Experimental study and chemical equilibrium calculation on the mineral transformation of high-alkali coal under oxy-fuel condition

Abstract

The study aimed to study the mineral transformation of high-alkali coal to address the ash-related issues in oxy-fuel combustion. However, the traditional ashing method alone is inappropriate to study the mineral transformation at the initial stage of coal combustion due to the high ashing temperature. Hence, both the plasma ashing method (<200 °C) and traditional ashing method (600–1200 °C) were employed in this research to prepare the ash samples for characterization. The plasma ashing method is an efficient way to determine the original minerals in coal because it can consume the organic matter at low temperature. The recirculation of flue gas in oxy-fuel combustion can lead to high SO2 and H2O concentrations, while their influences on mineral transformation need to be clarified. In this work, SO2 and H2O were injected into CO2 to simulate the recycled flue gas (RFG). Furthermore, the chemical equilibrium calculation was conducted besides multiple experimental tests to acquire more comprehensive information of mineral transformation. The results show that high sodium and calcium contents are determined in these high-alkali coals, and two of them are rich in iron. In O2/CO2 atmosphere, some sodium (e.g. NaCl) volatilizes into the gas phase and the others converts into aluminosilicates (e.g. NaAlSi3O8) during the heating process. The calcium salts (mainly CaCO3 and CaSO4) decompose and produce silicates, aluminosilicates, and magnesium silicates. The major iron-containing minerals are identified as FeS, FeS2, and Fe2O3. Both FeS and FeS2 are oxidized into Fe2O3 below 600 °C. Moreover, the kaolinite (Al2Si2O5(OH)4) in raw coals decomposes at low temperature (<600 °C). In O2/RFG atmosphere, the additions of SO2 and H2O enhance the sulfation of AAEMs (alkali and alkaline earth metals) and delay the decomposition of sulfates. The productions of some silicates, aluminosilicates, and magnesium silicates of AAEMs are inhibited to varying degrees, and there could be complicated salts containing sulfur (e.g. hauyne). The transformations of iron-containing minerals are little related to the presences of SO2 and H2O. This work obtained the transformations of main minerals in high-alkali coal under O2/CO2 and O2/RFG conditions, which helps to address the ash-related issues.