Calcium deactivation during the char-CO2 gasification and its influence on determining the apparent reaction order

Abstract

CO2 gasification reactivity and the apparent reaction order of chars derived from calcium-rich and calcium-poor lignites have been investigated via thermogravimetry analysis (TGA) in the temperature range from 825 to 900 °C and the CO2 partial pressure range from 0.015 to 0.1 MPa. Chemical forms of Ca species in raw and partially-gasified chars are characterized via X-ray diffraction (XRD). Results indicate that CaO carbonation during the char-CO2 gasification is a direct cause of Ca deactivation, which shows temperature and pressure dependence and follows the equilibrium regularity of CO2 over CaCO3 at different temperatures. The Ca in lignite chars plays a catalytic role at the CO2 partial pressures below 0.1 MPa at temperatures of 875 and 900 °C, while Ca deactivation occurred when the CO2 partial pressures are above 0.05 MPa at 825 °C and above 0.075 MPa at 850 °C. On the one hand, this fact suggests that Ca deactivation exhibits temperature and pressure dependence. On the other hand, char gasification may follow different catalytic mechanisms in the CO2 partial pressure range from 0.015 to 0.1 MPa, from which the determination of the apparent reaction order is less reasonable. The coefficient of determination (R2) reflects the goodness of r-PCO2 fitting, which becomes poor with the increase of CO2 partial pressure when CaO carbonation takes place. Choosing an appropriate CO2 pressure range for a given temperature is necessary to eliminate the effects of Ca deactivation on the determination of gasification reactivity and the apparent reaction order. The apparent reaction order of chars determined at conditions without Ca deactivation increases as the gasification temperature decreases.