If you have a means for mechanical removal of oxygen from feedwater like a DC heater during normal operation, you can fill the boiler full with de-oxygenated feedwater and use a nitrogen blanket for wet lay-up. Hydrazine in feedwater under ambient temperature conditions will not be effective.
OR, use a nitrogen sparger system to feed nitrogen into the feedwater and vent the excess nitrogen out the top of the boiler. You would need a continual supply of nitrogen until the dissolved oxygen level drops and you close the vent and bottle up the boiler with nitrogen.
We originally used nitrates and nitrites at a high Ph for wet lay up, but in the latter years a compound from Coted was used. I don’t have the specific compound but a quick call to Cortec show get you enough information to make a selection.
Here is just one of several paper from Cortec concerning the wet lay up of boilers.
I am involve in construction of Heat Recovery Steam Generator for combined cycle power plant. In my previous projects, we are using a demiwater with mixture of ammonia and hydrazine for the hydrostatic test of the boiler, during wet preservation and until the boil-out. In Qatar, hydrazine is forbidden to use. Therefore, I am inquiring what will be the alternative for this chemical.
Depends on plant but next best thing to hydrazine is carbohydrazide which will give you safe handling of raw material but will break down to hydrazine in the HRSG at running temperature and pressure. Down side is production of small amounts of CO2 but in my experience (12+ yrs hrsg power plant chemistry)I've not seen any detrimental effects. With this you can substitute directly and keep ammonia dosing
