Mechanistic study of PM2.5 reduction by kaolin-limestone compound additive in pulverized coal combustion

Abstract

A novel compound additive prepared from kaolin and limestone shows promising prospects for in-furnace abatement of combustion PM2.5 (particulate matter with aerodynamic diameters equal to or less than 2.5 μm). However, the mechanisms need further clarification. These were investigated by combustion of a low-alkali coal on a high-temperature drop tube furnace (DTF). Combustion experiments with the kaolin-limestone additive were carried out at 1300 °C and in simulated air. Baseline tests of individual additives (i.e., kaolin and limestone) were also performed for comparison. Particulate samples were collected and classified by a Dekati low pressure impactor (DLPI). Heat-treated additive samples were also collected by DLPI and filter. They were characterized by various techniques such as laser diffraction particle sizing, X-ray diffraction (XRD), and field emission scanning electron microscopy equipped with energy dispersive spectrometry (FESEM-EDS). The results show that the addition of kaolin or limestone suppresses the emission of aerosols between 0.3 and 2.5 μm (PM0.3-2.5), but unexpectedly increases that of ultrafine aerosols less than 0.3 μm (PM0.3). The latter is primarily attributed to the carryover of ultrafine additive particles. Due to different effects on the two particle size fractions, the individual additives are found to have overall insignificant effects on PM2.5 emission. In striking contrast, the addition of the kaolin-limestone additive leads to an apparent reduction of both PM0.3 (∼46%) and PM0.3-2.5 (∼37%). As a result, a significantly high reduction of PM2.5 by ∼ 40% is achieved. The superior performance of the compound additive, compared with the individual additives, is accounted for by enhanced interactions between kaolin-limestone, additive-vapor, and additive-ash. These mechanisms, together with the collision-induced particle capture mechanism proposed previously, are believed to play key roles in PM2.5 abatement in real boilers when the compound additive and fuel are premixed.