In the MFIX-Exa project, an application project under the Exascale Computing Project (ECP), a computational fluid dynamics, discrete element model (CFD-DEM) code that will run efficiently on exascale computers is being developed. MFIX-Exa builds on the multiphase modeling expertise embodied in NETL’s classic MFIX-DEM code. The core methodology has been both re-designed and re-implemented. The MFIX-Exa challenge problem is NETL’s 50 kW chemical looping reactor (CLR), a technology that can reduce CO2 emissions in power generation or hydrogen production. The challenge problem simulation will track 5 billion DEM particles in the full-loop CLR geometry.