Pore structure has a significant effect on transport phenomena inside porous media. This effect can be considered in simulations by using suitable transport coefficients. Some correlations are reported in the literature, which can be applied for simple pore geometries. In the present study, a pore-scale simulation approach is presented to determine mass diffusion coefficient considering molecular and Knudsen diffusion in a fibrous porous medium as a complex porous geometry. The methodology is employed for species in catalytic combustion of Methane inside a fibrous porous structure. The effect of Solid Volume Fraction (SVF), fibers orientation, and diameter are discussed in different temperatures. It is found that SVF plays the dominant role in mass diffusion, specifically above 600K. Mass diffusion coefficients obtained in the present study and flow permeability, conduction and radiation heat transfer coefficients from the previous study are used to simulate methane combustion inside the fibrous structure on the macro scale. An experimental setup is developed for validation. The results indicated that the simulation could well predict the temperature distribution, also 6.4% error in estimating the Methane conversion rate was observed. Due to the low Peclet number, the concentration of CH₄ and O₂ decreased unexpectedly before entering the catalytic zone.