A crucial part of current and future energy strategy involves the replacement of coal with biomass. However, the composition of biomass creates operational issues in large scale combustion, potentially creating severe slagging and fouling deposition and in worst case scenarios boiler shutdown. Additives have shown promise in improving the deposition of biomass ashes, by altering the composition of the ash and subsequently improving the melting behaviour and deposit properties. However, the effect of additives upon the performance of large scale combustion boilers is not fully understood. This research focuses on the impact of two promising aluminosilicate-based (Al-Si) additives, coal pulverised fuel ash (PFA) and Kaolin powder (KAO), upon the ash properties of different biomass types (olive cake, white wood, bagasse and a power station fly ash).
The first area of research was to determine the effect of Al-Si additives upon the electrical resistivity of the ashes across a range of temperatures, which can have a significant impact upon electrostatic precipitator (ESP) performance. A bespoke resistivity test was designed and built for this purpose, based on existing standards. Results showed that biomass ash resistivity is typically lower than that of coal ash by an order of magnitude or more. In some cases, the resistivity may be low enough to cause operational problems and increased particle emissions. The use of additives resulted in increased resistivities, thereby reducing the risk of lower ESP collection efficiencies. Although ESP loads would be increased, this would not be expected to negatively impact emissions. Analysis of the ash compositions indicated that, contrary to previous experience with coal ashes, potassium concentration is an important factor in biomass ash resistivity, meaning that current predictive models are inadequate for biomass and biomass-additive compositions. Therefore, an existing model has been modified using the experimental data and taking into account potassium concentration; this produced reasonable predictions, and showed promise in predicting the resistivity of both biomass and coal ashes.
The second area of research was focused upon ash melting behaviour. High temperature viscosity was used to determine ash flow behaviour at temperatures encountered in and around the combustion zone of large scale boilers. Results showed that KAO use with high potassium, high chlorine olive cake (OCA) would significantly improve the flow properties of the ash at combustion temperatures, resulting in ideal viscosities and significantly improved slagging deposition. Thermodynamic modelling data indicated that this was due to the decreased concentrations of magnesium and phosphorus and increased alumina concentrations within the ash, resulting in the formation of high melting temperature minerals and compounds. Ash fusion testing further indicated that KAO can significantly increase flow temperatures of biomass: however, PFA was observed to be less effective. In the case of high silica biomass, PFA was found to have a significant adverse effect upon flow temperature, which would lead to significantly worse slagging.
Sinter strength testing was investigated across a temperature range of 800-950°C. Both additives were found to improve the properties of OCA by binding potassium as silicates and aluminosilicates. This eliminates severe sintering caused by KCl sublimation and fluxing at 850°C by binding potassium as silicates and aluminosilicates. However, for the other biomass sinter strengths were increased with additive use. Although most results were below the strengths required for soot blower removal, high additive concentrations produced problematic sinter strengths. It was determined that kaolin has the greatest potential as an additive to reduce deposition issues from biomass combustion, due to its high kaolinite content. Coal PFA was determined to be less effective due to its high mullite and iron concentration. Finally, results indicated that Al-Si additive use is unsuitable for biomass containing high levels of SiO2, and should be used only on biomass with either low SiO2 or high KCl concentrations. However, lower additive rates need to be investigated in future.
