Download Systematic Approach to Optimization of Submicron Particle

Aerosol and Air Quality Research, 15: 2709–2719, 2015
Copyright © Taiwan Association for Aerosol Research
ISSN: 1680-8584 print / 2071-1409 online
doi: 10.4209/aaqr.2015.06.0418
Systematic Approach to Optimization of Submicron Particle Agglomeration Using
Ionic-Wind-Assisted Pre-Charger
Qianyun Chang1, Chenghang Zheng1, Xiang Gao1*, Penchi Chiang2, Mengxiang Fang1,
Zhongyang Luo1, Kefa Cen1
1
2
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, China
Graduate Institute of Environmental Engineering, National Taiwan University, Taipei, Taiwan
ABSTRACT
Electric agglomeration is the process in which particles are charged in electric fields and coagulate, which is enhanced
by electric force or turbulence. The collection efficiency of submicron particles is improved, which can be a solution to
submicron particle abatement in traditional electrostatic precipitators (ESPs), by using a pre-charger to increase median
particle diameter and realizing particle pre-charging. In this study, a laboratory bipolar pre-charger with a perforated plate
between discharging regions was designed to examine ionic-wind-assisted charge-induced agglomeration, and an ESP was
arranged afterwards to collect the fine particles. Experiments were conducted to investigate submicron particle charging,
agglomeration characteristics, and collection efficiency. Results indicated that a pre-charger with proper discharging
voltage match and plate porosity can optimize particle agglomeration and improve collection efficiency by about 12%
compared with the results obtained without a pre-charger. An optimal solution was achieved and a collecting efficiency of
96%–98% was obtained for all sizes by utilizing the turbulence caused by ionic wind and optimizing the experimental
operation conditions.
Keywords: Bipolar pre-charger; Electric agglomeration; Particulate matter; PM2.5; Electrostatic precipitator.
INTRODUCTION
Particulate matter pollution is a serious global problem
because fine particles can remain suspended in air and
spread with atmospheric motion, causing permanent health
and environmental damage(Kagawa and Ishizaka, 2014).
Particles less than 1 µm in diameter can penetrate the alveoli
where gas exchange occurs (Londahl et al., 2006; Cao et
al., 2013). These particles behave similarly to gas molecules
and affect gas exchange within the lungs, penetrating the
lung into the blood stream and translocating further into
the cell tissue and/or the circulatory system (Valavanidis et
al., 2008), which lead to significant health problems such
as asthma attacks, nonfatal heart attacks, and even cancer
(Gorbunov et al., 2013; Crilley et al., 2014; Kim et al.,
2015). Therefore, the problem of particulate matter pollution
requires urgent resolution.
Electrostatic precipitator (ESP), which is common for
particle abatement in power plants, is characterized by a
high overall mass collection efficiency of above 99.9%
*
Corresponding author.
Tel.: +86-571-87951335; Fax: +86-571-87951616
E-mail address: [email protected]
with large fluxes and low pressure drops (Mizuno, 2000;
Jaworek et al., 2007). However, conventional ESPs are not
sufficiently effective in collecting submicron particles (Le
et al., 2013; Chen et al., 2014; Wang et al., 2015). Thus,
upgrading existing technologies to enhance performance in
terms of removing submicron particles is of great importance.
Several ideas have been proposed to solve this problem
by enhancing particle agglomeration to increase mean particle
size, including acoustic coagulation to increase particle
vibration and agglomeration (Noorpoor et al., 2013), electric
agglomeration to increase particle electrostatic attraction
and agglomeration (Kanazawa et al., 1993; Hautanen et al.,
1995; Kildeso et al., 1995; Laitinen et al., 1996; Ji et al.,
2004; Lin et al., 2011; Thonglek and Kiatsiriroat, 2014),
and water vapor condensation to increase viscosity and
agglomeration (Stairman, 1968), etc. Electric agglomeration
is the process in which particles are charged in direct or
alternate current electric fields, and collide and coagulate
as driven by electric force. The agglomeration procedure is
often strengthened by an external electric field or
turbulators to increase particle collision in addition to that
caused by Brownian motion. Given that particles with
different polarities can attach together to form larger ones,
the collecting efficiency of submicron particles improves
through the pre-charger enhancement.
