**4. VOC removal by means of barrier discharges**

**Figure 10.** FT-IR spectra recorded in experiments with dc+ corona (+19 kV) in (a) pure synthetic air, used as reference spectrum, and in synthetic air containing 500 ppm of the following VOCs: (b) *n*-hexane; (c) toluene; (d) CH2Br2; (e)

**Figure 9.** Aldehydes and ketones detected as intermediates in the oxidation of *n*-hexane (500 ppm in dry synthetic air) induced by pulsed+, dc– and dc+. The data are displayed as a function of the fraction of decomposed *n*-hexane [18].

CF2Br2.

16 Current Air Quality Issues

The experimental results obtained with a surface DBD reactor developed at INP Greifswald are reported. The discharge arrangement consists of two metal woven meshes and a dielectric plate (mica) in between the two electrodes [43]. As shown in Figure 11 the surface DBD arrangement (110 x 80 x 3 mm) was installed in a gastight chamber made of poly(methyl methacrylate) (PMMA) where the gas mixture to be treated (mixed from gas cylinders by means of mass flow controllers) was conveyed with a total gas flow of 75 LN/h. In order to vary the humidity of the gas mixture the partial gas flow from oxygen gas cylinder was directed through a water containing bubbler. Water content of around 0.5% was realized in this way. Most of the gas flows along the electrodes configuration instead of entering the active plasma region between the electrodes and the dielectric as depicted by the arrows in Figure 11 (right). An advantage of this configuration is the very small back pressure, which is desired for the treatment of large gas flows.

The plasma reactor was energized with a programmable high-voltage power source and a high-voltage transformer. The frequency of the applied voltage was ranged from 400 Hz to 1 kHz. The power dissipated into the plasma was analyzed by recording the high voltage operating the reactor via a high voltage probe. Additionally, the voltage drop over a capacitor (capacitance 100 nF), connected in series with the reactor between the grounded electrode and protected earth, was recorded. By multiplying the voltage drop over the capacitor by the capacitance the transferred charge was obtained.

Samples of the gas mixture were analyzed by Flame Ionization Detector (FID) (Testa FID-2010T), measuring the total amount of organic carbon present in the exhaust gas. Addi‐ tionally, a Fourier Transform InfraRed spectrometer (FTIR) (Bruker Alpha, spectral resolution 1 cm-1, optical path length 5 m) was used to monitor the processed gas mixtures. The gas cell of the spectrometer was heated up to 40°C in order to avoid water condensation. Higher temperatures would be desirable to avoid water condensation but cannot be used otherwise ozone will decompose and give rise to sort of a "post-plasma" contribution to toluene oxidation which should be avoided.

**Figure 11.** Experimental setup for toluene removal studies (left) and detailed horizontal cut view of the Surface DBD reactor (right).

In order to obtain comparable results of the measurements under the selected conditions the operating voltage was chosen as the electrical parameter to be set for every measurement. The electrical data recorded during the experiments were investigated during the analysis procedure. The power input into the plasma reactor was calculated by integrating the area of the charge-voltage plot (Q-V plot) and multiplying the resulting value with the frequency of the applied voltage [44]. By this, it was found that the Q-V plots for different frequencies at the same driving voltage were almost identical.

Examples are given in the left part of Figure 12. The black curves show the Q-V plots recorded at a frequency of 400 Hz, whereas the red curves display that one recorded at 1 kHz. The operating voltage was 8.3 kV and 6.9 kV, respectively. The equality of the Q-V plots implies that the energy transferred per cycle into the plasma is almost identical. Thus, the power input for a fixed operating voltage should depend only on the frequency in a linear relation [44]. Using the frequency ratio 2.5 (1000 Hz divided by 400 Hz) and multiplying it with the power input measured at 400 Hz, one gets the calculated power at 1 kHz. In the right part of Figure 12 the power measured at 400 Hz (black boxes), the power calculated for 1 kHz (blue triangles) and the power measured at 1 kHz (red circles) are shown. The values calculated and measured at 1 kHz are in good agreement and can be taken as another evidence of the proportionality of frequency and power, as already mentioned by Kogelschatz [44] and Manley [45].

