Abatement of chlorinated volatile organic compounds from waste gas streams by post plasma catalysis

Air quality issues have become a huge concern of environmental legislation as a consequence of growing awareness in our global world. Exhausts, form outdoor sources (various chemical industries, painting and printing industries, cars) as well as from indoor sources, pollute the air with a variety of harmful substances like volatile organic compounds (VOCs) which pose a threat to human health and environment. The complex chemical and physical transformations that these VOCs can undergo in the atmosphere result in effects such as the formation of photochemical smog, secondary aerosols and ozone in the urban areas. They also play a part in the greenhouse effect, the destruction of the stratospheric ozone layer and acid depositions. Moreover, VOCs have been proved to be health hazards due to their potential toxicity, carcinogenicity and mutagenicity. Long exposure to VOCs can lead to a number of human diseases, including cancer and cardiovascular and several other potential diseases. These substances are therefore also classified as priority, toxic pollutants. European legislation has therefore imposed stricter objectives for VOC emissions, including for industrial emissions: VOC emissions must have decreased by 21 and 43% in 2020 for Belgium and France respectively compared with emissions in 2005. Moreover, the 2030 climate plan also provides a reduction of greenhouse gas emissions (of which VOCs form part) by 40% by 2030. The current policy to reduce VOC emissions is to give priority to the total or partial elimination of VOC emissions by removing the VOC source itself. However, if this is not possible for technical reasons, it is necessary to find solutions for the treatment of these emissions by applying new VOC decomposition processes that are adapted to low VOC concentrations and that can achieve complete degradation in an energy-efficient manner without the formation of by-products. To respond to this industrial problem, this doctoral thesis is focused on the development of a new, innovative treatment method adapted to industrial needs and based on the coupling of existing degradation methods. Conventional techniques used for end-of-pipe treatment of these VOCs are thermal oxidation, catalytic incineration, adsorption, condensation, bio filtration and membrane separation each process having its advantages and limitations. As a cost and energy efficient alternative, the use of non-thermal plasma (NTP) has been recognized to be relevant for the removal of VOCs from dilute atmospheric pressure gas streams. These so-called cold plasmas have proven to be more efficient than conventional techniques for the treatment of lightly contaminated waste gases, due to the lower energy consumption and its flexibility. The main advantage of NTP is that the supplied energy is used for the acceleration of electrons, instead of heating up the total gas volume. The energetic electrons collide with background molecules leading to the formation of reactive radical species, which in turn react with VOCs. The major drawback of the NTP removal technique is the formation of undesired by-products (e.g. ozone, NOx, aerosols, phosgene and other VOCs), that even increase the overall toxicity of the treated gas stream. In an effort to solve these issues, the combination of NTP and other techniques has been an active area of research since 2000. This thesis is focused in particular on linking plasma technology with heterogeneous catalysis (plasma catalysis), for the abatement of trichloroethylene (TCE), a typical chlorinated VOC. There are two ways to introduce the catalysts into the plasma: in plasma catalysis (IPC) or post plasma catalysis (PPC). Plasma assisted catalysis can obviously improve energy efficiency and suppress unwanted reaction byproducts in VOC decomposition. Furthermore, the development of a suitable catalyst will help to optimize the selectivity into environmentally more friendly end products. By placing the catalyst downstream of the discharge zone (PPC), the catalyst is able to decompose ozone formed in NTP into active oxygen species able to greatly improve the oxidation of both the target VOC and hazardous by-products. In this work, the abatement of TCE in a post plasma catalytic (PPC) system is studied. This PhD thesis first gives an overview of the air pollutants and their sources. The effect of a poor air quality on human health is summarized and illustrated by a more detailed section on trichloroethylene. Since regulatory initiatives are emerging, the available air cleaning technologies are also summarized. A general overview of the main fundamentals on non-thermal plasma and catalytic oxidation is given separately as introduction for the reader to this promising plasma catalytic system. The main outlines of the fundamentals which governed the plasma catalyst interaction are also discussed. This review is divided in two sections. In the first part, the concept of continuous treatment and sequential treatment is discussed and then compared. In the second part an overview of the literature dealing with only sequential treatment is given. And finally, the influence of critical process parameters on this new technique is summarized. In order to improve the performance efficiency of a plasma catalytic reactor, one has to understand the effect of the various parameters and choose an appropriate catalytic material depending on the type of the pollutants. Thus, one of the main purpose of this work is to identify the influence of various parameters on the CVOCs removal efficiency and screening for various suitable catalytic materials. In a first experimental chapter, TCE abatement was investigated in dry air with NTP using a 10-pin-to-plate negative DC corona discharge and CeO2 catalyst placed in downstream. As expected, NTP showed poor COx selectivity despite having a high abatement efficiency due to the formation of oxygenated intermediates such as phosgene, DCAC and TCAA, when operated alone. On the other hand, no activity for TCE oxidation over the CeO2 catalyst was observed when solely operating in the examined temperature. In comparison to the total catalytic oxidation and NTP process, PPC was found to be the best process to convert TCE into CO2 with all tested catalyst temperature. A clear synergy was observed in terms of TCE abatement and mineralization. The role of ozone in the plasma catalytic process was investigated and the synergistic reaction of O3 and catalysts was found to be the key point in the process. Additionally, it was found that with the assistance of NTP, CeO2 (at lower catalyst temperature) is only activated enough to selectively react with hazardous polychlorinated by-products (which need less energy to oxidise than TCE) to form the desired product (CO2). Furthermore, a successful long term test (40 h) proved that the combination of plasma with a CeO2 catalyst possesses excellent stability in terms of TCE abatement. These results evident that this plasma catalysis route shows great potential as air pollution control technology for low concentrated VOC air streams. The final part of this work describes the preparation, characterization and application of cryptomelane type manganese oxides placed downstream from a NTP reactor in the oxidation of TCE. Concerning catalyst preparation, it was observed that it is possible to tailor the shape, crystalline phase and chemical composition of manganese oxide materials by controlling the synthesis conditions, such as through the mode of doping metal (Fe) incorporation. The addition of iron to cryptomelane was performed by 2 different ways: (i) by a co-precipitation (Fe-K-OMS-2: goal is to incorporate Fe in K-OMS-2 structure instead of K or Mn) (ii) by impregnation (Fe/K-OMS-2: goal is to deposit FexOy particles at the surface of K-OMS-2). A reference cryptomelane (K-OMS-2) was also synthesized by a refluxing method at ambient. It was found that regardless of the synthesis route, all three catalysts in PPC configuration outperformed the NTP alone in total TCE oxidation. The catalysts can be ranked by increasing TCE conversion is as follows: K-OMS2 < Fe/K-OMS-2 < Fe-K-OMS-2. The superior performance of the Fe-KOMS-2 materials was attributed to improved textural properties such as high specific surface areas, amorphous state and structural disorder (presence of oxygen vacancies). These characteristics allow facilitating the production of active species from plasma generated ozone and the surface oxygen mobility to promote the degradation of TCE or reaction intermediates into CO2