Abstract: A method of removing nitrogen oxides from an exhaust which comprises the steps of contacting an exhaust containing nitrogen oxides and ammonia with a reducing catalyst received in a reactor to reduce the nitrogen oxides to nitrogen gas, wherein iron ore catalyst lumps having a particle size of 25 mm or less and forming a catalyst bed in the reactor are used as a reducing catalyst; the iron ore lumps of the catalyst bed are let to fall gravitationally through the reactor continuously or intermittently; and the iron ore catalyst lumps discharged from the reactor are either used as raw material for iron manufacture or fed back to the reactor to be reused as a reducing catalyst after being sieved.

Abstract: NO.sub.x gas remaining in a purified exhaust, which has been desulfurized, in accordance with a publicly known method using coke oven gas, by washing the original exhaust containing SO.sub.2 and NO.sub.x with a water solution mainly consisting of ammonium sulfite concentrated to about 30 percent by weight, the pH value and the temperature thereof being adjusted to 6.1 to 6.5 and 50.degree. to 55.degree.C respectively, can be reformed into harmless N.sub.2 by passing said desulfurized exhaust through a reducing catalyst at a temperaure of 200.degree. to 450.degree.C, wherein ammonia gas naturally brought into said desulfurized exhaust from the washing solution acts as a reducing agent.