The paper describes a study of the influence of Overfire Air (OFA) technology on heat and mass transfer parameters for a furnace chamber in the BKZ-75 boiler at the Shakhtinsk power plant (Kazakhstan); this plant is fueled with Karaganda high-ash coal. The computer simulation methods were applied for studying different air supply regimes through the air injectors operating in OFA design with different contributions: 0 (reference variant), 5, 10, 15, 18, and 20 %. The computer simulation of in furnace processes offers the distribution for total velocity vector, tempertaure field, NO2 concentration field for the entire furnace and the exit zone. The study demonstrated that the operation with air injection at 18 % of OFA is optimal for combustion of high-ash coal. This operation mode offers a higher tempertaure at the flame core and a gradual tempertaure reduction at the chamber outlet; the latter feature is significant for chemistry of combustion products. The boiler under this operation mode offers a reduction of nitrogen oxide NO2 emissions from the furnace.