**2. Overview non-thermal plasma-reactors and processes for depollution of gases**

The most common principles for the generation of "cold" non-thermal plasmas are dielectric barrier discharges (DBDs) and corona discharges. They are most suitable for treating exhaust gases from manufacturing processes and mobile emission sources, as they offer a compact design and good scalability.

DBD based devices consist of at least two electrodes enclosing a gas space which is filled with, or bound by, an insulating material [6]. Typically, dielectric materials such as glass, quartz, ceramics, enamel,plastics, siliconrubber orTeflonareusedas barriermaterials.There aremany possible DBD arrangements. Traditionally, DBDs were generated in parallel plate reactor geometries orin coaxial cylindricalreactor geometries, as shown in Figure 1 (left).Thedielectric barrier(s) can cover one or both electrodes entirely, but they can also be separated from both electrodes,forming two discharge gaps. The gap widths are typically in the range of 0.1 – 5 mm. When a sufficiently high voltage is applied between the electrodes, an electrical breakdown occurs and plasma is formed. Furthermore, both electrodes can be arranged in such a way that they are in direct contact with the barrier. In this case, the gas discharge is formed in the gas at theexposedelectrodeandpropagatesalongthedielectricsurface,andis thereforecalled'surface discharge' or 'surface DBD'. In 'co-planar DBDs' both electrodes are embedded in the insula‐ tor. The so-called 'sliding DBD' is based on surface DBDs but with a third electrode which is placed opposite to the top electrode on the dielectric surface [7]. This arrangement allows the discharge 'to slide' over the dielectric. The so-called 'packed bed reactor' is also often classi‐ fied as DBD-type plasma. Dielectric or ferroelectric pellets are packed in between the two electrodes. Due to polarization of the pellet material, regions with high electrical fields are generated, leading to gasdischarges in the voidspaces between the pellets andon their surfaces [8]. Porous ceramic foams can also be used instead of pellets beds [9]. Packed-bed reactors are relevant for depollution since the filling can feature catalytic properties.

various wavelength regions. Plasmas are generated artificially by supplying energy to gases,

In principle, gaseous plasma depollution can be done by an increase of the gas enthalpy in socalled "translational" plasmas, which are non-equilibrium plasmas (electrons and heavy particles have different mean kinetic energies) but with gas temperatures reaching several thousand K. Plasma torches, arcs, arc jets or gliding arcs are examples for such plasma-based incineration sources [4]. However in the following we will focus on non-thermal plasmas which stays at moderate gas temperatures, also referred to as "cold" non-thermal plasmas. Another approach, which is also excluded in this chapter, is the electron beam injection, which is under development for very large combustion facilities (coal fired power plants) [5].

The most common method of producing "cold" non-thermal plasmas for technological applications is based on the application of an electric field to a gas. If the applied field exceeds a certain threshold value (breakdown field strength) a gas discharge and thus plasma are generated. The specific feature of so-called non-thermal plasmas is that most of the coupled energy is primarily released to the free electrons which exceed in temperature that of the heavy plasma components (ions, neutrals) by orders of magnitude. Thus, strong non-equilibrium conditions are achieved in which the gas temperature remains nearly at or slightly above room temperature ("cold") while "hot" electrons initiate chemical processes resulting eventually in

This chapter is meant to provide some insight into the application of NTP for air remediation. After a brief introduction to summarize the principles of NTP generation, the main aspects of discharge physics and plasma chemistry involved in air treatment are described and discussed. Special focus is on the removal of volatile organic compounds (VOCs). The two major types of plasma sources for such applications, namely dielectric barrier discharges and corona discharges, are described in the next section. Various aspects of plasma generation and the ensuing chemical processes will be discussed in two separate sections based largely on work carried out by the Authors in Padova and in Greifswald. Finally, some conclusions and

**2. Overview non-thermal plasma-reactors and processes for depollution of**

The most common principles for the generation of "cold" non-thermal plasmas are dielectric barrier discharges (DBDs) and corona discharges. They are most suitable for treating exhaust gases from manufacturing processes and mobile emission sources, as they offer a compact

DBD based devices consist of at least two electrodes enclosing a gas space which is filled with, or bound by, an insulating material [6]. Typically, dielectric materials such as glass, quartz, ceramics, enamel,plastics, siliconrubber orTeflonareusedas barriermaterials.There aremany possible DBD arrangements. Traditionally, DBDs were generated in parallel plate reactor

liquids or solids.

4 Current Air Quality Issues

the oxidation of pollutants.

design and good scalability.

**gases**

perspective outlook on the field are given.