Further investigations showed that the slope from the bottom right corner to the upper right one of the charge-voltage plot, which gives the capacitance of the plasma reactor during the discharge period, increases with increasing the operating voltage (Figure 13, left). The reason is suggested to be the increase in the active area of the electrode, which means the surface of the electrode covered with plasma. Photographs of the plasma were taken (Nikon D5100, aperture 5.6, exposure time 30 s) and reworked with an image manipulation program (Gimp 2.8, color correction) to make the plasma visible (Figure 13, right). An analysis of the extension of the visible plasma was performed. The obtained values were normalized as well as the values of the measured capacitance. The result is given in the left part of Figure 13. The normalized capacitance (red circles) increases linearly with the increasing driving voltage. The normalized active area of the plasma (black boxes) increases almost linearly, except the value at 7.3 kV. The linear relation between the dielectric capacitance and the amplitude of the applied voltage is further confirmed by the fact that the bottom right corner as well as the left top corner of the Q-V-plot (which both correspond to the inset of the discharge in every half period of the applied voltage) are not sharp. This would be the case for a uniform breakdown of the gas discharge.

**Figure 11.** Experimental setup for toluene removal studies (left) and detailed horizontal cut view of the Surface DBD

In order to obtain comparable results of the measurements under the selected conditions the operating voltage was chosen as the electrical parameter to be set for every measurement. The electrical data recorded during the experiments were investigated during the analysis procedure. The power input into the plasma reactor was calculated by integrating the area of the charge-voltage plot (Q-V plot) and multiplying the resulting value with the frequency of the applied voltage [44]. By this, it was found that the Q-V plots for different frequencies at

Examples are given in the left part of Figure 12. The black curves show the Q-V plots recorded at a frequency of 400 Hz, whereas the red curves display that one recorded at 1 kHz. The operating voltage was 8.3 kV and 6.9 kV, respectively. The equality of the Q-V plots implies that the energy transferred per cycle into the plasma is almost identical. Thus, the power input for a fixed operating voltage should depend only on the frequency in a linear relation [44]. Using the frequency ratio 2.5 (1000 Hz divided by 400 Hz) and multiplying it with the power input measured at 400 Hz, one gets the calculated power at 1 kHz. In the right part of Figure 12 the power measured at 400 Hz (black boxes), the power calculated for 1 kHz (blue triangles) and the power measured at 1 kHz (red circles) are shown. The values calculated and measured at 1 kHz are in good agreement and can be taken as another evidence of the proportionality

of frequency and power, as already mentioned by Kogelschatz [44] and Manley [45].

Further investigations showed that the slope from the bottom right corner to the upper right one of the charge-voltage plot, which gives the capacitance of the plasma reactor during the discharge period, increases with increasing the operating voltage (Figure 13, left). The reason is suggested to be the increase in the active area of the electrode, which means the surface of the electrode covered with plasma. Photographs of the plasma were taken (Nikon D5100, aperture 5.6, exposure time 30 s) and reworked with an image manipulation program (Gimp 2.8, color correction) to make the plasma visible (Figure 13, right). An analysis of the extension of the visible plasma was performed. The obtained values were normalized as well as the

reactor (right).

18 Current Air Quality Issues

the same driving voltage were almost identical.

**Figure 12.** Left: Q-V plots recorded at 8.3 kV and 6.9 kV at 400 Hz and 1 kHz under dry conditions. Right: Power input under dry conditions at 400Hz and 1 kHz and at 1 kHz calculated based on the power of 400 Hz.

**Figure 13.** Left: Increase in active plasma area and increase in capacitance with respect to the operating voltage. Right: Photographs of the plasma operated at different voltages.

