Particle collection efficiency of the wet ESP at different aerosol flow rates

Figure 4.2 shows the collection efficiency of the wet ESP as a function of corn oil diameter when the aerosol flow rate is 5 and 10 L/min and the applied voltage is 4.3 kV under the initially clean condition. The residence time of the aerosol in the present wet ESP was calculated to be 0.39 and 0.19 s for aerosol flow rates of 5 and 10 L/min, respectively. As seen in Figure 4.2, decreasing the flow rate from 10 to 5 L/min (or increasing the residence time from 0.19 to 0.39 sec) had a substantial effect on the collection efficiency as the Deutsch-Anderson equation dictates. At the flow rate of 5 L/min, the collection efficiency was 96.9-99.7 % for particles from 16.8 to 615 nm in electrical mobility diameter. The minimum collection of 96.9 % corresponds to the particle diameter of 126.3 nm. As the flow rate is increased to 10 L/min, the minimum collection efficiency is reduced to 84.1 % at the corresponding diameter of 70 nm.

According to particle charging theory, particle charge decreases with decreasing particle diameter, while the mechanical mobility increases rapidly with decreasing particle diameter (Hinds 1999). Therefore, the collection efficiency of ESPs for particles in the size range of 0.1 to 1 μm has a U-shape efficiency curve, as shown in Figure 4.2. Saiyasitpanich et al. (2006) also found a U-shaped collection efficiency curve for the wet ESP with a minimum of about 95 % for particles between 80 and 250 nm in diameter when the supplied voltage was 70 kV, gas residence time was 0.4 s, and the particle concentration was 15.9 mg/Nm³.

3.2 3.4 3.6 3.8 4 4.2 4.4 4.

Applied Voltage (kV)

6 0

0.04 0.08 0.12 0.16

Corona Current (mA)

Initial clean condition Dry ESP Wet ESP After 2 hours particle loading test

Dry ESP

Figure 4.1 Corona current as a function of applied voltage in the dry and wet ESPs.

100 200 300 500 50

30 20

Particle diameter (nm) 70

80 90 100

Collection efficiency (%)

Aerosol flow rate 5 L/min 10 L/min

Figure 4.2 Collection efficiency of the present wet ESP for corn oil particles at the aerosol flow rate 5 and 10 L/min and the applied voltage of 4.3 kV. Each test was repeated 6 times.

Figure 4.3 shows the collection efficiency of the present wet ESP for corn oil particles in the size range of 16.8 to 615 nm under different applied voltages at the aerosol flow rate of 5 L/min. As shown in the figure, the corn oil particle collection efficiency also shows a U-shape efficiency curve for particles from 16.8 to 615 nm in electrical mobility diameter and the efficiency decreases with decreasing applied voltage because of the reduction of the electric field strength. When the applied voltage was decreased from 4.3 to 4.1 kV, the collection efficiency decreased slightly from 96.9-99.7 % to 93.9-99.4 %. The collection efficiency was further decreased to 74.2-82.4 % and 11.4-35.5 % for the same particle diameter range when the applied voltage was reduced to 3.9 and 3.8 kV, respectively. Based on these experimental results, it is certain that the present wet ESP can be operated to efficiently control fine and nanosized particles at an aerosol flow rate of 5 L/min, applied voltage of 4.3 kV, and scrubbing water flow rate per collection surface area of 2.31 L/min/m².

Figure 4.4 shows the collection efficiency of the present wet ESP for corn oil particles in the size range of 16.8 to 615 nm under different applied voltages at the aerosol flow rate of 10 L/min. The collection efficiency can be increased from 86.9-99.4 % to 98.2-100 % when the applied voltage was increased from 4.3 to 4.9 kV. This result suggested that the increase of aerosol penetration due to the decrease of residence time of particles in ESPs can be diminished by increasing the applied voltage. The spark over occurs when the applied voltage is increased to 5.5 kV, which is the operation limitation for the present wet ESP.

In the present wet ESP, high particle collection efficiency (99 %) was found for small nanoparticles (16.8 to 29.4 nm) with an applied voltage of 4.3 kV. This is attributed to the high electrostatic precipitation efficiency ηelec (dp) and diffusive deposition ηdiff (dp) in this size range, which can be calculated by the following equations:

%

% 100 )(%)

(  ^, 



in OFF out in p

diff C

C d C

 (4.2)

where C_out,OFF is the outlet particle number concentration of the wet ESP without supplying high voltages. As can be seen in Figure 4.5, the electrostatic precipitation efficiency decreased from 99.4 % to 97.2 % when the particle diameter increased from 16.8 to 615 nm.

The diffusive deposition was found to be negligible for particles greater than 29.4 nm, and it increased from 4.0 to 17.4 % as particles decreased from 29.4 to 16.8 nm. These results demonstrate that diffusive deposition plays an important role for very small nanoparticles in the wet ESP.

100 200 300 500 50

30 20

Particle diameter (nm) 0

20 40 60 80 100

Collection efficiency (%) 3.8 kV

3.9 kV 4.1 kV 4.3 kV

Figure 4.3 Collection efficiency of the present wet ESP for corn oil particles under different applied voltages. The aerosol flow rate is fixed at 5 L/min. Each test was repeated 6 times.

100 200 400 600 80

60 40 20

Particle diameter (nm) 70

80 90 100

Collection efficiency (%)

Applied voltage (kV) 4.3

4.5 4.7 4.9

Figure 4.4 Collection efficiency of the present wet ESP for corn oil particles under different applied voltages. The aerosol flow rate is fixed at 10 L/min. Each test was repeated 6 times.

100 200 300 500 50

30 20

Particle diameter (nm) 0

20 40 60 80 100

Collection efficiency (%)

Diffusive deposition (%)

Electrostatic precipitation (%) Total collection efficiency (%)

Figure 4.5 Electrostatic precipitation and diffusive deposition efficiencies of the polydisperse corn oil particles in the present wet ESP when the aerosol flow rate and the applied voltage are 5 L/min and 4.3 kV, respectively. Each test was repeated 6 times.