Hydrogen Production Rates in Ultramafic Rocks: Effect of pH

Summary Hydrogen is a clean fuel that can be burnt to produce energy without any carbon emissions. To minimize greenhouse gas emissions, humanity would need a significant amount of hydrogen as a fuel. The goal of this work is to understand the mechanisms of subsurface hydrogen production in ultramafic rocks by hydrothermal reactions. Hydrothermal reactions of peridotite rocks were conducted in stirred-tank and packed-bed reactors at 250°C and 1,200 psi. The effect of pH was studied. Micro gas chromatography (microGC) was used to analyze the produced gas for hydrogen (H2). The rocks were analyzed by scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS), X-ray fluorescence (XRF), and X-ray diffraction (XRD) before and after the reactions. The cations (Ca2+, Fe, Mg2+, and Na+) in the water were analyzed by inductively coupled plasma–optical emission spectroscopy (ICP-OES). The hydrothermal reaction produced a significant amount of H2, which varied with pH, with a minimum at neutral pH and a maximum under acidic conditions (0.80 ± 0.17 moles H2/kg of rock in 13 days). A packed-bed reactor of peridotite at 250°C and 1,200 psi yielded less H2 (0.16 moles H2/kg of rock in 13 days) than the stirred tank batch reactor for a similar amount of water and rock. The hydrogen production depends on pH (at a constant temperature) and is mass transfer-limited. A hydrogen production rate was estimated based on the estimated area of the packed bed, which is promising for hydrogen generation in ultramafic rocks at high temperatures.