In a first step the overall plasma chemistry of toluene removal was investigated by FTIR. In Figure 14 samples of selected spectra are presented showing the so-called fingerprint region (i.e. wavenumber 700–1500 cm-1). The left graph shows infrared-spectra taken at an applied voltage amplitude of 8.3 kVPP and a frequency of 400 Hz under dry conditions. In the untreated gas mixture (black curve) the absorption band related to toluene is the only detectable band. With plasma (red curve) the strong absorption band of ozone appears at 1053 cm-1, which is the main stable by-product of an NTP operated under ambient air conditions. Additionally, nitric acid HNO3 is detected. It is assumed that HNO3 is formed by the reaction of intermediate NOx with hydroxyl radicals produced by the toluene decomposition. The toluene absorption band is replaced by a broad absorption band whose origin could not be identified. Because no infrared absorption spectrum of the known products or intermediates (formaldehyde, benzaldehyde, benzoic acid, benzene, nitrobenzene, phenol, formic acid, and acetic acid) of the toluene removal process fits to the measured spectra it is assumed that it is a compound emitted from the material of the reactor housing (PMMA). According to the results given by the FID this analyzed gas mixture does not contain any hydrocarbons at all.

**Figure 14.** FTIR absorption spectra without plasma (black lines) and with plasma (frequency 400 Hz, operating voltage 8.3 kV, red lines) under dry (left) and wet conditions (right) of 50 ppm toluene in synthetic air.

Under wet conditions (right graph, same electrical parameters) there is no nitric acid detecta‐ ble. Moreover, the absorption of ozone is much smaller than under dry conditions. Both phenomena are attributed to the consumption of energetic electrons, which under dry conditions are used to produce NOx as a necessary intermediate for the production of nitric acid. These changes result in a considerable production of formic acid. The lower energy efficiency in toluene removal under wet conditions is also assumed to be due to the consump‐ tion of high energy electrons for the vibrational excitation of water.

FID is used to study the toluene removal since it is not sensitive to the main by-products of toluene removal that were identified by FTIR. With the concentration of toluene at about 50 ppm in the untreated gas mixture the molar fraction was calculated and plotted against the SIE. The results are shown in Figure 15. The removal efficiency increases with the SIE up to total removal at around 55 J⋅L-1 under dry conditions (black boxes). The same efficiency is achieved for 400 Hz and 1 kHz. Under wet conditions (red circles) the removal efficiency is smaller and about twice as much energy is needed to achieve complete removal of toluene which is only obtained at 1 kHz for this conditions. This dependency on the frequency is only found under wet conditions.

With plasma (red curve) the strong absorption band of ozone appears at 1053 cm-1, which is the main stable by-product of an NTP operated under ambient air conditions. Additionally, nitric acid HNO3 is detected. It is assumed that HNO3 is formed by the reaction of intermediate NOx with hydroxyl radicals produced by the toluene decomposition. The toluene absorption band is replaced by a broad absorption band whose origin could not be identified. Because no infrared absorption spectrum of the known products or intermediates (formaldehyde, benzaldehyde, benzoic acid, benzene, nitrobenzene, phenol, formic acid, and acetic acid) of the toluene removal process fits to the measured spectra it is assumed that it is a compound emitted from the material of the reactor housing (PMMA). According to the results given by

**Figure 14.** FTIR absorption spectra without plasma (black lines) and with plasma (frequency 400 Hz, operating voltage

Under wet conditions (right graph, same electrical parameters) there is no nitric acid detecta‐ ble. Moreover, the absorption of ozone is much smaller than under dry conditions. Both phenomena are attributed to the consumption of energetic electrons, which under dry conditions are used to produce NOx as a necessary intermediate for the production of nitric acid. These changes result in a considerable production of formic acid. The lower energy efficiency in toluene removal under wet conditions is also assumed to be due to the consump‐

