The Corrosion Challenge in Municipal Solid Waste Incinerators
Waste-to-energy (WtE) incinerators processing municipal solid waste (MSW) impose some of the most aggressive chemical and thermal environments encountered in industrial refractories. Unlike clean-fuel combustion systems, MSW contains chlorinated plastics, alkali-rich food waste, sulphur-bearing materials, and heavy metals — all of which generate corrosive combustion products that attack lining materials simultaneously. Chloride vapours (HCl, alkali chlorides KCl and NaCl), sulphur oxides (SO₂, SO₃), and reactive alkali fluxes (K₂O, Na₂O from fly ash) penetrate brick and castable matrices at operating temperatures ranging from 750 °C in the lower grate zone to 1,150 °C in the upper combustion chamber. Selecting the wrong refractory grade means unplanned shutdowns within 12–18 months; selecting correctly can extend campaign life to 4–6 years. This guide addresses zone-specific material selection, typical specifications, and practical installation recommendations for engineers and procurement managers specifying lining systems for grate-fired and rotary MSW incinerators.
Understanding the Attack Mechanisms: Chloride, Alkali, and Sulphur
Three principal corrosion mechanisms determine service life in MSW incinerators, and effective refractory selection must address all three concurrently.
Chloride Attack
Hydrogen chloride (HCl) concentrations in MSW flue gas routinely reach 500–2,500 mg/Nm³. At grate and lower furnace temperatures (800–1,000 °C), alkali chlorides condense on refractory surfaces as low-melting eutectics. Potassium chloride (melting point 770 °C) and sodium chloride (801 °C) penetrate open porosity and react with silica phases in the brick matrix, forming alkali silicates that cause volume expansion and spalling. Refractories with high free-silica content — standard fireclay and 40% alumina grades — are particularly vulnerable. High-alumina materials with Al₂O₃ exceeding 70% significantly reduce susceptible silica phases.
Alkali Attack
Fly ash in MSW combustion carries high loadings of K₂O and Na₂O — combined alkali contents of 3–8 wt% are common. These alkalis flux the refractory at temperatures above 900 °C, dissolving grain boundaries and forming glassy phases that reduce hot strength and creep resistance. Mullite-bonded refractories resist alkali penetration better than glass-bonded equivalents because the mullite crystal lattice does not readily accommodate alkali substitution. Phosphate bonding systems provide an additional defence, forming stable AlPO₄ phases that block penetration pathways.
Sulphur Attack
SO₂/SO₃ in the flue gas reacts with calcium-bearing phases (CaO, CaSO₄) and, at lower temperatures near the boiler entry zone, deposits sulphate condensates. Refractories with low CaO binder content outperform calcium aluminate–bonded castables in high-sulphur environments. Dense, low-porosity linings with apparent porosity below 15% restrict sulphate ingress and the associated spalling that follows thermal cycling.
Zone-by-Zone Refractory Requirements
An MSW incinerator is not a single environment — it comprises at least four thermally and chemically distinct zones, each demanding a specific refractory strategy.
| Zone | Operating Temperature | Primary Attack | Recommended Product | Typical Specification |
|---|---|---|---|---|
| Grate and lower furnace walls | 750 – 950 °C | Alkali chloride condensation, abrasion from waste bed | High-Alumina Brick 70 (HA-70) | Al₂O₃ ≥ 70%, bulk density ≥ 2.55 g/cm³, AP ≤ 18% |
| Primary combustion chamber | 950 – 1,150 °C | Combined alkali, chloride, sulphur flux; thermal shock | Corundum Brick 90 (CB-90) | Al₂O₃ ≥ 90%, CCS ≥ 80 MPa, AP ≤ 14%, RUL ≥ 1,450 °C |
| Post-combustion / burnout zone | 850 – 1,050 °C | Cyclic thermal shock, moderate alkali | Dense Castable 80 (DC-80) or HA-70 brick | Al₂O₃ ≥ 80%, HMOR ≥ 8 MPa at 1,000 °C, AP ≤ 16% |
| Roof, arches, and crown | 1,000 – 1,150 °C | Alkali vapour, creep under dead load | Corundum Brick 90 or Dense Castable 80 | Creep in compression ≤ 0.5% at 1,200 °C / 50 h |
| Repair patches and irregular geometries | Up to 1,100 °C | All of the above | Gunning Mix High-Alumina / Plastic Refractory Phosphate | Al₂O₃ ≥ 70%, bond: phosphate or hydratable alumina |
Material Selection: Grades, Properties, and Practical Trade-offs
High-Alumina Brick 70 — Grate Zone Workhorse
For the grate and lower furnace sidewalls, ThermalEast High-Alumina Brick 70 (HA-70) offers a cost-effective balance of alkali resistance, mechanical strength, and thermal shock tolerance. With Al₂O₃ at 70% minimum, the mullite content is sufficient to resist KCl/NaCl penetration without the cost premium of higher-alumina grades. The relatively lower operating temperature in this zone (below 950 °C) means that the higher creep resistance of corundum grades is not required. Specify shaped bricks with tight dimensional tolerances (±0.5 mm on critical faces) to minimise mortar joint thickness and reduce alkali infiltration pathways.
