An SCR system uses aqueous ammonia to convert toxic nitrogen oxides into harmless nitrogen gas and water vapour. The process can be summarised as follows: 1: A urea and water solution is injected into the exhaust stream 2: As the water evaporates, the urea is converted into ammonia 3: The ammonia bonds with the nitrogen dioxides in the exhaust gases 4: A catalytic converter reduces the toxic components in the exhaust gases to inert nitrogen gas and harmless water vapour 5: The nitrogen and water vapour are safely released into the atmosphere This process is described in more detail in our article on SCR system for ships.