Considering its simple equipment construction, electric
Chang et al., Aerosol and Air Quality Research, 15: 2709–2719, 2015
agglomeration has been studied and used to clean exhaust
gas. Kanazawa et al. (1993) studied a two-stage ESP with
a bipolar charging section, wherein the collection efficiency
for submicron particles is about 80% under optimum
condition. Maisels et al. (2004) used a developed Monte
Carlo method to simulate the process of simultaneous
charging and coagulation; he demonstrated that particle
size distribution is clearly changed by the coagulation of
high particle number concentrations. Xu et al. (2009) used
a positive pulsed ESP to enhance particle charging and
agglomeration to improve collection efficiency and achieved
a PM2.5 number reduction efficiency that exceeded 90%.
Zhu et al. (2010) designed a closed-loop lab ESP with a
bipolar pre-charger that yielded a grade collection efficiency
of 95%-98%. However, further detailed investigations on
submicron particle agglomeration technologies are still
necessary, and the design of agglomeration equipment must
be optimized.
Ionic wind produced by electrohydrodynamic (EHD)
flow is a noticeable phenomena from another aspect. In
ESPs, the ions created in an inhomogeneous electric field
near the discharge electrodes migrate toward the collecting
electrodes and provide momentum to neutral molecules,
which serve as the driving force to the fluid and cause the
secondary flow, called “ionic wind” (Robinson, 1962). The
ionic wind in ESPs can produce strong turbulence, which
significantly affects the flow field and complicates particle
movement (Mizeraczyk et al., 2003).
A laboratory bipolar pre-charger with a perforated plate
between discharging regions was used in this study to
examine ionic-wind-assisted charge-induced agglomeration.
Afterwards, an ESP was arranged to collect fine particles.
The effects of discharging voltage match and plate porosity
on particle agglomeration and collection were evaluated by
investigating the particle charging characteristics, median
particle diameter, and collection efficiency.
EXPERIMENTAL SETUP AND METHODOLOGY
Experimental Setup
A study on fine particle agglomeration and collection
was conducted in a laboratory ESP combined with a bipolar
pre-charger and a mixing region characterized by an acrylic
duct 20 cm in width, 10 cm in height, and 100 cm in length.
The experimental system (Fig. 1) can be divided into a precharging region, an agglomeration region, and an ESP.
Through the system, particles are positively or negatively
charged in the pre-charger, mixed and coagulated in the
static mixer, and finally collected by a traditional ESP.
Aerosol particles were charged in two separate parallel
corona charger chambers of the pre-charger. Each chamber
consisted of one discharging electrode placed in the middle
of each channel between grounding electrodes. The length,
width, and height of each chamber were 20, 20, and 10 cm,
respectively. The grounded stainless plate length was 6 cm,
and the wire-plate distance was 2.5 cm. Flow velocity was
0.6 m s–1, and particle residence time in the charger was
0.1 s, which ensured sufficient charging time and minimized
particle collection in the pre-charger (Long and Yao, 2010).
Fig. 1. Schematic diagram of the bipolar agglomeration system experimental setup.
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The discharging electrodes were ribbon electrodes made of
stainless steel (Fig. 2). The power supplies used in the precharger and the ESP were high-frequency power supplies
(Telsman, China).
In the agglomeration region, bi-polarly charged particles
were fully mixed through the turbulence caused by the static
mixer. Instead of traditional staggeredly arranged columnshaped pin fins, a set of specially designed inclined placed
cubics served as turbolators to optimize flow mixture and
to promote charged particle collision and agglomeration.
The structure of the static mixer is shown in Fig. 2. The
agglomerator was 20 cm in width, 10 cm in height, and 20
cm in length. The turbolators were 5 cm × 5 cm arylic cubes
with 3 cm thick, and guide plates were added for flow
uniformity.
The ESP, in which a set of four equidistant discharging
electrodes was used to collect enlarged particles, was 20
cm in width, 10 cm in height, and 60 cm in length. The
distance of adjacent discharging electrodes was 10 cm, and
the wire-plate distance was 5 cm.
