Chemical Composition of Surface Species in Pyrolyzed Brown Coals and Their Evolution during the Steam Gasification Reaction

In this paper, a detailed study focused on the characterization of the surface chemical species on the Kansk-Achinsk brown coal chars with different inherent mineral matters, calcium, in particular, and on the evolution of chemical species upon steam gasification at a mild temperature of 700 °C was performed by using surface-sensitive X-ray photoelectron spectroscopy , X-ray absorption near edge structure, and other techniques. It was found that the naturally occurring calcium surface species represented highly dispersed carbonate-like forms. Before gasification, they represented aragonite-like species. However, on steam gasification, they readily underwent solid-phase polymorphic transition into a calcite-like form. The main proportion of the surface carbon atoms (68–71%) on all the chars represented sp2-hybridized carbons in the graphene fragments, while the remaining 29–32% accounted for as oxidized carbon species of different configurations. Mainly, sp2-hybridized carbon atoms were found to be sensiti...