I. INTRODUCTION

The economical and cost effective diesel emission control system is mandated to meet the upcoming stringent regulations. The regulation for automobile diesel particulate matter (PM) emission was 0.01 g/kWh and NOx was 0.7 g/kWh by the year 2009. On the other hand, the marine engine regulation was 0.2~0.4 g/kWh for PM, 7.2~7.4 g/kWh for NOx, and 5 g/kWh for CO in the US and 1.0 g/kWh for PM, 9.8 g/kWh for NOx and 5.0 g/kWh for CO in Europe and Japan by MARPOL treaty in 2005. More stringent regulations are forced by TEER-3 by 2011 and TEER-4 by 2016 (80% of NOx reduction at the present level). The diesel emissions are from various sources such as automobiles, marine engines, power generation engines, and construction machines. The particles from diesel engine emission are low resistivity in nature, are very small in the range of 70~120 nanometers (nm) and can be penetrated into alveolus being extremely harmful to human health. The diesel particulate filter (DPF) was widely used for the collection of automobile diesel PM but was not economical, especially for the use of marine engine emission control where PM concentration is as high as 40 mg/m3. As an alternative technology, the conventional electrostatic precipitator has also problem for collection due to the reentrainment of low resistive particles (PM). The low resistive diesel PM, which is less than 1×105 ohm-cm, are detached from the collection plate where the electrostatic repulsion force due to induction charge exceeds particle adhesion force on the collection electrode, especially significant for large particles.

There are few literatures describing the control of particle reentrainment by electrostatic processes [1, 2]. Recently, two-stage ESP having particle charging zone by DC field, followed by the collection zone by applying a low frequency AC field or trapezoidal waveforms in the range of 1-20 Hz has been investigated for the collection of particles in a tunnel [3, 4], while the conventional ESP utilizes DC high voltage. The wet ESP was another strong candidate for this application but it creates the need for water treatment to be used. Based on reentrainment theory [5-8], the new electrohydrodynamically-assisted ESP (EHD ESP) was developed to overcome the reentrainment problem in the ESP [9, 10]. The EHD ESP utilizes the ionic wind to transport the charged particles effectively into the zero electrostatic field zone (or pocket zone) attached to the collection plate. The captured particles are trapped in the pocket or zero electric field, where no electrostatic repulsion force by induction charge is exposed. This is the major factor for the reduction of particle reentrainment. The effectiveness of the EHD ESP was demonstrated to show the significant suppression of particle reentrainment [9, 10]. As for NOx control, the conventional process such as a selective catalytic reduction (SCR) system was widely accepted but has practical limitations due to diesel engine start-up and low gas temperature operation such as bus or the use of turbo machine (<250oC). Ammonia leak and sulphur poisoning on catalysts are other factors. Laboratory and pilot-scale plasma-chemical hybrid processes for NOx removal have been investigated over the years and demonstrated to achieve a high NOx removal efficiency with negligible reaction byproducts [11, 12]. The operating cost was significantly lower than the selective catalytic reduction (SCR) system. The radical injection (often referred to as indirect plasma or remote plasma) means that air radicals are externally produced at ambient temperature and pressure, and

Diesel Emission Control using EHD-Assisted Electrostatic Precipitator Combined with Plasma Processes

T. Yamamoto, T. Mimura, S. Asada, and Y. Ehara

Department of Electrical and Electronic Engineering, Tokyo City University, Japan

Abstract—Diesel emissions consist of mainly particulate matter (PM) and NOx. The collection of low resistive PM

generated from marine and automobile diesel engines has been known to be difficult by the conventional electrostatic precipitators (ESPs) because of particle reentrainment. We developed an electrohydrodynamically-assisted ESP (EHD ESP) to prevent reentrainment caused by low resistive diesel PM. The EHD ESP showed an excellent collection efficiency for particle size ranging up to 4,000 nm and demonstrated significant reentrainment suppression, while the conventional ESP showed the severe reentrainment for particle size greater than 1,000 nm, resulting in negative collection efficiency. We also developed an extremely cost effective NOx removal system using adsorption and desorption, followed by nitrogen plasma processes. More than 90% NO reduction was achieved using a series of 6 and 12 surface discharge units. The energy efficiency was 3.4 g(NO2)/kWh with SV=24,000 hr-1, which is approximately 8.5 times more effective compared with 100 ppm of continuous plasma operation or at least one order of magnitude of energy cost of the conventional selective catalytic reduction (SCR) process. The EHD ESP combined with plasma process leads towards more economical and attractive diesel emission control.

