Investigation of K2CO3-modified CaO sorbents for CO2 capture using in-situ X-ray diffraction

Abstract

Alkali metal salt promoters, such as K2CO3 investigated in this work, are known to affect the cyclic performance of CaO-based sorbents – sometimes positively, sometimes negatively. Thus far, the influence of K2CO3 (and other salt-based promoters) is poorly understood, and detailed investigations on the interaction of K2CO3 with CaO are missing. We find that low amounts of K2CO3 in the sorbents improve the cyclic performance of CaO significantly (up to a factor of three), both under mild and harsh CO2 release conditions, while large amounts of K2CO3 (>1 mol.%) reduce the positive effect on the CO2 uptake performance. Aided by in-situ X-ray diffraction measurements, we observe that the gas environment has an important influence on the stability of K-containing phases in the sorbent. At typical carbonation conditions (650 °C, 15 vol% CO2/N2), CaO and K2CO3 readily form K-Ca double carbonates (K2Ca(CO3)2, which reacts further to form K2Ca2(CO3)3). When the calcination reaction is carried out in pure N2, the K-Ca double carbonates decompose above ∼730 °C, forming K2CO3 that becomes partially volatile near its melting point of ∼900 °C, and leads to a reduction of the K content in the samples with cycling. When the calcination reaction is carried out in a CO2-rich environment, the cyclic loss of K2CO3 is reduced substantially, but the sintering of the sorbent is increased. There are indications that very low quantities of K are incorporated into the CaO structure, and the formation of K-Ca double carbonates appears to influence the structural evolution of the CaCO3 phase formed during carbonation. With very low amounts of K2CO3 present in the sorbent, we observe an increase in pore volume for pore diameters between 20 and 80 nm, which ultimately may explain the improved CO2 uptake performance compared to the CaO sorbent without K2CO3.