Waste-to-Energy (WtE) and biomass boilers operate in some of the most chemically aggressive and thermally demanding environments. The combustion of municipal solid waste, refuse-derived fuels, and biomass introduces high concentrations of corrosive species such as chlorides, sulfates, alkalis, and acidic vapors into the flue gas. This results in accelerated fireside corrosion, erosion, and slagging, particularly in high-temperature components like waterwalls, superheaters, and economizers.
Maintaining asset reliability in these plants is critical, as unplanned shutdowns incur significant revenue losses and safety risks. Field-applied corrosion mitigation strategies are now essential, with weld overlay, HVTS® (High Velocity Thermal Spray), and anti-slagging coatings as leading solutions.
Understanding Corrosion in WtE and Biomass Boilers
Corrosion in WtE and biomass boilers is often caused by deposition and reaction of alkali salts, chlorine species, and fly ash. These form low-melting-point eutectics that aggressively attack protective oxide layers and metal substrates. Wastage is severe, especially in bull nose sections, walls, and burner zones with high gas velocities.
In addition to uniform corrosion, localized pitting and erosion accelerate degradation, leading to premature tube failure. Operators must protect these areas without extending turnaround windows or compromising already thinned tubes.
Weld Overlay – Durable but Challenging
Weld overlay applies corrosion-resistant alloy (CRA) layers like Inconel onto boiler tubes, forming a strong metallurgical bond resistant to corrosion and erosion.
However, it presents challenges in WtE and biomass environments:
- High heat input may distort thin-walled tubes.
- Slow application rate (0.5–2 m²/shift/machine).
- Post-weld heat treatment (PWHT) may be required.
- Substrate limitations if thinning or previous overlays exist.
Nonetheless, weld overlay remains useful in areas needing thick-layer restoration or pressure boundary repair.
HVTS – A Solution for Rapid, Durable Cladding
HVTS uses supersonic gas flow to apply metal particles onto prepared surfaces, creating dense, uniform, high-adhesion cladding. It avoids thermal distortion and heat-affected zones (HAZ).
Key benefits:
- Rapid application (3–6 m²/shift).
- No HAZ or distortion.
- Suitable for complex or confined geometries.
- Spot repairable without full reapplication.
HVTS alloys are customized for flue gas chemistry, temperature, and erosion profiles. Service life ranges from 10 to 15+ years under proper inspection regimes.
Anti-Slagging Coatings to Mitigate Fouling
Slag buildup on boiler waterwalls and heat exchangers is another major issue. Anti-slagging ceramic coatings reduce slag adhesion and fouling.
These field-applied coatings:
- Limit slag accumulation on waterwalls, burners, and OFA ports.
- Reduce sootblowing frequency.
- Lower furnace exit gas temperatures (FEGT).
- Protect superheaters and heat transfer.
Hybrid Protection Strategies
In high-risk areas like burner throats or bull noses, a hybrid approach is effective. Weld overlay restores metal thickness in the first pass, HVTS protects against corrosion and erosion in the second and third passes, and anti-slagging coatings reduce fouling and heat loss.
This three-tiered method balances protection, cost, and outage duration.
Engineering for Reliability
As WtE and biomass plants pursue higher availability and emissions targets, pressure part longevity becomes mission-critical. When integrated into outages and backed by inspection, HVTS, alone or combined with weld overlay and anti-slagging coatings, offers a robust defense against fireside degradation. This represents a strategic leap in reliability for harsh combustion environments.
