A three-dimensional computer model of a swirling fired type furnace where a pulverized petroleum coke and an ASTM No. 6 heavy oil are fed from two opposite side walls is presented. The predicted results are compared with measurements of exhausted NO concentration in an operating 125MW(e) boiler with and without air introduced tangentially from the bottom of the furnace (below the bottom burners) to convert from H2S to SO2 effectively. Agreement between prediction and experimental data is very satisfactory. Both of them show that the unburned ratio decreases with an increase in the flow rate ratio of the tangential air to the secondary air, and an increase in NO concentration. Our prediction also shows that when the tangential air is flowed from the bottom, the reduction of H2S formation rates are more significant in the low grade high sulfur content fuel such as heavy oil and petroleum coke.