Particle removal performance of a novel ESP type air cleaning system for indoor air quality in a subway station

A novel electrostatic precipitation type air cleaning technology with near zero ozone emission for indoor air quality of a subway station was developed. To minimize ozone emission and achieve high particle removal performance against ultrafine particulate matters (PMs), a multi-channel charging method using soft ionization by micrometre carbon fibers and a narrow gap collection method using high electric field by thin conductive plastic sheets as a high voltage electrode were used. In this study, the novel electrostatic charging and collection technologies were applied to a real scale air cleaning system in an Air Handling Unit (AHU) with air flow rate of 950 m3/min in Yuseong Oncheon subway station with an area of 3000 m2 in Daejeon, South Korea. The particle removal performance of the demonstration system was evaluated with real suspended particles in the subway station during test period with high PM concentrations in Korea over 40 μg/m3. Particle collection efficiency, pressure drop and ozone emission were evaluated, and compared to those of the a minimum efficiency reporting value (MERV) 10 and 14 filters used in the typical subway stations in Korea. In the field researches, the developed ESP achieved over 80% of the PM collection efficiency higher than those of the filters with MERV-10 and -14, and even 10% higher efficiency against PM1.0, while generating only 10% of pressure drop and also emitting nearly zero ozone. It is concluded that a novel ESP type air cleaning system with near zero ozone emission could be a promising air cleaning method for a large indoor environment such as a subway station where polluted air with ultrafine particles flowed in with a very high flow rate.


Introduction
As the number of deaths due to air pollution is increasing, the problem of harmfulness to the human body caused by airborne particles such as PM2.5 and PM10 is becoming very serious in the world [1].In addition, due to the recent global outbreak of COVID-19 and the scientific evidences of its airborne infection, importance in managing the concentration of the particulate matters is greatly increasing for indoor air quality [2].At 2021 World Health Organization (WHO) greatly strengthened the recommendation levels for the concentrations of PM2.5 and PM10 in the atmospheric air [3].
The indoor air quality of subway stations is greatly affected by the pollution level of the outside air because atmospheric air flows into the subway stations through air conditioners and ventilators or because the subway user's entrance doors are always open to the outside [4].When the atmospheric air quality is bad, the air quality inside the subway station also deteriorates, so it is very necessary to manage the air quality swiftly in the subway station which is relatively very large, compared to residential indoors.
An Air Handling Unit (AHU) in air conditioning and ventilation systems is used to manage air quality in subway stations.This facility circulates some air in the stations and introduces some cleaned air by the AHU into the stations from the outside to maintain the indoor temperature, humidity and carbon dioxide constant and to control the indoor air quality cleanly.A filtration system capable of collecting particulate matters from the outside and in the subway stations is used in the AHUs, and air filters are widely used as PM collectors.For example, low-and medium-grade air filters which are categorized as MERV-10 level of the US air filter standard are used to remove PM10 and bigger PMs, and the high-grade filters in the MERV-14 level to reduce PM2.5 [5].However, when the filters are used for a long time without frequent replacements, they are contaminated and cause a pressure loss so that a large fan power is required, so the filters must be replaced periodically, thereby continuously consuming economical costs [6,7].As an alternative to filters, two-stage electrostatic precipitators (ESPs) with nearly zero pressure loss are used to collect particulate matters in exhaust gas from industrial factories.However, since the ESPs generates a large amount of ozone over several ppms in the process of generating ions necessary to charge particles [8].In particular, WHO recommends that the concentration of indoor ozone be less than 50 ppb [9], and most indoor air purifier ozone emission standards also regulate the ozone emission from the air cleaning devices should be less than 50 ppb or less during the continuous operation for 24 hours, so it is technically difficult to apply electrostatic precipitators for indoor air quality purification [10].
In this study, a novel two-stage ESPs using fiber brush electrodes as pre-chargers and nonmetallic collection plates as collectors was developed to reduce ozone generation and to achieve high collection efficiency against PM2.5 for AHUs in subway stations.In addition, the novel ESPs were applied to an actual subway station in its actual size, and their particle removal and ozone emission performances with and without the EPS ON and OFF were investigated by measuring changes in PM and ozone concentrations of the real polluted air.

