Mixing narrow coarse and fine coal fractions – The maximum volume fraction of suspensions

•Maximum packing density is optimum at 70 wt% coarse for size ratio 5–10.•The trend supports the theoretical prediction for ideal bimodal system.•A yield stress method for determining maximum packing density is presented.

A main objective of coal-water slurry fuel (CWSF) preparation is to achieve maximum loading of coal. In the absence of a strong colloidal attractive force, the maximum loading is determined by the packing density of the particles which in turn is a function of the particle size distribution. In this study, coal fractions of different mean sizes with a narrow distribution were separated by sieving. Mixtures of different mean coarse to fine size ratios were then prepared. For each size ratio, different amounts of coarse particle contents were prepared. With these different mixtures, water was added to produce the CWSF. The maximum volume packing density, φmax, for each mixture was determined using a rheological vane yield stress technique. The determination of φmax involving the direct yield stress measurements of extremely high concentration suspensions is an entirely new and accurate approach. It was found that the highest φmax was obtained when the coarse to fine ratio was ∼10 located at a coarse coal content of 70 wt%. This result is consistent with that obtained by theoretical modelling of bimodal mixing of monodisperse size spheres with a size ratio of 10. At lower size ratio, φmax obtained at the optimum coarse coal content was lower.

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