Application End Evaluation of Electrostatic Precipitation for Control PM and NOx Emissions from Small-Scale Combustions

Abstract

Electrostatic precipitators (ESPs) have shown promise in reducing particulate matter (PM) emissions, but their potential for simultaneous NOx reduction in small-scale combustion systems remains underexplored. This study focuses on using non-thermal plasma generated in a corona discharge to reduce PM and NOx emissions from small-scale combustion. ESP was specifically designed for a commercially available 15 kW boiler with wood pellet combustion and used with both positive and negative discharge polarity to control emissions without any chemical additives. ESP performance was evaluated across a range of specific input energies (SIE) in terms of particle mass and number concentrations and NOx concentrations obtained by continuous gas analysis. ESP ensured the reduction in PM concentrations from 48 mg/m³ to the magnitude of PM content in the ambient air. The highest precipitation efficiency was observed for particles in the 20–200 nm range. Concurrently, NOx emissions were reduced by up to 78%, from 178 mg/m³ to 39 mg/m³. These results were achieved at specific input energies of 36 for positive and 48 J/L for negative corona, which is significantly lower than those reported for many existing separate PM and NOx control systems. This study demonstrates the potential of ESPs as a compact, energy-efficient solution for simultaneous PM and NOx removal in small-scale combustion systems, offering promising implications for improving air pollution control technologies for small-scale combustion systems.

1. Introduction

Small-scale combustions (with a heat–power output of less than 500 kW) generate dangerous pollution that significantly impacts the environment and human health, causing over 3 million premature mortalities, annually surpassing the toll of wars and violence before 2015 [1]. The most contributive emissions are particulate matter (PM), due to their harmful effect on the human cardiovascular system [2], and NOx, due to their contribution to respiratory diseases [3], so the present work focuses on them.

Legislative limits on atmospheric emissions for small-scale combustions were forced to be more stringent; therefore, new post-combustion gas cleaning technologies or improvements of used methods are required.

The problem of emissions from industrial combustion is mostly solved by the consecutively applied technologies for individual control, such as electrostatic precipitation of PM, ensuring up to 99.99% for particles larger than 1 μm [4], and selective catalytic reduction systems achieving NOx removal efficiencies of 80–90% [5].

Due to their operation and required heat output, small-scale combustion systems, including residential boilers, present unique challenges such as their instability in combustion gas volume and particle concentration and size distribution size constraints. Even though the physical background of gas cleaning for small-scale and industrial combustions is the same, expanding industrial technologies for pollution control on small-scale combustions may not be sufficient or technologically unattainable [6]. Therefore, a single compact gas cleaning system that removes both solid and gaseous contaminants is required to solve emissions from small-scale combustions.

Electrostatic precipitators (ESP) use an electric field to charge and remove particles. The mechanism of corona discharge formation is different due to the polarity. A constant voltage causes a negative or positive corona to accumulate a space charge in the vicinity of the discharge electrode. In the negative corona, the positive ions bombard the discharge electrode knocking out electrons from its surface. The positive corona also has positive ions distributed near the discharge electrode, but these are constantly pushed into the interelectrode space while electrons are drawn into the electrode. Moving electrons and generated ions represent the corona current, whose magnitude is appreciably higher for the discharge of negative polarity due to more developed ionisation.

Regardless of polarity, corona discharge in ESP generates non-thermal plasma (NTP), which initiates the dissociation and ionisation of gas molecules, and interactions among species in the corona discharge medium can result in chemical processes [7]. Particles arrive at this medium and acquire charges through collision with ions and molecule radicals. Therefore, the presence of particles reduces the capacity for chemical transformations in gases. This is especially evident for industrial ESPs, owing to the substantial concentrations of large particles present in combustion gases. In contrast, combustion gases from small-scale boilers contain low PM concentrations presented mostly by ultrafine particles with aerodynamic diameters less than 200 nm [8], which are not critical. Thus, gaseous contaminants can be chemically treated and converted from the gaseous phase to aerosols, which can then be precipitated with initial fly ash. Thus, a single ESP without chemical reagents can simultaneously remove PM and NOx promising sufficient and low-cost removal efficiency.

The potential of corona discharge NTP for NOx removal was stated previously: Wang, Sun, and Zhu [9] observed 60% removal for NOx, while Wu, Gao, Luo, Wei, Zhang, et al. [10] observed the 70% reduction for NOx; simultaneous control of PM and NOx and was explored before being applied to small-scale combustions [11]. Nevertheless, a significant research gap in the combined removal of these pollutants from small-scale combustion sources remains.

