Tangentially fired furnaces are vortex combustion units which have become more attractive in the field of power station firing systems in recent years. Although the application of tangentially fired furnace continuously increases, they have not yet been adequately investigated. The present work provides a numerical study of flow pattern and its effect on NOx emission in a single chamber square tangentially fired furnace. Details of the flow field, along with temperature and species concentration contour maps are obtained from the solution of the conservation equations of mass, momentum, and energy, and transport equations for scalar variables in addition to the equations of the turbulence model. Four cases with different inlet air velocities are studied. Combustion in a natural gas-fired horizontal furnace with circular cross-section is simulated to verify the simulation methodology and the solution algorithm. Results are compared with those of the existing references and good agreement is observed. Calculations for the tangentially fired furnace show that while the vortex created in the center of the furnace becomes stronger as the burner outlet air velocity increases, its size remains almost unchanged. Highest-temperature regions are favorably far from the furnace walls so that erosion and local over-heating can be minimized. According to the results, higher inlet air velocities lead to more uniform temperature distributions with lower peak temperatures, which in turn result in remarkable reduction of NOx emission of the furnace.