Fundamental study of NOx removal from diesel exhaust gas by dielectric barrier discharge reactor

Journal of Mechanical Science and Technology 26 (6) (2012) 1921~1928

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-012-0402-y

Fundamental study of NOx removal from diesel exhaust gas by dielectric barrier

discharge reactor† Tran Quang Vinh, Shuya Watanabe, Tomohiko Furuhata and Masataka Arai*

Department of Mechanical System Engineering, Gunma University, 1-5-1 Tenjin-cho Kiryu Gunma, 376-8515, Japan

(Manuscript Received January 3, 2011; Revised November 13, 2011; Accepted February 29, 2012)

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Abstract Due to increasingly stringent emission legislation, it is essential to find a solution of eliminating nitrogen oxides (NOx) from diesel ex-

haust gas. Non-thermal plasma (NTP) approach has been studied for years and has shown advantages. In this study, NOx removal from simulated diesel exhaust gas by a dielectric barrier discharge needle-to-cylinder reactor under room temperature condition is presented. The dielectric barrier consists of a diesel particulate filter (DPF) putting in the discharge field. A simulation gas of N2, NO and O2 com-bined with particulate matters (PM) was used as the test gas. PM was loaded from a diffusion flame PM generator. The effect of PM and oxygen fraction in exhaust gas on NOx removal characteristics was investigated experimentally. The results showed that PM promoted NOx removal reactions in the barrier discharge field but its effect was dropped following elapsed time. In addition, for PM composition, soluble organic fractions and sulphate were decreased with NOx removal. Besides, the chemical reaction mechanism inside reactor was discussed.

Keywords: Dielectric barrier discharge; Diesel particulate filter; Non-thermal plasma; NOx; Particulate matter ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Nitrogen oxides, commonly known as NOx, are one of the major air pollutants. NO2 and volatile organic compounds (VOCs) react in the presence of sunlight to form photochemi-cal air pollution. NOx also form nitric acid when dissolved in atmospheric moisture, forming a component of acid rain. No-wadays, removal treatment of NOx from engine exhaust gas has become a crucial issue because of progressively strict environmental regulations.

For years, non-thermal plasma (NTP) technique has offered an innovative approach to the solution of some environmental problems. In NTP, the electric supplied energy is mainly charged electrons (T ≈ 10.000K), whereas the neutral gas molecules remain almost cold (room temperature). In this case, an NTP also refers to as non-equilibrium plasma or cold plasma. A strong electric field causes ionization of an air stream and the formation of ions and radicals. These highly reactive species can lead to partial or complete oxidation of air pollutants. Because the typical concentration of the pollutants is mostly in the range of several hundred ppm, direct interac-tions between the electrons and pollutants can usually be ne-

glected [1]. The common ways to generate NTP are corona discharge,

dielectric barrier discharge and microwave discharge. Dors and Mizeraczyk [2] studied NOx removal with a hybrid sys-tem consisting of a DC corona discharge and a catalyst under room temperature in the presence of ammonia. The highest NOx removal (96% at energy density of 400 J/L) was ob-served when catalyst was saturated with ammonia. Vinogra-dov et al. [3, 4] used DC and pulsed corona discharges to in-vestigate NOx removal from diesel engines. They found that the cleanness (mass of NOx removed relative to its initial mass) is independent of engine load. It reached 55% with the optimal DC corona discharge needle-to-plane reactor.

Dielectric barrier discharge approaches have also been proved that they can remove NOx and particulate matters (PM) from diesel exhaust. Yao et al. [5] used the uneven dielectric barrier discharge reactors driven by positive-negative pulse plasma discharges to investigate PM removal. The maximum PM removal was 67% at 300W energy injection using reactor with a gap distance of 0.4mm. Mok and Huh [6] used a dielec-tric barrier discharge and catalysis hybrid system to simulta-neously remove NOx and PM from diesel engine exhaust. They suggested that the use of a porous thin steel sheet as one of the electrodes can increase the removal efficiency while consuming less electrical energy, although PM is removed well by the action of a dielectric barrier discharge. Song et al.

