Primary fragmentation of large coal particles
Badenhorst, Charlotte Johanne
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When rapidly exposed to high temperatures coal tends to fragment into several pieces due to thermal and volatile release stresses. This is known as primary fragmentation and is one of the major problems opposing the optimum performance in high temperature applications such as gasifiers. Hydrodynamic and pressure drop fluctuation problems, among other things, arise in gasifiers due to this primary fragmentation. It is therefore necessary to investigate this phenomenon in detail. In the study, “Primary fragmentation of large coal particles”, the fragmentation of large, almost spherical, coal particles (10, 15, 20, 25 and 30 mm) from five different coal origins in South Africa (low and high volatile, swelling and non-swelling and more or less the same ash yield samples) heated to 400, 600 and 900 °C respectively were tested. Five repeats of each combination were conducted for statistical accurate results. A horizontal tube furnace under a nitrogen atmosphere was used to test the samples (pre-heated so that heating rate was not controlled), while X-ray tomography was used to aid in the qualitative interpretation of results. From the tomography scans small cracks could be seen throughout the particle volume. These cracks are either natural cleats or fissures formed due to handling. Upon heating new cracks formed on these initial cracks, especially on cracks between coal-mineral interfaces. The cracks that formed were well structured and orientated, either perpendicular or parallel to the bedding planes. For the high volatile samples, a major crack parallel to the bedding plane was visible which, if enough stress was applied, fragmented it into two coarse fragments. This behaviour is associated with fragmentation due to volatile release stresses. For the low volatile samples, fragmentation due to thermal stresses was more prominent with the particles fragmenting into a multitude of small pieces under certain conditions. The amount of volatiles present relative to the pore structure of a particle influenced fragmentation due to volatile release stresses. The ratio between vitrinite and fusinite content in a particle had an influence on fragmentation due to thermal stresses. Since the ash yields for the different samples were very low and clustered around 10% the influence of mineral matter could not be determined. Fragmentation was quantified using a breakage index defined as the ratio between Sauter diameter after and before fragmentation. If the breakage index equalled one, zero fragmentation occurred, while if the breakage index was smaller or larger than one fragmentation and swelling respectively was present. The breakage index decreased with an increase in particle size. Larger particle sizes will thus fragment more than smaller sizes. Temperature also influenced the breakage index and it was concluded that, for the non-swelling samples, a temperature increase led to a decrease in breakage index. For the swelling samples, the breakage index, after being heated to 600 °C, was larger than for 400 °C, which was in turn larger than for the 900 °C situation. After being heated to 600 °C, swelling was able to reduce volatile release stresses and thus fragmentation, while for 900 °C volatile release and thermal stresses were too severe to be relieved by swelling. For particles heated to 400 °C swelling had not yet commenced and breakage was higher. The relationship proposed by Dakič et al. (1989:916) pertains to South African coal samples. From this relationship, it is predicted that the critical diameter (largest diameter for which there is no noticeable fragmentation) of a coal sample will decrease with an increase in pore resistance number. At certain temperatures, however, the low volatile samples did not follow this relationship since they are not affected as much as the high volatile samples by the pore resistance number. Take note also that, due to the lack of variability in South African coalfields only pore resistance numbers ranging between 0 and 10 were tested. The critical diameters based on a 75% probability (at least 75% of all repeated runs fragmented) for this study for particles heated to 900 °C ranged between 10 and 20 mm, while those heated to 600 °C ranged between 15 and 25 mm. The critical diameters for particles heated to 400 °C ranged between 15 and 30 mm. The critical diameters for 900 °C were successfully compared to literature which lay between 2 and 25 mm. Overall, the results correspond well to those from literature. For future studies, it was suggested that the influence of vitrinite and fusinite on the fragmentation behaviour should be investigated more thoroughly.
- Engineering