| Summary: | This project is aimed towards an understanding of ash deposition during air firing of high percentages of biomass with coal. Biomass resources are widely used as sustainable, renewable and environmentally friendly materials. There has been an increase in the use of biomass for power generation by means of co-firing with coal as well as by the combustion of 100% biomass. Despite the advantages of biomass in reducing carbon emissions from the electricity sector, the co-firing of high percentages of biomass can potentially aggravate ash related problems in the boiler. In order to develop mitigation strategies for the formation of deposits, an understanding of the ash behaviour during the combustion of high percentages of biomass is required. To understand ash deposition, the influence of the inorganics, crystal types, and complex compound formation should not be neglected. In this work, ash samples from El Cerrejon coal and pine, wheat straw, white wood pellet biomass were characterised for their inorganic composition by X-ray fluorescence (XRF) and wet chemical methods. Relationships between these two methods were found and a modification to the standard test method has been recommended to improve the accuracy of the XRF method. Furthermore, the melting behaviour of ashes from pure El Cerrejon coal, biomasses, and their blends were studied through ash fusion tests and via a method using a simultaneous thermal analyser coupled to mass spectrometer (STA-MS) for the evolved gas analysis. The inorganic composition were used to calculate indices to determine the slagging and fouling potential of pure fuel ash, ash blends and ash produced by ashing blended fuels (fuel blends ash). Base-to-acid ratio (Rb/a) results indicate that pine ash has a higher slagging potential than coal ash, which is not consistent with the experimental ash fusion measurements. Viscosity models appear to perform better for high-coal content blends than high-biomass content fuel, and further refinement is required for modelling the viscosity of pure biomass ash as well as high co-firing percentages. Thermodynamic modelling of slag formation was undertaken using the FactSage model and verified by XRD analysis for the solid phase. XRD showed complex interactions between inorganics which changed with biomass type, blend ratio and temperature. The FactSage model was successful in predicting the changes of gas, solid and liquid phases during pure biomass, coal and co-combustion, and for most of the blends studied the prediction of slag formation was within 100°C of the measured experimental ash melting window.
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