Selective Catalytic Reduction (SCR) Systems Information


Selective catalytic reduction (SCR) systems are post-combustion active emission control technology systems. The system uses a catalyst bed and an injected liquid-reductant agent to scavenge pollutants, nitrogen oxides (NOx), and produce diatomic nitrogen (N2) and water vapor (H2O).


Selective catalytic reducers (SCR) work in a manner similar to the way a catalytic converter works to reduce automobile emissions. Catalytic converters circulate un-oxidized portions of the exhaust stream in an oxidizinSelective Catalytic Reduction (SCR) Systemg environment to break down additional hydrocarbons and react with NOx. SCRs differ as they incorporate a gaseous or liquid reductant, generally ammonia or urea that is added to the exhaust gas stream. The exhaust gas mixes with the reductant and travels through several catalytic layers, where a reaction between NOx emissions and the injected reductant occurs. The reaction converts the NOx emissions into N2 and water vapor. The benign elements are then released into the air through a smokestack or exhaust pipes.


One common problem with selective catalytic reducers (SCR) is that they operate well only within narrow temperature bands. Consequently, control units are required to ensure the exhaust gas temperatures are within the range that will allow the catalytic reaction to occur.




Selective catalytic reducers (SCR) differ from selective non-catalytic reduction (SNCR) as they incorporate a catalyst to increase the reaction between NOx emissions and the liquid reductant, typically urea or ammonia. The use of a catalyst also allows an SCR system to operate at much lower temperatures (340° C to 380° C or 650° F to 720° F) than a SNCR system (870 °C to 1200° C or 1600° F to 2200° F). The SCR system increases the efficiency of NOx removal to achieve NOx reduction of up to 75% or more.


Common catalyst materials include porous ceramic honeycomb or plate substrates coated with base-metal oxides or metal-exchanged zeolites.


  • Base-metal oxides include vanadium and tungsten oxide. They exhibit moderate thermal durability and are suitable for some utility boiler applications.
  • Metal-exchanged zeolite, copper-zeolite and iron-zeolite catalysts have improved thermal durability and chemical stability over most other catalysts. They are able to operate at substantially higher temperatures and withstand prolonged operation at temperatures up to 900 K and transient temperatures up to 1120 K.




Reductants used in SCR systems include anhydrous ammonia, aqueous ammonia, and most and diesel exhaust fluid (DEF).


  • Anhydrous ammonia is toxic and unstable; however no conversion is needed and it will readily react with NOx. Large industrial processes may use anhydrous ammonium as they are able to incorporate safety control methods that isolate the fluid streams.
  • Aqueous ammonia must be vaporized before it will react with NOx emissions. It is safer to store and transport when compared to anhydrous ammonia.
  • DEF is an aqueous solution that is 32.5% urea and 67.5% deionized water. It is a consumable used to operate SCR converters in motor vehicles with diesel engines.



Selective catalytic reducers (SCR) are used to capture NOx in flue gases from power generations systems, manufacturing plants, and other industrial processes. Heavy-duty and light-duty diesel engines used in ether marine or commercial applications also use SCR technology to capture NOx and meet current emission standards.


Flue Gases


Flue gases exiting a power generation system or industrial processing equipment must be pretreated before entering the SCR system. Filtration units extend catalyst life by removing sulfur, soot, and other particulate matter. Air heaters are used to regulate the temperature of the flue gas before entering the SCR system. Industrial scale SCR systems that accommodate flow rates greater than 100 MMBtu/hr consume 20,000 to 100,000 gallons of reductants a week and have an average catalyst life of 25,000 to 40,000 hours.


Combustion Engines


Diesel and marine engines incorporate SCR technology. Thermal durability is particularly important for these SCR applications as the diesel particulate filter with forced regeneration generally results in increased operating temperatures. Combustion engines with SCR technology inject DEF or other urea based fluids into exhaust stream prior to the catalytic converter. Injection concentrations are controlled through a metering device and are proportional to the engines load and NOx emission measured upstream and downstream from the SCR system.



Selective catalytic reducers (SCR) are often the technology of choice for meeting emission standards. The Environmental Protection Agency (EPA) implemented the Clean Air Act in 1970 followed by amendments in1990 that required major stationary sources of NOx to install and operate reasonably available control technology. NOx emissions from industrial power plants in the United States then decreased by as much as 50% by 1995. New amendments to the clean air act target the automotive sector while global emission standards governing air pollutants are governed by regional authorities including European Union (EU), China's State Environmental Protection Administration (SEPA), and the Ministry of Environment (MOE) of Japan.




ISO 22241-1This part of ISO 22241 specifies the quality characteristics of the NOx reduction agent AUS 32 (aqueous urea solution) which is needed to operate converters with selective catalytic reduction, so-called SCR (selective catalytic reduction) converters, in motor vehicles with diesel engines. SCR converters are particularly suitable for selectively reducing the nitrogen oxide (NOx) emissions of diesel engines.


ISA 77.82.01This standard will address the control functions associated with the selective catalytic reduction systems on fossil-fired steam boilers greater than 200,000 lbs/hr and combustion turbines greater than 25 megawatts.


EPA—Air Pollution Control Technology Fact Sheet (pdf)

DieselNet Technology Guide—Selective Catalytic Reduction

Wikipedia—Selective Catalytic Reduction

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