Cold Plasma Gliding Arc Reactor System for Nanoparticles’ Removal from Diesel Cars’ Exhaust Gases

Abstract

The main goal was to investigate the ability of a non-thermal plasma reactor with gliding arc discharge to remove diesel exhaust particulates (DEPs). A conventional knife-shaped LTP GA (low-temperature plasma gliding arc) reactor was utilized. The following three cases were studied: 140 L/min, 70 L/min, and 14 L/min of air drawn through the reactor, and diesel exhaust fumes were sampled continuously. They were assayed in terms of concentration and number particle size distribution. The higher the residence times, the higher the energy input that may be utilized for DEPs’ removal. The reactor performance definitely lowered the concentration of DEPs (250–580 nm) and altered their number size distribution. There was no effect on the number concentration, nor the particle size distribution, of DEPs of 10–250 nm in size. Regarding the effectiveness of DEPs’ removal, decreasing the flow rate from 140 L/min to 70 L/min somehow altered the values. Achieving the airflow of 14 L/min led to a substantial improvement (even to a fourfold increase for 300–480 nm particles). Non-thermal plasma reactors with gliding arc discharge may be successfully adapted to the process of DEP treatment. Their performance may be optimized by adjusting the airflow at the inlet of the reactor to guarantee the longest aerosol residence times and the highest removal efficiency.

1. Introduction

Diesel engines, known for their efficiency and durability, are widely used in transportation and industrial applications, such as automobiles, marines, fishing boats, power generators, and mines. They are useful for reducing CO₂ emissions due to their high thermal efficiency and omnivorous consumption of different kinds of fuel, including biofuels [1,2,3].

However, the emissions generated by these engines pose significant environmental and health concerns. One of the most critical components of a diesel exhaust is the particulate matter suspended in the gas phase, which results from an incomplete combustion of fuels. Diesel exhaust particulates (DEPs) constitute the aggregates of the primary particles, with various compounds adsorbed on their surface. Diesel emission particulates mostly consist of dry soot (carbon, C), condensed organic or inorganic substances, a soluble organic fraction (SOF), hydrocarbon compounds (HCs), nitrates, and sulfates [4].

Diesel exhaust particulate matter is notoriously harmful when inhaled due to its minuscule size, since the diameter of the aggregates is in the range of 10 nm to 10,000 nm. This allows it to penetrate deep into the lungs and even enter the bloodstream. This capability is associated with significant respiratory and cardiovascular issues [5]. Furthermore, DEPs have been classified as a Group 1 carcinogen by the International Agency for Research on Cancer, solidifying its reputation as a severe public health hazard [6]. Studies have demonstrated that long-term exposure to diesel exhaust particulates can lead to chronic respiratory conditions, reduced lung function, and an increased risk of heart diseases [7]. Moskal and Penconek, in their previous study, provided critical insights into how these particles are deposited in the human respiratory tract. Their research emphasizes that the size and composition of diesel exhaust particulates vary with fuel type and combustion conditions, influencing the deposition efficiency and pattern within the respiratory system [8]. The adverse health effects associated with diesel exhaust particulates have been extensively studied, demonstrating a clear link to respiratory diseases, cardiovascular conditions, and even cancer. Therefore, their reduction has gained international interest.

In recent years, the focus has shifted towards innovative methods for mitigating these emissions. Air cleaners that incorporate high-efficiency particulate air (HEPA) filters and electrostatic precipitators are successful in decreasing the concentration of suspended particulate matter in the atmosphere. For DEPs in the exhaust gas of diesel engines, a diesel particulate filter (DPF) is installed in the exhaust pipe and its efficiency usually reaches 95% or more. Nevertheless, one must take into consideration the relatively large pressure loss associated with HEPA and DPF [9].

Among the emerging technologies, in-flight methods, including low-temperature (non-thermal) plasma (LTP) and reactors, have shown considerable promise. These reactors operate by ionizing the air at atmospheric pressure, producing reactive species capable of breaking down pollutants at relatively low temperatures [10,11]. This method is particularly attractive for its efficiency, scalability, and potential for integration with existing exhaust systems in diesel engines.

