1. Why is deoxygenation necessary for boiler feedwater?
Corrosion in boiler systems mainly originates from oxygen corrosion and weak acid corrosion. Oxygen in oxygen corrosion primarily comes from atmospheric dissolution in the feedwater; weak acid corrosion occurs when CO2 dissolves in water to form carbonic acid corrosion. In weak acid corrosion, CO2 either dissolves in the atmosphere or is present in the boiler feedwater, where bicarbonate ions decompose upon heating into CO2, which dissolves in the water.
Weak acid corrosion causes uniform thinning of the metal in boiler system equipment and pipelines. Oxygen corrosion causes pitting and under-scale corrosion on boiler heating surfaces, furnace tubes, system equipment, and piping networks. Both oxygen corrosion and weak acid corrosion can lead to excessive total iron levels in the boiler system water. When iron ions in the boiler system water exceed the standard, the boiler water turns yellow, and in severe cases, red. When iron ion levels in steam boiler system water exceed the standard (greater than 700 ug/L), the condensate color changes from pure yellow to red, and in severe cases, black. In cases of severe corrosion, the condensate will appear soy sauce-colored.
Deoxygenation is a crucial step in the boiler feedwater treatment process. Oxygen-corroded iron oxide can enter the boiler, depositing or adhering to the boiler tube walls and heating surfaces, forming insoluble and poorly heat-conducting iron scale. This reduces the heat exchange rate, lowers boiler output efficiency, and wastes energy. Furthermore, the poorly heat-conducting iron scale adhering to the boiler tubes can easily cause overheating and creep, leading to tube rupture accidents. Oxygen-corroded iron scale can also cause pitting and cratering on the inner walls of pipes. Severe corrosion can lead to pipe perforation, leakage, and even explosions.
2. Comparison and Analysis of Nine Boiler Feedwater Deoxygenation Methods
Diamonium Hydrochloride Regenerated Resin Deoxygenation
Diamonium hydrochloride regenerated resin deoxygenation is a resin deaerator using hydrazine as the regenerator.
Advantages of Diamonium Hydrochloride Regenerated Resin Deoxygenation: It is a low-temperature deoxygenation technology, resulting in a relatively lower feedwater temperature and no introduction of electrolytes.
Disadvantages of Diamonium Hydrochloride Regenerated Resin Deoxygenation: The water treated with hydrazine after resin deoxygenation is toxic; therefore, the steam and hot water produced by the boiler should not come into contact with humans. The regeneration methods of the resin are limited, making it difficult to achieve the required deoxygenation effect.
Sodium sulfite regenerated resin deoxygenation
Sodium sulfite regenerated resin deoxygenation is a resin deaerator that uses sodium sulfite as the regenerator.
Advantages of sodium sulfite regenerated resin deoxygenation: It is a room-temperature deoxygenation technology, resulting in a relatively lower feed water temperature and no introduction of electrolytes.
Disadvantages of sodium sulfite regenerated resin deoxygenation: The regeneration effect of sodium sulfite is limited; after a certain period of regeneration, the equipment needs to be activated with ammonium bisulfite; the regeneration process is relatively complex and cumbersome; and the rinsing after regeneration consumes a large amount of water.
Sponge Iron Deoxygenation
Sponge iron deoxygenation is a chemical deoxygenation method. It uses specially produced activated sponge iron to remove dissolved oxygen from water. The main component of sponge iron is iron, and its loose, porous internal structure provides a specific surface area 50,000 to 100,000 times larger than ordinary iron filings. When oxygen-containing water enters the sponge iron deoxygenator and passes through the sponge iron filter media layer, the dissolved oxygen in the water undergoes a rapid oxidation reaction with the iron, thus removing the oxygen from the water. The reaction equations are:
2Fe²⁺ + 2H₂O + O₂ → 2Fe(OH)₂
4Fe(OH)₂ + 2H₂O + O₂ → 4Fe(OH)₃
Advantages of sponge iron deoxygenation: It deoxygenates at room temperature, requiring no heating of the influent; the system can supply water at any time; the device is simple, can be installed in a low position, occupies a small area, is easy to operate, and saves investment; regeneration only requires backwashing, and the backwash effluent is non-toxic and non-corrosive; the consumption of sponge iron filter media is very low, consuming only about 25g per cubic meter of water treated, and generally only needs to be replenished once every 3-6 months depending on the amount and quality of water treated. Disadvantages of sponge iron deoxygenation: Unsoftened water can easily passivate the surface of the sponge iron filter media deoxygenator, slowing down its oxidation reaction and affecting the deoxygenation effect. Therefore, softening should be performed before deoxygenation. Treatment increases the iron ion content in the water. For steam boilers or feedwater deoxygenation with strict requirements on iron ion content, an iron removal device must be installed.
