Keywords
Electrostatic precipitator (ESP), spark discharge, polarity, methane (CH₄), carbon monoxide (CO), collection efficiency.
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Adil Abbas H and Adnan Abbas Q. Influence of spark polarity on the tubular Electrostatic precipitators characterization for CH4 and CO gases [version 1; peer review: awaiting peer review]. F1000Research 2025, 14:1370 (https://doi.org/10.12688/f1000research.173071.1)
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Research Article
Influence of spark polarity on the tubular Electrostatic precipitators characterization for CH4 and CO gases
[version 1; peer review: awaiting peer review]
PUBLISHED 08 Dec 2025
OPEN PEER REVIEW
REVIEWER STATUS
Abstract
Corresponding author:
Hadeer Adil Abbas
Competing interests:
No competing interests were disclosed.
Grant information:
The author(s) declared that no grants were involved in supporting this work.
Copyright:
© 2025 Adil Abbas H and Adnan Abbas Q.
This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
How to cite: Adil Abbas H and Adnan Abbas Q. Influence of spark polarity on the tubular Electrostatic precipitators characterization for CH4 and CO gases [version 1; peer review: awaiting peer review]. F1000Research 2025, 14:1370 (https://doi.org/10.12688/f1000research.173071.1)
First published: 08 Dec 2025, 14:1370 (https://doi.org/10.12688/f1000research.173071.1)
Latest published: 08 Dec 2025, 14:1370 (https://doi.org/10.12688/f1000research.173071.1)
Introduction
Although traditionally used for collecting particulate matter in industrial flue gas, the application of electrostatic precipitators (ESPs) for the removal of gaseous pollutants is uncharted, especially for light, non-particulate molecules such as CH4 and CO.1 Whereas solid particles can be collected through direct capture via electrostatic attractions, in contrast, the increase in electron temperature, density, and current generated more energetic electrons and reactive species (O•, OH•, and O3), which oxidized and decomposed the gas pollutants, improving gas removal efficiency.1 Current findings indicate that the operation of ESPs under partial vacuum conditions enhances plasma activity by extending the electron mean free path and increasing collision energy, which enhances effective decomposition of gas pollutants through chemistry.2 The magnitude and character of the electric discharge, particularly spark-type discharges, play an important role in creating gas-phase breakdown-stimulating high-density plasma.3 Despite this understanding, studies on the influence of discharge polarity (positive spark vs negative spark) on gas removal effectiveness in ESP have been scarce. Polarity influences plasma composition, ion mobility, and electron energies, and thereby affects overall reaction processes and rates.4
While most of the early studies on dielectric-barrier systems concerned either particulate collection or plasma-assisted gas treatment,1–4 only limited attention has been paid to the influence of discharge polarity in electrostatic precipitators operating under partial vacuum. This study, therefore, provides a new perspective by systematically evaluating the impact of positive and negative spark discharges on CH4 and CO removal efficiency, addressing a gap not covered in recent plasma-assisted ESP research.5–8 Recent investigations have begun exploring plasma-assisted gas treatment relevant to ESP configurations, but systematic polarity-resolved studies remain limited.5,6 Sub-atmospheric (partial-vacuum) operation has also been shown to intensify low-pressure corona/spark activity and improve energy efficiency.7 Insight into the effect of polarity on the performance of ESP is therefore invaluable in the design optimization of next-generation indoor air treatment systems for the removal of gaseous pollutants. Building on these findings, the present work advances the field by experimentally comparing positive and negative spark discharges in a cylindrical ESP operated under partial vacuum for CH4 and CO removal, addressing a gap not covered in recent plasma-assisted ESP studies.8 This study aims to assess the performance of a cylindrical ESP operating under spark discharge and partial vacuum conditions for the removal of methane CH4 and carbon monoxide CO of a cylindrical electrostatic precipitator operating through spark discharge and partial vacuum conditions in lowering concentrations of methane CH4 and carbon monoxide CO. Special attention is paid to the analysis required to gain insight into how gas removal performance is influenced by discharge polarity (positive vs. negative) for applied voltages of 15, 20, and 25 kV. Before and after treatment, gas concentrations were measured using MQ-2 and MQ-4 sensors. According to the manufacturer’s specifications, combustible gases such as methane and hydrogen are detected within ranges of approximately 200-10,000 ppm (MQ-4) and 300-10,000 ppm (MQ-2). Because the experiments were conducted in ambient air, the actual gas concentrations were expected to be much lower than these detection limits. Before each test, both sensors were baseline-calibrated in clean air to minimize drift and stabilize their response.
