Post-combustion CO2 capture using solid K2CO3: Discovering the carbonation reaction mechanism

Most authors consider that gas-solid K2CO3 carbonation occurs sequentially, with K2CO3·(1.5 H2O) as the intermediate to the formation of KHCO3. However, it is demonstrated here that K2CO3 hydration and carbonation occur in parallel, with no direct conversion from K2CO3·(1.5 H2O) to KHCO3. The novel results presented were obtained by separately tracking the individual uptake quantities of CO2 and H2O by K2CO3 derived from four different precursors. The K2CO3 precursors were subjected to 50 regeneration-carbonation cycles at the temperatures of 150°C and 50°C, respectively. In the initial cycles, K2CO3 derived from KHCO3 (phase I) showed higher reactivity and bicarbonate yield compared to K2CO3 derived from previously hydrated samples containing K2CO3·(1.5 H2O) (phase II). After sufficient cycling, however, the carbonation performances were independent of the K2CO3 precursors and stabilized at similar final conversions related to the carbonation conditions. This behavior is explained by the stabilization in the constituent phase compositions of K2CO3 produced by regeneration. X-ray diffraction analysis showed that phase I might be a slightly different monoclinic phase of K2CO3. Comparing the kinetic analysis of the observed uptake data with qualitative kinetic simulations of possible reaction mechanism scenarios revealed that carbonation and hydration of K2CO3 proceed as competing reversible reactions in parallel.