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Selective Catalytic Reduction System (SRC)


Selective Catalytic Reduction System (SCR)

In 2000 EPA (Environmental Protection Agency) published the Tier 2 Emission Standards for Light Duty and Heavy Duty Vehicles, mandate that regulates new strict emission control standards beginning January 1, 2010. Starting from that date, no new diesel-burning vehicles can be sold without meeting the Tier 2 emission standard: 0.2 grams of nitrogen oxide (NOx) per brake horsepower-hour.

Vehicle and engine manufacturers have developed Selective Catalytic Reduction (SCR) technology, which uses DEF to convert NOx into harmless nitrogen gas.

The “diesel exhaust fluid” (DEF), in Europe known as AdBlue, is a solution consisting of high purity urea (typically crystalline) dissolved and suspended within deionized water. The ratio of the mix is approximately 32.5% urea and 67.5% deionized water.

DEF is carried on board the vehicle in a separate tank. It is injected into the exhaust gases as a post combustion process through the SCR system, where it breaks harmful NOx (Nitrous Oxide) emissions down into mostly Nitrogen and Oxygen. Sensors in DEF tanks will notify the driver if volume is low or if the product concentration is not of good quality. Another sensor will indicate if the NOx level in the tailpipe exhaust is too high. These sensors will need to be maintained by fleet owners.

A potential problem in handling DEF is that it freezes at 11°F. Also elevated temperatures causes the DEF to decompose, releasing ammonia.

ISO standards limit the total ammonia to 0.2 percent, and so the shelf life of the fluid is limited if exposed to elevated temperatures for extended periods.

ISO standards have very strict limits on contamination, which limit the materials that the fluid can contact. Per ISO standards, the suitable materials are stainless steels, titanium, hastelloys and several plastics, so long as they are free of additives.

Materials specifically not recommended include non-ferrous metals and alloys (copper, aluminum, magnesium, silver, zinc, lead), solders containing non-ferrous metals, and nickel-coated plastics and metals. Bottles, drums and totes are most commonly made of polyethylene. Large storage tanks can be made of stainless steel or coated carbon steel.