Simultaneous removal of gaseous NOx and SO2 by gas-phase oxidation with ozone and wet scrubbing with sodium hydroxide

Abstract

In conventional coal-fired power plants, selective catalytic reduction (SCR) is used for NOx removal and wet scrubbing is used for SO2 removal. Because of the high operational cost of SCR, researchers have applied wet scrubbing for De-NOX, without SCR. Herein, NO was injected in a lab-scale reactor and oxidized to NO2 by reacting with ozone. Subsequently, wet scrubbing with NaOH solution was conducted in a lab-scale packed-bed scrubber. By varying the concentrations of ozone and SO2, which was also injected in the oxidation reactor, the NO and NO2 concentrations were measured using a gas analyzer. Ion chromatography, X-ray diffraction, and Fourier-transform infrared spectroscopy were used to characterize the scrubbing products. The effects of temperature and pH of NaOH on the NOx and SO2 removal efficiencies are discussed.In the absence of SO2, the injection of ozone at 60% with respect to the NO concentration resulted in the maximum NOx removal efficiency. This was attributed to the formation of trivalent N species (N2O3 and HNO2) and their fast absorption in NaOH. In the presence of 1000 ppm SO2, the ozone concentration needed to be increased to 90% of the NO concentration to maximize the NOx removal efficiency. Higher SO2 concentration led to greater NOx removal efficiency because the SO2 scrubbing product, Na2SO3, enhanced NO2 scrubbing. Higher pH and lower temperature increased the NOx removal efficiency, whereas SO2 was not as sensitive to temperature and pH.