Abstract: Multi-scale computational fluid dynamics (CFD) simulation bridges the gaps between particle and reactor scales in the modeling of biomass pyrolysis. Its accuracy depends on the coupling of sub-models in different scales. Recent progress includes (a) detailed kinetics with 32 reactions and 59 species; (b) efficient intra-particle models for thermally thick particles; (c) hybrid drag models for sand biomass and biochar; (d) convection heat transfer models for nonspherical particles; (e) shape-resolved models e.g., glued spheres and Superquadrics; (e) machine learning derived models. These sub-models are coupled with MFiX multi-scale CFD solvers: (1) coupling corrected 0-D intra-particle model with MFiX-CGP (coarse grained particle); (2) coupling 1-D intra-particle model with MFiX-PIC (particle in cell); (3) coupling 1D intra-particle model with Superquadrics MFiX-DEM (discrete element model); (4) coupling 3-D intra-particle model with glued spheres MFiX-DEM. These solvers are validated against experiments and used in the simulation of pyrolysis reactors processing varied feedstocks.