Furthermore, to utilize the ionic wind generated in the
pre-charger, perforated plates were used between positive
and negative discharging channels so that the ionic-windinduced turbulence could improve the mixture and collision
of bi-polarly charged particles in the pre-charger. The plates
were perforated by round holes in a consistent position with
the diameters of 6, 8, and 10 mm, such that the porosities
of perforated plates were 0%, 9.4%, 16.7%, and 26.2%.
Experiment Procedure
Particles used in the study were practically submicron and
were generated through incense burning. The stick incense
used in the study can burn steadily for more than 30
minutes with sufficient concentration and size distribution
stability. The number of particle distribution is shown in
Fig. 3. To investigate the effect of the pre-charger, power
supplies were first turned on once the particle concentration
was stabilized. The pre-charger performance could be
evaluated by comparing the particle charge and the median
diameter. Second, to investigate the combined effect of the
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pre-charger and the ESP, the power supply of the ESP was
turned on to evaluate the collection efficiency of ESP with
and without the pre-charger in order. All experiments were
performed under ambient conditions. The flow velocity in
the chamber was 0.6 m s–1.
The I-V characteristics of the pre-charger and the ESP
are shown in Fig. 4. In the pre-charger, the positive corona
starts at the voltage 8 kV and breaks down at nearly 17 kV,
whereas the negative corona starts at 6 kV and breaks down
at about 20 kV. In the ESP, the corona starts at 9 kV and
breaks down at about 25.5 kV. The average electric field
strength in the pre-charger and the ESP rose to 6 kV cm–1.
Corona discharge current increased with increasing
discharging voltage. Since the pre-charger in this study has
the functions of both particle pre-charging and particle
agglomeration. We chose the negative voltage to be as
high as possible (usually 80% of range from onset voltage
to the breakdown voltage) to ensure sufficient pre-charging
and operational stability and the voltage of the positive
channel was set from 9 kV to 15 kV (the operation region)
to evaluate the electric agglomeration effect in different
voltage matches. The discharging voltage of 25 kV was
applied to the four ribbon electrodes of the ESP.
Evaluation Methods
To determine the agglomeration effect and final collection
efficiency, ELPI+ (Electrical Low Pressure Impactor,
Dekati Ltd., Finland), a real-time particle size spectrometer
at scales of 6 nm to 10 µm with 14 stages, was used to
measure grade particle concentration. In an ELPI+, particles
were initially charged electrically by small ions produced
in a unipolar positive polarity charger. Afterwards, particle
size was classified in a low-pressure impactor based on their
aerodynamic diameter. The impactor stages were insulated
electrically and each stage was connected individually to
an electrometer current amplifier. The diagnostic current
value in each stage was proportional to the number of
particles and was converted to a size distribution by using
particle size dependent relations that describe the charger
properties and the impactor stages.
Fig. 2. Schematic diagram of (a) ribbon electrodes, (b) perforated plate, and (c) static mixer.
Chang et al., Aerosol and Air Quality Research, 15: 2709–2719, 2015
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Fig. 3. Particle number distribution under a mass concentration of 1.5 mg Nm–3 and a flow velocity of 0.6 m s–1.
Fig. 4. Voltage-current characteristics of (a) pre-charger and (b) ESP (with smooth grounding electrodes).
Therefore, when the initial concentration and outlet
concentration were measured, collection efficiency could
be calculated by Eq. (1).
η=
N inlet − N outlet
N inlet
(1)
where η is the grade collection efficiency, Ninlet is the initial
concentration of a certain diameter in (1 cm–3), and Noutlet is
the outlet concentration at the outlet of the ESP (1 cm–3).
To obtain in-depth insights on particle agglomeration,
the characteristics of fine particle charging by the precharger were evaluated by supposing that the same amount
of chargers were collected for same size particles. The
analytical relation between the number of collected charges
and the ELPI diagnostic current was derived early on as
I
n=
Q× N ×e
(2)
where n is the average charge units on particles, I is the
diagnostic current of ELPI+ with the charger off (A), Q is
the sample volume (cm3 s–1), N is the number concentration
of particles (1 cm–3), and e is the elementary electron
charge (1.602 × 10–19 C).
