Numerical analysis of the transient heating of steel billets and the combustion process under air-fired and oxygen enriched conditions

A three-dimensional analysis of the gas phase combustion and transient heating of steel billets in an industrial walking hearth furnace was done by CFD. An iterative solution procedure was used for the steady-state simulation of the reactive flow and the heat conduction in the billets for air-fired and oxygen enriched combustion with 25 vol% O2 and 75 vol% N2 in the oxidizer. Information about the turbulent flow, species concentrations, temperatures and heat fluxes in the furnace was obtained with low computational demand due to the used solution procedure. Modelling of the gas phase combustion considered 17 species and 25 reversible reactions including radical formation. According to operating conditions of the furnace, a gas saving of 8% was determined due to the oxygen enrichment. Although the fuel input was reduced with increasing oxygen concentration, the calculated heat fluxes to the billets were similar due to the oxygen enrichment. Furthermore, a higher heating rate was achieved in the heating zone compared to air-fuel combustion, as a consequence of the increased radiative heat transfer. The overall efficiency of the reheating process was raised from 57.6% (air-fuel) to 61.4% (oxygen enriched). Surface temperature and temperature uniformity of the billets were calculated using transient simulations and investigated for the two combustion cases. In air-fuel combustion, a faster heating of the billets was detected due to the higher convective heat flux in the pre-heating zone. This effect in the air-fuel case was compensated in the following heating zone in oxygen enriched combustion by radiation. This study shows that higher efficiency of reheating processes can be achieved by means of oxygen enrichment with a similar heating characteristic of the furnace load.