Dissolved Gases: The Hidden Threat to Boilers and Their Removal
Introduction
Feedwater quality is critical in boiler systems to ensure efficient and safe operation. While various treatment methods remove suspended undissolved particles and dissolved salts, dissolved gases such as oxygen, carbon dioxide, and nitrogen pose significant challenges. Oxygen is particularly harmful because it causes corrosion in boiler tubes. To address this issue, engineers employ deaeration and chemical dosing techniques. This section focuses on removing dissolved gases, especially oxygen, through low-pressure (LP) dosing and using chemicals like sodium sulfite and hydrazine.
The Problem of Dissolved Gases in Boiler Feedwater
Dissolved gases in boiler feedwater, especially oxygen and carbon dioxide, can cause severe damage to boiler components. Oxygen acts as a depolarizer, accelerating the corrosion process in metal tubes. The chemical reactions involved in oxygen-induced corrosion are as follows:
- Reduction of Oxygen: O2+4e−+2H2O→4OH−
- Formation of Ferrous Hydroxide: Fe2++2OH−→Fe(OH)2
Ferrous hydroxide further reacts to form iron oxide (rust), which weakens the boiler tubes and reduces their lifespan. Removing dissolved gases, particularly oxygen, from the feedwater is essential to prevent such damage.
Deaeration: The First Step in Gas Removal
Deaeration is the process of removing dissolved gases, primarily oxygen and carbon dioxide, from boiler feedwater using a device called a deaerator. The deaerator works by heating the water and reducing the solubility of gases, allowing them to escape into the atmosphere. However, deaeration alone may not eliminate oxygen, especially in trace amounts. Chemical methods are employed to ensure complete removal.
Low-Pressure (LP) Dosing: Chemical Removal of Oxygen
Low-pressure (LP) dosing is a chemical treatment method to remove residual oxygen from boiler feedwater. This process involves the addition of oxygen scavengers, such as sodium sulfite or hydrazine, to the feedwater. LP dosing is typically performed at the deaerator storage tank or the boiler feed pump suction, where the pressure is relatively low.
1. Sodium Sulphite as an Oxygen Scavenger
Sodium sulfite (Na2SO3) is an inorganic compound commonly used for deoxygenation. It reacts with dissolved oxygen to form sodium sulfate (Na2SO4), as shown in the following reaction:
2Na2SO3+O2→2Na2SO4
While sodium sulfite is effective, it has certain limitations. It increases the dissolved solids content in the feedwater, which may require additional blowdown to control. Its effectiveness depends on the feedwater temperature; higher temperatures reduce the sulfite required.
2. Hydrazine as an Oxygen Scavenger
Hydrazine (N2H4) is a preferred oxygen scavenger in many boiler systems due to its efficiency and lack of solid residue. It reacts with dissolved oxygen to form nitrogen gas and water, as shown below:
N2H4+O2→N2+2H2O
The nitrogen gas produced is inert and harmless, making hydrazine an ideal choice for deoxygenation. Hydrazine is available in various forms, including hydrazine hydrate (N2H4⋅xH2O), hydrazine hydrochloride (N2H4⋅HCl), and hydrazine sulfate (N2H4⋅H2SO4).
Advantages of Hydrazine:
- It does not contribute to dissolved solids in the feedwater.
- It is effective at temperatures above 100 °C and in alkaline conditions (pH > 7).
- It is typically dosed at the deaerator storage tank or feed pump suction, ensuring thorough mixing with the feedwater.
Implementation of LP Dosing
The LP dosing system consists of a dosing pump, a storage tank, and stainless steel pipelines to prevent corrosion. The dosing pump injects the required amount of hydrazine or sodium sulfite into the feedwater. Regular monitoring is essential to ensure the feedwater contains no dissolved oxygen and the optimal chemical dosage.
Key Considerations for LP Dosing
Operators must regularly test the presence of hydrazine in the boiler water to confirm the absence of dissolved oxygen. They should avoid overdosing on chemicals to prevent unnecessary chemical consumption and potential side effects. Engineers must design the system to handle the boiler’s specific requirements, including feedwater temperature and pressure.
Conclusion
Removing dissolved gases, particularly oxygen, from boiler feedwater is crucial to prevent corrosion and ensure the longevity of boiler components. Deaeration serves as the primary method for gas removal, but engineers often supplement it with chemical dosing techniques like LP dosing. Sodium sulfite and hydrazine are common oxygen scavengers, with hydrazine being the preferred choice because of its efficiency and lack of solid residue. Proper implementation of LP dosing and regular system monitoring ensures that harmful dissolved gases are eliminated from the feedwater, supporting boiler systems’ safe and efficient operation.
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