A process and a plant for producing ammonia, where an ASU (3) delivers an oxygen stream and a nitrogen stream; the oxygen stream (9) is fed to the secondary reformer of a front-end reforming section (1); the nitrogen stream (10) is used to wash a purge gas or tail gas taken from the synthesis loop (2), preferably in a cryogenic section; a methane-free and inert-free gas stream is recovered and recycled to the synthesis loop (2) or at the suction of the main syngas compressor, to recover the hydrogen contained therein. A corresponding method for increasing the capacity of an ammonia plant, by providing the ASU and feeding the oxygen stream to the secondary reformer and the nitrogen stream to a suitable purge gas recovery unit.