Moving Hearth Technologies

Moving Hearth Technologies: A Proven Path to Green Iron Production

Downstream Consulting Pty Ltd

July, 2025

The steel industry's journey toward decarbonization doesn't require reinventing the wheel. Moving hearth technologies – including rotary hearth furnaces, linear hearth systems, and rotary kilns – have been quietly operating for decades, processing millions of tons of iron annually. What's changed is our recognition of their unique suitability for green iron production and their role as practical stepping stones in the industry's sustainability transition.

The Moving Hearth Advantage: Proven Technology, New Applications

Moving hearth processes share fundamental characteristics that distinguish them from static blast furnace technology. Whether the hearth rotates in a circle, moves linearly, or tumbles in a kiln, these systems provide continuous, controlled processing with unprecedented flexibility in feedstock handling and process conditions.

Feedstock Flexibility Meets Sustainability: These technologies excel at processing materials that traditional blast furnaces struggle with – fine iron ores, recycled mill scale, iron-bearing dusts, and green pellets. This flexibility becomes crucial for green iron production, where unconventional feedstocks and sustainable reductants must be accommodated without compromising process efficiency.

Temperature Control for Hydrogen Integration: Operating at 1000-1400°C rather than blast furnace temperatures of 1600°C+, moving hearth processes provide the ideal thermal environment for hydrogen-based reduction. The controlled atmosphere and moderate temperatures optimize hydrogen utilization while minimizing energy requirements.

Continuous Processing Philosophy: All moving hearth technologies embrace continuous operation with precise residence time control. This consistency is essential for green iron production, where maintaining optimal reduction conditions with alternative reductants requires steady-state operation.

The Green Iron Transition Strategy

Moving hearth technologies offer multiple pathways for transitioning to green iron production, allowing operators to implement sustainability measures progressively rather than requiring massive overnight changes.

Phase 1: Alternative Reductants Existing moving hearth installations can immediately begin incorporating sustainable reductants. Biochar or biomass from agricultural waste, for example, can replace coal in self-reducing pellets. Research demonstrates that biochar's high reactivity and low impurity content make it particularly suitable for hearth-based reduction processes.

Phase 2: Hybrid Fuel Systems The flexible heating systems in moving hearth processes enable gradual transition from natural gas to hydrogen-natural gas blends. Operators can start with low hydrogen percentages and increase the ratio as supply infrastructure develops and operational experience grows.

Phase 3: Full Hydrogen Integration The ultimate green iron configuration uses hydrogen for both heating and reduction. However, moving hearth processes face practical challenges in maintaining pure hydrogen atmospheres due to sealing limitations. Air ingress through rotating seals, material feed points, and discharge areas can compromise hydrogen efficiency and create safety concerns. Successful implementations typically use hydrogen-rich atmospheres with positive pressure operation, nitrogen blanketing, or hydrogen-natural gas blends that are more tolerant of minor air contamination.

Phase 4: Renewable Integration When powered by renewable electricity – whether through electric heating or hydrogen produced via electrolysis – moving hearth processes can achieve near-zero carbon emissions. Some configurations with biochar and renewable energy can even result in net negative emissions.

Technical Realities: Addressing Hydrogen Implementation Challenges

While moving hearth technologies offer compelling advantages for green iron production, practical implementation faces significant technical challenges that must be addressed for successful hydrogen integration.

Air Ingress: The Critical Challenge Maintaining pure hydrogen atmospheres in moving hearth systems is complicated by inherent sealing difficulties. Rotary hearth furnaces must seal between stationary shells and rotating hearths, while rotary kilns face challenges with their open-end design and rotating shell interfaces. Even minor air ingress can be problematic since oxygen readily reacts with hydrogen, reducing efficiency and creating safety concerns.

Practical Engineering Solutions Successful hydrogen implementation requires adapted approaches rather than ideal conditions:

• Positive Pressure Systems: Maintaining slight overpressure with hydrogen-rich atmospheres minimizes air infiltration but increases hydrogen consumption and safety complexity

• Hybrid Atmospheres: Many operations use hydrogen-natural gas blends or hydrogen-steam mixtures that are less sensitive to air contamination while still achieving significant CO2 reduction

• Enhanced Sealing: Multiple barrier sealing systems, inert gas purging at leak points, and improved material handling designs can minimize air ingress

• Atmosphere Monitoring: Continuous oxygen level monitoring ensures process control and safety in real-world operating conditions

These engineering realities mean that commercial hydrogen applications in moving hearth processes typically operate with hydrogen partial pressures rather than pure hydrogen atmospheres, accepting some efficiency loss while still achieving substantial environmental benefits.