Keywords—Diesel emission, PM, NOx, plasma, EHD ESP, air pollution

Corresponding author: Toshiaki Yamamoto e-mail address: yama-t@tcu.ac.jp Presented at the Seventh International Symposium on Non-Thermal/Thermal Plasma Pollution Control Technology & Sustainable Energy, ISNTP-7, in June 2010

Yamamoto et al. 31

injected into the hot flue gas. It is extremely effective for NO oxidation especially when the flue gas temperature exceeds 300oC where NOx is generated when the plasma is applied at this temperature. The radical injection methods for the purpose of NO oxidization and reduction have been investigated using ozone (O3), ammonia (NH3), nitrogen (N2), and methane (CH4) and N2 mixture in both laboratory-scale and pilot-scale experiments [12-23]. To achieve more economical NOx reduction process, the concentration technique using adsorption and desorption, followed by NOx reduction process using N2 plasma was developed. The concentration technique, that is, the conversion of low concentration with high gas-flow rate to the high concentration with low gas flow rate, is able to achieve a significant reduction of the reactor size, power supply and power consumption. When the adsorbent was saturated with NO, a small amount of nitrogen gas flows through a series of 6 or 12 surface discharge plasma units to reduce a high concentrated NO flue gas to N2 and O2. The adsorption and desorption were repeated to confirm the regeneration of adsorbent and also to sustain superior NO reduction over the period. These PM and NOx reduction system was combined to achieve an economical and attractive diesel emission control.

II. EXPERIMENTAL SETUP

The total experimental system for controlling the

diesel PM and NOx emission is shown in Fig. 1. The diesel engine flue gas flows though the adsorbent tower to desorb the adsorbed NOx thermally by the hot flue gas. During the desorption process, small amount of N2 was introduced to achieve a high concentration of NOx, which is treated by the surface discharge plasma reactor. Then, the flue gas goes through the EHD ESP to remove PM from the diesel emission and then the adsorbent tower to adsorb the remaining NOx. The cleaned diesel emission is discharged to the atmosphere. When the adsorbent is saturated by NOx, the adsorbent tower is replaced with regenerated adsorbent. Typical adsorption time was 30 min, while NOx desorption time was in the order of 5 min. The adsorbent was preheated before desorption and cooled after regeneration.

A 2.3 L diesel engine using heavy oil of grade A (Hokuetsu Kogyo, PDS175S) was used for the use of compressor. The engine runs at 2,000 rpm, which the load was not able to change. The constituents of the diesel PM measured are 99% of C, 0.1% of Si, 0.07% of Fe, 0.1% of Ca, 0.4% of S, and 0.03% of Zn. In order to determine the number particle density in the ESP, the flue gas was diluted approximately 1,000 times by ambient air. Particle size-dependent number density before and after the EHD ESP was determined by a

Fig. 1. Diesel emission system using EHD-assisted ESP for PM collection, combined with nitrogen plasma system for NOx removal.

Discharge electrode

60mm

60 mm60 mm 15 mm

Pocket

Gas Flow

GND

H.V.

15mm

10mm

210mm

Discharge electrode

60mm

60 mm60 mm 15 mm

Pocket

Gas Flow

GND

H.V.

15mm

10mm

210mm

15mm

10mm

210mm

Fig. 2. Schematic diagram for EHD-assisted ESP.

