Plasma-Based Pollutant Degradation in Gas Streams: Status, Examples and Outlook

Contrib. Plasma Phys. 54, No. 2, 202 – 214 (2014) / DOI 10.1002/ctpp.201310059

Plasma-Based Pollutant Degradation in Gas Streams: Status,

Examples and Outlook

R. Brandenburg1∗, V. V. Kovacevic 2, M. Schmidt1, R. Basner1, M. Kettlitz1, G.B. Sretenovic 2,

B.M. Obradovic 2, M.M. Kuraica2, and K.-D. Weltmann1

1 Leibniz Institute for Plasma Science and Technology (INP Greifswald), F.-Hausdorf-Str. 2, D-17489Greifswald, Germany

2 University of Belgrade, Faculty of Physics, Studentski trg 12, P.O. Box 44, 11000 Belgrade, Serbia

Received 09 September 2013, revised 28 October 2013, accepted 07 November 2013Published online 10 February 2014

Key words Pollutant degradation, non-thermal plasma, dielectric barrier discharge, NOx-removal, waterfalling film reactor, VOC removal.

The status of the research into and the application of non-thermal plasmas for the pollutant degradation in gasesis discussed, including some fundamental topics and engineering issues.

Two reactor concepts, both based on dielectric barrier discharges (DBDs), are presented and investigatedfor various tasks. The DBD-stack reactor shows a very good scalability and was investigated for the oxidationof NO in combustion gases (shipping diesel engine exhausts). The oxidation processes were significantlyenhanced by the admixture of hydrocarbons. Significant NO conversion at low specific energy densities below100 J/L were achieved in laboratory and test bench studies. The water falling film reactor demonstrated itsfeasibility for the removal of hydrocarbon pollutants from gases. Undecane, a long-chain, harmful hydrocarbon,was decomposed. The conversion of non-soluble compounds into soluble ones (formic acid) is a promisingdevelopment towards a compact plasma-assisted scrubbing technology. These approaches are good progressnot only in the field of environmental plasma application, but also for indoor air quality, hygiene, and plasmasynthesis.

c© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

In 1857, the young engineer Werner Siemens (later Werner von Siemens) invented the first ozone generator. Thismilestone can be considered as the starting point of non-thermal plasma chemical technology. Today, more than150 years later, non-thermal plasmas (NTPs) are the state of the art for ozone generation in water cleaning orin bleaching facilities. Other established technologies in the industry are the incineration of waste gases andthe removal of dust from flue gases in electrostatic precipitators [1]. The potentials of ’cold’ NTPs or plasmaUV-radiation sources for the degradation of pollutants in gases are well-known and have led to new processes fordeodorization of off-gases from manufacturing processes.

However, air pollution remains a concern for public health and the environment. The European Commissionis currently reviewing the EU thematic strategy on air pollution and related policies, having designated 2013as the ’Year of Air’ [2]. Industry and transportation have a significant impact on the environment and health.Although Europe has made progress in tackling the emissions of some air pollutants (e.g. sulphur dioxide), alarge proportion of the population is still exposed to excessive concentrations of certain air pollutants (in particularVOCs, particulate matter, ammonia), leading to health risks and premature death in some areas. Acidificationand eutrophication have turned out to be more serious than anticipated, and ecosystems are more sensitive thanpreviously judged. As a result, environmental standards are continuously being raised.

NTP offers a promising approach to fulfill the legislative limitations of air purification. Plasmas containnumerous reactive species, in particular ions or radicals, which can decompose pollutant molecules and organicparticulate matter (e.g. soot). Since these chemical reactions can take place without extensive heating of thegas to be treated, they are very energy efficient. NTPs are able to remove gaseous compounds (nitrogen oxides,

∗ Corresponding author. E-mail: [email protected], Phone: +00 49 3834 5543818, Fax: +00 49 3834 554301

c© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Contrib. Plasma Phys. 54, No. 2 (2014) / www.cpp-journal.org 203

NOx; Volatile Organic Compounds, VOCs), as well as particulate matter. In contrast to many other establishedtechnologies of air cleaning, NTPs can be controlled more or less instantaneously by their electrical operationparameters, and they can thus be adjusted to fluctuating gas flow volumes and/or contamination levels.

Yet the application of NTPs for the pollutant degradation in gases remains a challenge, in two respects: scien-tific and engineering. The degradation of pollutants by means of plasma chemistry includes processes spanningcharacteristic time scales of about 12 orders of magnitude. In molecular gases at atmospheric pressure, the rel-evant NTPs show filamentary behavior which requires special efforts of plasma diagnostics. On the other hand,an industrial process must comply with the demands on efficacy and selectivity. The first part of this contribu-tion will discuss the state of the art regarding these different aspects. It will be shown that nearly all practicalprocesses of pollutant degradation in gases by means of NTPs are hybrid processes or a combination of NTPswith other technologies. In the second part, the ability of NTP reactors as oxidation stages for the treatment ofcombustion gases will be investigated. Combinations of NTPs with other technologies offer process synergies.This will be discussed in the third part of this paper and exemplified by a plasma-assisted scrubbing process. Thecontribution will close with a summary and an outlook to future research.

2 Plasma based pollutant degradation in gases: A scientific and engineering

challenge

2.1 Plasma sources for pollutant degradation and plasma filamentation

The most common non-thermal plasma sources for gas purification applications are dielectric barrier discharges(DBDs), corona discharges and gliding arc discharges [3, 4]. Another approach to plasma generation in wastegas streams is electron beam injection, known as Electron Beam Flue Gas Treatment (EBFGT), for very largecombustion facilities, such as coal fired power plants [5].

