Research On NO_x Conversion By DBD Non-thermal Plasma

Diesel engine has gradually become the one of the most important choices for vehicles,because of its power advantage,superior fuel economy and lower CO₂,HC and CO emissions.However,NO_x and PM emissions are high and difficult to control,and vehicle emissions regulations are increasingly stringent. Hence removal of harmful emissions from diesel exhaust,especially NO_x, becomes a research hotspot.Non-thermal plasma produced by dielectric barrier discharge is a promising emission control technique,which can simultaneously reduce various pollutants,has no secondary pollution,low energy consumption and other characteristics.In order to design plasma reactor,a variety of factors of dielectric barrier discharge(DBD) under atmospheric pressure at air were researched.Through establishing flat-panel-type dielectric barrier discharge device,the effects of applied voltage and frequency,the discharge gap distance and the thickness of barrier on discharge power,equivalent field,equivalent capacitance,single cycle charge transport and average discharge current were investigated with the q-v Lissajous figure.The experimental results demonstrate that discharge parameters are decided by applied voltage and frequency,the discharge gap distance and the thickness of barrier.Increasing applied voltage and frequency,using thin dielectric barrier and small discharge gap distance can get stronger electric field, larger single cycle charge transport and average discharge current.Taking into account the stability of discharge,discharge gap distance and the thickness of barrier had better be in the range of 1～3mm,excitation voltage is not more than 23kV,and excitation frequency is not more than 14kHz. A non-thermal plasma reactor was made and experiment equipment of post processing system for simulated exhaust of diesel engine was designed.The experiments of NO_x conversion were carried out in the N₂/O₂,N₂/NO,N₂/O₂/NO and N₂/O₂/NO/C₃H₆ systems at atmospheric,for aimed towards understanding the effects of discharge characteristics and diesel engine exhaust composition on NO conversion chemistry in non-thermal Plasma.Study results show that:the energy density has different effect on the NO_x conversion in different systems.Generally, when the energy density is at about 500J/L,both of reduction and oxidation conversion rate of NO have a peak.NO conversion rate is very low in N₂,and leads to significant energy consumption.O₂ concentration has significant influence on the NO conversion.When O₂ concentration increases,the oxidation trend which leads NO to NO₂ becomes stronger,as well as the NO reduction conversion rate decreases.The total NO conversion rate also declines with the oxygen concentration increasing.The existence of C₃H₆ can enhance conversion rate of reduction and oxidation,also can effectively reduce the energy consumption.In a word,NO can be hardly converted into N₂ directly by non-thermal plasma in oxygen enriched atmosphere of diesel engine exhaust,but can be effectively converted to NO₂,which create the conditions for catalytic reduction of NO_x.