Selective catalytic reduction

Exhaust system of the diesel engine with catalytic converters and urea injection, schematic representation

The term selective catalytic reduction ( English selective catalytic reduction , SCR ) refers to a technique for reduction of nitrogen oxides in exhaust gases from combustion plants , waste incineration plants , gas turbines , industrial plants and internal combustion engines . The chemical reaction on the SCR catalytic converter is selective, which means that the nitrogen oxides ( NO , NO ₂ ) are preferably reduced, while undesirable side reactions such as the oxidation of sulfur dioxide to sulfur trioxide are largely suppressed.

Ammonia (NH ₃ ), which is mixed with the exhaust gas, is required for the reaction . The products of the reaction are water (H ₂ O) and nitrogen (N ₂ ). The reaction is a comproportionation of nitrogen oxides with ammonia to nitrogen. There are two types of catalysts : the first consists mainly of titanium dioxide , vanadium pentoxide and tungsten dioxide to stabilize the titanium dioxide in its anatase form, the other uses zeolites . Another new development is a catalyst based on activated carbon . Before these processes were introduced, developments in other catalysts were carried out in Germany, but iron oxide catalysts in particular have not proven themselves on an industrial scale.

In the presence of gaseous halogens , catalysts made from titanium dioxide, vanadium pentoxide and tungsten oxide also oxidize the elementary mercury present in many power plant exhaust gases , which can then be better separated in the scrubbers of the flue gas desulphurisation systems or in electrostatic precipitators and only to a lesser extent (approx. 10%) Environment.

As a further technically used side reaction, dioxins and furans are broken down when flowing through a denitrification catalyst.

Chemical reaction

If urea is used, it must first be decomposed in a thermolysis and subsequent hydrolysis reaction in order to release the ammonia required for the SCR reaction.

Thermolysis and hydrolysis of the injected urea takes place on the path in front of the catalytic converter, the so-called hydrolysis path. Since the optimal atomization and evaporation of the aqueous urea solution is decisive for the implementation, various mixer designs are usually used to improve them.

Thermolysis of urea to ammonia and isocyanic acid

${\displaystyle \mathrm {(NH_{2})_{2}CO\longrightarrow NH_{3}+HNCO} }$

Subsequent hydrolysis , the isocyanic acid reacts with water to form additional ammonia and carbon dioxide

${\displaystyle \mathrm {HNCO+H_{2}O\longrightarrow NH_{3}+CO_{2}} }$

So-called hydrolysis catalysts are often used to improve the decomposition of urea, especially at cold exhaust gas temperatures. The urea decomposition can be further improved by only having part of the total exhaust gas flow through these catalytic converters; H. they are operated in a partial flow.

If the urea does not decompose completely, solid deposits of cyanuric acid (trimer of isocyanic acid) or melamine can form via the intermediate product isocyanic acid, which can lead to blockages in the exhaust system.

However, if the urea is completely decomposed, two ammonia molecules are formed from one urea molecule.

Reduction of nitrogen oxides by means of selective catalytic reduction in the reduction catalytic converter

Standard SCR (temperature over 250 degrees)

${\displaystyle \mathrm {4NO+4NH_{3}+O_{2}\longrightarrow 4N_{2}+6H_{2}O} }$

Fast SCR (temperature over 170 to 300 degrees, "Fast SCR")

${\displaystyle \mathrm {NO+2NH_{3}+NO_{2}\longrightarrow 2N_{2}+3H_{2}O} }$

"NO ₂ SCR"

${\displaystyle \mathrm {8NH_{3}+6NO_{2}\longrightarrow 7N_{2}+12H_{2}O} }$("NO ₂ SCR")

the ammonia reacts with the nitrogen oxides to form nitrogen and water.

In power plants and diesel engines, the proportion of NO ₂ in the total nitrogen oxides is usually only approx. 5%, so that only the standard SCR reaction is relevant without further measures. In order to enable the "Fast SCR" reaction nonetheless and thus lower the light-off temperature, so-called NO oxidation catalysts are used upstream of the SCR catalysts. These platinum-containing catalysts raise the NO ₂ content and thus enable the "Fast SCR" reaction. However, this only applies up to an NO ₂ proportion of 50% of the total nitrogen oxides. If the NO ₂ proportions rise above this limit, the so-called NO ₂ -SCR reaction takes place. Since this takes place much more slowly and the NH ₃ requirement also increases, NO ₂ proportions over 50% should be avoided.

In contrast to three-way catalytic converters, the use of the reducing agent ammonia makes it possible to reduce nitrogen oxides to nitrogen in the presence of oxygen.

