This work investigates the ideal performances that could be achieved in the coupling of a Fluidized Bed Combustor with a Stirling engine by placing the head of a Stirling engine, more specifically the elements of the hot side heat exchanger, in direct contact with the sand of the Fluidized Bed Combustor. This choice is primarily suggested by the heat exchange coefficients between the multiphase fluidized bed medium and the surface of the heat exchanger, much larger than those attained when the heat exchanger is located in the stream of hot flue gases. Moreover, the mechanical action exerted by the fluidized solid particles substantially reduces the fouling usually caused by impurities in exhaust gases of a biomass combustion process. A mathematical model, which covers a small size fluidized bed and the Stirling engine, is developed and used to optimize design and operating conditions with specific attention to the Stirling hot side heat exchanger. It is shown that the choice to place the heat exchanger in direct contact with the fluidized bed can lead to an improvement of performance in terms of efficiency and shaft power output, making the development of this kind of system attractive for the production of energy from renewable sources.