Explanation: Why Acid Dew Point (ADP) Can Be Neglected Below 5 ppmvol SO2

Introduction

The acid dew point (ADP) is a critical factor in assessing the potential for condensation and corrosion in systems handling flue gas. The ADP represents the temperature at which sulfuric acid (H₂SO₄) condenses, forming a highly corrosive liquid phase. Typically, the ADP depends on the concentrations of sulfur trioxide (SO₃) and water vapor in the gas.

However, when the concentration of sulfur dioxide (SO₂) in the flue gas is below 5 ppmvol, the associated corrosion rates become negligible. This write-up explains why the ADP can be neglected under these conditions and the implications for system design and operation.

Why ADP Can Be Neglected Below 5 ppmv SO₂

1. Low SO₃ Formation:

   • The conversion of SO₂ to SO₃ is typically very low in systems with minimal SO₂. For instance, even with a conversion efficiency of ~2%, the resulting SO₃ concentration is too small to form significant quantities of sulfuric acid.

   • Example: At 5 ppmvol SO₂, the corresponding SO₃ concentration is approximately 1.06 × 10^-7 vol fraction (assuming typical conditions), which is insufficient to raise the ADP significantly.
     
     

2. Negligible Corrosion Rates:

   • Laboratory and field data indicate that below 5 ppmvol SO₂, the corrosion rates of carbon steel (e.g., A36 or DC-01) are so low that it would take more than 50 years to corrode a 1.5 mm thick steel plate.

   • Corrosion rates under these conditions are typically below 0.03 mm/year, which is considered negligible in most industrial applications.
     
     

3. Minimal Acid Formation:

   • The amount of H₂SO₄ formed through SO₃ and water vapor reactions is extremely low at these SO₂ levels. This results in a dilute condensate that is less aggressive toward materials.

Practical Implications

1. Simplified System Design:

   • Below 5 ppmv SO₂, the system temperature can be maintained below the theoretical ADP without significant risk of corrosion. This allows for cost savings by reducing the need for expensive materials or coatings.
     
     

2. Operational Efficiency:

   • Processes can operate more flexibly at lower temperatures, improving heat recovery and energy efficiency without compromising equipment lifespan.
     
     

3. Material Selection:

   • Standard carbon steel materials can be safely used, avoiding the need for high-alloy or corrosion-resistant materials in low-SO₂ environments.

Conclusion

When the SO₂ concentration in flue gas is below 5 ppmv, the formation of sulfuric acid and the associated corrosion risks are negligible. As a result, the acid dew point can be disregarded in system design and operational considerations. This simplifies the engineering approach and reduces costs while maintaining safety and performance.

For flue gas systems operating above 5 ppmv SO₂, however, the ADP must be carefully managed to mitigate corrosion risks.