Principle Of Ozone Generation Engineering Essay

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Ozone or trioxygen (O3) is a triatomic molecules consisting of three oxygen atoms [1, 2]. It is an allotrope or different type of oxygen (O2) that is much less stable than the diatomic O2. Ozone quickly decays into diatomic oxygen, so that ozone must be generated on site. At ground level, ozone's lifetime in the air is about 20 minutes, depending on temperature. In many references, after ozone is generated, then it mixed with the effluent in a contact chamber.

The global equation representing the formation ozone is as follows:

3O2  2O3 H = 68 kcal …………(2.1)

The energy of 34 kcal necessary for the formation of one mole ozone cannot be brought thermically because the enthalpy of chemical reaction above has shown endothermic reaction. It means that on the creation of this type of the oxygen activation, overheating the gas during the procedures which favours the formation of ozone should be as weak as possible.

The cold plasma such as formed by the corona discharge in this ozone formation will fulfil the best for the process [3]. The energy in the reaction (2.1) must be furnished by other means such as by atoms of oxygen, the excited states of the molecule, or by the ions. The electron bombardment to oxygen molecules is needed for ozone formation[2].

The mechanism of the most referred to for the formation of ozone in electrical discharge involve three body reactions among O, O2 and a third collision partner M. This third collision partner can be O2, O3 or N2 in the air. [1, 2, 4, 5]: The common main chemical reactions with oxygen as input gas are written as:

e + O2 ï‚® O + O + e (2.2)

O + O2 + M ï‚® O3 + M (2.3)

Bombardment by electrons breaks oxygen molecules apart, and they recombine with each other or with the other oxygen molecules with ozone production.

In advance of the fundamental chemical reaction written above, the formation of ozone implicates the intermediate presence of excited oxygen in the ground state or it is known as a triplet 3∑u. Under electron bombardment, this triplet forms a free radical of oxygen at state 3P or 1D. The different stage of oxygen are illustrated at figure 1.

Figure 1. Potential curve of oxygen[1, 2]

This oxygen excitation process recorded in reference [2, 3, 6-8] give contribution to explain that ozone production by a pure oxygen contains two different mechanism two produce ozone. The first mechanism occurs at lower energy and it is written as:

e + O2  e +O2 (A3∑u+) (2.4)

e + O2 (A3∑u+)  e + O(3P) + O(3P) (2.5)

O2+ O(3P)  O3 (2.6)

The second mechanism occurs at higher energy, and it is written as:

e + O2  e +O2 (B3∑u-) (2.7)

e + O2 (B3∑u-)  e + O(3P) + O(1D) (2.8)

O2+ O(3P)  O3 (2.9)

O2+ O(1D)  O3 (2.10)

If the electron bombardment is too strong, the oxygen molecule will reach undesired level where there is deformation of ozone into oxygen. The excessive impact of electron lead to the following reaction[2, 7]:

e + O3  O2 + O(3P) (2.11)

e + O3  O2 + O(1D) (2.12)

O(3P) + O2  2O2 (2.13)

O(1D) + O2  2O2 (2.14)

Although the electron bombardment through electrical discharge occurs randomly during ozone generation, but it is important to give the optimum energy to initiate the discharge. This energy is expected to maintain the electrons keep breaking the oxygen molecule to make ozone formation and reduce ozone dissociation[4, 9].

The use of electron bombardment in cold plasma process to create ozone has some advantage behaviors such a low energy requirement and its capacity to induce physical and chemical reactions within gases at relatively low temperatures. In other literatures [3, 6, 10], the cold plasma is also known as non equilibrium plasma. The electrons in cold plasma can reach temperatures of 10,000-100,000 K (1-10 eV) while the gas temperature can remain as low as room temperature. Based upon mechanisms of which plasma is generated, the cold plasma have several types including glow discharge, corona discharge, dielectric barrier discharge, microwave discharge and radio frequency (RF) discharge [6, 11]. The glow discharge is operated at low pressure discharge; due to this characteristic the glow discharge is not very suitable for chemical synthesis [12]. The RF discharge operates at high frequencies (several megahertz) and very low pressure to achieve the non-equilibrium conditions. This discharge is also not suitable for chemical synthesis[12]. The microwave discharge operates at very high frequencies (e.g., 2.45 GHz) where only light electrons can follow the oscillations of the electric field. The performance of microwave discharge mostly depends on the type of microwave power applicators. Due to the complexity of its characteristics, the microwave discharge is not suitable for chemical synthesis in atmospheric pressure[13]. Only corona and dielectric barrier discharge can be initiated at atmospheric pressure.