The insulator suppresses large currents on the electrodes and thus keeps the plasma in the non-thermal regime [6]. Because of the capacitive coupling of the insulating material to the gas gap, DBD generation requires alternating or pulsed operating voltages. For the treatment of gas streams the gas is injected into the device flowing along the electrode arrangement. In industry DBDs are used for the generation of ozone, deodorization, surface treatment and many more.

Corona discharges are characterized by non-uniform electrical field geometries, e.g. needleto-plate or wire-in-cylinder electrode arrangements, as shown in Figure 1 (right). DC and low frequency AC operated corona discharges expand from the needle or wire electrode in the outer regions towards the plate or cylinder electrode. 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 the discharge is not supported anymore and is thus kept in the non-thermal regime [10]. DC-operated corona discharges are used in electrostatic precipitators. For environmental applications (e.g. VOC removal, water purification) corona discharges operated by pulsed high voltage are proposed since higher densities of reactive species can be achieved [11]. Pulsed corona discharges are characterized by plasma regions which fill a much larger fraction of the discharge gap than DC or low frequency corona discharges.

In molecular gases at atmospheric pressure, corona discharges and DBDs are typical examples of non-uniform, filamentary plasmas, consisting of many individual microdischarges or discharge channels. Each volume element of the flowing gas is repeatedly subjected to the action of these filaments as it passes through the reactor. Non-thermal plasma based reme‐ diation of air is due to chemical reactions with photons and active species created in the plasma, namely radicals or ions. The different physical and chemical processes associated with and induced by non-thermalizing discharges span a time range of about 12 orders of magnitude. The equilibrium of the electrons with the local electrical field is usually approached within

**Figure 1.** Top pictures: Cross sectional view of coaxial DBD (left) and DC corona discharge (right) arrangement; Bot‐ tom pictures: Coaxial DBD (left) and corona discharge (right) reactor for treatment of air streams.

picoseconds [6, 12, 13]. Ionization and electrical breakdown typically proceed at the nanosec‐ ond time scale via electron collisions. For example, DBD microdischarges in atmospheric air have duration of about 20 - 50 ns. The development of discharge channels and microdischarges is dominated by the build-up and spatio-temporal enhancement of volume space charges, resulting in propagating perturbation of the electric field, which has been investigated as a cathode directed streamer or ionization wave [12, 13]. In the filaments the highest electron density and electron temperature are achieved and electron-induced dissociation and thus formation of radicals occur. The pollutant degradation is initiated by secondary reactions with these free radicals and ions, mainly on a micro- to millisecond time scale, i.e. after the active discharge filament has faded [14]. Ion-molecule reactions occur on an intermediate time scale, typically in the range of 10 ns up to 1 µs. The pollutant molecules react with oxidizing atoms and radicals (e.g. O, OH) or with plasma-generated ozone (O3). Water vapor may play an important role, as it acts as the precursor for hydroxyl radicals (OH) and hydroperoxyl radicals (HO2). When hydrocarbons or other VOCs are present in the gas, other radicals are also produced and radical chain reactions occur. Beside electrons, photons or collisions with metastable or excited species can lead to ionization.

The chemical equilibrium including heat and mass transfer is commonly settled within milliseconds to seconds.

The application of NTP for pollutant degradation in gases in industry must comply with the demands on removal efficiency, energy efficiency and selectivity.

The removal efficiency is defined as the removed molar fraction of the pollutant related to the initial molar fraction Cin: η=(Cin-Cout)/Cin (Cout is the molar fraction of the pollutant after the plasma treatment) [2, 14]. Energy efficiency relates to the energy needed to achieve a given removal efficiency and can be expressed in several ways. For example the energy yield is the decomposed pollutant mass per dissipated energy. A high energy yield is not necessarily assuring sufficient removal efficiency as this is determined by the initial molar fractions of the pollutant [2]. The removal efficiency and the energy efficiency depend on the specific energy density of the plasma and are also determined by a number of conditions (gas composition, humidity and temperature; level of initial contamination) [2, 14].

Selectivity is defined as the fraction of the desired product of the plasma-chemical conversion to the total amount of products of the conversion process. The chemistry and fraction of desired products and undesired by-products can also be characterized by mass balances (e.g. carbon balance in the case of conversion of hydrocarbons). A high selectivity is required to achieve a reasonable performance in terms of energy efficiency and by-products [2]. High reactivity of radicals usually results in a poor selectivity, since competing reactions which result in the formation of undesired by-products happen simultaneously. One important reaction which consumes oxygen atoms alongside the reactions with pollutant molecules is the generation of O3. For some pollutants (e.g. NOx, alkenes and other unsaturated VOCs) O3 is an efficient oxidizer, but in other cases it constitutes an additional pollutant by-product.

