Design Aspects of a Low-NOx Burner for a Stirling Engine
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The Stirling engine is a promising prime mover for micro-scale combined heat and power. For Stirling engines with heat supply by combustion, the external heating system is one of the most important parts. It has major infulence on the overall performance. The central component of the external heating system is the burner. This thesis describes the theoretical and experimental studies in the developement of a gas fired burner for the external heating system that have been carried out. The focus was on low emissions and high system efficency. As a first step, a system analysis of the external heating system is presented based on fundamental considerations about the thermodynamics and practical aspects of the Stirling engine. The results of the analysis show that the expected NOx emissions are strongly determined by the system design. Without making any restrictions to the burner desing, a span of the NOx emissions with a ratio of 1:800 was found. Modern design methodology is then introduced in order to analyze a large number of different low-NOx burner concepts that were found in literature. The concepts are evaluated and classified with help of the methodology in order to find possible new low-NOx concepts by favourable combinations of generic principles. Based on this, the concept of the porous inert media (PIM) burner is chosen for further development as a burner for the Stirling engine. The selection is confirmed by an experimental benchmark study in which the PIM burner showes low NOx emissions and the lowest pressure drop compared to three other low- NOx burner concepts. The optimization of the design of the PIM burner is described. A fovourable combination of materials was found, which enables stable operation with a turn-down ration of 1:15 and a span of the excess-air ratio from 1.28 to 2.0 when methane is used as the fuel. Temperature and CO measurements inside the combustion region were made which enable conclusion about the stabilization of the combustion process. Finally, the behaviour of the PIM burner with different fuel gases like methane, propane, methane-hydrogen mixtures and syngas is presented based on experimental results. Stability and emissions of CO and NOx under different operating conditions are discussed. The possible pathways of formation of NOx are analysed. Strong correlation of the NOx emissions with the temperature at the burner outlet is found. Based on this, a way of predicting the NOx emissions by simple empirical correlations that are valid for all fuels gases that have been used is presented.