Externally-fired gas turbines (EFGT) are currently being investigated for co-generation from biomass, because of their ability to deal with low-grade fuels without the complexity of gasification. Main drawbacks of the technology are related to the high thermal stresses experienced by the heat exchanger. The present work proposes a computational fluid dynamics (CFD) analysis of a grate-fired furnace installed in a EFGT cycle, with the purpose to provide a tool for detecting the most critical regions in the furnace. The model is complemented with a process simulation of the entire EFGT cycle. Different approaches for treating the fuel bed and their impact on the CFD analysis are discussed and validated through the availability of in-flame measurements of temperature and chemical species. Predictions indicate the need for a detailed fluid dynamic characterization of the grate region, which was found to largely impact the furnace flow and thermo-chemical fields