Deactivation of V sub(2)O sub(5)-WO sub(3)-TiO sub(2) SCR catalyst at biomass fired power plants: Elucidation of mechanisms by lab- and pilot-scale experiments

In this work, deactivation of a commercial type V sub(2)O sub(5)-WO sub(3)- TiO sub(2) catalyst by aerosols of potassium compounds was investigated in two ways: (1) by exposing the catalyst in a lab-scale reactor to a layer of KCl particles or fly ash from biomass combustion; (2) by exposing full-length monolith catalysts to pure KCl or K sub(2)SO sub(4) aerosols in a bench-scale reactor. Exposed samples were characterized by activity measurements, SEM- EDX, BET/Hg-porosimetry, and NH sub(3) chemisorption. The work was carried out to support the interpretation of observations of a previous study in which catalysts were exposed on a full-scale biomass fired power plant and to reveal the mechanisms of catalyst deactivation. Slight deactivation (about 10%) was observed for catalyst plates exposed to a layer of KCl particles at 350 degree C for 2397 h. No deactivation was found for catalyst plates exposed for 2970 h to fly ash (consisting mainly of KCl and K sub(2)SO sub(4)) collected from an SCR pilot plant installed on a straw-fired power plant. A fast deactivation was observed for catalysts exposed to pure KCl or K sub(2)SO sub(4) aerosols at 350 degree C in the bench-scale reactor. The deactivation rates for KCl aerosol and K sub(2)SO sub(4) aerosol exposed catalysts were about 1% per day and 0.4% per day, respectively. SEM analysis of potassium-containing aerosol exposed catalysts revealed that the potassium salt partly deposited on the catalyst outer wall which may decrease the diffusion rate of NO and NH sub(3) into the catalyst. However, potassium also penetrated into the catalyst wall and the average K/V ratios (0.5-0.75) in the catalyst structure are high enough to explain the level of deactivation observed. The catalyst capacity for NH sub(3) chemisorption decreased as a function of exposure time, which reveals that Broensted acid sites had reacted with potassium compounds and thereby rendered inactive in the catalytic cycle. The conclusion is that chemical poisoning of active sites is the dominating deactivation mechanism, but physical blocking of the surface area may also contribute to the loss of activity in a practical application. The results support the observation and mechanisms of deactivation of SCR catalysts in biomass fired systems proposed in a previous study [Y. Zheng, A.D. Jensen, J.E. Johnsson, Appl. Catal. B 60 (2005) 253].