Mineral transformation, ash formation and characterisation during pulverised fuel combustion

• Main Author: Silvester, Laurence Paul Joseph
• Format: Dissertation
• Language: English
• Published: University of Nottingham 2020
• Subjects: TP Chemical technology

Due to the growing concerns about climate change, the power generation sector is increasingly facing pressure to reduce greenhouse gas emissions whilst maintaining energy security and supply. Co-firing biomass with coal is considered an intermediate term solution that can reduce CO2 emissions from fossil fuel utilisation and maintain use of existing power plant infrastructure. For power generators, ash deposition is still a major cause of reduced boiler efficiency and unscheduled shutdowns on heat transfer surfaces, and historically, less attention has focused on deposition due to biomass co-firing. In this research, a series of experiments were conducted on a suite of internationally traded coals, biomasses and industrial boiler fly ashes to characterise the particles formed during devolatilisation and char combustion at various residence times using a drop tube furnace (DTF) and mineral liberation analyser (MLA). The mineral transformations, associations and particle parameters determined aid the development of ash formation and erosion models. A unique mineral database using commercial coals, industrial fly ashes and biomasses was developed. This MLA database was used to assess particle parameters which influence erosion potential and determine the ash-deposition related mixed aluminosilicate species which contribute to ash deposit formation. Thermogravimetric analysis of DTF coal chars doped with biomass ashes was performed to investigate the effect of biomass-derived mineral species on the ignition temperature at varying co-firing ratios. DTF studies suggested that inherent minerals present in the biomass ash enhance the formation of sintered/deposit particles compared to firing only the DTF coal char, and that the deposit particle size increases with residence time in the DTF. Mineral composition data and the development of ternary phase diagrams showed that the DTF coal char has the added benefit of capturing metal fluxing elements such as K and Ca originally present in the biomass ash. MLA can serve as a useful tool to reveal the characteristics or mechanisms of mineral transformations, ash formation and deposition during biomass co-firing.