Results from tests of a Stirling engine and wood chips gasifier plant

The combination of thermal gasification and a Stirling engine is an interesting concept for use in small Combined Heat and Power (CHP) plants based on biomass, because the need for gas cleaning is eliminated and problems with fouling of the Stirling engine heater are considerably reduced. Furthermore, the overall electric efficiency of the system can be improved.

At the Technical University of Denmark a small CHP plant based on a Stirling engine and an updraft gasifier has been developed and tested successfully. The advantages of updraft gasifiers are the simplicity and that the amount of particles in the gas is small. Furthermore, the updraft gasifier can utilise fuels with a high content of water.

The disadvantage is that the gas has a large content of tar. Tar is a large problem for utilisation of gasification gas in an internal combustion engine, but the external combustion of the Stirling engine reduces the problems with tar to a minor problem in the design of the burner. The Stirling engine, which has an electric power output of 35 kW, is specifically designed for utilisation of fuels with a content of particles.

The gas burner for the engine is designed for low specific energy gas and for utilisation of preheated air. A mathematical simulation model has been developed as a tool for the analyses of the Stirling engine gasifier system. The paper presents results from a simulation of the Stirling engine demonstration plant in a typical operating condition, and the result of the simulation is compared with tests results from operation.

The comparison shows, that the simulation model is able to predict the performance of the plant with a reasonable accuracy.