FID is used to study the toluene removal since it is not sensitive to the main by-products of toluene removal that were identified by FTIR. With the concentration of toluene at about 50 ppm in the untreated gas mixture the molar fraction was calculated and plotted against the SIE. The results are shown in Figure 15. The removal efficiency increases with the SIE up to total removal at around 55 J⋅L-1 under dry conditions (black boxes). The same efficiency is achieved for 400 Hz and 1 kHz. Under wet conditions (red circles) the removal efficiency is smaller and about twice as much energy is needed to achieve complete removal of toluene which is only obtained at 1 kHz for this conditions. This dependency on the frequency is only

8.3 kV, red lines) under dry (left) and wet conditions (right) of 50 ppm toluene in synthetic air.

tion of high energy electrons for the vibrational excitation of water.

found under wet conditions.

20 Current Air Quality Issues

the FID this analyzed gas mixture does not contain any hydrocarbons at all.

**Figure 15.** Molar fraction of toluene under dry (black boxes) and wet (red circles) conditions at 400 Hz (circle) and 1 kHz (square).

In order to discuss the energy efficiency the energy constant parameter kE was evaluated according to the eq. (1). As reported in Table 2 the energy constants obtained under dry conditions are very similar. Those obtained under wet conditions are significantly smaller but also differ significantly.


**Table 2.** Reaction energy efficiency data, expressed as kE in L J-1 units, for DBD processing of toluene.

Quantitative analysis data for CO2 and CO produced under different experimental conditions are shown in Figure 16 as a function of the SIE. At dry conditions the production of CO is favored compared to CO2, while in humid conditions the amounts of both compounds are almost the same. The increased selectivity towards CO2 in humid conditions could be ex‐ plained as follows. In the presence of water vapor in the plasma area the production of OH radicals is higher. These radicals can react with the CO molecules to produce CO2, according to the eq. (5).

Usually the formation of OH radicals in the plasma area is also accompanied by an enhance‐ ment of the energy efficiency in the VOC removal process (e.g. see **Section 3** of this Chapter). In the case of this setup, as discussed above, the presence of water vapor in the process gas is obviously responsible of a decrease in the energy efficiency of toluene removal.

In Figure 17 the selectivity to CO2 of the plasma treatment for pollutant degradation is reported. The black vertical line is referred to a value of SIE of 55 J⋅L-1, the energy value at which the toluene is completely decomposed, but as reported in Figure 17 there is selectivity of 40% in dry conditions and of 60% in humid conditions. The complete oxidation of the toluene is

**Figure 16.** Left: CO (empty symbols) and CO2 (full symbols) production during toluene decomposition experiments. Comparison at 400 Hz (black) and 1 kHz (red) and between dry conditions (full line) and wet conditions (dashed line). Right: CO2/CO ratio as a function of SIE at 400 Hz (black) and 1 kHz (red). Comparison between dry condition (full symbols and straight lines) and humid conditions (empty circle symbols).

**Figure 17.** Carbon balance for the different experimental conditions being tested. 400 Hz (black symbols) and 1 kHz (red symbols) under dry (full square symbols) and wet (empty symbols and dashed lines) conditions.

achieved at a value of SIE of 150 J⋅L-1, where all the VOC is decomposed to CO and CO2. The carbon fraction which is missing is mainly formic acid that could be easily removed from the effluent gas by means of water scrubbing. Despite the reduction of the toluene removal efficiency made by the presence of water vapor, it is clear how the selectivity to CO2 production is improved (Figure 16, right), but also the carbon balance is clearly improved at least until the energy value of 100 J⋅L-1. Above the value of 150 J⋅L-1 the carbon balance is exceeding 100% (marked by the red horizontal line). This is due to some additional degradation of the acrylic housing. This effect was also noted in the IR spectra where an additional band around 700 cm-1 was recorded (see Figure 14).

