(74v) Role of Surface Properties in Predicting Viscosity of Concentrated Coal-Water Slurries

Abstract:

Coal-water slurries are highly concentrated suspensions of coal particles in water and are used as gasifier feed stock for advanced power generation methods using integrated gasification combined cycle. Recently, carbonaceous solids like petcoke and bitumen which are byproducts of the oil industry are also made into slurries and are used for gasification. These slurries have high solids loading in the range of 60-75%. For easier handling and pumping of these highly loaded mixtures, lower slurry viscosity is desirable. Predicting viscosities of such slurries can be very useful for process design and control. Published research on particle suspensions has mainly focused on determining empirical relationships between apparent viscosity and particle volume fraction. These models do not take into account surface characteristics of coal or petcoke and are therefore, inadequate for accurate viscosity predictions of such complex mixtures.

In this study it was observed that the hydrophobic and hydrophilic petcoke/coal when in suspension with water behaves entirely differently. The hydrophobic solids tend to aggregate in the presence of water. Particle aggregation effectively increases particle volume fraction because of entrapment of water in the aggregates and loss of water from the bulk volume of the suspension. Hydrophilic low rank coals, on the other hand, absorb and adsorb water which again results in the loss of water from the bulk volume and thus leads to an increase in the solid volume fraction. This study uses experimentally measured viscosity values and Krieger-Dougherty equation to determine a factor by which solid volume fractions of five different rank coals and petcoke when made into slurries, increase. This factor termed as the “volume fraction correction factor” increases with increasing ratio of oxygen to carbon of the carbonaceous solid. Volume Fraction Correction Factor (VFCF) is 1.28 for highly hydrophobic petcoke, 1.52 for bituminous Illinois#6 coal and 1.82 for Pust lignite coal. A correlation between VFCF and the Oxygen to Carbon ratio was obtained. It was established through this study that viscosities of medium to highly concentrated carbonaceous solids-water slurries can be predicted more accurately by using the appropriate VFCF in the Krieger-Dougherty equation than just Krieger-Dougherty equation. Maximum solid loading of any carbonaceous solid slurry with a known particle-size distribution can also be predicted using this method.