Experimental Set up
In this study, this technology was applied to an AHU for a waiting room in a subway station located in Yuseong-gu, Daejeon, Korea.Figure 1 shows the two-stage ESP in the AHU and the experimental schematic to measure particle removal, ozone emission, and pressure drop performances.The width and height of the AHU are 2.9 m and 1.85 m, respectively, and a return fan (Max.11 kW) and a supply fan (Max.18.5 kW) are used to circulate the air inside the waiting room with an area of approximately 3000 m 2 , and the total flow rate through the AHU was 950 m 3 /min.Shown in Figure 1, two high-voltage power suppliers (Max.-30 kV/10 mA, Korea Switching, Korea) were connected to the charging and the collection stages respectively to make corona discharges in the pre-charger and to generate electrostatic force between parallel plates.Sampling probes which were connected to an optical particle counter (Model 1.109, Grimm, Germany)were installed at the front and rear of the electrostatic precipitator to measure inlet and outlet particle concentrations with and without the applied voltages.An ozone monitor (O3 Analyzer T400, TELEDYNE, USA) was installed at the rear end of the collection stage to measure the ozone emitted from the electric precipitator, and a digital manometer (TESTO 480, TESTO, Germany) was installed to measure the pressure drop through the novel ESP system.To compare the particle removal and pressure drop performances of the novel ESP and a filter type system, the MERV 14 air filters which were used in the AHU and had the same size of the ESP was installed in the same location in the AHU.
Figure 2 shows a two-stage electrostatic precipitator composed of a charging and a particle collection stages (2.4 m x 1.8 m) used in this study.A carbon fiber with low ozone emission characteristic which was proven in our previous researches [11,12] was placed in the center of a 100 x 100 mm square metal channel, and 12 modules with a size of 600 x 600 mm composed of the 36 unit channels were placed in parallel.12 collection modules with a size of 600 x 600 mm composed of collection and high voltage plates were also places in parallel.The high voltage electrode (510 x 100 mm) were plastic plates coated with carbon to connected to the high voltage power supply and the grounded electrodes were metallic plates while they installed facing each other at intervals of 5 mm in one unit module.Herein, Cinlet and Coutlet are the particle number concentrations (#/m 3 ) of the upstream and downstream of the removal device with high voltages applied, respectively.
The pressure drop before and after the filtration methods the MERV filters and the novel two-stage ESP precipitator was estimated by the equation (2).
Herein, Δp is pressure loss through the filtration methods, and pupstream and pdownstream are the static pressures before and after them, respectively.

