Boiler Refractory: Package, Utility, CFB, Biomass and Waste-to-Energy Boilers and Incinerators

Boiler refractory is the castable, brick, plastic, gunning and insulating lining of package, utility, CFB, biomass and waste-to-energy boilers and incinerators, and about 90 % of it sits on the water walls (NIA Insulation Outlook, 2010). Which class goes into which zone, and why does a waste-to-energy grate need an alkali-resistant castable while a CFB cyclone needs silicon carbide (SiC)? Boiler linings draw on the same families as every furnace, defined under refractory materials.

The class is chosen per zone: conventional medium-weight castable on water walls, SiC where abrasion or heat transfer rule, alkali-resistant high-alumina castable where waste ash attacks.

Waste ash converts mullite below 1100 °C (2,012 °F), boiler castables are heated at 28 to 56 °C/h (50 to 100 °F/h), and erosion is tested to ASTM C704/C704M-15(2022)e1 or ISO 16282:2007.

Every value names its standard and edition, the product data sheet or the dated source it comes from; see our methodology and fact-checking process.

What Is Boiler Refractory?#

Boiler refractory is the nonmetallic lining that keeps heat, flame, ash and flue gas away from the pressure parts and casing of a boiler or incinerator, in service above 538 °C (1,000 °F) (ASTM C71-12(2024)). ISO 836:2001 (term 107) sets no temperature threshold.

Resco Products, a US producer, lists coal-fired, natural gas, waste-to-energy and circulating fluid bed boilers as its boiler applications. In all four, the lining protects tubes and casings rather than holding melt.

Why Most Boiler Refractory Sits on Water Walls#

About 90 % of boiler refractory is installed on the water walls, outside the tubes, and the most common product there is a conventional cement-bonded medium-weight castable (NIA Insulation Outlook, Bases 2010).

Specialty SiC or high-alumina castables cost about twice as much, a relative cost factor for the US in 2010, so they go only where abrasion, heat transfer or alkali attack demand them. How power compares with the other consuming industries is set out under refractories by industry.

Boiler and Incinerator Zones, Wear and Lining Classes#

Boilers and incinerators split into five lining situations, from utility water walls to incinerator kilns and the seals of every type, each with its own wear mechanism and lining class. Table 1 keeps this order.

Utility and Industrial Water-Tube Boilers#

On utility and industrial water-tube boilers the refractory covers the water walls outside the tubes, where conventional medium-weight castable is the default and SiC castable serves tube walls that need heat transfer. The failure points are erosion where refractory meets bare tube and failure of the anchors holding castable to the membrane wall.

Circulating Fluidized Bed Boilers: Lower Combustor, Cyclones and Loop Seals#

CFB boilers line the lower combustor, the cyclones and the loop seals with SiC-containing castables and dense castables or bricks, because circulating bed material erodes every surface it touches. Erosion of dense castables and bricks for CFBC boilers is tested to ASTM C704 (Prasad et al., Interceram 66, 2017).

Waste-to-Energy and Biomass Boilers: Grate, Roof and First Pass#

Waste-to-energy boilers use high-alumina low-cement castables with enhanced alkali resistance on the grate chamber sidewalls, gunning mixes from the roof down to the lower first pass, and SiC tiles on the first-pass water walls (Capital Refractories technical paper).

SiC tiles serve heat transfer and protect the water-wall tubes. Biomass boilers use SiC tiles on membrane walls and in the combustion zone for the same reasons, and the oxide bond of these tiles is their weak point in waste slags and gases (Journal of the European Ceramic Society, 2025).

Hazardous Waste Rotary Kiln Incinerators#

Hazardous waste incinerators run above 1000 °C (1,832 °F) (refractories WORLDFORUM paper). In one plant example, the rotary kiln carries 300 mm (11.8 in) of high-alumina brick graded from 60 to 90 % Al2O3 by zone on a 50 mm (2 in) hard firebrick backup. Brickwork of this kind answers for boiler brick in incinerator service, while steam-boiler water walls are castable-lined.

