分段电极介质阻挡放电CO_(2)重整CH4过程放电特性与反应性能研究

Discharge Characteristics and Reaction Performance of CH_() Reforming with CO_(2) in Dielectric Barrier Discharge with Segmented Electrodes

大气压低温等离子体可在温和条件下进行CO_(2)重整CH4反应,对环境保护和能源供应具有双重意义。介质阻挡放电(DBD)是进行该反应最常用的放电等离子体形式之一,但其工艺过程和反应性能受到反应器结构的显著影响。前期研究发现,分段电极DBD可以调节CO_(2)重整CH4反应过程的反应物转化率、产物分布及能量效率,但是分段电极数量和相邻电极间距对上述性能参数的影响机理尚不清晰。因此,该文设计了具有不同电极数量和不同相邻电极间距的分段电极DBD反应器,并用于CO_(2)重整CH4反应,从电学特性和温度特性的角度研究了不同实验条件下的放电特性,比较分析了对应条件下的CO_(2)重整CH4反应性能。结果表明,分段电极的引入可以增加放电边缘数量以增强边缘效应,且增加分段电极数量和增加相邻电极间距均可延长反应物的有效停留时间,上述因素均有助于提高等离子体CO_(2)重整CH4的反应性能。在施加电压11.0 kV条件下采用4段外电极时可获得最高的CO_(2)转化率(17.7%)和CH4转化率(29.5%),以及最大的CO选择性(36.0%)。而在2段电极结构中,当相邻电极间距为3 cm时,可获得最高的总反应物转化能量效率(0.334 mmol/kJ)。

CH4 reforming with CO_(2) is an effective way to convert the main two greenhouse gases(CO_(2) and CH_(4))into value-added chemicals.However,the traditional methods for this reaction have limitations in terms of operating conditions,reactant conversion and product selectivity,catalyst preparation and activity maintenance,due to its highly endothermic characteristics.Non-thermal plasma,as a novel molecule activation approach,has provided new routes for CH_(4) reforming with CO_(2).Dielectric barrier discharge(DBD)has attracted most attention for this process due to its simple structure,the potential plasma-catalysis synergy,and successful experience in industrial applications.The performance of this process is significantly affected by the reactor structure,and it has been demonstrated that using DBD with segmented electrodes is one approach to adjust the reactant conversion,product distribution and energy efficiency of this process,but the influence mechanism of the number of segmented electrodes and the distance between adjacent electrodes on the above performance parameters is still unclear.To deal with these issues,CH_(4) reforming with CO_(2) has been performed in DBD reactors with segmented electrodes.The discharge characteristics and the reaction performance are investigated in detail under different conditions(e.g.,the number of segmented electrodes and the distance between adjacent electrodes).The DBD reactors are self-designed with quartz tube,stainless-steel rod and mesh,which functions as the dielectric tube,high-voltage electrode and low-voltage electrode,respectively.The low-voltage electrode is grounded via a reference capacitor.In order to investigate their respective influences,the number of segmented electrodes is set as 1,2,3 and 4 when the total length of the low-voltage electrode is fixed at 120 mm;while the distance between adjacent electrodes is set as 10 mm,20 mm,and 30 mm when the number of segmented electrodes is 2.A custom-built AC power source is used to drive the DBD reactors.The applied voltage,the total current and the voltage across the reference capacitor are respectively sampled by a Tektronix high-voltage probe,a Pearson current coil monitor and a Pintech differential probe,and saved using a Tektronix digital oscilloscope.The electrical characteristics are obtained by analyzing the voltage-current wave forms and the corresponding Lissajous figure.The temperature of the outer electrode is measured by a Fotric infrared thermometer.The discharge characteristics are discussed from the perspective of electrical characteristics and temperature characteristics.The reactants and gaseous products are analyzed by a Techcomp gas chromatography(GC).The reaction performance is evaluated by the parameters of reactant conversion,product yield and selectivity,and energy efficiency.The following conclusions can be drawn:(1)Increasing the number of segmented electrodes or extending the distance between adjacent electrodes has little effect on the discharge duration time,but enhances the edge effect,which expands the discharge volume and shows obvious influence on the charge generation and transfer.There exists an optimum number of segmented electrodes that enables the DBD reactor to have the strongest capability of charge generation and transfer.(2)The outer surface of the DBD reactor shows the uniform temperature distribution when using the segmented electrodes,and no local high temperature areas are observed,which is beneficial to the long-term stability of reaction.With the combined effect of edge effect,residence time and charge generation and transfer ability,increasing the number of segmented electrodes or extending the distance between adjacent electrodes enhances the conversion of CO_(2) and CH_(4).(3)Increasing the number of segmented electrodes improves the yield and selectivity of H2 and CO,but has little effect on that of other products.While extending the distance between adjacent electrodes enhances the selectivity of C2H6,but has insignificant effect on that of H_(2),CO,and other hydrocarbons.(4)When the number of segmented electrodes is fixed at 2,extending the distance between adjacent electrodes decreases the discharge power and the capability of charge generation and transfer,but enlarges the discharge volume and increases the residence time of the reactant in the plasma reaction volume,which improve the possibility of collision between reactant molecules and reactive species,consequently promoting the plasma reaction and increasing the energy efficiency.The maximum total energy efficiency for converting reactants 0.334 mmol/kJ was obtained when the electrode spacing was 30 mm in the DBD reactor with 2 segmented external electrodes.