2.2. Ozone generation with Corona Discharge

The corona discharge can be initiated at atmospheric pressure using inhomogeneous electrode geometries, like a pointed wire electrode with a plate one as it is illustrated in figure 2.2.a. [6, 14-16]. When strong electric field is applied then a localized luminous crown discharge or corona is observed at near a sharp point as it is illustrated in figure 2.2 b

(a). (b) (c)

Figure 2.a. Front view of electrode arrangement for corona discharge chamber

Figure 2.b The corona discharge for single point at atmospheric pressure.

Figure 2.c. The corona discharge for fine wire at atmospheric pressure.

In general, whether in the single point or fine wire geometrical shape, the corona electrode is surrounded by an active region in critical radius [17]. At this radius, the electric field is equal to breakdown electric field of the working gas surrounding the electrodes. Ionization, excitation and production of active and radical species of gas can take place in this region which may more visible at higher current. A corona characteristically involves voltage from 10 kV to several tens of kV with current from 1mA/mm.

Figure 3. Type of corona discharge

When the applied voltage to electrode is increased, the corona discharge exist in several forms, depends on the polarity of the given field and geometrical shape of the electrode. For needle plate electrode configuration, when the positive field is given then the discharge start with burst pulse corona, streamer corona, glow corona and spark as the applied voltage increases. For negative field in the same geometrical electrode , the corona initially will start from trichel pulse corona, pulseless corona and spark as the applied voltage is increased. In fine wire - plate or fine wire pipe electrode configuration, when positive polarity injected into wire, the corona may visible as a tight sheath around electrode or as streamer moving away from the electrode. When the negative polarity applied, the corona may in the form of glow moving or it concentrated into small active spots called beads or tufts [18]. In other reference if the electrode is in negative polarity, or a more homogeneous glow in the case of a positive point [6, 19].

If spark occurs, then electric current flows from higher potential to lower potential or ground and this phenomena is similar with short circuit current and the terminal voltage at discharge chamber decrease close to zero [20, 21]. This large current arise the heat along the spark streamer which deforms the ozone into oxygen and the voltage become lesser than the voltage level needed to initiate discharge, so that during spark the ozone production will be decrease rapidly.

To generate positive or negative polarity, a high voltage direct current (HVDC) generator is required in corona discharge. Among of the common HVDC are simple high voltage rectifier and voltage doubler [20]. For a simple high voltage rectifier the electrical circuit with high voltage transformer, diode and capacitor are widely used as conventional HVDC generation as they are illustrated in figure.4

Figure 4 Simple high voltage rectifier

For voltage doubler circuit, there several topologies are generally employed such as Villard circuit and Greinacher circuit. The Villard circuit is composed of a simple high voltage capacitor and diode, but this circuit has poor ripple output. The Greinacher circuit is development of villard circuit which may consist of several stage of high voltage diode and capacitor. In this circuit, ripple is much reduced. Figure 5 shows the topologies of Villard and Greinacher circuit connected to the corona discharge chamber.

(a) (b)

Figure 5.a. Villard circuit

Figure 5.b. Greinacher circuit

It is important to note that both simple high voltage rectifier and voltage doubler require high voltage transformer to generate adequate HVDC to supply corona discharge chamber.

Figure 2 and 3 show that the volume exposed to the action of the corona is very much smaller than the total discharge volume. The applied voltage should not too high, since otherwise the corona might bridge the gap and produce a spark and breakdown. In this sense the corona discharge can be considered a partial breakdown. The electrodes in this discharge have direct contact to the radical ions or molecules from the result of discharge, this radical species can easily oxidise the metal electrode during discharges or sparks. This oxidation process will reduce the life time of electrodes. From illustration above, it may be concluded that some drawbacks of corona discharge are the small active volume around the electrode to produce ozone, the big air gap between the electrodes to avoid spark demands higher voltage to initiate the corona, and the partial discharges or sparks occurred can reduce the life time of electrodes. On power supply side, most of the HVDC generation utilize high voltage transformer and diode as rectifier which are considerably heavy and in large dimension.

