(107g) Production of Ferro-Hydrogen Via Ammonia Decomposition Using Hot Direct Reduced Iron

1. Introduction

A considerable amount of carbon dioxide (CO₂) is produced during the iron and steel manufacturing process, primarily because fossil fuels are used as reducing agents for iron ore. Accordingly, there is a global need for the production of CO₂-free hydrogen (H₂) to reduce iron ore in the ironmaking industry. Direct reduced iron (DRI), which is produced by reducing iron ore, is mainly composed of metallic iron (Fe). Fe is known to be active in the ammonia (NH₃) decomposition (Equation (1)), making DRI a potential catalyst candidate. The hydrogen produced through the decomposition of ammonia using DRI in ironmaking process is called “Ferro-Hydrogen”, which is an innovative hydrogen production way first suggested by the POSCO N.EX.T Hub of POSCO Holdings.

NH_{3 (g)} → N_{2 (g)} + 1.5H_{2 (g)} with ΔH^o_298K = 46 kJ/mol (1)

However, when the DRI is used as a catalyst for ammonia decomposition, a significant amount of iron nitride can form. In the field of steel manufacturing, a high nitrogen content in steel can diminish its toughness and ductility, and thus the addition of excessive denitrification processes can result in increased capital and operational expenditures (CAPEX & OPEX).

2. Methods

When using DRI as a catalyst for ammonia decomposition reactions, it is important to investigate not only the hydrogen productivity but also the nitridation characteristics of iron under various reaction conditions. Therefore, after reducing iron ore to DRI around 800°C under the hydrogen flow, we evaluated the ammonia conversion (Equation (2)) and nitridation behavior of iron species over the DRI. These evaluations were conducted with respect to reaction conditions such as ammonia/hydrogen co-feeding ratio, reaction temperature, and space velocity.

NH₃ conversion = 1- NH_3,out/NH_3,in (2)

Where NH_3,out and NH_3,in are the moles of NH₃ in the feed and outlet gases.

3. Results and discussion

When ammonia was decomposed using DRI, an ammonia conversion of over 90% was achieved. Furthermore, this ammonia conversion could be further enhanced by adjusting the process conditions (temperature, pressure, and space velocity). The formation of iron nitrides, such as Fe₄N phase, during ammonia decomposition is advantageous because it lowers the NH₃ dehydrogenation barrier, thereby enhancing ammonia decomposition. However, this nitridation occurs on the metal surface during ammonia decomposition at medium to high temperatures (> 450°C). This nitridation of the DRI could be suppressed by feeding additional hydrogen along with ammonia; nevertheless, its ammonia decomposition performance decreased as the partial pressure of hydrogen increased. We confirmed that the DRI was effectively active in ammonia decomposition within a specific range of ammonia/hydrogen co-feeding ratios. Furthermore, X-ray diffraction (XRD) and elemental analysis results of the DRI samples after ammonia decomposition revealed that the Fe₄N peak was barely detectable, and the nitrogen content in the DRI was also minimal.

4. Conclusion

This study demonstrates that Direct Reduced Iron (DRI) is an effective catalyst for ammonia decomposition, achieving over 90% conversion. The formation of iron nitrides, such as Fe₄N, enhances the process by lowering the NH₃ dehydrogenation barrier. However, nitridation at higher temperatures can be mitigated by co-feeding hydrogen, despite a decrease in performance with increased hydrogen partial pressure. Optimal ammonia/hydrogen ratios minimize nitridation, as confirmed by XRD and elemental analysis. These findings support the viability of the DRI for ammonia decomposition, contributing to the innovative production of "Ferro-Hydrogen" and advancing CO₂-free hydrogen production in the steel industry.

Acknowledgments

This research was supported by the National Research Council of Science & Technology(NST) grant by the Korea government (MSIT) (No. GTL24051-200).