The heat source for the HTG in flue gas fired LiBr Absorption units is high-temperature flue gas (typically above 300°C) discharged from oil or gas-fired boilers.

1. Root Cause: Sulfuric Acid Dew Point Corrosion

When high-temperature flue gas flows through the heat exchanger tube bundle of the HTG, it heats the LiBr solution inside the tubes. Simultaneously, the flue gas itself cools. When the flue gas temperature drops below its “acid dew point,” problems arise: SO₂ in the flue gas partially converts to SO₃. SO₃ combines with water vapor (H₂O) to form sulfuric acid (H₂SO₄) vapor.

This sulfuric acid vapor condenses into highly concentrated sulfuric acid droplets on metal surfaces (the outer walls of heat exchange tubes) below the dew point. This corrosion caused by condensed sulfuric acid is termed sulfuric acid dew point corrosion. It represents a severe form of low-temperature electrochemical corrosion with extremely rapid corrosion rates.

2. Why are ordinary steels (e.g., carbon steel) ineffective?

Carbon steel (e.g., Q235): Exhibits virtually no corrosion resistance in dilute sulfuric acid environments. Sulfuric acid rapidly reacts with iron to form ferrous sulfate, leading to accelerated wall thinning, perforation, and equipment failure.

Common stainless steel (e.g., 304): While resistant to general acid-alkali corrosion, it exhibits poor resistance to sulfuric acid, especially at medium concentrations. Chloride ions (potentially present in flue gas or the environment) may also induce stress corrosion cracking.

3. Advantages of ND Steel

Engineered for Sulfuric Acid Dew Point Corrosion

ND steel (09CrCuSb) is a corrosion-resistant steel specifically engineered for sulfuric acid dew point corrosion. It combats this specific corrosion through a unique alloy design: Core Alloying Elements: Copper (Cu) and Antimony (Sb) (Sb).

These elements enrich at the steel surface during early corrosion stages, forming a dense, stable, and highly adherent composite sulfate corrosion product protective layer.

This protective layer effectively blocks further contact between the sulfuric acid solution and the steel substrate, significantly slowing down the progression of corrosion. Performance Comparison with Carbon Steel:

Based on national standards and extensive engineering practice, under identical simulated sulfuric acid dew point corrosion conditions, ND steel exhibits corrosion resistance over ten times greater than ordinary carbon steel.

This means ND steel equipment can achieve several times longer service life in equivalent corrosive environments. It substantially reduces downtime, maintenance costs, and component replacement expenses caused by corrosion perforation, ensuring long-term stable operation of the unit.