Three-phase slurries provide a viable and immediate replacement for residual oil that is being used by existing boilers which are not capable of firing coal directly. This alternative fuel can be used economically, and has a great potential in terms of user convenience and environmental acceptability. Coal slurries have the same burning, heating, handling, and storage characteristics as fuel oil. Coal slurry technology often faces with the problem of instability and unfavorable rheological properties, such as high viscosity problems during processing and pumping. In order to overcome instability water as well as chemical additives have been introduced to coal slurries.The industrial application of coal slurries requires full knowledge of their flow behavior. Viscosity is the main parameter which determines the flowing, pumping, burning, and storing requirements. The rheological properties are influenced by many factors such as coal concentration, coal particle size, temperature, nature of the medium phase, chemical additives, and moisture contents. Experimental data on three-phase slurries in the literature are very scattered. Most of the published data are on coal/oil or water/oil slurries. Generally, coal slurries are non-Newtonian and mostly exhibit pseudoplastic behavior.This paper will present the results of an experimental investigation undertaken to study the rheological behavior of the practical coal/oil/water fuel slurries using a Weissenberg rheogoniometer with a cone and plate geometry. The effects of particle size, temperature, and polymer addition were also investigated. A synthetic polyacrylamide-type polymer named Pusher 700 was used in this study. Slurries were prepared using coal particles ranging from -200 mesh to -325 mesh, fuel oil No. 6, and distilled water. They consisted of 50% (by weight) coal, 40% (by weight) fuel oil, and 10% (by weight) water. The steady as well as the oscillatory shear were imposed to clearly define the non-Newtonian nature of the slurries.All the slurry systems studied were pseudoplastic and showed shear thinning behavior. Slurries made of finer coal particles were more viscous than those made of coarser grains. Temperature was found to decrease the apparent viscosity of the coal slurries studied. As expected addition of water increased the viscosity of the slurries. Addition of polymer resulted in further increase in the viscosity of the slurries. However, the slurries with polymer were less affected by coal particle size, but were more sensitive to temperature. Comparison of the oscillatory shear data with the steady state shear data revealed that the addition of polymer affected the dynamic properties of the slurries more than the steady shear properties. Furthermore, the effect of particle size was more pronounced on the dynamic viscosities of the slurries than on the shear viscosities. This was observed for slurries with polymer as well as without polymer.
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