Methods
Paschen’s law and breakdown threshold in cylindrical geometry
Paschen’s Law is used for the correlation of the breakdown voltage (Vb) with the pressure-electrode gap product (p * d). The minimum voltage needed for full electrical breakdown (spark discharge) in a gas-filled space between two electrodes is defined by the law. The equation for the law is9:
(1)
The collection efficiency
The collection efficiency ( η ) of the tubular precipitator was calculated according to the Deutsch-Anderson equation13:
(2)
Where C0 is the inlet gas concentration and CL is the outlet gas concentration after treatment (ppm).
Electron temperature
The ratio approach is one of the most often used optical emission spectroscopy (OES), and the Boltzmann equation is used to find the electron temperature as14:
(3)
The term gj refers to the high-level statistical weight associated with the transitional state, while Iji denotes the relative emission line intensity between energy levels iii and j. The wavelength is represented as λji (measured in nanometers), and Ej corresponds to the excitation energy of level j (expressed in electron volts). The parameter Aji signifies the probability of spontaneous radiation transition from level i to j. Additionally, N stands for the population density, and kB is Boltzmann’s constant.15
Electron number density
Can be calculated as16:
(4)
The Stark parameter’s full width is theoretically described by the spectral line w, where the electron density is estimated at approximately Nr≈1017 cm−3. The FWHM refers to the full width at half the maximum intensity of the spectral line.17
The specific energy input (SEI)
It is defined as the energy consumed per unit volume of gas treated, and is a key parameter in evaluating the energy efficiency of plasma reactors. It is calculated as11:
(5)
Where SEI: Specific Energy Consumption (J/L), P: Electrical Power (W = V × I), t: Operating Time (seconds), and Vgas Volume of gas processed during time (in liters).
Set up
The tubular electrostatic precipitator consists of two coaxial cylindrical electrodes encased in an outer Pyrex tube. The outer electrode is the hollow aluminum cylinder that acts as the collecting electrode, 28 cm long and 4.8 cm in diameter. The inner electrode, serving as the discharge electrode whose polarity is changed either positive or negative in the experiment, is a metallic rod, 37 cm long and 0.5 cm in diameter, that charges the air particles that enter the precipitator chamber. Both electrodes are encased in an outer Pyrex tube 38.2 cm long and 7 cm in diameter that provides electrical insulation and maintains an undisturbed environment for the experiment.
As illustrated in Figures 1 and 2, the tubular electrostatic precipitator was connected to a vacuum pump (Model: VE115N, China) through a pressure gauge for controlling the internal chamber pressure precisely. A high DC voltage of up to 25 kV was applied between the two electrodes using a high-voltage DC power supply to initiate the spark discharge under both positive and negative polarities, thus generating a very intense electric field. Due to the coaxial configuration, the electric field inside the precipitator is non-uniform; it is the strongest near the inner electrode and gets weaker toward the outer electrode. At an applied voltage of 25 kV, the field strength is in the range of approximately 4.6 kV/cm at the outer wall to 44 kV/cm near the inner rod surface. To sustain the required electric field, a high-voltage DC power supply Model: DY10, Cloudray (Nanjing) Laser Technology Co., Ltd. was employed, with AC input: 220V, DC output: 35kV 23mA max. The flow rate of the rotary vacuum pump Yangyi VP115 was approximately 50 L/min, while the internal chamber volume was approximately 1.8 L. Using these values, an estimated residence time of approximately 2.2 s for the gas inside the ESP chamber can be obtained. This is long enough to ensure sufficient interaction time between gas molecules and plasma. For the measurement of methane CH4 and carbon monoxide CO concentrations, four gas sensors have been used, comprising two MQ-2 gas sensors for detecting CO and two MQ-4 gas sensors for detecting CH4. The sensors have been well-positioned at the inlet and outlet points of the precipitator chamber so that the gas concentrations before and after the treatment can be recorded. For acquiring and processing the sensor output, an Arduino microcontroller was utilized, digitizing the sensors’ analog signals and transmitting the digital data to a computer in real-time, with the possibility to monitor gas concentrations within the chamber and record the data reliably.17
Figure 1. Tubular electrostatic precipitator.