*Corresponding author. Tel.: +81 277 30 1522, Fax.: +81 277 30 1521 E-mail address: [email protected]

† Recommended by Associate Editor Kyoung Dong Min © KSME & Springer 2012

1922 T. Q. Vinh et al. / Journal of Mechanical Science and Technology 26 (6) (2012) 1921~1928

[7] used a coaxial dielectric barrier discharge reactor supplied with high frequency AC power to study the effect of NTP on the reduction in NOx, HC and PM including carbon soot and soluble organic fraction (SOF) as a function of engine operat-ing conditions. They found that the maximum of PM, HC and NOx removal efficiencies could reach more than 80%, 75% and 65%, respectively. At most engine loads, the energy utili-zation efficiency is reduced with the increase of peak voltage. In another research, Grundmann et al. [8] used a wire-to-cylinder reactor putting in a cylindrical stainless steel vacuum vessel to investigate the soot decomposition process by ozone. They introduced the reaction scheme of soot treatment by ozone and concluded that plasma seems to reduce the activa-tion energy of these soot decomposition reactions.

The preceding studies of using dielectric barrier discharge to remove NOx and PM have not mentioned the interaction between PM and NOx reduction characteristics. Moreover, the combination between a dielectric barrier discharge reactor and a well-developed diesel particulate filter (DPF) should be con-sidered. In this study, needle-to-cylinder type reactors were used to investigate NOx removal characteristics, effects of PM mass and oxygen fraction in exhaust gas. A simulated diesel exhaust gas of N2, NO and O2 combined with PM was used as the test gas. In addition, the change of PM composition was examined.

2. Apparatus and method

The experiments were carried out in needle-to-cylinder type reactors which were supplied by 50 Hz alternative high volt-age in a range of 5 kV to 15 kV. Fig. 1 shows a schematic of experimental setup and detailed structure of the reactor. The barrier discharge reactor consists of two sections in series. A pre-reactor section on the left side consists of a steel needle (1.8 mm diameter) and a stainless steel foil of 105 mm length surrounding a cylinder with an inner diameter of 36 mm. In the main reactor section, a 9-wire bunch inside a DPF works as high voltage electrode and stainless steel foil with a length of 40 mm wrapped around a cylinder works as a ground elec-trode. The DPF has two functions, the filter function and the dielectric barrier function. The needle and the 9-wire bunch are connected inside the cylinder to the high voltage power source. Similarly, two stainless steel foils are connected to a common ground point.

All the experiments were carried out under room tempera-ture condition. The test gas is a mixture of N2, NO, and O2. The NO concentration is kept constant at 100 ppm while O2 fraction is changed from 0 to 20%. The gas flow rate of 0.5 L/min, 1 L/min and 2 L/min are controlled by flow meters (KOFLOC, RK 1250 series). After coming out from the reac-tor, the test gas is collected by a Tedlar bag and analyzed by an FT-IR exhaust gas analyzer (Horiba Ltd., MEXA-4000FT). In this paper, the NOx concentration is defined by the sum of NO and NO2 concentrations in ppm.

The DPF was used in experiments is a cordierite wall flow

type, down-sized for labo-scale experiment. The detailed structure of the DPF is shown in Fig. 2. Total volume of the DPF is 31.6 cm3 with 117 square holes of 1.5 mm by 1.5 mm.

To load PM on the DPF, a diffusion flame formation sys-tem [9] was used. The layout of the PM loading system is shown in Fig. 3. The liquid fuel from the reservoir was heated to 130℃ by a heater. The fuel temperature was controlled by a thermocouple. In the combustion chamber of 60 mm by 60 mm by 240 mm, the heated fuel mixed up with filtered com-bustion air and was ignited to form a diffusion flame of diesel fuel. The PM generated from the flame at the top of the cham-ber was pumped into the DPF to required mass.

