The impact of aluminosilicate-based additives upon the sintering and melting behaviour of biomass ash

The composition of ash arising from biomass combustion can cause significant slagging and fouling issues in pulverised-fuel boilers, particularly if high concentrations of alkalis are present. Al–Si additives have shown promise in improving the ash deposition characteristics of troublesome biomass, converting volatile potassium to potassium aluminosilicates. This article presents results of lab-scale testing for two high-potassium biomass ashes, olive-cake (OCA) and white-wood (WWA), combined with two promising additives, coal pulverised fuel ash (PFA) and kaolin powder, at 5% mass fraction. Ash fusion testing results show that the use of these additives consistently increases flow temperatures. For WWA, kaolin was observed to reduce deformation temperatures and increase flow temperatures to far above combustion temperatures. Sinter strength testing showed that additive use significantly improves the deposition properties of OCA, preventing the precipitation of KCl and formation of deposits that are highly undesirable for removal via sootblower. Sintering was eliminated at all temperatures measured with the use of kaolin. Both additives had negative effects upon the sintering of WWA, indicating that Al–Si additive use should be restricted to high K, high Cl biomass. High temperature viscometry of OCA, combined with thermodynamic modelling, showed that viscosities at combustion temperatures were far below ideal values due high Mg concentration and silicate formation. Kaolin at 5% mass fraction was predicted to significantly improve this behaviour, with aluminosilicate formation producing favourable viscosities. Results indicate that kaolin addition to high K, high Cl biomass such as OCA shows promise in making the ash compositions viable for pulverised-fuel combustion.