Effects of Ash Minerals on Coke Reactivity under Hydrogen Injection, Low CO2 Blast Furnace Conditions

The aim of this study was to quantify the effects of coke mineralogy on the reactivity of coke under H₂-injection, low CO₂ blast furnace (BF) conditions. This was carried out by reacting metallurgical coke samples and coke analogue samples at temperatures of 1100°C and higher using H₂- or H₂O containing gas mixtures.

Cokes were chosen to have distinct mineral compositions, with coke 114 having a high SiO₂ content, and coke 131 having high Fe. Five coke analogue samples were examined, with samples containing coke ash (analogue 114 and analogue 131), as well as single minerals (quartz, “magnetite” and alumina) prepared. It should be noted that the mineral in the fired analogue magnetite sample was metallic iron. The reaction conditions were chosen to represent a test similar to the standard CSR-CRI, as well as the lower zones of the BF (cohesive zone and hearth), but with gas mixtures that represented H₂-injection, low CO₂ BF conditions.

In general, all of the coke and coke analogue samples were more reactive at 1100°C (in very oxidising conditions) than at 1350°C and 1600°C (under less oxidising conditions). It was found that at 1100°C, an addition of 14% H₂O to CO₂ increased the reactivity of coke samples by 3-6% and coke analogue samples by 8-50%. The addition of 14% H₂ to the reaction gas at 1350°C increased the reactivity of Fe-rich samples by 48-122% compared to H₂-free conditions. Other samples showed less change. At 1600°C, the addition of 14% H₂ to the CO-Ar reaction gas increased the reactivity of the cokes by 61-173%. The effect on the single mineral coke analogue samples was much smaller.

The effects of the minerals on the reactivity of the coke and coke analogue samples can be summarised in more detail as:

• 1100°C, 86% CO₂-14% H₂O: the reactivity increased for all coke and coke analogue samples. The effects of the minerals on reactivity was similar to the effects in the standard CRI conditions. Coke 131 (high Fe) was more reactive than the coke 114 (high Si). In the single mineral coke analogue samples, analogue magnetite was more reactive than analogue alumina, which was more reactive than analogue quartz. However, it was also noted that the increase in reactivity in the 86% CO₂-14% H₂O gas mixture was greater for the high SiO₂ samples.
• 1350°C, 25% CO-2% CO₂-14% H₂-59% Ar: the effects of the minerals on reactivity were similar to those under H2-free conditions (and similar to their effects in the CSR-CRI conditions). Fe-rich samples were mostly more reactive that SiO₂-rich samples. The Feii rich samples also showed an increase in reactivity with H₂-containing gas compared to H₂- free conditions. The analogue alumina sample also showed an increase in reactivity using the H₂-containing gas.
• The reactions of the minerals with either the carbon or atmosphere were found to be very similar to those under H₂-free conditions, making it unlikely that these were fully responsible for the increase in reactivity of the samples. Gasification of carbon by H₂ to form hydrocarbons, in addition to gasification by CO₂, or possibly the water gas shift reaction affecting the Boudouard reaction may also have contributed to the increase in reactivity of the cokes and analogues in the H₂-containing gas. However, more work would be required to better understand these effects.
• 1600°C, 26% CO-14% H₂-60% Ar: the effects of minerals on reactivity were different to those found under H₂-free conditions. The highest reactivities were found in samples that contained SiO₂ (coke 114 and coke 131, analogue 114 and analogue 131, and the analogue quartz sample). These samples also all showed a distinct increase in reactivity over that found in the H₂-free conditions. It was unclear whether this increase in reactivity was caused by an increase in reduction of SiO₂ to either SiC(s) or SiO(g), or was due to an increase in gasification of carbon in the coke or analogue by H₂. Further work would be required to better understand these effects.

Examining the changes in the minerals in the coke and coke analogue samples after reactions by XRD, it was found that the changes under the H₂-injection, low-CO₂ BF conditions were found to be very similar to those under H₂-free conditions. At 1100°C, H₂O likely helped in the oxidation of the metallic iron species in the cokes and analogues with coke ash samples. This oxidation of metallic iron was most clear in the analogue magnetite sample. At higher temperatures, H₂ likely contributed to the reduction of SiO₂ and Al₂O₃.

The secondary aim of the project was to examine the low temperature reactivity of coke in the H₂- injection, low-CO₂ BF conditions. This was tested by reacting coke analogue with no mineral addition in the 86% CO₂-14% H₂O gas at temperatures from 800°C-1100°C. It was found that while reaction in the H₂O-containing gas was faster at 1100°C, at lower temperatures the differences were minimal.