Effect of kaolin additive on PM2.5 reduction during pulverized coal combustion: Importance of sodium and its occurrence in coal

Little work has been performed on the importance of sodium and its occurrence in coal to PM2.5 (particles less than 2.5 μm in aerodynamic diameter) reduction by kaolin during O2/N2 combustion and O2/CO2 combustion at high temperatures. In this study, the combustion experiment of a treated low-sodium coal with sodium aluminosilicate additive was conducted in a lab-scale drop tube furnace (DTF) at 1500 °C to reveal the contribution of mineral melting and coalescence to PM2.5 reduction. Meanwhile, two typical Na-loaded coals (in which the sodium was loaded in the form of NaCl and sodium carboxylate, respectively) with kaolin added were also burnt under O2/N2 and O2/CO2 atmospheres to investigate the effect of interaction between kaolin and different chemical form sodium on PM2.5 reduction. The results show that sodium aluminosilicate is able to promote the migration of PM0.5–2.5 (particles in aerodynamic diameter of 0.5–2.5 μm) to form coarse particles. Due to the stronger reactivity of sodium carboxylate reacting with kaolin than that of NaCl, PM0.2–0.5 (particles in aerodynamic diameter of 0.2–0.5 μm) decreases more significantly in the combustion when adding kaolin into the NaAc-loaded coal than into NaCl-loaded coal. In addition, the PM0.2–0.5 reduction in O2/CO2 combustion is lower than that in O2/N2 combustion owing to the less vaporization of metals and the slower diffusion rate of vapors in the O2/CO2 atmosphere in comparison to those in the O2/N2 atmosphere. The mineral coalescence varied in interactions of kaolin with NaAc and NaCl. Besides, the PM0.5–2.5 emission differed as a result of differences in coal characteristic and the atmosphere, and this would cause the difference of collision frequency between particles and additive. With the joint actions of mineral coalescence and particle collision, the NaAc-loaded coal has a higher PM0.5–2.5 reduction by kaolin than NaCl-loaded coal, especially under the O2/N2 combustion. An expression describing the relationship of PM0.5–2.5 reduction, mineral coalescence and particle collision was fitted and it is found that the mineral coalescence has a stronger influence than particle collision on PM0.5–2.5 reduction by kaolin.