dissolution and/or dispersion of sorbent particles in a molten salt matrix may enhance the reactivity of the sorbent with CO 2 because efficient gas‐liquid‐solid reactions can be employed . Various alkali and alkaline‐earth metal halide salts (CaCl 2 , CaCl 2 –CaF 2 , CaF 2 –NaF and CaF 2 –LiF) have previously been studied for the use as solvents for dissolution and/or dispersion of CaO to enhance sorbent activities in the CO 2 capture process . CaF 2 –NaF and CaF 2 –LiF eutectic mixtures have been used.…”