DBDs and corona discharges are most suitable for exhaust gases from manufacturing processes and mobileemission sources, as they offer a compact design and good scalability. The DBDs are sustained between twoelectrodes, with the space between these electrodes containing an insulating (dielectric) material. The insulatorsuppresses large currents on the electrodes and thus keeps the plasma in the non-thermal regime [6]. DBDs arecommonly operated by AC power supplies. Corona discharges are characterized by non-uniform electrical fieldgeometries, e.g. needle-to-plate or wire-in-cylinder electrode arrangements. DC and low frequency AC operatedcorona discharges expand from the needle or wire electrode in the outer regions towards the plate or cylinderelectrode. The energy is mainly dissipated in the high-ohmic region of non-ionized gas in the outer drift region,where the electrical field is lower than in the plasma region around the wire/needle electrode. In this region thedischarge is not supported anymore and is thus keeping non-thermal regime [7]. For environmental applicationscorona discharge operated by pulsed high voltage are proposed since higher densities of reactive species canbe realized [8]. Pulsed corona discharge are characterized by plasma regions thats fills a much larger fractionof the discharge gap than DC or low frequency corona discharges. DBDs are used for ozone generation, VOCremoval and deodorization. DC-operated corona discharges are used in electrostatic precipitators (ESPs) andpulsed coronas for VOC removal and water purification [9].

In molecular gases at atmospheric pressure, corona discharges and DBDs are typical examples of filamentaryplasmas, i.e. they consist of many individual, short-lived and thin microdischarges (DBDs) or discharge channels(corona) forming filaments. Each volume element of the flowing gas is subjected several times to the action ofthese filaments. Ionization and electrical breakdown typically proceed at nanosecond time scales. For example,a dielectric barrier microdischarge in atmospheric air has a duration of about 20 ns. The luminous filaments arethe active phase of the discharge treatment, with the highest electron density ne and electron temperature Te.These basic plasma parameters determine the plasma chemistry. Therefore, the investigation of microdischargesis of crucial importance for understanding and optimizing such technologies. Since most of the investigationsof single discharge events have been performed in unpolluted air-like atmospheres or other simple model gasmixtures [10–14], up to now there is a lack of knowledge on the behavior and properties of such events inpolluted gases. Such investigation must involve the application of plasma diagnostics with reasonable spatialand temporal resolution [11, 15] and the proper determination of the (instantaneous) electrical parameters (e.g.dissipated energy, charge and power), as has recently been recapitulated for DBDs by Pipa et al. [16].

www.cpp-journal.org c© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

204 R. Brandenburg et al.: Plasma-based pollutant degradation in gases

2.2 Plasma chemistry

Electron-induced dissociation and thus the formation of radicals happens within a few nanoseconds. The pollutantdegradation proceeds via secondary collision of the pollutant molecules with free radicals and ions, mainly ona micro- to millisecond time scale, i.e. after the active discharge has faded. Ion-molecule reactions occur atan intermediate time scale, typically in the range of 10 ns up to 1 μs. The pollutant molecules react with theoxidizing radicals (e.g. O, OH) or plasma-generated ozone (O3). Water vapor may play an important role, asit acts as the precursor for hydroxyl radicals (OH) and hydroperoxyl radicals (HO2). The chemical equilibriumincluding heat and mass transfer is commonly settled within milliseconds to seconds.

The rate coefficients of plasma chemical reactions depend on the plasma parameters, the gas composition,pressure and temperature. Thus, the identification and description of the dominant chemical pathways can bequite challenging.

The desired plasma-chemical conversion must be discussed with regard to two aspects, namely efficacy andselectivity. Efficacy means the energy needed to perform a certain number of chemical reactions. Alternativelythe energy yield EY as defined in equation (1) is the decomposed pollutant mass per dissipated energy. In thiscontext the specific energy density SED is derived from the power dissipated in the plasma PPlas and the volumeflow rate of the off-gas Qgas (equation (2)). The removal efficiency is η=(Cin-Cout)/Cin with Cin as the initialand Cout as the outlet molar fractions of the pollutant with the molar mass M. The term (p/R0T) describes theinverse molar volume Vm at temperature T and pressure p with the universal gas constant R0 (assuming idealgas). Furthermore, the energy consumption for a selected chemical gross process (e.g. the decomposition orconversion of a certain molecule) in (eV/molecule) or the so-called G-value (molecules per 100 eV) can be usedas a measure of efficacy and to estimate the energy consumption in real-scale processes. However, these valuesdepend on the SED and are determined by a number of conditions (gas composition, humidity and temperature;level of initial contamination).

An energy of about 10-30 eV is necessary to produce an OH-radical in humid air, which makes the decompo-sition of dilute pollutants quite energy consuming. The total energy consumption can be low for small concentra-tions of pollutant molecules, which means that NTP-based processes are feasible for low contamination levels.For volatile organic compounds, this level is about 100 mg/m3

N (N refers to the standard conditions for p andT) [3, 17].

EY =Cin η M

SED

p

R0 T(1)

SED =PPlas

Qgas(2)

Selectivity is defined as the fraction of the desired product of the plasma-chemical conversion to the totalamount of products of the conversion process. The chemistry and fraction of desired products and undesiredby-products can also be characterized by mass balances (e.g. carbon balance in the case of conversion of hy-drocarbons). A high selectivity is required to achieve a reasonable performance in terms of energy efficacyand by-products [18]. High reactivity of radicals usually results in a poor selectivity, since competing reactionswhich result in the formation of undesired by-products happen simultaneously. One important reaction whichconsumes oxygen radicals alongside the reactions with pollutant molecules is the generation of ozone O3. Forsome pollutants (e.g. NOx) O3 is an efficient oxidizer, but in other cases it constitutes an additional pollutantby-product.

2.3 Plasma enhanced and plasma based processing

Often, the removal efficacy and selectivity of plasma chemical reactions are not sufficient. Therefore, manycommercial plasma processes in exhaust treatment applications are a combination of an oxidizing plasma stage(NTP or plasma UV-light sources) with other technologies, e.g. catalyst, adsorbing agents or scrubbing. Suchcombinations are sometimes more than just a serial connection of different processes. The processes can beenhanced by the plasma activity and thus enable synergies.

