## 1 Introduction  
Ocean transportation undertakes more than 90% of the global freight transportation which has the advantages of low cost and high security [1]. Furthermore, ships generally use diesel engines as their main propulsion devices. Sometimes, high-sulfur oil is used as the alternative fuel in marine diesel engines for lower shipping costs. As a result, their exhaust emissions cause serious air pollutions [2; 3; 4]. To control the exhaust pollution of marine diesel engine, the International Maritime Organization (IMO) regulated the emission of NOx and SOX from marine diesel engine exhaust, as shown in Figure 1[5; 6]. For SOX removal, wet scrubbing can achieve a higher removal efficiency (more than 95%), while, for NOx removal, wet scrubbing has a low removal efficiency. This because more than 90% of NOx in the exhaust gas of marine diesel engine is NO, and NO has low solubility in water and does not react with alkali solution. Some studies have shown that NO\({}_{2}\) can promote the absorption of NO by alkaline solution. When the oxidation degree of NOx is increased to ~50%, the absorption efficiency of NOx by alkaline solution is the highest [7]. The technology of wet desulfurization and denitrification is easy to integrate with existing desulfurization scrubber, reducing the investment and space, which will be an important development trend in the future [4]. Therefore, non-thermal plasma (NTP) oxidation combined with wet scrubbing for simultaneous desulfurization and denitrification was widely studied [8; 9; 10]. Compared to electron beam (EB) and corona discharge (CD), dielectric barrier discharge (DBD) has higher NO removal efficiency at lower specific energy density (SED) and shorter residence time [11]. There are many types of DBD, such as volume dielectric barrier discharge (VDBD), surface dielectric barrier discharge (SDBD) and diffuse coplanar surface barrier discharge (DCSBD) [12; 13]. SDBD and DCSBD were widely used for surface modification. The VDBD reactor was widely used for the treatment of NO\({}_{\text{X}}\) and SO\({}_{\text{X}}\). The structure parameters of DBD reactor have great impact on NO removal efficiency and energy consumption, which has become a research hotspot [14; 15; 16; 17; 18; 19]. In 2011 and 2012, [PERSON] al. studied the effect of inner electrode shape on discharge of DBD reactor. They showed that the DBD reactor with multineedle-to-cylinder electrode had better discharge performance than the regular DBD reactor [14; 15]. In 2012 and 2013, [PERSON] et al. used the mixture of NO and N\({}_{2}\) to simulate the exhaust gas. The effects of electrode shape, electrode material and barrier medium thickness on NO removal were studied. They showed that tungsten electrode has a higher NO removal efficiency than copper and stainless-steel electrode with the same SED, which has a higher secondary electron emission coefficient. Screw electrode has a higher NO removal efficiency than rod electrodes, due to the smaller air gap capacitance, which makes the driving power of DBD reactor lower and dielectric loss lower [16; 17]. In 2015 and 2016, [PERSON] and [PERSON] et al. used the mixture of NO and N\({}_{2}\) as the simulated diesel exhaust. The effects of electrode length, electrode material and electrode diameter on NO removal were studied [18; 19]. Although the structure of DBD reactor has been studied in many previous investigations, most denitrification was carried out under the condition of NO\({}_{\text{X}}\)/N\({}_{2}\). In such circumstance, NO\({}_{\text{X}}\) is mainly removed by reduction reaction. However, the real exhaust gas contains a large amount of O\({}_{2}\), which makes the reduction effect of NO\({}_{\text{X}}\) by collision with N radicals ignorable. NO\({}_{\text{X}}\) mainly reacts with O, OH, HO\({}_{2}\) and other radicals produced by O\({}_{2}\) and H\({}_{2}\)O and needs wet scrubber for complete removal [20]. There are few studies on the effect of DBD reactor structure parameters on NO oxidation under oxygen-enriched condition.