Nitrogen oxides reducing agent and a phosphate

A process for reducing the concentration of NO.sub.x in an oxygen-rich gas stream by selective noncatalytic reduction at temperatures above 950.degree. C., comprising:

• injecting into the gas stream containing NO.sub.x an aqueous solution comprising both a nitrogenous NO.sub.x -reducing agent and at least 1.5% of an inorganic acid phosphate, wherein the NO.sub.x -reducing agent comprises an NH-containing composition comprising a member selected from the group consisting of ammonia, urea, carbamates, ammonium carbonate, ammonium bicarbonate, cyanurate, ammonium salts of organic acids and mixtures of these is introduced at a molar ratio of nitrogen in said NO.sub.x -reducing agent to the nitrogen as NO.sub.x in the gas stream of from about 0.8 to about 5, and the phosphate comprises a member selected from the group consisting of ammonium phosphates, calcium phosphates, and mixtures thereof with sufficient water solubility to permit their introduction in aqueous solution, and is introduced at a molar ratio of nitrogen in the NO.sub.x -reducing agent to phosphorous of from about 50:1 to about 1:2, as a spray of droplets having an average droplet size of from about 5 to about 10,000 microns, containing NO.sub.x to contact with the NO.sub.x in the gas phase and thereby reduce said NO.sub.x.