For example, the higher oxides of NO resulting from plasma treatment (e.g. NO2) have a better solubility inwater and can be scrubbed more easily. Another possibility for higher NOx reduction is the addition of NH3,

c© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cpp-journal.org


resulting in the formation of solid salt particles. These particulates can be precipitated and used as fertilizers.This approach is used in the electron beam injection process (EBFGT) [5]. Another technique for the treatment ofNOx-containing gas streams is the injection of O3 as an oxidizing agent, which is also referred to as Low-ThermalOxidation (LTO). The NO is oxidized to NO2, N2O4 or N2O5, which is then removed by scrubbing [19]. Theso-called Plasma Enhanced Selective Catalytic Reduction (PE-SCR) is likewise based on the conversion of NOinto NO2. NO2 can be more efficiently converted into N2 and H2O by catalysis (NH3-SCR) and at significantlylower temperatures [20–24]. Numerous studies on the combination of NTP with catalysts, in particular DBDsand dielectric packed-bed DBD reactors for VOC removal, have suggested synergies [25–28]. In this context,the combination of plasma treatment with adsorption methods has also been proposed for VOC abatement anddeodorization [17].

Prior to plasma exposure, the polluted gas is stripped of particulate matter and aerosols by means of clas-sical filtration (mechanical or physical methods). The plasma stage is responsible for the oxidation of pollutantmolecules. After the plasma stage, an active carbon filter or a metal oxide catalyst (e.g. manganese oxide) decom-poses the residual ozone or adsorbs other by-products. Several manufacturers offer such plasma-based devicesfor deodorization.

On the other hand, catalyst or adsorber beds can be regenerated by means of NTPs. This offers the possibilityto establish cyclic processes for the removal of low-concentrated pollutants [17]. In such processes, the low-concentrated pollutants are adsorbed and thus concentrated on solid matter in a storage phase. In the subsequentplasma phase, the adsorbed molecules are decomposed or desorbed by plasma activity. Since the retention timeof the pollutants in the plasma phase and their concentration are increased, less energy is consumed in sucha plasma-enhanced adsorption process. Such processes have been established to decompose different VOCsand NOx, as summarized in [29]. The decomposition of adsorbed ethanol on active carbon samples by meansof ozone generated in the plasma has been investigated in [30]. The regeneration of clinoptilolite (a naturalzeolite) loaded with NH3 has recently been shown by means of a packed-bed DBD reactor. The adsorbed NH3

is released at a relatively low temperature and low energy consumption [31]. A cycled adsorption and plasmaprocess using mineral granulates consisting of 80 % halloysite in a packed-bed DBD reactor for the removalof formaldehyde CH2O was investigated in [29]. Here, the adsorbed CH2O molecules were decomposed intoCOx and hydrocarbons in a N2-plasma. The total amount of decomposed CH2O and the selectivity towards CO2

increased with N2 gas space-times (i.e. the time required to process one packed bed volume of adsorbing materialwith gas) and with oxygen fraction in the carrier gas. The above examples demonstrate the high potential ofplasma-enhanced techniques, which can increase efficiency and lower operational costs. However, more researchand development are necessary in order to establish a wider industrial breakthrough.

3 NOx-conversion and exhaust treatment

3.1 NOx emissions

Effective ways to remove nitrogen oxides from combustion exhaust gases by means of NTPs have been studiedfor the last 15 years [18, 20–24, 32–34]. To remove nitrogen oxides from diesel engine exhaust gases, NO canbe oxidized to NO2 using an NTP-reactor to pre-process the exhaust for a subsequent catalytic treatment atmoderate temperatures. A plasma assisted lean NOx catalysis utilizing a hydrocarbon as a reducing agent wasinvestigated in [35] and the conversion of the hydrocarbons to an aldehyde have been the key to efficient catalyticreduction of NOx. However, a commercial plasma-based exhaust treatment system is still non-existent. Theemissions of marine diesel exhaust gases have now become an issue of public discussion. Especially shippingdiesels are subject to stronger legislative restrictions now and in the future [36], in particular concerning NOx,SOx and particulate matter. Engine modification mostly affects new vessels. Ammonia-based selective catalyticreduction (NH3-SCR) requires a significant amount of installation space, in addition to high costs. Consequently,the development of novel efficient exhaust after-treatment systems must be taken into account as well.

In this study, the plasma effect on synthetic exhaust gas compositions according to real marine diesel engineemissions was investigated. The aim was to study the influence of the plasma operation parameters, the gascomposition and the gas temperature on the NO oxidation in as realistic as possible situations [37]. Therefore,the full exhaust composition was investigated with systematic variation of the partial pressures changing with theoperation mode of the engine. Since the oxidation efficacy can be improved by adding hydrocarbons [38], which



is mainly due to the opening of reaction paths by the formation of peroxy radicals [39], propene C3H6 was usedas an additive. Continuing the study, investigations into the exhaust of a shipping diesel engine (HCP SULZER,6AL 20/24) have recently been performed at the Marine University in Szczecin.

3.2 DBD-stack reactor and experimental set-up

The reactor concept used in this study is a so-called stacked DBD reactor, as first described by Muller andZahn [40]. The DBD consists of dielectric plates (in this case Al2O3 ceramics) stacked alternatingly with stainlesssteel grid electrodes. The dielectric plates and grid electrodes are separated by spacers realizing discharge gapsof 1 mm distance at both sides of the dielectric plates. A scheme of a single DBD-stack is shown in figure 1 (a).The gas flows along the grid and plates and is treated by the plasma. This arrangement results in a fairly low backpressure, as is required for an exhaust after-treatment device [37].

