| Summary: | Situated between the casting and rolling operations, the reheating process ensures steel
billets or slabs are at temperatures high enough (~1200°C) to enable subsequent deformation pro-cesses
to be carried out economically. Industrial reheating furnaces have varying designs, utilize
different fuel mixtures and employ top, side or bottom firing. Under steady-state operating cond-itions,
the challenge is to achieve good temperature homogenization while minimizing fuel con-sumption
and maximising furnace throughput. During furnace stoppages, which are caused by
delays in therolling mill, there is a need to minimise fuel consumption and maintain discharge
temperatures.
To gain better insight into the operation of a billet reheating furnace, a transient, three-dimen-sional
thermal model has been developed. Radiative heat transfer in three-dimensions is
solved using Hottel's "Zone Method", employing a clear-plus-three-grey gas model to represent
the furnace atmosphere. In this method, the geometrical aspects of the problem are treated sepa-rately
to produce total exchange areas that can be stored for repeated use. The main module then
calculates the energy released through combustion, the heat transferred to the steel and the move-ment
of the charge during each time step. Gas temperatures are determined from energy balances
using a Newton-Raphson iterative technique. Conduction in the billets is solved in three-dimensions
by taking into account heat transfer in the gaps between and underneath the billets. The
model further evaluates heat losses through the furnace roof, walls and hearth.
The mathematical model was verified using industrial data obtained from plant trials con-ducted
at two Canadian steel mini-mills. Results from the plant trials indicated that the billets con-tinued
to increase in temperature during furnace stoppages. The model suggests that this is due to
continuous burner firing during these stoppages even with lower firing levels. For one of the furnaces,
the model predicts the thermal efficiency to be 31% for the heating of 0.15 m (6") billets,
with 68% of the combustion energy lost in the flue gases and the remainder lost through the
refractory. Improved performance could be realized through better control of the furnace atom-sphere,
with the air/fuel ratios maintained at levels closer to stoichiometric, as well as the installa- tion of a recuperator to preheat the combustion air.
Different delay firing strategies that focussed on the recovery of the furnace were examined
with the model and it was found that the sequential return to steady-state firing reduces the
extent of billet over-heating while ensuring newly charged billets reached adequate rolling temperatures.
The model was also used to examine the effect of air/fuel ratios in each of the furnace
control zones and the benefits of recuperatively preheating the combustion air or hot-charging,
where the billets are charged into the furnace soon after casting.
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