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State of the art of conversion of carbon dioxide
Actual state of the CO2 problem
Carbon dioxide is one of the main gases presenting in the atmosphere of our planet. Since the beginning of the industrial revolution in the middle of XIX century the antropogenic outcomes of carbon dioxide increased from 250 ppm in the very begin-ning of the industrial revolution up to 400 ppm in 2014 [1, 2]. The influence of the increase of the antropogenic emission of CO2 on the greenhouse effect is significant. Nevertheless, the idea that carbon dioxide is a source of secondary fuel rather than a pollutant waste has been acknowledged by scientific community and is developing nowadays. It is considered to be feasible to use different ways of conversion of carbon dioxide combined with other gases like water vapor or hydrogen back to the so called value-added chemicals such as methane and other hydrocarbons and thus to make possible the secondary use of fuels [3, 4]. The concept is illustrated in the Figure 1.1.
Consequently, the extraction and use of fossil fuels could be decreased significantly and the ecological situation could be improved. The problem of conversion of one of the main greenhouse onto value-added chemicals such as CO, CH4 and oxygen [1, 3] which could act as fuels and an oxidizer is therefore an actual scientific challenge of our time.
An alternative opinion on the problem of pollution of atmosphere of Earth by carbon dioxide is suggested in [5]. The idea of the opinion consists on obsolescence of methods which are used for calculation of heat flux caused by carbon dioxide emission. In fact, the calculation of heat flux change used in Paris treaty is based on CO2 concentration change only and doesn’t take into account other molecules like H2O whose spectral bands overlap with CO2 spectral bands. Disregard of this fact leads to overestimation of antropogenic global temperature change in 5 times ( T = 1.5 K instead of T = 0.3 K).
On the other hand, the XXI century became the era of space discoveries. For in-stance, much attention is paid to the exploration and future development of Mars whose atmosphere consists on 96% of carbon dioxide (with approximately 2% Ar and 2% N2 [6]) and several space missions from different countries are in progress. Currently, there are three automatic stations (rovers) exploring the surface of Mars: « Curiosity » (since 2012, August 6), « InSight » (since 2018, November 26) and « Per-severance » (since 2021, February 19). There are several space projects of a human mission to Mars planned for the next 10-20 years and the question of conversion of the Martian atmosphere to oxygen available for breathing and which could act as an oxidizer for the fuel combustion is of current interest. As for « Perseverance » rover which landed on Mars in the beginning of 2021, a prototype of such a reactor named MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment) has been launched and delivered to the Martian surface with the rover. The MOXIE project has been developed not only for the proof-of-concept that it is possible to supply future Martian astronauts with oxygen necessary to breath but also to understand how to produce fuel and oxidizer which are essential for the delivery of rockets with a sample of Martian soil back to the Earth. The MOXIE reactor is currently (March 2021) operating and is capable to produce approximately 10 grams of oxygen per hour [7]. The MOXIE reactor is based on the principle of the so called solid oxide electrolyzer cell (SOEC) but there are several other ways of conversion of carbon dioxide onto value-added chemicals such as thermal dissociation, photo-electrolysis, bio-fixation or plasma-assisted conversion whose detailed analysis can be found in [1, 3, 8]. The basic principles of the aforementioned mechanisms are described in the next subsection.
Different techniques of CO2 conversion
As it was mentioned above, there are several techniques of conversion of carbon diox-ide and the first one which will be discussed in the current Thesis is the thermal dissociation also known as thermolysis. It could be noticed that such a mech-anism is not very easy to make efficient enough if one take a look at the standard enthalpy of the net reaction of CO2 conversion [8]:
CO2(g)! CO(g) + 1
2O2 (g);H0 = 293 kJ mol 1: (1.1)
It could be seen that a strong thermodynamic barrier has to be overcome in or-der to make this highly endothermic reaction(1.1) possible. High temperatures are therefore required to shift the thermodynamic equilibrium of the reaction on the right-hand side. [8, 9] report the necessary values of temperature to be at least 2000 K. This fact can be clearly seen in the Figure 1.2 where the dependence of mole fractions of reactants and products of the reaction on temperature is given.