The gliding arc (GA) discharge is particularly effective due to its ability to generate high concentrations of reactive species in a relatively big area while maintaining a relatively low gas temperature. This low-temperature operation is beneficial for preventing the formation of secondary pollutants and reducing the thermal stress on engine components. The gliding arc system’s design also facilitates its seamless integration into existing exhaust mechanisms, thereby allowing for its application in various industrial and vehicular settings. Several studies have documented the effectiveness of LTP (low-temperature plasma) reactors in decomposing NO_x, SO_x, and hydrocarbons present in exhaust gases. However, specific attention to the application of these reactors for particulate matter control is still emerging. For example, Chae presented a review of the application of a non-thermal plasma technique to treat diesel exhaust [12]. Thomas et al. noted the effect of residence time on the oxidation of soot particles (particulate matter), in packed dielectric barrier discharge (DBD) reactors [13]. Pan et al. showed a great oxidative potential of LTP (obtained in DBD reactors) in decreasing the concentration of the carbon particles from diesel engine exhaust, which resulted in the effective conversion of carbon particles to CO and CO₂, while a series of complex chemical reactions with carbon particles lead to reducing the diameter of particles, as well as the number of C atoms [14]. Yamasaki et al. proposed an in-flight DBD plasma system that can provide a large filtering volume, improve the residence time characteristics of radicals, and increase the decomposition performance of the system [9]. They examined the particulate removal reactions and confirmed their combustion. Zhu et al. studied the application prospects of LTP reactors (illustrated by the case of dielectric barrier discharge LTP generator) in decreasing particulate matter concentrations produced under different diesel engine loads [15]. To our best knowledge, the effectiveness of non-thermal gliding arc reactors in decreasing particulate matter from diesel exhaust has not been yet reported.

Low-temperature gliding arc plasma reactors offer significant potential for reducing gaseous pollutants through the generation of high concentrations of reactive species at controlled temperatures. The advantages of high decomposition efficiency, scalability, and low thermal operation underscore their applicability in various industrial settings. However, challenges regarding energy consumption, electrode wear, and operational complexity remain to be addressed. Further research and optimization are essential to fully realize the potential of these reactors, with a focus on understanding and controlling plasma behavior to maximize efficiency and sustainability in pollution control.

The initial findings indicate that such reactors can indeed reduce particulate emissions, although comprehensive mechanisms and optimization strategies require exploration. The growing concerns over the environmental and health impacts of diesel exhaust necessitate innovative and effective solutions. Low-temperature plasma reactors, particularly those utilizing gliding arc discharge, offer a promising approach. By leveraging the unique properties of plasmas to decompose harmful pollutants, these systems can significantly contribute to cleaner diesel engine technologies. The main goal of this paper was to investigate the ability of a gliding arc LTP reactor to remove the DEPs, especially in the submicron size.

2. Materials and Methods

2.1. Characteristics of Low-Thermal Plasma Reactor with Gliding Arc Discharge

The low-temperature gliding arc plasma reactor is an innovative technology which can be used for the effective mitigation of gaseous and particulate pollutants, particularly those emanating from diesel exhaust emissions. This reactor utilizes principles of plasma physics to generate non-thermal plasma, consisting of various reactive species capable of decomposing a wide range of pollutants. The operation of a gliding arc plasma reactor begins with the ignition of an electric arc between a pair of diverging electrodes connected to a high-voltage power source (Figure 1).

The application of a high voltage ionizes the gas in the gap between the electrodes, forming an initial electric arc. (Figure 1) The introduction of a gas flow perpendicular to the arc axis causes the arc to glide along the length of the electrodes, elongating the arc and increasing the plasma volume. As the arc glides, it generates a non-equilibrium plasma characterized by a high electron temperature, which facilitates the creation of a variety of reactive species including ions, radicals, and excited molecules. These reactive species interact with pollutants in the gas stream, breaking molecular bonds and converting the pollutants into less harmful substances or completely oxidizing them to CO₂ and H₂O. The gliding arc plasma can be either thermal or non-thermal depending on the power and flow rate. Along with completely thermal and non-thermal modes, it is possible to operate the arc in the transitional regime when the discharge starts to become thermal, but during the space–time evolution it becomes a non-thermal one. The powerful and energy-efficient transitional discharge combines the benefits of both equilibrium and non-equilibrium discharges.