Backup Insulation, Ducts and Door Seals Across Boiler Types#

Behind and around the dense lining, boilers use AES wool for HRSG ducts, refractory ceramic fiber (RCF) for boiler, incinerator and stack linings, fiber modules for incinerators, microporous board for thin backup and fiber rope for door gaskets. RCF is Carc. 1B H350i (CLP Annex VI index 650-017-00-8); AES wool falls under index 650-016-00-2 and is not classified when Note Q is met.

A dense working lining faces flame, ash or bed material, and the insulating backup behind it limits casing temperature. Layer build-ups and interface temperatures are worked out under refractory lining design.

Materials by Zone: Refractory Classes for Boilers and Incinerators#

Boilers and incinerators are lined with six refractory families: conventional and low-cement castables, SiC castables and tiles, plastic refractories, gunning mixes, high-alumina brick and hard firebrick with mortar, and fiber insulation. Table 2 gives each class its zone and reason.

Conventional Medium-Weight and Low-Cement Castables in Boilers#

The most common castable refractory for boilers is conventional cement-bonded medium-weight castable (NIA Insulation Outlook, 2010), while waste-to-energy grate sidewalls take high-alumina low-cement castable with enhanced alkali resistance.

Medium-weight castables fall in a density band of 1,602 to 2,082 kg/m³ (100 to 130 lb/ft³), below the conventional dense family. They carry the most mixing water of the dense families, which makes boiler dry-out slow; the water figures follow under binders.

Silicon Carbide Castables for Tube Walls and CFB Zones#

Silicon carbide castables are low-cement castables with SiC grain, used on CFB lower combustors, cyclones, loop seals and boiler tube walls for abrasion resistance and high thermal conductivity toward the water-cooled tubes. The class is also called SiC castable, high conductivity castable or boiler tube wall castable.

In waste-to-energy double-layer systems, SiC tiles are dry-set and self-flowing SiC castable is cast behind them (NAWTEC 14 conference paper, 2006). Pressed oxide-bonded or nitride-bonded tiles have 10 to 20 % porosity, while siliconized SiSiC tiles are infiltrated with silicon metal to a pore-free body (Stephan, NAWTEC 15 conference abstract, 2007).

Plastics, Gunning Mixes and Brick in Boiler Service#

Power boilers also use phosphate-bonded plastic refractories of 50 to 90 % alumina, abrasion-resistant gunning mixes, and brickwork of high-alumina, super-duty and insulating firebrick laid in mortar. Brick serves incinerator kilns and brick-lined zones, yet most boiler refractory is monolithic, because the water walls that hold about 90 % of it are castable-lined.

Binders and Additives in Boiler Refractories#

Boiler refractories rely on four bonding systems: calcium aluminate cement in castables, SiC grain as a functional addition, phosphoric acid and monoaluminum phosphate in plastics, and cement bonds activated at the nozzle in gunning mixes.

Cement Level and Water Demand in Boiler Castables#

Water demand falls as cement level falls: class ranges are 15 to 18 % for medium-weight, 8 to 12 % for conventional dense and 4 to 8 % for low-cement castables (Jenkins 2011, Brimstone sulfur symposium paper). The low-cement family needs the least water to place. Cement grades and their hydrates are compared under calcium aluminate cement for refractories.

Steam occupies about 1,600 times the volume of its water (HWI 2020; Jenkins 2011), and a conventional castable uses about 25 % of its added water for hydration. The rest leaves as steam on dry-out.

SiC Grain, Phosphate Bonds and Nozzle Water in Gunning#

Silicon carbide grain is added to boiler castables for thermal conductivity, abrasion resistance and slag non-wetting, inside a low-cement hydraulic calcium aluminate bond. In plastics, phosphoric acid and monoaluminum phosphate react with alumina to form the bond.

