Performance of a fluidized bed-fired Stirling engine as micro-scale combined heat and power system on wood pellets

Abstract

Combined heat and power production in micro- to small-scale is one main part of the decentralization of energy production. Concepts using solid biogenic fuels to provide power and heat are manifold, but often focus on a nominal power output above 50kWel. Moreover, slagging or fouling issues in combustion processes provoke reduced electrical efficiencies and short maintenance intervals. This research paper presents an efficient and fuel-flexible micro-scale combined heat and power solution. The developed demo plant consists of a 45kWth fluidized bed combustion chamber and a 5kWel Stirling engine. Bringing the heat exchanger surfaces of the Stirling engine directly into the bubbling fluidized bed enables an efficient heat transfer, while a cooled combustion avoids exceeding low ash melting temperatures. This prevents the Stirling’s in-bed heat exchanger surfaces from fouling and slagging. The comprehensive lab experiments show carbon monoxide emissions below 100 ppm for full and part load operation, which emphasizes the flexibility of the micro-scale combustion. The integrated Stirling engine reached its nominal electrical power output of 5kWel. The evaluation of the electrical efficiency in a 72 h long-term lab test revealed 13–15% electrical efficiency, which is maximal in part-load operation and goes beyond known comparable approaches. The overall fuel utilization rate exceeded 85%.