Towards Achieving Better NOx Removal In Discharge Plasma Treatment Of Diesel Engine Exhaust
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In India, the expansion of industries and two-fold increase in motor vehicles over the last decade are posing a serious environmental crisis in the form of urban air pollution. Common pollutants include carbon monoxide, sulfur dioxide, chlorofluorocarbons (CFCs), and nitrogen oxides produced by industry and motor vehicles. Air pollution results from a variety of sources. The natural sources include volcanoes, forest fire, scattering soil, biological decay, lightning strikes, dust storms etc. and man-made sources include thermal power plants, vehicular exhausts, incinerators and various other industrial emissions. More than 60% of the air pollution is contributed by these man-made sources. Amongst the gaseous pollutants, the major concern and a challenging task is to control oxides of nitrogen, commonly referred to as NOx. In case of diesel engines, despite the modification in engine design and improvement in after treatment technologies, large amount of NOx continues is get emitted and attempts to develop new catalyst to reduce NOx have so far been less successful. Further, with the emission standards becoming more stringent, estimates are that NOx and particulate matter emission must be reduced by as much as 90%. In this context, the emergence of electrical discharge plasma technique in combination with the few existing technologies is providing to be economically viable and efficient technology. In this thesis emphasis has been laid on the discharge based non-thermal plasma for NOx removal. NOx from simulated gas mixture and actual diesel engine exhaust has been treated. The thesis mainly addresses the following issues. . • Performance evaluation of pipe-cylinder and wire-cylinder reactor for NOx removal . • Study of effect of plasma assisted adsorbent reactor on NOx removal . • Study of effect of adsorption and plasma based desorption using different adsorbent material and electrode configuration The first chapter provides introduction about the air pollutants and the existing NOx control technologies, a brief history of electric discharge plasma, a detailed literature survey and scope of the work. A detailed experimental setup consisting of voltage sources, gas system (simulated flue gas and diesel exhaust), gas analyzers, adsorbent materials are discussed in the second chapter. In the third chapter, NOx is treated by three different methods and are described in separate parts. In first part we have done a comparative study of NO/NOx removal using two different types of dielectric barrier discharge electrodes: a) wire-cylinder reactor, b) pipe-cylinder reactor. Investigations were first carried out with synthetic gases to obtain the baseline information on the NO/NOx removal with respect to the two geometries studied. Further, experiments were carried out with raw diesel exhaust under loaded condition. A high NOx removal efficiency 90% was observed for pipe-cylinder reactor when compared to that with wire-cylinder reactor, where it was 53.4%. In second part an analysis has been made on discharge plasma coupled with an adsorbent system. The cascaded plasma-adsorbent system may be perceived as a better alternative for the existing adsorbent based abatement system in the industry. During this study the exhaust is sourced from a diesel generator set. It was observed that better NO removal in a plasma reactor can be made possible by achieving higher average fields and subsequent NO2 removal can be improved using an adsorbent system connected in cascade with the plasma system. This part describes the various findings pertaining to these comparative analyses. The third and last part of chapter 3 consists of gas desorption from an adsorbent by non-thermal plasma, which is an alternative to conventional thermal desorption, has been studied in relation to diesel engine exhaust. In this process saturated adsorbent material is regenerated using high energetic electrons and excited molecules produced by non thermal plasma. The last Chapter lists out the major inferences drawn from this study.