Process for reducing nitrogen oxides [patent application]

A process for reducing nitrogen oxides in an exhaust stream, such as a vehicle exhaust stream, and apparatus for carrying out the process. The process comprises providing a first composition comprising urea and water, a second composition comprising ammonium carbamate and an exhaust stream comprising nitrogen oxides. The second composition is introduced into the exhaust stream (10) when the exhaust stream has a temperature below a threshold temperature and the first composition is introduced into the exhaust stream when the exhaust stream has a temperature at or above the threshold temperature. A process for producing ammonium carbamate is also provided.