Fig. 4 expresses the procedure of pre-treatment and PM loading on the DPF. It shows the steps to get the required PM mass for DPF. For experiments, the PM mass was from 50 mg

Fig. 1. Schematic of experimental setup.

Fig. 2. Structure of the diesel particulate filter (DPF).

T. Q. Vinh et al. / Journal of Mechanical Science and Technology 26 (6) (2012) 1921~1928 1923

to 150 mg. The reason is that a PM loading of 1 to 5 g/L was commonly reported in the literature as experimental data of the DPF. In this experiment the DPF used was a kind of down-sized model with a volume of 0.0316L (Fig. 2). Thus, PM loading was in the mentioned common range.

For loading PM, at first step, a new clean DPF is purged by nitrogen in 30 minutes at a flow rate of 1 L/min in order to

make sure there are no gas residues inside the DPF. Then it is put on the electric microbalance to determine tare weight. After being loaded by the PM generator in 20-25 minutes, the DPF with PM is once again purged in nitrogen to dry it. Fi-nally, the loaded DPF will be put on the microbalance to check if the PM mass is 100 mg or not. If the PM mass is still not enough, then the DPF should be additionally loaded with PM.

3. Results and discussions

3.1 Simulation gas with PM

In order to investigate NOx removal by barrier discharge re-actor in diesel exhaust gas, the simplified simulation exhaust gas of N2, NO and O2 was used. In addition, the DPF loaded with PM was also put into reactor to see how PM fraction influences in NOx removal characteristics.

Fig. 5 shows current-voltage characteristics of barrier dis-charge reactors. For this purpose, N2 gas went through the reactor with the flow rate of 1 L/min. Two DPFs, one without PM and the other loaded with 100 mg of PM, were used.

At the same applied voltage, the electric current through the reactors with DPF containing 100 mg of PM is significantly higher than the one without PM. The maximum current when reactor contained DPF without PM was nearly 200 μA while it reached 270 μA as reactor had DPF with 100 mg of PM. Be-cause in discharge field some active radicals were formed by electrons impacts with soot (mainly C) of the PM, so the cur-rent in case of DPF containing PM was much higher.

The DeNOx effect of PM following time in the case of mix-ture of N2, NO, and O2; NO = 100 ppm, O2 = 10%, and an applied voltage of 12.75 kV is shown in Fig. 6. For compari-son, two experiments were done: DPF without PM and DPF with 100 mg of fresh PM. In both experiments, test gas was sampled for 10 minutes every running hour.

Fig. 3. Layout of diffusion flame formation system for PM loading.

Fig. 4. Procedure of PM sampling.

0 5 10 150

100

200

300

Applied voltage E kV

Ele

ctric

cur

rent

I　μA

DPF with PM=100mg

DPF without PM

Fig. 5. Current-voltage characteristics of reactors.


The vertical axis expresses gas concentrations in ppm while the horizontal axis is running time in hours. Point [0] indicates the initial test gas concentration without applying an electric voltage on the reactor. The zero point (0) is timing when an electric voltage was applied and gas was sampled at once. It can be learned from Fig. 6 that the PM has a significant effect on NOx removal behavior but it just occurred when the PM was still quite fresh. When the PM was just loaded from the PM generator without running (so called PM composition was fresh), at the zero point (0), the NOx concentration was re-duced 50 ppm (equal to 44%) at an applied voltage of 12.75 kV (154.53 J/L). But after one hour running, at the same ap-plied voltage, the NOx removal effect disappeared.

Regarding the PM composition, an experiment was done by comparing the PM composition between a fresh PM loaded DPF and that DPF after two hours running with a gas mixture of N2, NO, and O2, NO = 100 ppm, O2 = 10% and a flow rate of 1L/min. The PM composition was measured by a particu-late analyzer (Horiba Ltd., MEXA-1370PM). Fig. 7 shows the experimental results in two cases: PM = 100 mg and PM = 150 mg. In both cases, sulphate fraction and soluble organic

fraction (SOF) were reduced whereas soot fraction was in-creased significantly. In short, the PM composition changed to one with reduced SOF and sulphate fraction.

