Development of Potassium- and Sodium-Promoted CaO Adsorbents for CO₂ Capture at High Temperatures

Abstract

Development of highly efficient adsorbents for the high temperature CO2 capture process is crucial for large scale implementation of this technology. In this work, development of novel potassium- and sodium-promoted CaO adsorbents (K–Ca and Na–Ca) is discussed, and their CO2 capture performance at high temperatures is presented. A series of K–Ca and Na–Ca adsorbents with various K/Ca or Na/Ca molar ratios were developed and tested for CO2 capture at high temperatures ranging from 300 to 400 °C. The structural, chemical, and morphological characteristics of the double salts were systematically evaluated before and after exposure to CO2. Our results indicated that CO2 capacity is largely influenced by both K or Na concentration and adsorption temperature. Maximum capacities of 3.8 and 3.2 mmol/g were obtained for K–Ca and Na–Ca double salts, respectively, at 375 °C and 1 bar. Further investigation of the effect of temperature revealed that the window temperature for operation ranges from 300 to 650 °C, while beyond 650 °C, the double salts start to decompose and lose capacity. Moreover, it was found that both adsorption kinetics and capacity improve with temperature, with CO2 uptake reaching a maximum at 10.7 mmol/g at 650 °C over K–Ca double salt. This study represents alkali metal-promoted CaO adsorbents as potential high-temperature adsorbents with similar performance to their MgO-based analogues.

Na2CO3-modified CaO-based CO2 sorbents: the effects of structure and morphology on CO2 uptake

“…Al 2 O 3 or MgO) 7,[11][12][13][14][15][16][17] or the addition of alkali metal salts. [18][19][20][21][22][23] Currently, there is very limited understanding of the effect of the addition of alkali metal salts on the CO 2 capture performance of CaO; indeed, several works have reported conflicting results. [18][19][20][21] For example, it has been reported that the addition of alkali metal chlorides and hydroxides (with the exception of Li-based salts) strongly increases the CO 2 uptake capacity of CaO, although no cyclic stability data was reported.…”

“…20 Upon the addition of Na 2 CO 3 , the double carbonate Na 2 Ca(CO 3 ) 2 can form under reaction conditions. 22,23 The formation of Na 2 Ca(CO 3 ) 2 has been linked to an increase in the kinetics of both the carbonation and calcination reactions and an increased cyclic stability of the sorbent when compared to pristine CaO. 22,23 Recent work has explored the effect of the addition of the double salt (Li-K) 2 CO 3 to CaO on its CO 2 uptake.…”

“…22,23 The formation of Na 2 Ca(CO 3 ) 2 has been linked to an increase in the kinetics of both the carbonation and calcination reactions and an increased cyclic stability of the sorbent when compared to pristine CaO. 22,23 Recent work has explored the effect of the addition of the double salt (Li-K) 2 CO 3 to CaO on its CO 2 uptake. 21 After 23 carbonation/calcination cycles a three times higher CO 2 uptake was observed when compared to pure CaO.…”

Mechanistic Understanding of CaO‐Based Sorbents for High‐Temperature CO2 Capture: Advanced Characterization and Prospects

“…The cyclic CO 2 uptake performance of alkali metal-modified CaO varies strongly depending on the nature of the alkali metal (i. e., Li, Na, K, Rb and Cs), the precursors used for synthesis (e. g., Na 2 CO 3 vs. NaCl) and the content of alkali metal salt added. [11,70,88] However, a general trend of a decreased CO uptake with increasing alkali metal content seems to hold for most studies. For high contents of Na and K (i. e., several mol %), independent of the precursor used, various authors have shown that the CO 2 uptake performance is decreased, possibly due to enhanced sintering effects.…”

“…Calcium oxide‐based solid sorbents that undergo cyclic carbonation and calcination reactions during CO 2 capture and regeneration, viz. CaO+CO 2 ↔CaCO 3 with Δ R H 0 298K =±179 kJ mol −1 , have emerged as a cost‐efficient class of CO 2 sorbents [9–12] . On the process level, the use of CaO‐based CO 2 sorbents is referred to as calcium looping (CaL).…”