Technology-Specific Green Iron Pathways

Rotary Hearth Furnaces: The ITmk3 process demonstrates commercial-scale green iron potential. Its ability to produce high-grade iron nuggets (>96% Fe) using hydrogen makes it ideal for premium green steel applications. However, sealing the rotating hearth against air ingress requires sophisticated sealing systems and often limits operations to hydrogen-rich rather than pure hydrogen atmospheres.

Linear Hearth Systems: These technologies may offer sealing advantages over rotary systems since the linear movement can be more easily contained. The Paired Straight Hearth (PSH) process represents emerging high-productivity approaches to green direct reduced iron production, with potentially better atmosphere control for hydrogen applications.

Rotary Kilns: While their robust heat transfer characteristics and proven scale-up experience make kilns attractive for large-volume green iron production, their open-end design presents the greatest challenges for hydrogen atmosphere maintenance. Successful hydrogen integration typically requires hybrid atmospheres and enhanced safety systems to manage potential hydrogen-air mixing.

Overcoming Traditional Ironmaking Limitations

Moving hearth technologies address specific blast furnace constraints that hinder green iron adoption:

Raw Material Preprocessing: While blast furnaces require expensive sintering and coking operations that are inherently carbon-intensive, moving hearth processes can directly utilize green pellets and self-reducing briquettes containing sustainable reductants.

Scale Flexibility: Unlike blast furnaces that require massive scale for economic viability, moving hearth systems can be deployed at regional scales. This enables distributed green iron production closer to renewable energy sources and sustainable feedstock supplies.

Operational Responsiveness: The ability to quickly adjust to varying renewable energy availability or hydrogen supply fluctuations gives moving hearth processes a crucial advantage in green iron applications where energy sources may be intermittent.

Product Adaptability: Different steel grades require different iron specifications. Moving hearth processes can adjust product characteristics to match downstream requirements, optimizing the entire green steel value chain.

Practical Implementation Strategies

Waste-to-Value Integration: Many steel plants already operate moving hearth systems for processing iron-bearing wastes. These existing installations provide immediate opportunities to incorporate green reductants and test hydrogen integration strategies without major capital investment.

Modular Expansion: Operators can add moving hearth capacity incrementally as green iron demand grows and technology confidence builds. This approach reduces financial risk while building operational expertise.

Hybrid Plant Designs: Some facilities are exploring configurations that combine moving hearth green iron production with existing blast furnace operations, allowing gradual transition while maintaining production flexibility.

The Innovation Pipeline

Current research is expanding green iron possibilities for moving hearth technologies:

Microwave Integration: Rio Tinto's BioIron™ process combines biomass reductants with microwave heating in moving hearth systems, demonstrating how advanced heating technologies can enhance sustainability.

Plasma Applications: University research on hydrogen plasma reduction shows potential for more efficient green iron production. Plasma systems may offer advantages by reducing dependence on maintaining perfect reducing atmospheres, as the high-energy plasma state can overcome some limitations of air contamination in moving hearth systems.

Advanced Reductants: Beyond hydrogen and biochar, researchers are exploring other sustainable reductants including synthetic gases from renewable sources and novel biomass preparations.

Market Reality and Opportunities

The steel industry produces over 1.8 billion tons annually, making the transition to green iron both urgent and challenging. Moving hearth technologies won't replace all blast furnace capacity overnight, but they provide proven pathways for immediate progress.

Current moving hearth installations already process tens of millions of tons annually. Scaling these proven technologies for green iron applications represents a lower-risk pathway than developing entirely new processes. The technology foundation exists – the challenge lies in optimizing it for sustainable operation and scaling deployment.

Strategic Positioning for the Future

Organizations serious about decarbonization should view moving hearth technologies as mature platforms ready for green iron optimization while acknowledging the engineering challenges of hydrogen implementation. The sealing and atmosphere control issues are not insurmountable but require careful engineering and may limit the theoretical maximum efficiency of hydrogen utilization. Success lies in designing systems that work effectively with realistic rather than perfect operating conditions.

Success will come to operators who leverage existing moving hearth experience while systematically implementing green iron modifications. This evolutionary approach builds on decades of operational knowledge while achieving revolutionary sustainability improvements