Heterogeneously catalyzed reactions over newly developed SCR DeNOx catalysts

Thesis from Tobias Schwämmle
University of Stuttgart, 2022

Catalysts for selective catalytic reduction (SCR) can, in addition to the reduction of NOx, also contribute to the oxidation of elemental mercury (Hg0) as well as to the undesired conversion of SO2 to SO3. By placing the catalysts in the high-dust configuration, oxidized mercury (Hg2+) can then be separated in downstream wet flue gas desulfurization units, allowing mercury to be removed efficiently from the flue gas.

The aim of this work is to show how mercury oxidation can be increased by newly developed SCR catalysts, which influences there are on Hg oxidation, and which mechanisms lie behind the three reactions over the SCR catalysts. The research with parameters derived from the experiences and conditions in power plant operation is carried out in synthetic flue gas in laboratory micro-reactors as well as in a lab-scale firing system under real flue gas conditions. The research forms a comprehensive examination of all reactions relevant in power plant operation with conventional as well as newly developed SCR catalysts. As a benchmark of the catalysts regarding all reactions, the performance indicator P3 is introduced.

The research is mainly conducted with standard SCR catalysts as reference and newly developed, modified honeycomb SCR catalysts, and supplemented by tests on plate-type SCR catalysts. In this research, modifications in the active component (V, Cu, Fe, Mn, Ce), of the promoters (W, Mo) as well as modifications of the base materials are studied. Through the dedicated application of the promoter molybdenum and modifications of the base material, a significant and clear increase in catalyst performance (high values of P3) can be achieved. An increased wall thickness of the catalyst also leads to an increase in Hg oxidation; however, the SO2/SO3 conversion is increased in parallel.

Examinations on the influences of the flue gas on the oxidation of Hg show a strong effect of the halogen content (HCl, HBr) in the flue gas. Likewise, the sulfation of the catalysts has a positive effect on the reactions over the catalysts. A parallel DeNOx reaction in the catalyst with the addition of NH3 and the presence of CO in flue gas inhibits Hg oxidation as well as SO2/SO3 conversion.

The oxidation of Hg over SCR catalysts seems to proceed according to an Eley-Rideal or Mars-Maessen mechanism: mercury adsorbs on the SCR catalyst and reacts with weakly adsorbed hydrogen halide or hydrogen halide species from the gas phase. The Hg adsorption and release can be correlated with the catalyst composition. The Deacon reaction might bring an additional contribution, but does not seem to be exclusively responsible for the measured effects.

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