Deactivation of Commercial SCR Vanadia-based Catalysts by Fuel Additives - K3PO4 effects
Advancing the chemical engineering fundamentals
Catalysis - I (T2-13a)
Keywords: Selective Catalytic Reduction, NOx, Catalyst Deactivation, Biomass Combustion
Nitrogen oxides emitted from combustion and high temperature processes continues to be a major environmental concern. Among the various technologies nowadays available for removing NOx from stationary sources, the Selective Catalytic Reduction (SCR) process applied to fossil fuel combustion is the best-developed and worldwide used. However, the application of this technology to the treatment of flue gas proceeding from biomass combustion is problematic. This fact is mainly due to the high rates of catalyst deactivation observed and related to compounds (e.g. alkali and alkaline earth metals, chlorides, etc.) introduced into the system by biomass.
Firing biomass has also another negative aspect. The ash produced during biomass combustion has relatively low melting points and is partially melted at the temperatures of the super-heaters. This means that this ash has a great tendency to stick on the super-heater surfaces forming deposits which decrease the heat transport through the pipes and consequently the global efficiency of steam production.
Despite these negative aspects, biomass firing is a way of reducing the net CO2 emissions. In Denmark, in order to follow the actual regulations, the power companies have to burn every year a fixed amount of biomass in their power plants. It is thus clear that a great amount of research is needed to overcome the problems related to biomass combustion.
In particular, the use of additives to the fuel is considered by Danish power companies as a promising way to minimize the ash deposition on the super-heaters. These additives may involve compounds such as phosphates of calcium and aluminum that can bind the alkali fraction of the flue gas to high melting temperature ashes with lower tendency to stick on the superheaters. However, little is known about the influence of these additives and the products of the addition process on the SCR catalysts. In other words, it is still unknown whether or not the additives themselves can deactivate the SCR catalyst.
One of the main compound which is potentially formed during the addition process is potassium phosphate tribasic (K3PO4). This compound has a melting point temperature of 1340°C and should not form sticky particles at the superheater temperatures. However, it is important to test its influence on the SCR catalysts. For this reason, commercial catalysts have been doped both in the laboratory and in a SCR pilot-scale setup. In the laboratory small catalyst plates have been doped by wet impregnation with different water solutions of K3PO4. The activity of the doped samples have then been measured and compared to a fresh one. In the pilot plant, instead, K3PO4 water solutions have been sprayed directly into a hot flue gas from a natural gas burner. By cooling the resulting flue gas at the SCR temperature (350°C), K3PO4 aerosols have been formed and measured by a Scanning Mobility Particle Sizer (SMPS). A commercial full-length SCR monolith was then exposed for more than 300 hours to the formed aerosols, which deposited on it and doped it in a more realistic way than the wet impregnation method. The activities of the resulting catalysts have been regularly tested during the whole exposure. Characterization of the doped catalysts has been carried out by chemical bulk analysis, SEM and FTIR.
Presented Thursday 20, 09:25 to 09:45, in session Catalysis - I (T2-13a).
