Page 302 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

10.2 Control of Particulate Matter Emissions 279

Fig. 10.1 Typical effect of 8
Effective drift velocity, m/min 4 2
resistivity of fly ash particles
on the electric drift velocity 6

0
7.5 8 8.5 9 9.5 10 10.5
Log (resistivity), ohm.m

When a selective catalytic reduction (SCR, to be introduced shortly) unit is
employed for NO x control at 350–400 °C, it is better to put the ESP upstream of the
SCR, because after the ESPs, fly ash particles are removed and it can improve SCR
catalyst lifetime, and thereby reduce SCR operation and maintenance problems.
Besides the size of particles, other properties of the particles also affect the
particle charging behavior and the consequent performance of ESPs. The actual
process is further complicated by interactions between the particle and the air. The
resistivity of the particles is the main factor that affects the performance of the ESPs
[25]. Figure 10.1 shows the relationship between the resistivity and the precipitation
speed of the particles. For the same device, the greater particle precipitation speed,
the greater particle removal efficiency. Overall, Fig. 10.1 shows that the particle
removal efficiency drops with the increase of particle resistivity [9].
For the effective separation of particles using an ESP, the resistivity of the
7
particles is preferably in the range 10 –10 12 Ωm. Particles with too high resistivity
cannot be charged effectively. In addition, a spark from the collection plate to the
discharge electrode may take place due to high intensity of the electric field
building up in the collected dust cake. On the other hand, particles with very low
resistivity may lose their charges rapidly to water in the gas or to other particles.
More importantly, these particles may be easily re-entrained from the collection
electrodes or the collected particle layer because they may rapidly lose their charges
or switch polarity of the charges.
The resistivity of particles depends on
• temperature
• moisture content
• chemical composition.
The resistivity of the particles first increases with the temperature and then drops.
Depending on the chemical composition of the fly ash particles considered, the
maximum resistivity is sustained between 140 and 170 °C.