The novelty of the present study is to introduce an innovative approach to addressing the existing gap by employing non-thermal plasma generated through corona discharge for the simultaneous reduction of particulate matter (PM) and nitrogen oxide (NOx) emissions from small-scale combustion systems. In contrast to the research above, which primarily focused on the separate removal of PM and NOx or required multiple treatment stages, this investigation examines the potential of a single electrostatic precipitator (ESP) to control both pollutants without the need for chemical additives.

The present study contributes to the existing knowledge and investigates the effectiveness of a specially designed ESP in simultaneously removing both PM and NOx from emissions of a 15 kW wood pellet boiler. The ESP performance with positive and negative coronas was compared for both pollutants, focusing on removal efficiency and energy consumption and compared to other technologies.

The study explores the mechanisms involved in the precipitation of particles of varying sizes and conversion processes associated with NOx within the context of emissions from small-scale combustion.

The potential of ESP for emission control in small-scale combustion systems is evaluated, aiming to provide valuable insights into a novel, integrated approach to emission control. This approach could offer a more effective and economical solution compared to separate technologies for PM and NOx control.

2. Materials and Methods

The method to control boiler emissions as described below was evaluated using the experimental setup in Figure 1.

The combustion gases were generated in the 15 kW automatic boiler and removed from installation by the fan, operated to maintain the negative pressure in the boiler stuck at 15 Pa.

To control PM and NOx from the boiler, the special honeycomb ESP was designed, in which collecting electrodes were formed by 20 hexagons with a side length of 25 mm; wire discharge electrodes of stainless steel wires (0.35 mm diameter) were installed along the axis of each hexagon cell. The electrodes had an active length of 1000 mm. The total collecting area of the designed ESP (A) was over 3 m². ESP was operated with negative/positive polarity. The ESP was powered by an XP Glassman high-voltage power supply, model PS/030R040-22, capable of delivering a maximum output voltage of 30 kV and a current of 40 mA. The current measurements of the HV unit have an accuracy of better than 1%, while the voltage measurements deviate by less than 2% of the rated output voltage, with recovery to within 0.1% in 1 ms. The measurements were undertaken when ESP was powered in five modes with current values of 1, 5, 10, 20, 30, and 40 mA.

All the tests were undertaken with stable boiler operation at steady CO concentration and flue gas temperature and repeated five times with the ESP off/on. Sampling points for detecting gas compounds, temperature and concentration of suspended particles were located downstream of the designed ESP in the straight duct section due to [12]. All samples were isokinetic and strictly from the centre of the duct. All measured concentration values were normalised to the volume unit of dry gas at 101.325 kPa, 0 °C, and reference O₂ at 10%.

The combustion gas compound was detected by a Non-Dispersive Infra-Red (NDIR) detector by ABB, allowing measurement concentrations of O₂, CO, NOx, and CO₂ within 1.5% accuracy. Before each experimental series, all NDIR analysers were calibrated using relevant calibration gases. The temperature of the combustion gases was measured with a thermocouple (type K).

The particulate matter content in combustion gases was evaluated in terms of both mass (PM) and number (PN) concentrations. The PM was measured gravimetrically, with 10% tolerance, using a 30 min isokinetic sampling, corresponding to the standard [12], which was adapted to suit the requirements for small-scale combustion units as specified in the standard [13]. The samples were taken from a straight section of duct downstream of the ESP, maintaining an isokinetic tolerance of 20%.

PN was defined by the Dekati^® electrical low-pressure impactor (ELPI), whose operation is based on particle pre-charging and the consequent inertial classification in a cascade impactor. ELPI allows online particle classification in 14 channels of aerodynamic diameter, so PN was evaluated as the total number concentration and particle size distribution within approximately 5% tolerance. A detailed description of ELPI can be found elsewhere [14].

The samples were adapted to the ELPI operating capabilities with a Dekati^® FPS-4000 fine particle sampler. The dilution system avoids problems associated with clogging and condensation in the downstream equipment. Samples were diluted with ambient air, pumped with a compressor, and then filtered with a HEPA filter to concentrations below 2000 #/cm³. The dilution ratio was settled at 1:80 and verified by comparing the CO₂ concentrations in the flue gas and the sample.

