Hydrazine (N₂H₄) Oxygen Scavenger Dosing Equation
Hydrazine (N₂H₄) is the long-standing oxygen scavenger for high-pressure boiler feedwater (>40 bar). Stoichiometric ratio: 1 ppm O₂ requires 1 ppm N₂H₄, but practice runs at 1.5–3× excess to maintain a 0.02–0.1 mg/L residual at the boiler drum. Note: hydrazine is a suspected carcinogen — many plants now use carbohydrazide, DEHA or food-grade alternatives.
Where:
- Flow = Main flow rate in m³/hr
- Dose_PPM = Target concentration in mg/L or ppm
- Strength_% = Percentage active ingredient of the stock chemical
- SG = Specific Gravity (density relative to water) of the stock
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Frequently Asked Questions
0.02–0.1 mg/L excess N₂H₄ residual at the boiler drum, after the deaerator has reduced O₂ to <0.007 mg/L. Stoichiometric reaction: N₂H₄ + O₂ → 2H₂O + N₂, so 1 ppm O₂ requires 1 ppm N₂H₄. Operating practice: 1.5× to 3× theoretical to maintain residual.
Hydrazine (N₂H₄): cheapest, fastest scavenger, suspected carcinogen — restricted in food/beverage and many EU plants. Carbohydrazide: hydrolyses to release hydrazine — safer to handle. DEHA (diethylhydroxylamine): non-volatile, no carcinogen flag — preferred for medium-pressure boilers and food industry. Hydroquinone: catalysed scavenger for pre-boiler protection.
OSHA classifies hydrazine as a 'reasonably anticipated' carcinogen (IARC 2B); REACH restricts industrial use in EU; food-contact regulations prohibit it in beverage / pharma steam. Most new utility plants use DEHA or carbohydrazide; existing plants are converting at the next major outage.
A well-tuned deaerator removes O₂ to <0.005 mg/L → minimal hydrazine needed (just 0.02 mg/L residual). A failing deaerator leaves 0.05–0.2 mg/L O₂ → hydrazine demand jumps 10–40×. Always check deaerator vent flow and tray fouling before increasing hydrazine.
Bulk hydrazine is supplied as 35% N₂H₄ in water (SG 1.02). PEL: 1 ppm in air (8h TWA), STEL: 2 ppm. Always dose from sealed nitrogen-blanketed tanks, with double containment, and PPE = full chemical suit + supplied air for any breaking-of-circuit work. Spills neutralised with weak hydrogen peroxide.