Numerical simulations on Oxy-MILD combustion of pulverized coal in an industrial boiler

The purpose of this study is to analyze the possibility of combining two innovative combustion technologies in large-scale pulverized coal fired plants: Moderate or Intense Low Oxygen Dilution (MILD) and oxy-combustion. The combination of both technologies, namely Oxy-MILD combustion, is expected to bring synergetic effects: NO_x reduction, CO₂ capture possibility, fuel flexibility and uniformity of heat fluxes and species concentrations. In this work, the predictable advantages of adopting this technology, with respect to conventional boilers, are evaluated by means of CFD modeling, in terms of pollutant emissions and uniformity of heat fluxes. In a previous work, the developed CFD model was validated against available experimental data of MILD combustion in a pilot-scale furnace and it was demonstrated that the proposed model captures the combustion features with good accuracy. In order to identify the effective potential of Oxy-MILD combustion and its possible uses on an industrial scale, an application in the boiler is analyzed in the current work. Results show that the temperature and species concentration distributions reach an acceptable level of uniformity in the boiler; similarly, the wall heat flux profile is uniform along the boiler height, as in the fluidized bed technology. The CO₂ concentration at the boiler exit, for an excess oxygen ratio of 1.1, is about 95.8%; this value is slightly increased for lower excess oxygen ratio, even though in this case, incomplete combustion occurs. Finally, lower NO_x (70 mg/MJ) than other technological solutions are obtained owing to the prevention of thermal and prompt NO_x, owing to the absence of N₂ in the oxidizer, and to the re-burning mechanism, which is predominant especially when recycled NO_x is considered.