Selective catalytic reduction is a catalytic reaction which uses ammonia to reduce oxides of nitrogen (NOx) into harmless nitrogen (N2) and water. Because of this function, the SCR catalyst is sometimes referred to as a NOx catalyst in the industry. It has been used in many industrial facilities including power plants and chemical refineries to reduce NOx emissions since the 1980’s. It has been used in automotive applications since the mid 2000’s to reduce NOx emissions from heavy duty and light duty diesel vehicles.
The active component of the catalyst itself can be one of several different materials; vanadium pentoxide is frequently used in industrial settings while copper and iron zeolites are frequently used in automotive applications. Different materials are used because of differences in operating conditions, temperatures, and resistance to impurities in the fuel and environment.
On industrial installations, SCR catalysts are installed along with an Ammonia Injection Grid, or AIG for short. The AIG sprays ammonia, typically stored as aqueous ammonia, anhydrous ammonia, or a urea solution, into the exhaust stream. The ammonia adsorbs onto the SCR catalyst, where it reacts with NOx and oxygen to form nitrogen and water. In a vehicle, onboard urea, known as Diesel Exhaust Fluid (DEF) or AdBlue, is sprayed into exhaust gas upstream of the catalyst brick and diffused using a mixer to optimize airflow and ensure thorough vaporization and even distribution in the exhaust prior to entering the catalyst.
If an Industrial SCR catalyst NOx removal efficiency declines or ammonia usage increases significantly, it could signal a serious problem with the catalyst and inspection and performance testing by a qualified company should be scheduled. See Environex’s SCR System Evaluation page for more information on our services.
In an industrial facility, the AIG may need to be tuned periodically to ensure proper distribution of ammonia into the SCR catalyst. If there are areas of high and low ammonia flow entering the system, the catalyst will not be used effectively and higher NOx emissions and ammonia may result. See AIG Tuning for more information.
NOx is short for oxides of Nitrogen. It is a class of pollutants which includes nitrogen oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). When emitted into the atmosphere, they can cause an array of problems, including acute and chronic health problems and smog. Nitrous oxide is also a potent greenhouse gas (GHG). For these reasons, NOx emissions are regulated in many areas of the world.
Nitrogen oxides are formed during combustion as high temperatures cause nitrogen from the atmosphere to oxidize. Because only temperature and heat are needed to form NOx, even lightning strikes form the compounds. NOx emissions from industrial facilities like power plants are regulated, as are the emissions from vehicles.
The level of regulation depends on the regulatory agency and the requirements of the area; more densely populated areas tend to require higher NOx reductions than rural areas. Geographic features also contribute to atmospheric NOx buildup, which makes some areas, such as Mexico City and Los Angeles, more susceptible to smog.