Because of the construction of the NTP-reactor with the electrode configuration in the middle of the discharge reactor and, therefore, a huge gas volume not in direct contact with plasma, the toluene to be removed hardly comes in contact with plasma. Thus, electron dissociation cannot be the main process. As ozone is generated in the air plasma it is a possible oxidizer of toluene. The effect of ozone on the toluene removal has been studied in a separate experiment. Therefore the reactor was operated with a pure oxygen gas flow which was mixed with the toluene polluted air in a separate reaction chamber (volume about 250 mL; not shown in Figure 11). The results of this experiment are presented in Figure 18.

**Figure 18.** Ozone and toluene concentrations as a function of SIE (frequency 1 kHz) at the surface DBD arrangement. Comparison of the direct plasma treatment (red symbols) with the ozone injection experiment (black).

Ozone concentration increases and toluene concentration decreases with increasing SIE. The ozone production is similar in both direct discharge and ozonation treatments, but the toluene removal varies significantly. In the case of indirect treatment with ozone a small reduction of toluene is obtained. In additional experiments a similar reduction was obtained even without plasma operation. Thus it must be that toluene is adsorbed somewhere in the system (reaction chamber, pipes etc). In case of direct treatment at 157 JL-1 toluene is completely removed. These results show that reactions with ozone are not dominant, which is in agreement with the fact that the rate coefficient of the reaction of toluene with ozone is small (eq. (8) [46]) compared to the reaction with atomic oxygen (eq. (2)).

$$\rm{C}\_{7}H\_{8} + O\_{3} \rightarrow \rm{Products} \qquad \rm{k}\_{298} = 1.5 \cdot 10^{-22} \text{ cm}^{6} \cdot \rm{molecules}^{-1} \cdot \rm{s}^{-1} \tag{8}$$

The reaction with atomic oxygen (eq. (2)) is considered to be the most important removal process.

achieved at a value of SIE of 150 J⋅L-1, where all the VOC is decomposed to CO and CO2. The carbon fraction which is missing is mainly formic acid that could be easily removed from the effluent gas by means of water scrubbing. Despite the reduction of the toluene removal efficiency made by the presence of water vapor, it is clear how the selectivity to CO2 production is improved (Figure 16, right), but also the carbon balance is clearly improved at least until the energy value of 100 J⋅L-1. Above the value of 150 J⋅L-1 the carbon balance is exceeding 100% (marked by the red horizontal line). This is due to some additional degradation of the acrylic housing. This effect was also noted in the IR spectra where an additional band around 700

**Figure 17.** Carbon balance for the different experimental conditions being tested. 400 Hz (black symbols) and 1 kHz

(red symbols) under dry (full square symbols) and wet (empty symbols and dashed lines) conditions.

**Figure 16.** Left: CO (empty symbols) and CO2 (full symbols) production during toluene decomposition experiments. Comparison at 400 Hz (black) and 1 kHz (red) and between dry conditions (full line) and wet conditions (dashed line). Right: CO2/CO ratio as a function of SIE at 400 Hz (black) and 1 kHz (red). Comparison between dry condition (full

symbols and straight lines) and humid conditions (empty circle symbols).

22 Current Air Quality Issues

Because of the construction of the NTP-reactor with the electrode configuration in the middle of the discharge reactor and, therefore, a huge gas volume not in direct contact with plasma,

cm-1 was recorded (see Figure 14).

Since for the production of these species energetic electrons are needed, the lower efficiency under wet conditions can be explained by their lower production efficiency due to the consumption of these electrons as well as a reduction of the electron temperature by water dissociation and vibrational excitation. More detailed investigation on the toluene by-products are needed to clarify the carbon balance and the selectivity of the process which remains to future investigations.

