Page 422 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

402 13 Nanoaerosol

particles to carry ions with same polarity, negative or positive. Charged particles are
subjected to electrical forces in an electrical field. The bipolar charging process
eventually leads to Boltzmann charge equilibrium if the particles experience suf-
ficient charging time. In such cases, they are considered neutralized. Highly charged
particles may be discharged by colliding with ions with different polarity [43].
There are many types of nanoaerosol chargers. Unipolar ions can be produced by
unipolar corona discharge, UV charging, carbon fiber ionizer, and separation of
ions produced by bipolar ions. Bipolar ions are usually produced by radioactive
sources such as Kr 85 or Po 210 , soft X-ray, AC corona discharge or dual electrode
corona discharge. According to the mechanisms of ion generation, the chargers can
be classified into:

• Corona discharge chargers,
• Radioactive chargers, and
• Photoelectric chargers.
Corona discharge is the most commonly used for high ion concentrations [22].
Bipolar charging method has a lower charging efficiency due to particle loss and
ions recombination. In the unipolar method, produced ions in the corona charger are
moved using the filtered air passing opposite the aerosol flow. Filtered air causes
aerosols to be diluted and decreases charging efficiency.
However, nanoparticles may be generated in a corona. Various studies have
investigated nanoparticle generation associated with bipolar and unipolar corona
chargers [26, 35, 45, 52] and have reported methods to reduce nanoparticle gen-
eration in corona charging. One reason for generating nanoparticles by the corona
charger is that a corona charger has enough energy to start gas-phase chemical
reactions in the charger region, such as forming ozone from oxygen, which may
lead to particle generation [45]. Moreover, sputtering of metal from the surface or
erosion of the electrodes is another reason which may cause particles to be gen-
erated [30].

13.4 Separation of Nanoaerosol from the Air

Separation of general aerosol particles and particulate matter has been introduced in
Chap. 6. Among the technologies introduced therein, separation based on inertia in
cyclone and gravity settling chamber have almost no effect on nanosized size range.
Under normal conditions, nanoaerosol particles follow the air due to their non-
continuum behavior.
It is technically challenging to remove nanoparticles from the air by electrostatic
precipitators (ESPs) only because nanoaerosol particle charging efficiency is low.
Furthermore, extra nanoaerosol particles are likely to be produced in corona
chargers, which is a critical component of an ESP. Passing nanoaerosol through a
liquid (e.g., water) column can effectively remove the unwanted particles from the