Considerable research efforts focus on modeling NOx formation/destruction and predicting NOx emission so that it can be controlled. A motivation for this numerical study was to examine the efficiency of combustion modifications in the furnaces of Kostolac B 350 MWe boiler units, tangentially fired by pulverized lignite. Numerical analysis was done by an in-house developed NOx submodel, coupled with differential comprehensive combustion model, previously developed and validated. The NOx submodel focuses on homogeneous reactions of both the fuel and the thermal NO formation/destruction processes. The submodel was validated by comparison of predicted NOx emissions with available measurements at the boiler units. Selected predictions of the emission, the furnace exit gas temperature, NO concentration, gas temperature, and velocity field are given for the case-study furnace under different operating conditions. The individual or combined effects of coal and preheated air distribution over the individual burners and the burner tiers, the grinding fineness and quality of coal, and the cold air ingress were investigated. Reduced emissions of up to 20-30% can be achieved only by proper organization of the combustion process. Obtained results were verified by the boiler thermal calculations. An optimal range of the furnace exit gas temperatures was proposed, with respect to the safe operation of the steam superheater. Simulations by means of a computer code developed for the purpose, showed that the air staging using overfire air ports might provide the NOx emission reduction of up to 24% in the test-cases with relatively high emission and up to 7% of additional reduction in already optimized cases.