Nitrogen oxide reduction



Nitrogen oxides or nitrogen-oxygen compounds, especially NO and NO2, cannot be converted into harmless materials by simple oxidation, but instead can only be converted back to the required elemental nitrogen by releasing oxygen, i.e. reduction.

Exhaust gases from engines which that in accordance with the diesel engine principle always have a systemic large amount of excess oxygen, meaning that the three-way catalyst principle for reducing nitrogen oxides under lack of oxygen cannot be used for diesel engines.

A similar principle, the NOx storage catalyst, has limited effectiveness with diesel engines (and thus limited application). Here, nitrogen oxides are temporarily stored in a special catalyst converter and reduced using brief rich phases at short intervals. The effectiveness and durability are, however, limited; the increased fuel consumption is not insignificant. For this reason, Exomission does not offer NOx storage catalyst systems.


SCR process

Selective Catalytic Reduction – SCR

Nitrogen oxide reduction using selective catalytic reduction (SCR) has been used successfully for industrial applications for decades and also in mobile applications for a few years now.

The SCR system can be roughly broken down into the SCR catalysts and the dosing system, including sensors. Extensive information about SCR catalysts can be found [here].

SCR technology injects ammonia in the form of a substrate (AdBlue®) into the exhaust gas system before an SCR catalyst converter, which is used to help reduce nitrogen oxides selectively to elemental nitrogen, with conversion rates of up to 90% and above.

The process works with a urea-water solution (AdBlue®), which stores ammonia in a non-toxic and space-saving form in a preliminary stage. SCR technology has been in use in various application areas for decades. The ammonia reacts with the nitrogen oxides in the catalyst to produce the harmless products of water and elemental nitrogen.

After the urea is injected into the exhaust gas system, NH3 must first be formed for the SCR reaction. Treatment is divided into the two following reaction steps:


Thermolysis     (NH2)2C­­­­O  → NH3 + HNCO


Hydrolysis         HNCO + H2O → NH3 + CO2


In thermolysis (the first reaction) urea is converted into ammonia (NH3) and isocyanic acid (HNCO) using the influence of temperature. In the second step, hydrolysis follows in the presence of water, also turning the isocyanic acid into ammonia and forming carbon dioxide (CO2). The last reaction takes place slowly, particularly at low temperatures of under 250°C without a catalyst. For this reason, a hydrolysis catalyst must either be connected ahead of the SCR catalyst or integrated into the SCR catalyst for low temperature applications.

After the urea is converted into NH3, it is then stored in the active centres of the SCR catalyst. This then results in the following definitive chemical reactions to reduce NOx using NH3:


Standard SCR    4NO + 4NH3 + O2 →  4N2 + 6H2O


Rapid SCR     NO + NO2 +2NH3→ 2N2 + 3H2O


The sequence of the reactions depends on various factors, including the NO2/NOx ratio. Since the raw exhaust gas is predominantly NO3, the standard SCR reaction is the dominant one. The rapid SCR reaction is the critical reaction equation at low temperatures under 300°C. This makes this reaction the favourable one with respect to a good rate of conversion in the otherwise difficult to control low-temperature range. This can be accomplished using an upstream diesel oxidation catalyst.


SCR systems


Our systems for the selective catalytic reduction of nitrogen oxides can be used in all areas of engine application in which fuel is combusted with surplus air. The hardware components of the AdBlue® dosing system include the dosing module (with an integrated control unit), the injector or the injection nozzle, the AdBlue® lines and the NOx and temperature sensors. Unlike other designs available on the market, both of the systems we offer are based on a sensor-based closed loop control strategy. The advantage of regulating with two NOx sensors is that the actual value and the target value can be constantly compared. This means that the control acts immediately, achieving the best possible nitrogen oxide reaction rate at all times in operation.


Two NOx sensors and one or two temperature sensors (depending on the requirement) form the basis of the control software. Various strategies can be selected to determine the exhaust flow depending on the application. In this way, there is a means of recording the air flow using a separate air flow sensor. If this is not possible, air flow can be calculated using data from the engine’s sensors or separate sensors. Put simply, these input variables are evaluated in the SCR storage model and the alpha control in the control unit in order to prepare the optimum quantity of AdBlue® required.


The systems we offer have been in used for many years and have proven themselves in diverse applications. Both systems have a display module which that the operator data such as a low tank level or malfunctions. The systems also have a diagnostics interface, which our service technicians can use to read all relevant data.


The following briefly describes the two SCR systems that we offer.

Air-assisted system

This system works with a fully high-grade steel nozzle. A separate compressed-air connection is required to operate the system. The system’s area of use is primarily stationary systems, but also mobile machines when on-board compressed air is available. A very fine spray is created in the dosing unit using the compressed-air. As a consequence of the complete abolition of the electronic control components, the nozzle does not have to be cooled separately, meaning that this system is particularly suited for very high temperatures.
















Airless system

This system is predominantly used for mobile applications, as it is operated without an additional air supply. The injector used is controlled using a pulse-width modulated signal in order to inject the AdBlue® quantity as required. The injector corresponds to a further development of a petrol engine injection nozzle and was modified with respect to AdBlue® compatibility. Depending on the location of the injector assembly in the exhaust gas system, it may additionally have to be cooled. This can be achieved using the engine’s cooling circuit or a separate cooling circuit.

















Ammonia slip catalysts

The critical parameter in the SCR process is the AdBlue® volume injected and the NH3 concentration formed as a result. If more reduction agent is prepared than is used in the reduction of NOx emissions, what is known as an ammonia slip occurs. This designation refers to the NH3 concentration in the exhaust gas that exists downstream in the SCR catalyst. As gaseous NH3 has a very low smell threshold of 15 ppm, unpleasant odours occur even with a small ammonia slip. NH3 is also toxic and leads to irritation of eyes, airways and skin at higher concentrations.

For this reason, it is essential that ammonia emission from the exhaust gas system is avoided at all costs. Despite careful application of the SCR system, we therefore recommend that you use a catalyst to prevent ammonia slip, especially when high NOx conversion rates are required.





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