## 3 Results and Discussion
### Spray Density  
The spray density is composed of the scrubbing solution spray per unit time and unit tower cross-sectional area, which affects the wetting degree of packings. The effects of the spray density on the desulfurization and denitration efficiency levels are shown in Figure 7. The experimental results show that when the spray density increases from 1.8 m\({}^{3}\)/(m\({}^{2}\cdot\)h) to 7.6 m\({}^{3}\)/(m\({}^{2}\cdot\)h), the desulfurization efficiency of the system increases from 99.1% to 100% and the denitration efficiency increases from 64.8% to 70.8%. In the range of 7.6-10.8 m\({}^{3}\)/(m\({}^{2}\cdot\)h), the desulfurization and denitration efficiency levels remain basically unchanged. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Diameter (mm) \(\times\) Height (mm)** & **Specific Surface Area (m\({}^{2}\)/m\({}^{3}\))** \\ \hline D\({}_{\text{g}}\) 10 mm Pall ring & 10 \(\times\) 10 & 482 \\ D\({}_{\text{g}}\) 6 mm Pall ring & 6 \(\times\) 6 & 905 \\ D\({}_{\text{g}}\) 6 mm Dixon ring & 6 \(\times\) 6 & 950 \\ D\({}_{\text{g}}\) 6 mm Cannon ring & 6 \(\times\) 6 & 910 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Packing parameters. Figure 6: Effect of packing height on desulfurization and denitration efficiency of scrubber (operating conditions: discharge power = 19.5 W; gas flow rate = 5 L/min, scrubbing solution = 1% NaOH solution, spray density = 7.6 m\({}^{3}\)/(m\({}^{2}\cdot\)h), packing type = D\({}_{\text{g}}\) 6 mm Dixon ring). For the packed scrubber using the NaOH solution, when the liquid/gas ratio reaches 2 L/m\({}^{3}\), the desulfurization efficiency reaches more than 99% [33]. In this experiment, the spray density of 1.8 m\({}^{3}\)/(m\({}^{2}\cdot\)h) means that the liquid/gas ratio reaches 3 L/m\({}^{3}\); therefore, further increases in spray density have little effect on the desulfurization efficiency. The increases in spray density have a greater impact on denitration efficiency, indicating that the demand for gas-liquid mass transfer area for NO\({}_{x}\) removal is higher. On the one hand, increases in spray density will mean the packings will be wetter and the actual gas-liquid mass transfer area will be larger; on the other hand, if the spray density is too high, the liquid film on the packing surface will thicken and flooding will occur, resulting in a sharp pressure drop in the scrubber and a sharp decrease in the gas-liquid mass transfer efficiency [34]. The flooding point is the operating limit of packed scrubber, while the flooding velocity is generally calculated using the Eckert general association diagram [32]. The abscissa can be calculated with the following formula:  
\[x=\frac{G_{L}}{G_{V}}(\frac{\rho_{V}}{\rho_{L}})^{0.5}, \tag{6}\]  
where \(G_{L}\) is the mass flow rate of scrubbing solution in kg/(m\({}^{2}\cdot\)s); \(G_{V}\) is the mass flow rate of the gas in kg/(m\({}^{2}\cdot\)s); \(\rho_{L}\) is the density of the scrubbing solution in kg/m\({}^{3}\); \(\rho_{V}\) is the density of the gas in kg/m\({}^{3}\). After the abscissa is calculated, the ordinate \(y\) value is obtained according to the flooding point line. The flooding velocity can be calculated according to the following formula:  
\[y=\frac{u^{2}\psi\phi}{g}\times\frac{\rho_{V}}{\rho_{L}}\mu_{L}{}^{0.2}, \tag{7}\]  
where \(u\) is the gas flow rate of the empty tower in m/s; \(\psi\) is the ratio of the density of the water to the density of the scrubbing solution; \(\phi\) is the packing factor in 1/m; \(g\) is the gravitational acceleration in 9.8 m/s\({}^{2}\); \(\mu_{L}\) is the viscosity of the scrubbing solution in mPa\(\cdot\)s.  
The calculation results show that when the spray density increases from 1.8 m\({}^{3}\)/(m\({}^{2}\cdot\)h) to 10.8 m\({}^{3}\)/(m\({}^{2}\cdot\)h), the flooding velocity decreases from 0.88 m/s to 0.48 m/s.