Simultaneous removal of SO₂ and NO_x from flue gas by wet scrubbing using a urea solution

ABSTRACT

Nitrogen oxides (NO_x) and sulfur dioxide (SO₂) are major air pollutants, so simultaneously removing them from gases emitted during fossil fuel combustion in stationary systems is important. Wet denitrification using urea is used for a wide range of systems. Additives have strong effects on wet denitrification using urea, and different mechanisms are involved and different effects found using different additives. In this study, the effects of different additives, initial urea concentrations, reaction temperatures, initial pH values, gas flow rates, and reaction times on the simultaneous desulfurization and denitrification efficiencies achieved using wet denitrification using urea were studied in single factor experiment. The optimum reaction conditions for desulfurization and denitrification were found. Desulfurization and denitrification efficiencies of 97.5% and 96.3%, respectively, were achieved at a KMnO₄ concentration 5 mmol/L, a reaction temperature of 70°C, initial urea solution pH 8, a urea concentration of 9%, and a gas flow rate of 40 L/h. The concentrations of the desulfurization and denitrification reaction products in the solution were determined. NO_x was mainly transformed into N₂, and the ${\text{NO}}_3^{-}$ and ${\text{NO}}_2^{-}$ concentrations in the solution became very low. The reactions involved in SO₂ and NO_x removal using urea were analyzed from the thermodynamic viewpoint. Increasing the temperature was not conducive to the reactions but increased the rate constant, so an optimum temperature was determined. The simultaneous desulfurization and denitrification kinetics were calculated. The urea consumption and ${\text{NO}}_2^{-}$, ${\text{NO}}_3^{-}$, and ${\text{SO}}_4^{2-}$ generation reactions were all zero order. The ${\text{NO}}_3^{-}$ generation rate was greater than the ${\text{NO}}_2^{-}$ generation rate. The simultaneous desulfurization and denitrification process and mechanism were studied. The results provide reference data for performing flue gas desulfurization and denitrification in factories.

KEYWORDS:

• Urea
• additive
• wet scrubbing
• simultaneous desulfurization and denitrification
• thermodynamic calculation
• kinetics

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The authors gratefully acknowledge support from the National High Technology Research and Development Program (‘863’ program) of China [grant number 2012AA06A115] and the China Postdoctoral Science Foundation [grant number 2017M610723].