SCR in power plant firing

In 1974 Masumi Saito, Sumio Tani, Tateo Ito and Shigeaki Kasaoka patented by the Japanese company Kurashiki Boseki Kabushiki Kaisha (Kurabo Industries Ltd.), based in Osaka, how ammonia can be mixed into the exhaust gas to convert the nitrogen oxides contained therein into harmless nitrogen and To convert water. Iron or copper sulfide was used as a catalyst. Methods were later added on how to correctly dose ammonia. Over time, urea became the state of the art for reducing nitrogen oxides in the exhaust gas in stationary power plants.

Depending on the firing concept (in coal-fired power plants: fluidized bed firing, dry dust firing , melting chamber firing), the fuel and the firing temperature, nitrogen oxides are generated in power plants through combustion , which have to be removed from the flue gas to protect the environment .

The systems required for this are referred to as "DeNO _x " systems and are among the secondary reduction measures for flue gas denitrification . In Germany, the SCR for flue gas denitrification has prevailed over other processes such as activated carbon or simultaneous processes.

When arranging the SCR in the flue gas flow of the power plant, one differentiates between three circuit variants:

1. High dust
2. Low dust
3. Tail end

High dust

In the high-dust circuit, denitrification of the flue gases is provided between the feed water preheater ( economiser ) and the air preheater (LuVo). In this concept, the systems for dust filtering are located behind the denitrification.

One of the advantages of this circuit is that the flue gas temperatures of 300 to 400 ° C required for the catalytic reaction are already present in the exhaust gas. The potential for mercury removal is best used with this circuit variant, since the systems for dust separation and flue gas desulphurisation are only arranged after the catalytic converter.

The disadvantage, however, is the high dust load, which noticeably reduces the service life of the catalyst . Furthermore, the sulfur dioxide (SO ₂ ) contained in this circuit has not yet been removed from the flue gas ( flue gas desulphurisation ). A small part (about 0.5 to 1.5%) of this is oxidized to sulfur trioxide. Since the NH ₃ required for denitrification is injected directly into the flue gas, an undesirable reaction of the SO ₃ with unused residual amounts of NH ₃ occurs in the cold area of ​​the air preheater to form ammonium bisulfate , which precipitates and leads to the air preheater being blocked.

Low dust

With the low-dust circuit, the flue gases are first passed through the dust separation system (usually electrostatic precipitators or bag filters ) before they hit the catalytic converter. This removes erosive components and extends the mechanical life of the catalytic converter. The temperature reduction of the flue gases necessary for the operation of the dust extraction system may have to be compensated for by appropriate reheating.

Tail end

In this concept, the SCR is arranged after the flue gas desulphurisation , so that the additional loads caused by most catalyst poisons and dust are eliminated - this extends the service life of the catalyst. The disadvantage of this circuit variant is that the flue gas only has temperatures of around 50 to 100 ° C for wet and around 140 ° C for dry flue gas cleaning with lime-based sorbents ( limestone , calcium hydroxide ). In order to reach the temperature required for the SCR, however, the gas must be preheated (e.g. duct burner), which reduces the overall efficiency of the system. In systems with dry RGR with NaHCO ₃ , the temperature is in the range from 180 to 190 ° C, which makes reheating superfluous.

SCR for use in vehicles and ships

SCR system for a Deutz-Fahr Agrotron K610 tractor

SCRi system - i = with integrated particle filter - for a Multicar Fumo (municipal vehicle ). The exhaust manifold from the engine and the turbocharger can be seen at the top right.

Tank for the urea solution on a truck

Tank for the urea solution in the spare wheel well on a car

history

Around 2000, the SCR technology for diesel engines was initially adapted for heavy commercial vehicles and in 2002 its practicality was tested in a field test. Since the introduction of the IMO Tier III limit values ​​for ocean-going vessels (corresponds roughly to the EuroV limit values ​​for trucks) in 2016, the procedure has also been widely used for ships. In contrast to the SCR process in power plants, no ammonia is used due to its toxicity, especially in engines used in vehicles. Rather, an aqueous urea solution is injected here, which releases the ammonia required for the SCR reaction in the hot exhaust gas.

In March 2003 OMV opened its first public filling station where, in addition to diesel fuel, the necessary urea solution could also be refueled at a pump.

The properties of the urea solution for exhaust gas cleaning from diesel engines were standardized in Germany from 2003 with DIN 70070 (initially as a pre-standard) and the composition with 32.5% pure urea in demineralized water as well as the neutral designation " AUS 32 " was specified for it. With ISO 22241, the DIN regulations and the designation were also adopted internationally.