2.3. Ozone generation with Pulse Discharge

The power of the continuous corona discharges discussed in the previous section is very low. Increasing this power will increase the voltage and current to build up of heat, give raising thermal emission and lead transition of corona into sparks. By making corona discharge occurs in pulse periodic scheme, the magnitude of power and voltage can be increased as well as the ozone production. Although the corona power in the sharp point or fine wire is small, when fine wires are very long then the total corona power becomes significant. This situation takes place on overhead high voltage transmission line, where this wire can generate corona discharge. This is the basic idea to generate ozone with pulsed corona discharge [12, 22]. In practical, the very short high voltage pulses are used and combined with a dielectric layer placed adjacent to the outer electrode to results a short lifetime of the corona discharges[5, 23-27]. The present of dielectric layer inside the chamber has made the corona discharges become streamers, so this kind of ozone generation is also known as pulsed streamer discharge (PSD).

To create pulsed streamer discharge in atmospheric pressure of oxygen, a sufficiently high voltage is applied to the central electrode of a coaxial concentric cylindrical electrode geometry with a dielectric sheet placed on the inner surface of the outer electrode. The dielectric sheet forms a barrier which prevent the development of the arc discharge so that a complete breakdown of the gap is also prevented. By this construction a short pulsed streamer discharge has ability to produce an intense corona effectively but not long enough to initiate an arc breakdown. The high-energy electrons in the streamers dissociate the oxygen molecules into atoms, which are prerequisite for the subsequent production of ozone by collisions with oxygen molecules and the third body. This is the basic prinsp of pulse streamer discharge (PSD) for ozone generation. The illustration od PSD chambers are shown in figure 6.

(a) (b)

Figure 6.a. Coaxial wire-cylinder configuration reactor for PSD

Figure 6.b. Spiral wire-cylinder. configuration reactor for PSD

The wide air gap between high voltage electrode and the ground layer electrode had given prerequisite that the input voltage should be high enough to initiate the streamer. In recent study of the use of high voltage pulse for generating pulsed streamer required the present of inception voltage from 17.5 to 57.9 kV with about 20 mm air gap distance between the electrodes[27]. A high power has to pass to through the wire to initiate corona discharges, so this matter becomes limitation of typical wire configuration chamber due to durability of the electrode. From figure 6a and 6b, the chambers for pulsed streamer discharge are not simple chamber to fabricate for operating at atmospheric pressure.

To generate pulsed corona discharge, it is important to provide pulse power supply which has capability to produce short voltage pulse with steep front and very short rise time. This kind of voltage pulse will sustain the development of electrons avalanches in corona discharge to the streamer. The total time to develop electrons avalanche to streamer transition and streamer propagation between electrode is about 100 - 300 nano seconds [28, 29], so the power supply must be able to provide a high voltage rise rate from 0.5 - 3 kV/ns to initiate corona and a sufficient short pulse width to avoid the corona - spark transition.

As a pulsed power supply principle is its energy storage system which can be released in the form of high power pulse to the load by means of switch, then with development of power electronics switch components, then thyristor, MOSFET and IGBT become a choice to generate pulse power to create corona discharge. The conventional pulse power supply design is to use gas switch such as Marx generator with a set of high voltage transformer, rotating spark gap, and thyratron.

The topology of pulsed power supply which is based on the use of power electronics switch component are pulse transformer topology, solid state Marx adder topology, magnetic pulse compressor (MPC) and Fitch impulse generator.

In Figure 7.a, Several the step up pulse transformer with lower voltage switch are used to avoid the need of high voltage switches and this circuit known as pulse transformer topology[30]. The drawback of this design are limited maximum pulse duration due to transformer volt second characteristics, limited pulse rising due to transformer inductance and the need to synchronize the gate of drive to avoid overloaded at one switch.

The topology of pulse generator omitting transformer between tank capacitor and the load is shown in Figure 7.b. This circuit is known as solid state Marx adder topology because it is similar with conventional Marx generator, but in the classic Marx generator several spark gaps are installed as switching devices[30-32]. The identical current flows through every switch then the voltage at every stage are added up. The weakness of this design are no galvanic isolation of the output and the complex gate drive design because no ground reference.

The Fitch pulse generator [30] is made from several resonant LC sets that are charge in parallel and discharge in series as it is shown in Figure 7. c. Half of capacitor are connected to power electronics switch and a series inductances. After one half cycle of resonant charge transfer the switches are closed, and at the time all the capacitor voltage are adding so total voltage at capacitors in column are now 2nV, where n is the number of stage and V is the charging voltage. The design of this topology has several weakness as Marx adder topology such as a complex gate drive design because no ground reference, the system must be careful design to get match resonant of every stage, each stage must be identical, and no galvanic isolation of the output.

Magnetic Pulse Compression system is designed to generate a relatively long pulse and to compress the pulse in the time domain by resonant charge transfer through saturable inductors. Figure 7.d. shows an example of MPC using two stages of magnetic compression, including a resonant transformer to step up the pulse voltage.

The disadvantage of this design are a careful design needed to make saturable inductor, the system only dedicated to certain load because when the load is change then the system will be not match, the design must accommodated the huge reflected pulse energy that is possible to happened in the system, the pulse width are not adjustable, and the shape of pulse is not rectangular [30, 33]. The high voltage and energy must be provided and stored in the capacitor C, and this demand adds complexity to design power converter for MPC power supply.

(b) (c)

(d)

Figure 7. Pulsed Streamer Discharge Power Supply

From the description above, the pulse discharge method need a high energy and voltage to initiate discharge to produce ozone so that the chamber construction is not simple. The design of power supply must fulfill difficult criterion and this contributes to the complexity of pulse power supply topology.

2.4. Ozone Generation with Dielectric Barrier Discharge

There is another method aside from employing pulsed voltage to avoid streamer channel during discharge which is based on the use of dielectric barrier in the discharge gap. The main elements of this chamber configuration consist of two electrodes, a gap in between and a dielectric layer covers at least one or sometimes both of electrodes. For this reason this discharge method is known as the dielectric-barrier discharge (DBD), or simply, barrier discharge (BD) [6, 34]. The dielectric is the key for the proper functioning of the discharge[3]. The DBD discharge can be performed in typical planar configurations or cylindrical as shown in figure 8.

Figure 8 Basic dielectric-barrier discharge configurations.

As a consequence of the presence of dielectric barrier, the dielectric, being an insulator, cannot pass a dc current and these discharges require alternating voltages for their operation [8, 34].

A consists of numerous non steady state local microdischarges distributed in the discharge volume. The physics of these microdischarges based on the initial avalanche-to-streamer transition that is followed by the streamer formation [17, 22]. An individual discharge is initiated when a high voltage is applied between the electrode such that the electric field in the gap equal or exceed the breakdown strength of the ambient gas. The electron emission from surface of dielectric placed on instantaneous cathode is stimulated by ion induced electron emission. In the electric field, there electron are accelerated to energies that equal or exceed the ionization energy of the gas and create an avalanche. In this avalanche the number of electron doubles due to ionizing collisions as illustrated in figure 9.a. The high mobility of electrons compare to the positive electrons or ions allows the electron swarm as in figure 9.b to move across the gap in nanoseconds and leave behind the slower ions and various excited and active species such as excited oxygen and ozone. When the electron swarm reaches the opposite electrode as illustrated in figure 9.c, then the electrons spread out over the insulating surface and counteract the positive charge on the instantaneous anode. Deposition electrons on instantaneous the anode results in charge accumulation and prevents new avalanche and streamer nearby until the cathode and anode are reversed. After the voltage polarity reverse, the deposite negative charge give facility to initiate the formation of new avalanche and streamer.

Figure 9 The electron avalanche development in DBD

In alternating current source, at the maximum and minimum of the applied voltage [8, 35] the displacement current is zero (dU/dt = 0) and the micro discharge activity stops, only to start again when the breakdown field is reached in the gap during the next half-wave as illustrated in Figure 10.

Figure 10 Symbolic presentation of micro discharge activity

In most applications the dielectric limits the average current density in the gas space. The value of dielectric constant and thickness, in combination with the time derivative of the applied voltage, dV/dt, determine the amount of displacement current that can be passed through the dielectrics. Thus the electrical conductivity are restricted to the micro discharges [3].

When the electric field in the discharge gap is high enough to cause breakdown, then a large number of micro discharge, in most gases, are occurred [6, 8, 22]. This micro discharge for ozone generation is preferred to happened when the pressure is around 1x105 Pa or at atmospheric pressure. The discharge gap itself has a typical width ranging from less than 0.1 mm to several centimeters [10, 36, 37], depending on the application. To initiate a discharge in such a discharge gap filled with a gas at about atmospheric pressure, voltages in the range of a few hundred V to several kV are required.

Preferred materials for the dielectric barrier are glass, silica glass, ceramic materials, thinned enamel or polymer layers [29, 35]. This dielectric material serves three functions namely limiting the amount of charge transported by a single micro discharge as mention above, distributing the micro discharges over the entire electrode area, and preventing the spark inside chamber.

The determination of the power dissipated in DBD has often proved to be difficult because in reality the power is consumed in a large number of short-lived micro discharges. The first form of DBD power formula shows that only the peak value of the applied voltage calculated and not its form [8, 15].

The power delivered [38] in ozone chamber can be obtained by approximated formula as follow:

(2.15)

The energy per cycle in the dielectric barrier discharge can be found by

(2.16)

Since the current flow through the ozone chamber can be expressed by

(2.17)

(2.18)

The energy loss per cycle inside the chamber can be found by

(2.19)

(2.20)

where Vchamber is the voltage across the chamber and q is the charge released in the chamber. After having the energy lost per cycle, the power consumed by the silent discharge can be obtained by multiplied the energy lost per cycle with the frequency (f) of applied voltage. The equation (2.20) become

(2.21)

From equation (2.7), for a given configuration and fixed peak voltage, the power is directly proportional to the frequency.

From the formulation of power dissipation in DBD, it can be inferred that the use of higher frequencies brought the advantage of lower operating voltages for a given input power. This resulted in reduced strain on the dielectrics. In addition the power densities were increased substantially. As a result, higher ozone production rates and much higher ozone concentrations are reliably attained in much more compact ozone generators.

It is descript above that the dielectric barrier discharge requires alternating voltages for their operation. The conventional alternating power supply is low-frequency high-turn-ratio transformers [7, 23, 39]. This power supply must give a very high voltage output to ozone chamber, since the system with this power supply must operate close to the sparking potential (about 20 kV for 1 mm gap) in order to reach the required discharge power density. Low frequency system also presents high volume, low efficiency and difficulty to control the ozone production.

Regarding the equation (2.21), the use of supply frequencies above 50-60 Hz allow to increase the power density applied to the electrode surface inside chamber and increase ozone production for a given surface area, while decreasing the necessary peak voltage. The use of switching converters based on fast power electronic devices such as metal oxide semiconductor field effect transistors (MOSFETs) or insulated gate bipolar transistor (IGBT) will give the possibility to increase the frequencies up to several kHz [40-43], thus it allow to increase the efficiency of the ozone generation system.

Among the different converters or inverter used in power electronics The possible resonant power converter topologies used to supply DBD ozone chamber are resonant converters. These resonant converters are divided in two groups namely current fed resonant inverter and voltage fed resonant converter [40, 44, 45]. The resonant converters are widely employed in applications such as dc-to-dc conversion, lamp ballast, electronic welders, electrostatic applications, induction heating, and discharge lamp [40, 46-50]. These converters employ a resonant circuit to obtain sinusoidal voltage and current waveforms in the power switches, therefore providing soft-switching and reducing losses and electromagnetic interference (EMI) or radio frequency interference (RFI).

Although the characteristics of discharge lamp in resonant converter application is resistive and ozone chamber is capacitive, but these converters have been proved to generate high voltage for the electric discharge. The three typical current fed resonant inverters are current-fed full-bridge inverter, current-fed push-pull inverter, and class E inverter as they are shown in figure 11.

(a) (b) (c)

Figure 11 Three typical current fed resonant inverters:

(a) current-fed full-bridge inverter, (b) current-fed push-pull inverter, and (c) class E inverter

Typical voltage-fed resonant inverters are push-pull, half bridge, full bridge inverter as hey are shown in figure 12.

(a) (b)

(c) (d)

Figure 12 Typical voltage-fed resonant inverters: (a) push-pull; (b), (c) half bridge; and (d) full bridge.

To analyze the voltage amplification of each type of resonant converter, the equivalent circuit is developed with modeling the topology of resistance load to the position of resonating component namely inductor, capacitor and source [40, 51].

For the current-fed push-pull inverter as the example of popular topologies of the current fed resonant converter, it is modeled in a parallel resonant tank supplied by current square wave as shown in figure 13.

Figure 13 Equivalent circuit of a parallel resonant inverter

In some references [40, 51-53], the ratio of output voltage and input voltage or voltage gain in a parallel resonant converter model is formulated as:

(2.22)

The Q is quality factor and n is the ratio between actual frequency () and resonant frequency o or .

(2.23)

Z is equivalent parallel impedance

The magnitude gain plot is shown in figure 14 as the function of frequency. The maximum output voltage that is greater than input voltage , is possible to obtain at resonant frequency with small quality factor.

Q = 0.2

Q = 0.5

Q = 2

Q = 1

Q = 3

Figure 14 magnitude gain plot

For voltage-fed resonant inverters, there are three typical resonant tanks equivalent circuit model [40, 51, 53]. Those are series-loaded resonant tank as shown in figure 15, the parallel-loaded resonant tank as shown in figure 16 and the series-parallel-loaded resonant tank in figure 17.

Figure 15 Equivalent circuit of series loaded resonant tank

Figure 16 Equivalent circuit of parallel loaded resonant tank

Figure 17 Equivalent circuit of series-parallel loaded resonant tank

The ratio of output voltage and input voltage or voltage gain in a series loaded resonant converter model is formulated as [40, 51]:

(2.24)

Q = 3

Q = 2

Q = 1

Q = 0.2

Q = 0.5

Figure 18 The magnitude gain plot of series loaded resonant inverter

From formula (2.24) and figure 18, the maximum output voltage of the series inverter such as half bridge series inverter occurs at resonant, but the output voltage will be never greater than input voltage.

In parallel loaded resonant tank, the voltage gain in a parallel resonant converter model is written as [40, 51]:

(2.25)

The magnitude gain plot of parallel loaded resonant frequency is shown in figure 19

Q = 8

Q = 5

Q = 3

Q = 2

Q = 1

Q = 0.5

Q = 0.2

Figure 19 The magnitude gain plot of parallel loaded resonant inverter

From figure 19, the parallel circuit can generate a very high output voltage, when the frequency is operated close to resonant frequency. As shown in formula (2.25), when the circuit is operated close the resonant frequency, the output voltage is equal to Q, and the output current will be formulated as input voltage divided by impedance. This means at resonant frequency, the parallel loaded resonant circuit behaves as current source whose the value depends only on the input voltage.

In the series-parallel loaded resonant circuit, the voltage gain in a series-parallel resonant converter model is formulated as [40, 51]:

(2.26)

where

Cequivalent is the series equivalent of two capacitors present in the series parallel loaded resonant inverter.

Q = 0.2

Q = 0.5

Q = 2

Q = 3

Q = 1

Q = 5

Figure 20 The magnitude gain plot of series parallel loaded resonant inverter

When  = 0.5 or Cseries = Cparallel, around the resonant frequency the circuit is able to produce maximum voltage with a maximum gain voltage about . The circuit also behaves as a current source equal with the input voltage divided by impedance. From the description above, when it is expected that the resonant inverter produce higher voltage than the input voltage, then parallel current fed resonant , parallel loaded voltage fed resonant, and seri parallel loaded voltage fed resonant circuit are possible to choose to supply DBD chamber.

During operational of the DBD chamber, when it is injected by alternating voltage power supply whether it is lower or high frequency [7, 39, 46, 54, 55], there is no local overheating, local shock generation, noise, and spark as long as it is operated above breakdown strength of gas but below breakdown strength of dielectric. Due to the reasons above DBD method is also called the silent discharge method. However, in practical the injected voltage used for DBD chamber at atmospheric pressure and ambient temperature is considerable high. For recent ozone generation with 50 or 60 Hz high voltage power supply, the voltage injected was about 3 - 10 kVrms or 8.5 - 28 kV peak to peak[7, 56, 57]. In reference[58], the the full bridge resonant converter operated at resonant frequency of 88 kHz was reported to generate ozone from from 4 - 6 kV peak to peak. The converter requires four MOSFET switches and high DC input voltage (340 Volt). To reduce voltage injected to the chamber, Alonso et. al.[46, 59] proposed a single hard switching converter driving an ozone chamber filled with special discharge gas such as Xe , Ar, or Ne operated at certain pressure. The dielectric layer was constructed in borosilicate glass or quarzt with a conductive thin layer on the surface. Although the voltage reduce to 1 kVrms or 2.8 kV peak to peak and the ozone yield was high, such chamber is considered complicated to construct. Due to hard switching mechanism, high losses occurred inside the converter and the voltage was not performed sinusoidal to produce longer discharge. The use of another single switch power supply based on class E basic shunt amplifier to produce ozone at frequency 27 kHz from 4.8 - 6.5 kV peak to peak was reported in reference [60]. However it appears that the paper lack of information on the type of ozone chamber and how the converter was applied to an ozone chamber. The author included additional passive element such as parasitic inductor and capacitor to generate voltage but in this reference there is no their design value and the performance of converter. In reference [54] the cylindrical ozone chamber made from stainless steel as electrode and aluminium ceramic as dielectric was proposed to be injected by current fed resonant converter. As the result, the ozone chamber need to be operated at higher voltage (more than 4.3 kV peak to peak) at range of frequency 5 - 7 kHz. It is important to note in this case and other similar system the application of higher voltage result in filamentary effect raising the heat inside chamber. For such case a cooling mechanism embedded to the ozone chamber is absolutely necessary [54, 61, 62], thus it complicates the overall ozone system generator.

From the description above, the DBD method is the most preferable choice to generate ozone at atmospheric pressure and ambient temperature because the construction of DBD chamber is considered simpler than other method and require reasonable power energy level without complicated power supply design. But, it is important to explore a further research to reduce the level of magnitude voltage required to initiate discharge in order to reduce voltage stress on insulation material, filamentary effect heating internal chamber and make overall system simpler.

2.5. Summary

From the description above, the basic reaction for ozone generation has been review. It is concluded that the ozone generation is best generated through electron bombardment in cold plasma process to create because it need low energy and conduct at relatively low temperatures.

The type of electron bombardment in cold plasma process has been intensively reviewed, but only corona discharge and dielectric barrier discharge which are possible to be conducted to produce ozone at atmospheric pressure and ambient temperature.

From previous literature, corona discharge method supplied by continuous dc has several weakness such as it was occurred at the small active volume around the electrode to produce ozone, demanded very high voltage to initiate the corona, the partial discharges or sparks occurred reduced the life time of electrodes, it utilized heavy and large high voltage transformer and diode as to generate dc.

From the previous references, the weakness in corona discharge supplied by continuous dc could be improved through pulsed periodic voltage generated by pulse streamer generator. However, pulse discharge method needs a high energy and voltage to initiate discharge. The chamber construction and pulse streamer generator is must fulfill difficult criterion and the construction of this system is considered complex.

The DBD method or silent discharge method has been thoroughly reviewed. This method becomes the most preferable choice to generate ozone at atmospheric pressure and ambient temperature, due to simple chamber that require reasonable power energy level without complex power supply design. Finally, the interest to select DBD method in this research has been drawn to get lower initial discharge voltage level and make overall system simpler.

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