Study On The Pulsed DBD Plasma Characteristics And It's Application On Dry Reforming Of Methane

With the industrial development and social progress,energy consumption has increased dramatically around the world.The massive burning of fossil energy has led to huge carbon dioxide emissions and significant greenhouse effect.Meanwhile,the natural gas,is abundant but relatively expensive to transport and store over long distances.Thus,methane is considered to be converted in situ into high value chemical products.Dry reforming of methane?DRM?can convert two greenhouse gases into syngas,which can serve as the raw materials for the fine chemicals,such as the methanol production and Fischer-Tropsch synthesis.The traditional DRM is mainly catalyzed by high temperature and high pressure,and there are many problems such as high requirements on equipment,high energy consumption,and complicated operation.A large number of high-energy active particles?electrons,excited ions and molecules,radicals,etc.?generated by non-thermal plasma?NTP?can activate two chemically stable gases at room temperature and atmospheric pressure.NTP catalyzed DRM technology is much favored by researchers.In this paper,an exploratory study on the application of plasma on DRM has been carried out in the literature.In view of the current research status of low conversion rate and low energy efficiency,this paper proposes the application of pulsed discharge plasma on DRM based on the rapid development of pulse power technology.In this paper,the atmospheric pressure microsecond and nanosecond pulse dielectric barrier discharge?DBD?plasma experimental systems were built respectively.The discharge characteristics,optical characteristics and conversion characteristics of atmospheric pressure pulsed DBD were studied by parameterization experiments.The discharge process and reaction process under different parameters were analyzed.The emission spectrum diagnostic technique is used to detected the active species produced in the nanosecond pulsed DBD plasma,and the reaction path is speculated and summarized in combination with the captured intermediate products.The discharge experimental results show that in a single pulse both the microsecond and the nanosecond pulsed discharges have two discharge processes,named as primary discharge and secondary discharge,respectively.Each discharge process includes pre-breakdown,breakdown and extinction.Both kinds of pulsed DBDs are typical filamentary discharges,and the discharge intensity is affected by the variation of discharge parameters.Among them,the secondary discharge of the microsecond pulsed discharge is obviously weaker than the primary discharge,and the pulse energy is mainly concentrated in the primary discharge process.When the pulse width is increased,the second discharge is enhanced.The energy distribution in the single pulse period of the nanosecond pulse is affected by the waveform?rise time,fall time and pulse peak width?.The shorter the rise time and the fall time,the stronger the initial electric field in the discharge space,and the higher the instantaneous power of the discharge.At the same time,increasing the pulse repetition frequency and peak voltage will enhance the discharge and obtain a higher energy injection.The experimental conversion results show that the nanosecond pulsed DBD can obtain higher energy conversion efficiency than the conventional AC DBD.This is because the shorter rise time of the pulse can quickly ionize the gas,so that the electrons are accelerated in an ultra-short time,while the heavy particles obtain less energy,thus the energy utilization rate is higher in the pulsed discharge.The conversion results of two kinds of pulsed DBD plasma show that the main gaseous products are H₂,CO and C₂H₆,accompanied by a small amount of C₃H₈ and C₄H₁₀.The conversion of CH₄ and CO₂ increases with the increasing energy injection during discharge,but at the same time,the increase in liquid product and high carbon hydrocarbons due to the deep oxidation of methane leads to a decrease in the selectivity of the main product and the balance of carbon and hydrogen.In the nanosecond pulsed discharge,when the repetition frequency is10 kHz,the conversion of CH₄ and CO₂ are as high as 42.5%and 26.5%,respectively,and the corresponding energy conversion efficiency is 6.8%.Besides,in the nanosecond pulsed discharge,higher instantaneous discharge power and lower total discharge power can be obtained by shortening the rise time or the fall time.A larger voltage change rate can provide a higher initial electric field,producing instantaneous high-density electron,thus both better conversion and energy conversion efficiencies can be obtained simultaneously.The results of optical emission spectroscopy show that H,CH,C₂,CO,CO₂⁺and O radicals are generated during the nanosecond pulsed DBD process,which verified the generation of some major products.The vibrational and rotational temperature of the CH are obtained by fitting the emission line of CH?A²??X²?,0-0,431.2 nm?and the results are6500±1000 K and 547±10 K,respectively.The large difference between the two temperatures indicates that the internal imbalance of the plasma is more significant.The electron density of the nanosecond pulsed DBD plasma obtained by fitting H?emission line is as high as 6.75×10²¹ m^-3,which is significantly higher than that in the traditional AC DBD plasma.This is due to short rise time of nanosecond pulse,on the other hand,is due to the tip of the threaded electrode to enhance the local electric field.