What are the static properties of SCR

Selective catalytic reduction

With SCR (selective catalytic reduction) is the name given to the technique of selective catalytic reduction of nitrogen oxides in exhaust gases from combustion systems and engines. The chemical reaction of the reduction is selective, i.e. not all exhaust gas components are reduced, only the nitrogen oxides (NO, NO2).

Ammonia (NH3) that is added to the exhaust gas. The products of the reaction are water (H.2O) and nitrogen (N2). From a chemical point of view, the reaction is a comproportionation of nitrogen oxides with ammonia to form nitrogen. There are two types of catalysts. One type consists essentially of titanium dioxide, vanadium pentoxide and tungsten oxide. The other type uses zeolites.

SCR in power plant firing

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

The necessary systems are called "DeNOx"Systems designate and count among the secondary reduction measures for flue gas denitrification. In Germany, the SCR for flue gas denitrification has prevailed over other processes (e.g. activated carbon or simultaneous processes, etc.).

When arranging the SCR in the flue gas flow of the power plant, a distinction is made between two circuit variants:

High dust

In the so-called 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 main advantages of this circuit is that the flue gas temperatures of 300-400 ° C required for the catalytic reaction are already present in the exhaust gas.

The disadvantage, however, is the high dust load, which noticeably reduces the service life of the catalyst. Furthermore, the sulfur dioxide (SO2) not yet withdrawn (flue gas desulphurisation). Since the NH needed for denitrification3 is injected directly into the flue gas, it can lead to an undesirable side reaction of the SO2 with unused residual amounts of NH3 come to ammonium bisulfate, which precipitates in the air preheater.

Low dust

In this concept, the SCR is arranged after the flue gas desulphurization, so that the additional loads from SO2 and dust is eliminated - this extends the service life of the catalytic converter. The disadvantage of this circuit variant is that the flue gas only has temperatures around 50-100 ° C (with wet RGR). In systems with dry RGR (with NaHCO3) the temperature is in the range of 180-190 ° C, which makes reheating superfluous. Otherwise, in order to reach the temperature required for the SCR, the gas has to be preheated (e.g. duct burner), which worsens the overall efficiency of the system.

SCR in vehicle technology

In vehicle technology, the SCR process is used to reduce pollutant emissions in diesel vehicles, especially commercial vehicles. This means that these diesel vehicles can meet the EU5 standard. The ammonia required is not here directly, i.e. H. in pure form, used, but in the form of a 32.5% aqueous urea solution, uniformly referred to by the industry as AdBlue®. The composition is regulated in DIN 70070. The solution is in front of the SCR catalytic converter in the exhaust system, z. B. by means of a metering pump or injector, injected. A hydrolysis reaction creates ammonia and water from the urea-water solution. The ammonia generated in this way can react with the nitrogen oxides in the exhaust gas in a special SCR catalytic converter at the appropriate temperature. 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 around 2 to 8% of the diesel fuel used, depending on the raw emissions of the engine. A corresponding tank volume must therefore be carried with you.

AdBlue® is a registered trademark of the Verband der Automobilindustrie e. V. (VDA).

The reason why the required ammonia is not carried in pure form is the dangerousness of this substance. Ammonia has a corrosive effect on the skin and mucous membranes (especially the eyes), and it forms an explosive mixture in air.

DaimlerChrysler recently owned a car in the USA, the E 320 Bluetec® introduced with a storage catalytic converter for post-engine denitrification. In the future, an SCR system with AdBlue injection is to be used in SUVs, which has a higher degree of efficiency in reducing nitrogen oxide. A study has already been shown as the Vision GL 420 CDI Bluetec.


By means of selective catalytic reduction, nitrogen oxides are largely removed from the exhaust gas. In contrast to the diesel particulate filter (DPF), there is no additional fuel consumption. This advantage also applies to the alternative technology for reducing nitrogen oxides by means of an NOxStorage catalytic converter, which, like the DPF, requires a temporary avoidance of optimal combustion conditions.


The main disadvantage of using SCR technology in trucks, for example, results from the ammonia required in the form of AdBlue. Due to its special properties, this additional operating material must be carried in a stainless steel or plastic tank and continuously sprayed into the exhaust gas flow. As a result, in addition to the SCR catalytic converter and the injection system, a second, usually smaller tank is required next to the diesel tank. There are a number of attempts to reduce the storage problem by using substances with a higher storage density or by generating ammonia directly from fuel in the vehicle. However, since these have so far failed to succeed, SCR technology with AdBlue tank, SCR catalytic converter and injection control has established itself in almost all major truck manufacturers to achieve the Euro 5 emissions standard.

Since the beginning of 2005, a number of providers have been starting to solve the problem of a comprehensive supply of AdBlue aqueous urea solution for heavy goods vehicles. Today, with the first Euro 5 trucks already in operation, there are AdBlue depots at haulage companies and some public filling stations.

Another disadvantage is that AdBlue has to be injected variably. So far, it has to have a feed ratio to the NOx be adjusted in the exhaust gas mass flow. If too much urea is added, the ammonia formed from it cannot react with NOx react. With this incorrect dosage ammonia can get into the environment. Since ammonia can be perceived in very small concentrations, this leads to an unpleasant odor. This can be remedied by installing an oxidation catalytic converter behind the SCR catalytic converter. This converts the NH in the event of an ammonia emission3 again in NOx around. 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. A NOx-Sensor behind the SCR catalytic converter has been developed to solve this problem. It will be used in SCR vehicles from 2006.


  • Helmut Effenberger: Steam generation. Springer-Verlag 2000, ISBN 3-540-64175-0
  • K. Strauss: Kraftwerkstechnik, 4th edition, Springer-Verlag, ISBN 3-540-64750-3
  • Kugeler, Phlippen: Energietechnik, 2nd edition, Springer-Verlag, ISBN 3-540-55871-3

See also

Category: Chemical-technical process