For laboratory experiments with small gas flows of up to 200 L/h, a single stack arrangement consisting of onedielectric plate and two grid electrodes was used (width 37 mm, length 120 mm, flow volume about 9 cm3) [37].The DBD single-stack reactor was placed into a tube furnace in order to simulate the exhaust conditions ofexhaust systems of internal combustion engines. Gas pipes were heated and the temperature was monitoredprior to, during and after the tube furnace. The different concentrations of the gas constituents and the C3H6-admixture were realized by utilizing pressurized gas cylinders, gas flow controllers and a moistening system.The DBD was operated by sinusoidal voltage power supply (400 Hz, up to 5.5 kVrms). The high voltage atthe powered electrode was measured by means of a high voltage probe (Tektronix P6015A, bandwidth 75 MHz,typical rise time less than 4.67 ns) connected to a digital oscilloscope (Tektronix DPO 4104, sampling rate 5 GS/s,bandwidth 1GHz). The total current was measured via a shunt between the grounded electrode and the groundingpoint, and recorded with the same digital oscilloscope as the voltage signal. The recorded voltage and currentsignals were averaged over 100 periods and multiplied with each other, which gives the mean value of PPlas. Thereliability of the power measurement was checked with charge-voltage oscillograms (so-called Lissajous figures).The processed gas was analyzed with a Fourier-Transform Infra-Red (FTIR, Thermo Fisher Antaris IGS, spectralresolution 0.5 cm−1) spectrometer (2 m cell with a volume of 250 mL at 185 ◦C).

For larger gas flows, the stack reactor could be readily scaled up by increasing the number of stacks. Forexperiments on a bypass of a diesel engine, nine modules each consisting of 44 stacks were realized. One moduleis shown in figure 1 (c). With nine modules, a bypass gas flow of about 260 m3/h can be treated.

3.3 Results and discussion

The results of NO and NOx (NOx= NO+NO2) conversion are shown in figure 1 (b) for a situation resembling10 % diesel engine load conditions. The gas composition and properties for these conditions are given in table1 [41]. The NO removal was dependent on the SED and saturates at about -15 % without propene admixture. Atthe same conditions, the NOx concentration increased for about 15 %.

Table 1 Exhaust gas composition and properties at 10 % engine load conditions

NO NO2 CO CO2 H2O N2+O2 Temperature348 ppm 102 ppm 195 ppm 3.9 vol.% 3.7 vol.% balance 160 ◦C

The admixture of about 730 ppm C3H6 significantly increased the NO conversion. About 50 % of initialNO was converted, which was already reached at about 30 J/L. At this point an EY of about 17 g/kWh (energyconsumption EC of about 50 eV/NO-molecule) was determined. Under the conditions under consideration, about24 % of engine load is expected to be consumed by the exhaust treatment in the case of full flow emission(assuming linear scaling with SED). The value of EC was found to increase with the SED, and to decrease withthe initial NO-to-NO2 ratio (also see [37]). The NOx concentration was slightly reduced in the case of propeneadmixture. The results showed that the converted NO was mainly oxidized to NO2 with a small production ofthe unwanted by-product N2O (less than 30 ppm). The NO2 can be scrubbed or reduced in a catalytic stage.The deoxidation of NO into N2 and O2 of about 20 % was observed for an SED larger than 50 J/L, which doesnot allow economic NOx removal. The results confirmed that the oxidation of NO to NO2 is enhanced by theadmixture of hydrocarbons. Propene forms peroxy radicals and hydroperoxy radicals via dehydrogenization due



to reactions with O atoms. These radicals increase the rate of oxidizing NO reactions [38, 39]. In additionthe removal of O-radicals by reactions with hydrocarbons reduces the rate of NO2 reduction via the reactionNO2 + O→ NO + O2. In case of a plasma assisted lean NOx catalysis utilizing a propene or propane admixturethe conversion of the hydrocarbons to an aldehyde has to be considered [35].

a) b)

c) d)

Fig. 1 NO-conversion by means of a stacked DBD reactor: (a) Scheme of stacked DBD reactor; (b) Laboratory results ofNO-conversion with and without admixture of propene; (c) Stacked DBD reactor module for test bench studies consisting of44 stacks; (d) Results of test bench studies (NO and NOx removal).

Figure 1 (d) shows the results of the test bench studies performed at the Marine University of Szczecin for10 % engine load conditions (720 revolutions per minute) in the bypass with a partial gas flow of 133 m3/h. Theexperiments were performed with and without propene admixture (108 L/h, i.e. about 810 ppm) and at a gastemperature of 110 ◦C. Similar to the laboratory experiments, a poor reduction of NO and NOx was observedwithout propene admixture (ΔNO= -10 % at about 7 J/L), but with propene admixture a much better NTP effectwas achieved. About 50 % of the initial NO was removed at about 9 J/L, i.e. the efficacy was much higher than inthe laboratory studies (EY about 60 g/kWh; EC about 11 eV/NO-molecule, corresponds to about 8 % of enginepower). Such an energy efficiency could enable an economic exhaust gas treatment system.

The NO2 concentration remained nearly constant at 120 ppm but NOx was significantly reduced to up to40 %. In the test bench studies, the particle matter (PM) concentration was measured in real-time (gas analyzerHORIBA MEXA-1230PM based on diffusion charging method and flame ionization detectors) and a significantPM reduction by NTP activity was observed. The reduction and/or adsorption of NO2 at particles (soot, solubleorganic fraction) are assumed to be responsible for the significantly better performance of the DBD stack reactorin the real exhaust. The possibility of such reactions has already been described [42–45], but further studies areneeded to validate this hypothesis.

The results demonstrate that NTPs are capable to reach a significant NOx removal under real conditions, witheconomically feasible energy requirements. A dual, synergistic effect on both particles and NOx could support



the development of an efficient after-treatment system of shipping diesel engines, as well as other combustionprocesses.

4 Plasma assisted scrubbing

4.1 Water falling film reactor

This section discusses the application of a water falling film reactor for pollutant degradation in gases. The waterfalling film reactor, shown in figure 2, was originally designed as an atmospheric pressure NTP reactor for waterpurification by Kuraica et al. at the University of Belgrade [46]. The reactor consists of a sealed quartz cylinder(thickness 1.7 mm) with a concentric hollow metal (stainless steel) tube electrode inside and a metal electrodeoutside (wrapped aluminum foil). The distance between the inner quartz wall and the metal tube is about 3 mm.Water or other liquids are pumped through the inner tube electrode by means of a peristaltic pump, with a flowrate of 100 mL/-min. The liquid flows through the tube opening, forming a laminar flowing layer along theouter wall of the inner metallic tube which is grounded. The outer electrode is powered by a high voltage and afilamentary plasma with a length of 40 cm is formed between the inner glass vessel wall and the liquid film, i.e.the plasma is in direct contact with the liquid.

