Development and validation of a combustion model for a fuel cell off-gas burner
A low-emissions power generator comprising a solid oxide fuel cell coupled to a gas turbine has been developed by Rolls-Royce Fuel Cell Systems. As part of the cycle, a fraction of the unreacted fuel (the off-gas) and oxidizer streams is reacted in a burner, which is the main source of pollutant formation. In this thesis a computational model of the burner has been developed which captures the formation of NOx and the oxidation of CO. This model gives accurate predictions at low computational cost, making it suitable for use as a design tool in future burner design optimization through parametric studies. A key factor in increasing computational efficiency was the development of a reduced H2/CO/N2 kinetic mechanism; from a starting mechanism of 30 species to 10 and 116 reactions to 6. The results of laminar opposed-flow diffusion flames have been used to validate the reduced mechanism. Several different turbulent combustion models have been evaluated by creating an interface between the reduced kinetic mechanism and the commercial CFD solver FLUENT. Comparison of model predictions with well-characterized turbulent syngas flames, which share a similar fuel composition to the experimental work conducted on the off- gas burner, shows acceptable agreement. These studies have demonstrated the sensitivity of modelling constants. Improved predictions were achieved by calibrating these constants and including radiative heat losses. Following suitable modification to reflect the predominantly laminar flow present in the current burner design, the relevant modelling approaches were applied to the off- gas burner. Comparison was made to previous detailed measurements, showing that the important trends of NOx and CO are captured in general. The model was extended to high pressure conditions, similar to those in the actual off-gas burner, with the emissions predictions within design limits. The outcome of this work is a fast, accurate design tool for CFD which has capabilities to simulate beyond the laminar burner studied here. It may be applied to more general types of off-gas/syngas burners where turbulence-chemistry interaction is expected to be more significant.