A commercial V2O5-WO3-TiO2 corrugated-type SCR monolith has been exposed for 1000 h in a pilot-scale setup to a flue gas doped with KCl, Ca(OH)(2), H3PO4 and H2SO4 by spraying a water solution of the components into the hot flue gas. The mixture composition has been adjusted in order to have P/K and P/Ca ratios equal to 2 and 0.8, respectively. At these conditions, it is suggested that all the K released during biomass combustion gets captured in P-K-Ca particles and the Cl is released in the gas phase as HCl, thus limiting deposition and corrosion problems at the superheater exchangers during biomass combustion. Aerosol measurements carried out by using a SMPS and a low pressure cascade impactor have shown two distinct particle populations with volume-based mean diameters equal to 12 and 300 nm, respectively. The small particles have been associated to polyphosphoric acids formed by condensation of H3PO4, whereas the larger particles are due to P-K-Ca salts formed during evaporation of the water solution. No Cl has been found in the collected particles. During the initial 240 h of exposure, the catalyst element lost about 20% of its original activity. The deactivation then proceeded at slower rates, and after 1000 h the relative activity loss had increased to 25%. Different samples of the spent catalyst have been characterized after 453 h and at the end of the experiment by bulk chemical analysis, Hg-porosimetry and SEM-EDX. NH3-chemisorption tests on the spent elements and activity tests on catalyst powders obtained by crushing the monolith have also been carried out. From the characterization, it was found that neither K nor Ca were able to penetrate the catalyst walls, but only accumulated on the outer surface. Poisoning by K has then been limited to the most outer catalyst surface and did not proceed at the fast rates known for KCl. This fact indicates that binding K in P-K-Ca compounds is an effective way to reduce the negative influence of alkali metals on the lifetime of the vanadia-based SCR catalysts. On the other hand, P-deposition was favoured by the formation of the polyphosphoric acids, and up to 1.8 wt% P was accumulated in the catalyst walls. Deactivation by polyphosphoric acids proceeded at about 0.2% day(-1). About 6-7% of the initial activity was lost due to the accumulation of these species. However, the measured relative activity reached a steady-state level during the last 240 h of exposure indicating that the P-concentration in the bulk reached a steady-state level due to the simultaneous hydrolysis of the polyphosphoric acids.
- Polyphosphoric acid