SELECTION GUIDE FOR OXYGEN SCAVENGERS
Corrosion by oxygen in the boiler can be controlled by the addition of an “oxygen scavenger” to the preboiler section of the steam generating system. It is generally fed, along with other treatment chemicals, as an aqueous solution to the feedwater either just upstream or, preferably, justdownstream of the deaerator, although it is sometimes added into the return lines to scavenge oxygen in the condensate. The most widely used materials in this application are sodium sulfite (Na2SO3) and hydrazine (N2H4), both of which are usually sold as catalyzed systems to enhance reactivity with oxygen at lower temperatures and pressures. Quinones and cobalt salts are typically used as catalysts.Sulfite is the least expensive and most active (when catalyzed) for lower and medium pressure boilers [up to 600 psig (42 bar abs)]. In its reaction with oxygen, sodium sulfite produces sodium sulfite produces sodium sulfate, which contributes solids to the circulating boiler system:
2 Na 2 SO 3 + O 2 → 2 Na 2 SO 4
The oxygen scavengers used in the higher pressure boilers, and the ones withwhich diethylhydroxylamine (DEHA) competes most directly are hydrazine and catalyzed hydrazine. Hydrazine does not produce corrosive gases at high temperatures and pressures, and in application, reacts with oxygen to form nitrogen and water:
N 2H 4 + O 2 → 2 H 2O + N 2
In calculating the theoretical requirement of hydrazine for scavenging oxygen, a value of 1 part per part oxygen is obtained:32 g/mole hydrazine = 1 32 g/mole O 2
In operation, a 100% excess of hydrazine is used. Boiler residuals of 1 ppm hydrazine are typically maintained. Hydrazine does not contribute solids to the system, so boiler blowdown, or the mechanical removal of solids from the after-boiler section as sludge, is reduced. It also promotes the formation of the protective magnetite film on the boiler tubes anddrum, and converts red iron dust (hematite) to magnetite. It is because of these passivation effects that an excess of scavenger to oxygen is required when changing a boiler system form a non-passivating scavenger to one which passivates. Hydrazine is not without limitations. It is not considered “volatile”, so it does not leave the boiler with the steam to scavenge oxygen and passivate metalthroughout the system. In boilers operating above 400˚F (205˚C), it can degrade to ammonia and volatilize with steam, and, in the presence of oxygen, attack metals containing copper:
2 N 2 H 4 → 2 NH 3 + N 2 + H 2
Thus, in high pressure and supercritical boilers, where any solids constitute a severe problem, sulfite cannot be used. The theoretical dosage of sodium sulfite, or the number of partsof Na2SO3 required to consume 1 part of O2, can be calculated based on its reaction with oxygen described above:
2(126 g/mole Na 2 SO 3 ) = 7.88 theoretical 32 g/mole O 2
Therefore, about 8 parts of Na2SO3 are fed to the boiler to consume each part of oxygen. Typically, residual concentrations of sulfite of up to 20 ppm are maintained in the boiler. Sulfite also breaks down at pressures as lowas 600 psig (41 bar abs) resulting in the formation of sulfur dioxide or hydrogen sulfide, by either of two routes:
Na 2 SO 3 + H 2 O → SO 2 + 2 NaOH 4 Na 2 SO 3 + 2 H 2 O → H 2 S + 2 NaOH + 3 Na 2 SO 4
Both are corrosive gases which leave the boiler with steam, resulting in low pH steam and condensate and potential attack throughout the system. Sulfite is an effective oxygen scavenger, butit is nonvolatile and does not leave the boiler with the steam, thus providing no protection in the condensate system. Sulfite also does not reduce hematite to magnetite and is ineffective in repassivating boilers with existing rust.
Finally, and most importantly of late, is the inclusion of hydrazine on the OSHA and NIOSH lists as a suspect carcinogen. Papers and sales literature presenting...
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