NUMERICAL PARAMETRIC INVESTIGATION OF A FLEXIBLE LIGNITE-FIRED BOILER OPERATING WITH PRE-DRIED LIGNITE OR BIOMASS AS SUPPORTING FUEL

This work presents a numerical parametric investigation of an indirect firing scheme using pre-dried lignite and biomass (olive pomace) as supporting fuel for a lignite-fired boiler during its operation at a thermal load of 35% of the nominal one, lower than the current minimum one. The parametric investigation is important in order to evaluate technically the optimal firing concept and the ideal fuel blend for the flexible operation of the unit at lower than technical minimum thermal loads. Based on the numerical results, it can be concluded, that the utilization of both types of supporting fuel can ensure the stable operation of the boiler at thermal loads, lower than the technical minimum of the unit. Moreover, it can be remarked, that the injection of biomass as supporting fuel improves the thermal efficiency of the boiler. Furthermore, the injection of pre-dried lignite through all vapour burners increases NOx emissions due to the developed temperature field. Finally, simulation results indicate that the injection of biomass particles through all vapour burners induces high thermal loading on boiler surfaces. Therefore, it is proved that the firing strategy plays a key role to the operation of a boiler using these specific fuel blends.