At Hamaoka Unit 4 and 5, the hydrogen concentration in the outlet of off-gas recombiner had increased, and the reactors could not continue start-up operation. Therefore, we investigated the causes of the deactivating the recombination reaction and selected appropriate countermeasures to the plants. From our investigation, two types of deactivation mechanism are found. One of the causes was decreasing the active surface area of alumina as support material by the dehydrative condensation. The other cause was poisoning of the catalyst by organic silicon compound. The organic silicon was introduced from organosilicon sealant used at the junctions of the low-pressure turbine. We also found that the boehmite rich catalyst was deactivated more easily by the organic silicon than gamma alumina because boehmite had a lot of hydroxyl groups. Finally, we estimated that the deactivation of the hydrogen recombination catalysts was caused by combined two factors, which are characteristics of boehmite catalyst support and the poisoning by the organic silicon on the catalyst surface. As the countermeasures, the boehmite was changed into more stable gamma alumina by adding the heat treatment in hydrogen atmosphere at 500°C for 1 hour, and the source of organic silicon, organosilicon sealant, was removed. At Hamaoka Unit 4 and 5 improved catalysts were applied. Moreover, linseed oil that used to be used at the plants was applied again as sealant of the low-pressure turbine casing instead of the organosilicon sealant. As a result of application of these countermeasures, the reactors could be started without increase of the hydrogen concentration at these plants.

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