## 3 Results and Discussion
### Mass Fraction of NaOH Solution  
The effects of the mass fraction of the NaOH solution on desulfurization and denitration efficiency levels are shown in Figure 8. When the mass fraction of the NaOH solution increases from 0% to 1.0%, the desulfurization efficiency increases from 97.2% to 100% and the denitration efficiency increases from 51.0% to 71.8%. With a further increase in the mass fraction of the NaOH solution to 1.5%, the desulfurization efficiency is maintained at 100% and the denitration efficiency slowly increases to 72.6%.  
In the absorption process of NO\({}_{x}\) and SO\({}_{2}\) from the gas phase to the liquid phase, the total mass transfer resistance is the sum of the gas phase mass transfer resistance and liquid phase mass transfer resistance [10,32]:  
\[\frac{1}{K_{y}}=\frac{1}{k_{y}}+\frac{m}{k_{x}}, \tag{8}\]  
where \(\frac{1}{K_{y}}\) is the total mass transfer resistance; \(\frac{1}{k_{y}}\) is the gas phase mass transfer resistance; \(\frac{m}{k_{x}}\) is the liquid phase mass transfer resistance; \(m\) is the phase equilibrium constant.  
When the absorption capacity of the scrubbing solution is weak, the phase equilibrium constant (\(m\)) is large and the liquid phase mass transfer resistance \(m/k_{x}\gg 1/k_{y}\). Under this condition, the mass transfer resistance is mainly concentrated in the liquid film:  
\[\frac{1}{K_{y}}\approx\frac{m}{k_{x}}. \tag{9}\]  
When the scrubbing solution has strong absorption capacity for solutes in the gas phase, \(m\) is small and the liquid phase mass transfer resistance \(m/k_{x}\ll 1/k_{y}\). Under this condition, the mass transfer resistance is mainly concentrated in the liquid phase:  
\[\frac{1}{K_{y}}\approx\frac{1}{k_{y}}. \tag{10}\]  
The addition of NaOH in the scrubbing solution makes the desulfurization and denitration process change from physical absorption to chemical absorption. The chemical absorption involves a faster absorption rate. NaOH improves the selectivity and solubility of the scrubbing solution to NO\({}_{x}\) and SO\({}_{2}\). With the increase in the NaOH mass fraction in the scrubbing solution, \(m\) gradually decreases and the liquid phase mass transfer resistance  
Figure 8: The desulfurization and denitration efficiency levels at various mass fractions of the NaOH solution (operating conditions: discharge power = 19.5 W, the gas flow rate = 5 L/min, spray density = 7.6 m\({}^{3}\)/(m\({}^{2}\)-h), packing type = D\({}_{8}\) 6 mm Dixon ring, packing height = 80 cm).  
\(m/k_{x}\) decreases. When the mass fraction of the NaOH solution exceeds 1.0%, the mass transfer resistance is mainly concentrated in the gas phase; continuing to increase the mass fraction of NaOH solution has little effect on the whole mass transfer process [33].  
Table 3 shows the pH values of various mass fractions of NaOH solutions. It can be seen that the pH value of the NaOH solution increases rapidly in the range of 0-1%, which greatly promotes the chemical absorption of NO\({}_{x}\) and SO\({}_{2}\) in the liquid phase and reduces the mass transfer resistance in the liquid film. In the range of 1.0-1.5%, the further increase in the NaOH solution mass fraction has little effect on its pH value and the growth of the denitration efficiency slows down significantly.