Shimizu et al. 45

Application of Atmospheric Microplasma for Indoor Air Treatment

K. Shimizu, M. Kanamori, and M. Blajan Innovation and Joint Research Center, Shizuoka University, Japan

Abstract—Removal of low concentration formaldehyde, which exists in room air, is investigated by using microplasma. Microplasma is generated with a pair of metallic electrodes, covered with a dielectric barrier at a relatively low discharge voltage of around 1kV, and has an advantage of reducing the power and downsizing the entire plasma system. In this paper, electric characteristics and ozone generation are also studied. The advantage of the microplasma electrode was estimated by measuring the removal of very low concentration formaldehyde with different air flow rates. The concentration of formaldehyde is measured by the HPLC, and humidity conditions of the flowing air are changed. With this microplasma electrode, formaldehyde could be purified with low discharge voltage and power, while ozone and NOx generation are near zero. In addition, byproduct analysis is confirmed by using the FT-IR, and smell analysis is confirmed by the Fragrance Flavor Analyzer. CO, N2O, and HCOOH were the major byproducts of formaldehyde treatment and a change in the smell of formaldehyde was obtained.

Keywords—Microplasma, dielectric barrier discharge, ozone generation, formaldehyde

I. INTRODUCTION II. ABOUT MICROPLASMA

Sick-Building Syndrome (SBS) has become an Atmospheric microplasma is a type of dielectric environmental issue worldwide in recent decades [1]. barrier discharge (DBD) [12, 13]. The discharge gap is Much research on this syndrome has been carried out, set to an order of micrometers, which is extremely and it is has since come to be generally believed that narrow, enabling the plasma to start at a discharge since buildings have became more airtight for voltage of around 600 V. Streamers between the improvement of air-conditioning and heating efficiency, electrodes are also very small (in the order of the harmful effect of Volatile Organic Compounds micrometers), resulting in a relatively compact and dense (VOCs) diffusions from building materials has increased, plasma. Fig. 1 is an image of the microplasma during resulting in many symptoms of SBS. Control of these discharge. Streamers were generated form not only indoor air pollutants is necessary to maintain Indoor Air between the electrodes but also around the holes of the Quality (IAQ). electrodes. Formaldehyde (HCHO) is one of the most common Discharge gap was set based on the Paschen’s law, VOCs indoors. This substance is emitted from resins, which indicates the minimum sparking voltage and plastics and often building materials such as plywood, discharge gap for various gases at atmospheric pressure. chipboard, and paneling, and is one of the main factors in High reduced electric fields were readily obtainable with SBS [2]. Decompositions of VOCs have been researched such small discharge gaps, resulting in a reduction of low recently [3-8], and one method to control VOCs is the energy electrons (1-2 eV) which dissociate ozone [14, application of non-thermal plasma. Non-electrical 15]. This microplasma electrode has the advantage of methods such as decomposition of formaldehyde with Pt/TiO2 alumite catalyst could be used [9], but the catalytic process requires the replacement of the catalyst after a determinate period of time. Atmospheric non- thermal plasma has the advantage of generating ozone and free radicals effectively in ambient air, which enhances dissociation of pollutant substances. Especially, non-thermal plasma with streamers in the order of micrometers is called microplasma, and requires a relatively low voltage than other discharge methods. Recently, investigation of treating NOx has been carried out using microplasma [10, 11], and the aim of this research was to investigate the possibility of reducing low concentration HCHO by using atmospheric microplasma.

Corresponding author: Kazuo Shimizu Fig. 1. Image of the microplasma electrode (Vdis = 1 kV). e-mail address: [email protected] Picture is taken by a digital camera with 5 seconds shutter exposure.

