Oxygen Scavengers & Reducing Agents

Picking Up Where Deaeration Leaves Off

Module 1.1 introduced the deaerator's job: mechanically stripping the bulk of dissolved oxygen from feedwater by heating it near saturation temperature. Mechanical deaeration typically gets dissolved oxygen down to a few hundred ppb but rarely to zero, and even a few ppb of residual oxygen is enough to drive meaningful corrosion over time in a system that operates continuously for years. Oxygen scavengers are the chemical cleanup step that finishes the job mechanical deaeration starts, reacting directly with residual dissolved oxygen to remove it from solution.

Hydrazine — The Historical Standard

Hydrazine (N₂H₄) reacts with dissolved oxygen to form nitrogen gas and water — genuinely clean byproducts that add no dissolved solids to the system, which made it attractive for decades across all pressure classes including high-pressure and supercritical units. The complication is toxicity: hydrazine is classified as a suspected human carcinogen, requiring strict handling, storage, and exposure control procedures. This health profile is the single biggest reason the industry has broadly moved toward alternatives over recent decades, even though hydrazine's chemistry performance was never really the problem.

Carbohydrazide — The Modern Default

Carbohydrazide decomposes in feedwater conditions through intermediate compounds before ultimately reacting to remove oxygen, producing nitrogen and water as final byproducts — the same clean, TDS-free outcome as hydrazine, without hydrazine's toxicity profile. This combination (hydrazine-equivalent performance and byproduct cleanliness, without hydrazine's health hazard) is why carbohydrazide has become the common default choice for units needing an oxygen scavenger across pressure classes, including high-pressure and supercritical service where TDS-free byproducts matter most.

Sodium Sulfite — Simple, but Limited by TDS

Sodium sulfite reacts with dissolved oxygen to form sodium sulfate, a straightforward, low-toxicity, easy-to-handle reaction. Its limitation is exactly what carbohydrazide and hydrazine avoid: sodium sulfate is a dissolved solid that adds directly to boiler water TDS. On a lower-pressure drum boiler already running phosphate treatment with blowdown handling TDS control, that addition is manageable and sulfite's low handling burden makes it an attractive, simple choice. On a high-pressure or once-through unit, that same TDS addition is disqualifying.

Dosing Considerations

• Scavenger feed rate needs to track actual residual oxygen load, not run at a fixed rate regardless of conditions — underfeeding leaves oxygen unaddressed, overfeeding wastes chemical and, for reducing agents, can push the system further into reducing conditions than intended.
• On AVT(O) programs specifically (Module 2.4), scavenger feed is typically absent or minimal by design, since the program relies on a small amount of residual oxygen for the protective oxide layer — a reminder that "more scavenger" is not universally the safer choice.
• Excess reducing agent feed carries its own corrosion considerations in certain metallurgies, which is why dosing is tuned to actual demand rather than fed generously as a margin of safety.