Numerical examination of an operationally flexible lignite-fired boiler including its convective section using as supporting fuel pre-dried lignite

This work simulates the main furnace and the convective section of the lignite-fired Ag. Dimitrios V boiler located in Kozani, Northern Greece, under three different thermal loads (100%, 60% and 35% of the nominal one) using the Ansys Fluent 15.0® software program. The lowest thermal load investigated, i.e. 35% of the nominal one, is lower than the current technical minimum of the unit (~ 55%). In order to ensure the furnace stability and efficiency in this low thermal load, a co-combustion scheme of pre-dried with raw lignite is implemented. Due to sub-stoichiometric operating conditions in the main furnace zone in the full-load case and the formation of a two-stage over-fire air (OFA) system, the simulation of whole the furnace, including both the main furnace and the convective section of the boiler, is necessary, to investigate the char conversion and the furnace performance thoroughly. The simulation of the convective section is based on the porous media analysis using the Macro Heat Exchanger Model provided by Ansys Fluent 15.0®. The results regarding the total heat transfer towards the working medium and the flue gas average temperature at the main furnace outlet surface and at characteristic transversal surfaces of the convective section are compared against thermodynamic model predictions and experimental values showing good agreement, since the maximum percentage difference between the results of the two software tools and with the experimental values is approximately equal to 10%. Based on these results, it is indicated that the injection of pre-dried lignite fuel ensures the combustion stability and high efficiency of the boiler in lower thermal load (35%) than the current technical minimum of the unit.