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pubblicazioni - Presentazione

Air Plasma Sprayed Catalytic Coatings for Denox Applications

pubblicazioni - Presentazione

Air Plasma Sprayed Catalytic Coatings for Denox Applications

E’ brevemente descritta l’attività di sviluppo di rivestimenti catalitici depositati con tecnica plasma spray per l’abbattimento degli NOx presenti nei gas di scarico delle centrali termoelettriche, con particolare riferimento alla possibilità di retrofitting di impianti a ciclo combinato

The selective catalytic reduction (SCR) of NOx by NH3 is the best developed and most widely applied method for removing these pollutants formed from flue gases of fossil fuelled power plants and engines. Commercial catalysts are composed mainly of TiO2, WO3 and V2O5 plus other compounds without catalytic function.

TiO2 in the anatase form acts as high specific surface support material and shows the most effective dispersive capacity for the catalytically active phases (V2O5 and WO3). Catalyst activity can be easily improved by increasing the V2O5 loading which, however, is instead maintained low (typically below 3% wt.) to limit the unwanted SO2 to SO3 oxidation, a source of problems downstream from the reactor such as plugging (formation of ammonium sulphate) or corrosion.

Depending on the SO2 content in the feed, the catalyst activity can be tuned by decreasing the V2O5/WO3 ratio in the catalyst to maximize the activity in the reduction of NO, while minimizing SO2 oxidation activity. The WO3 is also used to provide thermal stability to the catalysts by increasing the transition temperature from anatase to rutile phase of the TiO2.
In the temperature range 300 – 450°C, at the catalyst surface the following chemical reactions occur.

4NO+4NH3+O2 → 4N2+6H2O

2NO2+4NH3+O2 → 3N2+6H2O

Nowadays the installation of SCR DeNOx plants is usually limited to coal, oil and waste fired power plants. In fact, NOx emissions of natural gas combined cycles (NGCC) are still below the threshold fixed by the law (50 mg/Nm3 for natural gas fuelled gas turbine). Since in Italy most of the electric power is produced by high efficiency combined cycles if the admitted NOx emission threshold – continuously lowered in the last decades – will be further reduced, new combined cycles will have to be equipped with DeNOX plants or with very low NOx burners. On the other hand, to upgrade the already existing NGCC, a retrofitting technology is required.

In this perspective, a possible solution would consist in depositing a catalytic coating on heat exchange pipes in the heat recovery steam boiler of combined cycle power plants in the region where the temperature of flue gases is in the range of operation of the catalyst. In this work the first results of the development of air plasma sprayed catalytic coatings will be presented. The efforts have been focused both on powder preparation (different powder production routes have been investigated) and on the set up of the deposition parameters. In particular, microstructure, phase composition and NOx reduction efficiency of different coatings have been investigated.