Experimental
and theoretical analyses with empirical correlations were framed for
diffusion of gas species CH4 and CO2 in coal
samples from Jharia coal fields, India, considering the intrinsic
pore parameters. Coefficient of diffusion (D) and
diffusivity (D
eff) for a single and binary
component coal–gas system were estimated by adopting unipore
gas kinetic models for gas flow on the integration of Fick’s
law and Langmuir relation. The rigorous study was carried out in estimating
crossover pressure, which is dominant in distinguishing the flow regime
for two primary types of diffusion: Knudsen and molecular as well
as the transition between two regimes. Investigation reveals that
experimental values of coefficients of diffusion of CH4 and CO2 in random homogeneous isotropic sphere packing
of coal samples are in good agreement with the results of theoretical
calculations. For the pressure range investigated, variation of coefficient
of diffusion was found to follow a dual nature with a stable trend
at pressures above 3500 kPa and an increasing trend for lower pressures.
The practical implication of the investigation for the pressures that
are characteristically encountered in the Jharia coalfields is a positive
finding for the concomitant recovery of coalbed methane with CO2 sequestration. Additionally, the dynamic relation between
sorption–diffusion reveals that the coefficient of diffusion
significantly depends on the pore structure and pore size distribution,
exhibiting a negative relationship with pressure variation.
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