In old ammonia plants the purge gas of the synthesis loop after recovering of the ammonia content in a flash drum is vented; whereas, in new ammonia plants there are the subunits to recover the NH3 and H2 content of the purge gas and then use the retentive gas as a fuel in furnaces. The installation of a purge gas recovery unit not only will increase ammonia production capacity but also it will be beneficial and will save considerable amount of Natural Gas (NG) (as a common fuel) and decreases CO2 emissions compared with the original older ammonia plant design. Hereby in this article, the advantages of the retentive gas using as a fuel gas has been considered and economical and environmental benefits earned by CO2 emissions reduction also have been calculated.