Received; December 25, 2009

46 International Journal of Plasma Environmental Science & Technology, Vol.4, No.1, MARCH 2010

generating high concentrations of ozone with low III. EXPERIMENTAL SETUP discharge voltage and power. Fig. 2 is a schematic image of the microplasma The experimental set-up is presented in Fig. 3. Air electrode. Two perforated metal plates covered with was made to flow through a diluted formalin solution by dielectric materials were placed facing each other, and an an air pump or gas cylinders, supplying a constant low alternating voltage (about 25 Hz, 1 kV) was applied. concentration of formaldehyde to the microplasma Innumerable streamers were generated between the reactor. electrodes, which could excite ozone and various radicals The treated gas was then sent to an ozone monitor (O*, N*, etc). These radicals could react with the flowing (Ebara Jitsugyo, EG-2001B), NOx analyzer (Shimadzu, gas and detoxify them [16]. The electrode had a diameter NOA-7000), FTIR (Shimadzu, IRPrestige-21), HPLC of 45 mm and a thickness of 1 mm. Since this electrode (Agilent, 1100 series) and Fragrance Flavor Analyzer had a large aperture (aperture ratio: about 30 %), the (Shimadzu, FF-2A) for investigation of the gas pressure loss through the electrode was extremely low composition change, identification and quantity analysis (about 25 mmH2O at gas flow 10 L/min). This could of by-products, and distinction of the gas smell. enable large volume gas treatment phases. Also, an oscilloscope (Tektronix, TDS 3014) was used to measure the discharge voltage, current, and power. Lissajous figures were used for the estimation of

Fig. 2. Schematic image of the microplasma electrode. The discharge gap is about 10 µm.

Fig. 3. Experimental setup for formaldehyde treatment.

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Fig. 5. Electrical characteristics of microplasma.

Fig. 4(a). Waveform of discharge voltage and current.

Fig. 6. Ozone and NOx concentration versus discharge voltages at various air flow rates. Air humidity is 0 %.

Fig. 4(b). Waveform of spike currents. discharge power.

IV. ELECTRICAL CHARACTERISTICS OF MICROPLASMA

Fig. 4 shows a waveform example of a discharge voltage and discharge current. Spike currents, which could be occurred by streamers convoluted on the current waveform, were confirmed in addition to the capacitive current at the steepest slopes of the waveform [10, 11]. This is recognized as the discharge current shown in Fig. 4(b). The electrical characteristics such as discharge voltage, current, and power are sorted in Fig. 5. The Fig. 7. Ozone and NOx concentration versus discharge voltage at discharge current was measured at the peaks of spike various air flow rates. Air humidity is 60 %. currents. For example, the discharge power at 1 kV was about 15 W. However, this practical discharge power was substances such as HCHO. When generating O3 in the mainly due to the capacitive current. atmosphere, the generation of small amount of nitrogen Therefore, further research must be done to estimate oxides (NOx) was confirmed. NOx is usually produced the real discharge current, which is only affected by the by the exhausts gases from car engines or factories, and a spike-like currents. high concentration of NOx is harmful to human. When treating indoor air, it is desirable not to produce high concentrations of either NOx or O3. V. OZONE GENERATION Fig. 6 and 7 show the characteristics of O3 and NOx generation by microplasma at various air flow rates and Ozone (O3) has the most oxidation ability next to humidity. The gas flow rate was 2, 5, 10 L/min, and the fluorine, and is very effective to detoxify harmful measurement time was 3 minutes after the discharge start

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to measure the equilibrium condition. Humidity of air was changed between 0 % and 60 %.

In this case, NOx was defined as the sum of NO and NO2. From Fig. 6 and 7, both ozone and NOx concentration had a trend to increase as the discharge voltage increases, although, when there was humidity in the air, the concentration of O3 was lower compared to the concentration of NOx. This could be because the dissociated O3 reacts with H2O in the air, rather than O2. A trend can been seen in which there was a generation peak for O3 generation at lower air flow rates. When the discharge voltage was set to more than 900 V, O3 concentration started to decrease. This could be because when the electric field becomes too high, the number of electrons with an energy of more than 9 eV Fig. 8. Removal efficiency of formaldehyde by microplasma with increase, thus dissociating nitrogen (N2). The dissociated and without humidity. Air flow rate is set to 5 L/min. N2 reacts with O3, resulting in an increase of NOx.

VI. FORMALDEHYDE TREATMENT

There are many reports on treating hazardous substances, such as cigarette smoke and VOCs [17, 18]. Most of these methods require high voltages and perform poorly in attempting to handle large volume. Therefore, we have carried out a treatment experiment of HCHO using a microplasma reactor, which is more compact, and has low energy consumption. The following major plasma chemical reactions are known to take place in the treatment of HCHO [19].