Fig. 2 Coaxial water falling film reactor for liquidtreatment and plasma assisted scrubbing.

The water falling film reactor has been demonstrated to be capable of decomposing pollutants in the liquidphase. Water containing phenols or arsenic and textile dyes was purified [47–50]. A similar configuration withouta water layer was successfully applied to the simultaneous removal of NOx and SO2 from flue gases of coal-firedpower plants [51]. In the current study the capability of such reactors for gas purification is investigated.

The linear arrangements of NTP-stage and scrubber sections for the VOC removal have already been describedin the literature, e.g. [52]. In principle, the NTP can convert non-soluble compounds into soluble ones which canbe scrubbed. The aim of our study was to investigate whether this can be done in one stage. This would enablea much more compact design for an exhaust gas treatment system, which can easily be scaled up to the requiredcapacity by parallel connection of multiple reactors. Furthermore, the direct contact between the plasma andthe liquid could increase the efficacy of plasma treatment (e.g. by higher density of OH radicals) and lead to acleaning of the liquid phase due to the impact of ozone and other reactive species. To investigate this hypothesis,a non-soluble hydrocarbon was chosen as a model system. Undecane CH3(CH2)9CH3 is a liquid alkane with aboiling point at 196 ◦C, but it is also emitted into the air. Undecane is a constituent of gasoline and diesel fuel,which is declared as harmful as it can be fatal when entering airways (respiratory irritation, skin or eye irritation).



4.2 Experimental set-up and procedure

The water falling film reactor described above was operated with an AC power supply consisting of a frequencyinverter and a high voltage transformer. For the presented reactor configuration, the optimal operating conditionswere found to be at a frequency of 300 Hz. The DBD was generated by applying high voltage with an amplitudeof up to 20 kV, recorded by a high voltage probe and a digital oscilloscope (see previous section). The innerelectrode was grounded via a capacitor of 470 nF in order to record voltage-charge plots for the determination ofPPlas. Alternatively the power could be measured as already described for the stack reactor. The mixture of airand undecane was prepared by bubbling compressed air through a sample of liquid undecane, which was heatedin a water bath (40 ◦C). The gas stream containing undecane was diluted with compressed air in order to obtainan initial undecane concentration of about 50 ppm. The total gas flow rate was 800 L/h. Prior to the experimentswith plasma operating it was checked that absorption of undecane by the circulating water could be omitted.

Gas analysis was performed by means of a Fourier Transform Infra-Red (FTIR) spectrometer and a flameionization detector (FID). The FTIR devices (Alpha by Bruker Optik GmbH, Gasmet Cr-2000 by Ansyco) givequalitative as well as on-line quantitative gas analyses of IR-active compounds with a sensitivity of over 2 ppm.The total concentration of organic carbon was measured by FID (2010T, TESTA GmbH) with a sensitivity downto 0.2 ppm (related to the calibration gas propane).

For analysis of water treated in air plasma 1 liter of distilled water was recirculated six times in such a waythat the currently treated amount of water was never mixed with the amount of water treated in the previous pass.After each recirculation, 50 mL of the water was sampled for analysis after the amount of 500 mL has passedthrough the DBD reactor. After each pass the following sample was made immediately ensuring that air doesnot enters through the water. For each pass through the reactor, the energy density introduced into the water was39 kJ/L. Immediately after each pass the pH-value and the conductivity were determined. Five minutes after eachrecirculation concentrations of chemical species nitrate [53], nitrite [54] and hydrogen peroxide [55, 56] weremeasured by methods based on color forming reactions with reagents.

4.3 Results and discussion

Typical voltage and current waveforms of water falling film DBD without water and with water film are shownin figure 3. In all experiments plasma had filamentary character. When water flowed over the inner electrode thefilaments appeared thicker and their number was reduced. This could be observed by looking through metallicmesh as an outer electrode instead of aluminum foil and by comparing the current signals in figures 3 a) and b).

a) b)

Fig. 3 Voltage and current signals of water falling film DBD a) in air and b) in air with water.

Furthermore, the voltage amplitude decreased slightly when water was introduced. Consequently, the mean powerdissipated into the plasma was reduced when discharge was operated with water. The dissipation of electricalpower in the water layer was estimated up to 20 % of the total discharge power for the reactor without water forlower applied voltages (about 12 kV), while for higher applied voltages (17-20 kV) energy dissipated in waterlayer reduces to only few percents. This is due to the fact that plasma-liquid interaction influences both - gasdischarge characteristics and water properties. For this reason chemical analysis of water treated in air plasma



was performed. Measurement of pH and conductivity values showed a steep decrease of pH from 6 to less than3, while conductivity increased linearly up to 1200 S/cm with SED (see figure 4 a). Air plasma treated waterbecomes acidic and contains reactive oxygen species (ROS) and reactive nitrogen species (RNS) such as nitrite(NO−

2 ), nitrate (NO−3 ) and hydrogen peroxide (H2O2). In air plasma ROS and RNS are generated in gas phase and

transferred to water layer through plasma-liquid interactions [57, 58]. Dissolution of these reactive species leadsto the acidification of the liquid exposed to the plasma. Consequently, the presence of ionic species increasesconductivity of treated water, and leads to transformation of water layer from dielectric barrier, at the beginningof plasma treatment, to conductive part of the electrode system.

a) b)

Fig. 4 Changes of conductivity and pH values a) and measurement of nitrate, nitrite and hydrogen peroxide b) in distilledwater treated in water falling film DBD

a) b)

Fig. 5 Conversion of total hydrocarbons (THC) (a) and the generation of ozone (b) in a water falling film reactor as a functionof SED (full symbols: without water layer; empty symbols: with water layer).