### **5. Conclusions and outlook**

The feasibility of NTP for the removal of VOCs has been demonstrated by means of two different gas discharge concepts, namely DBD and corona discharge. There are evident peculiarities in the two different approaches and, notably, also among different types of coronas. Different discharge regimes create NTPs with distinct properties and compositions (type and density of reactive species), which reflect on the process chemical outcome. This knowledge is important for mastering the process product selectivity, i.e. the chemical composition of the treated gas. Likewise important is the process efficiency. Unfortunately, up to now it is possible to make a comparison between the two reactor arrangements only for the specific case of toluene. There are reviews in the literature in which the efficiency of different NTP processes is compared [17].

Regarding the main chemical reaction involved in the decomposition process of toluene a similar mechanism for DBD and dc– corona can be proposed. Here, the most important reactive species are radicals. The main difference in the overall efficiency of the decomposition process is when water vapour is present in the treated gas, but this opposite effect is totally in agree‐ ment with the physical properties of the DBD. In the case of the DBD configuration the volume of gas which is directly affected by plasma is small compared to dc– corona. Accordingly, in the case of the corona a much higher probability of collisions between reactive species and substrate molecules (toluene) exists, but also a high probability to generate OH radicals. In the case of DBD the presence of water vapour is quenching the high energetic electrons and reducing the chance to generate more reactive species with an overall result of a decrease in energy efficiency. Despite this, if we calculate the Energy Yield (EY) according to the equation reported in [47]:

$$EY = \frac{C\_{\text{in}} \cdot \eta \cdot M \cdot 0.15}{SIE} \tag{9}$$

where Cin is the starting concentration of the pollutant to be treated (ppm), η is the removal efficiency and M is the molar mass of the pollutants. For the DBD configuration EY value is 13.6 g/kWh while for the corona reactor, in pulsed+ mode, the value is 6.9 g/kWh.


**Table 3.** Selected results on toluene removal with NTP in dry air.

In Table 3 are summarized selected results on toluene removal with NTP taken from [47] to better evaluate the EY of the two setups reported above. In general, the DBD setups present higher values compared to the corona ones. What is really interesting to note from this table is that not only the values of EY obtained with the setups developed by the authors are among the highest ones, but also these are the only cases in which the complete removal of toluene is reached.

**5. Conclusions and outlook**

24 Current Air Quality Issues

NTP processes is compared [17].

reported in [47]:

The feasibility of NTP for the removal of VOCs has been demonstrated by means of two different gas discharge concepts, namely DBD and corona discharge. There are evident peculiarities in the two different approaches and, notably, also among different types of coronas. Different discharge regimes create NTPs with distinct properties and compositions (type and density of reactive species), which reflect on the process chemical outcome. This knowledge is important for mastering the process product selectivity, i.e. the chemical composition of the treated gas. Likewise important is the process efficiency. Unfortunately, up to now it is possible to make a comparison between the two reactor arrangements only for the specific case of toluene. There are reviews in the literature in which the efficiency of different

Regarding the main chemical reaction involved in the decomposition process of toluene a similar mechanism for DBD and dc– corona can be proposed. Here, the most important reactive species are radicals. The main difference in the overall efficiency of the decomposition process is when water vapour is present in the treated gas, but this opposite effect is totally in agree‐ ment with the physical properties of the DBD. In the case of the DBD configuration the volume of gas which is directly affected by plasma is small compared to dc– corona. Accordingly, in the case of the corona a much higher probability of collisions between reactive species and substrate molecules (toluene) exists, but also a high probability to generate OH radicals. In the case of DBD the presence of water vapour is quenching the high energetic electrons and reducing the chance to generate more reactive species with an overall result of a decrease in energy efficiency. Despite this, if we calculate the Energy Yield (EY) according to the equation

> 0.15 *C M in EY SIE*

13.6 g/kWh while for the corona reactor, in pulsed+ mode, the value is 6.9 g/kWh.

