A system for vaporizing and optionally decomposing a reagent, such as aqueous ammonia or urea, which is useful for NOx reduction, includes a cyclonic decomposition duct, wherein the duct at its inlet end is connected to an air inlet port and a reagent injection lance. The air inlet port is in a tangential orientation to the central axis of the duct. The system further includes a metering valve for controlling the reagent injection rate. A method for vaporizing and optionally decomposing a reagent includes providing a cyclonic decomposition duct which is connected to an air inlet port and an injection lance, introducing hot gas through the air inlet port in a tangential orientation to the central axis of the duct, injecting the reagent axially through the injection lance into the duct; and adjusting the reagent injection rate through a metering valve.