Deactivation and regeneration of SCR catalysts used in bio fuel power plants

The SCR technique is used (1998) in five Swedish wood fired CHP plants. These plants are among the first references in the world for SCR in combination with bio fuels. The experiences show some problems with the catalyst function because of deactivation and ash-plugging. The deactivation has been handled by seasonal washing of the catalyst. In this work the cause for and the extent of catalyst deactivation has been investigated when using 100 % wood as fuel. The field tests have been performed in the CFB boiler in Norrkoping, firing forest residues, and in the boiler in Jordbro, firing pulverised wood (PC). Samples of four different commercial catalyst types have been exposed to flue gas in a test rig connected to the convection section of the boiler. The results after 2 100 hours show a large difference in deactivation trends between the two plants; when using a conventional honeycomb catalyst 80 % of the original activity remains in the CFB boiler but only 20 % remains in the PC boiler. The deactivation in the CFB boiler is about 3 - 4 times faster than what is expected for a conservative design for a coal fired boiler. Potassium neutralising the acid sites of the catalyst is suspected to be the reason for the deactivation. The potassium on the catalyst dissolves easily in both water and sulphuric acid. Washing of deactivated catalyst samples with water resulted in higher activity than for the fresh samples if it was supplemented by sulphatization with sulphur dioxide. The use of sulphatization, periodic washing and the use of an optimised catalyst are promising methods to increase the catalyst lifetime and to reduce the operation costs for SCR in bio fuel fluidised bed based power plants.