Coal reactivity and selection for solid-based pre-reduction of sponge iron

Abstract

Solid-based direct reduction of iron ore requires the reductant coal to have a suitable CO2 reactivity to achieve optimum pre-reduction within a rotary kiln. The CO2 reactivity is affected by numerous factors including coal properties and operating conditions, and is traditionally determined using pulverized samples. The CO2 reactivity of nine South African coals, using 20 mm coarse coal particles, was measured. For this, an in-house constructed large particle thermogravimetric analyzer was used, at typical pre-reduction conditions: 1050°C and 25 vol.% CO2. The initial specific reaction rate was used to quantify the CO2 reactivity and a statistical analysis was performed to determine correlations between coal and char properties and coarse coal reactivity. For the coals, the chemical and petrographic characteristics, and for the char, the chemical and structural properties, showed the most significant relations. Multiple linear regression was applied to derive empirical equations from which the initial specific reaction rate could be determined as a function of coal or char properties. For the coal, the reactivity was a function of the fuel ratio and reactive maceral index, while for the char the initial specific reaction rate was a function of the nitrogen content and the carbon-based micropore surface area