HCHO + O → HCO + O k = 1.75×10-13 [cm3 s-1] (1) Fig.9. Removal efficiency of formaldehyde by microplasma with and 1 without humidity. Air flow rate is set to 10 L/min. HCHO + OH → HCO + H2O -11 3 -1 k2 = 1.11×10 [cm s ] (2) HCO + O → CO + H was 87% at the maximum, at a discharge voltage of 900 2 3 -11 3 -1 V and energy density 52 mJ/cm . This could be because k3 = 5.00×10 [cm s ] (3) of the concentration of generated O was higher at 0 % HCO + O → CO + OH 3 humidity, as shown in Fig. 6 and 7. k = 5.00×10-11 [cm3 s-1] (4) 4 To purify HCHO in atmospheric indoor air, it is HCO + OH → H2O + CO -11 3 -1 necessary not to generate too much O3, since O3 can also k5 = 5.00×10 [cm s ] (5) be harmful for humans. The amount of HCHO was estimated by the DNPH When the discharge voltage is set to 700 V, energy method using the HPLC [20]. Fig. 8 and 9 show the density is 16.8 mJ/cm3, at an air flow rate of 10 L/min. In results of the HCHO treatment using microplasma this condition, there were only a few amount of O3 (0.08 treatment. ppm) generated (See Fig. 6, gas flow rate 10 L/min). The The gas flow rate was 5 and 10 L/min, humidity was regulated value of HCHO was reached to 0.08 ppm, and 0 % and 60 %, and the measurement time was 10 minutes ozone is 0.1 ppm for indoor air. At this discharge voltage due to the specification of the DNPH cartridge. and 10 L/min gas flow rate, the removal ratio of HCHO The initial concentration of HCHO was about 0.25 is 80 %, corresponding to an achieved concentration of ppm, which is about 3 times higher than the regulated 0.052 ppm for the air with 0% humidity. This value for indoor air in Japan. For both air flow rates, concentration fulfills the condition imposed by the indoor HCHO removal efficiency was higher when there was no air quality standards. For the treated air with 60 % humidity. At a discharge voltage of 800 V, 100 % of humidity, the HCHO removal ratio of 40 % corresponds HCHO was removed. This removal ratio is achieved to an achieved concentration of 0.11 ppm, at a gas flow 3 when the energy density is above 33 mJ/cm , at an air rate of 10 L/min. flow rate of 10 L/min, 0 % humidity. On the other hand, with the existence of humidity of 60 %, the removal ratio

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VII. BYPRODUCT ANALYSIS

Fig. 10 and 11 show the analysis results obtained by the FTIR. In this case, the initial concentration of HCHO was set to 10 ppm to observe the spectrum difference clearly, and the discharge voltage was set to 600, 800, and 1000 V, respectively. The gas flow rate was set to 2.0 L/min and humidity conditions were changed. From the analysis results, carbon dioxide (CO2: 2250-2400 [cm-1]), carbon monoxide (CO: 2000-2250 -1 -1 [cm ]) and nitrous oxide (N2O: 2175-2250 [cm ]) were discovered as byproducts without humidity. CO2 and CO could be a result of equations (3-5). The generation of Fig.10. FT-IR analysis (2000-2450 cm-1) after formaldehyde N2O according to (6) is realized by the reaction of treatment without humidity (0 %) in the flowing air. excited species of N2 with O2. The presence of N2 excited species in microplasma was demonstrated by the + measurement of N2 second positive band system and N2 first negative band system for microplasma discharge in air [21]. N2O was a byproduct which derives also from reaction (7).

N2* + O2 → N2O + O -14 3 -1 k6 = 3.00×10 [cm s ] (6) NO2 + N → N2O + O k = 3.00×10-12 [cm3 s-1] (7) 7 When there was humidity in the flowing air, formic acid (HCOOH: 1100 [cm-1]) is also confirmed as a (a) Wavenumber (2000-2300 cm-1) byproduct (Fig. 11(b)). Electron impact dissociation of H2O leads to the production of H and OH radicals and 1 also excited state O ( D) dissociated H2O to generate OH. Thus formaldehyde reacts with generated OH radicals and HCOOH was formed from the following equation: HCHO + OH → HCOOH + H 3 -1 k8 = 2.00×10-13 [cm s ] (8) Table I and II show the concentration of these byproducts and its corresponding ozone, NOx, and HCHO concentration. Measuring conditions were the same as the FT-IR analysis (gas flow rate: 2 L/min, HCHO: 10 ppm) for Table I and II. It can be seen that the total concentrations of the (b) Wavenumber (950-1150 cm-1) products were higher when there was no humidity. From another point of view, differences in the concentration of Fig. 11. FT-IR analysis after formaldehyde treatment with humidity CO and N O were not so remarkable. In addition, the (60 %) in the flowing air. 2 initial concentration of HCHO was 10 ppm, which was TABLE I lower than the CO value at 1000 V. This could be CONCENTRATION OF EACH PRODUCT AT 0% HUMIDITY because some CO are dissociated into CO. NOx CO N O 2 Vdis [V] O [ppm] 2 It is a not desirable to produce byproducts which are 3 [ppm] [ppm] [ppm] harmful to humans. Therefore, there should be a second 600 0 0.25 0.01 0.14 800 42.09 5.78 6.55 2.58 treatment process for these byproducts, or the electron 1000 37.82 28.06 11.57 9.21 energy should be controlled in order not to dissociate N 2 and CO . These issues will be investigated in subsequent 2 TABLE II research. CONCENTRATION OF EACH PRODUCT AT 60% HUMIDITY NOx CO N O Vdis [V] O [ppm] 2 3 [ppm] [ppm] [ppm] VIII. SMELL ANALYSIS 600 4.85 0.61 1.56 0.55 800 34.24 7.38 8.17 2.42 HCHO is known to have a sweet smell, and can be 1000 23.48 24.01 10.45 9.07 detected when there is 0.08 ppm or more in the air. After