Dry gunning mixes with conventional or low cement levels receive their water at the nozzle; wet shotcrete pumps a deflocculated low- or no-cement castable and flocculates it at the nozzle. Grit grades and free carbon and silicon limits are covered under silicon carbide grain for refractories.

Wear Mechanisms in Boiler and Incinerator Linings#

Boiler and incinerator linings wear by four mechanisms: alkali attack from fuel and waste ash, oxidation of SiC in steam-bearing flue gas, erosion by circulating solids and at refractory-tube transitions, and anchor failure.

In one slag analysis from the Capital Refractories technical paper, a municipal solid waste surface slag contains 39.40 % SiO2, 36.79 % CaO and 8.68 % Al2O3.

Alkali Attack Below and Above 1100 °C#

Below 1100 °C (2,012 °F), sodium from waste ash precipitates as thenardite and converts mullite to albite plus secondary alumina, while potash forms kaliophilite; above 1100 °C, beta-alumina (Na2O·11Al2O3) forms. The same paper is the source.

Waste-to-energy sidewalls therefore take high-alumina castables with enhanced alkali resistance, and oxide-bonded SiC tiles lose their bond in these slags and gases. Test methods for corrosion and penetration are explained under slag and chemical attack.

SiC Oxidation in Steam#

Silicon carbide oxidizes actively and passively in boiler flue gas, and steam accelerates both. Passive oxidation swells SiC tiles until the expansion joints between them disappear.

Post-mortem X-ray diffraction finds cristobalite and feldspars that crack the glaze, and slag adheres to the tiles with gas vesicles (Coggin 2025 paper).

Erosion at Tube Transitions and in CFB Loops#

Erosion concentrates where refractory meets bare tube and wherever CFB bed material circulates through combustor, cyclone and loop seal. Erosion at refractory-tube transitions is one of four failure modes recorded for SiC castables in boilers. How abrasion and erosion are measured is covered under abrasion and erosion resistance.

SiC castables and dense castables or bricks for CFB zones are selected on ASTM C704 results, reported as eroded volume in cm³.

Lining Quantity, Cost Factors and Price Trend for Boiler Linings#

About 90 % of boiler refractory sits on water walls, and the main cost lever is class choice: specialty SiC or high-alumina castables cost about twice as much as conventional medium-weight castable (US, 2010). Table 3 lists the dated metrics; no dated source gives campaign life or consumption per megawatt-hour for boiler linings.

Installation quotes add area, thickness, tonnage, placement method, access and scaffolding, anchors, shifts, outage window, curing, dry-out equipment and fuel, tear-out tonnage and waste class, quality control and mobilization.

The US BLS producer price index for clay refractories (WPU1352, 1982 = 100) stood at 405.818 in September 2026, a trend, not a price.

Installing, Drying Out and Repairing Boiler Linings#

Boiler linings are cast, pumped, gunned, rammed or tiled onto anchored membrane walls and casings, dried out with slow ramps and holds sized by the thickest section, and repaired at outages after visual and hammer inspection.

Placement Methods and Tile Fixing on Membrane Walls#

SiC and dense castables in boilers are placed by vibration casting, pumping, shotcrete or self-flow, and monolithic linings on walls and roofs are held by anchors. Anchors sit at least 152 mm (6 in) from expansion joints (RAI anchor producer guidance). Plibrico (2022) gives an anchor length of 75 to 85 % of the dense layer and a metallic expansion about three times that of alumino-silicate refractory; alloys and spacing are set out under refractory anchors.

Waste-to-energy tile fixing evolved from bolt-on to hidden-clip to back-cast systems (NAWTEC 14, 2006). Tiles hang on membrane walls by anchors or ventilated hangers with a mortar or castable backfill.

