Transformation of alkali and alkaline-earth metals during coal oxy-fuel combustion in the presence of SO2 and H2O

The occurrence modes of alkali and alkaline-earth metals (AAEMs) in coal relate to their release behavior and ash formation during combustion. To better understand the transformation of AAEMs, the release behavior of water-soluble, HCl-soluble, HCl-insoluble AAEMs during Shenmu coal (SM coal) oxy-fuel combustion in the presence of SO2 and H2O in a drop-tube reactor was investigated through serial dissolution using H2O and HCl solutions. The results show that the release rates of AAEMs increase with an increase in temperature under the three atmospheres studied. The high release rates of Mg and Ca from SM coal are dependent on the high content of soluble Mg and Ca in SM coal. SO2 inhibits the release rates of AAEMs, while H2O promotes them. The effects of SO2 and H2O on the Na and K species are more evident than those on Mg and Ca species. All three types of AAEMs in coal can volatilize in the gas phase during coal combustion. The W-type AAEMs release excessively, whereas the release rates of I-type AAEMs are relatively lower. Different types of AAEM may interconvert through different pathways under certain conditions. Both SO2 and H2O promote the transformation reactions. The effect of SO2 was related to sulfate formation and the promotion by H2O occurs because of a decrease in the melting point of the solid as well as the reaction of H2O. (C) 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.