When burning any fossil fuels, one of the most harmful combustion products are nitrogen oxides NOx, which damage both the environment and human health in particular. Reduction of NOx emissions from fuel combustion at TPPs plays an important role in reducing the total level of nitrogen oxides NOx emitted into the atmosphere. One way to reduce the concentration of nitrogen oxides NOx is the stepwise combustion of the pulverized coal mixture, in particular the «Over fire Air» technology. The essence of this method is that the main volume of air is fed into the pulverized burners, and the rest of the air is further along the height of the torch through special nozzles. Structurally, the method of stepwise combustion of fuel can be carried out in boilers with a two-tier arrangement of burners along the height of the combustion chamber. In this case, practically no significant reconstruction of the boiler is required, which is associated with additional costs. In the present work, computational experiments on the use of modern overfire air technology (OFA) in the combustion chamber of the PK-39 boiler of the Aksu TPP were carried out and the fields of the main characteristics of heat and mass transfer, as well as the influence of the mass flow of the oxidant through the OFA injectors on the combustion process were obtained. When burning any fossil fuels, one of the most harmful combustion products are nitrogen oxides NOx, which damage both the environment and human health in particular. Reduction of NOx emissions from fuel combustion at TPPs plays an important role in reducing the total level of nitrogen oxides NOx emitted into the atmosphere. One way to reduce the concentration of nitrogen oxides NOx is the stepwise combustion of the pulverized coal mixture, in particular the «Over fire Air» technology. The essence of this method is that the main volume of air is fed into the pulverized burners, and the rest of the air is further along the height of the torch through special nozzles. Structurally, the method of stepwise combustion of fuel can be carried out in boilers with a two-tier arrangement of burners along the height of the combustion chamber. In this case, practically no significant reconstruction of the boiler is required, which is associated with additional costs. In the present work, computational experiments on the use of modern overfire air technology (OFA) in the combustion chamber of the PK-39 boiler of the Aksu TPP were carried out and the fields of the main characteristics of heat and mass transfer, as well as the influence of the mass flow of the oxidant through the OFA injectors on the combustion process were obtained.