The particle concentration data are presented as total particle number concentration DN and fractional concentrations as dN/dlog(Dp), where dN is the particle number concentration for each measured size bin, and dlog(Dp) is the difference in the log of the channel width of the particle diameters Dp. Different instruments may use different bin widths for particle size ranges. dN/dlog(Dp) normalises these differences, allowing for direct comparisons between measurements from various instruments.

To evaluate the NOx abatement efficiency, combustion gases were sampled at on/off operation regimes of the special ESP and results were analysed:

$${\eta }_{\left[NOx\right]}=\left(1-{\displaystyle \frac{{\left[NOx\right]}_{ESP on}}{{\left[NOx\right]}_{ESP off}}}\right)\times 100\%$$

(1)

The same Equation (1) was used to determine the particle removal efficiency. The concentration values were normalised to the volume unit of dry gas at 101.325 kPa, 0 °C, and reference O₂ at 10%.

The specific input energy SIE [J/L], or Becker parameter, was evaluated as follows

$$SIE={\displaystyle \frac{I\times U}{V}}$$

(2)

where U is applied voltage [kV], I is electric current in ESP [mA], and V is the flow rate of combustion gas [L/s].

3. Results and Discussion

3.1. Parameters of Combustion Tests

Information about emissions in the combustion gases at the ESP-off operation regime is presented in

Table 1. The experimental conditions.

Parameter	Unit	Negative/Positive Corona
Flue gas temperature	°C	130
Gas flow rate	L/s	10
H2O content	vol%	10.3
O2 content	vol%	12
NOx * ESP-off regime	mg/m3	178
PM * ESP-off regime	mg/m3	48
PN * ESP-off regime	#/cm3	1.5 × 107
Electric current in ESP	mA	1	5	10	20	30	40
Voltage value	kV	7/8.5	8.5/9	9/10	10/11	11/12	12/-
Specific input energy	J/L	0.7/0.85	4.25/4.5	9/10	20/22	33/36	48/-

* in dry flue gas (0 °C, 101.3 kPa); at reference O₂ = 10% vol.

. The measurement results are considered representative since the emission characteristics of the studied boiler were typical for small-scale heating units, and the ESP electrical parameters were aligned with previously published data.

The current–voltage characteristic of the studied ESP is presented in Figure 2 for both polarities.

The minimum current of 0.1 mA was observed at voltages of 5.0 kV and 7.7 kV for negative and positive polarity, respectively. In the positive polarity, the corona discharge was limited by sparks and breakdowns at 11.9 kV and 30 mA. In contrast, the negative corona discharge remained stable up to 12 kV, constrained by the XP Glassman current limit of 40 mA.

3.2. Changes in NOx and Particle Concentrations

Figure 3 demonstrates changes in the concentration of PN and NOx with ESP operation referred to SIE for positive and negative corona discharge.

Fuel was dosed into the boiler at regular intervals, ensuring combustion of each dose processing through three different combustion stages: (i) initial combustion, a highly unstable stage with by suboptimal thermal conditions and improper air settings, (ii) optimal combustion with the correct air/fuel ratio and stable temperature, and (iii) smouldering, which is characterised by incomplete oxidation. Processing through three different combustion stages exhibited variations in combustion gas composition, along with fluctuations in particle concentration and NOx levels. The weighted average pollutant concentrations are presented in Figure 3 against the background of fluctuations due to combustion stages.

NOx removal efficiency can be observed as a sublinear function of SIE for negative corona in the range from 5% to 77% and for positive corona in the range from 11 to 76%. Demonstrated ESP efficiency can be supported by other researchers, including [15], who observed a similar reduction in soot and NOx concentrations by NTP treating gases exhausted from a diesel engine, and [16], who investigated NOx removal from coal combustion gases. More information on NOx removal in NTP generated by different types of discharges can be found in a comprehensive review [17].

In the studied ESP, the energy consumption of NOx removal with the same efficiency was higher for the negative corona: maximal 78% removal efficiency was achieved with the SIE of 36/48 J/L for positive/negative corona. The efficiencies of NOx removal and particle precipitation were calculated due to Equation (1), and values for SIE were calculated due to Equation (2).

The mechanism of NOx removal in developed ESP can be broadly divided into several key stages: plasma generation, active species formation, NOx oxidation, acid formation, and nucleation.

