Selective catalytic reduction

Selective catalytic reduction

Selective catalytic reduction (SCR) is a means of converting nitrogen oxides, also referred to as chem|NO|x with the aid of a catalyst into diatomic nitrogen, chem|N|2, and water, chem|H|2|O. A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is absorbed onto a catalyst. Carbon dioxide, chem|CO|2 is a reaction product when urea is used as the reductant.

Selective catalytic reduction of chem|NO|x using ammonia as the reducing agent was patented in the United States by the Englehard Corporation in 1957. Development of SCR technology continued in Japan and the US in the early 1960’s with research focusing on less expensive and more durable catalyst agents. The first large scale SCR was installed by the IHI Corporation in 1978. [Steam: Its Generation and Uses. Babcock and Wilcox.]

Commercial selective catalytic reduction systems are typically found on large utility boilers, industrial boilers, and municipal solid waste boilers and have been shown to reduce chem|NO|x from 70-95%. [Steam: Its Generation and Uses. Babcock and Wilcox.] More recent applications include large diesel engines, such as those found on large ships, diesel locomotives, combustion turbines, and even automobiles.

The Reaction

The chem|NO|X reduction reaction takes place as the gases passes through the catalyst chamber. Before entering the catalyst chamber the ammonia, or other reductant, is injected and mixed with the gases. The chemical equation for a stoichiometric reaction using either anhydrous or aqueous ammonia for a selective catalytic reduction process is as follows:

:4NO + 4NH_3 + O_2 ightarrow ; 4N_2 + 6H_2O

:2NO_2 + 4NH_3 + O_2 ightarrow ; 3N_2 + 6H_2O

:NO + NO_2 + 2NH_3 ightarrow ; 2N_2 + 3H_2O

With several secondary reactions

:2SO_2 + O_2 ightarrow ; 2SO_3

:2NH_3 + SO_3 + H_2O ightarrow ; (NH_4)_2SO_4

:NH_3 + SO_3 + H_2O ightarrow ; NH_4HSO_4

The reaction for urea instead of either anhydrous or aqueous ammonia is as follows::4NO + 2(NH_2)_2CO + O_2 ightarrow ; 4N_2 + 4H_2O + 2CO_2

The ideal reaction has an optimal temperature range between 675 and 840°F, but can operate from 450 to 840°F with longer residence times needed. The minimum temperature is effected by the various fuels, gas constituents and catalyst geometry. Other possible reductants include cyanuric acid and ammonium sulfate [”Environmental Effects of Nitrogen Oxides”. Electric Power Research Institute, 1989]


SCR catalysts are manufactured from various ceramic materials used as a carrier, such as titanium oxide, and active catalytic components are usually either oxides of base metals (such as vanadium and tungsten), zeolites, and various precious metals. All catalyst components have their own unique advantages and disadvantages.

Base metal catalysts, such as the vanadium and tungsten, lack high thermal durability, but are less expensive and operate very well at the temperature ranges most commonly seen in industrial and utility boiler applications. Thermal durability is particularly important for automotive SCR applications that incorporate the use of a diesel particulate filter with forced regeneration. They also have a high catalyzing potential to oxidize chem|SO|2 into chem|SO|3, which can be extremely damaging to its acidic properties. [ DOE presentation] ]

Zeolite catalysts have the potential to operate at significantly higher temperatures than base metal catalysts, with the ability to withstand long term operational temperatures of 1200°F, and transient conditions of up to 1560°F. Zeolite also have a lower potential for potentially damaging chem|SO|2 oxidation. [ DOE presentation] ]

Recently developed iron and copper exchanged zeolite urea SCRs have been developed with approximately equal performance to that of vanadium urea-SCRs if the fraction of the chem|NO|2 is 20% to 50% of the total NO_X. [ [ SAE Technical Paper 2001-01-0514] ]

The two most common designs of SCR catalyst geometry used today are honeycomb and plate. The honeycomb form usually is an extruded ceramic applied homogeneously throughout the ceramic carrier or coated on the substrate. Like the various types of catalysts, their configuration also has advantages and disadvantages. Plate type catalysts have lower pressure drops and are less susceptible to plugging and fouling than the honeycomb types, however plate type configurations are significantly larger and more expensive. Honeycomb configurations are significantly smaller than plate types, but have higher pressure drops and plug much more easily. [Steam: Its Generation and Uses. Babcock and Wilcox.]