32 International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011

Scanning Mobility Particle Sizer (SMPS, Model 3034) for the particle size ranged 20-800 nm and the particle counter (PC, Rion KC-01C) for the particle size of 300-5,000 nm, respectively. The gas velocity at the duct of the EHD ESP inlet was measured by a hot wire anemometer (Kanomax).

The EHD ESP used for this experiment is shown in Fig. 2 and their dimensions were designated in the figure. The EHD ESP consists of five teeth shaped electrodes and the collection plate with six pockets. The channel width was 60 mm and its effective height was 200 mm. The 10 mm deep and 20 mm long pocket are attached to the collection plate with every 60 mm interval. The discharge electrode was the saw type and their teeth were equally spaced with the interval of 10 mm. The distance between the discharge electrode and the upstream backend of the pocket was maintained at 20 mm. The overall dimension of ESP section was 300 mm high and 420 mm in the gas direction without hopper and inlet and outlet transitions. The flue gas was connected to the inlet and outlet of the ESP through a transition where 50% opening perforated plates were placed to achieve a uniform flow at the EHD ESP inlet. The bottom section of the EHD ESP has a hopper section with buffer plates, so that particle sneakage was minimized. Sneakage was defined that the gas sneaks through the upper and the lower section of the ESP without gas treatment. At the same time the fraction of particle collection can be measured for each electrode section. The top section was made out of plexiglass for visualizing EHD flows and particle transport phenomena in the EHD ESP.

As for NOx reduction system, the NO/N2, N2, and zero air were prepared and the flow controller was set at each line to obtain the desired flow rate and concentration. The adsorption/desorption chamber is made out of a 13 mm diameter stainless steel tube which contains 5.0 g of molecular sieve pellets coated with Cu and MnO2 catalysts. The heating tape was wrapped around the stainless steel adsorption chamber. The flow rate in the adsorption process was set at 1.0, and 2.0 L/min, while the flow rate in the desorption process was set at 0.4 L/min. N2 gas was used for thermal desorption from adsorption/desorption chamber and gas temperature was set at 200oC for thermal desorption to achieve a high concentrated NO. The surface discharge consists of a series of 6 and 12 units, which were reduced to NO to N2. The input power of 6 and 12 units of the surface discharge power supply were 75 and 150 watts, respectively.

III. RESULTS AND DISCUSSION Fig. 3 shows the particle-size dependent number

density for the EHD ESP, which was measured by SMPS when the applied voltage was V = -10 kV and the gas velocity was 2.0 m/s. Note that the spark-over voltage was 15 kV. One order of magnitude reduction was achieved for particle size in the range of 30-500 nm. The particle size-dependent number density measured by the

PC was in the range of 300-5,000 nm and the results were shown in Fig. 4. The collection efficiency was excellent up to 300-1,000 nm but decreased as particle size increased. When particle size was greater than 3,000 nm, the collection efficiency was slightly negative, indicating particle agglomeration, reentrainment and recapture in the EHD ESP. From the previous work the conventional ESP showed significantly lower collection efficiency for particle size greater than 1,000 nm due to particle reentrainment [10]. Note that the number density at 3,000-5,000 nm for EHD ESP was small so that the fluctuation of efficiency was significant.

Figs. 5 and 6 show the corresponding weight-base particle density distribution. The collection efficiency of 92.9% was achieved for the particle size measured by SMPS and 89.5% by PC. The overall collection efficiency was 92.7% for the EHD ESP. Negative collection efficiency indicated that the agglomerated large particles captured at the electrostatic field exposed were detached and reentrained by the repulsion force caused by the induction charge. After 20 min of operation, the majority of particles were collected in the first two stages of the EHD ESP and approximately 70% of particles were captured in the pockets visually. Fig. 7 shows the view of particle collection on the discharge

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

10 100 1000Particle size (nm)

Num

ber

part

icle

/Δｌ o

gD (

part

s/cm

3 )

upstream

downstream Vo=2.0 m/s V=-10 kV

Fig. 3. Particle size-dependent number density for the EHD ESP, measured by the PC when V = -10 kV and Vo = 2.0 m/s.

