Specialist boiler maintenance
Waste-to-energy (WtE)boilers operate in environments where corrosion is an inherent and ongoing consideration
The combination of chlorine-rich fuels, variable feedstock composition, and elevated steam conditions accelerates material degradation compared with conventional power generation. This abbreviated article looks at how to circumvent the issue. To read the full article visit www.engineerlive.com.
The focus for maintenance teams
For maintenance teams, the focus is on effective corrosion management throughout the asset’s operational life. This requires a clear understanding of degradation mechanisms, supported by appropriate inspection strategies and timely intervention.
In this context, material selection and the application of weld overlay, including re-overlay, are established approaches to maintaining component integrity. When implemented correctly, these measures can help manage degradation rates, support planned maintenance strategies, and extend the service life of critical boiler components.
The role of chlorine in high-temperature boiler corrosion
In European WtE boilers, chlorine-induced high-temperature corrosion is the primary driver of fireside damage.
Municipal solid waste contains significant amounts of chlorine, largely from PVC and other chlorinated materials. During combustion, chlorine forms hydrogen chloride (HCl) and reacts with alkali and heavy metals such as zinc, lead, sodium, and potassium. These reactions produce low-melting-point metal chlorides, including ZnCl2 and PbCl2.
These compounds form molten deposits on tube surfaces and
- Flux and destabilise protective oxide layers (e.g. Cr2O3)
- Promote active oxidation mechanisms
- Accelerate metal wastage
In practice, this manifests as general wall thinning, localised pitting, and, in severe cases, rapid loss of tube integrity, particularly in high-temperature regions.
The role of fuel variability
Unlike conventional fuels, waste streams are inherently inconsistent. Variations in chlorine content, alkali metals, moisture, and ash composition can significantly alter deposit chemistry. Biomass co-firing, now increasingly common in Europe, introduces additional potassium and sodium, further influencing deposit melting behaviour.
This variability means that corrosion rates are not constant. A boiler may experience relatively mild conditions during one run and aggressive degradation in the next.
From an operational standpoint, corrosion behaviour in WtE boilers is highly plant-specific and cannot be reliably predicted from design conditions alone.
Understanding erosion-corrosion
In addition to chemical attack, many WtE boilers experience erosion-corrosion, particularly in areas exposed to high-velocity, particle-laden flue gases.
This is commonly observed in:
- Superheater banks
- Economisers
- Gas flow turning zones
In these regions, fly ash particles remove protective oxide layers from tube surfaces. The freshly exposed metal is then subjected to accelerated corrosion, creating a self-reinforcing degradation mechanism.
The severity of erosion-corrosion depends on gas velocity, ash loading, and local flow geometry, and is often intensified in modern plants operating at higher efficiencies.
Inspection strategies and corrosion monitoring
Given the variability of both fuel composition and operating conditions, regular inspection is essential.
Effective inspection programmes typically include:
- Visual assessment of exposed surfaces
- Ultrasonic thickness (UT) mapping
- Evaluation of protection system (weld overlay, thermal spray) condition and integrity
- Identification of pitting and localised attack
Establishing corrosion trends is critical. In many cases, operators observe measurable wear rates in protective overlays, while base material degradation can accelerate rapidly once exposure occurs.
Without structured inspection, maintenance strategies tend to become reactive, often leading to unplanned repairs or premature component replacement.
Conclusion
As European WtE plants continue to evolve, operators who adopt a proactive, lifecycle-focused approach, supported by high-quality welding capabilities, will be best positioned to maintain reliability and optimise performance over the long term.
Lech Biegus is a senior operations and quality professional with over 20 years of experience delivering complex welding, weld overlay, and Coke Drum repair projects across Oil & Gas, Power, and Waste-to-Energy facilities worldwide. As Regional Operations Lead at Integrated Global Services (IGS), he specialises in large-scale turnaround execution and quality control leadership.
For more information visit: https://integratedglobal.com/en
Protection strategies for WTE boilers
1) Panel replacement: Full panel replacement remains the most invasive option. It involves removing sections of waterwall tubing and installing new prefabricated panels, often with shop-applied overlay, followed by additional on-site welding.
While this approach effectively resets the asset’s condition, it introduces significant logistical complexity, costs, and outage duration.
2) On-site weld overlay: Weld overlay has become the industry standard for protecting carbon steel components in WtE boilers.
In this process, a corrosion-resistant nickel-based alloy, most commonly Alloy 625, is deposited onto the tube surface. The overlay acts as a sacrificial barrier, protecting the base material from aggressive combustion environments.
On-site application offers flexibility since it means operators can extend the life of existing components and target specific high-risk areas.
Contributed by Lech Biegus, operations lead at Integrated Global Services (IGS)
