In the present work the behavior of a lignite boiler under both full and partial load conditions is investigated by means of CFD modeling. The simulated furnace is that of the Megalopolis Unit IV, a 300 MWe power plant operated by Public Power Corporation S.A. (PPC) and located in Peloponnese, Southern Greece. Due to the extremely low quality lignite utilized in the particular power plant, a special system for dried lignite dust separation from vapors — composing of a) an additional vapor ESPs – partial vapor discharge and b) a dry lignite dust recirculation and firing system, is designed and operated in the particular boiler. Hence, pre-dried lignite is continuously used as the additional supporting fuel, through a special feeding, dosing and firing system along with separation and vapor discharge.
Scope of the present work is the evaluation and optimization of different firing modes for the decrease of minimum load operation lower than 40%. The numerical analysis is focused on the induced flue gas flow field characteristics, associated with average exhibited temperature, main gas species concentrations, wall heat flux and particle burnout. The boundary conditions for partial load operation are provided by Mitsubishi Hitachi Power Systems Europe (MHPSE) and the basic design data of the furnace are provided by PPC S.A. In total, eight operating conditions are investigated corresponding to five boiler thermal loads, thus covering a wide range of thermal load operation. Furthermore, the boiler performance under different air and fuel distributions among the burners is also examined, in order to identify useful trends regarding the optimum conditions, under which a stable and safe furnace operation for very low load operation (less than 40%) can be assured. The CFD results are compared against design values provided by MHPSE. Finally, conclusions concerning the furnace membrane wall erosion and accretion rates under various operating configurations are derived, providing useful information about potential operating issues that could be expected, if the examined firing cases were applied. Finally, the case of a thermal load equal to 40% is of interest, for which a stable pulverized lignite flame can be achieved when two mills instead of three are operating.