Mathematical modelling of ignition behaviour in pulverised fuel flames

Coal is going to play an important role in meeting future energy demands but it produces carbon dioxide, which is considered to be a major contributor towards climate change. Oxyfuel technology is a promising carbon capture technology where the N2 in the oxidiser stream is replaced with CO2 to enhance the capture process at the exit of the power plant. The oxyfuel technology results in changing the combustion environment, which may affect the flame characteristics, fuel efficiency, the emissions and overall boiler performance. The aim of this research was to develop an ignition model, which provides further understanding of the flame characteristics due to fuel switching and changes in the combustion environment.\\

An investigation was conducted analysing predictions of different coal devolatilisation models. The three models analysed were CPD, FG-DVC and PC-Coal Lab where the models simulated hot wire mesh experimental conditions. The results showed that the PC-Coal Lab and CPD is able to accurately predict the devolatilisation behaviour for a broad range of coals whereas FG-DVC is less effective. The CPD model is chosen for further investigations over PC-Coal Lab as it entails a licensing cost whereas CPD is open source and proves to be less demanding.

An ignition model is developed to understand the fundamental ignition mechanisms of a single particle of a solid fuel, which are categorised as either homogeneous or heterogeneous. The model accurately couples kinetics, heat and mass transport phenomena between the interior and exterior of the particle. The study accounts for variation in ambient conditions (including oxyfuel conditions) and fuel properties where the results are validated against experimental data. On extending the ignition model to different particle size, a correlation is obtained for the ignition mechanism based on particle size and ambient oxygen concentration. The correlation developed is useful in investigating ignition in pilot/full scale boiler assisting in any design and operational changes.

Baseline CFD simulations were conducted on an IFRF and Utah furnace replicating their respective experimental flows. The simulations were repeated with integration of the correlation using the methodology developed in previous study. The results are compared against the experimental data suggesting that ignition model improves the overall predictions of the flame lift (i.e ignition zone) and thus the model can be used for further investigating novel combustion environments and fuels. The validated ignition model is applied in investigating the pilot scale burner, which indicated a small improvement in the species prediction but a further development will be required in the turbulent chemistry model.