The initial breakdown in gas at the distance of a few millimeters between the two electrodes (Figure 1) begins the dynamic cycle of the gliding arc. After about 1 μs, low-resistance plasma is formed and the voltage between the electrodes falls. The equilibrium stage occurs after the formation of the plasma channel. The gas flow pushes the plasma column with a velocity of about 10 m/s; their velocities, however, can be slightly different. The length of the arc increases together with voltage; the power increases up to the maximum value provided by the power supply. During the quasi-equilibrium stage, the gas temperature does not change significantly (remaining around 3000 K). The non-equilibrium stage starts when the length of the gliding arc exceeds its critical value. Heat losses from the plasma column begin to exceed the energy supplied by the source, and it is not possible to sustain the plasma in quasi- equilibrium. Then, the plasma rapidly cools down to 1000–2000 K, while conductivity is sustained by high electron temperature, ~1 eV. After the decay of the non-equilibrium discharge, a new breakdown takes place at the shortest distance between the electrodes, and the cycle repeats [11,16].

One of the primary advantages of gliding arc plasma reactors is their high decomposition efficiency, resulting from the enhanced interaction between reactive plasma species and pollutants. The reactor’s design, which promotes the continuous movement of the arc, ensures a uniform distribution of reactive species, thereby maximizing pollutant degradation. Additionally, the reactor’s ability to operate at relatively low temperatures prevents the formation of secondary pollutants often associated with high-temperature processes and reduces thermal stress on reactor components. The non-thermal nature of the plasma ensures that most of the energy is concentrated in the electrons, allowing for the efficient generation of reactive species without significantly increasing the gas temperature. The gliding arc discharge has a higher current and higher electrical, as well as lower applied voltage compared to a glow or corona discharge [17]. Therefore, a gliding arc discharge ensures high reaction selectivity and energy conversion, while easily conquering the limits of both gas pressure and required energy input [11].

Despite the promising applications of gliding arc plasma reactors, there are several limitations to consider. Maintaining a stable electric arc and sustaining the plasma requires significant energy input could be a limiting factor in large-scale applications. Furthermore, continuous operation leads to the wear and degradation of the electrodes due to high-intensity electric arcing, necessitating routine maintenance and periodic replacement. In addition, the control and optimization of plasma properties and gas flow dynamics can be complex, requiring sophisticated control systems and a comprehensive understanding of the underlying plasma physics.

The plasma behavior within gliding arc reactors is crucial for optimizing reactor performance and pollutant degradation efficiency. The plasma operates in a non-equilibrium state, with electron temperatures significantly higher than those of the ions and neutral gas. This discrepancy facilitates efficient energy transfer to gas molecules, promoting the dissociation and ionization processes essential for pollutant degradation. The dynamic nature of the gliding arc, characterized by its continuous motion along the electrodes, ensures a steady distribution of reactive species and enhances the interaction time between the plasma and pollutants. The high density of energetic electrons in the plasma promotes frequent collisions with gas molecules, generating a myriad of reactive species necessary for effective pollutant breakdown.

2.2. Characteristics of Diesel Engine and Utilized Fuel

The diesel exhaust particles were obtained from combustion of sulfur-free diesel oil in a diesel engine. The fuel was always taken from one and the same gas station of the most popular petrol refiner and retailer in Poland. The characteristics of utilized fuel are given in

Table 1. Fuel characteristics.

Parameter	Unit	Value
Cetane number	-	Min. 51
Cetane index	-	Min. 46
Density at 15 °C	kg/m3	820.0–845.0
Kinematic viscosity at 40 °C	mm2/s	2.0–4.5
PAH (polycyclic aromatic hydrocarbons) content	% (m/m)	<7
Sulfur content, mg/kg	mg/kg	<10
Fatty acids methyl esters content	% (v/v)	<7

.

DEPs were generated with the use of the four-cylinder diesel engine Mercedes Benz 240D (1982), adapted to be used in a laboratory, on a motor test rig (engine displacement 2399 cm³, engine power 53 kW at 4200 rpm, torque 137 Nm at 2400 rpm). Motor emissions were generated under idle speed conditions.

