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Impact of SCR-DeNOx catalysts on mercury behaviour

Within the project the efficiency of emission control technologies with respect to mercury removal from flue gases was evaluated. At this the impact of SCR-DeNOx catalysts was studied at pilot- as well as full-scale facilities.

In order to determine how SCR affects mercury speciation, measurements were carried out on different facilities and the effect of fuel composition, SCR operation conditions (SV, temperature, ammonia injection), SCR catalyst age and type, were investigated.

The decrease of elemental mercury through the SCR and then the ESP has been clearly assessed.

A clear correlation between the fuel Cl content and the SCR mercury conversion has been assessed meaning that chlorine takes part to the oxidation reactions on the catalyst, although the mechanism is not understood yet. Space velocity strongly affects the mercury oxidation: the larger the SV the lower the mercury oxidation. The effect of SV on mercury oxidation confirms that conversion process is influenced by a catalytic reaction.

By running tests at a test facility at different ammonia injection rates resulting in different NOx/NH3 ratios it was seen, that the NOx reduction and the mercury oxidation reaction are competing. From this it was concluded for full-scale plants, that at a new catalyst the NOx reduction reaction takes place at the first catalyst layer.

After that the ammonia is consumed and the Hg oxidation can take place at the following catalyst layers. In case of deactivation of the first layer the NOx reduction is moved to the second layer and the mercury oxidation can only take place at the third layer. This means, that catalyst deactivation first affects mercury oxidation before the NOx reduction capacity of a catalyst significantly decreases.

Documentation and Information on this result is available in the deliverable reports D07 and D10 which are available on the project website.

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University of Stuttgart, IVD
Pfaffenwaldring 23
70569 Stuttgart
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