Method for removing nitrogen oxides in waste gas or flue gas

The invention relates to a method for removing nitrogen oxides in waste gas or flue gas, comprising the following steps of: connecting an absorbing tower in series on a waste gas or flue gas conveyingpipeline at the front of an exhaust pipe, adding NO2 at the front of the absorbing tower and introducing the NO2 into flue gas or waste gas flow for mixing after being metered, then using a removingagent to treat the mixed gas by absorbing and removing of the absorbing tower with the removing agent and then discharging the mixed gas from the exhaust pipe; and adopting the following technical scheme: connecting the absorbing tower in series on the waste gas or flue gas conveying pipeline, connecting a nitrogen dioxide cylinder laterally at the front of the tower to directly add NO2 into the waste gas or flue gas conveying pipeline or connect a catalytic converter and an oxidizer laterally at the front of the absorbing tower, oxidizing NH3 released by a liquid ammonia cylinder into NO2 which is then mixed in the fl