The positive effect of a fresh PM on NOx removal behavior will be useful on a real engine because the PM composition is always renewed when the engine is running. Then, the follow-ing comparison was made with a fresh PM, in other words; a comparison was made using the data of a sampling time at (0).

The experimental results of the two cases: a DPF without PM and a DPF with 100 mg of fresh PM at a gas flow rate of 1 L/min are shown in Fig. 8. The testing gas was a mixture of NO = 100 ppm, O2 = 10%, and N2 left. The graph expresses gas composition concentrations in ppm following the energy density in J/L that is defined by supply power per unit of gas flow rate.

When the energy density increased, the NO reduction in both cases is increased. In case of DPF without PM, NO con-centration was reduced from 92 ppm to 33 ppm at the energy density of 132 J/L. When DPF loaded with 100 mg PM, NO reduction was 56 ppm at 194 J/L from the initial concentration of 94 ppm. However, there’s a big gap between these cases is the NO2 concentration. In the case of a DPF without PM, the initial NO2 concentration was about 10 ppm. When the sup-plied energy increased, the NO2 concentration also increased rapidly. As a result, the total NOx concentration is reduced insignificantly. Conversely, in the case of a DPF with 100 mg PM, the NO2 concentration is first reduced slightly and then starts to increase from the energy density over 140 J/L. For that reason, the total NOx reduction was 37 ppm from an ini-tial concentration of 105 ppm.

Fig. 9 plots the effect of PM on NOx removal behavior when calculating the NO and NOx removal efficiency in per-centage. It can be seen that although in the case of a DPF without PM, the NO reduction rate is higher but the total NOx

Fig. 6. Effect of running time in case of N2, NO, and O2 mixture.

Fig. 7. PM composition of new DPF and the DPF after 2 hours run-ning.

Fig. 8. NOx removal by barrier discharge reactor in two cases: DPF without PM and DPF with PM = 100 mg.


reduction rate is still much lower than in case of a DPF with 100 mg of PM. The reason is that, with the existence of PM inside the reactor, O radical will be shared to react with HC or soot besides oxidation of NO into NO2 as normal. So the NO2 formation process was slower and consequently, the NOx re-duction rate was higher. Accordingly, PM has worked as a reactive agent in the barrier discharge field.

The variation of NOx removal efficiency with PM mass loading on DPF is showed in Fig. 10. The PM masses of 0, 50, 100 and 150 mg were examined. In all four cases, when en-ergy density increases, the NOx removal is also increased but with different rates. It is showed that the higher PM mass loading on DPF, the better NOx removal efficiency obtained.

While it differs between DPF without PM and DPF with PM, there is a small change (about 5%) when shift the PM mass from 50 mg to 100 mg or 150 mg.

3.2 Simulation gas of N2 and NO

The gas mixture of 100 ppm of NO and remaining N2 was used to study the NOx removal at a flow rate Q of 0.5 L/min, 1 L/min and 2 L/min. In these tests, DPF without PM was used. The experimental results at Q = 1 L/min are shown in Fig. 11.

The maximum energy density was limited by the applied voltage and when the voltage was elevated beyond 13.5 kV (137 J/L) at the gas flow rate, spark discharge was emitted. When the voltage applied to the reactor was increased, the discharge field was changed from a dielectric barrier discharge to a spark discharge, the maximum energy that could be sup-plied as dielectric discharge energy was limited by the thresh-old voltage of the reactor and it depended on a schematic ar-rangement of electrodes and the flow rate of test gas.

When the energy density increased, the NO concentration decreased. Also NOx concentration, which was defined as the sum of NO and NO2 concentrations, decreased with NO. In this experiment, N2O concentration is not relevant since it was observed that N2O was almost zero during the experiments. However, NO2 was detected, even though no oxygen was contained in the mixture. This is because that small amount of NO was changed by a reaction scheme of 2NO → NO2 + N.

