Selective catalytic reduction

The term selective catalytic reduction (english selective catalytic reduction, SCR) refers to a technique for reducing nitrogen oxides in exhaust gases from combustion plants, waste incineration plants, gas turbines, industrial equipment and motors. The chemical reaction at the SCR catalyst is selective, that is, it is preferred that the nitrogen oxides (NO, NO2 ) is reduced, while undesirable side reactions (such as the oxidation of sulfur dioxide to sulfur trioxide ) are substantially suppressed.

The end of the reaction, ammonia (NH3 ) is required, which is admixed to the exhaust gas. The products of the reaction are water (H2O ) and nitrogen (N2). Chemically, it is in the reaction to a comproportionation of nitrogen oxides with ammonia to nitrogen. There are two types of catalysts. One type consists essentially of titanium dioxide, vanadium pentoxide and tungsten dioxide. The other type uses zeolites.

Catalysts of titanium dioxide, vanadium pentoxide and tungsten oxide oxidize in the presence of gaseous halogens also present in many power plant emissions elemental mercury, which then can be better deposited in the scrubbers of flue gas desulphurization plants or in the electrostatic precipitators and only to a lesser extent (about 10% ) is released to the environment.

Another technology used side reaction is that dioxins and furans are degraded when flowing through the Entstickungskatalysators.

• 2.1 Advantages
• 2.2 disadvantages

SCR in the Kraftwerksfeuerung

Depending on the Firing (with coal-fired power plants: fluidized bed combustion system, Trockenstaubfeuerung, slag tap, etc.), the fuel and the firing temperature result in power plants by combustion nitrogen oxides, which must be removed to protect the environment from the flue gas.

The necessary facilities are referred to as " DeNOx " facilities and are among the secondary mitigation measures in the flue gas denitrification. In Germany, the SCR for flue gas denitrification compared to the other methods (eg, activated carbon, or simultaneous processes, etc. ) has prevailed.

In the arrangement of SCR in the flue gas stream of the power plant, there are three circuit variants:

High-dust

In the so- called high - dust configuration, the denitrification of flue gases from the feedwater ( economizer ) and the air preheater ( LuVo ) is provided. The plants for dust filtration are in this concept behind the NOx removal.

Among the advantages of this circuit include above all, that the flue gas needed for the catalytic reaction temperature of 300 to 400 ° C are present in the exhaust gas in any case. The potential for mercury removal is used in this circuit variant is best, since the systems for dust removal and flue gas desulfurization only after the catalyst are arranged.

However, a disadvantage is the high dust load, the appreciably reduces the life of the catalyst. Furthermore, the flue gas, the sulfur dioxide contained ( SO2) was in this circuit not yet withdrawn ( FGD ). This is oxidized to a small extent ( about 0.5 to 1.5 %) of sulfur trioxide. Since the necessary for denitrification NH3 is injected directly into the flue gas, it is in the cold area of the air preheater to an undesired reaction of SO3 with unused residual amounts of NH3 to ammonium bisulfate, which precipitates, and leads to the blockage of the air preheater.

Low -Dust

In the so-called low - dust configuration, the flue gases are first passed through the plant for dust deposition ( usually electrostatic precipitators or bag filters ) before they hit the catalyst. Characterized erosive constituents are removed, and extends the mechanical life of the catalyst. The power required to operate the Enstaubungsanlage temperature reduction of the flue gases must be compensated by a corresponding possible reheating.

Tail end

In this concept, the SCR is arranged after the flue gas desulfurization, so here account for the additional burden of most catalyst poisons and dust - this increases the life of the catalyst increased. The disadvantage of this type of connection is the fact that the flue gas only has temperatures of 50 to 100 ° C in wet and around 140 ° C in dry flue gas cleaning with lime-based sorbent (limestone, calcium hydroxide). To achieve the required temperature for SCR, however, the gas must be preheated (for example, duct burner ), which deteriorates the total efficiency of the system. In systems with dry RGR with NaHCO3, the temperature in the range 180 to 190 ° C, which makes the post-heating unnecessary.