2.3. Characteristics of Non-Thermal Plasma Reactor with Gliding Arc Discharge

A conventional knife-shaped LTP GA reactor was utilized for experimental purposes and is shown in Figure 2a. The reactor wall was made of a glass tube and was thermally insulated, as it was proven to improve reactor energetic efficiency (however, in DBD reactors). The plasma volume was calculated by knowing the volume of an inverted cone with a height value equal to 120 mm and a base radius equal to 23.5 mm (based on Figure 2). The residence times in the plasma area (of an estimated volume equal to 69.4 cm³) in the plasma reactor are 0.03 s, 0.06 s, and 0.3 s when the flow rates of the exhaust gas are 140, 70, and 14 L/min, respectively. The LTP GA is equipped with two flat electrodes and their dimensions are presented in Figure 2b. Knife-shaped electrodes were sharpened at the end, their thickness value was 3 mm. They were made of acid-resistant stainless-steel type 1.4301/1.4307 (According to EN), 304 (According to AISI), 0H18N9 (According to PN). The reactor is equipped with two AC high-voltage SIET transformers (50 mA, 10 kV, Brollo siet, Caerano di San Marco TV, Italy) for the power supply (connected in parallel, 2 × 300 W). The electric power applied to our experimental setup was measured with a power meter. To drive the discharge, an AC high voltage was generated with a maximum of 10 kV (rms). The frequency of the applied voltage was 50 Hz. When a high-voltage power supply is turned on, the gas flow initiates the gliding of the arc, thus forming GA plasma.

2.4. Experimental Setup

Figure 3 shows a schematic diagram of the experimental setup for testing the DEPs treatment system incorporating a non-thermal plasma reactor with gliding arc discharge.

The diesel exhaust fumes containing DEPs produced by the diesel engine were directed to the chamber (1 m³) after they were stabilized in a buffer tank. The diesel engine and buffer tank were situated outside the building. The airflow was generated by a compressor and controlled by a mass flow controller (MFC) (SFC5500–200 slm, Sensirion, Zurich, Switzerland). Leaving MFC, compressed air was at the first inlet of the mixing tee, where the suction nozzle ensured the continuous sampling of diesel exhaust fumes from the chamber through the second inlet. Fumes were then diluted in the compressed air and entered the gliding arc LTP reactor. The dilution rate was dependent on the compressed airflow and its obtained values were: 1:0.05 for 140 L/min, 1:0.2 for 70 L/min, and 1:0.15 for 14 L/min, respectively. Treated diesel exhaust fumes containing DEPs were collected in the chamber prior to testing. They were assayed in terms of their concentration and number particle size distribution using two spectrometers: Grimm aerosol counter (Model 1.109, Grimm Aerosol Technik GmdH, Ainring, Germany) and Fast Automotive Particles Emission Spectrometer—FAPES (Model 5.601, Grimm Aerosol Technik GmdH, Ainring, Germany). Exhaust gases were directed to waste using a fan. The reactor is equipped with two AC high-voltage SIET transformers.

2.5. Assessment of Time–Space Evolution of the Gliding Arc Discharge

Firstly, gliding arc evolution with the transition from quasi-thermal to the non-thermal regime of the discharge in the LTP reactor was measured using a camera that made a series of consequent snapshots with the frequency of 30 Hz.

2.6. Measurement of DEPs’ Number Particle Concentration and Size Distribution

Secondly, the study encompassed the examination of particle number concentrations as a function of time.

The Fast Automotive Particles Emission Spectrometer—FAPES (Model 5.601, Grimm GmbH, Mettmenstetten, Germany) was utilized to assay the diesel exhaust fumes at the reactor’s outlet by measuring the concentration of DEPs in the range of 10–640 nm in size diameter. In this study, a sampling rate of 0.2 Hz was used. The measurement technique is based on 10 Differential Mobility Analyzers (DMAs) with in-line Faraday Cup Electrometers (FCEs).

The FAPES did not detect any particles from the range of 370–640 nm due to their concentration being too low. For this reason, the second spectrometer (Grimm Model 1.109, Mettmenstetten, Germany) was used to determine the number of particle size distribution and concentration of the DEPs in the gas leaving the reactor, in the range of 250 nm–32 µm. As stated before, the DEP samples were diluted before measuring. It was the requirements of the spectrometers that prevented them from damage caused by severe aerosol exposure.