However, particle concentration or collection efficiency
failed to clearly reflect the agglomeration effects. Considering
that the agglomeration procedure was frequently accompanied
by increased median particle diameter (i.e., production of
enlarged particles), a parameter called Agglomeration
Enhancement Ratio was defined to demonstrate the
enlargement ratio of median particle diameter:
Rae =
Dagg
D0
(3)
where Rae is the agglomeration enhancement ratio, D0 is
the median particle diameter before agglomeration procedure
(µm), and Dagg is the median particle diameter after
agglomeration procedure (µm).
Chang et al., Aerosol and Air Quality Research, 15: 2709–2719, 2015
Median particle diameter D can be calculated by Eq. (4):
14
D=
∑ ( Ni × Di )
i =1
(4)
14
∑ Ni
i =1
where Ni is the particle number of each grade measured by
ELPI+, and Di is the mean particle diameter of each stage.
The agglomeration effects of the pre-charger could be
further analyzed through considering the Agglomeration
Enhancement Ratio. For example, Rae > 1 illustrates an
increase of the main diameter and an achievement of effective
agglomeration.
RESULTS AND DISCUSSION
Particle Charging Characteristics
Ionic wind produced near discharging electrodes can
cause strong turbulence toward grounding plates, which
significantly affects flow field and particle migration. To
utilize ionic wind for agglomeration enhancement, perforated
plates were used between positive and negative discharging
channels. In this manner, the ionic-wind-induced turbulence
can drive the particles toward the grounding plates, move
through the holes on the plate, and penetrate into the other
channel with the opposite polarity. Thus, the chances of
positively and negatively charged particles mixing and
colliding are increased.
The discharging characteristics are shown in Fig. 5.
Note that the porosity of perforated plates significantly
affect the corona current. With rising porosity, the amount
of positive ions that penetrate the negative channel increases,
moving to the opposite direction and resulting in heightened
current density. The current of the negative channel also
increases with positive channel voltage, which indicates
the increase in positive ions were produced and penetrated
the negative channel.
To understand the particle charging procedure in the pre-
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charger, the characteristics of mean particle charge were
investigated by the number of collected charges and ELPI+
diagnostic current, the analytical relationship of which was
derived early through Eq. (2). Typical examples of the
charging properties are shown in Fig. 6. Given the perforated
plate porosity, the collected charges increases with the use
of perforated plates. The particle charging procedure in the
pre-charger is a complicated process which is affected by the
electric field distribution, flow field, particle agglomeration,
ion concentration, etc. Perforated plates enables the ions
produced by positive and negative power supplies move
into the other channel through the plate which would be
collected by the imperforated grounding plate otherwise,
providing a higher chance for uncharged particles getting
charged. On one side, since the amount of negative ions and
the current of negative channel is higher than those of positive
channel, the mean particle charge increased comparing to
smooth plate. On the other side, in agglomeration process,
particles get the charge from smaller particles (the amount
of negative ions is larger than positive ones) without
change of diameter magnitude, i.e., in the same detection
channel, therefore the calculated mean particle charge after
agglomeration should be larger than theoretically the result
without agglomeration. For example, the porosity 9.4% in
this study was proved to be most effective for particle
agglomeration, the mean particle charge unit increased
most after agglomeration, which can be the reason of why
the mean particle charge of porosity 9.4% is usually larger
than that of 16.7%. In considering voltage matches, collected
charges always decrease with increasing applied positive
voltage, which is caused by the neutralization of bipolar
particle agglomeration.
Agglomeration Characteristics
Effect of Applied Voltage
To evaluate and obtain suitable voltage matches for the
pre-charger, as well as ensure that particles were fully precharged, a negative voltage of –15 kV was applied to the
negative channel. Positive voltage ranged from 9 kV to 15 kV
to investigate the effect on agglomeration. The particle
Fig. 5. Effect of positive channel voltage on the current of negative channel.
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Fig. 6. Particle charging characteristics under (a) different plate porosities and (b) different voltage matches.
