Hydrazine, N2H4, is a volatile oxygen scavenger that contributes no dissolved solids to feed water (eg: needed if feed water is used for superheated steam attemporation). Hydrazine can decompose to ammonia and water, with decomposition in boiler water systems begininng at 400F, and being rapid at 600 F. The alkaline ammonia does not attack steel, but, particularly with dissolved oxygen, can be aggressive to copper alloys. Close control of hydrazine feed helps limit formation of ammonia.
Hydrazine reaction times are very slow at low temperatures. For example, at 5:1 N2H4:O2 ratio, pH of 9.5, reaction time for 90% dissolved oxygen scavenging is 0.3 minutes at 400F, 0.7 minutes at 300 F, 1.4 minutes at 250 F. At 150 F, you need 100:1 N2H4:O2 ratio to achieve this in 1.4 minutes. At 300 F you can get 90% oxygen scavenging in 1.5 minutes with a 1.6 N2H4:O2 ratio.
An alternate volatile oxygen scaventer is hydroquinone. Hydroquinone performs better at low temperatures (it is even used effectively in cold wet equipment lay-ups), and it is less hazardous to handle. Any degradation at higher temperatures produces CO2, not ammonia.
This is in view of steam cycle application
1.Is it advisable that shifting from Hydroquinone to Hydrazine dosing once water temperature is picked up?
2.Is it necessary to change Deaerator design?
Hydroquinone is my preference for a hydrazine substitute mainly because it so effective at building a magnatite layer at low temperatures. I have observed it carrying through the steam turbine and promoting the formation of magnatite in the condenser.
That said, many of the steam turbine suppliers will void the warranty on new units if you use anything other than hydrazine and ammonia for steam cycle conditioning. This is mainly due to research from Europe indicating formation of organic acids due to thermal decomposition of the hydrazine substitutes, cyclohexamine, morpholine, etc. The concern is that the organic acids will attack turbine blading especially if you have chloride carryover problems resulting is stress cracking of the low pressure blading.
You have to decide which risk is greater, the possibility tossing a turbine blade or exposing your operations personnel to a known carcinogen.
Several of the large water treatment chemical suppliers have engaged the turbine manufacturers in discussions to resolve the issue but I believe that they are still stalemated.
Hydroquinone is especially good for wet layup of steam generators.
I presume that you could switch from hydroquinone to hydrazine but that would add the requirement to store two chemicals on site for the same function and add unnecessary operational complexity.
Your DA should have been specified to achieve a defined level of DO2 at the outlet, usually 0.007 mg/l, without the use DO2 scavengers.
