Abstract
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.
Highlights
Biomass is increasingly being used to replace coal in many commercial and industrial applications such as power generation
It is observed that the additives increase the temperature of initial deformation, with an increase of ∼120 °C observed with pulverised fuel ash (PFA) and ∼220 °C with kaolin powder (KAO)
Ash fusion testing indicated that the additives consistently increased the flow temperature of the ash compositions, with kaolin having a greater effect than the PFA
Summary
Biomass is increasingly being used to replace coal in many commercial and industrial applications such as power generation. Alkali and alkaline earth metals are troublesome: they may react with silica in the ash, producing eutectics that have initial melting temperatures far below typical combustion temperatures, such as potassium silicate (600–980 °C) and potassium-sodium silicate (540–1170 °C) systems [1]. Such low temperature systems will result in molten or sticky ash particles that agglomerate, impact or condense upon heat transfer surfaces, reducing the efficiency of the boiler. Volatilised alkali metals from biomass with high Cl content can produce hydroxides and carbonates that, in turn, react with HCl in the flue gas to form alkali chlorides [2]
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