Hydrogen from Underground Coal Gasification in Supercritical Water: CCD-RSM Optimization

Deep-seated coal resources, exceeding 3 trillion tonnes in China alone, remain largely unexploited due to the technical and economic limits of conventional mining. Supercritical water underground coal gasification (SCW-UCG) offers a promising route to unlock these stranded reserves while producing hydrogen-rich syngas. To investigate the optimized process for hydrogen production from coal gasification under supercritical water (SCW) conditions, this study employed a batch-type high-temperature and high-pressure autoclave using pulverized coal as feedstock. Four critical parameterstemperature (380-480 °C), initial pressure (1.0-2.6 MPa), moisture content (80-90%), and reaction time (30-90 min)were systematically examined for their effects on gasification product distribution. The experimental design adopted a four-factor five-level central composite design (CCD) within response surface methodology, comprising 30 experimental runs. Product composition was analyzed via gas chromatography, and quadratic regression mathematical models were established for both hydrogen (H₂) yield and total hydrocarbon gas (C₁-C₄) production. Analysis of variance demonstrated that both models achieved high statistical significance (P < 0.001). Reaction temperature emerged as the most decisive factor influencing H₂ yield and total hydrocarbon gas production, with its effect substantially exceeding that of other parameters. The optimal process window was determined as approximately 480 °C, 60 min, 85% moisture content, and 1.8 MPa, achieving a H₂ molar production of 1.769 mmol/g and a mole fraction of 51.63%. This research provides systematic experimental data and theoretical foundations for process parameter optimization and mechanistic understanding of hydrogen production via supercritical water underground coal gasification technology.