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Grading of single element contribution to overall deactivation

Fuel mixes contain a range of deactivating elements. However, these may be bound in stable compounds that they are inert and are not released during the combustion process. On the other hand, additional catalyst poisons can be released, which increase the deactivation rate, or even more harmful compounds are generated leading to even higher deactivation.

A literature study has shown that alkali metals are among the strongest poisons to SCR catalyst.

Studies in this project find that potassium both in the form of chlorides and sulfates is a strong poison for the catalyst. Exposure to these compounds happens when the catalyst is used in straw fired power plant. Aerosol measurement finds that particles in the flue gas of straw fired power plant consist of almost pure potassium chloride and sulphate with minor amounts of phosphorous. NH3 chemisorption tests show that it is the Brøndsted sites that are affected by potassium, since the amount of NH3 chemisorbed on the catalyst decreases with an increasing amount of potassium.

Alkaline earth metal react with SO3 adsorbed on the catalyst forming CaSO4, which causes catalyst surface masking, preventing the reactants from diffusing into catalyst interior surface.

Phosphorus has also been shown to decrease the catalytic activity; however, its poisoning effect is not as strong as alkali metals.

Especially increased concentrations of Na, K, Ca and P are found to be responsible for faster inactivation during co-combustion whereas the main chemical reasons of inactivation during hard coal burning are Si, Ca and sometimes As.

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