Diesel exhaust fluid (DEF; also known as AUS 32 and marketed as AdBlue) is a liquid used to reduce the amount of air pollution created by a diesel engine. Specifically, DEF is an aqueous urea solution made with 32.5% urea and 67.5% deionized water. DEF is consumed in a selective catalytic reduction (SCR) that lowers the concentration of nitrogen oxides in the diesel exhaust emissions from a diesel engine.

Pritish Kumar Halder explained the chemistry, operations safety and storage procedure for the diesel exhaust fluid system below:

Other names

In the international standard defining DEF (ISO 22241), it is referred to as AUS 32 (aqueous urea solution).DEF is also sold as AdBlue, a registered trademark of the German Association of the Automotive Industry.

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 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 → 2NH3 + CO2

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

4NO + 4NH3 + O2 → 4N2 + 6H2O  and
6NO2 + 8NH3 → 7N2 + 12H2O

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 + 6NO2 → 7N2 + 8 H2O + 4 CO2

Ad Blue System

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 c Safety and storage

The urea solution is clear, non-toxic and safe to handle. Since urea has corrosive impact on metals like aluminum, DEF is stored and transported in special containers. These containers are typically made of stainless steel. Vehicles’ selective catalytic reduction (SCR) systems and DEF dispensers are designed in a manner that there is no corrosive impact of urea on them. It is recommended that DEF be stored in a cool, dry, and well-ventilated area that is out of direct sunlight. Bulk volumes of DEF are compatible for storage within polyethylene containers (HDPE, XLPE), fiberglass reinforced plastic (FRP), and steel tanks. DEF is also often handled in intermediate bulk containers for storage and shipping.

DEF is offered to consumers in a variety of quantities ranging from containers for single or repeated small usage, up to bulk carriers for consumers requiring a large amount of DEF. As of 2013, many truck stops have added DEF pumps. These are usually adjacent to fuel pumps so the driver can fill both tanks without moving the truck. Coolant flow within 70 minutes.

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

Another method to thaw DEF (and thus allow for full SCR operation) is to integrate an electrical 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.

Safety and storage

The urea solution is clear, non-toxic and safe to handle. Since urea has corrosive impact on metals like aluminum, DEF is stored and transported in special containers. These containers are typically made of stainless steel.  Vehicles’ selective catalytic reduction (SCR) systems and DEF dispensers are designed in a manner that there is no corrosive impact of urea on them.

It is recommended that DEF be stored in a cool, dry, and well-ventilated area that is out of direct sunlight. Bulk volumes of DEF are compatible for storage within polyethylene containers (HDPE, XLPE), fiberglass reinforced plastic (FRP), and steel tanks. DEF is also often handled in intermediate bulk containers for storage and shipping.

DEF is offered to consumers in a variety of quantities ranging from containers for single or repeated small usage, up to bulk carriers for consumers requiring a large amount of DEF. As of 2013, many truck stops have added DEF pumps. These are usually adjacent to fuel pumps so the driver can fill both tanks without moving the truck.

Reference

https://en.wikipedia.org/wiki/Diesel_exhaust_fluid