How does FLUENT calculate turbulent mass diffusivity (m2/s) for the granular phase
 

Hi, I'm using Eulerian-Eulerian model to simulate coal combustion with k-epsilon dispersed turbulent model. The solid phase is treated as granular. I have 7 species in the solid phase and the species transport equations are solved.
How does the FLUENT calculate the effective mass diffusivity?
And what value should I set to the mass diffusivity in the Material panel?