Milling of Blends
ACARP Project Number: C13063      Published: July 05
 Philip Bennett, Graham O'Brien, Don Holcombe
Extended Abstract
Evaluating the potential performance of coal blends in both the thermal and PCI market requires knowledge of the size distribution of the organic and mineral matter components of a blend, especially when there are significant differences in the HGI of the component coals. The size distribution of the organic matter impacts on combustibility of thermal and PCI coal blends and handleability of PCI coal blends.
Petrography techniques were used to examine four size fractions from the PF of single coals and blends. The development of curve fitting algorithms to the vitrinite and inertinite reflectance distributions allowed these maceral groups from individual coals to be identified in the sized PF samples. From this data the size distribution of vitrinite and inertinite was determined in single coals and blends.
For most coals, a good estimate of a blend's size distribution can be made assuming that the size distribution of the individual coals, milled under the same conditions, are added together in the proportions of the blend. The exception is when a very soft coal (HGI 90) is blended with a very hard coal (HGI 35). In this case preferential milling (more reporting to the smaller size fractions) of the softer coal occurred.
All coals studied in this project show some sign of preferential grinding of the softer maceral group when the coal was milled individually or in a blend. This preferential grinding of macerals is due to differing strengths of the macerals which dictates how the size reduction of the maceral varies with energy used in the breakage of the particle.
Breakage characteristic curves (change in size reduction per unit of energy) for vitrinite and inertinite were determined from the milling data of the coals and blends. For these curves the mill specific power was proportioned to the maceral groups based on the petrographic analysis and blend composition. These curves have similar trends as those found for the breakage of lithotypes. Generally, these trends are for the particle size to initially decrease rapidly then approach a constant size with increasing breakage energy. The results indicate that the breakage characteristic curves of maceral groups in individual coals do not change when they are blended with other coals.
It is only when the reduction in breakage energy proportioned to a maceral group of a coal in the blend moves to the steeper region of its breakage characteristic curve that the preferential milling of a coal in a blend is observed in the size distribution of the blend. This would also explain the non-linearity of Hardgrove Grindability Index (HGI) determined on some blends when compared to their component coals as HGI is a measure of size reduction for a fixed energy.
The results show that the breakage of a coal particle can have three mechanisms, these are:
  • Vitrinite and Inertinite Breakage : The breakage of both vitrinite and inertinite consumes energy in the milling process.
  • Vitrinite Dominated Breakage : The breakage energy of the coal is dominated by the breakage of the vitrinite.
  • Inertinite Dominated Breakage: The breakage energy of the coal is dominated by the breakage of inertinite.
The results also explain why some coals, those with an inertinite dominated breakage mechanism, do not follow the generally observed trend between HGI and the maximum vitrinite reflectance.
It was shown that relationships between mill specific power and HGI, Rosin Rammler parameters and vitrinite reflectance and the breakage characteristic curve of vitrinite and inertinite allows one to determine the mill performance of a coal or a blend. Currently it is not possible to estimate the breakage characteristic curve from petrographic analysis. Further milling testing, under fixed mill conditions, for wider range of coals will assist identifying methods to predict the breakage characteristic curves for the vitrinite and inertinite maceral groups.
The ability to predict the size distributions of individual maceral groups within a coal or a blend could greatly assist in the prediction of the combustion performance of blends and the handleability of blends of PF in blast furnace injection systems. In advanced low NOx burners NOx formation is very dependent on how the coal composition varies with the size of the particles. These burners concentrate the coal, mostly the larger particles, in the inner core of the primary air stream while the finer particles stay in the outer region of the primary air stream. This outer region forms a high temperature flame that envelopes the inner core driving the rapid volatile release from the coal thus ensuring minimum NOx formation when the primary air mixes with the secondary air. By selected blending a plant operator could control the composition ( nitrogen content and volatiles) of the coal that reports to the inner core and the outer region of the flame for greater NOx reduction for these burners.