Why Boiler Needs External Feedwater Treatment?
Introduction
Have you ever wondered what happens to the water before it enters a boiler? It’s not just about the water inside, you know! Boilers, those powerful machines that generate steam, are the backbone of many industries, from powering factories and generating electricity to providing heat for our homes. But like any machine, boilers need the right fuel to operate efficiently. And in the case of boilers, that fuel is water.
The quality of the water entering the boiler, known as feedwater, significantly impacts its performance and lifespan. Impure feedwater can lead to serious problems, hindering the boiler’s ability to generate steam effectively and even causing costly damage.
That’s where ‘external feedwater treatment’ comes in – a crucial step to ensure only the purest water reaches the boiler. This involves a series of processes designed to remove impurities from the raw water source before it enters the boiler system. 1 Let’s delve deeper into how we clean up this feedwater before it even gets a chance to boil.
Why Boiler Feedwater Needs Treatment
Raw water sources such as rivers, lakes, and groundwater often contain a variety of impurities that can adversely affect boiler performance. These impurities include:
- Suspended solids: Particles like dirt, mud, and other debris.
- Dissolved solids: Minerals such as calcium, magnesium, and silica are dissolved in the water.
- Gases: Oxygen, carbon dioxide, and other gases that can react with boiler components.
If left untreated, these impurities can cause significant problems:
- Scale formation: Minerals like calcium and magnesium can precipitate as hard deposits, reducing heat transfer efficiency and potentially damaging boiler components.
- Corrosion: Dissolved gases, especially oxygen and carbon dioxide, can accelerate the corrosion of metal parts, weakening the boiler’s structure.
- Fouling: Suspended solids can settle and clog pipes and equipment, disrupting the water flow and impairing system operation.
To mitigate these risks, feedwater must be treated before entering the boiler. Rigorous purification processes, such as filtration, softening, and chemical dosing, help remove impurities, ensuring efficient, safe, and reliable boiler performance while extending its lifespan.
External Feedwater Treatment Techniques
To ensure feedwater is suitable for use in boilers, several external treatment methods are employed to remove impurities effectively:
Clarification
- Sedimentation: This process allows suspended particles like dirt and mud to settle at the bottom of a tank under gravity, leaving more transparent water on top.
- Filtration: Water is passed through fine filters to trap and remove remaining suspended particles, ensuring cleaner water.
- Softening: This step targets the removal of minerals that cause hardness, such as calcium and magnesium.
- Lime-soda process: Chemicals are added to react with and precipitate these minerals, making the water softer.
- Ion exchange: Specialized resins replace hardness ions with non-hardness ions, typically sodium, ensuring effective softening.
Aeration: Dissolved gases like oxygen and carbon dioxide, which can contribute to corrosion, are removed by exposing water to air. This process allows the gases to escape.
What Is Demineralization?
Using cation and anion exchangers, demineralization removes dissolved salts and minerals from raw water. This method is more efficient and economical than other purification techniques, making it the preferred choice in most steam-generating plants. The water produced in this process, called DM water, is crucial for boiler feedwater, especially in once-through boilers, where strict water quality parameters must be maintained.
The Demineralization Process
Raw Water Composition:
Raw water contains salts like magnesium, calcium, and potassium in chlorides, sulfates, carbonates, and nitrates. These salts are categorized as:
Cations: Ca²⁺, Mg²⁺, Na⁺
Anions: CO₃²⁻, SO₄²⁻, Cl⁻
Cation Exchanger
Raw water passes through a cation exchanger containing resin charged with hydrogen ions (H⁺). The resin replaces cations (like Ca²⁺ and Mg²⁺) with H⁺ ions, making the water acidic. For example:
CaCO₃ + R-H→Ca-R + H₂CO₃
CaCO₃ + R-H→Ca-R + H₂CO₃
- Degasser Unit: The acidic water then enters the degasser, where carbonic acid (H₂CO₃) is broken down into water (H₂O) and carbon dioxide (CO₂). A blower helps vent the CO₂, and the water collects in a degasser tank.
Anion Exchanger
The degassed water moves through an anion exchanger containing resin with hydroxide ions (OH⁻). The resin replaces anions (like Cl⁻ and SO₄²⁻) with OH⁻ ions, neutralizing the water:
Cl⁻ + R-OH→R-Cl + OH⁻
Cl⁻ + R-OH→R-Cl + OH⁻
Mixed Bed Polisher
The water passes through a mixed bed containing cation and anion resins to ensure maximum purity. This step removes any remaining salts.
Types of Exchangers Used
Cation Exchangers:
Strong Acid Cation (SAC) removes all cations associated with strong and weak acids. Weak Acid Cation (WAC) removes only cations from weak acids like silicic acid.
Anion Exchangers:
Strong Base Anion (SBA): Removes all anions.
Weak Base Anion (WBA): Removes only strong acid anions.
Regeneration of Resin Beds
After processing a certain quantity of water, the resin beds become saturated with ions and require regeneration:
Cation Resin: Regenerated using hydrochloric acid (HCl) or sulphuric acid (H₂SO₄).
Anion Resin: Regenerated using caustic soda (NaOH).
Backwashing, slow rinsing, and fast rinsing ensure the resin is clean and ready for reuse.
Benefits of the Degasser Unit
CO₂ Removal: Reduces the load on the anion exchanger.