The conversion of total hydrocarbons (THC) determined from the FID results as a function of SED is presentedin figure 5 (a). The conversion was studied with and without water layer. Although the undecane admixtureprocedure was indentical in all experiments the initial undecane concentration varied slightly. In case of waterfilm operation it was 40 ppm while 56 ppm in the experiment without water. In the discharge without water flow,the conversion of undecane and all other possible organic by-products reached 87 % at a SED of 83 J/L (EYabout 13.5 g/kWh assuming undecane as the main organic compound). The same level of conversion by means ofdischarge with water layer was reached at SED 112 J/L corresponding to an EY of about 7 g/kWh. The maximalconversion level of 98% was reached in both cases, and for this value EYs of 6 g/kWh and 3.3 g/kWh werecalculated for discharges operating without water and with water film, respectively. The reduction of efficacywhen water film was present in the discharge could be related to the change of the plasma parameters. TheFTIR measurements revealed an ozone concentration of about 1,200 ppm at a SED of 100 J/L without waterfilm, but only of about 600 ppm at the same SED in the case of water film operation (see figure 5 b). Ozonemay oxidize undecane and dissolve in water. But ozone can also be seen as a ’kind of measure’ for the plasmachemical activity. Obviously, less atomic oxygen and other radicals are generated when the microdischarges



are operated in contact with a water film. More detailed investigations of microdischarge properties under suchspecific conditions are necessary to confirm the above suggestion, which will be left to future research.

Although the presence of water had an adverse effect on energy efficacy of undecane conversion, the FTIRanalysis showed that its presence enhanced the gas purification process. Figure 6 presents selected FTIR spectraof plasma treated air (a) and undecane polluted air (b). By-products generated in the unpolluted, humid air in (a)included O3, nitric acid HNO3 and nitrous oxide N2O. Similar results have been obtained in other studies, e.g.indirect treatment of liquids by means of surface DBDs [57]. The spectrum in (b) reveals CO2, CO and formicacid HCOOH as the major degradation products of undecane. The absorption bands marked as residual undecanein figure 6 (b) refer to non-oxidized undecane and alkanes with lower numbers of carbon atoms (e.g. decane,nonane and octane). Low-carbon compounds have almost the same FTIR-spectra as undecane, and thus it wasnot possible to distinguish these compounds in this study.

Fig. 6 FTIR-spectra taken at a water falling film reactor (with and without water film in operation) gas outlet for (a) air and(b) undecane diluted in air. The boxed areas mark the most significant changes.

When comparing the two spectra in figure 6 (a) it can be observed that by-products were significantly reducedfor DBDs operating with water film. In particular the HNO3 signals were lower. Nitric acid is soluble in waterand can be scrubbed efficiently. The same results were obtained in the FTIR-spectra of undecane diluted in air infigure 6 (b). Furthermore, formic acid emission caused by undecane oxidation was reduced (see the boxed partin the figure). Without water film operation, the formic acid concentration increased with SED. At an SED of250 J/L, about 28 ppm of HCOOH were measured. At these conditions about 85 ppm of CO and 140 ppm of CO2

are generated while 37 ppm of total hydrocarbons are converted. This results in a selectivity SCO2 for formationof CO2 of about 55 % and a carbon balance (CB) of 62 % (see equations (3) and (4) [59]), i.e. about 38 % ofinitial carbon is not contained in the CB. One possible reason for this could be the deposition of carbon layersin the reactor, or the sticking of undecane or other compounds to the reactor walls, gas tubes etc. It should benoted that the measurement of THC concentrations at the reactor exit also includes alkanes with lower number ofcarbon atoms, which could be generated due to the decomposition of undecane. For this reasons, the presented



values of selectivity and CB can be considered only as a upper limit and lower limit values, respectively.

SCO(2) =[CO(2)]

[CO2] + [CO] + [HCOOH]× 100 (3)

CB =[CO2] + [CO] + [HCOOH]

11 ([C11H24]in − [C11H24])× 100 (4)

In the case of water film operation, the gas phase HCOOH concentration never exceeded a value of 8 ppm.Aldehydes and organic acids can be considered to be intermediate products of undecane oxidation. Since theyare soluble in water, they are removed from gas phase before complete oxidation can occur. At an SED of 250 J/L30 ppm of CO, 51 ppm of CO2 and 4 ppm of formic acid were detected. Consequently, the selectivity Sx of COand CO2 was enhanced to 60 % and the CB changed to 24 %. The large difference of the CB was due to theremoval of soluble formic acid and intermediate products by dissolution in water. The concentrations of COand CO2 were significantly lower for nearly the same amount of removed undecane (about 30 ppm) as withoutwater layer operation. One possible explanation for this is that intermediate plasma chemical compounds weredissolved in water before the total oxidation of this species could proceed. Indeed the pollutant molecules areremoved from the gas phase but not completely decomposed. A fraction of by-products is polluting the water andplasma-assisted scrubbing will be feasible in industrial sites where polluted water is generated anyhow.

5 Summary and Outlook

The current state of the research into and the application of non-thermal plasmas to the pollutant degradation ingases was discussed. Some fundamental topics and engineering issues were addressed, such as the physics andchemistry of filamentary plasmas in polluted gases or the possibilities of hybrid plasma processes. A great chal-lenge in this field is to merge a profound understanding of the breakdown and formation in filamentary plasmaswith the plasma chemistry. Multidisciplinary approaches that includes plasma physics and chemical engineeringare required but will advance the development not only in the field of environmental plasma applications, butalso for plasma synthesis, indoor air quality and hygiene. Plasma processes for pollutant degradation in oxygen-containing gases must be interpreted as oxidation processes for the enhancement or support of other technologies.The combination of plasmas with catalysts or adsorbing materials offers promising technologies for the depollu-tion of gases.

