Numerical investigation on post-combustion in a burner for heat treatment furnaces with a reducing gas atmosphere

Scale formation in reheating and heat treatment furnaces causes a significant metal loss on semi-finished metal products. The presented project is about the development of a burner that produces a low oxidizing / reducing atmosphere in the furnace with a thermal efficiency comparable to state-of-the-art recuperative burners. The concept combines direct fuel rich firing and indirect heating with radiant tubes in a recuperative burner. Unlike existing recuperative burners, the concept-burner is equipped with an open radiant tube (ORT) forming an annular gap between the burner and the tube, where the off-gas is post-combusted by the addition of secondary air. The heat of reaction is either transferred to the furnace by radiation of the ORT or recuperated to heat up the primary and secondary combustion air. In the presented numerical study, the impact of the primary equivalence ratio, the furnace temperature and the total equivalence ratio ϕtotal on the post-combustion process in the annular gap is evaluated. The total equivalence ratio has the most significant influence on post-combustion. The results for the variation of the total equivalence ratio ϕtotal show a faster post-combustion can be reached by using a lower total equivalence ratio of ϕtotal = 0.87 instead of ϕtotal = 0.91. Another parameter on post-combustion is the primary equivalence ratio ϕprimary. At ϕprimary = 1.43 the amount of H2 and CO in the primary off-gas is higher, resulting in a slower post-combustion. At the same time the combustion at ϕprimary = 1.11 is delayed because of lower mixing rates and kinetics. A variation of the primary off-gas inlet temperature has a low impact on the post-combustion. Decreasing temperature from 1050 °C to 950 °C delays the reaction. The results show that there is a good correlation between the calculations and the measurements at the experimental setup. Therefore, the numerical model is capable to show the impacts of the different parameters on the post-combustion.