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

100 1000 10000Particle size (nm)

Num

ber

part

icle

/Δｌ o

gD (

part

s/cm

3 ) upstream

downstream Vo=2.0 m/s V=-10 kV

Fig. 4. Weight-base Particle density distribution for EHD ESP measured by SMPS when V = -10 kV and Vo = 2.0 m/s.

Yamamoto et al. 33

electrodes, looking upstream from the EHD-ESP outlet. It is clear that there is little particle deposition on electrodes after third electrodes, indicating that the EHD ESP showed significant reduction of reentrainment over the conventional ESP.

Regarding NOx reduction, we need to confirm that N2 plasma using the surface discharge was able to reduce a high concentration of NOx effectively. Fig. 8 shows NO removal efficiency for the concentration ranging from 100 to 4,000 ppm. Six and 12 surface discharge units were used at the applied voltage of V = -12 kV, while the flow rate was varied between 1 and 2 L/min,

respectively. The NO removal efficiency was maintained at approximately 90% regardless of NO concentration when the flow rate was 1 L/min. When the flow rate was increased to 2 L/min, NO removal efficiency was somewhat reduced to about 60-80%, irrespective of concentration. The concentration technique using adsorption and desorption NOx treatment allowed reduction in the reactor size and power consumption.

A 300 ppm concentration of NO with flow was selected for a more realistic NO removal process. A 5.0 g

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

10 100 1000Particle size (nm)

dM/d

logD

(μg/

m3 )

upstream

downstream Vo=2.0 m/s V=-10 kV

Fig. 5. Weight-base particle density distribution for EHD ESP measured by SMPS when V = -10 kV and Vo = 2.0 m/s.

1.0E+01

1.0E+02

1.0E+03

1.0E+04

100 1000 10000Particle size (nm)

dM/d

logD

(μg/

m3 )

upstream

downstream Vo=2.0 m/s V=- 10kV

Fig. 6. Weight-base particle density distribution for EHD ESP measured by PC when V = -10 kV and Vo = 2.0 m/s.

Fig. 7. View of particle collection on discharge electrodes, looking upstream from the EHD-ESP outlet.

0

20

40

60

80

100

100 1000 10000NO concentration (ppm)

NO

Rem

ova

l Eff

icie

ncy

(%)

1 L/min (6 units)

2 L/min (6 units)

1 L/min (12 units)

2 L/min (12 units)

Fig. 8. NO removal efficiency for NO concentration, ranging from100 to 4,000 ppm.

0

500

1000

1500

2000

2500

3000

0 30 60 90 120 150 180 210

Time (min)

NO

con

cent

rati

on (

ppm

)

Adsorption

Desorption

Removal by 6 units

Removal by 12 units

Fig. 9. NO concentration with 5 repetitive adsorption and desorption, followed by N2 plasma reduction with the flow rate of

1.0 L/min using adsorbent A.

0

500

1000

1500

2000

2500

3000

0 30 60 90 120 150 180 210Time (min)

NO

con

cent

rati

on (

ppm

)

Adsorption

Desorption

Removal by 6 units

Removal by 12 units

Fig. 10. NO concentration with 5 repetitive adsorption and desorption, followed by N2 plasma reduction with the flow rate of

2.0 L/min using adsorbent A.

34 International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011

of adsorbent was used for a 13 mm diameter tube with the aspect ratio of 2.8. The flow rate was set at 1.0 L/min for adsorption and 0.4 L/min for N2 thermal desorption, which results in the space velocity (SV) of 12,000 hr-1. Adsorption time was stopped when NO concentration reached 10% of the initial NO concentration, while desorption time was at 10 min throughout the experiments with the flow rate of 0.4 L/min after the adsorbent temperature at 200oC was reached. This adsorption and desorption process was repeated 5 times. The NO concentration increased as adsorption and thermal desorption repeated as shown in Fig. 9. The results of NO concentration with N2 plasma with 6 and 12 surface discharge units was shown in the same figure

when 6 and 12 surface discharge units were used. An excellent NO reduction was achieved for treating greater than 2,000 ppm NO for five repetitive operations. When the flow rate was increased to 2.0 L/min, the adsorption time was decreased to about one half although the adsorption characteristics were significantly reduced in the range of 1,500 ppm as shown in Fig. 10. Accordingly, SV value was increased to 24,000 hr-1. Although the maximum NO concentration was reduced, NO removal efficiency was significantly improved.

