General performance for partial oxidation of methane (POM)

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Naturally the organic matter has an anaerobic decay what formed an important renewable source of methane. The methane obtained from these processes is usually called biogas. Almost all organic matter can be used as a biogas feedstock. The use of waste products could be particularly advantageous as it could prevent the unnecessary waste of useful energy sources and offer increased financial profits to plant operators. Industries that generate biogas from waste products could use it directly on-site as a fuel and/or for electricity generation [10]. Waste biomass that have potential for industrial biogas generation include:
Food waste – Waste materials from food processing industries, agricultural processes and forestries all have the potential to generate biogas through the use of anaerobic digesters. In the U.K, several plants of this type are in operation including the use of brewery by-products, potato peelings, fish waste, sugar cane waste and other food wastes from kitchens [11].
Wastewater treatment – the sludge generated by the treatment of wastewater has to be chemically treated and disposed of, in a process that represents a considerable financial cost. One encouraging alternative is the generation of the biogas from wastewater sludge in anaerobic digestion tanks. This process has been considered economically feasible [10], this kind of processes are already operated in several sites across the U.K.
Animal manure – One important source of biogas is the manure which has important uses in farming as a fertiliser, and recently also as a source for biogas generation on farm. The usual practice is to store the manure for several months until it is needed. During this time, gases that are produced from the manure can be released straight into the atmosphere, if they are not properly collected. To make use of these gases, the manure can be transferred to an anaerobic digester for biogas generation. The remaining substrate after biogas production still contains nutrients that give it value as a fertiliser [12].

Fischer-Tropsch processes

The Fischer-Tropsch process was established in 1923 by German researchers, Franz Fischer and Hans Tropsch. They discovered that syngas CO + H2 could be converted into a mixture of linear and branched hydrocarbons and alcohols using various metal catalysts at elevated temperatures. For commercial F-T synthesis, iron and cobalt catalysts are used at temperatures of 200 – 300 °C and pressures of 1000 – 6000 kPa. A syngas ratio of H2/CO = 2 is generally required. Potassium and iron catalysts are used to promote the water-gas shift reaction which is used to modify the H2/CO ratio [11]. The main reactions are shown in table 1.1.

Generation of non-thermal plasma by electric fields

The most used method for the formation of non-thermal plasma is by the application of an external electric field between two electrodes surrounded by a volume of gas. The plasma can be operated either at low pressures (10-3 Pa) or at atmospheric pressure and above.
The breakdown voltage (Vb) defines the minimum voltage required to breakdown a gas (or mixture of gases) to form a plasma discharge. Vb is dependent on the gas pressure (p) and the distance between the electrodes (d). This relationship is described by Law of Paschen, where a and b are constants that are dependent on the gas type [36]. 𝐕𝐛=𝐚 𝐩 𝐝𝐥𝐧(𝐩 𝐝)+𝐛 e Eq. 1.1.
An applied voltage causes free electrons that exist to some extent in a gas volume as a result of an interaction with cosmic radiation, to become accelerated. At the point where the breakdown voltage is reached, the current flow will increase sharply due to an intensive avalanche of electrons in the discharge gap between the electrodes. These high energy electrons will collide with gas molecules leading to the formation of new “active” plasma species including excited molecules and atoms and their relevant degrees of freedom, radicals, ions and new stable gas molecules. These collision processes are shown in Table 1.3.

Townsend mechanism of electric breakdown

Consider breakdown in a plane gap d by DC voltage V corresponding to electric field 𝑬= 𝑽𝒅 Eq. 1.2 Occasional primary electrons near a cathode provide low initial current i0. The primary electrons drift the anode, ionizing the gas and generating avalanches. The ionization production of electrons per unit length along the electric field: dnc/ dx = αne, ne(x) = ne0 exp (αx). The Townsend ionization coefficient is related to the ionization rate coefficient ki (E/n0) and electron drift velocity vd as:
𝜶= 𝒗𝒊𝒗𝒅= 𝟏𝒗𝒅 𝒌𝒊(𝑬𝒏𝟎)𝒏𝟎= 𝟏𝝁𝒆𝒌𝒊 (𝑬𝒏𝟎)𝑬/𝒏𝟎 Eq. 1.3
Where vi is the ionization frequency and μe is electron mobility, which is inversely proportional to pressure. The Townsend coefficient α is usually presented as similarity parameter α/p depending on the reduced electric field E/p. Dependences α/p = f (E/p) for different gases can be found in Fridman and Kennedy [36].
Each primary electron generated near a cathode produces exp (αd) -1 positive ions moving back to the cathode. The ions lead to extraction of y* [exp (αd)-1] electrons from the cathode due to secondary electron emission characterized by the Townsend coefficient y. Typical y- values in discharges are 0.01-0.1. Taking into account the current of primary electrons i0 and electron current due to the secondary electron emission from the cathode, the total electronic part of the cathode current icath is 𝒊𝒄𝒂𝒕𝒉= 𝒊𝟎+ 𝒚𝒊𝒄𝒂𝒕𝒉⌈𝐞𝐱𝐩(𝜶𝒅)−𝟏⌉ Eq. 1.4.
Total current in the external circuit is equal to the electronic current at the anode, where the ion current is absent. The total current can be found as i=icath exp (αd), which leads to the Townsend formula: 𝒊= 𝒊𝟎𝐞𝐱𝐩(𝜶𝒅)𝟏−𝒚[𝐞𝐱𝐩(𝜶𝒅)−𝟏] Eq. 1.5.

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.

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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.

Micro-gas chromatography

Micro-GCs have several advantages over conventional instruments for gas chromatography, such as the small size of the required sample and of the instrument, which allow micro-GCs to be used for bench-top or portable applications. The run time for each sample is 5 minutes and it is not necessary to work with temperature programs which samples can be run back to back without the need for column cool down, which is necessary with conventional GC instruments.
A micro gas chromatography (Agilent technologies 490 micro GC) equipped with a thermal conductivity detector (TCD) has been used in this thesis for the gaseous analysis equipped which is presented schematically in figure 2.5. The micro-system contains two columns of different materials that use different carrier gases (Ar and He). The first one was used to calculate carbon monoxide (CO), hydrogen (H2) and oxygen (O2) concentrations (molecular sieve), the second column (HP-plot U column) was used for the quantification of methane (CH4) and carbon dioxide (CO2).

Plasma milli-reactor cellule fabrication

Borosilicate was purchased from Codex International (ε=4.6 20°C/1 MHz, transformation temperature equal to 510°C, 2 mm thick) and the gold target to use in the sputtering in order to make the electrodes deposition was purchased from NEYCO (99.999% w/w).The S1818 photoresin and Microposit 351 developer were obtained from Shipley®. The liquid adhesive NOA 81 was purchased from Norland Products Incorporated. It is sensitive to the entire range of long wavelength light from 320 to 380 nanometers with peak sensitivity around 365 nm. In order to complete the sealed processes of the millireactor, a UV exposure (30, 2 minutes) is required.

Table of contents :

General Introduction : Literature review
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 Corona discharges Gliding arc discharges Dielectric barrier discharges (DBD) Microdischarges 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 DBD model for the simulation of one microdischarge 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 Energetic aspects Production of primary radicals
5.5.3. Comparison of sinusoidal model and multi-time scale model Energetic aspects 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
General conclusions and outlooks


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