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Title: Non-thermal plasma technology for nitric oxide removal
Author: Zhou, Linghe
ISNI:       0000 0004 7226 3862
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2018
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Non-thermal plasma, as a potential nitric oxide (NO) removal technology, has been researched for more than one decade. The advantage of direct non-thermal plasma treatment is that it is able to generate reactive species from the existing components in the flue gas without additional catalyst, oxidant or reductant, so any NO removal system based upon this technology is simple and easy to operate. However, the energy efficiency of non-thermal plasma technology is lower than the most commonly used selective catalytic reduction system for NO removal. In order to understand the possible reasons, it is important to investigate the mechanism of NO removal by direct non-thermal plasma treatment. Two of the most commonly used non-thermal plasma sources, dielectric barrier discharge (DBD) and corona discharge, are investigated. The most important reactive species include oxygen atom (O), ozone (O3) and hydroxyl radical (OH). Different reactive species lead to different chemical reaction pathways for NO removal. Under different NO concentration and discharge configurations, the dominant reactive species was found to change from one to another. For dielectric barrier discharge, when the initial NO concentration was higher than 420 ppm under dry condition, it was found that O was the dominant reactive species for NO oxidation and NO oxidation was independent on O2 concentration. When initial NO concentration was lower than 100 ppm under dry condition, it was found that O3 was the dominant reactive species and NO oxidation was dependent on O2 concentration. When NO concentration was in the range of 120 ppm to 190 ppm, there was a synergistic effect of O and O3 on NO oxidation. NO removal depended on the initial NO concentration. However, no matter what the initial NO concentration was, the NO removal energy efficiency was lower than 25g/kWh. When water vapour (H2O) was introduced into the gas mixture, reactive species OH was generated and provided an alternative chemical reaction pathway for NO removal. When initial NO concentration was 1000 ppm, NO removal was in the range of 150 ppm to 200 ppm, but the energy efficiency was in the range of 7 to 12 g/kWh. With an increase of temperature in DBD reactor, the effect of OH on NO removal was promoted. To further investigate the OH effect, a novel pin to water corona discharge configuration was used. The effect of discharge modes from Trichel pulse, pulseless and arc discharge was investigated. Under arc discharge mode, 200 ppm NO was generated at 6W discharge power. Under Trichel and pulseless discharge modes, NO removal increased with increasing discharge power. When initial NO concentration was 1000 ppm, the highest NO removal achieved was 715 ppm with 5.5 g/kWh energy efficiency. In addition, it was found that the energy efficiency did not reduce with increasing discharge power. In order to increase the possibility of chemical reaction between NO and reactive species, higher initial NO concentration was used. To obtain higher NO concentration a process of NO absorption by activated carbon and thermal desorption was used. This increased the NO concentration from 1000 ppm up to 6%. It is found that at 6% level, NO could be partially oxidized by oxygen molecule (O2) and higher O2 concentration would obtain higher NO oxidation rate. Direct non-thermal plasma treatment can be used for NO removal but the energy efficiency (less than 30g/kWh) is too low to compete with the mature technologies including selective catalytic reduction (SCR) and low temperature oxidation (LoTOx) whose energy efficiencies are higher than 60 g/kWh. Although the energy efficiency is not improved in this research, the mechanism and chemical reaction pathways of NO removal are quantitatively analysed under different initial NO concentration levels by two different non-thermal plasma technologies (DBD and corona discharge). The dominant reactive species for NO removal can shift from O, O3 to OH. In addition, a novel technology which is a combination of non-thermal plasma, NO absorption and desorption processes is developed in this research. It offers a new mechanism for NO removal, because increasing the concentration of NO from ppm level to a few percentages creates a regime where NO removal can be effectively done by O2 rather than strong oxidants like O and O3. As the formation of O and O3 is more expensive than that of O2, this is a promising research direction for NO removal. However, based on the investigation in this research, some challenges are found. One is the poor selection between NO and H2O for activated carbon and the other one is high energy consumption for the desorption process.
Supervisor: Wang, Tao Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral