Thermodynamic analysis of solar thermochemical hydrogen production using coal-based reductants

The two-step solar thermochemical cycle is a promising solar energy technology that converts solar energy into chemical energy with high theoretical efficiency. It not only enables efficient energy storage and transportation but also maximizes the advantages of solar energy, offering strong support for sustainable energy transformation. Nevertheless, this technology is confronted with issues such as the high reduction temperature and high energy consumption for reducing oxygen partial pressure, which restricts the further development of thermochemical cycles. In this paper, a thermochemical cycle hydrogen production method utilizing coal-based reductants is proposed to address the aforementioned challenges. The preparation of coal-based reductants is analyzed from both material and energy perspectives. Three coal-based reductants—carbon (C), carbon monoxide (CO), and methane (CH₄)—are investigated for their auxiliary effects and compatibility with various oxygen carriers in thermochemical cycles. A comparative analysis of different reductant-oxygen carrier pairs is conducted using indicators such as Gibbs free energy, equilibrium components, reductant conversion rate, and thermochemical cycle efficiency. The results indicate that the coal conversion efficiency can reach 89 % when the coking temperature is 700 °C and the gasification temperature is 1100 °C. The addition of reductants significantly reduces the reduction temperatures of iron-based, tin-based, zinc-based, and cerium-based oxygen carriers. The oxygen carriers Fe₃O₄/FeO and SnO₂/SnO are used for gas and solid reductants, respectively, achieving a solar-to-fuel conversion efficiency of 60.3 %, with a hydrogen production efficiency of up to 65.9 %. This study provides reference value for the research in the domain of coal clean utilization and the participation of reductants in thermochemical cycles.