In the present work, a complete three dimensional fluid dynamics simulation of reacting fluid flow in post-combustion chamber of an electric arc furnace is presented. The chemical composition of the simulated off-gases was representing typical crucial load. The gap (where oxygen enters to combust hydrogen and carbon monoxide) size was an independent variable. The optimal gap size is desirable: if the gap size is too large, the thermal efficiency diminishes. On the other hand, if the chosen gap size is too small, oxygen deficiency occurs, which lead to incomplete combustion of carbon monoxide and hydrogen. Herein established, that by means of CFD calculation, the optimal gap size can be evaluated for a particular steelmaking furnace.