Behavior of Mineral Matters in Chinese Coal Ash Melting during char-CO2/H2O Gasification Reaction

The typical Chinese coal ash melting behavior during char-CO2/H2O gasification reaction was studied by using TGA, XRD, and SEM-EDX analysis. It was found that ash melting behavior during char gasification reaction is quite different from that during coal combustion process. Far from the simultaneously ash melting behavior during coal combustion, the initial melting behavior of ash usually occurs at a middle or later stage of char-CO2/H2O reaction because of endothermic reaction and more reactivity of char gasification reaction as compared with that of mineral melting reactions in ash. In general, the initial melting temperature of ash is as low as 200-300 K below the deformation temperature (T-def) of ash with ASTM test. The initial molten parts in ash are mainly caused by iron bearing minerals such as wustite and iron-rich ferrite phases under gasification condition. Along with the proceeding of ash melting, the melting behavior appears to be accelerated by the presence of calcium to form eutectic mixtures in the FeO-SiO2-Al2O3 and CaO-SiO2-Al2O3 system. The different states of iron are the dominant reason for different melting behaviors under gasification and combustion conditions. Even under both reducing conditions, the ash fusion temperature (AFT) of coal under char-CO2 reaction is about 50-100 K lower than that under char-H2O reaction condition. The main reason of that is the higher content of CO under char-CO2 reaction, which can get a lower ratio of Fe3+/Sigma Fe in NaO-Al2O3-SiO2-Fe-O melts.