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Leistungen
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.
The countermeasures concerning the catalyst deactivation largely depend on the deactivation mechanism of catalyst poisons. Therefore, the understanding of the mechanisms leading to deactivation is essential.
Within the consortium, the behaviour of alkali metals, especially of potassium as a constituent of biomass was the focus of investigations of the Scandinavian partners. The primary deactivating compounds were identified and the deactivation potential of their aerosols in a synthetic flue gas was verified.
The activities of the German partners concentrated on the element phosphorus as a deactivating compound present in secondary fuel such as meat-and-bone meal (MBM) and sewage sludge. Lab scale tests showed the deactivation potential of volatile phosphorus, which was further confirmed in co-combustion tests.
Sodium as a compound of MBM was also considered but the deactivation would be similar to potassium. Therefore, only investigation on its gas-phase concentration were carried out.
The results of the research provided the basis for the application of appropriate countermeasures either of primary nature, i.e. fuel additives or modified catalyst compositions, or secondary measures such as catalyst regeneration.
According to the composition of the consortium, both ways were pursued.
The catalyst structure and the chemical composition have a significant influence on the NOx reduction capability, the SO2/SO3 conversion as well as on the resistance towards deactivation and the capability to oxidise elemental mercury.
A possible way to circumvent the loss of activity could be to increase the operating temperature of the catalyst. Another way to prolong the lifetime of the catalyst could be to increase the content of vanadium. Thereby the catalyst would be able to take up more potassium on an absolute level and remain active. The latter approach would only be possible for fuels with low sulfur contents since increasing vanadium content accelerates the undesired oxidation of SO2 to SO3 over the catalyst.
However, studies of this project show that simply increasing the operating temperature or the vanadium content in the catalyst cannot fully compensate the loss of catalyst activity. Increasing the temperature hardly increases the conversion of NO for the strongly poisoned catalysts and partly deactivated catalysts with high vanadium content become active for oxidizing NH3 to NO. For these reasons, it was decided to proceed the activities aiming at a change in chemical composition or in the physical structure of the catalysts not further. Changing a certain parameter included a large risk for manufacturers concerning the overall performance in a DeNOx reactor. The catalyst composition of standards types is very reliable and the performance in power plant operation can safely be guaranteed. This would otherwise not be the case and would require comprehensive demostration beyond the scope of this project.
Concerning the influence of the catalyst concerning the adsorbtion and oxidation or mercury sample catalysts with a specific composition of active components and surface composition have been tested. The results indicate that primarly elemental mercury is adsorbed on the sample catalyst with only one active component while the adsorption was insignificant on the standard catlyst mixture.
Previous studies of VAB in a circulating fluidised bed plant firing forest residues and a plant firing pulverised wood in Sweden found that after 2100h, when using a conventional honeycomb catalyst 80% of the original activity remained in the circulating fluidised bed boiler but only 20% remained in the pulverised wood boiler. The tendency is that a boiler with short residence time between burnout zone and catalyst and/or high flame temperatures gives a faster deactivation. Elsam has carried out SCR catalyst tests on catalysts exposed to, respectively, high and low-dust conditions, in a pulverized fuel power plant co-firing of straw and coal. The deactivation of catalyst elements was faster than for pure coal firing but lower than for pure biomass firing. Experience with fuels having a high amount of volatile potassium show that the combustion conditions are very important for the speciation of potassium and the deactivation rate.
During full scale tests in slip stream reactor in High-Dust as well as in Low-Dust configuration however actual investigations carried out within this project did not show a significant decrease in catalyst activity during 7% thermal straw addition.
Concerning the release of phosphorus, reducing conditions as for instance during air staging for primary NOx reduction showed no detectable influence on catalyst deactivation during bench scale tests.
In full scale measurements on the behaviour of phosphorus could not detect a certain behaviour, which could solely be contributed to the combustion conditions. However, parameter variations in full scale are extremely difficult given enough time for stabilisation of the equilibrium in the boiler. Parametric studies would have exceeded the budget of the current project.
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