Fig. 5a also indicates that the particles number concentration
first increases and then decreases with the particle size. And there
is a highest level of the particles number in the particle size range
of about 0.1–1.0 lm. Besides the formation of aerosol particles, it is
also related to the removal mechanism of particles in the WFGD
system. This is caused either by inertia or diffusional force depending on the size of the particles to be separated. For particles with a
size below 0.1 lm, the only mechanism of particle removal is diffusional
forces, which increase with decreasing particle size and
hence the enhanced particles number decreases. For particles larger
than 1 lm, the removal by inertial forces dominates. With
increasing particle size the inertial force of the particles increases
and hence the particles number decreases. Therefore, the particle
number concentration for particle size below 0.1 lm and larger
than 1 lm is lower than that of particle size range of 0.1–1.0 lm
at the outlet of the WFGD system. For the case of steam addition
in the particle growth region, the improvement of grade removal
efficiency first increases and then tends to stable with the particle
size.