Heat Transfer in a Bed of Dry Iron Ore Pellets

Abstract

Iron ore pellets are used as an intermediate product in the production of steel. Their size of approximately 10 mm diameter combines ease of handling with the ability to control air flow and heat distribution in a blast furnace. Previous studies have investigated the thermal properties of iron ore pellets over from 30 to 800°C, i.e. over a temperature range relevant for blast furnaces.1) It was found that the thermal conductivity and diffusivity decreases with increasing temperature, and that the specific heat peaks at around 680°C due to a phase transition. Heat and mass transfer of individual pellets exposed to a stream of air has been subject to computational fluid dynamics simulations.2) In earlier energy transfer measurements, the diffusivity of individual iron ore pellets was found to range between 2.9 and 3.6×10–7 m2 s–1 over a temperature range from room temperature to 680°C.3) This study focuses on heat transfer over a temperature range commonly observed during transport of iron ore pellets. Specifically, KPBO pellets of Luossavaara Kiirunavaara Aktiebolag (LKAB) are commonly shipped shortly after production, i.e. while warm (30°C). However, some batches may have been stored outside at sub-freezing temperatures commonly observed to be as low as –30°C. Here, heat transfer between a cold layer of pellets underlying a warm layer is measured and compared with heat transfer simulations. Such layering may be observed in storage silos. Previous measurements of thermal properties of individual pellets are not directly applicable to heat transport though a bed of pellets. At atmospheric pressure, effective thermal conduction through granular media combines the effects of conduction through solid particles, conduction through gas, advective heat transfer through the gas, and radiative heat transfer between solid particles.4,5) The significance of individual contributions depends on pore size, absolute temperature and material properties, and varies with air pressure and temperature.5) This work focuses on the question whether vertical heat transfer through a bed of KPBO pellets can be described as a homogenous material with temperature-independent thermal properties. Laboratory experiments were performed and compared with one-dimensional heat transfer simulations.