SCR in automotive engineering

In vehicle technology, the SCR process is used to reduce nitrogen oxide emissions from diesel vehicles. Using this technique, commercial vehicles to the Euro V standard, Agriculture meet the Tier 3b and standard car the very stringent U.S. BIN5 emissions standard as well as the Euro - 6 standard.

The ammonia required for the SCR reaction is not directly, that is, in pure form, used but consistently referred to in the form of a 32.5 percent aqueous urea solution from the industry with AdBlue. The composition is specified in DIN 70070. This aqueous solution is injected upstream of the SCR catalyst in the exhaust system, e.g. by means of metering or injector. From the urea -water solution produced by a hydrolysis reaction of ammonia and CO2. The ammonia thus produced can react in a special SCR catalyst at an appropriate temperature with the nitrogen oxides in the exhaust gas. The amount of injected urea is dependent on the motor and thus the nitrogen oxide emission of the instantaneous rotational speed and the torque of the motor. The consumption of urea -water solution is dependent on the raw emissions of the engine about 2-8 % of the amount of diesel fuel used. It must therefore be accompanied by a corresponding tank volume. To achieve high NOx reduction rates, it is important that AdBlue is dosed in proportion to nitrogen oxide emissions of the engine. Since SCR catalysts can store up to a certain limit NH3, the dosage must meet the average NOx emission. The dosage is too low, the efficiency of the nitrogen oxide reduction decreases, the urea is added too much, it may not react with NOx and escape into the environment, the ammonia formed therefrom. Since ammonia has a pungent smell and can already be perceived in very small concentrations, this would result in an overdose to an odor nuisance in the vicinity of the vehicle. Remedy is by an oxidation catalyst installed downstream of the SCR catalyst. This converts in the case of ammonia overdose, NH3 back into nitrogen and water. Another way to prevent the so-called ammonia slip is greater design of the catalyst, in order to obtain a certain amount of memory function.

Hydrolysis of the urea solution:

Reduction of nitrogen oxides:

Nitrogen oxide reduction takes place without changing the engine combustion and thus receives the very high efficiency of diesel engines.

In October 2006, DaimlerChrysler in the U.S. a car, the E 320 BlueTec ® first introduced with a memory catalyst for post-engine denitrification.

In the future, an SCR system with AdBlue injection will be used, which has a higher efficiency in the reduction of nitrogen oxides in passenger cars. Since 2007, various BMW models are available with the 3.0 -liter diesel engine with AdBlue injection. Since 2009, the used "Audi Q7 3.0 TDI clean diesel" this system. Also the Audi A4 and Audi Q5 SCR systems are installed as standard. The corresponding models are designated "clean diesel". Also since the beginning of 2009, the VW Passat BlueTDI is on the market. Compared to the version without SCR system the motor has a slightly higher output and lower by about 3 to 4% of consumption as well as lower CO2 emissions.

Benefits

By a selective catalytic reduction of nitrogen oxides are removed from the exhaust gas with a high efficiency. In contrast to the diesel particulate filter ( DPF), no increase in fuel consumption is established. This advantage also applies to the alternative technology for the reduction of nitrogen oxides by means of a NOx storage catalytic converter which, like the DPF requires a temporary departure from optimum combustion conditions. The installation of an SCR system for NOx reduction makes it possible to operate the engine in fuel-efficient operating points. This reduces consumption depending on driving style 3-8 %.

Disadvantages

The main disadvantage of the use of SCR technology arises from the need of AdBlue. This must be kept in another tank and demand sprayed into the exhaust stream. This results not only from the SCR catalyst and the injection system is the need for a second smaller tank next to the fuel tank.

There are a number of attempts to reduce the storage problem by the use of materials with higher storage density or circumvent by the direct production of ammonia from fuel in the vehicle. Since this date, however, were unsuccessful, the SCR technology with AdBlue tank, SCR catalyst and injection control has prevailed in almost all major truck manufacturers to meet the Euro 6 emissions standard.

The initially feared problem of universal coverage with aqueous urea solution AdBlue for the heavy traffic has been solved since the beginning of 2005 by a number of vendors. Today, as the first Euro -6 trucks are already operating, there is AdBlue account with trucking companies and many public service stations, which are usually on long-haul routes. In addition to pumps for AdBlue can now be found in Europe at many petrol stations also refill.