Sodium sulfite deoxygenation
Sodium sulfite deoxygenation is done by adding chemicals in the furnace. The reaction equation for sodium sulfite and oxygen is:
2Na₂SO₃ + O₂ → 2Na₂SO₄.
Based on this reaction equation, removing 1g of dissolved oxygen requires 7.88g of 100% anhydrous sodium sulfite, while simultaneously producing approximately 9g of sodium sulfate in the water. To ensure a more complete reaction, a residual concentration of 20–40 ppm is typically maintained in the boiler water to guarantee deoxygenation. Since sodium sulfite reacts with oxygen to form the stable salt sodium sulfate, it increases the soluble solids in the boiler water, deteriorating water quality. This necessitates more frequent blowdowns, leading to waste of chemicals and increased fuel costs. When the boiler operating pressure exceeds 6.2 MPa, sodium sulfite decomposes, producing corrosive hydrogen sulfide and sulfur dioxide. These gases are discharged with steam, causing corrosion of downstream equipment:
Na₂SO₃ + 2H₂O → 2NaOH + H₂SO₃
H₂SO₃ → H₂O + SO₂
Sodium sulfite may also undergo its own redox reaction, producing sodium sulfate and sodium sulfide:
Na₂SO₃ → The reaction 3Na₂SO₄ + Na₂S produces sulfur dioxide and sodium sulfide, both of which are corrosive. Therefore, using sodium sulfite as a deoxygenating agent is essentially replacing one type of corrosion with another. Furthermore, injecting feedwater containing sodium sulfite into superheated steam to regulate temperature can lead to the deposition of sodium sulfate and other salts in the superheated steam header and turbine. Sodium sulfite offers no passivation protection for metals.
Advantages of sodium sulfite deoxygenation: Can deoxygenate at room temperature; low investment, simple operation.
Disadvantages of sodium sulfite deoxygenation: The dosage and cycle are difficult to control, resulting in unstable deoxygenation effects; increased boiler water salinity leads to increased blowdown volume and heat waste; because sodium sulfite decomposes into harmful gases at high temperatures inside the boiler, causing metal corrosion, it is only suitable for medium and low-pressure boilers, not high-pressure boilers. It is generally used in small boiler rooms and thermal systems with high water quality requirements as an auxiliary deoxygenation method, unsuitable for many situations with specific water quality requirements; sodium sulfite is a strong reducing agent, and its storage and preparation should be carried out in sealed containers away from air to prevent oxidation and failure. In practice, cases of sodium sulfite oxidation and failure are common.
Hydrazine deoxygenation
Hydrazine is a strong reducing agent in alkaline aqueous solutions, capable of reducing dissolved oxygen in water. The reaction equation is:
N₂H₄ + O₂ → N₂ + 2H₂O.
The reaction products N₂ and H₂O pose no harm to the thermal system or equipment. Hydrazine deoxygenation is often used as an auxiliary measure after thermal deoxygenation to completely remove residual oxygen from water without increasing the salinity of boiler water. High-pressure boilers frequently use hydrazine deoxygenation. However, because hydrazine is toxic and volatile, it cannot be used for deoxygenation in drinking water boilers or domestic water boilers. Many boiler users are restricting or ceasing this method.
Advantages of hydrazine deoxygenation: Hydrazine does not increase the salinity of boiler water; the reaction produces nitrogen and water, which helps prevent further corrosion of the boiler.
Disadvantages of hydrazine deoxygenation: Hydrazine is a suspected carcinogen, volatile, toxic, and flammable. Safety precautions must be taken during its use; it cannot be used in domestic boilers. The excess amount of hydrazine should be appropriate; too much excess may introduce incompletely reacted hydrazine into the steam.
Acetone oxime deoxygenation
Acetone oxime, abbreviated as DMKO, also known as dimethyl ketone oxime, has strong reducing properties and readily reacts with oxygen in feedwater, reducing the dissolved oxygen content. It is non-toxic and an ideal product to replace traditional chemical deoxygenators such as hydrazine in medium and high-pressure boiler feedwater. When using it, the excess acetone oxime in the feedwater should be controlled at 15–40 μg/L.