Two reactor concepts, both based on dielectric barrier discharges, were presented. The DBD-stack reactorshowed a very good scalability. The oxidation of NO was investigated under as realistic as possible conditions.The enhancement of oxidation processes regarding efficacy and selectivity by the admixture of hydrocarbonswas confirmed for both laboratory and test bench investigations. The DBD-stack reactor could be extended bycatalytic coatings on the electrodes enabling a plasma-enhanced catalysis. This is currently being studied further.

The water falling film reactor demonstrated its feasibility for the purification of water and gases. A long-chain, harmful hydrocarbon was successfully decomposed with reduced emission of by-products. The conversionof non-soluble compounds into soluble ones is a promising development towards a compact plasma-assistedscrubbing technology in combination with water cleaning procedures.

Plasma technology is an environmental technology. The application of non-thermal plasmas for the purifica-tion of gases and liquids shows a great potential for solving some of our environmental problems. The investiga-tion of the physical properties of filamentary plasmas and microdicharges in polluted gases as well as the studyof the interaction mechanism in hybrid processes will enhance the potential for future applications. In particular,a quantitative understanding of plasma-catalyst or plasma-adsorbent interaction is desired.

Acknowledgements This work was supported by the transnational project ”PlasTEP Dissemination and fostering of plasma-based technological innovation for environmental protection in the Baltic Sea region” (partly financed by the European Union- the European Regional Development Fund) and the Federal German Ministry of Economy and Technology (BMWi) withinthe project ERA-Net Martec PBCT (03SX288A). The water falling film reactor studies were supported by the Ministry ofEducation and Science of the Republic of Serbia, through Projects 171034 and 33022.

The technical assistance of, and cooperation with, Wolfgang Reich, Bernd Nehmzow, Helge Grosch, Tomas Hoder,Alexander Schwock, Hans Hoft (all INP Greifswald), Robin Summerer (neoplas GmbH), Miroslaw Dors (Center for Plasma



and Laser Engineering Gdansk), Tadeusz Borkowski (Marine University Szczecin), Marcin Hołub (West Pomeranian Univer-sity of Technology Szczecin), Ivan B. Krstic and Borivoje Trbovic (all University of Belgrade) is highly appreciated.

References

[1] U. Kogelschatz, Yu.S. Akishev, and A.P. Napartovic, in Non-Equilibrium Air Plasmas At Atmospheric Pressure, editedby K.H. Becker, U. Kogelschatz, K.H. Schoenbach, and R.J. Barker, Series in Plasma Physics (Institute of PhysicsPublishing, Bristol and Philadelphia, 2005; now Taylor & Francis Group: Boca Raton), chap. 2.

[2] European Environment Agency: 2013: Kicking off the Year of Air; http://www.eea.europa.eu/highlights/2013-kicking-off-the-2018year; conducted 25th July 2013

[3] A. Fridman, Plasma Chemistry (Cambridge University Press, Cambridge, 2008), chap. 4.5 and 4.3.8.[4] J.J. Chang, in Industrial Plasma Technology, edited by Y. Kawai, H. Ikegami, N. Sato, A. Matsuda, K. Uchino, M.

Kuzuya, and A. Mizuno (Wiley-VCH Verlag GmbH & Co. KGaG, Weinheim, 2010), chap. 3.[5] A.G. Chmielewski, Rad. Phys. Chem. 46, 1057-1062 (1995).[6] U. Kogelschatz, B. Eliasson, and W. Egli, J. Phys IV France 7, C448-C466 (1997).[7] S. Nijdam, E.v. Veldhuizen, P. Bruggeman, and U. Ebert, in Plasma Chemistry and Catalysis in Gases and Liquids,

edited by V.I. Parvulsecu, M. Magureanu, and P. Lukes (Wiley-VCH GmbH & Co. KGaG, Weinheim, 2012), chap. 1.[8] W.F.L.M. Hoeben, F.J.C.M. Beckers, A.J.M Pemen, E.J.M van Heesch, and W.L. Kling, J. Phys. D: Appl. Phys. 45,

055202 (2012).[9] A. Mizuno, Plas. Phys. Contr. Fusion 49, A1-A15 (2007).

[10] G.J. Pietsch and V.I. Gibalov, Plasma Sources Sci. Technol. 21, 2 024010 (2012).[11] K.V. Kozlov, in Nonthermal Plasma Chemistry and Physics, edited by J. Meichsner, M. Schmidt, R. Schneider, and

H.-E. Wagner (Taylor & Francis Group, Boca Raton, 2013), 286-296.[12] T. Hoder, M. Cernak, J. Pailliol, D. Loffhagen, and R. Brandenburg, Phys. Rev. E 86, 055401 (2012).[13] M. Kettlitz, H. Hoft, T. Hoder, K.-D. Weltmann, and R. Brandenburg, Plas. Sour. Sci. Technol. 22, 025003 (2013).[14] J. Mizeraczyk, S. Kanazawa, and T. Ohkubo, J. Advanced Oxid. Technol. 7, 20-30 (2004).[15] P. Bruggeman and R. Brandenburg, J. Phys. D.: Appl. Phys. 46, 464001 (2013).[16] A.V. Pipa, J. Koskulics, R. Brandenburg, and T. Hoder, Rev. Sci. Instr. 83, 115112 (2012).[17] H.H. Kim, A. Ogata, and S. Futamura, Int. J. Plas. Environ. Sci. Technol. 1, 46-51(2007).[18] H.H. Kim, Plas. Process. Polym. 1, 91-110 (2004).[19] E. Stamate, W. Chen, L. Jørgensen, T.K. Jensen, A. Fateev, and P.K. Michelsen, Fuel 89, 978-985 (2010).[20] T. Hammer and S. Broer, SAE Technical Paper 982428 (1998).[21] S. Broer and T. Hammer, Appl. Catal. B: Environmental 28, 101-111 (2000).[22] B.M. Penetrante, R.M. Brusasco, B.T. Merritt, W.J. Pitz, G.E. Vogtlin, M.C. Kung, H.H. Kung, C.Z. Wan, and K.E.