Describes the shape and dimensions of the manufactured electrostatic precipitator.
Figure 2. Set-up.
Represents the working part.
Despite the promising results, several limitations should be acknowledged. The MQ-2 and MQ-4 sensors used for gas concentration measurements exhibit limited sensitivity and selectivity at low CH4 and CO levels, which may introduce measurement uncertainty. In addition, the voltage range (15–25 kV) restricted the exploration of higher discharge intensities that might further influence gas removal efficiency. Minor fluctuations in ambient temperature and humidity could also have affected plasma stability and sensor response.
Results
Voltage–current characteristics and the influence of gas pressure and polarity on spark discharge in a tubular ESP
In this section, a detailed voltage-current (V-I) curve of a tubular electrostatic precipitator (ESP) at ambient air and under both negative and positive polarity is analyzed. The central rod electrode was made negative and then positive in turn, the cylindrical collector being kept at ground potential. The experiment was aimed at investigating the influences of various air pressures 380 torr, 406 torr, and 508 torr and discharge polarity on the behavior of sparks within an applied voltage range of 14 to 25 kV.
Spark discharge in negative polarity
Figure 3 shows negative polarity in which spark discharge began at the minimum voltage applied (14 kV), with current varying between 0.011 and 0.013 mA. This means that the applied voltage was above the breakdown voltage of air, and thus it went directly to the spark discharge regime without passing through the corona regime.18–20 The spark discharge is initiated by the rapid acceleration of free electrons, which leads to avalanche ionization.21 As the voltage was increased to 25 kV, the current value continuously grew in a linear trend, which characterizes a stable spark discharge mode in which no destructive arc is observed.22 At 25 kV, the current across all tested pressures converged to approximately 0.023 mA, reflecting a state of ionization saturation, beyond which an increase in current cannot be achieved due to the limited number of available neutral molecules.23,24
Figure 3. Voltage-current characteristics for a negative spark.
Describes the voltage-current curve for a negative spark.
Spark discharge in positive polarity
Under positive polarity, the system showed similar linear I–V characteristics Figure 4, but with slightly lower current values at all voltages. The starting current at 14 kV was in the range of 0.011–0.012 mA. This increased gradually and linearly with voltage, reaching 0.022–0.023 mA at 25 kV. Similar to negative polarity, pressure affects current considerably at low voltages: pressure of 508 Torr gave a higher current because of longer electron mean free paths and more efficient ionization.20 At higher voltages, pressure had little effect, and the current values converged, which means full ionization saturation occurred.24
Figure 4. Voltage-current characteristics for a positive spark.
Describes the voltage-current curve for a positive spark.
In both polarities, gas pressure had a clear influence at voltages below 20 kV. Lower pressures favored higher current due to reduced collision frequency and enhanced ionization by more energetic electrons.13 At 25 kV, the saturation occurred, and the impact of pressure became negligible. This confirms that with the increase of the applied voltage, the spark discharge makes a transition from the pressure-sensitive to the saturation-dominated one.24,25
Although both polarities followed similar spark discharge behavior, key differences were observed. Negative polarity at the same voltage always produced higher current than positive due to the higher mobility of electrons compared to that of positive ions.25 Positive spark discharge was slightly more stable and quieter than negative discharge, which, though stronger, may exhibit Trichel pulse patterns under certain conditions.19 Both polarities showed a drop in pressure sensitivity with increasing voltage; this effect was, however, more pronounced in the negative mode due to the dominance of electron-driven ionization.
These results emphasize that polarity and gas pressure are key variables in the operation and optimization of ESPs for many applications where the stability and efficiency of charged particles are concerned.
Gas collection efficiency
As can be seen in the figures below 5-10, the collection efficiency of a cylindrical electrostatic precipitator (ESP), operating with spark discharge under negative and positive polarities, is strongly affected by the discharge voltage, internal pressure, and operating polarity. In particular, in the case of methane CH4, the maximum collection efficiency was obtained at 88% negative spark discharge at 380 Torr and 20 25 kV. Likewise, carbon monoxide CO achieved 87% efficiency under the same conditions. This voltage dependence is expected based on electrostatic theory, where the voltage is seen to increase the strength of the electric field, and thus is important to enhance ionization and particle charging, both of which are important in pollutant removal in ESPs.26
Figure 5. The collection efficiency of CO2 and CH4 for a negative spark at a pressure of 380 Torr.