3. Results

3.1. Assessment of Time–Space Evolution of the Gliding Arc Discharge

First of all, time–space evolutions of the gliding arc discharge in the studied LTP reactor are illustrated in Figure 4, Figure 5 and Figure 6, for 140 L/min, 70 L/min, and 14 L/min, respectively, by a series of 18 snapshots per each studied flow with frequency of 30 Hz.

Secondly, two indicators of energetic yield in the LTP GA reactor were discussed for the purpose of all three cases, namely for 14 L/min, 70 L/min, and 140 L/min, being compared in further analysis of results. These were the power input for DEPs’ removal and the specific power input per molecule. For this reason, the specific power input per molecule (SPI) was normalized—as the ratio of power input in the studied case versus the highest power input obtained (SPI_max for the “14 L/min” case). The aforementioned are shown in Figure 7 and superimposed onto the presented trends of the indicator are the chosen representative snapshots for every studied airflow.

3.2. Measurement of DEPs’ Number Particle Concentration and Size Distribution

The total particle number concentrations were scrutinized in real time and presented in Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13, so as to show the influence of plasma occurrence on the removal of DEPs of different sizes. Hence, marked are the time periods of turning on and turning off the studied non-thermal plasma reactor with gliding arc discharge (“plasma on” and “plasma off”, respectively).

Moreover, Figure 14, Figure 15, Figure 16 and Figure 17 illustrate the number particle size distributions for two fractions of DEPs: 10–250 nm (measured by FAPES spectrometer) and 250–580 nm (measured by Grimm 1.109 particle spectrometer). The concentration of particles higher than 580 nm was too low to draw any conclusions, thus they are not shown in the present work.

3.3. The Effectiveness of Gliding Arc Non-Thermal Plasma Reactor in Removal of Nanoparticles from the Diesel Exhaust Fumes

The next focus of the study is to discuss the effectiveness of the gliding arc non-thermal plasma reactor in the removal of nanoparticles from diesel exhaust fumes. This parameter is described by the following equation:

E = 100% ∗ (C_{“plasma off”} − C_{“plasma on”})/C_{“plasma off”},

(1)

where

• E denotes the removal efficiency of DEPs due to plasma’s occurrence [%];
• C_{“plasma on”} denotes the number DEPs’ concentration detected when the GA reactor was working (during plasma occurrence) [particle/liter];
• C_{“plasma off”} denotes the number DEPs’ concentration detected when the GA reactor was not working (no plasma occurrence) [particle/liter].

The findings, their mean values and standard deviation, attained for all studied airflows, are presented collectively in Figure 18 for particulates ranging from 250 to 580 nm in size.

4. Discussion

4.1. Assessment of Time–Space Evolution of the Gliding Arc Discharge

First of all, from Figure 4, Figure 5 and Figure 6, it appears that in the case of the lowest airflow rate, 14 L/min, the classic Jacob’s ladder is clearly visible. This result shows that there is some critical value of the Reynolds number for gas, which is responsible for changing the character of the plasma behavior in the reactor. We can distinguish two regimes of plasma behavior: “laminar” with characteristic Jacob’s gliding of arc between the knives of the reactor, and “turbulent” presented in Figure 4 and Figure 5. From the results obtained for the three studied flows, it is observed that in both the “70 L/min” and “140 L/min” cases, the area of the plasma channel reaches a lower level than in the “14 L/min” case. It also appears that the characteristics of the plasma flow are more turbulent, and the color of nitrogen triplets (purple) is evident. The average residence time’s value would be far higher when the airflow drawing DEPs is lower (14 L/min). Based on the results in Figure 7 the main observation is as follows: the higher the residence time’s value (the lower the airflow’s value), the higher the energy input that may be successfully utilized for DEPs’ removal in the LTP reactor with gliding arc discharge. Moreover, it is noteworthy that the highest achieved value of specific power per molecule was 400 W (Figure 7, for the “14 L/min” case), which is substantially low when compared to the diesel engine power (e.g., 100 horsepower, corresponding to about 75 kW). This presents as a relatively attractive, cost-effective way of diesel exhaust treatment.

