Heat Recovery and Gasification Processes in Molten Slag Systems

Molten slag, the high‐temperature by-product of metallurgical operations, contains substantial thermal energy at temperatures often exceeding 1 500 °C. Harnessing this energy through heat recovery and gasification transforms waste heat into valuable chemical or mechanical work, reducing both fuel consumption and greenhouse gas emissions. Two principal approaches dominate current research: physical heat transfer via specialised exchangers and chemical conversion through endothermic gasification reactions. In the former, slag’s low thermal conductivity and intermittent availability drive the design of robust heat‐exchange surfaces and heat carriers. In the latter, molten slag serves simultaneously as heat source, reactant medium and catalyst, enabling steam or CO₂ gasification of carbonaceous feedstocks—char, coal, biomass or sewage sludge—into syngas rich in CO and H₂. Recent advances have elucidated thermodynamic equilibria, kinetic mechanisms and catalytic roles of slag constituents (FeOx, CaO, MgO), yielding optimised parameters for temperature, steam-to-carbon ratio and particle residence time. Integration schemes now envisage circular networks in which slag‐driven gasification supplies syngas for direct‐reduction iron, power generation or hydrogen production, while residual solids contribute to cementitious or mineral products. Despite promising pilot demonstrations of energy savings and emission reductions, challenges remain in scaling continuous reactors, controlling slag crystallisation and achieving stable catalytic performance under industrial throughput. Further innovation in reactor design, process coupling and materials resilience is needed to realise the full potential of slag-based heat recovery and gasification in a decarbonising global steel and energy sector.

Research from Nature Portfolio

Recent studies have demonstrated the feasibility of using hot slag as both heat carrier and reaction medium for waste conversion. Investigations into sewage sludge gasification revealed that slags at around 920 °C enable staged syngas release—volatile evolution, char transformation and fixed-carbon reaction—with enhanced CO and H₂ yields under CO₂/O₂ atmospheres. This work highlighted sulfur retention in slags and stabilisation of toxic elements in residues, proposing integrated steel-sludge systems that achieve substantial annual coal savings and CO₂ reduction in large steel plants. Complementary research on coupling coal gasification with steel-slag heat recovery identified that slags not only transfer heat but also catalyse char gasification, lowering apparent activation energy from approximately 96 kJ mol⁻¹ to 12 kJ mol⁻¹. Oxidation of FeO to Fe₃O₄ within slag released additional energy and enabled magnetic separation of iron phases. Finally, a conceptual Fe-C-Ca cycle was formulated, linking metallurgical and cement industries through interdependent loops of carbon, iron and calcium. This cycle quantified theoretical potentials for energy saving, emission reduction and iron recovery, framing molten-slag gasification as a keystone of circular industrial ecosystems.

Heat Recovery and Gasification Processes in Molten Slag Systems publication trend

The graph below shows the total number of articles in heat recovery and gasification processes in molten slag systems across all publications each year (not limited to Nature Index journals).

Technical terms

Molten slag: A high-temperature liquid waste stream from metal smelting, rich in metal oxides and containing recoverable sensible heat.

Gasification: A thermochemical process converting carbonaceous materials into syngas (a mixture of CO and H₂) through reaction with steam, oxygen or CO₂ at elevated temperatures.

Syngas: Synthetic gas composed primarily of carbon monoxide and hydrogen, used as fuel or chemical feedstock.

Char: The solid carbonaceous residue remaining after initial devolatilisation of biomass or coal, subject to further gasification.

Activation energy: The minimum energy barrier that must be overcome for a chemical reaction—such as char gasification—to proceed.

Basicity: A measure of the ratio of basic to acidic oxides in slag, influencing catalytic behaviour and slag fluidity.

References

1. Achieving waste to energy through sewage sludge gasification using hot slags: syngas production. Scientific Reports (2015).
2. Integration of coal gasification and waste heat recovery from high temperature steel slags: an emerging strategy to emission reduction. Scientific Reports (2015).
3. A Fe-C-Ca big cycle in modern carbon-intensive industries: toward emission reduction and resource utilization. Scientific Reports (2016).
4. Thermodynamic Analysis of Hydrogen Production from Bio-Oil Steam Reforming Utilizing Waste Heat of Steel Slag. Processes (2023).
5. The Thermodynamic Characterizations of Hydrogen Production from Catalyst-Enhanced Steam Reforming of Bio-Oil over Granulated Blast Furnace Slag as Heat Carrier. Processes (2023).
6. Sludge Gasification Using Iron Bearing Metallurgical Slag as Heat Carrier: Characteristics and Kinetics. Energies (2022).