why-is-hydrazine-hydrate-added-to-boiler-feed-water
Hydrazine hydrate is a colorless, transparent, oily liquid with a light ammonia smell. In industry, hydrazine hydrate aqueous solution or hydrazine salt with a content of 40% to 80% is generally used. Relative density 1.03 (21℃); melting point -40℃; boiling point 118.5℃. Surface tension (25℃) 74.OmN/m, refractive index 1.4284, heat of formation -242.7lkj/mol, flash point (open cup) 72.8℃. Hydrazine hydrate is strongly alkaline and hygroscopic. Liquid hydrazine hydrate exists in the form of dimer, miscible with water and ethanol, insoluble in ether and chloroform; it can corrode glass, rubber, leather, cork, etc., and decompose into Nz, NH3 and Hz at high temperature; hydrazine hydrate has extremely strong reducing properties, and reacts violently with halogens, HN03, KMn04, etc. It can absorb C02 in the air and produce smoke.
In order to prevent the boiler water system and pipelines from oxygen corrosion, and to ensure the complete elimination of the dissolved oxygen remaining after thermal deoxidation and the oxygen leaking into the feed water due to the looseness of the pump and the water supply system, it is necessary to add an appropriate amount of hydrazine to the feed water. Hydrazine (N2H4) is also called hydrazine, and its hydrate hydrazine hydrate (N2H4·H2O) is commonly used.
Since hydrazine hydrate is a strong reducing agent in alkaline aqueous solution, it reacts with dissolved oxygen in water as follows: N2H4 + O2 → N2 + 2H2O. Therefore, the role of adding hydrazine is to remove dissolved oxygen by chemical methods. The products of the reaction, N2 and H2O, have no harm to the thermal system. In addition, at a high temperature of more than 200°C, hydrazine can also reduce Fe2O3 to Fe3O4 or Fe to prevent the formation of iron scale in the furnace.
Hydrazine hydrate can also reduce CuO to Cu2O or Cu to prevent copper scale in the furnace. In actual production, 40% hydrazine (N2H4·H2O) is usually used and added to the suction port of the boiler feed water pump or the outlet pipe of the deaerator. The amount of addition is usually controlled based on the N2H4 content in the economizer inlet feed water not exceeding 50μg/L. N2H4 is toxic, flammable, and volatile, so special attention should be paid when using it. N2H4·H2O is not easy to burn when it is <40% (mass fraction). The deoxygenation reaction of hydrazine hydrate is very fast when the temperature is greater than 150℃, but the reaction speed is slow when the temperature is low. Therefore, catalytic hydrazine is sometimes used. That is, when adding hydrazine, additives that promote the reaction are added at the same time, such as hydroquinone, quinone compounds, 1-phenyl-3-pyrazolidone, p-aminophenol, etc.