The maximum NO reduction was 35 ppm at 137 J/L (at 13.5kV) while the NO2 concentration increased to 5 ppm and the total NOx concentration was reduced by 30 ppm. This indicates that the NO elimination process was dominant.

Fig. 12 shows the NOx removal efficiency at Q = 0.5 L/min, 1 L/min, and 2 L/min. The NOx removal efficiency in percent is calculated by NOx reduction in ppm divided the initial NOx

Fig. 9. Effect of PM on NOx removal by barrier discharge followingremoval efficiency.

Fig. 10. NOx removal efficiency following energy density at variousPM mass.

Fig. 11. NOx removal by barrier discharge with N2 and NO mixture, flow rate Q = 1 L/min.


concentration in ppm. In this graph, the horizontal axis is the supplied energy density in J/L and the vertical axis represents the NOx removal efficiency. When the energy density in-creased, the NOx removal efficiency increased in all experi-mental conditions. The highest NOx removal efficiencies re-corded were 20.28% at 70 J/L, 31.37% at 137 J/L and 43.46% at 157 J/L with gas flow rates of 2 L/min, 1 L/min and 0.5 L/min, respectively.

Regarding NOx removal mechanism in a barrier discharge reactor with a gas mixture of N2 and NO, it has been reported in many papers [2, 10, 12-14]. The following reactions are assumed to occur inside the reactor:

N2 + e → N + N + e (R1) NO + N → N2 + O (R2)

where e is an electron. At beginning, the N2 molecules in the gas mixture were ex-

cited by the discharge field to form N radicals. These radicals then react with NO to transform it into N2 and O as in reaction (R2). This process is sometimes called the primary mecha-nism for NO destruction [10]

It can be seen from Fig. 12 that at the same supplied energy, when the gas flow rate increased, the NOx removal efficiency also increased. This means that residence time or flow stagna-tion in the reactor does not control the NO reduction rate. One of the actual reasons is the limitation of the N radical in the gas. If the saturation limit of the N radical was very low and too much energy is supplied, the total NO radicals formed with the same electrical energy is proportional to the gas flow rate. Nevertheless, due to the threshold applied voltage of the reactor is 13.5 kV and different residence time at various gas flow rates so the maximum energy densities were not the same as see in Fig. 12.

3.3 Simulation gas of N2, NO and O2

To investigate the NOx removal efficiency with simulation gas of N2, NO, and O2, a gas mixture of NO = 100 ppm, O2 = 10% and remaining N2 was used. Experiments were done under the gas flow rates of 0.5 L/min, 1 L/min and 2 L/min.

Fig. 13 shows the experimental results at a gas flow rate of 1 L/min. When the energy density increased, NO was reduced 60 ppm from an initial concentration of 92 ppm to 32 ppm at the energy density of 125 J/L (at 13.5 kV). On the contrary, NO2 increased 44 ppm from 10 ppm to 54 ppm. Therefore the NOx concentration decreased 16 ppm from 102 ppm to 86 ppm. Moreover, in this condition, the N2O concentration can be negligible.

In brief, the NOx removal by the barrier discharge field with a gas mixture of NO = 100 ppm, O2 = 10% and remaining N2 is mostly to convert NO into NO2. The N2O concentration is very small or negligible. These results, which are far different from the results shown in Fig. 11, can be explained by the existence of O2 in the gas mixture.

The NOx removal mechanism in this case, in addition to re-actions (R1) and (R2), is described by chemical reactions as below [10, 13-14]:

O2 + e → O + O + e (R3) O2 + O → O3 (R4) NO + O3 → NO2 + O2 (R5) NO + O → NO2 (R6) NO2 + N → N2O + O (R7) NO2 + N → N2 + O2 (R8) NO2 + O → NO + O2 (R9) 2NO2 + O3 → N2O5 + O2 . (R10) The O radicals formed in reaction (R3) combine with O2 to

Fig. 12. NOx removal efficiency by barrier discharge with N2 and NO mixture.

