Abstract: The present paper presents a three-dimensional numerical investigation of a pulverized-fuel, tangentially-fired utility boiler located at Florina/Greece under air, partial and full oxy-fuel conditions. Heat and mass transfer and major species concentration, such as CO2, CO and O2 are calculated; whilst the results for the reference air case scenario studied are in good agreement with the corresponding operational data measured in the plant, both for combustion calculations and NOx emissions. Results for the partial and full oxy-fuel operation scenarios are in line with similar experimental and numerical investigations found in the recent literature. This numerical investigation of oxy-fuel conditions scenarios prior to their implementation under real scale conditions demonstrates the utmost of its importance, since significant results regarding the operation of a boiler in terms of lignite particle trajectories and burning rates are attained. Furthermore, NOx calculations have been performed for all the examined case studies.

Abstract: Lignite pre-drying and dry lignite firing is considered as an important development step for the next generation of lignite power plants. The integration of a predrying system in future lignite power plants, which utilizes low temperature steam for the drying process, combined with the further thermal utilization of lignite’s evaporated moisture, may bring an efficiency increase of four to six percentage points compared with today’s state of the art. Firing predried lignite is however expected to cause certain changes in the combustion behavior of large scale boilers. The investigation of these changes in an existing Greek utility boiler through experimental activities and numerical simulations is the scope of the present work. The investigations take place in a 75 MWth lignite fired Greek boiler. The specific unit is equipped with dedicated dry lignite burners. The measurements are performed with a dry lignite thermal share of 6%. Higher cofiring thermal shares of up to 20% are further simulated and the effect of cofiring on the combustion behavior is evaluated by specific parameters including temperature fields, wall heat flux, fuel’s burnout, and NOx emissions. No clear effects of cofiring on boiler operation are observed during the experimental campaign indicating that the operational behavior in low cofiring shares remains constant. Some trends are observed in the simulations of the dry coal cofiring cases. Increased temperature peaks in the near burner region and higher furnace outlet temperatures are predicted. A clear increase of the total fuel burnout when firing dry coal is also foreseen by the simulations, which is a strong argument for the coutilization of dry lignite as a supportive fuel. The overall examinations imply that dry lignite cofiring for low thermal shares up to 20% is feasible in the case of Greek boilers without major technological difficulties. For the realization of a lignite predrying concept in an existing Greek boiler however, a detailed study on each particular boiler is necessary.