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Aqueous urea solution can be injected into the exhaust gas stream of a combustion process in order to lower its emissions of nitrous oxides (NOx).
In the hot flue gas stream, urea solution decomposes into carbon dioxide and ammonia. Ammonia reacts with the nitrous oxides, usually over a catalyst, decomposing them into the harmless components water and nitrogen.
For reducing NOx emissions from diesel engines, a suitable urea solution is referred to as DEF (diesel exhaust fluid), and in Europe a solution of this kind is marketed under the name AdBlue®.