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

Highlights: • Release of Na, K, Ca and their effects on char-CO₂/H₂O gasification are revealed. • Release of Na and K is insensitive to the change in gasification atmosphere. • Ca compounds in char maintain a better dispersion in the presence of H₂O. • CO₂ and H₂O atmospheres favor the formation of micropores and mesopores respectively. • Water-soluble Na and K species show a better catalytic activity in the gasified char. - 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.