Amato, Luigi (2021) NOx ADSORPTION AND NTP DESORPTION-REDUCTION. [Tesi di dottorato]

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NOx emissions have multi-fold hazards for both environment and human health, due to their role in the formation of fine particles, ozone smog, acid rain and eutrophication. The increasing level of emissions and the improved comprehension of NOx toxicity have led to a progressive reduction of allowed emissions from stationary and mobile anthropic sources. NOx emissions can be controlled by altering the combustion process (e.g. Selective Non-Catalytic Reduction) or by post-combustion treatment of the flue gases (e.g. Selective Catalytic Reduction). These two technologies are the most commonly used for industrial processes in a wide range of applications. SCR has a higher capital cost than SNCR but allows a superior reduction of NOx concentration and it is the preferred technique in many industrial applications when high-quality standards are required. Due to the recent stricter European regulations for NOx emissions, SNCR is almost inadequate for the new standards, and cost-effective and alternative methods to SCR are under investigation. The NOx adsorption and non-thermal plasma desorption/reduction strategy was proposed and it is investigated in this thesis. The process consists of the adsorption of NOx on a sorbent material followed by the sorbent regeneration using non-thermal plasma in N2 flushing. The presence of the plasma has the double effect of promoting the NOx desorption from the sorbent surface and reducing the NOx desorbed to N2 and O2. A further reduction of NOx can be performed in a DBD plasma reactor. The desorption and reduction are promoted by the high reactive nitrogen gas discharge particles that react with NOx. A granular activated carbon (GAC) was investigated for NOx adsorption from simulated flue gases. The adsorption capacity for NOx was negligible (less than 4mg/g) if a mixture of NO diluted in N2 was considered; however, it was possible to calculate the adsorption isotherms at 30 and 120°C. A great improvement in the adsorption capacity was calculated as oxygen was fed (up to 66mg/g). The tests demonstrated that part of the NO was catalytically oxidized to NO2 in the presence of oxygen, with a consequent improvement in NOx adsorption. Even the adsorbent impregnation with copper (GAC-Cu) played a positive role in the NO oxidation, hence improving the capacity to remove NOx from the gas stream. Dielectric Barrier Discharge reactors demonstrated a high efficiency (up to 100%) in NOx reduction. The configurations with one and two dielectrics were investigated, highlighting the high performance of the one dielectric barrier discharge reactor and the high flexibility of the two dielectric barriers discharge. A numerical analysis was performed to model the plasma condition promoted in the realistic reactors. The results in terms of electric parameters were consistent with those estimated in the gas discharge, e.g. electric field, electron density and electron temperature. Cyclic tests of adsorption and sorbent regeneration plasma-assisted demonstrated the effectiveness of the process by using GAC-Cu for NOx removal. The amount of NOx adsorbed during the adsorption stage was desorbed and converted during the desorption plasma-assisted process. Higher effectiveness (≈100%) was estimated in the case of GAC-Cu of commercial size (1.5-3mm), while the effectiveness decreased in the case of lower granular size (0.3-0.6mm). The possibility to perform the process at low temperature makes this technology suitable for those gas treatment plants that require a retrofit to accomplish the new regulations. The gas would be treated with the designed reactor to obtain further NOx reduction, without risks of ammonia slip that represents the main problem of high-efficiency conventional technologies.

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