Numerical Investigation of NOx Emissions for a Flexible Greek Lignite–Fired Power Plant

Abstract

This work simulates the Megalopolis IV power plant boiler operation in terms of NOx emissions for a wide range of thermal loads, and with different operational patterns of the firing system, i.e., different numbers and variation of the open burners’ positions. This work provides feedback on thermal loads that are lower than the current technical minimum (∼55%) using numerical tools (CFD software packages) for both 40 and 35% partial-loads. For the lowest investigated load, it also examines contemporary combusting systems, with the simultaneous firing of raw and stored dry lignite to achieve flame stability. The results of the current study primarily focus on the effect of lower than 100% boiler operation on the NOx emissions because the majority of works in literature investigates boiler NOx emissions only under full-load conditions. Furthermore, it focuses on the variation of the NOx emissions among different partial-load conditions and the influence of the firing system operational patterns on these. To reduce the required computational resources, the decoupled approach is adopted, which distinguishes the main combustion mechanisms from the NOx emission mechanisms themselves. The results indicate that the NOx emissions concentration reduced at 6% dry oxygen molar concentration at the main furnace outlet surface increases as the thermal load gets lower. The results also prove the direct relation between temperature values and NOx emissions because the highest NOx concentrations are tracked in the hot spot areas; primarily attributed to thermal NOx production mechanisms. Additionally, the firing system operational pattern for each specific thermal load affects the NOx emissions concentration, as the results regarding the 35% partial-load case imply. In particular, for the 35% partial-load case, the even distribution and injection of stored dry lignite from all six vapor burners result in lower NOx concentrations compared to the case for which the injection of the stored dry lignite happens through the inlets of the two open vapor burners. In conclusion, the results of this work can provide valuable information about the boiler performance in terms of its NOx emissions behavior under partial-load conditions, and provide potential measures that can be implemented for NOx emissions reduction, especially under low-thermal conditions.