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H+H SCR Technology for lower NOx emissions
  • SCR Catalyst Design Basics

To limit the emission of nitrogen oxides from combustion, incineration or chemical processes the SCR (selective catalytic reduction) method is very common. Ammonia is needed as reduction agent. The ammonia is introduced into the exhaust gas stream so the reaction on the catalyst occurs. NOx is converted to harmless nitrogen and water.

Without the presence of a catalyst the SCR reaction cannot take place at temperatures in an combustion engine, boiler, etc.

To design an SCR catalyst several input boundaries must be considered. In simple words the catalyst volume is based on

  • Exhaust gas volume and Temperature
  • NOx emitted from the engine, boiler, process
  • NOx out of the catalyst – clean gas value
  • Maximum allowed back pressure

Another factor that needs to be considered is the fuel type of the emission source, the fuel type and quality gives a first indication what type of catalyst can be used.

We can use different types of catalyst, catalysts based on ceramics or metal supports with vanadium pentaoxide or zeolites as active component. Either type can be manufactured with different size of the channels, common units for the size is either SW40x40 – that refers to 40 channels per edge, or 100 CPSI, this refers to 100 channels/cells per square-inch.

SCR Catalyst Types

The mentioned specific surfaces are just an exemplary indication and might vary between manufacturers

With all that known parameters we can start to design the catalyst to fit for purpose. Our catalyst modelling tool will calculate the needed “active” surface. The value for the surface is mainly influenced by the exhaust gas temperature.

The lower the exhaust temperature the more surface will be needed.

Knowing the surface, exhaust gas quality, load profile, etc., we can pick the type of catalyst that we will use. Based on the value m²/m³ the formula will give us a catalyst volume.

The challenge for us is to find a geometrical form that fits into the vessel/building structure and keeping the needed catalyst volume. Additional, we need to consider the optimum velocity of the exhaust gas passing the catalyst. If the catalyst is, e.g. very short, but wide – the velocity will be very low. This results in laminar flow and the molecules will not enter the pores to react.

If the catalyst is very long, but narrow – the velocity will be very high. This results in very high backpressure and the molecules will not have enough time to enter into the active pores.

Thus we need to find a compromise between needed volume and exhaust gas velocity.

With the help of a profound CFD - Computational Fluid Dynamics - it is possible to represent a velocity profile, the concentration distribution of the reducing agent, but also the conversion of urea to NH3. Only in this way can the catalytic converters be safely installed in the smallest installation space with extreme requirements, e.g. in retrofitting, and the required limit values be achieved.

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