Figure 6. The collection efficiency of CO2 and CH4 for a negative spark at a pressure of 406 Torr.
Figure 7. The collection efficiency of CO2 and CH4 for a negative spark at a pressure of 508 Torr.
Figure 8. The collection efficiency of CO2 and CH4 for a positive spark at a pressure of 380 Torr.
Figure 9. The collection efficiency of CO and CH4 for a positive spark at a pressure of 406 Torr.
Figure 10. The collection efficiency of CO and CH4 for a positive spark at a pressure of 508 Torr.
Figures 5-10 illustrates the precipitator efficiency for several pressures of two different gases.
Regarding the pressure, the efficiency of collection reduced as the pressure decreased. For illustration, with CH4, the efficiency decreased from 88% at 380 Torr to 63% at 508 Torr at 25 kV, while with CO, it reduced by 87% to 53% under similar conditions. This is because the low gas pressures decrease the collision frequency and the effective ionization cross-section, hence weakening the performance of plasma generation and collection. These findings are in good agreement with theoretical simulations and experimental observations of the U.S. Environmental Protection Agency that have indicated ESP performance generally is better at moderate and high pressures due to improved particle charging and migration behavior.27
Polarity was also found to be a critical factor, with the negative spark discharge being superior to the positive in all voltages and pressures tried. As an example, the collection efficiencies of CH4 and CO were 88% and 87% negative polarity at 25 kV and 380 Torr, compared to 81% and 83% positive polarity. This superior performance of negative polarity is well documented in the plasma engineering literature and is explained by the more intense and stable electron avalanches produced by cathode-driven discharges, leading to denser ionization and an increased reaction rate with pollutant molecules.28
External plasma discharge experiments also corroborate these results. Ono and Oda (2017) studied CO, CH4 conversion with a multi-electrode dielectric barrier discharge and directly increased the gas conversion rates with discharge voltage. The mechanism of discharge was different (dielectric barrier discharge vs. spark), but the overall effect of the voltage on the plasma intensity and the efficiency of chemical processing resembles the current findings. Their findings affirm that the optimization of discharge parameters is central to the maximization of the efficiency of gas-phase pollutant treatment systems.29
Table 1 describes the highest collection efficiency of methane CH4 and carbon monoxide CO for different applied polarities and voltages for a 120-second time of treatment.
Table 1. Highest collection efficiency for CH4 and CO with different polarities.
The data present that the most effective condition for the elimination of pollutants by a cylindrical electrostatic precipitator (ESP) with spark discharge is by negative polarity, at a higher voltage of 25 kV, and at a pressure of 380 Torr. Such a condition ensured the highest reduction of concentrations of CH4 and CO throughout the plasma treating process.
All the experiments were repeated three times for each condition to be reproducible. Mean value and standard deviation were calculated accordingly. The statistical analysis also confirmed the significance of variation of collection efficiency and plasma parameters between positive and negative polarity, which indicates better performance under negative discharge conditions.
They agree on the need to set both the electrical conditions (such as voltage and polarity) and the physical conditions (for example, pressure) in a correct manner for the utmost performance of the ESP-based systems for treating gaseous pollutants.
Plasma parameters and their influence on gas removal efficiency
Plasma generated by spark discharge inside the cylindrical electrostatic precipitator was characterized using optical emission spectroscopy (OES), specifically through analysis of selected O II emission lines. This method enabled the extraction of key plasma parameters, including electron temperature (T e) and electron density (n e), across varying discharge voltages, pressures, and polarities.
As summarized in Table 2, the results revealed a clear increase in both Te and ne with increasing voltage and pressure; values were significantly higher under negative polarity at all tested voltages and pressures. For instance, at 25 kV and 380 Torr, the negative spark discharge produced a Te of 0.366eV and an electron density of 2.6 × 1017 cm−3, whereas the positive polarity under the same conditions yielded only 0.225 eV and 0.99 × 1017 cm−3. These values are consistent with previously reported ranges for high-pressure spark discharges, where Te typically lies between 0.5 and 3 eV and ne between 1015 and 1019 cm−3.10,28
Table 2. Plasma parameters (T e and ne) under different conditions of voltage, pressure, and polarity.
