Study On Methane-carbon Dioxide Conversion By Gliding Arc Discharge Plasma Coupling Ni Catalyst

Carbon dioxide（CO₂）and methane（CH₄）are the two most important greenhouse gases,accounting for more than 90%of the total global greenhouse gas emissions.It is of great significance to recycle them for solving the environmental problems caused by greenhouse gases.The conversion of CO₂ and CH₄ into CO and H₂（i.e.,CH₄-CO₂ reforming reaction）is an important CH₄ and CO₂ conversion method to generate industrial raw material syngas while reducing two greenhouse gases.Gliding arc discharge（GAD）plasma is a kind of discharge plasma between thermal plasma and non-thermal plasma with high electron density and low gas temperature.It has the advantages of high energy efficiency,mild reaction conditions and fast start-up and shutdown for CH₄-CO₂ reforming reaction.GAD coupling catalyst is an effective method to improve the energy efficiency of plasma reactions and regulate product distribution.In this paper,single-support nickel catalysts and composite support nickel catalysts were prepared.The research of CH₄-CO₂ conversion by GAD plasma coupling with nickel catalysts were carried out.The process of GAD,CH₄ and CO₂ reforming performance and reaction mechanism,as well as the energy efficiency were studied.The influence of steam addition on CH₄-CO₂ reforming reaction were exploered to optimize the product distribution,and inhibit the carbon deposition on catalysts.The specific conclusions are as follows:（1）The discharge characteristics,arc motion characteristics and refroming performance of GAD plasma were analyzed under CH₄-CO₂ atmosphere.The discharge voltage-current waveform and arc motion images showed that a gliding period included arc generation,stretching and extinction.The discharge tends to be stable when the arc stretched smoothly and each gliding period are similar in time scale.Optical emission spectrum diagnosis showed that CH₄-CO₂ GAD possessed high electron density and OH rotational temperature.At the condition of power about 50 W,gas flow rate 1 L/min,the electron density was 1.08×10¹⁶ cm^-3,and OH rotational temperature was about 3000 K.Through the CH₄ and CO₂ reforming experiment on discharge voltage,inlet gas flow,the mole ratio of CH₄/CO₂ and the distance between the electrode and electrode thickness and length of the electrode is optimized,the discharge voltage10.5 k V,gas flow rate 0.6 L/min,CH₄/CO₂=1/1.5,minimum electrode gap 2 mm,electrode thickness 2 mm,electrode length is 4 cm,CH₄ and CO₂ reforming effect optimal,conversion rate were 55.8%and 48.5%respectively.The main products were H₂ and CO.Their selectivities reached 65.7%and 71.4%,respectively.Energy efficiency（EE）was 1.65 mmol/k J,and energy conversion efficiency（ECE）was 36.7%.（2）Four Ni-based catalysts Ni/BaTiO₃,Ni/TiO₂,Ni/ZrO₂ and Ni/Al₂O₃ were prepared,for investigating CH₄-CO₂ reforming by GAD plasma coupling with single-support nickel catalysts.When the catalyst was placed downstream of the arc and the minimum distance between catalyst and electrode end was 5 mm,all of the four catalysts can be activated by GAD plasma and enhanced CH₄ and CO₂ conversions.The catalytic activity was ordered as Ni/ZrO₂>Ni/BaTiO₃>Ni/TiO₂>Ni/Al₂O₃,and the catalyst stability sequence was Ni/ZrO₂>Ni/BaTiO₃>Ni/TiO₂≈Ni/Al₂O₃ under.Shortening the distance between catalyst and arc would be beneficial to conversion,but showed negative effect on discharge stability.When loaded with Ni/BaTiO₃,the GAD could maintain stable at the catalyst-electrode end distance of 2 mm.And the conversion of CH₄ and CO₂ reached the maximum values of 60.7%and 53.7%,respectively,which were 12.7%and 17.6%higher than those of GAD alone.H₂ and CO selectivities were67.9%and 73.2%,respectively.EE and ECE were 1.89mmol/k J and 39.8%,respectively.（3）Composite-support nickel catalysts Ni/BaTiO₃-ZrO₂ were prepared.Compared with Ni/BaTiO₃,the specific surface area of Ni/BaTiO₃-ZrO₂ were increased by 60～80%,and the particle size of Ni was reduced by 47%.The content of ZrO₂ affected the catalytic activity of Ni/BaTiO₃-ZrO₂.Ni/BaTiO₃-ZrO₂ with ZrO₂ content of 30 wt.%（Ni/BTZ-3）got the highest catalytic activity and best stability.In the experiment of CH₄-CO₂ reforming by gliding arc discharge coupling with Ni/BTZ-3,the conversion of CH₄ and CO₂ reached 65.6%and 57.0%,respectively,which were about 12%higher than those with Ni/BaTiO₃.EE and ECE reached2.02 mmol/k J and 44.1%.After 10 h of experiment,Ni/BTZ-3 still showed high catalytic activity.The conversion of CH₄ and CO₂ stabilized at 63.9%and 54.8%.The carbon deposition on the catalyst surface decreased by 8.4%compared with Ni/BaTiO₃。（4）The effect of steam on GAD plasma and its coupling Ni/BTZ-3 catalyst for CH₄-CO₂reforming was studied.Optical emission spectrum diagnosis results showed that steam increased the number of OH radical and H atoms in plasma area.The CH₄-CO₂ reforming results showed that when the steam to carbon ratio（S/C）increased from 0 to 2.3,the selectivity of H₂ and CO increased by 23%and 11%,the carbon balance increased,and the yield of acetic acid and formic acid in the liquid phase products increased by 2 times and 5 times,respectively.When S/C=0.58,the GAD plasma can operate stably at CH₄/CO₂=3/1,and the H₂/CO ratio ranges from 0～1.2 in the absence of steam to 0～2.9.After 10 h reaction,the measured carbon content on the surface of Ni/BTZ-3 was 25%lower than that of Ni/BTZ-3 used under no steam condition.The addition of steam effectively extended the service life of the catalyst.