Since 2004, SCR emission control has been used as standard in truck engines from the Euro 4 emission standard . The aqueous solution is in front of the SCR catalytic converter in the exhaust system, z. B. by means of a metering pump or injector, sprayed. A hydrolysis reaction produces ammonia and CO ₂ from the urea-water solution . The ammonia generated in this way can react with the nitrogen oxides in the exhaust gas in the downstream SCR catalytic converter at the appropriate temperature . Nitrogen and water are created. The amount of urea injected depends on the engine's nitrogen oxide emissions and thus on the current speed and torque of the engine. The consumption of urea-water solution is - depending on the raw emissions of the engine - around 2–8% of the amount of diesel fuel used. A corresponding tank volume must therefore be carried with you.

2007 was the Model 320 Bluetec Mercedes-Benz E in the United States in car range for the first time in a vehicle with a diesel engine used an SCR exhaust aftertreatment, in the first version still with storage catalyst. From 2008 Mercedes used urea injection.

The nitrogen oxide reduction takes place without changing the engine's combustion and thus maintains the very good efficiency of diesel engines.

At exhaust gas temperatures of up to 550 ° C (trucks, ships, construction machinery), vanadium-based SCR catalysts are mostly used, and above that, zeolite-containing catalysts (cars).

As of 2019, SCR exhaust gas aftertreatment will be used in diesel engines in cars, commercial and rail vehicles, and ships.

technology

To achieve high NO _x reduction rates, it is important that the solution is dosed in the correct proportion to the nitrogen oxide emissions from the engine. Since SCR catalytic converters can store NH ₃ up to a certain limit , the average dosage must correspond to the NO _x emissions. If the dosage is too low, the efficiency of the nitrogen oxide reduction decreases; if too much urea is added, the ammonia formed from it cannot react with NO _x and get into the environment. Since ammonia has a pungent odor and can be perceived even in very small concentrations, an overdose would result in an unpleasant odor in the vicinity of the vehicle. This can be remedied by installing an oxidation catalytic converter behind the SCR catalytic converter. In the event of an ammonia overdose, this converts the NH ₃ back into nitrogen and water. Another possibility to prevent the so-called ammonia slip is a larger design of the catalytic converter in order to obtain a certain storage function.

	Car
standard	Euro 6	Tier 2 Bin 5
CO	500 mg (per km)	2113 mg (per km) 3400 mg (per mile)
( HC + NO x )	170 mg (per km)
HC		47 mg (per km) 75 mg (per mile)
NO x	80 mg (per km)	31 mg (per km) 50 mg (per mile)

Urea solution OFF 32

The properties of the urea solution for the emission control of motor vehicle engines were standardized with DIN 70070 (first as a pre-standard) in Germany from 2003 and the composition with 32.5% pure urea in demineralized water as well as the neutral designation " AUS 32 " was determined. With ISO 22241, the DIN regulations and the designation were also adopted internationally.

The freezing point of the urea solution AUS 32 is −11.5 ° C, which is why additional heating is necessary for vehicles in temperate zone countries with freezing temperatures in winter. The storage tank is heated for this purpose and the line system can be emptied. The emptying process is implemented, for example, by reversing the submersible pump in the storage tank: the solution in the pipe system is returned to the heatable storage tank after the ignition is switched off.

The aqueous solution does not pose any particular risk in terms of European chemicals law. Also according to the transport law it is not a dangerous good. Skin contact should be avoided; any residues can be washed off with water.

Urea solution OFF 40

In the maritime sector, a 40% urea solution (AUS 40 - ISO 18611) with a melting point of 0 ° C has become established due to its higher urea content. Due to the fact that the tanks for the urea solution are arranged within the ship's structure and thus almost never reach 0 ° C., there is usually no risk of the solution freezing.

SCR system in cars
1 = SCR tank
2 = line
3 = injector computer
4 = injector
5 = catalytic converter

Urea consumption

Consumption depends primarily on the engine's raw NOx emissions, so that different values ​​can be found in the literature depending on the engine design (e.g. with or without EGR ). The Robert Bosch GmbH are at 5 percent of the amount of the diesel fuel employed, 7 percent is assumed in agriculture tractors. The Association of the Automotive Industry gives 1.5 liters for cars per 1000 km, other sources speak of up to 4 liters per 1000 km for appropriate cleaning.

The manufacturers' tank sizes vary from 12 to 25 liters for cars, 50 to 100 liters for trucks and several cubic meters for ocean-going ships. According to a study by TNO , the SCR tank in Euro 6 cars is between 45 and 80 percent too small if refilling between maintenance intervals is to be avoided. In commercial vehicles, the use of urea solution enables the start of injection earlier and thus reduces fuel consumption by around 6%.