4.2. Measurement of DEPs’ Number Particle Concentration and Size Distribution

In general, from the analysis of Figure 8, Figure 9, Figure 10 and Figure 14, it is clear that in every studied case the produced plasma state in the reactor had no effect on the number concentration, nor on the particle size distribution of DEPs, being in the range of 10 to 250 nm. On the other hand, based on the graphs presented in Figure 11, Figure 12, Figure 13 and Figure 15, Figure 16, Figure 17, it was noted that reactor performance definitely lowered the concentration of bigger DEPs in the range of 250 nm to 580 nm. It is reflected in the alterations of the number size distribution of these particulates. Furthermore, it is also worth noting that the decrease in the value of airflow drawn through the reactor favors the removal of the DEPs.

4.3. The Effectiveness of Gliding Arc Non-Thermal Plasma Reactor in Removal of Nanoparticles from the Diesel Exhaust Fumes

Based on the findings presented in Figure 18, it is clear that decreasing the flow rate from 140 L/min to 70 L/min somehow altered the values of effectiveness, but the results remained within statistical error margins and no trend was observed for increasing the particle diameter. Achieving the airflow of 14 L/min led to a substantial improvement in the removal’s effectiveness, corresponding to an almost fourfold increase in this parameter for particles that were 300–480 nm in size. Smaller particulates experienced removal increasing with their diameter, then a considerable, quick jump in the process effectiveness in the aforementioned size range, while bigger particles were absent in the assessed aerosol clouds of diesel fumes.

The above analysis suggests that the differences between the results for studied airflows are associated with different residence times acting as a key attribute of the evaluated process. The higher the residence time value (the lower the airflow value), the higher the power input per molecule that may be successfully utilized for DEP removal (the removal by its combustion) in the LTP reactor with gliding arc discharge. To understand the results of the ongoing LTP GA plasma reactor’s performance, adequate discussion of major mechanisms governing DEPs’ removal must be provided. From the literature review mentioned in the introduction section [9,12,13,14,15], it is known that the non-thermal DBD plasma reactor can affect the oxidation of particulate matter in diesel exhaust gas at a low temperature and it can be detected that the diameter of the carbon particles became smaller, and the number of C atoms was significantly reduced under the treatment of NTP technology. Further insights into the mechanisms occurring in another kind of NTP apparatus, that is the gliding arc reactor, must be gained in future research.

5. Conclusions

From the above analysis, it was clearly proven to be true that non-thermal plasma reactors with gliding arc discharge may be successfully adapted to the process of diesel exhaust particulate treatment. The most valuable result is the reduction in most penetrated particles to the level of 70%. Those particles are more dangerous due to their availability and potential long life in the atmosphere. It shows that plasma reactors can be a promising tool herein, since the low-pressure reduction in carbon particles at the level of the exhaust line is considered one of the present challenges of the diesel engine. This constitutes the great advantage of incorporating plasma reactors over treatment using diesel particulate filters or HEPA filters causing high pressure when installed in the exhaust pipe. Another desired effect is the cost-effectiveness of plasma reactors in terms of the needed power input per molecule, being about 100 orders of magnitude lower than the diesel engine power. The obtained results show that the process parameters of resident time in the plasma region and power introduced into the reactor play a crucial role in the optimization of cold plasma systems for exhaust treatment. The performance of non-thermal gliding arc reactors may be optimized by adjusting the airflow at the inlet of the reactor, in such a way as to guarantee the longest aerosol residence time in the plasma region and, thus, the highest DEPs’ removal efficiency. In future investigations, the effect of flow rate on DEPs’ removal should be examined in detail. Specifically, experiments with a lower flow rate of the air are needed to obtain the solution for the scalability of the treatment system, potentially consisting of multiple GA reactors connected in parallel.

Our findings underscore the promising potential of GA technology in reducing emission of DEPs, improving the environmental impact of diesel engines, as well as improving occupational safety and health levels by ensuring a reduction in the concentration of airborne harmful DEPs in the respirable size range. It should be mentioned that results of reducing the particles form the exhaust are achieved with an almost negligible pressure drop, so it gives those methods an advantage compared to traditional filtration.