Iron Transformation and Ash Fusibility during Coal Combustion in Air and O₂/CO₂ Medium

Little work has been performed on the transformation of iron and ash fusibility during oxy-coal combustion, which is of great significance to assessing ash deposition propensity. A high-iron bituminous coal was burnt at 1300 °C in a laboratory drop-tube furnace under four conditions: (1) 21 vol % O2/79 vol % N2 (air-firing), (2) 21 vol % O2/79 vol % CO2 (oxy-firing), (3) 27 vol % O2/73 vol % CO2 (oxy-firing), and (4) 32 vol % O2/68 vol % CO2 (oxy-firing). The bulk ash samples were subjected to X-ray fluorescence and Mössbauer spectroscopic analyses. The effects of changing from air combustion to O2/CO2 combustion and the effects of varying the O2 level in O2/CO2 combustion on iron transformation and ash fusibility were investigated. The results show that varying the combustion condition has insignificant effects on the elemental composition of coal ashes but has appreciable effects on the relative proportions of iron combustion products that were identified as hematite, magnetite, and Fe–glass phases. This indicates that speciation analysis is as important as bulk analysis for thorough ash characterization. Replacing N2 in air with CO2 results in a higher content of hematite but a lower content of magnetite. Increasing the O2 level in O2/CO2 combustion increases the formation of hematite but decreases the formation of magnetite. In contrast, the amount of Fe–glass phases remains almost unchanged. An appreciable fraction (about 8% Fe) of hematite/magnetite seems to crystallize out of the molten glass phases during combustion, and it is not significantly affected by changing combustion conditions. The fusion temperatures of the oxy-fired ashes are higher than those of the air-fired ash and increase with the inlet O2 level. A positive correlation between the hematite content and the ash fusion temperatures is observed.