XAFS and thermogravimetry study of the sintering of calcium supported on carbon

An X-ray absorption fine structure (XAFS) and thermogravimetry (TG) study has been carried out to analyze the sintering of calcium supported on carbon during pyrolysis treatment and during CO2-carbon gasification. The following variables have been studied: (a) the heating rate effect during pyrolysis on calcium dispersion as a function of calcium content; (b) calcium sintering and deactivation during CO2-carbon gasification as a function of the reaction temperature and of the initial calcium dispersion, and (c) the effect of the chemical state of calcium (CaCO3 or CaO) on the calcium sintering rate. The results confirm the well-known effect of the heating rate during a pyrolysis treatment on calcium dispersion: the higher the heating rate, the higher the calcium dispersion. The study of calcium dispersion evolution (measured by CO2 chemisorption) during CO2-carbon gasification allows us to understand the mechanism of calcium deactivation. XAFS results show the great effect of the chemical state of calcium on the sintering rate. In this sense, it is observed that CaCO3 has a much higher mobility on the surface of the carbon than CaO due to the great difference in their Tammann temperatures.