The riser tube solar receiver of a circulating fluidized bed solid particle absorption solar thermal energy system was numerically modeled for analyzing hydrodynamic and heat transfer behaviors of the solid particles in the riser. Hydrodynamics of the model is validated by comparing radial distribution of void fractions with an experimental study. For the heat transfer from the opaque walls of the receiver that is heated to high temperatures by the solar rays concentrated by the heliostat field, a simple fractional model is used in which radiative transfer is neglected and total heat flux is distributed to phases according to the instantaneous volume fractions at the boundary cells. MFIX: Multiphase Flow with Interphase eXchanges code of NETL is used with a 2.5D Eulerian-Eulerian computational model for transient simulations. The 2.5D grid is a combination of planar cells and cylindrical cells with the determined optimum fraction of planar cells of 0.15. For the solar receiver riser, transient and time averaged results of void fraction and. gas and solid phase temperature distributions were numerically obtained and analyzed.