Power generation by Stirling engine during fluidized bed combustion of wood pellets

A system consisting of a Stirling engine (SE) and a fluidized bed combustor (FBC) for combined heat and power (CHP) generation has been experimentally investigated. The heat generated by combustion of wood pellets is used as source for the SE that converts part of the thermal energy into mechanical and then electric energy. This system, having the heat exchanger of the SE located inside the sand bed of the FBC, presents several advantages: (1) very high bed-to-external surfaces heat exchange coefficients; (2) absence of fouling on the heat exchange surface due to the cleaning action exerted by the fluidized sand particles; and (3) FBCs are able to use a wide variety of biomass fuels. The FBC used in this investigation can develop a thermal power in the range 15-40 kW feeding wood pellets as fuel and changing fluidization conditions and fuel feeding rate. Bed operation temperature was varied in the range 750-850 degrees C. The SE adopted is a gamma-type with the heater in form of tube bundle. The performances of this integrated system have been assessed in terms of gaseous emissions and of SE efficiency varying the bed temperature and the pressure of SE working fluid. A mathematical model able to simulate the integration of the FBC with the SE for CHP generation has been developed to quantify the heat fluxes among the different components of the system.