Air Preheaters (APH) play a crucial role in boiler systems, facilitating the transfer of heat from flue gases to the combustion air before it enters the furnace. This process improves thermal efficiency by recovering waste heat from the boiler’s flue gas, thereby minimizing energy losses, and reducing operational costs. However, a significant challenge faced in Tubular Air Preheaters (TAPH) is corrosion, particularly in the low-temperature zone.
This blog explores the factors contributing to corrosion in APH and how an innovative sacrificial layer design effectively mitigates this issue.
The Corrosion Challenge in Tubular Air Preheaters
In Power Plants, TAPH units, which can be either horizontal or vertical crossflow heat exchangers, utilize their tubes to preheat water using heat recovered from flue gases. When sulfur-containing fuels are combusted, flue gases enter the preheater tubes at approximately 160°C while the average temperature of the tube surfaces remains around 120°C. These conditions create an environment conducive to the formation of corrosive sulfuric acid leading to material degradation.
Here’s why corrosion happens:
• SO₃ gas present in flue gases condenses at lower temperatures.
• When the temperature of the tube wall drops below the dew point (~120°C), sulfuric acid is generated.
• This acid aggressively corrodes the tube surfaces, resulting in significant material degradation.
Key Failure Zone in Tubular Air Preheaters
The majority of issues with Tubular Air Preheater arise within the initial 5% of the inlet end of the tube, typically located at or just beyond the tubesheet. This region is subjected to the most severe corrosive and erosive conditions, making it a critical point of failure. Traditionally, addressing issues in this area required complete retubing of the bundle, resulting in significant maintenance costs and extended downtimes.
Conventional Repair Methods vs. Innovative Sacrificial Layer Design
Traditionally, when corrosion was detected, the entire tube bundle required replacement, a process that was both expensive and time-consuming. However, a new cost-effective solution has been developed: the integration of a removable sacrificial layer at the cold end of the bundle. This sacrificial layer consists of tubes made from Corten steel or enamel-coated steel which are designed to withstand corrosion for an extended period.
The key advantages of this approach include:
Localized Replacement: Only the sacrificial layer needs to be replaced, rather than the entire bundle.
Cost Efficiency: Eliminates the need for a full bundle retubing, reducing maintenance expenses.
Extended APH Lifespan: Protects the main Airpreheater tubes from premature failure.
Engineering the Sacrificial Layer: CFD & Analytical Calculations
To determine the optimal length of the sacrificial layer, analytical and Computational Fluid Dynamics (CFD) calculations are performed.
This evaluation considers various factors such as:
• The composition and temperature of flue gases
• The temperature distribution across the tube walls
• Acid dew point of the sulfur-laden gases
Through CFD simulations and Analytical calculations, a sacrificial layer length will be identified that effectively prevents corrosion in the lower bundle region of the TAPH.
Guide for Implementing the Sacrificial Layer
To minimize corrosion and improve efficiency, the existing tube bundle will be completely removed and replaced with a new one that features two distinct sections:
- Sacrificial Layer: Designed to endure corrosive conditions, this section is made from specialized corrosion-resistant materials such as Corten steel or enamel-coated steel. It is intended to absorb damage over time and can be replaced independently.
- Main Bundle Region: The remaining portion of the bundle will continue to function under standard heat transfer conditions and will remain protected due to the presence of sacrificial layer.
Implementation Steps
Removal of the Existing Bundle: The old and corroded tube bundle is removed to facilitate the installation of the upgraded system.
Preparation of the New Bundle: A fresh bundle is manufactured with two clearly defined sections: the sacrificial layer and the main bundle region.
Installation of the New Bundle: The newly designed bundle is installed in place, ensuring proper alignment and connectivity with the existing system.
Conclusion
The integration of sacrificial layers in Tubular Air Preheaters marks a significant advancement in boiler maintenance and efficiency. By replacing only the corroded section, Power Plants can significantly minimize downtime, reduce costs, and improve operational reliability.
This advanced corrosion mitigation approach, supported by CFD and analytical studies ensures that boiler systems operate efficiently while minimizing maintenance expenses. With the growing demand for sustainable energy solutions, the adoption of such cost-effective technologies can help Power and Process Plants in achieving emission reduction goals and enhancing overall energy efficiency.
In summary, TEFUGEN Technologies‘ advanced sacrificial layer technology for Tubular Air Preheaters (TAPH) effectively prevents corrosion, extends equipment lifespan, and reduces operational costs.