Comparative analysis on characteristics in non-thermal plasma reactor with oxygen and air

Diesel engines have been favored in heavy trucks and passenger cars due to their low fuel consumption, high power output and durability. However, particulate matter (PM) and nitrogen oxide (NOx) emitted from diesel bring about deterioration of air pollution, which has an adverse impact on human health. Stricter emission regulations have been implemented worldwide. As a solution, diesel particular filter (DPF) has become a mainstay in PM control. However, there are some problems with DPF regeneration technologies, such as thermal damage, sulfur poisoning of the catalyst and low regeneration efficiency. So it is meaningful to find out a new regeneration method. Recently, non-thermal plasma (NTP) has become a research focus in the field of diesel emission control with its high efficiency, safety, no secondary pollution and a wide range of application. The active materials, mainly including O3, NO2, OH and O, can start complex chemical reactions, which is impossible in a conventional condition. So, it can be used to remove PM deposits in DPF and realize DPF regeneration. In term of NTP reactor, dielectric barrier discharge is widely used in the laboratory and industry for its simple type, safety and reliability. There are many influence factors concerning discharge, such as discharge voltage and frequency, gas type and flow, materials of barrier and electrode type. In this paper, a coaxial type NTP reactor was designed. In order to have a detailed recognition of NTP reactor, comparative analysis on oxygen and air dielectric discharge were investigated, with the studies on the influence of discharge electrode area (SE), peak-peak voltage (Up-p) and volume flow rate (qv) on discharge power (P), charge flux (Q), ozone concentration, ozone output and ozone output efficiency. SE was changed by the length of wire tightly wrapped around the barrier, Up-p was adjusted by a plasma source and qv was controlled by gas valves and flow meters. The results indicated that SE had a similar effect both on oxygen and air dielectric discharge. With the increase of SE, P and Q had a linear growth but there were lower values and growth rate in air discharge. Ozone concentration increased as SE increased while its output efficiency decreased both in oxygen and air discharge. Up-p had remarkably positive impacts on P and Q, both of which had a rising growth rate. Ozone concentration rose at first and decreased afterwards with increasing Up-p while ozone output efficiency declined, which indicated a contradiction between them. Under the different discharge frequency, the maximum ozone mass concentration in oxygen discharge was larger than 55 mg/L while the maximum ozone mass concentration in oxygen discharge was between 4-8 mg/L. Excessive increase in Up-p was useless after the maximum ozone concentration point. In addition, under the same Up-p, ozone concentration increased with growing discharge frequency in oxygen discharge but it showed an opposite trend in air discharge. Ozone output efficiency was higher than 100 g/kW·h when Up-p was lower than 17.5 kV in oxygen discharge and it maintained at 20 g/kW?h when Up-p was lower than 17 kV in air discharge. Oxygen and air discharge differed when qv changed. P and Q showed a sharp climb and then kept steady with the growth of qv in oxygen discharge while they kept growing in air discharge. Ozone concentration fell with increasing qv and its output grew at first and then flattened in oxygen discharge, but they increased to the highest, and then descended, and its output grew with a smaller rate in air discharge. The same was that their output efficiency rose slowly with increasing qv. In consideration of the trade-off between ozone concentration, ozone output and output efficiency, qv should better not exceed 10 L/min when the gas source was oxygen and 9 L/min could be chosen when the gas source was air. This study founds a theoretical and experimental basis for optimization of non-thermal plasma injection system and its application on diesel particular filter regeneration.