| Summary: | Biomass combustion has increasingly been used in pulverised fuel coal fired power stations as a way of addressing a wide range of emissions reduction targets. The reuse of existing equipment such as coal mills is essential to minimise the costs of conversion. However the fundamental fracture mechanics involved in biomass comminution are completely different to coal. Thus a thorough knowledge of the comminution properties of all biomass types in coal and biomass mills is necessary in order to minimise operational issues and to optimise milling and combustion. This thesis provides extensive novel characterisation on densified biomass before and after milling. The study analysed 9 densified biomasses, 2 non-densified biomasses, and a sample coal in five different mills; planetary ball mill, Hardgrove Grindability Index testing mill, Bond Work Index ball mill, cutting mill, and a ring-roller mill. Milling was found to have little impact on particle shape, even when an order of magnitude change in particle size was observed. Particle shape is inherent to the particles which comprise a pellet, and is determined by the pre-densified comminution processes. Milling had little impact on compositional particles of herbaceous or wood pellets. Olive cake had the most spherical of all the materials. Thermal pre-treatments of woody biomass not only saw a significant improvement in grindability in all mills, but also enhanced shape factors. The Hardgrove Grindability Index is a poor indicator of the grindability of biomass. The Bond Work Index can be used to analyse the choking potential of biomass pellets prior to full scale mill trials. To optimise milling in coal mills, biomass pellets should be composed of particles close to the required size so that only the pellet comminution stage occurs. The milling behaviour of densified biomass in a laboratory scale ring-roller mill with dynamic classification was investigated for the first time. The milling studies showed that knowledge of a materials critical particle size for comminution through compression is essential to understand its milling behaviour in different mills. The results presented in this thesis not only provide new insight and addresses significant gaps in knowledge, they also provide useful and practical guidance for addressing operational issues such as mill choking, as well as ways to optimise biomass comminution in laboratory and full scale mills, such as mill classifier optimisation based on real particle characteristics.
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