How Is Corrosion Managed in Refuse-burning Heat Exchangers?

Corrosion in refuse-burning (waste-to-energy) heat exchangers is a major challenge because the flue gas contains aggressive compounds such as chlorides, sulfates, heavy metals, and acidic condensates, in addition to high temperatures. Effective corrosion management combines material selection, design strategies, operational control, and chemical treatment. Here’s a detailed breakdown:

1. Material Selection

The choice of materials is critical because conventional carbon steel cannot withstand the highly corrosive environment in waste incineration.

• High-Alloy Stainless Steels (e.g., 316L, 904L):

• Resistant to chlorides and acidic condensates.

• Often used in superheater and economizer sections where temperatures are moderate (200–450°C).

• Nickel-Based Alloys (e.g., Inconel, Hastelloy):

• Used for high-temperature sections or where chlorine-induced corrosion is severe.

• Expensive but very effective for long-term durability.

• Cladded or Coated Tubes:

• Carbon steel tubes can be cladded with stainless steel or coated with ceramic layers to reduce cost while enhancing corrosion resistance.

2. Design Strategies

Proper design reduces the risk of corrosion:

• Gas Flow and Temperature Control:

• Avoid cold spots where acidic condensates can form.

• Ensure the flue gas stays above dew point of corrosive acids, especially hydrochloric acid (HCl) and sulfuric acid (H₂SO₄).

• Tube Geometry:

• Use smooth surfaces and optimized fin spacing to minimize ash and deposit accumulation, which can lead to under-deposit corrosion.

• Cleaning Access:

• Include soot blowers, mechanical cleaning systems, or online washing to remove corrosive deposits.

3. Operational Control

Operations significantly influence corrosion:

• Combustion Optimization:

• Maintain complete combustion to reduce corrosive species like HCl.

• Control temperature to minimize fly ash condensation and chloride deposition.

• Water Chemistry (for water/steam-side corrosion):

• Control pH and oxygen content in boiler water to prevent internal corrosion.

• Use phosphate treatment or oxygen scavengers where appropriate.

• Load Management:

• Avoid frequent startups/shutdowns that can produce condensation of acids on heat exchanger surfaces.

4. Chemical and Protective Measures

• Acid-Resistant Coatings: Epoxy or ceramic coatings on heat-exchanger surfaces to resist acid attack.

• Additives: Sulfite or phosphate-based additives in water/steam cycles to form protective layers.

• Corrosion Monitoring: Online corrosion probes and deposit sampling help identify corrosion rates early.

5. Regular Maintenance and Inspection

• Frequent visual inspections and non-destructive testing (e.g., ultrasonic thickness measurement) identify thinning of tubes.

• Deposit removal (via soot blowers or chemical cleaning) prevents localized corrosion.

Summary:
Corrosion in refuse-burning heat exchangers is managed through a combination of high-alloy or coated materials, careful design to avoid condensate formation, operational control of temperature and combustion, chemical treatments, and regular cleaning/inspection. Each measure targets the specific corrosion mechanisms caused by chlorides, acids, and particulate deposits.