An experiment on fully thermal dissociation of carbon dioxide where a rate of the reaction (1.1) was several times increased at T=1300 K with a corundum tube is reported in [10]. The corundum tube was acting as a catalyst. The dissociation fraction at T=1300 K increased more than 3 times (0.02 % with the catalyst instead of 0.006 % without it). The hypothesized mechanism of the catalysis consisted on two main steps: formation of oxygen vacancies in the metal oxide during the heat-ing and the consequent dissociative adsorption of CO2 on defective surfaces of the catalyst. Nevertheless, despite the fact that the catalyst significantly increased the dissociation fraction, its value is still relatively low. Hence, either a significant in-crease of temperature or use of other mechanisms of conversion of carbon dioxide is required.
Another possibility to dissociate CO2 is its electrochemical reduction which is a technique using a direct electric current which makes non-spontaneous redox re-actions to pass in the gaseous, aqueous or non-aqueous phase with or without a catalyst [1, 8, 11, 12]. The basic principle of any way of electrochemical conversion of pure carbon dioxide without admixtures is the following. The reaction on the cathode can be written as: CO2 + 2e ! CO+O2 : (1.2)
On its turn, the reaction occurring on the anode can be written as follows: O2 ! 1 O2 + 2e ; (1.3)
which results in the net reaction (1.1) occurring in an electrolyte.
The most common way of electrochemical conversion of pure gaseous CO2 is the so called solid oxide electrolyzer cell (SOEC). A detailed review on this topic can be found in [13]. The main pecularity of the method is use of a dense porous ionic conductor as an electrolyte. The most wide-spread option of the electrolyte is yttria-stabilized zirconia (YSZ) which becomes a conductor for O2 ions at elevated temperatures (around 1000 K). The SOEC concept has been first proposed in 1970 [14] for in situ production of CO and oxygen from the Martian atmosphere. Since the beginning of the XXI century, research groups from different countries are de-veloping their exploratory projects on to Mars. As it was mentioned in the section 1.1.1, the MOXIE reactor [7] launched on Mars in 2021 with the « Perseverance » rover is operating successfully. The MOXIE reactor is based on SOEC principle with use of YSZ as an electrolyte.
The main advantages of the electrochemical way of dissociation of carbon dioxide are its controllabilty by reaction parameters such as temperature or an electrode potential. The reactors are relatively cheap, can be easily integrated with other sources of sustainable energies and can be used for different functions [1, 13]. On the other hand, as it is mentioned in [13], the SOEC technique has several serious disadvantages such as relatively low efficiency of the dissociation and insufficient ac-tivity and stability of possible catalysts. In the case of MOXIE, it is neither possible to operate at atmospheric pressure of Mars ( 6 mbar) and preliminary gas com-pression is required. Item, the necessity to heat the reactor cell to relatively high temperatures entails possible recombination of products of dissociation and other technical challenges of operation.
One more possibility of CO2 conversion is photochemical dissociation [1, 9, 15] which can be also combined with the termochemical approach [16]. A detailed re-view describing the technique of photochemical dissociation of carbon dioxide can be found in [17]. The technique is based on the solar-activated photocatalytic reduction of carbon dioxide with water at room temperature and atmospheric pressure [15]. Briefly, it is possible to describe the mechanism in the following way. When photons of solar radiation illuminate a surface of the catalyst (most generally, TiO2), they can excite electron-hole pairs in the latter. The electrons are excited in the con-duction band of the catalyst and can furthermore reduce CO2 molecules into CO, CH4 or other fuels. In their turn, the holes excited to the valence band of the cata-lyst can oxidize water into oxygen. The main advantages of this mechanism are its adaptability for different functions and a relatively low cost. Nevertheless, there are many requirements to be met for a successful operation of a setup. First of all, the electronic band gap of the catalyst material must be low enough for efficient photon collecting from the solar radiation and at the same time high enough such that the excited electrons could have sufficient energy to dissociate an H2O molecule. Second, the band edges must fit the water electrolysis redox potentials, and the photoelec-trode must be resistant to corrosion in water. Several developments of the technique such as doping of TiO2 by metals or metal oxides can increase [15] the efficiency of the mechanism but still, all aforementioned limitations constrain the efficiency of photochemical conversion of carbon dioxide.
