dc.description.abstract | 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. | |