It was observed that when the density of the medium reached the coal density, the amount of the floated material was decreased significantly.

In addition, it was concluded that when the specific gravity of dense medium reached the coal density (1.50 g/[cm.sup.3]), the amount of floated material was decreased significantly.

Properties of the surrounding rock Unit Value Roof/floor density kg/[m.sup.3] 2500

Coal density kg/[m.sup.3] 1300 Roof/floor modulus GPa 10 Coal modulus GPa 4 Roof/floor Poisson ratio 0.2 Coal Poisson ratio 0.3 Coal cohesion MPa 1 Coal friction angle [degrees] 25 Coal tension MPa 0.5 Properties of the interface unit Value Normal stiffness GPa 673 Shear stiffness GPa 673 Cohesion MPa 0.1 Friction angle [degrees] 10 Tension MPa 0

The coal density increases with adsorption pressure increasing.

The main reason for the decrease of coal density is the expansion deformation of coal internal.

Table 1--Parameters of Gas Reservoir and Fluid Parameters Value Coal rock elastic modulus, E(GPa) 2.735 Poisson's ratio of coal, [upsilon] 0.3756

Coal density, [Rho](kg/ [m.sup.3]) 1500 Seam depth, h (m) 850.4 Seam thickness, H(m) 9.2 Coal and rock porosity, [phi] 2.0% Coal permeability, K ([10.sup.-3] [micro][m.sup.2]) 6.2 Seam initial pressure, P(MPa) 12.56 Strata water viscosity, [[mu].sub.W] (Pa.

Parameter Value Coal elastic modulus 2813 E (MPa) Matrix elastic modulus 8439 [E.sub.s] (MPa) Poisson's ratio v 0.34

Coal density [[rho].sub.c] 1250 (kg/[m.sup.3]) Air density (kg/[m.sup.3]) 0.717 Facture stiffness 4800 [K.sub.f] (MPa) Dynamic viscosity 1.84 x [10.sup.-5] u (Pa x s) Bulk modulus K (MPa) 5400 Matrix width [a.sub.0] (m) 5 x [10.sup.-6] Frack width [b.sub.0] (m) 0.005 Parameter Parameter Coal elastic modulus Adsorption pressure E (MPa) constant [P.sub.L] (MPa) Matrix elastic modulus Adsorption volume constant [E.sub.s] (MPa) [V.sub.L] ([m.sup.3]/kg) Poisson's ratio v Adsorption pressure strain [[epsilon].sub.L]

Coal density [[rho].sub.c] Initial matrix porosity.

where [[phi].sub.f] is the porosity, [[rho].sub.ga] is the gas density, [[rho].sub.c] is the

coal density, and [V.sub.sg] is the absorbed content.