Dry-Out Ramp and Holds for Boiler Castables#

Boiler castables cure for 24 hours and are then heated at 28 to 56 °C/h (50 to 100 °F/h), with each hold lasting 1 hour per 25 mm (1 in) of the thickest section (NIA Insulation Outlook, Bases 2010). The first hold sits at 104 to 204 °C (220 to 400 °F) for cement-bonded castables; this is trade guidance, not an API 936 requirement, and the maker's product schedule applies.

The slow ramp gives the steam a path out before pressure builds. Hold temperatures by castable type are set out under refractory dry-out and heat-up schedules.

Outage Inspection and Repair of Boiler Linings#

Boiler linings are inspected at outages by visual and hammer sounding, and in service by shell thermography and embedded thermocouples.

Ceramic linings fail in tension, so a lining is not cooled just to look for cracks (Hotwork). API RP 982, 1st edition (2023), covers inspection and repair assessment of refractory linings in refinery equipment, including boilers and incinerators.

Cold repair cuts back to sound material, replaces anchors, re-casts or guns, and dries locally. Hot and cold repair methods are compared under refractory repair and maintenance.

Standards and Quality Control for Boiler Refractories#

Boiler refractories are tested for erosion to ASTM C704/C704M-15(2022)e1 or ISO 16282:2007, for cold crushing strength to ASTM C133-24 or ISO 10059-1:2025, and for thermal conductivity to ASTM C201-24 or ISO 8894. Table 4 sets them side by side; no pair is equivalent.

Between laboratories, C704 results vary by ±47 % for high-alumina brick and ±78 % for conventional castable, and the ASTM C08 authors call the method not yet reliable as a specification.

Castables are classified under ASTM C401-12(2022) and ISO 1927-1:2012, using CaO content among other measures. Sampling plans and in-service checks are covered under refractory inspection and quality control.

Contractor Match and Supplier Quotes for Boiler Linings#

Boiler relines, dry-outs and outage repairs are contracted per zone, and a request names the boiler type, zone, lining class, area or tonnage and outage window.

How Does Boiler Refractory Connect to Other Refractory Questions?#

Boiler refractory shares its classes, binders and practice with every other industry, while the boiler-specific answer always starts from the zone: water wall, CFB loop, waste-to-energy grate and first pass, or incinerator kiln.

The trade term "boiler refractory cement" covers mortars, castables and patching products. Which product the term means for a given job is explained under refractory cement.

Which Hazards Arise When Boiler Linings Are Torn Out?#

Tear-out of oxidized SiC linings can release respirable crystalline silica (cristobalite), and used fiber insulation needs the controls for its fiber type. For quartz (crystalline silica), ECHA gives: "No Annex VI entry; product-specific, see supplier SDS (ECHA C&L: no majority; not classified 27.5 %, H372 25.2 %, H373 18.8 %, H350 7.7 %)"; for cristobalite, see the supplier SDS. OSHA sets 50 µg/m³ (0.05 mg/m³), action level 25 µg/m³, 8 h TWA (29 CFR 1910.1053, effective 2016-06-23); the EU limit is 0.1 mg/m³, respirable fraction (Directive (EU) 2017/2398, transposition by 2020-01-17).

Aluminosilicate RCF is on the REACH Candidate List since 2010-01-13, with an EU limit of 0.3 f/ml, 8 h TWA (Directive (EU) 2017/2398, transposition by 2020-01-17). Three pages cover controls and waste routes.

• crystalline silica in refractories: silica phases and limits
• ceramic fiber health rules: RCF and AES controls
• spent refractory recycling: routes for torn-out linings

How Do Boiler Linings Differ From Other Industry Linings?#

Eight other industry pages line their vessels with partly the same classes, listed below.

• steel plant refractories: iron and steel
• cement kiln refractory: cement and lime kilns
• glass furnace refractories: glass furnaces
• refractories for aluminium: aluminum
• copper smelter refractory: copper and nickel
• petrochemical refractory: refinery and petrochemical
• induction furnace lining: induction furnaces
• coke oven refractories: coke ovens