DC corona discharge generated in an ESP creates a non-thermal plasma in a thin region in close vicinity to discharge electrodes. This plasma consists of electrons, ions, and neutral species in an excited state. The energetic electrons in the plasma collide with gas molecules, leading to dissociation, ionisation, and excitation processes. Electron-induced dissociation, driving the formation of active species, is highly dependent on the electric field strength. These processes generate active species such as atomic oxygen (O), atomic nitrogen (N), hydroxyl radicals (OH), and ozone (O₃), which play crucial roles in NOx removal.

The nature of this plasma differs slightly between positive and negative corona discharges: in a negative corona, electrons are accelerated away from the discharge electrode, resulting in a higher average electron energy, while a positive corona produces a more uniform distribution of reactive species throughout the inter-electrode space [18]. Moreover, the ionisation in the remaining wide zone of the interelectrode gap outside this thin region becomes more intensive for the negative corona. Although this ionisation has minimal impact on NOx decontamination, it contributes to an increase in corona current, thereby raising the overall energy consumption for NOx removal.

More than 700 reactions are known for corona discharge in combustion gases composed typically of CO₂, N₂, O₂, and H₂O [19]. Eliasson and Kogelshatz [20] examined ionic processes, which are dynamic over time, adding further complexity to the chemical kinetics involved and making the comprehensive corona discharge model quite difficult to implement. A comprehensive study on the chemical kinetic of NOx removal can be found elsewhere [21].

The present work uses a previous chemical model previously developed by Mok, Won, and In-Sik [22], applied for positive pulsed corona discharge demonstrating a 10% agreement with experimental measurements [23], which was subsequently validated in a further study [24].

The primary mechanism for NOx removal in an ESP involves the oxidation of NO to higher nitrogen oxides, to NO_2, NO₃, and N₂O₅, which all react to OH radicals and water vapour generating vapours of nitric acid (HNO₃).

For simplicity, reactions negligibly involved in NOx decontamination [25] and reactions with low reaction rates or poor concentrations of substances [22] were omitted in the present work. The primary reactions involved in NOx conversion can be summarised as follows:

NO oxidation:

NO + O + M → NO₂ + M

(R1)

NO + O₃ → NO₂ + O₂

(R2)

NO₂ conversion:

NO₂ + O → NO + O₂

(R3)

NO₂ + N → N₂O + O

(R4)

N₂O₅ formation:

NO₂ + O₃ → NO₃ + O₂

(R5)

NO₂ + NO₃ + M → N₂O₅ + M

(R6)

Acid formation:

N₂O₅ + H₂O → 2HNO₃

(R7)

NO₂ + OH + M → HNO₃ + M

(R8)

The rate constants (R1–R8) for these reactions are temperature dependent and their value can be found elsewhere [26].

The corona discharge environment stimulates ion-induced nucleation, which transfers the gaseous pollutants to liquid aerosols (HNO_x)_a(H₂O)_w [27]. Gamero-Castaño and de la Mora [28] studied ion-induced nucleation demonstrating that casters (HNO_x)-(H₂O) can be formed at concentrations well below the individual component’s 100% vapour saturation point, leading to the formation of secondary particles in the size range of a few nanometres. This is consistent with the work of Chang [29], who observed the generation of secondary particles ranging in size from 0.43 to 6.51 nm formed with a charge between 1 and 5 elementary units. The resulting acid–water clusters undergo further growth and charging processes. Newly formed particles are consequently precipitated in the studied ESP together with initial fly ash particles.

The sublinear relationship between NOx removal efficiency and SIE observed in experiments can be explained by the complex plasma–chemical processes occurring within the ESP including the competition between NOx removal reactions (R1, R2, and R5–R8) and reverse reactions (R3, R4).

The chemical mechanisms in positive and negative corona discharges, while fundamentally similar in their basic reactions, differ in several key aspects that influence their NOx removal efficiency and energy consumption.

In the context of electron energy distribution, negative corona discharges facilitate the acceleration of electrons away from the discharge electrode, resulting in a higher average electron energy compared to positive corona discharges. This increase in energy may lead to enhanced ionisation and dissociation of gas molecules, thereby promoting the production of reactive species such as O and OH radicals.

Regarding ion composition, the negative corona predominantly generates negative ions (notably O²⁻ and O⁻), whereas the positive corona produces positive ions (including N²⁺ and O²⁺). This distinction has important implications for ion–molecule reactions and the subsequent formation of intermediate species involved in the conversion of NOx.