The elevated electron temperature indicates a higher energy distribution among free electrons, enabling efficient dissociation of stable molecular species like CH4 and CO. Meanwhile, the increased electron density supports the generation of reactive species such as O• and OH•, which are essential in oxidizing and decomposing these gases. Notably, the highest CH4 removal efficiency of 88% was achieved under the same conditions where T e and n e peaked, directly linking enhanced plasma activity with pollutant breakdown.
These findings confirm that gas removal in this system is not governed by electrostatic collection alone, but rather by plasma-induced chemical reactions driven by the energy and density of the electron population.
Specific energy input and its effect on pollutant removal efficiency
The obtained Specific Energy Input (SEI) (calculated as per Equation 4) indicates that there is a high correlation between the energy demand in liters of gas and the efficiency of pollutant removal. In the case of both CH4 and CO, the SEI values showed that the higher the value, the better the removal efficiency, especially at 25 kV and 380 Torr. In addition, under the same conditions, negative polarity always produced higher SEI and higher removal efficiency than positive polarity. Such an effect is in line with what is known about spark discharges, in which the negative polarity increases the strength of the electric field and facilitates the production of high-energy electrons and reactive species.10,28
Also, when the operating pressure was raised to 380 Torr (compared to 508 Torr), both SEI and pollutant breakdown were enhanced, probably owing to enhanced collisional interactions and elevated plasma density. These results affirm that the electrical discharge geometry, in addition to the energy input, is critical to the maximization of the gas removal efficiency of ESP-based plasma systems.
As an example, SEI rose as the negative polarity rose at 380 Torr, 574 J/L at 15 kV, and 873 J/L at 25 kV. The same tendency was observed in positive polarity when SEI values increased from 505 J/L to 812 J/L. These values were highly associated with CH4 removal efficiencies that reached 88% at the most severe SEI condition.
Effect of plasma on temperature and relative humidity
During electrical discharge operation, the relative humidity (RH) was maintained constant at 36%. Measurements were performed using an HTC-2 Thermo-Hygrometer, Figure 10, which simultaneously records both temperature and RH with digital precision. The results showed in Figure 11 a gradual increase in outlet gas temperature from 45.0°C to 46.6°C over 120 seconds, whereas the inlet temperature remained stable at 26.3°C. This indicates that the temperature rise was induced by the energy dissipated during the discharge process.30
Figure 11. Temperature variation of inlet and outlet air with time at RH = 36%.
It shows the humidity ratio of the air entering and leaving the precipitator.
Such a temperature increase is attributed to non-thermal energy transfer mechanisms within the ESP chamber, where high-energy electrons impart kinetic energy to gas molecules through frequent collisions. Similar trends were observed by Jaworek et al., who reported that ion drift and corona current lead to localized gas heating in enclosed systems.22
According to the U.S. Environmental Protection Agency, the internal temperature rise in ESPs is primarily a result of corona current. Since RH remained unchanged, it can be inferred that the heating was not caused by moisture transfer, but rather by discharge energy.31 This interpretation aligns with plasma discharge theory outlined by Kogelschatz, which states that even non-thermal discharges can induce localized heating due to cumulative energy deposition from the electric field.
Plasma chemistry and reaction mechanisms
Methane CH4, is a potent greenhouse gas with a global warming potential approximately 28 times greater than CO2 over 100 years. It is also chemically stable and non-polar, which makes its removal through conventional filtration or absorption techniques highly inefficient.32
Carbon monoxide CO, a colorless and odorless gas, is a major product of incomplete combustion. It is toxic at concentrations above 50 ppm and is difficult to oxidize at low temperatures without the aid of catalytic surfaces.33
These airborne pollutants require advanced ionization or oxidation strategies for effective decomposition. The use of corona discharge in a vacuum-operated electrostatic precipitator (ESP) enables such processes by producing a range of reactive plasma species such as atomic O•, ozone O3, OH•, and electrons (e−), which are capable of fragmenting or converting CH4 and CO molecules.14
When a high-voltage corona discharge is maintained in air containing CH4 and CO, various non-equilibrium plasma reactions can be initiated. For example:
Reactions involving CH4:
Reactions involving CO:
These reactions are driven by energetic electrons and reactive radicals formed in the discharge zone. Their efficiency is influenced by operational parameters such as pressure, electron temperature, and plasma density, which in turn depend on voltage polarity and vacuum conditions.34,35
In addition to O• and OH• radicals, ozone O3 also plays a significant role in the oxidation of CH4 and CO molecules within the plasma zone. The formation and participation of O3 in these reactions further enhance the overall efficiency of gas decomposition under electrical discharge conditions.