Several reductants are currently used SCR applications including anhydrous ammonia, aqueous ammonia or urea. All three reductant are widely available in large quantities.

Pure anhydrous ammonia while extremely toxic and difficult to safely store, need no further conversion to operate within an SCR. Since it requires no further conversion to be useful, it is typically favored by large industrial SCR operators. Aqueous ammonia must be hydrolized in order to be used but it is significantly safer to store and transport than anhydrous ammonia. Urea is the safest to store, but requires conversion to ammonia through thermal decomposition in order to be used as an effective reductant. [Steam: Its Generation and Uses. Babcock and Wilcox.]

Technical problems with automotive SCR units

In order to ensure that the SCR unit remains free from contaminants, correct materials of construction must be used for both storage and dispensing. Manufacturers of the SCR unit have specified that, without using compatible materials of construction, ions can be passed from the dispensing materials into the porous head on the SCR unit. This can render the SCR unit ineffective and reduce its life expectancy by more than 60%.

The biggest issue with SCR is the necessity to tune the SCR system to the engine operating cycle. This requires running the engine through a simulation of the operating cycle of the machine it will be fitted to. The simulation can be run on a dynamometer, or on an actual piece of equipment during its normal work day (data logging). Even at best, data logging tends to be inaccurate, as no two operators will use the equipment in the same way. Even when used for the same general purposes (i.e., a truck delivering goods to stores in a city), small differences in the route such as hills, one-way streets, amount unloaded, etc., can make the engine loads different enough that effectiveness of the system will suffer.

Another common problem with all SCR systems is the release of unreacted ammonia referred to as ammonia slip. Slip can occur when catalyst temperatures are not in the optimal range for the reaction or when too much ammonia is injected into the process. Additional oxidation catalyst, so called slip catalyst, is typically fitted downstream an SCR system to reduce such slip.

Another common problem especially in passenger cars applications is relatively low temperature of exhaust gas resulting in low SCR catalyst monolith temperature outside optimal range. This is a problem especially in cold-start conditions.

Power plants

In power stations, the same basic technology is employed for removal of NOx from the flue gas of boilers used in power generation and industry. The SCR unit is generally located between the furnace economizer and the air heater and the ammonia is injected into the catalyst chamber through an ammonia injection grid. As in other SCR applications, the temperature of operation is critical. Ammonia slip is also an issue with SCR technology used in power plants.

Other issues which must be considered in using SCR for NOx control in power plants are the formation of ammonium sulfate and ammonium bisulfate due to the sulfur content of the fuel as well as the undesirable catalyst-caused formation of SO3 from the SO2 and O2 in the flue gas.

A further operational difficulty in coal-fired boilers is the blinding of the catalyst by fly ash from the fuel combustion. This requires the usage of sootblowers, sonic horns and careful design of the ductwork and catalyst materials to avoid plugging by the fly ash.

ee also

* Acid Rain
* AdBlue
* Automobile emissions control
* Environmental Engineering


External links

* [ Association for Emissions Control by Catalyst]
* [ - News, reports, updates and companies involved with Selective Catalytic Reduction diesel emissions technology]
* [ Selective Catalytic Reduction]
* [ Selective catalytic reduction for NOx control]
* [ The definitive web site for Diesel Emissions information]
* [ Free technical references and guidelines for the correct handling of AdBlue and how to prolong the life expectancy of any SCR.]
*MSDS Sheet for [ AdBlue] (as used in a vehicles SCR)
*Independent report on differences between ERG & SCR ( [ AdBlue] ) Technology.

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