Fig. 13. NOx removal by barrier discharge with N2, NO, and O2 mix-ture, O2 = 10%, flow rate Q = 1 L/min.


form ozone through reaction (R4), or oxidize NO to form NO2, following reaction (R6). Ozone also reacts with NO to form NO2 through reaction (R5). These NO2 can be reduced by N radicals from reaction (R1) to form N2O through reaction (R7), or N2 following reaction (R8). NO2 might be also oxidized by O3 as reaction (R10). Reaction (R9) is assumed to occur to form secondary NO [10].

Fig. 14 shows the NOx removal efficiency in percentage fol-lowing the energy density in J/L. There is a small difference of efficiencies between two cases of gas flow rate Q = 1 L/min and Q = 2 L/min. When energy supplied is increased, the NO reduction rate was also increased. Compared to the gas mix-ture of NO and N2, the NO reduction was significant higher, can reach 100% at energy density of 250 J/L under gas flow rate of 0.5 L/min. As explained in 3.2, under different gas flow rates and at the same threshold applied voltage of reactor (13.5 kV) the maximum energy densities were different be-cause of the effect from residence time inside reactor.

3.4 Effect of oxygen fraction on NOx removal efficiency

The gas mixture of N2, NO = 100 ppm, and O2 fractions of 10%, 15%, and 20% were used to investigate how oxygen affects NOx removal characteristics in the dielectric barrier discharge field. The gas flow rate was 1 L/min. The experi-mental results are shown in Fig. 15. The horizontal axis is energy density in J/L while the vertical axis expresses the NOx removal efficiency in percentage.

It can be seen from Fig. 15 that for the NO removal effi-ciency, it differed dramatically between cases of gas mixture without oxygen and the one with oxygen. On the other hand, the NOx removal efficiency was close. This implies that with the existence of oxygen the NO oxidation process prevailed.

4. Conclusions

The simulation gas of N2, NO, and O2 at different composi-tions was used to investigate the NOx removal characteristics from simulated diesel exhaust gas under room temperature condition. The gas flow rates were 0.5 L/min, 1 L/min, and 2 L/min. By using a dielectric barrier discharge needle-to-cylinder reactor combined with a DPF, the effects of PM and oxygen on NOx removal efficiency were studied. Through the experimental results, the following conclusions could be ob-tained.

(1) PM strongly enhances NOx removal reactions in the di-electric barrier discharge field. However, only fresh PM has a significant effect on NOx removal behavior. This will be use-ful with real running engines where PM is always renewed.

(2) Under the condition of a barrier discharge field and gas mixture of N2, NO and O2, soluble organic fractions and sul-phate in the PM composition are slightly reduced.

(3) Oxygen plays an important role in the NO oxidation process that is dominant in NOx removal characteristics with a gas mixture of N2, NO, and O2.

(4) As gas flow rate up to 2 L/min, gas mixture without oxygen and the applied voltage under 13.5 kV, the residence time or flow stagnation in the reactor does not control the NO reduction rate.

Nomenclature------------------------------------------------------------------------

K : Kelvin temperature scale Q : Gas flow rate T : Temperature

Fig. 14. NOx removal efficiency by barrier discharge with N2, NO, and O2 mixture, NO = 100 ppm, O2 = 10%.

Fig. 15. Effect of oxygen fraction on NOx removal efficiency.


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Tran Quang Vinh is a Ph.D Candidate in the Department of Mechanical System Engineering of Gunma University, Japan. He received the B.S. degree in 1997 and MSc degree in 2007 from Hanoi University of Science and Technology, Vietnam. His main research interest is the engine exhaust

treatment.

Masataka Arai received his B.S. in 1971 and M.S. in mechanical engineer-ing from Gunma University in 1973. He then received his Ph.D. degree from Tohoku University in 1977. Dr. Arai is currently a Professor at the Graduate School of Engineering at Gunma Uni-versity, Japan. His research interests are

combustion, fuel atomization and sprays, engine emission and energy conversion.

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Fundamental study of NOx removal from diesel exhaust gas by dielectric barrier discharge reactor