Castables for Waste-to-Energy Boilers: Three Zones, Three Different Mixes

Castables for Waste-to-Energy Boilers: Three Zones, Three Different Mixes

Most refractory problems in waste-to-energy boilers come from copying coal-fired practice. A coal boiler fights wear and heat; a waste incinerator fights two more enemies — HCl and alkali chlorides. Raw municipal waste produces HCl at levels commonly in the 400–1200 mg/Nm³ range in the flue gas, and KCl/NaCl-bearing fly ash deposits on the lining. The result is acid gas penetration, salt crystallising inside pores and expanding, and thermal shock from daily start-stops, all at once. These three damage modes weigh differently by location, so one mix cannot be laid across the whole furnace — selection has to be by zone.

1. Separate the Damage Mechanism by Zone First

Waterwall and furnace side walls: 800–1000℃, with the main threat being HCl and chloride salt penetration at the hot face, plus a steep gradient because the tubes are water-cooled behind the lining. The failure pattern is surface spalling over a loosened interior — not abrasion.

Grate zone, front and rear arches: the hottest area at 1000–1200℃, with slag scraping and tumbling on top of the heat. This is abrasion plus high temperature, and what matters is aggregate hardness and slag resistance.

Flue-gas path and horizontal pass turns: 400–700℃, where fly-ash erosion dominates, but low-temperature chloride dew-point corrosion also appears — below the dew point, condensed hydrochloric acid forms, and failure usually starts at the anchor root, corroding the stud before the castable falls away.

2. Mix Direction and Key Numbers for the Three Zones

Waterwall zone: SiC 50%–65%, Al2O3 30%–40%, low-cement or sol-gel bonded, water held at 5.0%–5.5%. Thickness 80–120 mm, anchors in 310S or 253MA heat-resistant steel, 8–10 mm diameter, staggered at 200–250 mm spacing. SiC here is not only for wear: the glassy phase it forms reduces molten salt wetting and is the main defence against chloride ingress.

Grate zone: SiC raised to 60%–75%, critical aggregate size up to 8–10 mm, thickness 120–150 mm. Sacrifice some placement flowability to keep the aggregate skeleton — the matrix is what wears away, the aggregate is what resists the scouring.

Flue-gas path: SiC 30%–45% is enough, or a high-alumina wear-resistant castable, at 60–80 mm. There is no point stacking SiC here; the priorities are low porosity and protected anchors.

3. Water Addition and Density: The First Gate Against Chlorides

Chloride attack works through permeability, and permeability works through connected pores. On the same mix, going from 5.2% to 6.0% water typically adds 3–5 percentage points of apparent porosity, and the penetration depth of chloride multiplies. Three firm rules on site: measure water by bucket within the datasheet limit, never fill from a hose; vibrate until the surface glistens and large bubbles stop, then stop — over-vibration segregates the mix into layers; seal the formwork joints, because a leaking joint is usually where spalling appears first.

Expansion joints go in by area: one 3–5 mm joint every 1.5–2 m, with dedicated joints at tube penetrations, irregular corners and anchor-dense zones, filled with ceramic fibre paper. Never leave joints with timber or foam — they burn out and leave a void. Check every anchor weld by hammer, and orient V or Y ends downward so ash does not collect.

4. Dry-Out: Do Not Copy the Coal-Boiler Curve

Incinerator linings are thick, carry more water and sit against waterwalls, so heat-up has to balance free-water removal against tube-wall temperature. A curve you can use directly: ambient to 150℃ at 10–15℃/h with a 6–8 hour hold; 150–350℃ at 10℃/h with a 6 hour hold; 350–600℃ at 15–20℃/h, then a 4 hour hold at target. Total is usually 60–90 hours. The critical points are the slow ramp below 150℃ and the plateau near 350℃ — the first drives off free water, the second drives off chemically bound water. Skip either and spalling within the first month of operation is close to certain. Before start-up, establish waterwall circulation so the tube wall heats with the lining; a cold wall against hot castable is the most damaging combination there is.

5. Acceptance and In-Service Monitoring

Three acceptance checks: hammer every point for hollow sound, and cut out and replace any hollow area over 100 cm²; treat 0.5 mm as the crack-width limit, recording and tracking anything wider; check thickness with a probe, allowing no more than 5% negative deviation from design. In service, inspect at every six-month shutdown, focusing on residual thickness of the side wall above the grate and on anchor roots in the flue-gas path — these two determine the next outage's workload. Plot all measurement points on one unfolded drawing, and by the second maintenance cycle you can see which zone needs a different mix, instead of guessing.