Selective catalytic reduction
Encyclopedia
Selective catalytic reduction (SCR) is a means of converting nitrogen oxide
s, also referred to as with the aid of a catalyst into diatomic nitrogen
, , and water
, . 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
, is a reaction product when urea is used as the reductant.
Selective catalytic reduction of 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 1960s with research focusing on less expensive and more durable catalyst agents. The first large-scale SCR was installed by the IHI Corporation in 1978.
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 by 70-95%. More recent applications include diesel engine
s, such as those found on large ships, diesel locomotives, gas turbine
s, and even automobiles.
), 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 and 720 K
, but can operate from 500 to 720 K with longer residence time
s. The minimum effective temperature depends on the various fuels, gas constituents, and catalyst geometry. Other possible reductants include cyanuric acid
and ammonium sulfate
.
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
), zeolite
s, or 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 catalysing potential to oxidize into , which can be extremely damaging due to its acidic properties.
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 oxidation.
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 is 20% to 50% of the total .
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 drop
s 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.
or urea
. All those three reductants 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 hydrolysed
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.
Most SCRs require tuning to properly perform. Part of tuning involves ensuring a proper distribution of ammonia in the gas stream and uniform gas velocity through the catalyst. Without tuning, SCRs can exhibit inefficient NOx reduction along with excessive ammonia slip due to not utilizing the catalyst surface area effectively. Another facet of tuning involves determining the proper ammonia flow for all process conditions. Ammonia flow is in general controlled based on NOx measurements taken from the gas stream or preexisting performance curves from an engine manufacturer (in the case of gas turbines and reciprocating engines). Typically, all future operating conditions must be known beforehand to properly design and tune an SCR system.
Ammonia slip is an industry term for ammonia passing through the SCR un-reacted. This occurs when ammonia is: over-injected into gas stream; temperatures are too low for ammonia to react; or catalyst has degraded (see above).
Temperature is one of the largest limitations of SCR. Gas turbines, cars, and diesel engines all have a period during a start-up where exhaust temperatures are too cool for NOx reduction to occur.
s, the same basic technology is employed for removal of from the flue gas of boiler
s used in power generation and industry. In general, the SCR unit is 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 that must be considered in using SCR for 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 from the and in the flue gas.
A further operational difficulty in coal
-fired boilers is the binding 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.
continuing to manufacture engines after this date have chosen to utilize SCR. This Includes Detroit Diesel
(DD13, DD15, and DD16 models), Cummins
(ISX, ISL9, ISB6.7, and ISC8.3 line), PACCAR
, and Volvo
/Mack. These engines require the periodical addition of Diesel Exhaust Fluid (DEF, a urea solution) to enable the process. DEF is available in a bottle from most truck stops, and some have put in bulk DEF dispensers near the Diesel Fuel pumps. Navistar has chosen to utilize Enhanced Exhaust Gas Recirculation (EEGR) to comply with the Environmental Protection Agency (EPA) standards.
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
s, also referred to as with the aid of a catalyst into diatomic nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, , and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, . A gaseous reductant, typically anhydrous ammonia, aqueous ammonia
Ammonium hydroxide
Ammonia solution, also known as ammonium hydroxide, ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or simply ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3...
or urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
, is added to a stream of flue
Flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...
or exhaust gas
Exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...
and is absorbed onto a catalyst. Carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, is a reaction product when urea is used as the reductant.
Selective catalytic reduction of using ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
as the reducing agent was patented in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
by the Englehard Corporation
Engelhard
Engelhard Corporation is a former American Fortune 500 company headquartered in Iselin, New Jersey, USA. It is credited with developing the first production catalytic converter. In 2006, the German chemical manufacturer BASF bought Engelhard for $US5 billion....
in 1957. Development of SCR technology continued in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
and the US in the early 1960s with research focusing on less expensive and more durable catalyst agents. The first large-scale SCR was installed by the IHI Corporation in 1978.
Commercial selective catalytic reduction systems are typically found on large utility boilers
Fossil fuel power plant
A fossil-fuel power station is a power station that burns fossil fuels such as coal, natural gas or petroleum to produce electricity. Central station fossil-fuel power plants are designed on a large scale for continuous operation...
, industrial boilers
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
, and municipal solid waste boilers
Incineration
Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials. Incineration and other high temperature waste treatment systems are described as "thermal treatment". Incineration of waste materials converts the waste into ash, flue gas, and...
and have been shown to reduce by 70-95%. More recent applications include diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
s, such as those found on large ships, diesel locomotives, gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
s, and even automobiles.