Ozone generation in ESP using electrical discharge
A cylindrical electrostatic precipitator (ESP) was operated using both corona and spark electrical discharges under fixed sub-atmospheric pressures of 380, 406, and 508 Torr. The duration of operation was kept constant at 120 seconds per test. A rotary vacuum pump (Yangyi VP115) with a maximum flow rate of approximately 50 L/min was employed to stabilize the internal pressure and ensure adequate residence time of the gas mixture within the discharge chamber.
The objective was to evaluate the generation of O3 during electrical discharge and to compare the output concentrations across different polarities and voltage levels. An MQ-131 semiconductor sensor was used to measure ozone concentrations with a high sensitivity range of 10-1000 ppb (0.01-1 ppm).
The obtained results indicated that the concentration of ozone rose as the applied voltage rose. Moreover, negative discharge polarity was always able to generate higher O3 levels as compared to positive polarity at equal pressure and voltage conditions. This is due to more energetic electron avalanches and an enhanced production of atomic oxygen in negative discharges, a precursor to ozone formation via the following reactions:
A summary of the O3 concentrations for different operating conditions is provided in the following.
These values indicate that the negative polarity formed a little more concentration of O3 as compared to the positive polarity at the same voltage. Table 3 shows that this result is consistent with known mechanisms of discharges, in which negative discharges tend to produce high electron densities, and therefore should be more effective at forming ozone.
Table 3. Ozone Concentration in ppm generated during different polarities at different voltages.
| Voltage (kV) | O3 Concentration in ppm | ||
|---|---|---|---|
| Positive polarity | Negative polarity | ||
| 15 | 0.144 | 0.147 | |
| 20 | 0.146 | 0.161 | |
| 25 | 0.148 | 0.164 | |
The results agree well when compared to other data that had been published earlier. For instance, Li et al. (2019) reported that non-thermal corona discharge used to treat CH4 and CO generated ozone concentrations in the range of 0.1–0.3 ppm during the initial minutes of operation. The ozone was instrumental in oxidizing pollutants into less harmful compounds such as CO2 and H2O.36
Similarly, Wang et al. (2014) confirmed that ozone produced under low-pressure electrical discharge conditions effectively reduced CO levels, converting it into CO2 via ozone’s strong oxidizing potential, even under ambient temperatures.37
When the ESP operates in air containing CH4 and CO, O3 is produced primarily through reactions involving molecular O2 and energetic electrons or UV photons generated during electrical discharge. In such a system, ozone is not a pollutant, but a catalytic oxidizing medium to break down CH4 and CO. As confirmed by Li et al.36
“Ozone was found to play an essential role in methane oxidation during non-thermal plasma treatment.”
Thus, the ozone generated within the ESP serves as a chemically active component that enhances the system’s ability to remove gaseous pollutants, in addition to aiding particulate collection. Due to its oxidative power, ozone facilitates reactions such as:
The presence of O3 in the discharge chamber directly contributes to reducing toxic emissions and improving overall air quality. These findings support the conclusion that intentional and optimized operation of ESPs can effectively utilize plasma-generated O3 for the treatment of low-concentration combustion gases in non-thermal air purification systems.
Analysis of discharge behavior
Although Paschen’s Law predicts the breakdown voltage for air at the tested pressures and electrode spacing to be in the range of 163–236 kV, the experimental spark discharge was consistently initiated at much lower voltages, between 15 and 25 kV.16 This observation suggests that the discharge is not a classical full-gap breakdown, but instead represents a localized spark event facilitated by strong electric field enhancement at the rod electrode tip.
