An apparatus for producing hydrogen using a retrofitted steam methane reformer (SMR) via ammonia cracking is provided. The apparatus can include: means for providing a pressurized and gaseous ammonia stream to a plurality of reactor tubes; a furnace having the plurality of reactor tubes and a plurality of burners, wherein the furnace is configured to catalytically crack the ammonia within the reactor tubes to produce a crude process gas and a flue gas; a plurality of waste heat recovery sections; and a pressure swing adsorption (PSA) unit disposed downstream the furnace, wherein the PSA unit is configured to receive the crude process gas, or a gas derived therefrom, and produce a hydrogen product stream and a PSA offgas.