Coal char gasification in H2O/CO2: Release of alkali and alkaline earth metallic species and their effects on reactivity

Abstract

The release of alkali and alkaline earth metallic species (AAEMs) and their effects on char gasification reactivity in different atmospheres were investigated. Chars from a bituminous coal pyrolysis were gasified under 30% H2O, 30% CO2, and 15% CO2+15% H2O atmospheres at 800°C to obtain a series of gasified chars with different carbon conversions. The AAEM content and morphology of the gasified chars were analyzed by atomic absorption spectroscopy and scanning electron microscopy, respectively. The physicochemical structural features of gasified chars were characterized by Raman spectroscopy and N2 adsorption. The reactivities of the gasified chars were determined using a thermogravimetric analyzer. The release behavior of Na and K is insensitive to the change in atmosphere, whereas the release of Ca in CO2 atmosphere is obviously higher than that in H2O or mixed H2O/CO2 atmosphere. Ca compounds agglomerated and sintered during the char-CO2 gasification, whereas they maintained good dispersion in the presence of H2O. Gasification under CO2 atmosphere produced micropores, whereas that under H2O atmosphere favored the formation of mesopores. The water-soluble Na and K species showed a better catalytic activity in the gasified char, leading to a higher reactivity of gasified char than the raw coal char.