Integrated fluidized bed combustor and Stirling engine for micro-scale cogeneration

The paper deals with the direct integration between a Stirling engine and a fluidized bed combustor for micro-scale cogeneration. A pilot-scale facility integrating a fluidized bed combustor (up to 60 kWt ) and a gamma-type Stirling engine (0.5 kWe) was set up and tested to demonstrate the feasibility of this solution and investigate the critical concerns of the combined system. The Stirling engine was installed at a lateral wall of the combustor in contact with the bed region. An experimental campaign has been carried out to assess the performance of the innovative integrated system. The paper reports on the experimental results that can be summarized in: i) very high combustion efficiency with biomass feeding, ii) enhanced heat transfer rate to the engine, iii) conversion to electric power close to the upper performance limit of the engine. Moreover, aspects of the dynamics of the integrated system were investigated and mathematically modeled, the dynamics of the thermal system being dominated by the fast response of the Stirling engine, which rapidly reacts to the slow changes of the fluidized bed. Under the tested conditions (i.e., a thermal power of about 13 kWt), only a limited fraction of power (2 kWt) was subtracted from the engine to the combustor, leading to a dynamic response of the system with a thermal time constant of the order of 10 min.