Process for treatment of a gas stream containing nitrogen oxides

Claim 1

An oxidation-reduction process for the removal of nitrogen oxides, NO x , contrained in a gas stream additionally containing oxygen, comprising the step of countercurrently oxidizing and absorbing said gas stream in aqueous solution in a gas-liquid contact device, wherein in that section of said gas-liquid contact device where the NO x content in the gas stream is less than about 6000 vpm, said process further comprises the steps of (a) increasing for values of the ratio NO/NO 2 greater than 1, the ratio K, which is the ratio of the liquid retention volume in m 3 to the flow rate of the gas stream in Nm 3 /hour, so that the NO x are preferentially absorbed according to the reaction: NO+NO.sub.2 +H.sub.2 O→2HNO.sub.2 (liquid phase) (b) then incorporationg as oxidizing agents nitric acid and hydrogen peroxide, said hydrogen peroxide being incorporated such that, with reference to FIG. 2 of the drawing, the representative point of hydrogen peroxide concentration (V) by volume relative to the normality (n) of the resultant nitric acid solution containing hydrogen peroxide is situated in zone (B) above curve (C1), showing a threshold value of (N) corresoonding to each value of (V), and above curve (C2) which represents the appearance of NO 2 in the gaseous phase and inducing oxidation to reduce the value of the ratio NO/NO 2 and to restore the conditions of steo (a), wherein in steps (a) and (b), the residence time of the aqueous solution is increased to a value such that said ratio K is greater than 5×10 -4 hour for 6000 vpm; and (c) repeating step (a) at least once.