In the following paragraphs the main aspects of pollutant degradation by means of DBDs and corona discharges are discussed using selected examples. The discussion is focussed on VOCs as a class of contaminantspresentinmanydifferentindustries (e.g. semiconductormanufactur‐ ing, chemical processing, painting and coating) as well as in indoor air (outgassing of paint, carpets etc.). VOCs contribute to the generation of photochemical smog and to certain health diseases like nausea and skin irritation. Some are associated with high cancer risk [15, 16]. Conventional methods for VOCs removal are thermal oxidation, condensation, absorption and biofiltration.The thermal oxidationandcondensationare economic only for situations in which VOCs are present in moderate to high concentrations. The absorption process does not destroy VOCs but only transfers them to another medium. In addition, this technology suffers from problems arising by deposits of dirt or clog on filters. Biofilters are useful only for VOCs that have some solubility in water and they are cost-effective if the volume of air to be treated is in the range of 104 – 105 m3 /h. In NTP energy of about 10-30 eV are needed to produce an O-atom or an OH-radical in (humid) air, which makes the decomposition also energy consuming [14]. However, the total energy consumption can be low in case of small concentrations of pollu‐ tants. Thus NTP-based processes are feasible forlow contamination levels. For VOCs, this level is about 100 mg/mN 3 (N refers to standard conditions for pressure and temperature) [2, 14].

picoseconds [6, 12, 13]. Ionization and electrical breakdown typically proceed at the nanosec‐ ond time scale via electron collisions. For example, DBD microdischarges in atmospheric air have duration of about 20 - 50 ns. The development of discharge channels and microdischarges is dominated by the build-up and spatio-temporal enhancement of volume space charges, resulting in propagating perturbation of the electric field, which has been investigated as a cathode directed streamer or ionization wave [12, 13]. In the filaments the highest electron density and electron temperature are achieved and electron-induced dissociation and thus formation of radicals occur. The pollutant degradation is initiated by secondary reactions with these free radicals and ions, mainly on a micro- to millisecond time scale, i.e. after the active discharge filament has faded [14]. Ion-molecule reactions occur on an intermediate time scale, typically in the range of 10 ns up to 1 µs. The pollutant molecules react with oxidizing atoms and radicals (e.g. O, OH) or with plasma-generated ozone (O3). Water vapor may play an important role, as it acts as the precursor for hydroxyl radicals (OH) and hydroperoxyl radicals (HO2). When hydrocarbons or other VOCs are present in the gas, other radicals are also produced and radical chain reactions occur. Beside electrons, photons or collisions with

**Figure 1.** Top pictures: Cross sectional view of coaxial DBD (left) and DC corona discharge (right) arrangement; Bot‐

tom pictures: Coaxial DBD (left) and corona discharge (right) reactor for treatment of air streams.

The chemical equilibrium including heat and mass transfer is commonly settled within

The application of NTP for pollutant degradation in gases in industry must comply with the

The removal efficiency is defined as the removed molar fraction of the pollutant related to the initial molar fraction Cin: η=(Cin-Cout)/Cin (Cout is the molar fraction of the pollutant after the plasma treatment) [2, 14]. Energy efficiency relates to the energy needed to achieve a given removal efficiency and can be expressed in several ways. For example the energy yield is the decomposed pollutant mass per dissipated energy. A high energy yield is not necessarily

metastable or excited species can lead to ionization.

demands on removal efficiency, energy efficiency and selectivity.

milliseconds to seconds.

6 Current Air Quality Issues

Among the different VOCs which are being routinely monitored for air quality, toluene (methylbenzene, C7H8) is one of the most important ones. Toluene is widely used as feedstock in the chemical industry for the synthesis, among others, of drugs, dyes, explosives, and as a solvent (e.g. thinner, paints, adhesives). Exposure to toluene is known to affect the central nervous system and may cause tiredness, confusion, weakness, memory loss, and nausea. Toluene is water-insoluble and thus cannot be scrubbed. For its wide use, diffusion and well known properties and reactivity, toluene has become sort of a standard for testing and comparing non-thermal plasma based air treatment for VOCs removal. Thus, the discussion in the following sections will focus largely on experiments with toluene. The conversion of toluene in hybrid systems in which NTP is combined with a catalyst is also being extensively studied and has been reviewed [17]. Such hybrid processes will not be covered in this chapter.