Advantages of acetone oxime deoxygenation: Non-toxic, no environmental pollution; acetone oxime undergoes a passivation reaction with metals and can be used as a passivating agent after boiler acid washing.
Disadvantages of acetone oxime deoxygenation: As a new deoxygenator, experience and methods for its use need further accumulation and improvement; some users have reported cases of boiler tube rupture due to corrosion of the heating surfaces after using DMKO.
Carbohydrazide(CHZ) for Deoxygenation
Carbazide, also known as diaminourea or Carbonic dihydrazine, is a derivative of hydrazine. It was patented by Nalco Chemical Company in 1981 and is superior to hydrazine in both deoxygenation and metal purification. In water treatment, carbazide is used as a deoxygenating agent for boiler water and as a passivating agent for metal surfaces to reduce corrosion rates. Carbazide reacts with dissolved oxygen in water to produce carbon dioxide, nitrogen, and water. The reaction between carbazide and oxygen is as follows:
CON4H6 + 2O2 = 2N2 + 3H2O + CO2
Carbazide is used after thermal deoxygenation. The dosage of carbazide is 0.5 mol for 1 mol of oxygen.
Advantages of carbazide deoxygenation: low toxicity, high melting point, safe and environmentally friendly.
Disadvantages of carbazide deoxygenation: generally used as an auxiliary agent in thermal deoxygenation.
Acetaldehyde oxime Deoxygenation
Acetaldehyde oxime molecular formula: C2H5NO. Acetaldehyde oxime is a low-toxicity reducing agent. In the 1990s, it replaced the highly toxic hydrazine as a boiler water deoxygenator, and its deoxygenation effect is forty times that of hydrazine. Therefore, it is widely used in thermal power plants in China as a new type of deoxygenator. Adding acetaldehyde oxime to the unit via feedwater can achieve excellent deoxygenation results, especially when demineralized water is directly added to the condenser feedwater. Utilizing the characteristics of acetaldehyde oxime’s rapid deoxygenation at low temperatures, most of the oxygen can be removed in the condenser, mitigating corrosion of the high-pressure heater and its piping.
Advantages of acetaldehyde oxime deoxygenation: Deoxygenation at low temperatures; rapid deoxygenation; low toxicity.
Disadvantages of acetaldehyde oxime deoxygenation: Steam mixed with air can be explosive; flammable; releases toxic nitrogen oxide gases when heated; stored away from fire and heat sources; storage temperature should not exceed 30℃.
Isoascorbic acid and its sodium salt for deoxygenation
Isoascorbic acid and its sodium salt are low-volatility deoxygenating agents. The use of isoascorbic acid as a deoxygenating agent was patented by Nalco Chemical Company in the United States in 1981. It is an isomeric compound of vitamin C (L-ascorbic acid). Its reaction with dissolved oxygen is complex, requiring several intermediate steps, and the mechanism is not yet fully understood. Due to its safety, it is widely used as a deoxygenating agent in food and feed. In my country, some manufacturers also use it as a boiler water deoxygenating agent. Isoascorbic acid and its sodium salt are superior to hydrazine in reducing iron ion content and slowing the corrosion rate of feedwater systems, and its final decomposition product is CO2, thus avoiding the organic acid corrosion problem in the low-pressure cylinder of steam turbines.
Advantages of isoascorbic acid and its sodium salt for deoxygenation: Fast deoxygenation speed; non-toxic and safe.
Disadvantages of using isoascorbic acid and its sodium salt for deoxygenation: The need to control microbial growth in dilute solutions; the decomposition of sodium isoascorbate at high temperatures producing corrosive dissolved solids, with decomposition products at 300℃ including 71.07% lactic acid, 20.48% acetate, and 8.44% formate; the sodium salt will affect the conductivity of water and steam; and it only deoxygenates without passivation.
There are many methods for deoxygenating boiler feedwater. To prevent boiler system corrosion and improve the boiler’s efficiency, economy, stability, and safe operation, it is essential to consider the boiler type and the user’s actual conditions, taking into account boiler thermal parameters, water quality, tonnage, load variations, and economic factors, and selecting the appropriate method based on local conditions.