Figure 12 shows that enhancement lowers the effective local breakdown threshold and permits inception-type spark discharges to be started early within the gradient system, and those are compatible with streamer propagation models seen in high-gradient discharge systems.38,39
Figure 12. Evolution of electrical discharge shape in a cylindrical electrostatic precipitator under varying voltage (15–25 kV).
This represents the shape of the plasma generated at different voltages, starting with 15 kV, 20 kV, and 25 kV, respectively.
This behavior is used to explain why spark discharge is always created in both polarities at relatively low voltages, and it also indicates how the electrode geometry and field concentration can dictate the discharge properties in the compact ESP designs. This significant deviation from Paschen’s prediction can be explained by the non-uniform electric field distribution inherent to the rod–cylinder geometry of the ESP, which produces localized regions of extremely high field intensity near the electrode tip. Paschen’s law assumes a uniform field and constant gas composition, conditions that are not strictly satisfied in this setup. Under partial-vacuum operation, the increased electron mean free path and the presence of surface irregularities promote micro-inception and streamer development at much lower voltages than the theoretical uniform-gap breakdown threshold. Consequently, the experimentally observed sparks represent localized inception-type discharges rather than full-gap breakdown, consistent with streamer-assisted discharge models reported in non-thermal plasma systems.40–42
Conclusion
This study established that a tubular electrostatic precipitator (ESP) can be used under spark discharge and sub-atmospheric pressure conditions to achieve efficient removal of low levels of methane CH4 and carbon monoxide CO in air. Experimental findings indicated that negative spark polarity always gave better performance than positive polarity, with higher current, electron temperature, and electron density being produced, along with higher O3 concentrations. These aspects led directly to an increase in the target gases’ removal efficiency. A rise in the applied voltage, increasing the applied voltage from 15 to 25 kV, enhanced plasma activity due to the strengthening of the electric field, increasing the electron energy and density, as well as increasing the ionization and radical generation. The maximum removal efficiencies were obtained at 25 kV and 380 Torr, where CH4 attained 88% and CO attained 87% in negative polarity.
The results indicate that the removal of CH4 and CO was primarily due to plasma-chemical reactions rather than electrostatic collection. The reaction with these reactive species, atomic O•, OH•, and O3, oxidized CH4 and CO to CO2 and H2O. Higher gas pressure resulted in a more stable plasma and a more uniform discharge, whereas low pressures caused low ionization and poor removal capabilities. Specific energy input (SEI) rose with the voltage and reached a maximum of the SEI value of 873 J/L was obtained at the best-performing operating conditions of 25 kV, 380 Torr, and negative polarity, which were associated with maximum gas removal efficiency. In terms of thermal effects, the system exhibited a measurable rise in outlet gas temperature, increasing from 26.3°C to 46.6°C over 120 seconds, a net gain of 20.3°C, while relative humidity remained constant at 36%. This heating is attributed to energy transfer by ion drift and electron-neutral collisions within the discharge, consistent with non-thermal plasma behavior in enclosed ESP systems. Therefore, the study confirms that the combination of negative spark discharge, high voltage, and moderate vacuum conditions yields a highly efficient plasma-assisted system for gaseous pollutant removal. These insights can inform future designs for compact, low-energy air purification technologies targeting CH4 and CO.
Beyond the removal efficiencies, the plasma diagnostics indicated that the electron temperature (Te) ranged from about 1.8 to 2.3 eV and the electron density (ne) increased up to approximately 7.5 × 1011 cm−3 at 25 kV, accompanied by an ozone concentration of about 0.4 ppm under negative polarity. These values confirm the strong coupling between discharge energy and plasma reactivity in the ESP system.
The novelty of the proposed setup lies in its hybrid operation mode a tubular electrostatic precipitator functioning as a spark-discharge plasma reactor under partial vacuum. This configuration has not been systematically examined in previous studies and enables high pollutant conversion at moderate voltage and pressure levels, providing an energy-efficient alternative to conventional DBD or corona reactors.
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Adil Abbas H and Adnan Abbas Q. Influence of spark polarity on the tubular Electrostatic precipitators characterization for CH4 and CO gases [version 1; peer review: awaiting peer review]. F1000Research 2025, 14:1370 (https://doi.org/10.12688/f1000research.173071.1)
NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article.
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