The most ’conventional’ and ’well-known’ mechanism of biochemical conversion of carbon dioxide is photosynthesis. However, mass cultivation of plants directly for processing of carbon dioxide by photosynthesis is difficult due to the fact that a significant part of the arable land is used for the production of crops. Use of mi-croalgae [1, 17, 18] which need only sunlight and freely available raw resources such as freshwater, saltwater or wastewater where the microalgae can be grown might be a reasonable and relatively efficient compromise. The microalgae can hence con-tribute to the solution of the problem of water overuse. Nevertheless, there is no any common technology which colud be used directly for biological conversion of carbon dioxide by microalgues. Currently, the costs must be shared by several industries such as food, pharmaceuticals and cosmetics industry.
Several approaches to conversion of CO2 have been briefly reviewed in the current subsection. As it could be seen, all them have their scopes and can be applied for different functions. All them have advantages and disadvantages. The next subsections are devoted to the review of main principles and achievements in the domain of plasma-assisted conversion of carbon dioxide which is the topic of the current Thesis. As it will be seen further, plasma-assisted conversion of carbon dioxide is not an exception and has its own drawbacks and points of superiority.
Theoretical aspects of conversion of carbon dioxide in low temperature plasma
Generally speaking, plasma is a state of matter where the behaviour of electrons, ions and neutral particles is different. Moreover, different internal degrees of freedom of ions and neutral particles might be excited at different stages and this difference de-pends on many conditions such as gas composition, pressure, electric field etc. This variety entails the fact that plasma in general must be described by several separate temperatures such as temperature of electrons (Te), rotational temperature (Trot), vibrational temperature (Tvib), temperature of ions (Ti) and temperature of neutral gas (T0) [19, 20, 21]. If the temperature of electrons is equal to the temperature of ions and neutral gas, the plasma is called thermal or equilibrium plasma since such plasma fits the conditions of thermodynamic equilibrium. In the case of strong inequality of temperatures (Te Tion T0) plasma is called non-thermal plasma since no thermodynamic equilibrium is established.
Another way of classification of plasma is its ionization degree which is defined as the ratio of electron density and total number density of particles: = ne : (1.4) ntotal
If 1, plasma is classified as strongly ionized. In the other limit case, when 1, plasma is called weakly ionized. Hereinafter, the case of non-thermal weakly ionized plasma (in most cases, < 10 2) where the average kinetic energy of electrons doesn’t exceed significantly the first ionization potential of the gas will be called low temperature or cold plasma. It is conversion of carbon dioxide by low temperature plasma which is the object of the current literature review and the Thesis.
Dissociation of a CO2 molecule is a strongly endothermic process. Decomposition of a molecule starts with the following process: CO2 ! CO + OH = 5.5 eV/mol. (1.5)
Then, the formed O atom can recombine with another O atom or with a CO2 molecule. According to the section 1.1, the total entalpy required for decomposition of one CO2 molecule on carbon monoxide and ’a half’ of molecular oxygen is equal to E=2.9 eV/mol. Therefore, if the dissociation fraction is defined as: = [CO] ; (1.6) [CO] + [CO2]
the energy efficiency of CO2 decomposition can be written as: = E : (1.7)
Here, E=2.9 eV/mol corresponds to the minimal average energy required for for-mation of carbon monoxide and molecular oxygen, w is the energy deposited into plasma per one particle. As it will be evident now and further, the counterbalance between dissociation fraction and energy efficiency is an object of essential matter.
There are several mechanisms of conversion of carbon dioxide in low temperature plasma and the first one which will be the object of the discussion is conversion of CO2 via excitation of the vibrational degrees of freedom. A molecule of carbon dioxide is a triatomic one and it has three different vibrational modes: asymmet-ric stretching (! = 2396:32 cm 1), symmetric stretching (! = 1354:31 cm 1) and double degenerate bending (! = 672:85 cm 1) [22, 23, 24]. All additional constants such as coefficients of anharmonicity of vibrations are given in [22]. A detailed ap-proach to all vibrational modes of CO2 and, in particular, to vibrational kinetics of CO2 will be given in Chapter 9 but it is important to notice that the asymmetric mode plays the most essential role in the vibrational dissociation of carbon dioxide [20].
The Figure 1.3 illustrates the configuration of electronic terms of a CO2 molecule. It should be noted that since the molecule has 3 atoms, the Figure cannot precisely rep-resent the configuration of terms and the curves should be interpreted as projections of 3D potential surfaces. Nevertheless, this illustrative sketch allows to have an idea about relative position of terms and energy thresholds of various chemical processes.