Energy Efficiency: Placing the degasser at an elevation allows water to flow by gravity to subsequent exchangers, minimizing pumping requirements.
Why DM Water is Essential for External Feedwater Treatment
DM water is indispensable for boiler feedwater systems, as it prevents scaling, corrosion, and inefficiencies caused by impurities. Its quality is critical in ensuring the longevity and reliability of power plant operations.
Understanding Anion Exchangers, Mixed Bed Exchangers, and Reverse Osmosis in Boiler Feedwater Chemistry
Water treatment for boiler feed involves complex processes to remove impurities, prevent scaling, and maintain the water’s chemical properties. This blog post delves into three critical aspects: anion exchangers, mixed bed exchangers, and reverse osmosis (RO) systems. Let’s explore these components step by step.
Anion Exchanger: The Acid Neutralizer
After water passes through the cation exchanger, it becomes acidic due to the formation of sulfuric acid, hydrochloric acid, and silicic acid. The anion exchanger neutralizes these acids using a special resin known as anion resin, represented as 𝑅+𝑂𝐻−.
The following reactions illustrate this process:
H2+ SO4+ R+OH– → R+ SO–4 + H2O
H+Cl–+ R+OH– → R+Cl–+ H2O
H2 SiO–3+ R+ OH– → R+ SiO–3 + H2O
The following reactions illustrate this process:
Regeneration of Anion Resin
Over time, the resin becomes exhausted and needs regeneration using a caustic soda solution (NaOH). The process involves:
- Backwashing: Removes impurities and loosens the resin bed.
- Caustic Soda Treatment: A 4–5% caustic soda solution is passed through the resin to regenerate it:
R+SO–4+Na+OH−→ Na+SO4−+ R+OH−
Mixed Bed Exchanger: The Silica Guard
The mixed bed exchanger purifies water by removing residual silica and fine impurities. It contains mixed cation and anion resins. However, its regeneration process is more intricate due to the presence of two types of resins.
Steps in Regeneration
Backwashing and Bed Separation: Backwashing separates the heavier cation resin (which settles at the bottom) from the lighter anion resin (which moves to the top).
Acid Injection: Acid is introduced to regenerate the cation resin.
Alkali Injection: Caustic soda regenerates the anion resin.
Mixing and Final Rinsing: High-pressure air mixes the resins, followed by a final rinse with water.
The purified water from the mixed bed exchanger is stored in a demineralized (DM) storage tank and dosed with morpholine to maintain the desired pH before being fed into the boiler.
Reverse Osmosis: Advanced Purification
Reverse Osmosis (RO) is a powerful water treatment method that removes both dissolved and colloidal impurities, making it suitable for high-pressure boilers.
How Does Reverse Osmosis Work?
In natural osmosis, water flows from a lower salt concentration to a higher one through a semi-permeable membrane. In RO, this flow is reversed by applying pressure, forcing water from a high-salt area through the membrane to the low-salt side. The result:
- Pure water (permeate): Passes through the membrane.
- Concentrate (reject water): Contains the removed impurities.
- Pretreatment for RO:
- To extend membrane life and improve efficiency, water undergoes pretreatment:
- Sand and Carbon Filters: Remove suspended particles.
- Ultrafiltration: Eliminates fine colloidal particles and macromolecules. Pore sizes range from 0.001 µm to 0.02 µm.
- Cleaning Enhanced Backwash (CEB): Removes fouling with cleaning chemicals like sodium hypochlorite, HCl, and NaOH.
RO System Operation
After pretreatment, water is pumped through the RO unit, where 80% becomes permeate, while the rest is rejected water with high Total Dissolved Solids (TDS).
Maintaining RO Efficiency
Regular maintenance is crucial:
- Membrane Cleaning: Prevents fouling and scaling.
- Chemical Dosing: Biocides and antiscalants minimize biological growth and scaling.
- Proper Shutdown: Rinsing membranes with permeate water prevents salt deposition and biofouling during downtime.
Understanding these processes highlights the intricate steps involved in ensuring high-quality boiler feedwater. Power plants can maintain operational efficiency by efficiently combining anion exchangers, mixed bed exchangers, and RO systems while protecting their equipment from scaling, fouling, and corrosion.
Conclusion
Understanding these processes highlights the intricate steps involved in ensuring high-quality boiler feedwater. Power plants can maintain operational efficiency by efficiently combining anion exchangers, mixed bed exchangers, and RO systems while protecting their equipment from scaling, fouling, and corrosion.
Understanding why boilers require external feedwater treatment is vital for ensuring their safety, efficiency, and longevity. Impurities in untreated water, such as dissolved salts, silica, and suspended solids, can lead to scaling, corrosion, and operational inefficiencies, causing costly downtime and damage. Processes like anion exchange, mixed bed exchange, and reverse osmosis provide comprehensive solutions to eliminate these contaminants and maintain the chemical balance essential for optimal boiler performance. External treatments safeguard boilers from potential failures and enhance their thermal efficiency by removing impurities and ensuring water quality.
“We greatly appreciate you taking the time to read our article! “Why Boiler Needs External Feedwater Treatment?” To help us reach a wider audience and continue creating valuable content, we kindly ask that you like and share this article on your preferred social media platforms. Your feedback is also invaluable. Please take a moment to leave a comment below, sharing your thoughts and insights on the article in 100 words or less. Your input helps us understand what resonates with our readers and allows us to improve our future content.”