Coal evaluation and reactivity for direct 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 in order to achieve optimum pre-reduction within a rotary kiln. CO2 reactivity is affected by numerous factors including coal properties and operating conditions. Relating CO2 reactivity to coal/char properties will assist in proposing suitable coals/blends. The CO2 gasification reactivity and coal/char characteristics of nine different coals originating from the Highveld, Witbank and Ermelo coalfields were determined and compared to a benchmark coal previously utilised for pre-reduction. The experiments were divided into two phases. The first was a screening process whereby the reactivity of all nine samples (20 mm particles) was measured and compared at both 950 and 1050 °C utilising a large particle thermo gravimetric analyser. From the results, two coals along with the benchmark coal were selected for further kinetic studies. For these coals, 6 mm and 212 μm char particles were gasified at 900, 950, 1000 and 1050 °C. The influence of particle size was investigated, and the activation energy was determined for the different samples and kinetic modelling of the conversion experiments was executed. Statistical analysis of the phase one results showed that the coal volatile and vitrinite content had the most significant influence on reactivity at 1050 °C, while the fixed carbon and inertinite content had the greatest influence at 950 °C. For the char analysis it was observed that chemical and physical properties had the greatest influence on reactivity at both 950 and 1050 °C. Multiple linear regression was used to derive empirical equations that correlate the initial specific reaction rates at both 950 and 1050 °C as a function of coal/char properties. The equations derived for 1050 °C were able to more accurately predict the reactivity. The results of phase two indicated that increased particle sizes decreased CO2 reactivity and that the rate of internal mass transfer was not negligible at the given experimental conditions. The apparent activation energies appeared to decrease with particles size and were estimated as 211 – 224 kJ/mol for 6 mm and 174 – 227 kJ/mol for 212 μm particles, which compared well with previous studies. Lastly, the Wen model showed accurate predictions of the rate of gasification. The conclusion was made that CO2 gasification reactivity is dependent on both internal (coal/char properties) and external (temperature and particles size) properties. Chemical and petrographic coal properties had the greatest influence, while for the char properties the chemical and structural properties had the greatest impact. From the results and conclusions it was recommended that the two coals (AC-5-72 & FC-2-21) selected for phase two are blended in order to achieve desired pre-reduction within the rotary kilns.