Collection Characteristic of Nanoparticles Emitted from a Diesel Engine with Residual Fuel Oil and Light Fuel Oil in an Electrostatic Precipitator

: The purpose of this study was to investigate the collection characteristics of nanoparticles emitted from a diesel engine in an electrostatic precipitator (ESP). The experimental system consisted of a diesel engine (400 cc) and an ESP; residual fuel oil and light fuel oil were used for the engine. Although, the peak value of distribution decreased as the applied voltage increased due to the electrostatic precipitation e ﬀ ect, the particle concentration, at a size of approximately 20 nm, increased compared with that at 0 kV, in the exhaust gas, from the diesel engine with residual fuel oil. However, the e ﬃ ciency increased by optimizing the applied voltage, and the total collection e ﬃ ciency in the exhaust gas, using the residual fuel oil, was 91%. On the other hand, the particle concentration, for particle diameters smaller than 20 nm, did not increase in the exhaust gas from the engine with light fuel oil. ﬃ ciency 65%. This result shows that an optimum voltage exists for collection e ﬃ ciency. This is probably due to ion-induced and binary homogeneous nucleation, as already described.


Introduction
Nano-particles are included in various diesel exhaust gases from diesel automobiles [1], power generation engines, marine diesels and construction machines. These particles can penetrate into alveolus, and are harmful to human health. Thus, electrostatic precipitators (ESPs) have been used or developed.
ESPs have been used for cleaning gas, which was emitted from diesel automobiles, in road tunnels [2]. The collection efficiency estimated by dust weight reached 90 % at a gas speed of 9 m/s. Ehara et al. investigated to collect nano-particles emitted from a diesel engine with light fuel oil in order to improve an ESP for road tunnels, and the efficiency for nanoparticle whose size was between 20 and 800 nm was greater than 90 % at a gas speed of 10 m/s [3].
ESPs have also been developed for power generators, form engines, construction machines or marine diesels. Although an ESP combined with an after-cyclone dust collector [4], a system of a mechanical filter located after an ESP [5] and the electrostatic cyclone DPF for marine engine [6] were proposed, nano-particle collection efficiency was not investigated. Yamamoto and Ehara suggested an EHDassisted ESP [7] and a hole-type ESP [8], respectively. They measured collection efficiency for nano-particles emitted from a diesel engine with light fuel oil. Authors investigated nano-particle collection efficiency for exhaust gas from a diesel engine with residual fuel oil [9]. As described so far, there are some literatures for nano-particle collection efficiency. However, influences of the kind of fuel oil, applied voltage and electrode length on nano-particle collection efficiency were not investigated, and the efficiency for the particle smaller than 20 nm was not also investigated.
In this study, the experiment was carried out using exhaust gases from a diesel engine with residual and light fuel oils to clarify the collection characteristic of nano-particles in an ESP.

Experimental Setup
The schematic of the experimental system is shown in Fig. 1. The system consists of a water-cooled 4-cycle diesel engine (DA-3100SS-IV, Denyo Co., Ltd; displacement, 400 cc; output, 5.5 kW; Load, 0%) and an ESP. Residual fuel oil ((ENEOS LSA fuel oil. sulphur content: 0.61 %) and light fuel oil (ENEOS Light fuel oil; sulphur content: 0.0009%) were used. The temperature of the exhaust gas was between 130 o C and 150 o C, and the wind velocity inside the ESP was approximately 4.4 m/s. Fig. 2 shows the structures of the ESP. Type A has a coaxial cylinder structure consisting of high voltage needle electrode (Stainless. D: 1.6 mm, point angle: 14 o , radius of curvature: 22 m) and the grounded case (Stainless. L: 80 mm, inner diameter: 58 mm). Negative DC high voltage of maximum -18 kV was applied to the needle electrode. Type B has a coaxial cylinder structure consisting of high voltage application wire electrode (Tungsten, diameter: 0.26 mm) and the grounded case (Stainless, L: 80, 130 mm. inner diameter: 58 mm). Negative or positive DC high voltages of maximum 10 kV was applied to the wire electrode.
A portion of the exhaust gas was drawn (2.7 L/min) to measure the particle size distribution. The temperature of the sampling tube was controlled by the tape heater so that it would be equal to the temperature of the drawn gas, in order to prevent the natural cooling of the sampling tube which may possibly cause condensation inside the tube and change the ratio of components. The sampled gas was cooled to room temperature after 10-fold dilution by the diluter at the same temperature as the exhaust gas. The particle size distribution was measured using a scanning mobility particle sizer (TSI, Model 3936). The SMPS can measure the concentration for the particle size between 6 nm and 200 nm. The collection efficiency  was calculated by equation (1).
where, N is the particle number concentration [part/m 3 ] after applying the voltage, and N0 is the particle number concentration [part/m 3 ] before applying the voltage.

