Advanced design methods for rotary hearth furnaces

*Tenova LOI Italimpianti, Italy, **Centro Sviluppo Materiali SpA, Italy ENVIRONMENTAL concerns and rising prices of fossil fuels have focused the steel industry’s attention on minimising emissions and improving heating efficiency in reheat furnaces. Additionally with the rise in competitiveness within the world steel market product quality can be a key strategic factor to gain a market, a factor which arises in the reheating furnace by ensuring the correct heating rates, ultimate temperature, uniformity of temperature and furnace atmosphere. All those targets need to be fulfilled without increasing capital expenditure and operating costs beyond acceptable limits. As a consequence, a successful project for a reheating furnace must accomplish all these targets to minimise energy consumption, limit emissions and ensure product quality as far as is practically possible. This scenario poses various questions when designing or modifying a reheating furnace, many of which can provide valuable answers through integration of advanced design and verification methods into the common engineering workflow. Various attempts to evaluate the heating process inside a furnace have been made in past years with different levels of detail and accuracy. As it is well understood that radiation is the main mechanism of heat transfer involved in heating the charge, simplified models are commonly used to analyse performance and efficiency of reheating furnaces. The well-established zonal methods range from single or two dimensional models for preliminary estimation of charge temperature to three dimensional models able to account for edge effects in slab and square bloom or billet passing through a walking beam furnace as well as the cooling effect of the skids that carry the charge through the furnace[1-4]. Nevertheless, such models cannot give an indication about the local combustion processes which develop inside the furnace, nor can they give an indication of expected emissions levels. From the experimental side, existing methods of measuring cannot provide continuous measurements of temperature inside the furnace. Trailing thermocouples or data loggers can be used only with test charges to give an indication of the heating process at a few points measured inside the charge, while optical pyrometers or thermal cameras give information only about the surface temperature of the charge and their accuracy is greatly affected by the presence of the surface oxide layer (scale) on the charge. Therefore in recent years two main developments are on-going thanks to continuously increasing computational power: These are: Fig 1 TRGX flame temperature iso-surface coloured in terms of CO2 mole fraction. Top conventional long flame burner bottom short compact burner Fig 2 FlexyTech TRX4 burner