Chemistry

DEF is a 32.5% solution of urea, (NH2) 2CO. When it is injected into the hot exhaust gas stream, the water evaporates and the urea thermally decomposes to form ammonia (NH3) and isocyanic acid (HNCO):

(NH2) 2CO → NH3 + HNCO

The isocyanic acid reacts with the water vapor and hydrolyses to carbon dioxide and ammonia:

HNCO + H2O → CO2 + NH3

Overall, thus far:

(NH2)2CO + H2O → 2 NH3 + CO2

Ammonia, in the presence of oxygen and a catalyst, reduces two different nitrogen oxides:

4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O ("standard SCR") and

6 NO2 + 8 NH3 → 7 N2 + 12 H2O ("NO2 SCR selective catalytic reduction")

NO + NO2 + 2 NH3 → 2 N2 + 3 H2O ("fast SCR")

The overall reduction of NOx by urea is then:

2 (NH2)2CO + 4 NO + O2 → 4 N2 + 4 H2O + 2 CO2 and

4 (NH2)2CO + 6 NO2 → 7 N2 + 8 H2O + 4 CO2 and

(NH2)2CO + NO + NO2 → 2 N2 + 2 H2O + CO2

The ratio between NO2 and NO determines which reactions take place and how fast. The highest conversion rates are achieved if equal amounts of NO2 and NO are present, especially at temperatures between 200°C and 350°C. If there is more NO than NO2, fast SCR and standard SCR take place sequentially. If there is more NO2 than NO, fast SCR and NO2 SCR take place sequentially, however, NO2 SCR is slower than standard SCR, and ammonium nitrate can form and temporarily deactivate the catalytic converter.

Operation in winter time
DEF freezes at −11 °C (12 °F). For the SCR exhaust cleaning system to function at low temperatures, a sufficient amount of the frozen DEF must be melted in as short time as possible, preferably on the order of minutes. For example, 2010 EPA emissions requirements require full DEF coolant flow within 70 minutes.

In Europe, Regulation (EC) No 692/2008[19] specified in Annex XVI point 10 that DEF from a frozen tank at a core temperature of −15 °C (5 °F) must become available within 20 minutes when starting the engine at −15 °C (5 °F).

Typically, the frozen DEF is melted by heat from the engine, e.g. engine coolant passing through the DEF tank, governed by a thermostatic coolant control valve. This method may take significant time before the SCR exhaust cleaning system is fully operational, often up to an hour.

Another method to thaw DEF (and thus allow for full SCR operation) is to integrate an electric heater into the DEF tank. This heater must be sized, positioned, and powered adequately to rapidly melt sufficient frozen DEF. It should preferably be self regulating not to overheat if (part of) the heater is outside of the liquid. It should also preferably be self regulating to eliminate any complicated sensor and temperature regulating systems. Furthermore, the heater should not exceed 50–60 °C (122–140 °F), as DEF begins to decompose at around 60 °C (140 °F). PTC heaters are often used to achieve this.

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