Results and Discussions
Figure 3 shows the voltage-current curve at the charging stage of the two-stage electrostatic precipitator used in this study.Even though a high voltage was simultaneously applied to 462 discharge channels composed of carbon fiber discharge electrodes at the square ducts with a total size of 100 x 100 mm, the V-I curve followed a well-known Townsend's law [13], the onset voltage was approximately -2 kV, and the total corona current when -20 kV was applied was approximately 24 μA.In order to minimize the ozone concentration emitted from the electrostatic precipitator to less than 5 ppb, the voltage applied to the chargers was fixed at -15 kV and the applied voltage at the collection plates was -8 kV.The discharge current at the collection stage was almost zero because only the electric field was applied between the collection plates.
Figure 4 shows the distributions of mass and number concentrations of particulate matters flowing into the AHU in the subway station of the demonstration site.Based on the mass concentration distribution, the highest concentration was observed in the micrometer and the 0.3 μm size ranges.On the other hand, based on number concentration, the concentration of particles less than 0.3 μm was the highest.This shows the typical distributions of the mass and number concentrations of atmospheric particles [14].In this study, the concentration of PMs entering the AHU was approximately 40 μg/m 3 , and the collection efficiency was calculated using the mass concentrations.Figure 5 compares the PM collection efficiencies by particle sizes according to the PM collection methods, and the PM collection methods were the ESP developed in this study and the MERV-14 and MERV-10 filters used in AHUs of general subway stations.The voltages applied to the charging and collecting stages of the ESP were -20 kV and -8 kV, respectively to minimize ozone emission less than 5 ppb which is nearly zero according to a UL standard [15].The air flow rate entering the PM collection devices in the AHU was 950 m 3 /min, and the its face velocity was 3 m/s with a standard deviation of 0.75 m/s due to an uniform flow distributor located at the upstream of the filtration systems.In the case of the MERV-10 filter, the PM collection efficiency was in the 20% range for 0.3 μm-particles, about 50% for 1 μm-particles, and more than 95% for particles larger than 5 μm.In the case of MERV-14, the efficiency was in 70-80% for 0.3 μm-particles and very high over 95% for particles larger than 1 μm.These efficiencies were very similar to those corresponding to the existing MERV 10 and 14 standards [5].On the other hand, in the case of the electrostatic precipitator used in this study, the collection performance was similar to that of MERV-14 for the particle size larger than 1 μm however the efficiencies were 10 to 15% higher in the particle size of 0.3 μm or less, compared to those of the MERV filters.
Figure 6 shows the collection efficiency for PM10, PM2.5, and PM1.0 of the MERV-10 and -14 filters and the electrostatic precipitator.The MERV-10 filter is widely used in subway air conditioners because it does not have a large pressure loss and is inexpensive, however in this study its collection efficiency for particulate matters of 10 μm or less is very low, less than 16%, and the MERV-14 filter in subway stations used to reduce fine particles of PM10 or less showed a collection efficiency of about 70%.On the other hand, the electrostatic precipitator developed in this study showed an efficiency of over 80% for PM10 and 2.5, and especially for PM1.0, it showed 11% higher efficiency than the MERV14.
In particular, as shown in Figure 7, the pressure loss of the ESP is 15.5 Pa, which is only 1/10 of the 157 Pa of the MERV 14.The fan power consumption is proportional to the flow rate and pressure drop [16], so it is possible to save the 90% of the power consumption by replacing the MERV filter by the novel ESP.The figure 8 shows the change in ozone concentration emitted from the outlet before and after applying -20 kV to the chargers and -8 kV to the collection plates of the ESP developed in this study.Before applying the high voltages to the electrostatic precipitator, it increased up to 23 ppb, however after applying the high voltages, the concentration rather decreased.The decrease in the ozone concentration with the ESP on indicated that the ozone emission generated by the operation of the electrostatic precipitator was negligible because the concentration of ozone in the air flowing into the subway station from the outside has changed.
These results indicate that the novel ESP in this study which has nearly zero ozone emission, low pressure drop, and high efficiency against ultrafine particles can be a promising technology to clean high flow rate air for the large indoor area such as the subway stations.

Conclusions
Herein, a two-stage ESP was developed for the particle removal device in an AHU in the subway station.In order to achieve nearly zero ozone and high PM removal efficiency, a multi-channel chargers with carbon brush electrodes and 0.1m x 0.1m square channels and a very narrow collection plates with a carbon coated plastic plate and aluminum ground plates were used for the novel ESP.The real scale ESP pilot system in the AHU with the face velocity of 3 m/s through the area of 2.9 m x 1.8 m was evaluated in terms of particle removal, ozone emission, and pressure drop performances with real air which were contaminated by the suspended particulate matters in the subway station.The ESP performances were also compared with the filtration systems which were used in typical AHUs of the subway station in Korea.From the experiments it was found that particles suspended in the subway station were mostly ultrafine particles less than 1 μm and fine smaller than 10 μm.The MERV-10 showed only 16, 14, 11% removal efficiencies against PM10, PM2.5 and PM1.0 respectively, while those of the MERV-14 were 78, 73 and 69%.However, the efficiencies of the ESP were over 80% against all PMs, even 11% higher than that of the MERV-14 for PM1.0 while generating only 10% of pressure drop, 15 Pa.Moreover, the ESP showed nearly zero ozone emission because when the ESP was turned on the ozone was rather decreased.
It is concluded that the novel ESP type air cleaning system with near zero ozone emission and high PM removal performances could be a promising air cleaning method for a large indoor environment such as a subway station.

Figure 1 .
Figure 1.Experimental set up for measuring performances of the novel ESP and MERV air filters

Figure 2 .
Figure 2. Schematic of the charging and collection stages in the ESP used in this study

Figure 3 .
Figure 3. Corona current curve according to the voltages applied to the multi-channel precharger.

Figure 4 .
Figure 4.The distributions of mass and number concentrations of particulate matters used in this study flowing into the AHU in the subway station.

Figure 5 .
Figure 5. PM collection efficiencies by particle sizes according to the PM collection methods tested in this study.

Figure 7 .
Figure 7.Comparison of the pressure drop between the filter with MERV-14 and the ESP developed in this study.

Figure 8 .
Figure 8. Changes in the ozone concentration emitted from the AHU with the ESP ON and OFF.