Selective catalytic reduction

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An aqueous ammonia SCR Process Overview; note that a vaporizer would not be necessary when using anhydrous ammonia

Selective catalytic reduction (SCR) is a means of converting nitrogen oxides, also referred to as NO_x with the aid of a catalyst into diatomic nitrogen, N₂, and water, H₂O. 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, CO₂ is a reaction product when urea is used as the reductant.

Selective catalytic reduction of 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. ^[1]

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 NO_x by 70-95%.^[1] More recent applications include diesel engines, such as those found on large ships, diesel locomotives, gas turbines, and even automobiles.

[edit] Chemistry

The NO_x reduction reaction takes place as the gases pass through the catalyst chamber. Before entering the catalyst chamber the ammonia, or other reductant (such as urea), 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:

With several secondary reactions:

The reaction for urea instead of either anhydrous or aqueous ammonia is:

The ideal reaction has an optimal temperature range between 630 K and 720 K, but can operate from 500 K to 720 K with longer residence times. The minimum effective temperature depends on the various fuels, gas constituents and catalyst geometry. Other possible reductants include cyanuric acid and ammonium sulfate. ^[2]

[edit] Catalysts

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. Each catalyst component has 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 SO₂ into SO₃, which can be extremely damaging due to its acidic properties. ^[3]

Zeolite catalysts have the potential to operate at substantially higher temperature than base metal catalysts; they can withstand prolonged operation at temperatures of 900 K and transient conditions of up to 1120 K. Zeolites also have a lower potential for potentially damaging SO₂ oxidation. ^[3]

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 NO₂ is 20% to 50% of the total NO_x.^[4] 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, but plate configurations are much larger and more expensive. Honeycomb configurations are smaller than plate types, but have higher pressure drops and plug much more easily.^[1]

[edit] Reductants

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 is extremely toxic and difficult to safely store, but needs no further conversion to operate within an SCR. It is typically favored by large industrial SCR operators. Aqueous ammonia must be hydrolyzed in order to be used, but it is substantially 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.^[1]

[edit] Limitations

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 SCR units have stated that ions can pass from the dispensing materials into the porous head on the SCR unit if unsuitable materials are used in the manufacture of the units^{[citation needed]}. This spoils the SCR's efficacy and can reduce its lifespan by more than 60%.^{[citation needed]}

The biggest challenge 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 to which it will be fitted. The simulation can be run on a dynamometer, or on an actual piece of equipment during its normal work day using data logging. Data logging tends to be inaccurate, as no two operators use the equipment in exactly the same way. Even when used for the same general purposes, such as 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. This is called 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. An additional oxidation catalyst called a slip catalyst is typically fitted downstream of an SCR system to reduce such slip.

Another common problem, especially in passenger car applications, is relatively low temperature of exhaust gas resulting in SCR catalyst temperature below the optimal range. This is a problem especially in cold-start conditions.

[edit] Power plants

In power stations, the same basic technology is employed for removal of NO_x 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 NO_x 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.

[edit] See also

• Acid Rain
• AdBlue
• Automobile emissions control
• Environmental Engineering
• selective non-catalytic reduction

[edit] External links

• Links to Cormetech Inc. SCR Description

[edit] References