Coal slurries applications

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.

Unlike other coal slurry applications where the slurry must be kept fluid and pumpable, coal slurry in impoundments needs to be stabilized to prevent flow. Treatments of coal waste slurry, therefore, concentrate on promoting complete settling, strong interparticle interactions and high viscosities so that impoundment breaks will not release the material as a damaging flow. 

The sulfonated lignin products function primarily as dispersants in aqueous systems and help to form stable dispersions of a number of insoluble materials. For example, lignin dispersants find use in pigments, carbon black, gypsum, ceramics, coal slurry and water treatment systems to mention some of the more prominent applications. 

Direct-Liquefaction Kinetics All direct-liquefaction processes consist of three basic steps (1) coal slurrying in a vehicle solvent, (2) coal dissolution under high pressure and temperature, and (3) transfer of hydrogen to the dissolved coal. However, the specific reaction pathways and associated kinetics are not known in detail. Overall reaction schemes and semiempirical relationships have been generated by the individual process developers, but applications are process specific and limited to the range of the specific data bases. More extensive research into liquefaction kinetics has been conducted on the laboratory scale, and these results are discussed below. 

The term coal slurry, therefore, includes all mixtures of coal and water that can flow as a fluid. There are four main types of high-percent-solid coal slurries discussed in the literature, with different characteristics depending on the application ... 

Of the four applications for coal slurries given here, coal slurry fuels have the most stringent requirements. Hence, the following discussion of slurry properties is with reference to coal slurry fuels. 

Figure 9.4.1 Relative viscosity of a bidisperse coal slurry made up of a colloidal fine fraction of mean diameter 2.3 /j,m and a noncolloidal coarse fraction of 200—300 m particles of mean diameter about 250 fim as a function of shear rate. The volume fraction of the colloidal particles = 0.30 and of the coarse particles , = 0.52. The solid line is a mean curve through the measured viscosities of the colloidal fraction. The triangles are the experimental points for the measured viscosity for the fine plus coarse mixture. The dashed line is the fine relative viscosity experimental curve redrawn through the data points to illustrate the parallelism. The upward shift of this curve corresponds to a coarse relative viscosity log 77, = 2.13. [After Sengun, M.Z. Probstein, R.F. 1989. Bimodal model of slurry viscosity with application to coal-slurries. Part 2. High shear limit behavior. Rheol. Acta 28, 394-401. Steinkopff Darmstadt. With permission.]...

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

These humic acids are not dissolved because the pH of this slurry is in the range of 4 to 9. Small amounts of fulvic acids are formed, and these are soluble in the water of the slurry. The coal-derived humic acids find applications as drilling fluid dispersants and viscosity control agents, whereas the coal-derived fulvic acids may be used to produce plasticizers and petrochemicals. 

At the beginning of the 1990s, Houston Industries developed an enzymatic process ( Enzymatic Coal Desulfurization ) protected in Canada and US [83,84], Although, the application was focused to coal desulfurization it may also be applicable to crude oil and fossil fuel-derived liquids. The processes claim the removal of both, organic as well as inorganic sulfur species. The process was described as using ground coal (10-50 p,m) slurried with water, while the oil was treated in an aqueous emulsion. 

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