This paper presents the results of calculation system of main processes in power steam boiler, before and after application of air staging. Modified air injection scheme was implemented during 2015 on the power steam boiler within unit 1 of TPP Kostolac B. Measurements performed on site showed that applied reconstruction led to a permanent loss of the steam boiler power. This study was performed in order to define the cause of such an occurrence and to consider the possibility for regaining the designed steam parameters along with keeping NOx concentration in prescribed limits. This paper discusses the influence of repositioning the air injection location on processes within the furnace. Furthermore, the influence of the redistribution of injected airflow along the furnace height on important boiler operation parameters has been analyzed. Analysis showed that, with appropriate dosing of air along the height of the existing furnace, it is possible to achieve the optimum of the boiler’s operation parameters. Results of research showed that air staging throughout the furnace height in best test case additionally reduces NOx concentration (195/225 mg/Nm3) and increases the power of considered boiler (828.8/751.1 MW) with an insignificant decrease of the boiler’s efficiency (86.27/86.77 %). Furthermore, the designed temperatures of superheated (540/498 °C) and reheated (540/518 °C) steam have been reached again, whereby the safety of the boiler’s operation has been significantly increased. Results of this study improve the present explanation of the processes occurred in the furnace with applied primary measures. They also give directions on defining the most influential parameters on considered processes with the final purpose to increase the efficiency and availability of the entire plant. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. TR-33018: Increase in energy and ecology efficiency of processes in pulverized coal-fired furnace and optimization of utility steam boiler air preheater by using in-house developed software tools]