Relevance of structure, fragmentation and reactivity of coal to combustion and oxy-combustion

The nature and extent of the microstructural changes of coal upon oxidation under conditions typical of combustion systems, and the relation between microstructure and reactivity towards oxygen and carbon dioxide have been extensively addressed. The first major changes in morphology, structure and chemical composition of the solid fuel occur in the early instances of combustion/gasification processes, during heat up and devolatilization. At this stage particles can be broken into smaller fragments as a consequence of thermal stress and pressure generated by volatiles release. This phenomenon, regarded to as primary fragmentation, is considered responsible of important changes in the fuel particle size distribution within reactors. At longer time scales the concurrent heating and heterogeneous reactions may further affect the structure and the reactivity of chars: on one side the prolonged heat exposure induces thermal annealing with progressive graphitization and loss of reactivity. On the other side, additional phenomena, referred to as secondary fragmentation and attrition by abrasion can occur in parallel with char burning. Eventually, for large extents of internal burning, the increase of particle voidage can induce percolative fragmentation. The distinction between the individual fragmentation processes, just as that between pyrolysis and thermal annealing, is more operational than conceptual and depends on the timescale of the processes and on the nature of the fuel. On the whole, the course of combustion/gasification of a solid fuel is ruled by the competition between heterogeneous reactions of the solid fuel and a number of complex phenomena that modify char structure and reactivity. Limited information is available in the literature on the relationship among structure, fragmentation, and reactivity of coal in oxy-combustion conditions. The transferability of the lessons learnt under conventional combustion and gasification conditions to the case of oxy-combustion is here discussed to stress the need of additional investigation on this topic.