The aim of the present study is to better understand the mechanisms involved during silicon deposition from silane, SiH4, into agglomerates of sub-micrometer-size particles treated by fluidized bed (FB)CVD. Two models of silicon deposition into agglomerates assumed either stable, or in permanent formation/desegregation, are developed. In the first case, classical equations of diffusion/reaction in a porous medium are solved, whereas in the second case, the multifluid Eulerian code, MFIX, is used. By comparison with experimental energy dispersive X-ray (EDX) data, modeling results show that the limiting step is not gaseous diffusion into the agglomerates. Very high local deposition rates near the silane entrance probably explain their formation. These results allow us to propose original deposition conditions involving much lower local deposition rates, which should limit agglomeration due to CVD.