Voss, SAE Technical Paper 982508 (1998).[23] Y.S. Mok and Y.J. Huh, Plas. Chem. Plas. Process. 25, 625-639 (2005).[24] M.S. Cha, Y.-H. Song, J.-O. Lee, and S.J. Kim, Int. J. Plas. Environ. Sci. Technol. 1, 28-33 (2007).[25] J. van Durme, J. Dewulf, Ch. Leys, and H. Van Langenhove, Appl. Cat. B: Environ. 78, 324-333 (2008).[26] C. Whitehead, in The 2012 Plasma Roadmap. J. Phys. D: Appl. Phys. 45, 253001 (2012).[27] F. Holzer, U. Roland, and F.-D. Kopinke, Applied Catalysis B: Environmental 38, 163-181 (2002).[28] M. Magureanu, in Plasma Chemistry and Catalysis in Gases and Liquids, edited by V.I. Parvulsecu, M. Magureanu,

and P. Lukes (Wiley-VCH GmbH & Co. KGaG, Weinheim, 2012), chap. 4.[29] K. Saulich and S. Muller, J. Phys. D: Appl. Phys. 46, 045201 (2013).[30] R. Basner, A. Akimalieva, and R. Brandenburg, Surf. Coat. Tech. 234, 126-131 (2013).[31] S. Muller, K. Saulich, and T. Kruger, Chemie Ingenieur Technik 84, 2190-2197 (2012).[32] M. Okubo, T. Kuroki, K. Kitaura, and T. Yamamoto, J Environ. Eng. 1, 29-38 (2006).[33] J.O. Chae, J. Electrostat. 57, 251-262 (2003).[34] R. McAdams, P. Beech, and J. Shawcross, Plas. Chem. Plas. Process. 28, 159-171 (2008).[35] J.W. Hoard and A. Panov, SAE SAE Technical Paper 2001-01-3512 (2001).[36] International Maritime Organization (IMO) ’Prevention of Air Pollution from Ships’ [Online],

http://www.imo.org/OurWork/Environment/PollutionPrevention/AirPollution/Pages/Air-Pollution.aspx; conducted28th August 2013.

[37] M. Schmidt, R. Basner, and R. Brandenburg Plas. Chem. Plas. Process 33, 323-335 (2013).[38] Y.S. Mok, V. Ravi, H.-C. Kang, and B.S. Rajanikanth, IEEE Trans. Plasma Sci. 31, 157-165 (2003).[39] R. Dorai and M.J. Kushner, J. Appl. Phys. 88, 3739-3747 (2000).[40] S. Muller and R.-J. Zahn, Contr. Plas. Phys 47, 520-529 (2007).[41] T. Borkowski, Marine University Szczecin, private communication.[42] P.M. Borrell and P. Borrell (editors), Heterogeneous Interaction of Ozone, NO2 and N2O5 wit Soot Aerosol (Proceed-

ings of the Eurotrac Symposium 1998, Southampton, WIT-Press, 1998).[43] O. Mohler, K.-H. Naumann, H. Saathoff, and U. Schurath, J. Aerosol Sci. 28, 309-310 (1997).



[44] V.H. Grassian, J. Phys. Chem. A 106, 860-877 (2002).[45] J. Grundmann, S. Muller, and R.-J. Zahn, Plas. Chem. Plas. Process. 25, 455-466 (2005).[46] M.M. Kuraica, B.M. Obradovic, D. Manojlovic, D.R. Ostojic, and J. Puric, Vacuum 73, 705-708 (2004).[47] D. Manojlovic, D.R. Ostojic, B.M. Obradovic, M.M. Kuraica, V.D. Krsmanovic, and J. Puric, Desalination 213, 116

(2007).[48] D. Manojlovic, A. Popara, B.P. Dojcinovic, A. Nikolic, B.M. Obradovic, M.M. Kuraica, and J. Puric, Vacuum 83,

142-145 (2008).[49] B.P. Dojcinovic, D. Manojlovic, G.M. Roglic, B.M. Obradovic, M.M. Kuraica, and J. Puric, Vacuum 83, 234 (2009).[50] B.P. Dojcinovic, G.M. Roglic, B.M. Obradovic, M.M. Kuraica, M.M. Kostic, J. Nesic, and D. Manojlovic, J. Hazard.

Mat. 192, 763-771 (2011).[51] B.M. Obradovic, G.B. Sretenovic, and M.M. Kuraica, J. Hazard. Mat. 185, 1280-1286 (2011).[52] A. Fridman, Plasma Chemistry (Cambridge University Press, Cambridge, 2008), chap. 11.8.6.[53] A.E. Greenberg, R. Rhodes Trussell and L.C. Clesceri, in Standard Methods for the Examination of Water and Wastew-

ater, ed. by M.A.H. Franson (APHA & AWWA & WPCF, Washington, 16th ed., 1985), Method 419, 404-406.[54] http://www.caslab.com/EPA-Methods/PDF/EPA-Method-3521.pdf.[55] G.M. Eisenberg, Ind. Eng.Chem. Anal. Ed. 15, 327-328 (1943).[56] P. Lukes, M. Clupek, V. Babicky, and P. Sunka, Plasma Sources Sci. Technol. 17, 1-11 (2008).[57] K. Oehmigen, J. Winter, M. Haehnel, C. Wilke, R. Brandenburg, K.-D. Weltmann, and T. von Woedtke, Plas. Process.

Polym. 8, 904-913 (2011).[58] C.A.J. van Gils, S. Hofmann, B.K.H.L. Boekema, R. Brandenburg, and P.J. Bruggeman, J. Phys. D: Appl. Phys. 46,

175203 (2013).[59] H.H. Kim, A. Ogata, and S. Futamura, J. Phys. D: Appl. Phys. 38, 1292-1300 (2005).


Plasma-Based Pollutant Degradation in Gas Streams: Status, Examples and Outlook

Documents

Transcript of Plasma-Based Pollutant Degradation in Gas Streams: Status, Examples and Outlook