Puzzling question: solid density

For the fluidized bed with pyrolysis reaction and using TFM model, it is suggested to choose constant or variable as the density of biomass particles. What’s the difference between the two options? If variable is selected, what formula is used to solve the biomass density? I am confused and hope to be solved! Thank you !

The variable density is computed from the inert species mass fractions (subscript I) and the baseline densities of each species (superscript B).

image

Thank you so much for your reply.