The vibrational pumping has several steps [25, 26]. First, energy of the applied electric field is transferred to kinetic energy of electrons which reaches 1-3 eV. The electrons accelerated by the electric field transfer their energy to the lower vibrational levels of CO2 molecules which collide with each other and exchange by vibrational quanta (V-V exchange). Therefore, the vibrationally excited molecules can reach the dissociation threshold (5.5 eV) and decompose onto CO(X1 +) and O(3P) via the CO2(3B2) electronic state. The process is illustrated in the Figure 1.3 (orange arrow).
Table of contents :
Acknowledgements
R´esum´e de th`ese
Abstract
1 Literature review
1.1 State of the art of conversion of carbon dioxide
1.1.1 Actual state of the CO2 problem
1.1.2 Different techniques of CO2 conversion
1.1.3 Theoretical aspects of conversion of carbon dioxide in low temperature plasma
1.1.4 Experiments on conversion of carbon dioxide in low temperature plasma
1.2 Nanosecond discharges. Fast ionization waves. Dissociation of gases in nanosecond discharges
1.2.1 Fast ionization waves
1.2.2 Dissociation of molecular nitrogen and oxygen in nanosecond discharges
1.2.3 Dissociation of carbon dioxide in nanosecond discharges
1.3 Numerical modeling of CO2 discharges
1.4 Conclusions of the literature review
2 Experimental methods
2.1 Discharge cell
2.1.1 General observations
2.1.2 Thin capillary
2.1.3 FTIR capillary
2.2 Electric measurements
2.2.1 Back current shunts and high voltage cables
2.2.2 Capacitive probe
2.3 Optical emission spectroscopy
2.4 Fourier Transform Infrared Spectroscopy(FTIR)
2.4.1 Basic principles of FTIR
2.4.2 Experimental procedure and data treatment
2.5 Reconstruction of radial profiles of active species
2.5.1 Experimental procedure
2.5.2 Inverse Abel transform
3 Electric characteristics of the discharge
3.1 Applied voltage and electric current
3.1.1 Thin capillary
3.1.2 FTIR capillary
3.2 Electric field
3.2.1 Thin capillary
3.2.2 FTIR capillary
3.3 Deposited energy
3.3.1 Thin capillary
3.3.2 FTIR capillary
3.4 Conclusions
4 Measurements of CO2 dissociation fraction and its energy efficiency by FTIR 84
4.1 Dissociation fraction and energy efficiency at different pressures at low pulse frequency regime
4.2 Dissociation fraction and energy efficiency at high pulse frequency regime
4.3 Dissociation fraction and energy efficiency at different parameters
4.3.1 Different flow rates
4.3.2 Different applied voltages
4.4 Conclusions
5 Study of the discharge by optical emission spectroscopy
5.1 Optical emission spectra
5.1.1 Thin capillary
5.1.2 FTIR capillary
5.2 Temporal behaviour of the main electronically excited species in the thin capillary
5.3 How big is the electron energy?
5.4 Behaviour of the Swan band in the FTIR capillary
5.4.1 Variable pressures
5.4.2 Varied delay between pulses
5.5 Conclusions
6 Measurements of the temporal profile of the electron density in the thin capillary
6.1 Description of the technique
6.2 Experimental results
7 Analysis of the radial profile of the electron density in the thin capillary on the basis of OES measurements
7.1 Experimental setup
7.2 Experimental results
7.3 Relation between the emission profiles and the electron density. Validity of gas temperature measurements
7.4 Conclusions
8 Measurements of the gas temperature as the function of time by the means of optical emission spectroscopy
8.1 FTIR capillary
8.2 Thin capillary
8.3 Conclusions
9 Numerical modeling of the discharge
9.1 Principles of 0D modeling
9.2 Kinetic scheme
9.2.1 Set of reactions
9.2.2 Vibrational kinetics
9.2.3 Vibrational distribution function
9.3 Validation of the model
9.3.1 Calculations of gas temperature profile
9.3.2 Electron density
9.3.3 Predictions of CO2 dissociation fraction and energy efficiency
9.3.4 Temporal profiles of the electronically excited species
9.4 Conclusions
10 General conclusions
List of Figures