Fig. 7. Particle percent distributions under different voltage matches.
percentage distributions under different voltage matches
are shown in Fig. 7. The effect of agglomeration can be
observed in the decrease of percentages in the first and
second stages, as well as in the increase of larger particles
in the percentage of other stages, which implies that
smaller particles diminish and become larger particles. The
agglomeration can be further proven by the Rae ratios, which
were all above 1, indicating the improvement of average
particle diameter.
The agglomeration enhancement ratio Rae was compared,
as shown in Fig. 8, to compare and to obtain the appropriate
voltage match. The increase of positive discharging voltage
increased Rae when the positive voltage changed from 9 kV
to 12 kV. This increase suggests that particles with high mean
Chang et al., Aerosol and Air Quality Research, 15: 2709–2719, 2015
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agglomerate on charged particles because of polarization.
However, when overcharged, an excess of positive ions are
produced and those parts of agglomeration are inhibited.
Fig. 8. Agglomeration enhancement ratio under different
voltage matches.
positive charges are produced with high discharging voltages,
which mix better and agglomerate further with negatively
charged particles under the effect of Coulomb force.
However, when the positive discharging voltage was above
12 kV, Rae decreased from 1.33 to 1.10. This finding can
be presumably explained through submicron particles in
the positive channel, where multiple neutral particles can
Effect of Perforated Plate
Four types of plates were used to evaluate the effect of
perforated plate with various porosities. The voltage of the
negative channel was –15 kV, whereas that of the positive
channel was +12 kV. Particle percentage distributions
under different perforated plate porosities are shown in
Fig. 9. With the pre-charger on, the effect of agglomeration is
evident across all porosities, which demonstrates improved
median particle diameter.
To compare the effect of different porosities, the
agglomeration enhancement ratio Rae under different
porosities is shown in Fig. 10. For all cases, the highest Rae
is under the voltage match (+12 kV, –15 kV), which agrees
with previous voltage match results. Considering the
porosities under varying voltage matches, generally perforated
plates with porosities under 16.7% achieved better
agglomeration results compared to regular smooth plates,
while the maximum porosity under nearly every voltage
match obtained the lowest agglomeration enhancement ratio.
This finding indicates that perforated plates with proper
porosities can promote flow exchange and particle collision in
Fig. 9. Particle percent distributions under different perforated plate porosities (discharging voltage match: +12 kV, –15 kV).
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Fig. 10. Agglomeration enhancement ratios with different plate porosities.
discharging regions. Furthermore, high porosity is less
effective probably because large holes in the plates cause
inhomogeneity of the electric field, as well as disturbance
and excessive neutralization with ions. To optimize particle
agglomeration under this experimental situation, the voltage
match (+12 kV, –15 kV) and perforated plate of porosity
9.4% can be chosen as an optimal solution.
Effects of Pre-Charger on ESP Performance
Effects of Pre-Charger on ESP Corona Current
The discharging characteristics between positive channel
voltage and ESP corona current are shown in Fig. 11. The
diagnostic current of ESP falls from 380 µA to 367 µA
with the increase in positive discharging voltage of the precharger. Given that ions and particles with positive charge
can neutralize negative ions and negatively charged particles
produced in the ESP, we can infer that higher positive
channel voltages neutralize more negative ions and particles,
which reduce the collected ions and particles of collecting
electrodes and lead to decreased diagnostic ESP current.
This phenomenon can similarly affect particle charge and
capture processes.
Effects of Pre-Charger on ESP Collection Efficiency
The grade and total collection efficiencies of the ESP
under different applied voltages without the pre-charger are
shown in Fig. 12. The discharging voltage for ESP ranged
from –9 kV to –25 kV. Results show that grade efficiency
increases as discharging voltage rises. Total collection
efficiency was 84.3% at a discharging voltage of –25 kV.
In contrast to the collection efficiency without the precharger application, Fig. 13 shows the typical effects of prechargers on grade collection efficiency. When considering
the voltage matches applied on the pre-charger, the voltage
of negative channel was controlled at -15 kV to ensure that
fully negatively charged particles and positive channel
voltages ranged from +9 kV to +15 kV. The discharging
voltage for ESP was –25 kV to collect agglomerated particles.
