Page 185 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

160 6 Separation of Particles from a Gas

The particles can be charged by the ions generated by a corona discharger. There
are two distinctive charging mechanisms, one being diffusive charging and another
field charging. For either one, there is a saturation of charging because a particle can
carry only certain amount of ions. With more and more ions charged on the particle,
they also create another electric field preventing more ions from coming closer to
the particle.
Ions charged to a particle can be positive, negative, or both. Depending on the
polarity of the ions, the charging process is defined as unipolar or bipolar charging.
Unipolar charging is much more effective than bipolar charging and thereby widely
employed in industrial ESPs. Although there is not much difference between the
effectiveness of positive and negative charging processes, positive charging will
generate ozone, which is considered as secondary air pollutant. Therefore, negative
charging is preferred and widely used.

6.3.2.1 Diffusive Charging

Airborne ions share the thermal energy of the gas molecules and obey the same law
of kinetics theory. Diffusion of the ions in the air may result in collisions between
the ions and the airborne particles, and thereby the attachment between the particles
and the ions. This process is referred to as diffusive charging.
Consider an ion that is approaching a particle already being charged with n ions.
The potential energy of the air with a distance r away from the particle is

K E ne 2
P ¼ ð6:30Þ
r
2
2
9
where K E ¼ 9 10 Nm /C is a force constant.
According to White (1951), the spatial distribution of the concentration of air-
borne ions in a potential field is
P

N i rðÞ ¼ N i0 exp ð6:31Þ
kT
where N i0 = ion concentration in the charging zone, k = the Boltzmann constant,
T = absolute temperature in K, and P = potential energy in J.
Substitute Eq. (6.30) into (6.31), ion concentration near the particle at a radial
distance of r becomes

2
K E ne 1
N i rðÞ ¼ N i0 exp ð6:32Þ
r kT