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was 10 L/min of 0% humidity air. Ozone concentration

was 0.1 ppm and the initial concentration of HCHO was 0.25 ppm. The numbers in the radar graph stands for the similarity percentages against the nine standard gases. The smell of HCHO had a strong similarity with hydrogen sulfide, ester, and aldehyde. After microplasma treatment, the similarities of these three contents have decreased, and the similarities of hydrocarbons and aromatic properties increased. It is evident that the smell of the sample gas has changed after microplasma treatment. Ozone has a strong similarity with hydrocarbons, which could be the reason for this increase. According to Fig. 7, at a discharge voltage of 700 V, 0.08 ppm of O3 was generated. The O3 concentration was below the regulated value, although O3 smell can be detected above 0.01 ppm. Therefore, O3 smell may be affecting the smell of the gas after microplasma treatment. Fig. 12. Smell similarity analysis of room air and room air with 0.1 ppm ozone obtained by the Fragrance Flavor Analyzer.

IX. CONCLUSION

In this study, low concentration treatment of HCHO was confirmed by atmospheric microplasma. O3 and NOx generation is investigated with different air flow rates and humidity. The following conclusions were obtained by the series of experiments. (1) HCHO was purified using atmospheric microplasma with very low energy, generating very little O3 and NOx as byproducts. For air with 0 % humidity, the ozone concentration was 0.08 ppm and the HCHO concentration was 0.052 ppm after the microplasma treatment. The values have been achieved at a discharge voltage of 700 V, an energy density of 16.8 mJ/cm3 and at a gas flow rate of 10 L/min. These values fulfill the standards for indoor air quality regulations in Japan. Higher values of the ozone concentration were measured with the increase of the discharge voltage, but so were Fig. 13. Smell similarity analysis of 0.25 ppm HCHO before and after microplasma treatment, obtained by the Fragrance Flavor higher removal rates of formaldehyde. Ozone could be Analyzer. removed from treated air by combining the microplasma treatment with active charcoal or other filters. microplasma treatment, a few byproducts were (2) There was a generation peak of O3 at lower air confirmed. This fact suggests the smell of the treated gas flow rates. In addition, ozone generation was decreased could also be changed. A smell analysis was carried out with the existence of humidity in the air. to confirm the difference of smell before and after (3) CO2, CO and N2O were found to be byproducts microplasma treatment. in dry air, by the FT-IR analysis. When there is humidity Sample gas was collected and analyzed by a in the air, HCOOH is also formed as a byproduct. Also, a Fragrance Flavor Analyzer, which has ten different change in the smell after microplasma treatment is semiconductor sensors with different sensitivities. Nine confirmed. standard gases, which each have a different kind of smell, are imputed in this device and the analyzed sample gas is compared with them, outputting the similarity against ACKNOWLEDGMENT each standard gas. It is able to compare the change of the smell [10]. This paper was partly supported by the Ministry of The smell similarity of room air and ozone is Education, Science, Sports and Culture, Grant-in-Aid for presented in Fig. 12, and the similarity of HCHO before young Scientists (A), 2007 and was also supported by the and after microplasma treatment is shown in Fig. 13. Japan Science and Technology Agency, Grant Program The discharge voltage was set to 700 V, gas flow rate

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