Chemistry
The reduction reaction takes place as the gases pass through the catalyst chamber. Before entering the catalyst chamber the ammonia, or other reductant (such as ureaUrea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
), 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:
- 4NO + 4 + → 4 + 6
- 2 + 4 + → 3 + 6
- NO + + 2 → 2 + 3
With several secondary reactions:
- 2 + → 2
- 2 + + →
- + + →
The reaction for urea instead of either anhydrous or aqueous ammonia is:
- 4NO + 2 + → 4 + 4 + 2
The ideal reaction has an optimal temperature range between 630 and 720 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, but can operate from 500 to 720 K with longer residence time
Residence time
Residence time is the average amount of time that a particle spends in a particular system. This measurement varies directly with the amount of substance that is present in the system....
s. The minimum effective temperature depends on the various fuels, gas constituents, and catalyst geometry. Other possible reductants include cyanuric acid
Cyanuric acid
Cyanuric acid or 1,3,5-triazine-2,4,6-triol is a chemical compound with the formula 3. Like many industrially useful chemicals, this triazine has many synonyms. This white, odorless solid finds use as a precursor or a component of bleaches, disinfectants, and herbicides...
and ammonium sulfate
Ammonium sulfate
Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...
.
Catalysts
SCR catalysts are manufactured from various ceramicCeramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
materials used as a carrier, such as titanium oxide
Titanium oxide
Titanium oxide may refer to:* Titanium dioxide , TiO2* Titanium oxide , TiO, a non-stoichiometric oxide* Titanium oxide , Ti2O3* Ti3O* Ti2O* δ-TiOx...
, and active catalytic components are usually either oxides of base metals (such as vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
and tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
), zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
s, or various precious metals. Each catalyst component has advantages and disadvantages.
Base metal
Base metal
In chemistry, the term base metal is used informally to refer to a metal that oxidizes or corrodes relatively easily, and reacts variably with diluted hydrochloric acid to form hydrogen. Examples include iron, nickel, lead and zinc...
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
Diesel Particulate Filter
A diesel particulate filter is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. Wall-flow diesel particulate filters usually remove 85% or more of the soot, and under certain conditions can attain soot removal efficiencies of close to 100%...
with forced regeneration. They also have a high catalysing potential to oxidize into , which can be extremely damaging due to its acidic properties.
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 oxidation.
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 is 20% to 50% of the total .
The two most common designs of SCR catalyst geometry used today are honeycomb
Honeycomb
A honeycomb is a mass of hexagonal waxcells built by honey bees in their nests to contain their larvae and stores of honey and pollen.Beekeepers may remove the entire honeycomb to harvest honey...
and plate. The honeycomb form usually is an extruded
Extrusion
Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section...
ceramic applied homogeneously
Homogeneous (chemistry)
A substance that is uniform in composition is a definition of homogeneous. This is in contrast to a substance that is heterogeneous.The definition of homogeneous strongly depends on the context used. In Chemistry, a homogeneous suspension of material means that when dividing the volume in half, the...
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 drop
Pressure drop
Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...
s 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.
Reductants
Several reductants are currently used in SCR applications including anhydrous ammonia, aqueous ammoniaAmmonium hydroxide
Ammonia solution, also known as ammonium hydroxide, ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or simply ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3...
or urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
. All those three reductants 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 hydrolysed
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
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.
Limitations
SCR systems are sensitive to contamination and plugging resulting from normal operation or abnormal events. Many SCRs are given a finite life due to known amounts of contaminants in the untreated gas. The large majority of catalyst on the market is of porous construction. A clay planting pot is a good example of what SCR catalyst feels like. This porosity is what gives the catalyst the high surface area essential for reduction of NOx. However, the pores are easily plugged by a variety of compounds present in combustion/flue gas. Some examples of plugging contaminates are: fine particulate, ammonia sulfur compounds, Ammonium Bisulfate (ABS) and silicon compounds. Many of these contaminants can be removed while the unit is on line, for example by sootblowers. The unit can also be cleaned during a turnaround or by raising the exhaust temperature. Of more concern to SCR performance is poisons, which will destroy the chemistry of the catalyst and render the SCR ineffective at NOx reduction or cause unwanted oxidation of ammonia (forming more NOx). Some of these poisons are as follows: Halogens, Alkaline Metals, Arsenic, Phosphorus, Antimony, Chrome, Copper.Most SCRs require tuning to properly perform. Part of tuning involves ensuring a proper distribution of ammonia in the gas stream and uniform gas velocity through the catalyst. Without tuning, SCRs can exhibit inefficient NOx reduction along with excessive ammonia slip due to not utilizing the catalyst surface area effectively. Another facet of tuning involves determining the proper ammonia flow for all process conditions. Ammonia flow is in general controlled based on NOx measurements taken from the gas stream or preexisting performance curves from an engine manufacturer (in the case of gas turbines and reciprocating engines). Typically, all future operating conditions must be known beforehand to properly design and tune an SCR system.
Ammonia slip is an industry term for ammonia passing through the SCR un-reacted. This occurs when ammonia is: over-injected into gas stream; temperatures are too low for ammonia to react; or catalyst has degraded (see above).
Temperature is one of the largest limitations of SCR. Gas turbines, cars, and diesel engines all have a period during a start-up where exhaust temperatures are too cool for NOx reduction to occur.
Power plants
In power stationFossil fuel power plant
A fossil-fuel power station is a power station that burns fossil fuels such as coal, natural gas or petroleum to produce electricity. Central station fossil-fuel power plants are designed on a large scale for continuous operation...
s, the same basic technology is employed for removal of from the flue gas of boiler
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
s used in power generation and industry. In general, the SCR unit is located between the furnace
Furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
economizer
Economizer
Economizers , or economisers , are mechanical devices intended to reduce energy consumption, or to perform another useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, powerplant, and heating, ventilating, and air-conditioning uses are...
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 that must be considered in using SCR for control in power plants are the formation of ammonium sulfate
Ammonium sulfate
Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...
and ammonium bisulfate
Ammonium bisulfate
Ammonium bisulfate, also known as ammonium hydrogen sulfate, is a white, crystalline solid when it is completely pure, with formula HSO4. It is commonly collected as a byproduct of flue gas desulfurization, formed by partial neutralization of sulfuric acid aerosols by gaseous ammonia, NH3. ...
due to the sulfur content of the fuel as well as the undesirable catalyst-caused formation of from the and in the flue gas.
A further operational difficulty in coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
-fired boilers is the binding of the catalyst by fly ash
Fly ash
Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
from the fuel combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
. This requires the usage of sootblowers, sonic horns, and careful design of the ductwork and catalyst materials to avoid plugging by the fly ash.
SCR and EPA 2010
Beginning with diesel engines manufactured on or after January 1, 2010, the engines are required to meet lowered NOx standards. All of the heavy-duty engine (Class 7-8 trucks) manufacturers except for Navistar InternationalNavistar International
Navistar International Corporation is a United States-based holding company that owns the manufacturer of International brand commercial trucks, MaxxForce brand diesel engines, IC Bus school and commercial buses, Workhorse brand chassis for motor homes and step vans, and is a private label...
continuing to manufacture engines after this date have chosen to utilize SCR. This Includes Detroit Diesel
Detroit Diesel
As a corporation, Daimler Trucks North America has decided to rename the company "DETROIT".Detroit Diesel Corporation is an American-based diesel engine producer headquartered in Detroit, Michigan, USA...
(DD13, DD15, and DD16 models), Cummins
Cummins
Cummins Inc. is a Fortune 500 corporation that designs, manufactures, distributes and services engines and related technologies, including fuel systems, controls, air handling, filtration, emission control and electrical power generation systems...
(ISX, ISL9, ISB6.7, and ISC8.3 line), PACCAR
PACCAR
PACCAR Inc is the third largest manufacturer of heavy-duty trucks in the world , and has substantial manufacture in light and medium vehicles through its various subsidiaries.-History:...
, and Volvo
Volvo
AB Volvo is a Swedish builder of commercial vehicles, including trucks, buses and construction equipment. Volvo also supplies marine and industrial drive systems, aerospace components and financial services...
/Mack. These engines require the periodical addition of Diesel Exhaust Fluid (DEF, a urea solution) to enable the process. DEF is available in a bottle from most truck stops, and some have put in bulk DEF dispensers near the Diesel Fuel pumps. Navistar has chosen to utilize Enhanced Exhaust Gas Recirculation (EEGR) to comply with the Environmental Protection Agency (EPA) standards.
See also
- Acid rainAcid rainAcid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...
- AdBlueAdBlueDiesel Exhaust Fluid is an aqueous urea solution used in Selective Catalytic Reduction to lower NOx concentration in the exhaust emissions from diesel engines...
- Environmental engineeringEnvironmental engineeringEnvironmental engineering is the application of science and engineering principles to improve the natural environment , to provide healthy water, air, and land for human habitation and for other organisms, and to remediate polluted sites...
- Selective non-catalytic reduction
- Vehicle emissions control