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Abstract

An improved method is described for reducing the nitrogen oxide content of an effluent gas stream wherein the temperature of the gas varies. The improvement comprises alternately, depending upon the temperature of the gas, contacting the gas with (A) formaldehyde when the gas is at an elevated temperature which is sufficient to promote the reaction of formaldehyde with the nitrogen oxide in the gas and to reduce the nitrogen oxide content in the gas to a desired level, or (B) formaldehyde-derived free radicals capable of reacting with nitrogen oxide when the temperature of the gas is lower than the temperature in (A) and the reaction with formaldehyde does not reduce the nitrogen oxide content to the desired level, whereby the nitrogen oxide content of the gas is reduced to the desired level. The free radicals may be generated by reacting formaldehyde with nitrogen or nitrogen oxide in the presence of a catalyst. Preferred catalysts comprise zirconium, cerium, phosphorus, vanadium, or mixtures thereof. Catalysts comprising zirconium in the plus four oxidation state, cerium in the plus four oxidation state, or mixtures thereof are particularly preferred catalysts for the reaction of formaldehyde with nitrogen oxide. In another embodiment, free radicals are obtained by reacting formaldehyde with nitrogen in the presence of a catalyst comprising vanadium or zirconium or a mixture thereof.

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First Claim