Aalborg Energie Technik a/s (AET)

Aalborg Energie Technik a/s (AET)

Emmission Reduction Services

Reduction of NOx in Flue Gas; At the combustion of all types of fuels occur, besides the heat energy, unwanted substances which through the flue gas reach the atmosphere and thereby cause environmental pollution. One of the main pollutants is NOX. Depending on the temperature conditions in the furnace room, it is possible to reduce the content of NOX in the flue gas by adding urea or ammonia water to the combustion.

Different technologies are used for the reduction of the NOX content, each of which have their own specific advantages and disadvantages. The two main technologies employed are SCR and SNCR.

The SCR process (Selective Catalytic Reduction) works by injecting ammonia water which is vaporised and fed into the flue gas. The evaporated ammonia water will then pass the catalyst elements, whereby NOx in the exhaust gas is transformed into nitrogen and water.


4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O

NO + NO2 + 2 NH3 → 2 N2 + 3 H2O

The Advantages are:

  • Reduction of NOX emissions by up to 98%
  • Low NH3 slip

The Disadvantages are:

  • Purchase cost in the range 5-10 times higher than an SNCR plant
  • High operating costs
  • Bulky installation

The SNCR process (Selective Non-Catalytic Reduction) utilises the reduction capacity in the reducing media.  The substance is injected as an aqueous solution in the hot flue gas in the furnace room at temperatures between 900 and 1100 °C.

In order to obtain a good NOX reduction, the aim is to attain a consistent distribution of the reducing media over the cross section of the furnace. The reduction proceeds as a homogeneous gas phase reaction, preferably with the following reaction sequence, where molecular nitrogen, water vapour, and carbon dioxide by urea is formed.


CO (NH2)2 + 2NO + 1/2OÞ 2N2 + CO2 + 2H2O


4NO + 4NH3 + O2 Þ 4N2 + 6H2O

2NO2 + 4NH3 + O2 Þ 3N2 + 6H2O

As shown in Figure 1, the reduction proceeds best over a limited temperature range between 900 and 1100 OC. At higher temperatures the secondary reaction steps into the foreground, and nitrogen dioxide is formed. On the other hand, lower temperatures will cause a deterioration of the reaction rate and unconverted urea or pyrolysis products form ammonia (NH3).

The Advantages are:

  • The reducing agents can be stored without pressure and can be regarded as water hazard class 1.
  • Since the reducing agents are produced in large quantities, the security of supply is good.
  • Low operating costs
  • Extremely competitive costs
  • No residues to be disposed of.
  • Can reduce NOX emission by 50-75%

The Disadvantages are:

  • High NH3 slip at low emission.
  • Reduction is limited to approx. 80%

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