Influence of fuel oil on distribution
The relationship between the applied voltage and the discharge current in Type A, which is the needlecylinder structure, in the exhaust gas when the residual fuel oil was used is shown in Fig. 3. The corona onset voltage was approximately -5 kV, and the spark voltage was approximately -19 kV. Thus, the experiment was carried out at voltages between -6 kV and -18 kV in the needle-cylinder structure. The size distribution for various applied voltage is shown in Fig. 4. The distribution at a voltage of 0 kV had a peak of approximately 6.0x10 6 part/cm 3 at a size of 57 nm. The peak value decreased with increasing applied voltage due to electrostatic precipitation, and the value at 57 nm was 2.1x10 5 part/cm 3 at -16 kV. The collection efficiency was greater than 95%. However, the value at a size of 27 nm increased to 2.2x10 5 part/cm 3 at -18 kV from 4.6x10 4 part/cm 3 at -16 kV.
Thus, the experiment using the light fuel oil was carried out in order to investigate this cause. The particle size distribution for various applied voltage when the light fuel oil was used is shown in Fig. 5. The distribution at the voltage of 0 kV had a peak of 6.7×10 6 parts/cm 3 at the size of 71 nm. At the voltage of -18 kV, the particle concentration at 71 nm decreased to 1.5×10 5 parts/cm 3 due to the electrostatic precipitation effect. Furthermore, the particle concentration for the size of approximately 27 nm did not increase in comparison with Fig. 4.   It is known that fine particles are generated by ioninduced and binary homogeneous nucleation when corona discharge is generated in the gas including SO2 [10]. Analysis of the components in the exhaust gas revealed that the SO2 concentration in the exhaust gas using residual fuel oil was 37 ppm, whereas it was 2 ppm in the case of light fuel oil. Therefore, the increase of the concentration for the particle size of approximately 27 nm in the exhaust gas using residual fuel oil may be due to ion-induced and binary homogeneous nucleation.
However, the total number collection efficiency in the exhaust gas using the residual fuel oil was as high as 87%, and the ion-induced nucleation did not affect the total collection efficiency.

Influence of polarity on distribution
It was shown in Figs. 4 and 5 that the particle concentration at the size of approximately 27 nm increased due to corona discharge. Thus, the influence of the polarity of the applied voltage in the exhaust gas when residual fuel oil was used was investigated. The experiment was carried out using Type B, which was wire-cylinder structure ESP. The electrode length was 80 mm.
The relationship between the applied voltage and the discharge current when negative and positive voltages were individually applied is shown in Fig. 6. The corona onset voltages with both polarities were approximately 6 kV, and the spark voltages were approximately 11 kV. The discharge current in negative polarity was greater than that in positive polarity.
The size distribution for various applied voltages in negative polarity is shown in Fig. 7. The distribution at the voltage of 0 kV had a peak value of 7.0x10 5 part/cm 3 at a size of 76 nm. The peak value decreased as the applied voltage increased, and that was 2.7x10 4 part/cm 3 at -10 kV due to electrostatic precipitation effect. However, the concentration at the size of 20 nm at -10 kV increased to 3.4x10 4 part/cm 3 .
The size distribution for various applied voltages in positive polarity is shown in Fig. 8. The overall tendency was similar with the result in negative polarity as shown in Fig. 7.
The comparison between the results of negative and positive polarities at the voltage of 10 kV is shown in Fig. 9. Fig. 9-a shows the comparison of the size distributions. In this figure, the size distributions with both polarities at 0 kV was not the same, so it is difficult to investigate the influence of the polarities on the increase in the particle concentration of approximately 20 nm. Therefore, the enhancement rate  is defined as equation (2). (2) where, N is the particle number concentration [part/m 3 ] after applying the voltage, and N0 is the particle number concentration [part/m 3 ] before applying the voltage. Fig. 9-b shows the comparison of the enhancement rates between negative and positive polarities. The maximum value of enhancement rate at negative polarity was 29 at a size of 16 nm. That at positive polarity was 22 at a size of 13 nm. This result indicates that the increase of fine particles due to corona discharge in negative polarity is greater than that in positive polarity. This is most likely because the discharge current in the negative corona is greater than that in the positive corona at the same voltage as shown in Fig. 6.

Collection efficiency
Although the collection efficiency for the total number of particles is high as describe so far, the increase of fine particles is undesirable for an ESP. Thus, the effects of the applied voltage and the electrode length on the efficiency were investigated. The polarity of the voltage was negative.
The collection efficiency as a function of particle diameter for various applied voltage with the electrode length of 80 mm is shown in Fig. 10. The collection efficiency at a voltage of -10 kV decreased with decreasing the particle diameter, and had a minimum value of approximately -2900 % at the particle size of 16 nm. The negative collection efficiency means that the particle number concentration after the application of the voltage is greater than before. Thus, the concentration at 16 nm increased to 30 times after the application of the voltage of -10 kV. However, the efficiency at the size of approximately 16nm increased as the applied voltage decreased. The total number collection efficiency as a function of applied voltage is shown in Fig. 11. The efficiency increased with increasing applied voltage, and reached 91 % at -8 kV, while the efficiency at -10 kV was 65 %. This result shows that the optimum voltage exists for the collection efficiency. This is probably due to ion-induced and binary homogeneous nucleation as describe so far.
The collection efficiency as a function of particle diameter for the electrode lengths of 80 mm and 130 mm is shown in Fig. 12. The collection efficiency at the electrode length of 80 mm decreased with decreasing the particle diameter. On the other hand, the collection efficiency at 130 mm significantly improved due to the increased electrode length in comparison with that at 80 mm, although it was negative efficiency. Therefore, the increase of fine particles can be dramatically suppressed by increasing the collection electrode length. a) Size distribution b) Enhancement rate Fig. 9 Comparison between results of negative and positive polarities.

Conclusion
The experiments were carried out to investigate the collection characteristic of nano-particles emitted from diesel engine with residual fuel oil and light fuel oil in the ESP. Results are follows; 1) The peak concentration at 57 nm of the size distribution in the exhaust gas from residual fuel oil decreased with increasing the applied voltage. However, the fine particle concentration at the size of 27 nm increased. On the other hand, fine particle concentration did not increase in the exhaust gas when light fuel oil was used. These results indicate that the increase of the fine particle concentration in the exhaust gas using residual fuel oil may be due to SO2 in the gas.
2) The amount of increase of fine particles due to corona discharge in negative polarity is greater than in positive polarity at the same voltage. 3) An optimum voltage to suppress the fine particle concentration exists for the collection efficiency.
Although increase of fine particle concentration due to an ESP, this can be easily suppressed by increasing the collection electrode length.