A process for reducing nitrogen oxides in an exhaust stream such as an exhaust stream from a mobile combustor, such as a vehicle exhaust stream or an exhaust stream from a stationary combustor, such as a utility boiler or diesel generator. The process comprises providing a first reservoir having a first composition therein, the first composition comprising an aqueous solution of urea and water; transferring a portion of the first composition along a flow path in communication with a second reservoir; heating the portion of the first composition to produce a mixture comprising ammonia, carbon dioxide and water, the heating taking place in the flow path or in the second reservoir; cooling the mixture to generate a second composition, the second composition comprising an aqueous solution of ammonium carbamate and the cooling taking place in the flow path or in the second reservoir; and introducing the second composition into the an exhaust stream (10) comprising nitrogen oxides when the exhaust stream has a temp