The spatial distribution of reactive species also varies between the two types of coronas. Negative corona predominantly confines the active region of electron impact reactions to the area adjacent to the discharge electrode, in contrast to the more expansive active region of positive corona, which may result in a more uniform dispersal of reactive species throughout the interelectrode space.

Secondary electron emission represents another critical divergence; negative corona experiences this phenomenon at the cathode, enhancing overall ionisation efficiency, a process that is absent in positive corona. Moreover, the positive corona is more prone to forming streamers than its negative counterpart. While this can create localised zones of heightened reactivity, it may also increase the risk of spark formation at elevated voltages.

Ozone production adds another layer to these differences, as negative corona generally yields greater quantities of ozone due to the elevated electron energies involved. This ozone can significantly influence the oxidation pathways of NOx, particularly in reactions such as R2.

Furthermore, the varying ion compositions between positive and negative coronas can impact the ion-induced nucleation process, leading to discrepancies in the formation and growth of secondary aerosols.

The distinct features above clarify the observed lower energy consumption associated with NOx removal in positive corona, despite comparable removal efficiencies. The enhanced uniformity in the distribution of reactive species in positive corona may facilitate more effective energy utilisation for the conversion of NOx, whereas the markedly higher ionisation in negative corona might result in energy losses due to excessive ionisation outside the primary reaction zone. Nonetheless, the slightly improved particle removal efficiency noted in negative corona, particularly for particles around 100 nm, can be ascribed to the greater concentration of ions and the more intense electric field present near the discharge electrode.

The intricate interplay among these mechanisms highlights the necessity of optimising corona polarity for specific applications. In this instance, positive corona appears to strike a favourable balance between NOx removal efficiency and energy consumption for small-scale combustion emissions, while still achieving considerable efficacy in particle removal.

ESP demonstrated pretty high removal efficiency for both polarities of corona discharge: the minimal ESP energisation regime of 0.7/0.85 J/L in Figure 3 ensured the removal of 97/95% of the total particle number for the negative/positive corona, respectively. The further increase in the energy supply of the studied electrostatic precipitator led to an increase in particle removal up to 99.95/99.8% of the total PN removal efficiency for the negative/positive corona, respectively.

The total PN concentration was reduced to approximately 18 × 10³ #/cm³ during operation regimes with a voltage of 12 kV. This value is comparable to the particle concentration in ambient air, which ranged from 14 × 10³ to 21 × 10³ #/cm³ during the experimental campaign.

PM concentrations with the ESP-off regime were obtained at levels of 48 mg/Nm³. However, under the minimal ESP energisation regime of 0.7/0.85 J/L, the 30 min sampling did not produce results sufficient for an accurate evaluation of precipitation efficiency. The mass increase on the fibre filter after each of the five samplings was comparable to the margin of error of the measuring equipment. Doubling the sampling time similarly resulted in a filter weight increase that was undetectable by the available equipment. This can be attributed to the fact that larger particles, constituting the majority of the mass concentration, were fully precipitated at low energisation levels of the studied ESP. As a result, the PM data considered statistically insignificant PM concentrations in cleaned combustion gases were accepted at 0 mg/Nm³ and were not discussed.

Figure 4 illustrates the changes in particle size distribution as a function of ESP energisation for both negative and positive corona discharges along with the detection limit of the ELPI defined by MarjamäkiKeskinen, Chen, and Pui [30], which is given as a blue dotted line. The initial particle size distribution, measured without the ESP-off operation regime, was typical to small-scale combustion, demonstrating unimodal distribution with a peak of approximately 1.5 × 10⁷ #/cm³ at a particle diameter of 70 nm.

For the negative corona, particle removal was most effective in the 20–200 nm range. At 0.7 J/L, the concentration of 70 nm particles decreased by 98% to 3 × 10⁵ #/cm³. Concurrently, a new peak formed at approximately 20 nm, with a concentration of 1 × 10⁶ #/cm³. At 48 J/L, concentrations across all particle sizes were reduced to 1 × 10⁴ #/cm³ or lower.

The positive corona exhibited similar trends but with slightly lower efficiency. At 0.85 J/L, 70 nm particles were reduced by 96.7% to 5 × 10⁵ #/cm³, while a new 20 nm peak emerged at 8 × 10⁵ #/cm³. At 36 J/L, all particle sizes were reduced to about 2 × 10⁴ #/cm³ or lower.

