| With the developing and increasing application of hydrogen and hydrogen fuel cells, the development of high-efficient hydrogen production system for small-scale distributed site (e.g., hydrogen refueling station) or portable site (on-board hydrogen production) is becoming more and more urgently required. Traditional catalytic method for hydrogen generation has a lot of drawbacks, such as high capical costs, large equipment size, complex process, and rapid loss of catalyst activity. Plasma-assisted hydrogen production technology, which is highly efficient, simple, flexible, independent of catalyst and allows a rapid start-up and shutdown of the process, provides an attractive alternative to the conventional catalytic route, especially for some small-and medium-scale sites. In this work, a novel rotating gliding arc (RGA) warm plasma codriven by tangential flow and magnetic field was developed to overcome the problems that are encountered in a traditional flat gliding arc plasma (e.g., limited plasma area, inhomogeneous distribution of reactive species, low retention time, small processing capacity). A detailed investigation of the physical characteristics of the RGA plasma was performed, and then the RGA plasma was used for hydrogen production from methane/methanol reforming, providing experimental guidance and theoretical basis for the potential industrial application. The contents and conclusions of this dissertation are listed as below.(1) Characterization of the physical characteristics of the RGA plasma. A high-speed camera, oscilloscope, and optical emission spectroscopy (OES) were employed to investigate the motion behavior, V-I characteristics, intermediate species distribution, and spectroscopic property of the RGA plasma. The results have shown that the arc can rotate rapidly around the inner electrode with a frequency of up to 100 rotations per second, generating a stable "plasma disc" area. The plasma area and the retention time of the reactant in plasma area are both significantly enhanced compared to the traditional flat gliding arc, improving the reactor efficiency. Two motion modes of the arc can be observed with increasing flow rate. At a relatively low flow rate, the arc anchors near the tip of the inner electrode and rotates rapidly with a long arc length. The electron temperature, electron density, and gas temperature of the RGA plasma are 1～2 eV,1013～1015 cm-3, and 1600-2850K respectively, indicating that the RGA belongs to the so called "warm" plasma, which could provides a high reaction efficiency, while maintaining a considerable processing capacity.(2) Hydrogen production from methane decomposition was performed in the RGA plasma and the effects of feed flow rate, load resistance, reactant compositon, and the type of carrier gas on the plasma reaction performance were investigated. Increasing flow rate can lead to a decrease in both the methane conversion and power consumption for hydrogen production. With increasing CH4/N2 ratio, the methane conversion with a 40 kΩ resistance showed a continuous decrease, whereas for a 70 kΩ resistance the methane conversion first increased and then decreased. The maximum methane conversion could be up to 87.5%, while maintaining a processing capacity (feed flow rate) of 2-3 magnitudes higher than traditional non-thermal plasmas, which definitely benefits the potential industry applications. In addition, the high value-added C2H2 can be obtained simultaneously, with a power consumption of 21.8～114.1 kWh/kg, which is just a little higher than that of the industrial arc discharge method (10～80 kWh/kg).(3) OES analysis and kinetics modeling study were performed to give insight into the reaction mechanisms of the RGA assisted methane decomposition process in N2. The RGA CH4/N2 spectra are dominated by the CN(B2∑-X2∑), C2 (d2∏g'a2∏u),and CH (B2∑->X2∏) spectral lines. The modeling study showed that H atom plays a key role in the conversion of methane and the contribution of the H involved reaction:CH4+H'CH3+H2 is up to>90%. H2 forms primarily from the combination of H atoms with CH4 or C2H4, and in which the reaction:CH4+H' CH3+H2 dominates in the generation of H2 with a contribution of higher than 85.9%. The CH3 radicals play a key role in the formation of C2H6, C2H4 and C2H2。(4) Methanol decomposition process was conducted in the RGA reactor for hydrogen production. The effects of different operation parameters on the plasma reaction performance were investigated. A mathematical model was established and sensitivity analysis was performed to get insight into the relative importances of different parameters for this process. The results have shown that increasing flow rate or input methanol concentration significantly decreased the methanol conversion, whereas the increase of applied voltage or operating current facilitated the decomposition of methanol. The most important parameters for the methanol decomposition process are methanol concentration, discharge power, and feed flow rate. The RGA assisted methanol decomposition process shows significant advantages in comparison to other non-thermal plasmas. A maximum methanol conversion of 95.6% could be obtained. The processing capacity is enhanced by several magnitudes while maintaining a higher reaction efficiency. Compared with methane, the methanol decomposition process has advantages in terms of energy efficiency and operation stability of the system.(5) OES analysis was used for understanding the reaction methanisms of the RGA assisted methanol decomposition process. The vibrational temperature of the RGA CH3OH/N2 plasma was shown to be up to 8930(±1300)～14300(±800) K, which is much higher than that of typical non-thermal plasmas, indicating that the vibrationally excited species (e.g., N2(X, v)) probably substantially contributes to the decomposition of methanol.(6) The CO2 reforming (dry reforming) of methanol, which can produce syngas and convert CO2 to CO in one step, was investigated for the first time using the RGA reactor. The conversions of methanol and CO2 are mutually facilitated and the maximum methanol and CO2 conversion are 64.4% and 18.6%, respectively. The reverse water gas shift reaction can significantly contribute to the conversion of CO2. The reaction efficiency for the conversion of CO2 could be up to 34.0%-62.4%, which is much higher than that in traditional non-thermal plasmas (0.95%～19.33%), while maintaining a CO2 flow rate of 1-3 magnitudes higher. The RGA assisted methanol dry reforming process provides a promising method for high-efficient CO2 conversion. |
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