Moisture content and corresponding liquid induced adhesive forces strongly affect flowability of powders during their storage and handling. During coal handling in thermal power stations, coal particles tend to contain considerable amount of moisture from processes involved in mining and transportation. Resulting wet coal powders when stored in the silos are known to offer series of flow problems while flowing from bunkers. Therefore for trouble free operation of these bunkers it is very important to quantify effects of moisture content and resulting capillary forces on flowability of coal along with time consolidation effects for deciding suitable bunker configuration. In the present paper effects of moisture content on the instantaneous unconfined yield strengths of coal samples used in thermal power station bunkers are discussed.

Flow properties of coal samples like major consolidation stress (σ₁), unconfined yield strength (σ_c), effective angle of internal friction (δ) at various consolidation levels are evaluated using Jenike type Shear Cell in the moisture content range of 4-28%. Resulting ‘Flow functions' of the moist coal samples are observed to be having ‘critical moisture content' representing maxima in the relation between unconfined yield strength and moisture content. For given coal samples the critical moisture content is observed to be around 23%. Further, effect of median particle diameter (d₅₀) of moist coal samples on critical moisture content is examined for various coal samples in the range of 25-200 μm. Finally, an attempt is also made to estimate magnitudes of liquid bridge induced adhesive capillary forces between particles (F_c) and resulting tensile stresses (σ_z) at various moisture contents for prediction of unconfined yield strength (σ_c) using a mathematical model. Magnitudes of adhesive capillary forces between two particles at varying moisture levels are estimated in terms of half filling angle (β) by approximating the geometry of the liquid bridge to be toroidal.