Comparing the two corona types, the negative corona demonstrated marginally higher efficiency for particles around 100 nm, achieving approximately 99.9% removal at 48 J/L compared to 98.7% at 36 J/L for the positive corona. However, for particles smaller than 20 nm, the negative corona resulted in approximately 20% higher concentrations of newly formed particles at lower SIE levels.

These observations can be attributed to differences in ion concentration between negative and positive coronas. The higher ion concentration in the negative corona, which is typically observed in negative corona discharges and reported in our previous work [10], likely contributes to the more intensive particle charging attributed, resulting in slightly superior precipitation efficiency. Conversely, this higher ion concentration also leads to enhanced nucleation of secondary aerosols, explaining the 20% higher concentration of sub-20 nm particles observed in the negative corona discharge at lower SIE levels.

Nucleation mode particles, with diameters of less than 30 nm, have a limited ability to acquire a charge because of their small size. Nevertheless, the precipitation efficiency for both initial and secondary aerosols was observed to be quite high, 86%/92%, even at the low energisation of the studied ESP of 0.7/0.85 J/L positive/negative polarity, respectively. This could be facilitated by electrohydrodynamic turbulence, which is quite important for such fine particles [8], as well as by coagulation which can contribute to about 10% of particle removal in the electric field of ESP [31].

3.3. An Evaluation of the Performance of the ESP under Study

The performance of the presented ESP technology for simultaneous PM and NOx removal can be evaluated against other practically used technologies in terms of removal efficiency and energy consumption.

For particle removal, the presented results align with those of Vicente, Duarte, Tarelho, Nunes, Amato, Querol et al. [32], who found removal efficiencies of up to 98% for PM2.5 in small-scale biomass combustion using an ESP. However, the ESP developed in the present work achieved this high efficiency along with simultaneous NOx removal at lower energy consumption, suggesting an improvement in overall energy efficiency.

Regarding NOx removal in the studied ESP, an efficiency of 78% at 36–48 J/L compares favourably with other plasma-based technologies. For example, Penetrante, Hsiao, Merritt, Vogtlin, Wallman, Neiger et al. [33] reported NOx removal efficiencies of about 60% in pulsed corona reactors for simulated diesel exhaust, but at higher energy inputs of 60–100 J/L. Similarly, Mizuno, Shimizu, Chakrabarti, Dascalescu, and Furuta [34] achieved approximately 70% NOx removal efficiency in dielectric barrier discharge plasma, but at significantly higher energy consumption (about 100 J/L). More recent studies have shown improvements in plasma-based NOx removal. Obradović, Sretenović, and Kuraica [16] developed a dielectric barrier discharge system for NOx removal from coal combustion flue gas and reported energy consumptions of 120–150 J/L for similar NOx removal efficiencies. Recent research by Zhang, Wang, Yang, Zhang and Chang [35] compares the energy efficiencies of dielectric barrier discharge reactors of various designs, demonstrating that SIE exceeding 100 J/L is required for comparable NOx removal by this technology.

Chmielewski, Licki, Pawelec, Tymiński, and Zimek [36] used electron beam technology for NOx removal and reported efficiencies of up to 70%, with an energy consumption of approximately 28.8–43.2 J/L.

Forzatti [37] studied the NOx removal through Selective Catalytic Reduction and reported efficiencies of 80–90% for these systems. However, this technology operates at temperatures of about 300–400 °C and requires ammonia as a reducing agent.

Bhowmick, Badiwal, and Shenoy [38] presented a low-temperature oxidation with ozone injection and subsequent scrubbing and achieved 95% NOx removal. However, this technology requires a separate scrubber for removing the oxidized NOx and has high operational costs due to ozone generation.

Mok, Koh, Shin, and Kim [39] used a two-stage system of a pulsed corona plasma reactor followed by a monolith V₂O₅/TiO₂ catalyst for reducing NOx in a gas mixture. While they achieved approximately 80% NOx abatement with a similar energy input, their approach required separate units, increasing complexity and cost, and did not treat the particulate matter.

Over the technologies for separate control of particulate matter and NOx, some technologies are known for the simultaneous removal of these pollutants. Skalska, Miller, and Ledakowicz [21] reviewed various wet scrubbing forces with other technologies, reporting efficiencies ranging from 60–95%. However, these systems generate liquid waste and often require chemical additives.