Financial aspects

The technical equipment of vehicles with an SCR system incurs costs as well as weight and space requirements for tank, lines, sensors, electronics. The prices of diesel vehicles with SCR catalytic converters were significantly higher in 2015 than vehicles with gasoline engines.

Scams

Since the use of the urea solution causes additional costs, fraudulent diesel truck operators are illegally using electronic shutdown devices. With the prohibited change in pollutant emissions, several fines are affected in Germany, e.g. B. in the area of ​​road traffic and motor vehicle tax law; In addition to and independently of fines , the toll amount not paid in the required amount will be levied retrospectively by the Federal Office for Goods Transport for trucks subject to toll .

From September 2015 it became known that Volkswagen AG was using an illegal shutdown device in the engine management of its diesel vehicles in order to minimize the consumption of the solution: If the software detects that the vehicle is on a roller dynamometer, the exhaust gas aftertreatment with SCR Catalysis carried out so that the legal limit values ​​are complied with, but with normal movement it is switched off - see exhaust scandal . A similar approach was later found at other German and international manufacturers.

General aspects of exhaust aftertreatment using SCR

A selective catalytic reduction removes nitrogen oxides from the exhaust gas with a high degree of efficiency. In contrast to the diesel particulate filter (DPF), there is no additional fuel consumption. This advantage also applies to the alternative method for reducing nitrogen oxides by means of a NO _x storage catalytic converter, which, like the DPF, requires a temporary avoidance of optimal combustion conditions. The installation of an SCR system to reduce NO _x makes it possible to operate the engine at more economical operating points. This reduces consumption by between 3% and 8%, depending on the driving style.

The necessary AUS 32 urea solution can be obtained from haulage companies and many public filling stations. In addition to filling pumps for the urea solution, refill canisters can be found at many filling stations across Europe.

literature

• Helmut Effenberger: Steam generation. Springer, Berlin a. a. 2000, ISBN 3-540-64175-0 .
• Karl Strauss: Power plant technology. For the use of fossil, regenerative and nuclear energy sources. 4th edition. Springer, Berlin a. a. 1998, ISBN 3-540-64750-3 .
• Kurt Kugeler, Peter-W. Phlippen: energy technology. Technical, economic and ecological basics. 2nd Edition. Springer, Berlin 1993, ISBN 3-540-55871-3 .

Individual evidence

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3. ↑ ^{a b} Andreas Döring, Eberhard Jacob: GD-KAT: Exhaust aftertreatment system for reducing particulate and NOx emissions in commercial vehicle diesel engines . In: Leipertz (Hrsg.): Engine combustion - current problems and modern solutions . BEV - House of Technology, HdT Essen 2001, p. 513-528 .
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14. ↑ Mun Kyu Kim: Design of an urea decomposition chamber using urea decomposition catalyst in NoNOx-LP SCR system for 2-stroke engine . Ed .: Cimac congress. Helsinki 2016.
15. ↑ Changseong Ryu: The World's First Commercialized Low Pressure SCR system on 2-Stroke Engine, DelNOx System . Ed .: Cimac Conference. Helsinki 2016.
16. ↑ OMV is building Europe-wide infrastructure for low-emission trucks . Retrieved November 24, 2014.
17. ↑ ^{a b} DIN 70070 diesel engines - NOx reducing agents AUS 32 - quality requirements.
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20. ^ Diesel Exhaust Fluid (DEF) - How it works. , March 25, 2010.
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25. ^ Robert Bosch GmbH: Kraftfahrtechnisches Taschenbuch. 28th edition, Springer Vieweg, Wiesbaden 2014, ISBN 978-3-658-03800-7 , p. 719.
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27. ↑ AdBlue (PDF), vda.de, p. 9.
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29. ↑ TNO Report 2015 R10838 Emissions of nitrogen oxides and particulates of diesel vehicles (PDF), TNO.nl (accessed on August 27, 2017)
30. ^ Rolf Gscheidle et al .: Expertise in automotive technology. 30th edition, Europa-Lehrmittel, Haan-Gruiten 2013, ISBN 978-3-8085-2240-0 , p. 745.
31. ↑ TradersCity.com: Adblue Scr Emulation Module Emulation Nox Original, Not Chinesse Copy Not Fake | SpecialDiagnostic | TradersCity. Retrieved May 16, 2017 . 
32. ↑ Alpine Initiative - The Federal Council must act. Retrieved May 16, 2017 . 
33. ↑ Spiegel.de: Authority suspects illegal defeat device in the Mercedes Vito , February 14, 2018, accessed on March 30, 2018
34. ↑ Spiegel.de: BMW is also said to have manipulated exhaust gas cleaning , December 5, 2017, accessed on March 30, 2018
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