Corundum Brick 90 — Primary Combustion Chamber
The combustion chamber crown and upper sidewalls demand the most chemically resistant material in the system. ThermalEast Corundum Brick 90 (CB-90), with Al₂O₃ ≥ 90% and a corundum-dominant microstructure, resists flux attack from combined alkali, chloride, and sulphur at temperatures up to 1,400 °C service. Apparent porosity below 14% limits penetration depth even under prolonged exposure. At typical combustion chamber operating temperatures of 950–1,150 °C, CB-90 provides a significant safety margin against eutectic melting and creep deformation. The refractoriness under load (RUL T₀.₅ ≥ 1,450 °C) ensures structural integrity through temperature excursions during upset combustion conditions.
Dense Castable 80 — Monolithic Flexibility
ThermalEast Dense Castable 80 is the preferred monolithic solution for post-combustion zones, transition areas, and complex geometric sections — boiler entry throats, offtake ducts, and air injection ports — where shaped brickwork is impractical. Formulated with tabular alumina aggregate and low-cement (LC) or ultra-low-cement (ULC) bonding systems, DC-80 delivers Al₂O₃ ≥ 80%, apparent porosity ≤ 16%, and hot modulus of rupture (HMOR) ≥ 8 MPa at 1,000 °C. Low CaO content (≤ 1.5% in ULC variants) minimises sulphate-phase formation. Specify a curing and heat-up schedule with dwell periods at 110 °C, 350 °C, and 600 °C to drive out free and chemically bound water without explosive spalling.
Gunning Mix and Phosphate Plastic for Maintenance
WtE incinerators typically schedule hot-face inspections at 12–18 month intervals. Localised erosion, edge spalling, and joint opening are addressed using ThermalEast Gunning Mix High-Alumina for pneumatic application and Plastic Refractory Phosphate for hand-packing of irregular cavities. The phosphate binder in the plastic refractory provides immediate green strength without a formal cure cycle — critical when turnaround time is limited to 48–72 hours. Gunning mix rebound loss should be specified below 15% to control material wastage on vertical surfaces.
Practical Recommendations for Procurement and Installation
- Minimise silica content: Specify maximum SiO₂ of 22% for HA-70, 8% for CB-90, and 10% for DC-80 — free silica is the primary chloride attack vector.
- Low apparent porosity is non-negotiable: Specify AP ≤ 14% for combustion chamber bricks; higher-porosity materials extend no more than 18 months before significant flux penetration.
- Mortar matching: Use high-alumina air-setting mortar with ≥ 70% Al₂O₃ for HA-70 brickwork; corundum-grade mortar (≥ 85% Al₂O₃) for CB-90 to prevent mortar joints from becoming preferential attack sites.
- Expansion joints: Allow 10–12 mm/metre of ceramic fibre expansion joint (1,260 °C-rated) in the combustion chamber lining to accommodate differential thermal growth and prevent compressive spalling.
- Anchor design for castables: Use SS310 or Inconel alloy V-anchors at 250 mm grid spacing for DC-80 in zones above 900 °C; coat anchors with bituminous paint prior to casting to allow thermal movement.
- Incoming material verification: Require mill certificates with XRF chemical analysis, cold crushing strength (CCS), and apparent porosity for each production lot. Retain samples for 12 months for post-incident analysis.
Summary
Effective refractory performance in MSW incinerators depends on matching material chemistry and microstructure to the specific attack mechanisms present in each zone. High-alumina compositions above 70% Al₂O₃ are the minimum baseline for any hot-face lining; corundum-grade materials at 90% Al₂O₃ are the correct specification for primary combustion chamber environments. Dense, low-porosity linings combined with phosphate or low-cement bonding systems resist the simultaneous chloride, alkali, and sulphur attack that defeats standard refractories within a single campaign. Maintenance programmes using matched gunning mixes and phosphate-bonded plastics protect capital investment between scheduled outages.
ThermalEast supplies the complete product range required for MSW incinerator lining systems — from HA-70 shaped bricks and CB-90 corundum bricks through to DC-80 dense castable, high-alumina gunning mixes, and phosphate plastic refractories — with technical datasheets, XRF certification, and application engineering support. Whether you are designing a new lining system or optimising refractory campaign life on an existing unit, contact ThermalEast to discuss your zone-by-zone requirements and request a detailed material quotation tailored to your operating conditions.