**DBD (this work, sec. 4) 50 >99 13.6** DBD packed with glass pellets [47] 1100 80 11.5 **Pulsed Corona (this work, sec. 3) 500 >99 6.9** DBD packed with glass beads [47] 240 36 6.8 DBD [47] 400 23 5.2 DC corona [47] 5 – 200 93 0.4

**range (ppm)**

**Plasma type Concentration**

**Table 3.** Selected results on toluene removal with NTP in dry air.

where Cin is the starting concentration of the pollutant to be treated (ppm), η is the removal efficiency and M is the molar mass of the pollutants. For the DBD configuration EY value is

×h× × <sup>=</sup> (9)

**Maximum removal efficiency (%)**

**Energy Yield (g/kWh)**

In contrast to many other established technologies of air cleaning, NTPs can be controlled more or less instantaneously by their electrical operation parameters, and they can thus be adjusted to fluctuating gas flow volumes and/or contamination levels. However, nearly all practical processes of pollutant degradation in gases by means of NTPs are hybrid processes or a combination of NTPs with other technologies. In such combinations the NTP acts as an oxidation stage. However, the combination is not only a processing by means of subsequent methods but also offer multiple process synergies. Therefore NTP can be coupled with catalysts, adsorbing agents or scrubbing. For example, the oxidation of non-soluble VOCs results in soluble by-products such as formaldehyde or formic acid. This can also be used for the removal of NOx [48]. The so-called Plasma Enhanced Selective Catalytic Reduction (PE-SCR) of NOx offers many synergies between plasma and catalyst.

In this context, the combination of plasma treatment with adsorption methods has also been proposed for VOC abatement and deodorization [49]. Several manufacturers offer devices for deodorization which sometimes combine a NTP with an active carbon or molecular sieve stage. The odour reduction of so-called indirect plasma treatment was also demonstrated. Indirect treatment means that the plasma processed air is injected in the VOC containing off-gas. In such case short lived radicals and ions may be less involved in the decomposition processes but the operation lifetime of such system is much longer. During the direct plasma decompo‐ sition aerosols can be formed by nucleation of intermediate products and deposit as layers on the electrodes which interfere with the plasma generation. This is avoided by indirect treat‐ ment. Many of such DBD-based installations are worldwide used for deodorization in several factories for producing food for fattening, fish meal and flavouring substances. The installa‐ tions are low-maintenance and need about one third to one fifth of the space as conventional technologies. In [50] the investment- and running cost of numerous waste air purification processes for a gas flow of 50,000 mN³/h and for <100 mg VOC/m³ in the flavour processing industry were determined and compared. NTP installation had the lowest investment costs (about 400,000 € compared to at least 700,000 € for combustive methods, biological filter or molecular sieve filtration) and second lowest operating cost (about 8 €/h, compared to 70 €/h for combustion and biological filtration with 35 €/h). Although the applicability of NTPs is devoted to low-contaminated gas streams, these examples show the high economic relevance and potential of such technologies.

Furthermore, the combination of NTP with absorbers offers the possibility to establish cyclic processes for the removal of low-concentrated pollutants [49]. In such processes, the lowconcentrated pollutants are adsorbed and thus concentrated on solid matter in a storage phase. In the subsequent plasma phase, the adsorbed molecules are desorbed and decomposed by plasma activity. Since the retention time of the pollutants in the plasma phase and their concentration are increased, less energy is consumed in such a plasma-enhanced adsorption process. Such processes have been established to decompose different VOCs and NOx, as summarized in [51]. The decomposition of adsorbed ethanol on active carbon samples by means of ozone generated in the plasma has been investigated in [52]. The regeneration of clinoptilolite (a natural zeolite) 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 [53]. A cycled adsorption and plasma process using mineral granu‐ lates consisting of 80 % halloysite in a packed-bed DBD reactor for the removal of formalde‐ hyde CH2O was investigated in [51]. Here, the adsorbed CH2O molecules were decomposed into COX and hydrocarbons in 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 material with gas) and with oxygen fraction in the carrier gas. The above examples demonstrate the high potential of plasma-enhanced techniques, which can increase efficiency and lower operational costs. However, more research and development are necessary in order to establish a wider industrial breakthrough.
