Thèse de doctorat
Résumé : One of the major challenges of modern society is to find solutions for the severe problem of theglobal warming. This detrimental climate change is generated by the emission of greenhousegases (GHG) of which CO2 is the most abundant. There are various ways to solve or miti-gate this problem, such as the reduction of CO2 emission by using alternative renewable energysources, the capturing and storage of CO2, and the valorization of CO2. The combination ofthese techniques can cover up the technological shortcomings of each individual technique andconsequently create the best solution for global warming. For instance, the intermittent nature ofrenewable energy sources poses major challenges to maintain the balance between the electricsupply and demand. Thus, the development of energy storage technology to store the surplus ofgreen electricity generated on sunny and/or windy days seems to be an indispensable part of thesolution. One of the ways of using this surplus is to feed plasma sources to valorize CO2. Thevalorization of CO2 consists in converting it into other molecules that can be used to fabricatevalue-added fuels with high energy density storage capabilities. Afterward, these value-addedfuels can be employed for the energy generation on days with less favorable weather conditions.The first step in the valorization is the dissociation of CO2, thus the splitting of CO2 into CO and O.The direct electronic dissociation of CO2 requires a significant amount of energy and there-fore is not an energy-efficient pathway. One efficient and less expensive way to dissociate CO2and convert it into CO goes along the vibrational channels. The asymmetric mode of CO2 isof special interest as its inherent anharmonicity makes the energy spacing between vibrationallevels decreases for increasing vibrational quantum number. The quenching can afterward redis-tribute the energy among the vibrational levels and give rise to highly populated upper states. Themechanism behind this is the so-called vibrational-vibrational (VV) ladder climbing which impliesthat by VV collisions the energy released by the de-excitation of the molecule in the lowest stateis more than enough to promote the high-energy molecule to an even higher vibrational state.The bonds in high vibrationally excited CO2 molecules are very weak such that these moleculescan be dissociated very easily. The VV ladder climbing, and thus the CO2 dissociation, can bestimulated by gas dynamics in supersonic nozzles.The numerical analysis of CO2 dissociation in supersonic nozzles constitutes the main focusof this thesis. Two types of configurations are considered: combustion-expansion and microwave-induced plasma expansion (MIP-expansion). In the first approach, the end-product of combustionprocesses is expanded. The dissociation degree is low in this case. However, as the energy inputis strictly zero, this approach can reach high energy efficiencies. Regarding the second approach,the supersonic expansion of microwave-induced CO2 plasmas is considered as the most energyefficient plasma-assisted CO 2 dissociation solution. The reason is twofold: first, the operatingconditions in microwave discharges can be tuned such that the electron energy is as low as 1-3eV to preferentially excite the vibrational levels of the asymmetric mode of CO2, which is imme-diately followed by ladder climbing VV dissociation. Second, shock-free supersonic expansion ofmicrowave-induced CO2 plasmas can prevents nullification of dissociation in the afterglow. Thisis a recurrent and notorious problem addressed in the field of plasma-assisted CO2 valorization isthat once CO2 is dissociated in the active zone of plasma, it can be destroyed in the downstreamregion; Shock-free supersonic expansion of microwave CO2 plasmas not only prevents nullifica-tion of dissociation, but also it gives a boost to further dissociation. Moreover, it is found that thedissociation degree mainly depends on the vibrational distribution function (VDF) temperature ofCO2 at the inlet of the supersonic expansion. It will be come out that an extra CO2 dissociation of15-25% can be obtained by employing shock-free supersonic nozzle after a microwave dischargesystem.It will be shown that the proper design of supersonic nozzle is of paramount importance. Thelatter must be conceived in a way that shock formation within the diverging part of the nozzle isavoided. Shock waves are detrimental for CO2 dissociation as they create heating. The instruc-tions on the design of shock-free supersonic nozzles get special attention in this thesis. Moreover,it is found that in case of MIP-expansion, the microwave should be put prior to the expansion, oth-erwise the vibrational energy would be converted into thermal energy due to vibrational transfer(VT) collisions. Compared to the VV mechanism, VT collisions lead to the opposite trend; a pull-down of the VDF-tail, which is also detrimental for dissociation.The effect of supersonic expansion on the VV-VT competition and how this changes the CO2VDF is studied in this thesis. It is found that the expansion length, and consequently the shock-free supersonic nozzle, must be sized such that the residence time in the expansion is longenough for VV processes to take place and short enough to avoid VT processes.Additionally, the CO2 plasma creation zone is studied and links are established between theplasma externally applied steering parameters and the resulting plasma internal properties. Weshow in this thesis how the steering parameters can be chosen in order to provide an optimumcondition for CO2 dissociation.The platforms to carry out the numerical modelings are OpenFOAM and Matlab. Two differentmodel-types are developed: Full Numerical (Fu-N) and Semi-Analytical (S-An). The Fu-N gives amore complete self-consistent description but needs much longer conversion-times. Comparisonbetween the models’ results shows a good agreement between two models. This justifies theapplication of the S-An approach for a fast parameter space exploration of experimental settings.