Ash and slag: the enemies of the refractory lining in biomass plants

Why biomass ash is so aggressive

Biomass ash differs fundamentally from coal ash. Whereas coal ashes consist predominantly of relatively inert silicon and aluminium oxides, biomass ashes contain high concentrations of alkalis, especially potassium and sodium, as well as chlorine, sulphur and phosphorus. At combustion temperatures these elements form low-melting compounds that attack the refractory lining as a liquid phase. The chemical attack takes place through infiltration of the melt phase into the pore structure of the refractory material, where it reacts with the matrix constituents and destroys the structure.

The attack mechanisms in detail

• Infiltration: liquid slag penetrates the open porosity of the refractory material through capillary forces. The depth of penetration depends on the viscosity of the slag and the pore size distribution of the material
• Corrosion: the infiltrated slag reacts with the matrix constituents of the refractory material and forms new mineral phases with altered properties. Frequently this produces compounds with a greater volume, which generate stresses in the structure
• Erosion: ash particles carried along in the flue gas stream act like blasting grit and mechanically remove the surface. Areas with high flow velocities and changes of direction are particularly affected
• Slagging: adhering layers of slag alter the geometry of the combustion chamber, impair heat transfer and, if they detach in an uncontrolled way, can damage the lining beneath

Protective measures against ash and slag

The most effective protection begins with the choice of fuel and the way combustion is managed. Fuel quality that is as constant as possible, with a known and controlled ash content, reduces the peak loads on the lining. The combustion temperature should be controlled so that it lies below the ash softening point wherever possible, although in practice this cannot always be achieved. On the material side, dense refractory materials with low porosity offer the best protection against infiltration. Dense, alkali-resistant fireclay and andalusite grades, as well as materials with a high silicon carbide content, have proven effective against alkali corrosion; high-alumina bricks, by contrast, tend to form volume-increasing phases under alkali attack that damage the microstructure. In particularly critical areas, special washes or ceramic coatings can form an additional barrier against slag attack.

Operational measures and regular maintenance

Besides the right choice of material, operational measures are indispensable. Removing slag deposits regularly, before they develop into massive build-ups, extends service life considerably. Thermographic inspections allow the early detection of hotspots that point to advanced infiltration. And systematic documentation of the wear rates in different zones provides the data basis for continuously optimising the materials concept. At SBS Refractory Service we combine the experience of hundreds of combustion chamber projects with a deep understanding of the underlying materials technology. In this way we develop an individual strategy for each plant against its specific ash loads.

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