Moreover, Fig. 4a also indicates that the removal efficiency first increases and then tends to stable with the particle size. This can be explained as follows. The fine particles from coal combustion are mainly insoluble aluminosilicates, which are not perfectly wettable substances [17]. According to the classical heterogeneous condensation theory of Fletcher [18,19], the critical supersaturation Scr of partially wettable, insoluble spherical particles can be expressed as follows: