Influence of the argon percentage on the electrical characteristics of the discharge

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Gliding arc discharges

A gliding arc is an auto-oscillating discharge between at least two diverging electrodes submerged in gas flow. Self-initiated in the upstream narrowest gap, the discharge forms the plasma column connecting the electrodes. This column is dragged by the gas flow toward the diverging downstream section. The arc grows with the increase interelectrode distance until it extinguishes, bit it reignites itself at the minimum distance between the electrodes to start a new cycle as shown in Figure 1.11 [29].
Gliding arcs may be thermal or non-thermal, depending on the applied power and gas flow rate. It is also possible to operate in the transitional regime, whereby the discharge has thermal characteristics in the lower part of the gliding-arc and evolves into a non-thermal discharge as it proceeds up the electrodes [29]. Gliding arc discharges are suitable for applications that require relatively large gas flows (several L min-1) and can be either DC or AC driven.

Dielectric barrier discharges (DBD)

The corona to spark transition is prevented in a pulsed corona by employing nanosecond pulse power supplies. Another approach which avoids spark formation in streamer channels is based on the use of a dielectric barrier in the discharge gap that stop current and prevent spark formation. Such a discharge is called the DBD. The presence of a dielectric barrier precludes DC operation of DBD, which usually operates at frequencies of 0.05-500 kHz. Sometimes DBDs are called the silent discharges due to the absence of the spark, which are accompanied by local overheating and the generation of local shock waves and noise.
The dielectric barrier discharge (DBD) or silent discharge (as it was originally known) is a strongly non-thermal plasma that can be operated at atmospheric pressure. DBDs are able to form stable discharges in a range of different gases at relatively high discharge powers, making them particularly suitable for many industrial applications.
The DBD reactor consists of two electrodes with one or more dielectric barriers positioned in the discharge gap (in the path of current flow). Materials with high relative permittivity such as quartz, glass and ceramics are suitable for use as dielectric barriers. Several DBD configurations are possible including planar, cylindrical and surface discharges, as illustrated in Figure 1.12. The spacing in the discharge gap can vary from hundreds of micrometers to several centimeters.

Microdischarges

The term microdischarge is not clearly defined; sometimes it just indicates the generation of plasma smaller than 1 mm. According to such a definition, even DBD can be considered a microdischarge; the DBD gaps are often smaller than 1 mm, and the DBD filaments have a typical diameter of 0.1 mm [29].
The DBD is a non-uniform plasma discharge, it consists of many tiny breakdown channels known as microdischarges or filaments that cover the entire surface of the dielectric material and extend across the discharge gap. The dielectric barrier limits the flow of current causing the microdischarges to become extinguished, leaving significant charge deposition on the dielectric surfaces. As the polarity of the electrodes is rapidly changing, the microdischarges are reformed at the point where the breakdown voltage is reached in the next half cycle of the AC voltage sine wave. This results in the continuous formation of nanosecond microdischarges at a frequency which is twice that of the applied frequency [34]. The microdischarges appear as “spikes” on the current waveform. In appearance, the microdischarges are randomly distributed over the surface of the dielectric. In reality, the position of the microdischarge formation is dependent on the residual charge distribution on the dielectric surface [40], [41].
Where DBD plasmas are concerned, exclusive use of the term “dielectric constant” is used, but the terms “relative permittivity” and “dielectric constant” are synonymous, meaning the ability of a material to store electrical charge relative to a vacuum.

Memory effect in DBD plasma

The formation of the filaments is a complex process. Avalanches are first initiated, followed by cathode-oriented streamers bridging the gap. They form conducting channels of weakly ionized plasma until the local electric field is collapsed caused by the charges accumulated on the dielectric surface and ionic space charge. After electron current termination, there is still a high level of electronic excitation in the channel volume, along with charges deposited on the surface and ionic charges in the volume, allowing this region to be separated from the rest of the volume.
The fact that the remnant is not fully dissipated before the formation of the next microdischarge is called memory effect, which will facilitate the formation of a new filament in the same location. It is possible that with the increase in the flow rate value these remnant can displace through the channel and facilitate the appearance of discharge [42], [43].

Plasma chemistry for methanol production

There is currently promising research on the partial oxidation of methane in a non-thermal discharge, especially on DBD plasma. These experiments indicate that this kind of systems are able to create a highly reactive environment at low temperatures and therewith opens up an alternative, highly flexible and environmentally friendly processing route.
However, atmospheric pressure plasmas have a tendency to become unstable due to rapid transition to arcs. Confinement of high pressure plasma to dimensions below about one millimeter is useful to avoid instability problems and maintain a self-sustaining discharge. Such a plasma is often referred to as microplasma. In this respect, utilization of microplasma in microreaction technology can bring unconventional thermochemical conditions to materials processing, enabling better control over process parameters to selective synthesis of desirable products [44].

Plasma reactor designs for partial oxidation of methane (POM)

Various designs of plasma reactor for methane and oxygen conversion to methanol have been proposed shown in Figure 1.13. The typical tubular discharge reactor consists of two concentric cylinders. The outer cylinder functions as ground electrode and the inner cylinder usually made from glass or quartz tube, serves as the dielectric. A steel or copper metal rod is located inside the inner cylinder and performs as another electrode. When plasma is turned on, the micro-discharges appear on the surface of the inside electrode.
The DBD plasma ability to reduce the required temperature and pressure needed for reactions to occur as well as its ability to control the products selectivity could be considered as a catalytic effect following the idea of Larkin et al. [48].
The partial oxidation of methane to methanol with oxygen or air was also investigated experimentally and theoretically by Zhou et., al. [47].
The reactor presented in figure 1.13c, consists in an annular discharge gap of 1 mm width formed by an outer steel cylinder of 54 mm inner diameter and an inserted cylindrical quartz tube of 52 mm outer diameter and 2.5 mm wall thickness. The length of the discharge gap is 310 mm, giving a discharge volume of about 50 ml. The outer steel cylinder serves as the ground electrode and an alternating sinusoidal high voltage of up to 20 kVpp amplitude and about 30 kHz frequency is applied to the HV electrode, a metal foil mounted inside of the quartz tube [47]. Is reported the partial oxidation of methane in two different types of reaction mixtures the first one consists in methane and oxygen and the second one in methane and air. The highest value of methanol yield of 3% was achieved in CH4/O2 (8/2) mixture. In CH4/O2 (7/3) mixtures 2% was obtained.
Another interesting reactor design was also proposed by Nozaki et al. (2004) [45], called micro-plasma reactor, shown in figure 1.13a. In their studies, the DBD reactor was used to synthesize methanol from methane and oxygen. It consists of a Pyrex thin glass tube with an internal diameter of 1.0 mm and a length equal to 60 mm, a twisted metallic wire inside the tube serves as electrode. The reactor is secured in a heat reservoir to maintain a constant reaction temperature. A high-voltage sine wave is applied between the twisted metallic wire and the grounded heat reservoir. The principle of generating plasma was similar to the DBD which was characterized by a variety of filamentary micro discharges. In this experiment, methanol was the major product, whose selectivity reached 34% at 30% of methane conversion. However, methanol, oxidized partially from methane, could further react to form other oxygenates such as formic acid and formaldehyde.
Another type of DBD reactor was proposed independently by Aghamir et al. (2004) and Okumoto et al. (1997) [49,50]. The general scheme of the reactor is presented in Figure 1.13b. The discharge reactor consisted of a 10 mm as an inner diameter quartz tube, which was also used as dielectric barrier. A stainless steel rod and a 15 cm long aluminum foil, formed respectively the anode and cathode of the discharge. The aluminum foil was tightly wrapped around the quartz tube, which had outer diameter of 12 mm. The stainless steel electrode was placed on the symmetric axis inside the tube. Two gas inlets located at the top and the bottom of the reactor were provided as an inlet of the reactants and an outlet of products [46]. C2 hydrocarbons and methanol were mainly produced and the selectivity of methanol reached 30% with high voltage pulses of approximately 10-10 s of rise time, +25 kV of peak voltage, 440 Hz of pulse frequency and argon concentration of 30 % (v/v).

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Table of contents :

1.1. Introduction
1.2. Natural gas
1.3. Global climate change
1.3.1. Biogas
1.4. Fischer-Tropsch processes
1.5. Industrial methanol synthesis
1.6. Introduction to plasma
1.6.1. Applications of plasma
1.6.2. Types of plasma
1.6.3. Generation of non-thermal plasma by electric fields
1.6.4. Townsend mechanism of electric breakdown
1.6.5. Continuous and pulsed direct current discharges
1.6.5.1. Corona discharges
1.6.5.2. Gliding arc discharges
1.6.5.3. Dielectric barrier discharges (DBD)
1.6.5.3.1. Microdischarges
1.6.5.3.2. Memory effect in DBD plasma
1.7. Plasma chemistry for methanol production
1.7.1. Plasma reactor designs for partial oxidation of methane (POM)
1.7.2. CH4/O2 ratio
1.7.3. Noble gas effect
1.8. Conclusion
2.1. Plasma power measurements
2.2. Intensified charge coupled device (ICCD)
2.3. Gas chromatography
2.3.1. Micro-gas chromatography
2.3.2. Thermal conductivity detection
2.3.3. Flame ionization detection
2.4. Conclusion
3.1. Introduction to milli-reactor development
3.2. Reactor fabrication
3.2.1. Plasma milli-reactor cellule fabrication
3.2.2. Borosilicate glass engraving
3.2.3. Electrode patterning
3.2.4. General description of the sputtering process
3.2.5. Electrode deposition for POM
3.2.6. Electrode deposition for discharge characterization
3.2.7. Sealing step
3.3. Electrical characterization of the reactor
3.3.1. Influence of the argon percentage on the electrical characteristics of the discharge
3.3.2. Influence of the O2/CH4
3.4. Characterization of discharge uniformity by ICCD measurements
3.4.1. ICCD measurements in pure gases
3.4.2. ICCD measurements CH4/O2/Ar mixture
3.5. Conclusion
4.1. General performance for partial oxidation of methane (POM)
4.2. Methane to methanol in a plasma milli-reactor
4.2.1. Influence of flow rate on methanol selectivity (ICCD)
4.2.2. Influence of argon concentration on methanol selectivity
4.2.3. Influence of the O2/CH4 ratio on methanol selectivity
4.3. Conclusion
5.1. Introduction to the simulation of a DBD plasma milli-reactor
5.2. Description of the filamentary aspect of DBD plasma
5.3. The DBD plasma modelling
5.3.1. Governing equations
5.3.2. Electron transport equations
5.3.3. Diffusive transport equations for heavy species
5.3.4. Poisson’s equations and surfaces boundary conditions for DBD plasma
5.3.5. Chemical kinetics and source term treatment
5.4. Introduction to numerical simulation (plasma module COMSOL Multiphysics 5.1) and description of the different modeling approaches
5.4.1. Sinusoidal model
5.4.2. Multi-time scale model
5.4.2.1. DBD model for the simulation of one microdischarge
5.4.2.2. 0D model for time evolution of chemical species
5.5. Simulation results
5.5.1. Results obtained with the sinusoidal model
5.5.2. Results obtained with the multi-time scale model
5.5.2.1. Energetic aspects
5.5.2.2. Production of primary radicals
5.5.3. Comparison of sinusoidal model and multi-time scale model
5.5.3.1. Energetic aspects
5.5.3.2. Production of stables species at low constant SIEM
5.6. Comparison of multi-time scale model results and experimental results
5.6.1. Methane conversion
5.6.2. Influence of Argon percentage on methanol selectivity
5.6.3. Influence of O2/CH4 ratio on methanol selectivity
5.7. Conclusion

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