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. Shimizu et al. 47 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 48 International Journal of Plasma Environmental Science & Technology, Vol.4, No.1, MARCH 2010 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 -13 3 -1 Fig.9. Removal efficiency of formaldehyde by microplasma with and k1 = 1.75×10 [cm s ] (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.