Coal slurries rheology

Besides coal properties, a number of other variables also have significant effects on the behavior of CWS, such as coal PSD, the preparation process of the slurry, and the type and volume of chemical additives. Accordingly, CWS coal concentration can be increased by means of seeking effective additives and optimizing PSD and preparation processes. Unfortunately, knowledge in this area is mainly empirical. The inherent relationship between slurry rheology and the preparation process is also unknown. 

The properties of coal slurries that are of most interest are 1) the viscosity/rheology, which controls the ease of pumping and atomization 2) the stability against settling, which controls the ability of the slurry... 

Fig. 1 Classes of rheological behavior that can be shown by coal slurries, as they appear when plotted on a shear rate/ shear stress graph. It is desirable for coal slurries to be Bingham plastic or pseudoplastic with yield, as such slurries flow readily at high shear rates (such as during pumping or atomization), while remaining stable against settling at low shear rates because of their yield stress. Dilatant slurries are completely unsuitable for coal slurry applications because they are extremely difficult to pump.

The size distribution of particles will control the amount of liquid needed to fluidize a given quantity of coal. In general, a fine size distribution will produce a more viscous slurry than a coarse size distribution at the same wt% solids, and the fine particles will produce a more non-Newtonian rheological curve. This can be seen in the laboratory results shown in Fig. 3, which compares a coarse coal slurry to a fine coal slurry. It is clearly seen that the fine slurry is much more viscous, its pseudoplastic character is very pronounced, and its yield value is high, while the coarse coal slurry is clearly a Bingham plastic. ... 

Fig. 3 Comparison of the rheological curves for a fine coal slurry (80% passing 34 gm, top size lOOgm, 52wt% solids) and for a coarse coal slurry (58wt% solids). Neither slurry used any additives. Because it is extremely difficult to measure the rheology of unstable slurries with conventional rheometers, these results were obtained using a continuous-pressure-vessel rheometer, which was specially designed for this purpose. The fine coal curve is the average of 10 measurements and the coarse coal curve is the average of 5 measurements, and the standard error of the shear rate measurements was approximately 1.0 Pa for these slurries. The fine coal slurry is clearly pseudoplastic with a yield value of approximately 18 Pa, while the coarse coal slurry is Bingham plastic with an estimated yield value of 4 Pa.

Usui, H. Tasukawa, T. Saeki, T. Katagiri, K. Rheology of low rank coal slurries prepared by an upgrading process. Coal Prep. 1997, 18, 119-128. 

PROBSTEIN, R.F. SENGUN, M.Z. 1987. Dense slurry rheology with application to coal slurries. PhysicoChem. Hydrodynamics 9, 299-313. 

Sengun. M.Z. Probstein, R.F. Bimodal model of slurry viscosity with plication to coal-slurries. 1. Theory and experiment, Rheologica Acta. 28 382-393, 1989. I.equeux, F. F.mnlsion rheology. Curr. Opin. Colloid Interface Sci., 3 408-411. 1998. 

The rheological behaviour of a coal slurry (1160kg/m ) can be approximated by the Bingham plastic model with Tq = 0.5 Pa and /ng = 14mPa-s. It is to be pumped through a 400 mm diameter pipe at the rate of 188kg/s. Ascertain the nature of the flow by calculating the maximum permissible velocity for laminar flow conditions. 

Pulido, J.E. Rojas, C.P. Acero, G. Duran, M. Orozco, M. Rheology of Columbian coal-water slurry fuels effect of particle-size distribution. Coal Sci. Technol. 1995, 24, 1585-1588. 

Nguyen, Q.D. Logos, C. Semmler, T. Rheological properties of South Australian coal-water slurries. Coal Prep. 1997, 18, 185-199. 

Mishra, S.K. Senapati, P.K. Panda, D. Rheological behavior of coal water slurry. Energy Sour. 2002, 24, 159-167. 

Stabilizers, dispersants, and particle size distributions can have tremendous impacts on the elongational rheology of coal-water fuels while minimally impacting the shear rheology. In particular, stabilizer structure and aspect ratios of the particles present in the slurry may exert profound influences on the atomization of... 

Preisad, M., B.K. Mall, A. Mukherjee, S.K. Basu, S.K. Verma, and K.S. Narasimhan. 1998. Rheology of petroleum coke-water slurry. Proc. 23 International Technical Conference on Coal Utilization and Fuel Systems. Coal Technology Association. Clearwater, FL. March 9-13. pp. 1109-1116. 

The flow behaviour of concentrated fine-grained slurries (clay, coal, ash, tailings, ores) in laminar regime can be approximated by rheological models. For the turbulent regime a turbulent model must be employed. Two models based on rheological parameters (Slatter, Wilson) were analysed. 

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