Setup of an integrated Stirling Engine - Fluidized Bed (SE-FB) experimental system

This paper presents a new concept for the integration of a combustion chamber based on renewable fuels and a Stirling Engine (SE). Biomass is adopted as fuel to be burned in a fluidized bed (FB) combustor in the preferred form of wood pellets. This combustion technology has proven to be flexible, efficient and clean in a variety of configurations. The heat generated by combustion is used as source for the Stirling engine that converts part of the heat into mechanical and then electric energy. The novelty of the proposed setup consists in placing the heat exchanger directly into the sand bed of the fluidized combustor. This choice is motivated by the very high heat exchange coefficients attainable with the fluidized bed and by the absence of fouling due to the cleaning action exerted by the fluidized sand. A prototype system has been built to prove these concepts. The fluidized bed combustor has a square section of 290x290 mm2 in the bottom and 390x390 mm2 in the top; the total height is 1850 mm. All components are made in stainless steel. The combustor is equipped with an electric pre-heater, devices for temperature, pressure and flow measurements as well as a dedicated gas analyzer. The thermal power can be varied in the range 15-40 kW by changing the operating conditions of the fluidized bed and the fuel feeding rate. The typical operation temperature of the bed is in the range 800-850 °C. The heater of the Stirling Engine, produced by El.Ma. Srl, is immersed in the bed section of the combustor. A specifically designed support and connector, able to dump the engine oscillations and to allow the same engine to follow the movements of the insertion point determined by the thermal expansion of the combustor, has been constructed. First tests of heating-up, combustion and co-generation were performed, providing positive confirmations about the technical solutions adopted for the prototype and the possibility to achieve better performance when the SE heater is placed in contact with the hot flue gases. The system is the core of a larger research project named MEGARIS, aimed at developing a micro-cogeneration system, which is exclusively based on renewable energy sources and able to guarantee heat and electric power continuously 24 hours a day.