Generally, the bipolar corona-induced agglomeration can
improve collection efficiency by 8%−12%. With increased
positive discharging voltage, the grade collection efficiency
increases to the highest at the match (+12 kV, –15 kV), and
then drops down at the match (+15 kV, –15 kV), which is
identical to the agglomeration results. The pre-charger also
promotes particle charging in advance. Thus, negatively
pre-charged particles are collected in the ESP faster, which
indicates better collection efficiency compared to normal
ESP. Furthermore, increased positive discharging voltage
is adverse to particle pre-charging (as discussed in section
3.1), such that the discharging voltage match of (+15 kV,
–15 kV) performed in a manner that was not as effective as
the optimal solution.
Similar results were obtained in the case of porosity.
When comparing the collection efficiency using perforated
plates with different porosities at the match (+12 kV, –15 kV)
proven to be the best match for particle agglomeration, the
highest collection efficiency was achieved at a porosity of
9.4%, and a higher porosity was proven with less effective
probability because of the inhomogeneity of the electric
field, disturbance, and excessive neutralization with ions
caused by large holes on the plate. Utilizing the turbulence
caused by ionic wind and optimizing the experimental
operation conditions resulted in an optimal solution a
collecting efficiency that could be as high as 96%−98% for
all size particles. The total mass collection efficiency was
also at 96.8%. The improvement was nearly 12% compared
with the condition without the pre-charger.
In addition, the wall loss in the experimental system was
about 4% to 10% at different stages and 6% of mass loss in
total with no DC voltage applied. To consider the mass
balance in this study, 6% of particles were attached to the
surface of reactor through diffusion, and the fine particle
collection efficiency was improved to about 84%-96% with
the enhancement of ESP and pre-charger, and the remaining
6% to 14% was released through off-gas.
CONCLUSIONS
According to the presented experimental investigations
on fine particle charge, agglomeration, and collection, the
Chang et al., Aerosol and Air Quality Research, 15: 2709–2719, 2015
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Fig. 11. Effect of positive channel voltage on ESP corona current.
Fig. 12. Collection efficiency of the ESP without pre-charger: (a) grade collection efficiency and (b) total mass collection
efficiency under different discharging voltages.
Fig. 13. Collection efficiency with pre-charger and ESP under (a) different voltage matches and (b) different plate porosities.
following conclusions are obtained.
1) By utilizing ionic-wind-induced turbulence to improve
collision of positively and negatively charged particles,
the current of negative channel increases with the increase
of positive channel positive, as well as the decrease of
mean particle charge with the increasing of positive
discharging voltage.
2) With the use of perforated plates, the corona current in
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Chang et al., Aerosol and Air Quality Research, 15: 2709–2719, 2015
the negative channel increases with the heightened
porosity of the perforated plates, leading to increased
mean particle charges.
3) The positive and negative discharging voltage matches
in the pre-charger can affect agglomeration and
collection procedure. The positive electric field strength at
4.8 kV cm–1 (12 kV) can effectively improve particle
agglomeration because it can increase the median particle
diameter and ensure pre-charged particle collection.
4) The porosity of perforated plates in the pre-charger
with proper porosity can affect the discharging current
and particle agglomeration procedures. The porosity of
9.4% is chosen as an optimal solution for the increased
mean particle diameter by 39%.
5) Given that the current of ESP is inhibited by positive
discharging voltage, it can affect the collection efficiency
in addition to the particle charge and agglomeration
results. An optimal solution is achieved, and the collecting
efficiency increases as high as 96%−98% for all size
particles. Total mass collection efficiency is also
improved by nearly 12%.
ACKNOWLEDGMENTS
This work was supported by the National Science Fund
for Distinguished Young Scholars (No.51125025), the
National Program on Key Basic Research Project (973
Program) (No. 2013CB228504), and the National Hightech R&D Program of China (863 Program) (No.
2013AA065002).
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Received for review, June 24, 2015
Revised, August 30, 2015
Accepted, August 30, 2015