The effect of alkali and alkaline earth metals (AAEMs) on combustion and PM formation during oxy-fuel combustion of coal rich in AAEMs

This study focused on the effect of alkali and alkaline earth metals (AAEMs) on combustion and particulate matter (PM) formation during oxy-fuel combustion of coal rich in AAEMs. The combustion characteristics and PM formation were investigated using thermogravimetric analyzer and drop tube furnace, respectively. The results showed a catalytic role of AAEMs in coal combustion. Addition of AAEMs can decrease ignition temperature and burnout temperature and increase the comprehensive combustion index. The catalytic effect of alkali metals was stronger than alkaline earth metals. The optimal addition ratio of AAEMs was 3 wt%. Pretreatment, such as water or acid washing, effectively reduced the formation of PM0.4. Increasing furnace temperature or O2 concentration can increase particle temperature during oxy-fuel combustion. Due to Na's higher volatility compared to Mg and Ca, increasing particle temperature cannot promote the vaporization of Na, but can enhance the reactions between Na-containing precursors and coarse particles. Thus, the content of Na in PM0.4 decreased. However, increasing particle temperature can promote the reduction reactions of Ca and Mg oxides, which was the primary reason why a higher furnace temperature or O2 concentration resulted in increased PM0.4 during oxy-fuel combustion of AAEMs-rich coal.