Enhancing Thermal Energy Storage with Modified Steel Slag: A Sustainable Solution

The search for effective sustainable energy solutions has gained increasing momentum as the world struggles with the challenges posed by climate change and the limited availability of fossil fuels. One promising avenue for addressing these issues is the development of thermal energy storage systems, which can effectively store and release thermal energy, making it possible to harness intermittent and renewable energy sources like solar and wind power efficiently. These systems are vital for ensuring a stable and reliable energy supply, especially in regions where these renewable sources are abundant. Thermal energy storage (TES) encompasses various technologies designed to store thermal energy in different forms, including sensible heat storage (SHS), latent heat storage, and thermochemical heat storage. Among these, SHS is the most established and widely employed method due to its maturity and cost-effectiveness. However, the quest for high-performance SHS materials at a reasonable cost is ongoing, as the existing materials often fall short in terms of thermophysical properties and economic feasibility. A recent study published in the Journal of Cleaner Production, led by student Junlei Wang and Dr. Yun Huang from the State Key Laboratory of Multiphase Complex Systems at the Institute of Process Engineering – Chinese Academy of Sciences. The authors investigated the potential of utilizing recycled solid waste resources, specifically steel slag, as a sensible heat storage material for thermal energy storage. Moreover, it introduces a novel modification process using sodium carbonate (Na₂CO₃) to enhance the thermal properties of steel slag.

Solid waste materials have gained recognition as valuable resources for thermal energy storage applications, primarily because they offer a dual benefit. Firstly, they can enhance the efficiency and cost-competitiveness of renewable energy systems and waste heat recovery processes. Secondly, they contribute to the reduction of processing costs for solid waste management and mitigate environmental problems associated with landfills. Several types of solid waste materials, including fly ash, asbestos waste, concrete, and metallurgical slag, have been extensively studied for their potential in medium-high temperature heat storage systems. Metallurgical slag, particularly steel slag, holds great promise as a sensible heat storage material. Steel slag is a byproduct of steelmaking processes, characterized by its porous structure and excellent thermal stability. It primarily consists of oxides such as calcium oxide (CaO), silicon dioxide (SiO₂), iron oxide (Fe₂O₃), alumina (Al₂O₃), magnesium oxide (MgO), and other compounds. Despite its potential, steel slag has been underutilized, with less than 30% of the annual production in China being effectively utilized. Most of it ends up in landfills, posing significant environmental challenges. Previous research efforts have focused on electric arc furnace steel slag, demonstrating its stability at high temperatures and its potential for sensible heat storage applications. However, little research has explored the use of converter steel slag, and there have been limited attempts to modify steel slag to improve its thermal performance.

In the study conducted by Wang and Dr. Huang, converter steel slag was examined for its suitability as a sensible heat storage material, and a novel Na₂CO₃ modification process was developed to enhance its thermal properties. The authors had several key findings emerged from their research, first the X-ray fluorescence analysis revealed that the main elements of steel slag  in terms of oxides are CaO, SiO₂, Fe₂O₃, Al₂O₃, and MgO. X-ray diffraction analysis of the slag both before and after sintering identified various mineral phases. After sintering, the primary phase of the modified steel slag was identified as Na₂CaSiO₄, indicating that molten Na₂CO₃ reacted chemically with the steel slag at high temperatures. They conducted thermodynamic analysis of the possible reactions between steel slag components and carbonates, specifically Li₂CO₃, Na₂CO₃, and K₂CO₃, provided insights into the modification process. Na₂CO₃ was chosen for the experimental study due to its cost-effectiveness and potential for enhancing thermal properties. Thermodynamic calculations suggested that Na₂CO₃ could react with steel slag components to form Na₂CaSiO₄, enhancing its thermal performance. Moreover, the researchers tested the thermal cycle stability of various sodium carbonate-modified steel slags (SMSs), and SMS-35 was identified as having the best thermal cyclic stability. SMS-35 exhibited no significant changes in morphology after 200 thermal cycles, making it a promising candidate for further investigation. Furthermore, when the researchers conducted differential scanning calorimetry and thermogravimetric analysis to evaluate the thermal properties of both steel slag and SMS-35. The equivalent specific heat of SMS-35 was found to be 25.32% higher than that of steel slag in the range of 400–900°C. Furthermore, the thermal conductivity of SMS-35 significantly surpassed that of steel slag, showing a 32.7% increase at 500°C. These improvements are attributed to the modification process, which altered the material’s composition and structure, resulting in enhanced thermal performance. According to the authors, after 200 thermal cycles, SMS-35 exhibited excellent thermal cycle stability. Its morphology remained unchanged, and the phase composition was consistent before and after the thermal cycles. The equivalent specific heat values for SMS-35 across different temperature segments tended to be consistent after thermal cycling, indicating that the internal structure of SMS-35 became more stable, and the reaction between steel slag and Na₂CO₃ became more complete.

In conclusion, the study led by Wang and Dr. Huang presents a compelling case for the utilization of steel slag as a sensible heat storage material for thermal energy storage applications. Through the innovative modification process using Na₂CO₃, the thermal properties of steel slag were significantly enhanced, making it a promising candidate for concentrated solar power and industrial waste heat recovery systems.