When the adsorbent was changed from a cylindrical shape (A) to a spherical shape (B), the adsorption characteristics were almost doubled. Fig. 11 shows the NO concentration for five consecutive adsorption and desorption repetitive processes when the flow rate was 1.0 L/min. Fig. 12 shows the NO concentrations with N2 plasma with 6 and 12 surface discharge units. Although desorption characteristics were significantly reduced, the overall NO removal efficiency was about 90% regardless operating conditions. Figs. 13 and 14 show NO concentrations with five consecutive adsorption and desorption processes, and plasma reduction, respectively when the flow rate was increased to 2.0 L/min. A similar trend for the case of 1 L/min was observed. Energy efficiency for various operation modes was summarized in Fig. 15. The results indicated that the use of the adsorbent B gives the highest energy efficiency, which is

0

300

600

900

1200

1500

1800

0 100 200 300 400 500

NO

con

cen

trat

ion

(p

pm

)

Time (min)

Adsorption

Desorption

Fig. 11. NO concentration with 5 repetitive adsorption and desorption with the flow rate of 1.0 L/min using adsorbent B.

0

300

600

900

1200

1500

1800

0 100 200 300 400 500Time (min)

NO

con

cen

trat

ion

(p

pm

)

AdsorptionRemoval by 6 unitsRemoval by 12 units

Fig. 12. NO concentration with 5 repetitive adsorption and N2 plasma NO reduction with the flow rate of 2.0 L/min using

adsorbent B.

0

300

600

900

1200

1500

1800

0 50 100 150 200 250 300Time (min)

NO

con

cen

trat

ion

(p

pm

)

AdsorptionDesorption

Fig. 13. NO concentration with 5 repetitive adsorption and desorption with the flow rate of 1.0 L/min using adsorbent B.

0

300

600

900

1200

1500

1800

0 50 100 150 200 250 300

NO

con

cen

trat

ion

(ppm

)

Time (min)

AdsorptionRemoval by 6 unitsRemoval by 12 units

Fig. 14. NO concentration with 5 repetitive adsorption and N2 plasma NO reduction with the flow rate of 2.0 L/min using

adsorbent B.

0

0.5

1

1.5

2

2.5

3

3.5

4

6 units 1.0 L/min

6 units 2.0 L/min

12 units 1.0 L/min

12 units 2.0 L/min

Ene

rgy

Yie

ld (

g(N

O2)

/kW

h)

w/o adsorbentMS 13X(A)MS 13X(B)

Fig. 15. Comparison of energy efficiency in g(NO2)/kWh between adsorbent A and B.

Yamamoto et al. 35

3.4 g(NO2)/kWh when 6 units of surface discharge units with the flow rate of 1.0 L/min were performed. The energy yield can be increased by improving the heat transfer of the adsorbent. Much higher NO regeneration can be achieved by increasing the adsorbent regeneration temperature as high as 250oC without destroying the adsorption characteristics of the adsorbent.

IV. SUMMARY

The innovative diesel PM and NOx control system was developed using combined EHD ESP and N2 plasma processes. The EHD ESP demonstrated more than 90% collection efficiency and showed a significant suppression of particle reentrainment. As for NOx treatment, the concentration technique followed by the surface discharge plasma reactors was able to achieve more than 90% NO removal efficiency. More than 8.5 times higher energy efficiency was achieved in comparison with the continuous plasma operation. These combined process leads to extremely compact, energy efficient and practical diesel PM and NOx treatment system possible.

ACKNOWLEDGMENT

Authors wish to thank you for support by Grant-in-Aid for Scientific Research (B) of the Japanese Society for the Promotion of Science.

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