Control of nitrogen oxide emissions by hydrogen peroxide-enhanced gas-phase oxidation of nitric oxide

Nitrogen oxides (NOx) and sulfur oxides (SOx) are criteria air pollutants, emitted in large quantities from fossil-fueled electric power plants. Emissions of SOx are currently being reduced significantly in many places by wet scrubbing of the exhaust or flue gases, but most of the NOx in the flue gases is NOx which is so insoluble that it is virtually impossible to scrub. Consequently, NOx control is mostly achieved by using combustion modifications to limit the formation of NOx, or by using chemical reduction techniques to reduce NOx to N-2. Low NOx burners are relatively inexpensive but can only achieve about 50% reduction in NOx emissions; selective catalytic reduction (SCR) can achieve high reductions but is very expensive. The removal of NOx in wet scrubbers could be greatly enhanced by gas-phase oxidation of the NO to NO2, HNO2, and HNO3 (the acid gases are much more soluble in water than NO). This oxidation is accomplished by injecting liquid hydrogen peroxide into the flue gas; the H2O2 vaporizes and dissociates into hydroxyl radicals. The active OH radicals then oxidize the NO and NO2. This NOx control technique might prove economically feasible at power plants with existing SO2 scrubbers. The higher chemical costs for H2O2 would be balanced by the investment cost savings, compared with an alternative such as SCR. The oxidation of NOx by using hydrogen peroxide has been demonstrated in a laboratory quartz tube reactor. NO conversions of 97% and 75% were achieved at hydrogen peroxide/NO mole ratios of 2.6 and 1.6, respectively. The reactor conditions (500 degrees C, a pressure of one atmosphere, and 0.7 seconds residence time) are representative of flue gas conditions for a variety of combustion sources. The oxidized NOx species were removed by caustic water scrubbing.