## 3 Results and Discussion
### The Influence of Power Frequency on NOx Removal Efficiency  
According to the ionization mechanism of Townsend, when a high voltage alternating current is applied between two electrodes, the strong electric field will accelerate the collision between the electrons and the neutral gas particles near the electrodes [21]. The variation of the power supply frequency will change the collision frequency (referring to the non-isotropic collision frequency affected by the applied electric field) and the free stroke of the electrons. Specifically, when the power frequency is very low, the collision frequency of the high-energy electrons with the neutral particles is too low for the electrons to be ionized. When the frequency is too high, the free path of the electrons is shortened and the energy obtained by the electrons is less, which will also increase the difficulty \begin{table}
\begin{tabular}{c c} \hline \hline
**KCL Solution Concentration (mol/L)** & **Electrical Conductivity (ms/cm)** \\ \hline
0.001 & 0.25 \\
0.01 & 1.37 \\
0.1 & 10.00 \\
1.0 & 72.80 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrical conductivity of the KCL solution at different concentrations (20 °C). of ionization. Therefore, a discharge frequency test was performed on the DBD reactor before the experiment. The results showed that the plasma reactor did not discharge when the power supply frequency was lower than 6 kHz. Since in DBD discharge, subsequent discharges are more likely to occur than initial discharges due to wall charges (residual charges) attached to the dielectric barrier. Therefore, in the subsequent experiments, the measured voltages are all continuous discharge voltages, which are the plasma driving voltages. However, when the power frequency exceeded 10 kHz, the sustaining voltage at the start of discharge was so high that the subsequent experimental voltage could easily overtake the maximum value of the plasma power supply. Based on this phenomenon, four power frequencies of 7 kHz, 8 kHz, 9 kHz, and 10 kHz were selected for the follow-up experiments. As displayed in Figure 4, the sustaining voltage of the first discharge in each case is the very first point of each curve in Figure 4: the sustaining voltage during the first discharge is 30 V at 7 kHz, 50 V at 8 kHz, 110 V at 9 kHz, and 220 V at 10 kHz. Moreover, as the frequency increases, the sustaining voltage at which discharge starts is greater. Furthermore, under the same power frequency, the energy density (SED) increased almost linearly as the voltage increased. The influence of power frequency on NO removal efficiency is exhibited in Figure 5a, where the concentration of KCL solution is 1.0 mol/L. It can be seen that when the discharge frequency is lower than 10 kHz, the final NO removal efficiency can reach 100%. Moreover, the NO removal efficiency increases with the energy density. When the input energy  
Figure 4: The variation of DBD discharge density with discharge voltage under different discharge frequencies. Figure 3: DBD plasma reactor electrical parameter curve. density is the same, the overall NO removal efficiency is the best at the discharge frequency of 8 kHz. Figure 5b shows the profiles of the variation of NOx removal efficiency with power frequency and energy density. On the one hand, the NOx removal efficiency at each power frequency increases continuously with the energy density. On the other hand, when the energy density is kept constant, the NOx removal efficiency presents a characteristic of first increasing and then decreasing as the power frequency increases. This can be explained by the fact that at the discharge frequency of 7 kHz, the collision frequency of high-energy electrons and neutral particles is very low, which makes it difficult for the gas molecules to ionize. Therefore, the reaction rate of the main reaction occurring in the DBD reactor is slower than that when the discharge frequency is 8 kHz, and the removal efficiency is lower.