Reducing the central mode particulate matter in coal combustion by additives: Impacts of nano Al2O3 and TiO2 addition

Abstract

Current additives generally have lower efficiency on reducing PM2.5 than the ultrafine PM as they could not effectively reduce the central PM. To find additives that could effectively reduce the formation of central PM, two kinds of nano particles (namely Al2O3 and TiO2) were screened out, and coal combustion with the nano additives were performed. The generated PM10 were sampled to characterize their particle size distribution, mass yield, size-resolved composition and micro morphology with element map. The results showed that adding a small dose (0.6%) of nano Al2O3 and TiO2 reduced the mass yields of PM0.1-2.5 (i.e., central PM) and PM2.5 by 24.38%-26.06% and 12.85%-19.59% respectively, indicating quite different PM reduction characteristics than the conventional clay-mineral-derived additives. The nano additives on the fuel surface promoted the melting and coalescence of small ash particles, promoting their migration into coarse PM. Moreover, the added nano additives covering the coal particle scavenged the mineral vapors releasing from the coal particle during its pyrolysis/combustion, and thereby reduced their migration into central PM via the following heterogenous condensation. Finally, the added nano additives mainly migrated into the large ash particles and enhanced heterogeneous condensation/reaction of the released mineral vapor via providing larger specific surface area.