Since the oil crisis in 1970's, utilization of coals for alternative fuels has drawn the attention of industry and researchers on energy related matters. Although coal has several advantages over oil, its solid form makes the handling, transportation and storage more difficult and expensive. Besides it causes considerable environmental problems. To minimize these problems coal slurry fuels have been investigated for the replacement of oil. CWM has been considered the most economic coal slurry fuel and its possible applications include gas turbines, diesel engines, fluidized bed combustors, and gasification systems.
CWM fuel is usually composed of 60 to 70wt% of coal particles, 30 to 40wt% of water, and chemical additives less than 1wt% that are used for improving sedimentation stability and lowering viscosity of the fuel. In order to prevent particles from sedimentation, to reduce pressure loss in pipeline transportation and to finely atomize CWM droplets in atomizer it has been reported that it is desirable for CWM to have pseudoplastic(shear-thinning) behavior. Rheological and hydrodynamic behavior of CWM is influenced by various parameters such as coal properties, coal loading, type and amount of additives, temperature, and coal particle size and its distribution.
In this study, the effects of additives(surfactant, stabilizer, and electrolyte) on the rheological properties of CWM were investigated to determine the optimum composition. In most case anionic and nonionic surfactants are used to reduce the viscosity of CWM. If an anionic surfactant is added to CWM, the coal particle surface becomes negatively charged by the adsorption of hydrophilic site of surfactant and electric double layer is formed on the surface. Repulsive force by the electric double layer helps prevent the aggregation of particles and reduce the slurry viscosity. The steric effect of polymeric stabilizer also influences the stability and the fluidity of CWM. Electrolyte also affects the rheological properties of CWM by changing the ionic strength of suspending medium.
In suspension rheology a very large number of experimental and theoretical papers have been published to determine the relative viscosity as a function of solids volume fraction. But it is very difficult to apply these results to CWM because it is a very turbid slurry composed of polydisperse coal particles which absorb a lot of water. In this study, a correlation of weight fraction to volume fraction was made by defining new parameters and applying them to two semi-empirical expressions(Krieger-Dougherty and Frankel-Acrivos equation).
By fitting experimental data, parameters of these equations could be determined and Krieger-Dougherty and Frankel-Acrivos equations were found to be appropriate for representing the solid weight fraction viscosity relationship for concentrated CWM fuel.