However, it is important to note that direct comparisons can be challenging due to differences in experimental conditions, gas compositions, and scales of operation. For instance, Park, Kim, Lee, Chun, and Chun [40], working with diesel exhaust, found that the presence of hydrocarbons significantly enhanced NOx removal efficiency in plasma treatment. A comparison of the discussed emission control technologies is summarised in

Table 2. Comparison of Emission Control Technologies.

Technology	Capital Cost	Operating Cost *	SIE	Efficiency	Additional Considerations	References
Presented ESP	Moderate	Low	36–48 J/L	PM: Up to 99.99% NOx: Up to 78%	− No chemical additives − Compact design − Simultaneous PM and NOx removal
Selective Catalytic Reduction	High	Moderate to High		NOx: 80–90%	− Requires ammonia − Separate PM control needed	[37,41,42,43]
Wet Scrubbing Systems	Moderate to High	High	Generally higher than ESP	PM and NOx: 60–95%	− Generates liquid waste − requires chemical additives	[21,44]
Pulsed Corona and Dielectric Barrier Discharge	Moderate to High	Moderate	60–150 J/L	NOx: 60–70%	− May require separate PM control	[33,34,35]
Electron Beam Technology	Very High	High	28.8–43.2 J/L	NOx: Up to 70%	− Complex technology − Safety concerns due to radiation − require separate PM control	[36,45]
Low-Temperature Oxidation	High	High	Generally higher than ESP	NOx: Up to 95%	− Requires separate scrubber − High operational costs − May need separate PM control	[38,46,47]

* includes typical components such as consumables/reagents, water consumption, waste management, maintenance and repair.

.

While a comparison highlights certain limitations, it also underscores several significant advantages of the designed ESP. The presented system offers simultaneous removal of both particulate matter and nitrogen oxides, presenting a comprehensive emission control solution with energy consumption, which is notably lower than many plasma-based NOx removal systems, low-temperature oxidation, and scrubbing. Moreover, the presented ESP operates without the need for additional chemical reagents, unlike selective catalytic reduction or wet scrubbing technologies, simplifying operation and reducing costs. Additionally, the system can function efficiently at lower flue gas temperatures than what is required for some catalytic technologies. The compact design makes the presented ESP technology particularly suitable for small-scale applications where space is limited. While the NOx removal efficiency is slightly lower than some specialised DeNOx technologies, the ESP’s high particle removal efficiency, combined with its energy efficiency, compactness, and operational simplicity, positions it as a highly competitive option for small-scale combustion systems.

4. Conclusions

This study presents a novel approach to simultaneously control PM and NOx emissions from small-scale biomass combustion using a specially designed ESP with corona discharges of positive and negative polarity. ESP demonstrated sufficient efficiency in simultaneously removing both PM and NOx from a 15 kW wood pellet boiler.

The experimental approach entailed systematic testing of the ESP across a range of energisation levels, quantified by the specific input energy. Particle removal efficiency was evaluated using gravimetric analysis for PM, alongside electrical low-pressure impactor (ELPI) measurements to determine particle number concentration and size distribution. Continuous gas analysis employing a non-dispersive infrared detector was used to evaluate NOx removal efficiency.

The ESP exhibited remarkable performance in particle precipitation, achieving removal efficiencies of up to 99.99%. This resulted in the reduction of PM concentrations from an initial value of 48 mg/m³ to levels comparable to ambient air. The study highlighted variations in removal efficiencies across different particle size ranges, with the ESP demonstrating particularly high effectiveness for particles within the 20–200 nm range. A notable 78% reduction in NOx emissions was fixed, with concentrations decreasing from 178 mg/m³ to 39 mg/m³.

These removal efficiencies were achieved at SIE values (36/48 J/L for positive/negative polarity) significantly lower than those reported for many existing separate PM and NOx control systems. The positive corona exhibited lower energy consumption, providing valuable insights into optimising ESP design for the simultaneous removal of multiple pollutants.

The developed ESP is a compact, energy-efficient solution for simultaneous PM and NOx removal and offers a promising solution for improving air quality in various settings, from residential areas to small industrial applications.

Future research directions should focus on extended durability tests to assess the system’s performance over time and under varying operational conditions. Also, comprehensive studies on the composition and potential uses or safe disposal of captured pollutants should be conducted.