Reaction Pathways Study On CO₂-CH₄ Reforming By Non-Thermal Plasma For Syngas Production

Non-thermal plasma（NTP）reforming of CO₂-CH₄ to syngas and high-value products is an emerging technological pathway to realize the resource utilization of two greenhouse gases.Due to the complex physical and chemical reaction processes inside the NTP,the reaction pathway of NTP reforming of CO₂-CH₄ to syngas with different feeding gas composition,gas residence time and operation temperature has been thoroughly investigated with comprehensive employing in-situ optical emission spectroscopy（OES）measurements and methods based on non-isothermal continuous low discharge plasma reaction kinetics analysis and on-line detection of reforming products in this thesis.Decoupling of thermal effects and gas residence time from electric field in NTP enhanced dry reforming process would also be initially investigated in this work.The major studies and findings of this thesis are as follows.Experimental research of CO₂-CH₄ reforming by atmospheric dielectric barrier discharge（DBD）plasma was conducted to study the effects of different feeding gas composition,gas residence time and operation temperature on feeding gas conversion performance,energy efficiency and distribution of products.It was shown that increasing the feeding CH₄ ratio improved the production of C₂ and C₃ hydrocarbons and produced more high-carbon hydrocarbons,while the percentage of room temperature condensable components in the product also increased.With a constant specific energy input（SEI）,increasing gas residence time led to larger promotions in CH₄ and CO₂ conversion,as being compared to increase of input power.The CH₄ and CO₂ molecules experienced more successive micro-discharges with increasing gas residence time,so more energy would be devoted to CH₄/CO₂ conversion;while more energy was dissipated as heat when the discharge power was roughly increased due to the limit of reaction time.A CH₄ conversion as high as 44.76%could be achieved without catalyst at an operation condition of T=300°C,SEI=78 J/cm³ and X_CH4,in=50%,which was attributed to the effective synergistic effect of electric and thermal fields,while CO₂ conversion continuously decreased with increasing gas temperature due to the high gas temperature promoting CO and O complexation.OES was employed to diagnose in-situ the intermediate active particles in the NTP CO₂-CH₄ reforming process,and the trend of the intensity of the characteristic particle bands with the feeding CH₄ ratio and the operation temperature was investigated.The results indicated that the main leap spectral bands in the CO₂-CH₄-N₂ plasma spectroscopy diagram were CH（B²Σ→X²Π）,CH（A²Δ→X²Π）,CO（B¹Σ→A¹Π）,C₂Swan band（d²Π→a³Π）and CN Violet（B²Σ→X²Σ）.The relative intensities of the spectral bands induced by the de-excitation of CH,CO and C₂ increased monotonically with the increase of the feeding CH₄ ratio.The peak intensity of the CH（A²Δ→X²Π）spectral band was significantly enhanced when the operation temperature was increased,which was mutually verified with the experimental phenomenon of CH₄ conversion promoted at higher gas temperatures.Based on the kinetic calculations of the NTP reforming CO₂-CH₄ reaction,the effects of pulse discharge reaction behaviors on the transient evolution processes of reactant molecules,intermediate active particles and steady-state products were investigated,and the pathways of key species generation and consumption were thoroughly explored.The reaction pathway analysis showed that CH₄ conversion was mainly induced by electron impact dissociation and ionization in the discharge stage,while CO₂ conversion was mostly converted by the recombination reaction CH₂+CO₂→CH₂O+CO in the afterglowing stage and the electron impact vibration reaction e+CO₂→e+CO₂（vn）.The continuous pulsed discharge effectively promotes the conversion of CO₂ and CH₄ as well as the production of CO and H₂ due to the accumulation effects.The H₂ production was derived from the collisional dissociation of high-energy electrons with CH₄ and the reaction of N₂（a’）with CH₄.The reaction of CH₂ with CO₂ and the reaction of N₂（A）with CO₂ are the main pathways for the CO formation.53.25%of C₂H₆ was generated via the CH₃ coupling reaction,while the recombination reaction of CH₃ with C₂H₃ contributed 78.8%to the C₃H₆ formation.99.85%of CH₃OH was derived from the compound reaction of CH₃ with OH.CH₃radicals plays a crucial role in the production of C₂ and C₃ hydrocarbons.The formation of C₂ hydrocarbons products was supposed to follow the reaction path of CH₄（?）CH₃/CH→C₂H₆（?）C₂H₅（?）C₂H₄→C₂H₂.