Characteristics Diagnosis Of Atmospheric Nanosecond Pulsed Surface DBD And Investigation Of Application In CH₄/CO₂ Reforming

Atmospheric nanosecond pulsed surface dielectric barrier discharge（SDBD）can generate diffuse and non-equilibrium plasma at low temperature with high electron energy,which has wide applications.However,the development process of discharge,space-time resolved characteristics and energy relaxation of atmospheric nanosecond pulsed SDBD still need further investigation.The mechanism of reaction,energy transition and energy dissipation of reforming CH₄/CO₂ by discharge plasma are still essential to study.In this paper,a tube-based SDBD structure and expanding to a large-area discharge have been given for generating stable and diffuse discharge plasma excited by nanosecond pulse power in atmospheric air and nitrogen.The characteristics of discharge plasma,time-space evolution,and energy relaxation process have been diagnosed.Besides,the generated nanosecond pulsed SDBD plasma is also applied to CH₄/CO₂ reforming for producing H₂/CO syngas at low-temperature.The main research results are described as follows:1.Stable and diffuse SDBD plasma under tube-based annular structure is generated by nanosecond pulse power in atmospheric air and nitrogen.Waveforms of discharge voltage and current have been measured,and optical emission spectra（OES）have been diagnosed for investigating the characteristics of excited SDBD plasma.The rotational and vibrational temperatures are simulated by N₂（C-B,?ν=-2）.The time-space resolved characteristics of nanosecond pulsed SDBD plasma in atmospheric nitrogen are investigated.The results show that width of main discharge current and photocurrent are both about 60 ns during both positive and negative pulses.The OES intensities increase when the pulse peak voltage rises.When applied voltage and discharge time vary,gas temperatures of discharge plasma keep at room temperature around.The space-time resolved OES intensities of N₂（C³Π_u→B³Π_g）and N₂⁺（B²Σ_u⁺→X²Σ_g⁺）of nanosecond pulsed SDBD plasma in atmospheric nitrogen have different behaviors,which may be caused by the threshold voltage of excited states and the distribution of electron energy.The electron-to-vibrational energy relaxation happens during the diagnosis period.Spatially,with the direction from high-voltage to ground electrode,vibrational temperatures increase around the high-voltage electrode then become oscillatory,and finally decrease.Meanwhile,the rotational temperatures nearly keep constant.Combining the space-resolved behaviors of OES intensities of N₂（C³Π_u→B³Π_g）and N₂⁺（B²Σ_u⁺→X²Σ_g⁺）,and vibrational temperatures,it can be deduced that the electrons with high energy exist around the middle of discharge gap.2.An array wire-to-wire SDBD structure is used for generating diffuse and stable plasma with large-area by nanosecond pulse power in atmospheric air,the discharge area of which can be adjusted by varying the quantity of electrode group.The discharge development of single wire-to-wire SDBD is captured by ICCD camera.For investigating the electrical and optical characteristics of discharge plasma,the discharge voltage and current are measured,the OES are diagnosed,and the discharge images are captured.The discharge power,discharge area and OES intensity under single and multiple electrode groups are calculated.The rotational and vibrational temperatures are simulated by N₂（C-B,?ν=-2）.The effects of the quantity of electrode group and pulse peak voltage are investigated.The results show that during rising edge of pulse the discharge of single wire-to-wire SDBD has a shorter duration compared with that of decaying edge,but has higher discharge intensity and larger discharge area.The discharge current has two main peaks during single pulse and becomes to zero in an oscillatory mode.When the quantity of electrode group increases,the peak value of discharge current increases and the oscillation time prolongs.As the single wire-to-wire structure forms into array discharge,discharge area and OES intensity of the whole array and the single wire-to-wire structure show different trends when the quantity of electrode group increases.For the whole array of discharge,discharge area and OES intensity increase when the quantity of electrode group rises.However,these parameters both decrease for single wire-to-wire structure.The decrease of discharge area of single wire-to-wire structure may be caused by the discharge repulsion and the decrease of discharge power.The decrease of OES intensity may be caused by the decreases of discharge area and discharge power.When the quantity of electrode group rises,the rotational temperatures nearly keep around 330 K.As the pulse peak voltage rises,the OES intensity and discharge area increases for chain extemnsion of electron avalanche and increase of luminous range;The rotational temperatures increase and meanwhile the vibrational temperatures decrease.3.Spiral wire-to-wire SDBD structure is used for generating stable and diffuse discharge plasma by nanosecond pulse power and applied in reforming CH₄/CO₂ producing H₂/CO syngas at low temperature.Electrical and optical characteristics of nanosecond pulsed SDBD plasma generated in atmospheric CH₄/CO₂ are investigated for analyzing micro-processes of the reforming reactions.The effects of pulse voltage and pulse frequency on the convention rates of reactants,and selectivities of H₂ and CO are evaluated.The energy efficiency and energy conversion efficiency of syngas are also investigated.Nanosecond pulsed discharge in atmospheric nitrogen is used for 10%Cu/γ-Al₂O₃ and 10%Cu/ZSM5 preparation based on equal-volume impregnation method,and the reforming results are compared with those of traditional calcine method.The results show that the main reactive species of nanosecond pulsed spiral wire-to-wire SDBD plasma are CH（A）,CH（B）,CH（C）,CO（B）,CO（b）and CO₂⁺（A）.As the pulse voltage or pulse frequency rise,the conversion rates of reactants increase.However,when the pulse voltage increases,the selectivity of H₂ first increases and then decreases,and the energy efficiency and energy conversion efficiency of syngas decrease.The increase of pulse frequency dose not cause the decreases of selectivities of H₂ and CO,and the energy efficiency and energy conversion efficiency of syngas nearly keep constant as pulse frequency increases.As the 10%Cu/γ-Al₂O₃ and 10%Cu/ZSM5 are calcined for 3 hours in nitrogen discharge plasma,the selectivities of H₂ and CO are both higher than those calcined in nitrogen without discharge plasma,which means diffuse discharge plasma can regulate and control the productions of CH₄/CO₂ reforming by participating in catalyst preparation.Besides,the catalyst and the atmospheric nanosecond pulsed SDBD plasma can have an effective synergistic effect on the reforming reaction.