A Selective Catalytic Reduction system comprising an SCR reactor, an inlet flow system, and a vaporizer module, wherein: a. the SCR reactor comprises at least one SCR catalyst and is in communication with the inlet flow system and the vaporizer module; b. the inlet flow system comprises one or more entrances for exhaust gases from an engine, wherein the entrance is configured to distribute the flow of exhaust gases through at least one flow duct (11, 12) located around the SCR reactor, wherein the flow of exhaust gases through the flow ducts (11, 12) around the SCR provides heat to the vaporizer module in which urea, or an ammonia precursor, is introduced into the vaporizer module to be converted to ammonia prior to being introduced into the SCR reactor, and the flow of exhaust gases in each of the flow ducts (11, 12) is introduced into the SCR reactor after the introduction of an ammonia-laden gas stream and after passing the exhaust gas containing the ammonia-laden gas stream through a plurality of baffle plates to provide an approximately uniform gas velocity and ammonia concentration profile across the cross section of the SCR reactor; and c. a vaporizer module comprising a means for introducing urea, or the ammonia precursor, into a pre-reactor comprising a portion of the vaporizer module so that the pre-reactor is heated by a stream of cleaned exhaust gas and in which the urea, or the ammonia precursor, is at least partially decomposed and subsequently fed into an exhaust gas stream prior to introducing the exhaust gas stream to the SCR reactor, a means for removing a portion of the clean exhaust gas exiting the SCR reactor, a means for collecting the portion of the clean exhaust gas under pressure, a means for feeding the pressurized clean exhaust gas into a vaporizer tube and a means for injecting urea, or the ammonia precursor, into the clean exhaust gas in the vaporizer tube, wherein the vaporizer tube is located in at least one of the flow ducts (11, 12) and wherein water and urea, or the ammonia precursor, are vaporized and urea, or the ammonia precursor, is decomposed into ammonia inside the vaporizer tube. a. the SCR reactor comprises at least one SCR catalyst and is in communication with the inlet flow system and the vaporizer module; b. the inlet flow system comprises one or more entrances for exhaust gases from an engine, wherein the entrance is configured to distribute the flow of exhaust gases through at least one flow duct (11, 12) located around the SCR reactor, wherein the flow of exhaust gases through the flow ducts (11, 12) around the SCR provides heat to the vaporizer module in which urea, or an ammonia precursor, is introduced into the vaporizer module to be converted to ammonia prior to being introduced into the SCR reactor, and the flow of exhaust gases in each of the flow ducts (11, 12) is introduced into the SCR reactor after the introduction of an ammonia-laden gas stream and after passing the exhaust gas containing the ammonia-laden gas stream through a plurality of baffle plates to provide an approximately uniform gas velocity and ammonia concentration profile across the cross section of the SCR reactor; and c. a vaporizer module comprising a means for introducing urea, or the ammonia precursor, into a pre-reactor comprising a portion of the vaporizer module so that the pre-reactor is heated by a stream of cleaned exhaust gas and in which the urea, or the ammonia precursor, is at least partially decomposed and subsequently fed into an exhaust gas stream prior to introducing the exhaust gas stream to the SCR reactor, a means for removing a portion of the clean exhaust gas exiting the SCR reactor, a means for collecting the portion of the clean exhaust gas under pressure, a means for feeding the pressurized clean exhaust gas into a vaporizer tube and a means for injecting urea, or the ammonia precursor, into the clean exhaust gas in the vaporizer tube, wherein the vaporizer tube is located in at least one of the flow ducts (11, 12) and wherein water and urea, or the ammonia precursor, are vaporized and urea, or the ammonia precursor, is decomposed into ammonia inside the vaporizer tube.