How to Use Carbohydrazide as a Boiler Oxygen Scavenger

Overview

Carbohydrazide (carbonohydrazide, CAS 497-18-7) is a non-toxic, non-carcinogenic oxygen scavenger developed specifically as a replacement for hydrazine in medium and high-pressure boiler systems. With a purity of ≥ 98% and a clean white powder form, it is the preferred choice for facilities transitioning away from hydrazine due to occupational health and regulatory concerns.

Carbohydrazide scavenges dissolved oxygen through the reaction: (N₂H₃)₂CO + 2O₂ → 2N₂ + H₂O + CO₂. Critically, it also reduces ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), which is subsequently precipitated as magnetite (Fe₃O₄) — the same protective passive film that hydrazine builds on boiler surfaces. This two-step mechanism makes carbohydrazide a genuine functional replacement for hydrazine, not merely an O₂ scavenger.

Carbohydrazide is accepted under FDA 21 CFR 173.310 for use in boilers generating steam that contacts food. It is not classified as a carcinogen by IARC, NTP, or OSHA. This regulatory clarity is a major reason for its growing adoption in food, beverage, and pharmaceutical manufacturing plants.

Stoichiometry: 1 ppm carbohydrazide scavenges approximately 1.4 ppm dissolved O₂. In practice, dose at 3–5x stoichiometric to achieve a meaningful residual and ensure complete reaction at feedwater temperatures.

Preparation & Dissolution

Carbohydrazide is supplied as a white crystalline powder (melting point 153–155°C) and must be dissolved before injection.

Stock solution preparation: 5–10% w/v in deaerated demineralized water.

1. Fill a clean HDPE or stainless steel dissolving tank with demineralized water (pre-warmed to 30–40°C speeds dissolution)
2. Add the calculated mass of carbohydrazide powder slowly while stirring
3. Mix for 20–30 minutes until completely dissolved (solution will be clear and colorless)
4. Deaerate the water used for dissolution to avoid oxidizing the product during preparation
5. Do not prepare solutions exceeding 10% — higher concentrations dissolve slowly and can salting-out at lower temperatures
6. Use within 7–14 days; solution stability is good at ambient temperature in sealed tanks
7. Clearly label: concentration (% w/v), batch date, pH

Approximate dissolution guide: At 25°C, carbohydrazide dissolves freely in water up to at least 50 g/100 mL. A 5% solution requires dissolving 50 g per liter of water.

Dosing Guide

Boiler Pressure	Feedwater O₂ (after deaerator)	Carbohydrazide Dose	Boiler Water Residual
Medium pressure (20–60 bar)	7–20 ppb	0.5–1.5 mg/L feedwater	0.05–0.2 mg/L
High pressure (60–120 bar)	≤ 7 ppb	0.3–1.0 mg/L feedwater	0.02–0.1 mg/L
Hydrazine replacement (1:1 N₂H₄)	—	1.3 mg carbohydrazide per 1 mg N₂H₄	Match existing residual target
Food/pharma steam systems	As needed	Per FDA 21 CFR 173.310 limits	Maintain minimum effective residual
Wet layup / mothballing	—	5–15 mg/L	3–5 mg/L in layup water

Transition from hydrazine: When switching from hydrazine, the molar conversion factor is approximately 1.3 kg carbohydrazide per 1 kg hydrazine (N₂H₄ basis). Begin at the equivalent dose and monitor iron and residual for the first 2 weeks as the passive film transitions.

Application Procedure

1. Test feedwater dissolved oxygen at deaerator outlet — target < 7 ppb before chemical scavenging. Confirm deaerator is operating correctly.
2. Prepare carbohydrazide stock solution at 5–8% concentration in demineralized water.
3. Prime the metering pump (peristaltic or diaphragm, HDPE wetted parts) and calibrate flow rate against target dose.
4. Set injection point downstream of the deaerator and storage tank, upstream of the boiler feed pump. This gives maximum residence time at temperature for reaction completion.
5. Inject continuously at the calculated dose rate based on feedwater flow.
6. After 24–48 hours of steady operation, sample boiler water and test carbohydrazide residual using the appropriate colorimetric method.
7. When transitioning from hydrazine: run parallel residual monitoring for both compounds for the first week to confirm the transition is clean.
8. For passivation of a new boiler or a boiler that has been opened for inspection, dose at 3–5 mg/L for the first 200–500 operating hours to build the magnetite layer.

Monitoring & Control

Parameter	Frequency	Target	Method
Dissolved O₂ (deaerator outlet)	Daily	< 7 ppb	Inline DO probe
Carbohydrazide residual (boiler water)	2x weekly	0.02–0.2 mg/L	Colorimetric kit (DMAB method)
Iron in feedwater	Weekly	< 50 ppb Fe	ICP or colorimetric
Iron in boiler water	Weekly	< 100 ppb Fe	ICP or colorimetric
Boiler water pH	Daily	10.5–11.5	pH meter
Boiler water conductivity	Daily	OEM limits	Inline meter
Hydrazine residual (during transition only)	Daily for first 2 weeks	Declining to zero	Colorimetric (DMAB or p-DAB)

Common Mistakes

• Using a hydrazine residual test kit to measure carbohydrazide: The standard p-dimethylaminobenzaldehyde (p-DAB) test kit for hydrazine gives a cross-reaction with carbohydrazide but with different sensitivity. Use the DMAB method validated for carbohydrazide, or use a specific kit labeled for this product to avoid under- or over-dosing.
• Dissolving in aerated tap water: Preparing the stock solution in oxygenated tap water wastes product — dissolved O₂ in the makeup water will partially consume the carbohydrazide before it even reaches the boiler. Always use deaerated or freshly boiled demineralized water.
• Underdosing during transition from hydrazine: Many operators cut the dose too early. During the first 2–4 weeks after switching from hydrazine, maintain slightly elevated carbohydrazide dose (1.5–2x normal) to rebuild the magnetite passivation layer that may have been disturbed during the changeover.
• Ignoring elevated iron as a signal: Rising iron in feedwater or boiler water indicates the passive film is breaking down — either the oxygen scavenger dose is insufficient or the deaerator is failing. Do not attribute it to the product change and continue without investigation.
• Storing powder in humid conditions: Carbohydrazide powder is hygroscopic and will cake if exposed to moisture. Store in sealed bags inside a dry warehouse. Once a bag is opened, reseal or transfer to a sealed container.

Storage & Handling

• Shelf life: 24 months in original sealed 25 kg fiber drums under dry conditions
• Temperature: 5–35°C, away from heat sources and open flame
• Container: Original sealed fiber drums or HDPE bins. Avoid metals.
• Safety: Low acute toxicity (LD50 oral rat > 2,000 mg/kg). Not classified as carcinogen (IARC Group 3). Slight eye and respiratory irritant — wear safety glasses and dust mask when handling powder. Keep away from strong oxidizers and acids.