Study On Discharge Mode And CO₂ Reduction Reaction With H₂ In Magnetically Driven Gliding Arc Plasma

Gliding arc plasma is an atmospheric non-thermal plasma with high energy efficiency,and has emerged in recent years to exhibit broad application prospects in the fields of environment and energy.However,the fundamental understanding of gliding arc plasma is currently very lacking,especially for aiming at the reaction system for environment and energy applications.In this thesis,a model reactor of magnetically driven gliding arc powered by a constant-current DC power supply is designed,and its discharge modes and CO₂ reduction with H₂ are studied.The main research content and results are as follows:1.In the magnetically driven gliding arc plasma,three discharge modes are observed:（1）in ModeⅠ,the cathode arc spot rotates ahead of the anode arc spot,and the arc channel is bent forward;（2）in ModeⅡ,the cathode arc spot and the anode arc spot are synchronal,and the arc channel is straight;（3）in ModeⅢ,the cathode arc spot falls behind the anode arc spot,and the arc channel is bent backward.The discharge mode depends on the magnetic field,independent of the discharge current.When the discharge current is constant,the arc length in ModeⅡis the shortest and its discharge power is the lowest.In addition,the rotational frequency of the arc channel in ModeⅡis the highest,and the gas treatment performance is the best.Therefore,ModeⅡis the optimal mode for the magnetically driven gliding arc.Combining the optical diagnosis and the simulation of the magnetic field,the current I of the gliding arc and the magnetic flux density B are obtained.A mechanical model based on torque balance of arc rotation is established,and the rotational frequency of the arc channel is calculated.The calculated and measured arc rotational frequencies agree well in all three modes.Furthermore,the criterion of the optimal mode of the magnetically driven gliding arc is proposed.2.Using the optimal mode of gliding arc,the experimental study of CO₂ reduction with H₂ in plasma is carried out.and the influence of parameters such as H₂/CO₂ molar ratio on the reaction performance,arc channel diameter,rotation frequency and optical emission characteristics are investigated.When H₂/CO₂ molar ratio increases from 0 to 4,CO₂conversion rises significantly from 6.6%to 48.9%.The rate of CO produced varies with the H₂/CO₂ molar ratio in a volcano curve peaking at H₂/CO₂=1,which is more than 10 times that of pure CO₂（H₂/CO₂=0）.The arc channel presents obvious characteristics of glow discharge.As the H₂/CO₂ molar ratio increases,the diameter of the arc channel decreases from 1.3 mm for pure CO₂ to 0.6 mm for pure H₂,and the rotational frequency increases from 22 Hz for pure CO₂ to 65 Hz for pure H₂.By the Stark broadening method of the H_βline,the electron density is on the order of 10¹⁴cm^-3.According to the fit of of CO（B-A）band,the gas temperature of the arc channel is about2200-3600 K.When the H₂/CO₂ molar ratio is very low,strong emission of O atom lines and CO-O chemluminescence are observed.At H₂/CO₂?1,the O atom line and the CO-O chemluminescence nearly disappear,and the CO₂ conversion increases significantly.3.In view of the characteristics relevant to the model reactor of magnetically driven gliding arc,based on the Boltzmann equation and the mass conservation equation,a zero-dimensional plasma chemical kinetic model including molecular vibrational dynamics is established,to disclose the mechanism of CO₂ reduction with H₂ in gliding arc plasma.A good agreement between the simulated and experimental results for excited species and stable products verifies the reliability of the model.By solving the Boltzmann equation,it shows that,under the experimental conditions,the electron temperature of the gliding arc plasma is about 1 e V,and hardly changes with the H₂/CO₂ molar ratio.With such electron temperatures,the mechanism of electron energy loss is dominated by the vibrational excitation of molecules.Via efficient vibrational excitation of CO₂ molecules,the population of high vibrational levels of CO₂ increases and CO₂ dissociation is speeded up.The simulation results of the time-dependent changes in number densities of the main species show that,in pure CO₂,although a large amount of CO₂ is dissociated to CO in the plasma zone,most of the CO is oxidized to CO₂ after leaving the plasma zone,leading to the very low CO₂ conversion.However,in the discharge gas with high H₂/CO₂ molar ratios,the abundant H₂ greatly reduces the number densities of O atoms and O₂ molecules,then significantly s the reaction of CO oxidation to CO₂ is remarkably inhibited,resulting in the greatly enhanced conversion of CO₂.