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Gray hydrogen is a high-emissions form of hydrogen produced using steam methane reforming

Gray hydrogen is produced through steam methane reforming (SMR), a process where natural gas, mainly methane (CH4), reacts with steam (H2O) at high temperatures (700–1,000°C) in the presence of a catalyst. This reaction yields hydrogen (H2) and carbon monoxide (CO), which is subsequently converted to carbon dioxide (CO2) through the water-gas shift reaction.1 The CO2 produced is typically released into the atmosphere, making gray hydrogen a significant source of greenhouse gas emissions.

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Footnotes​

  1. U.S. Department of Energy. "Hydrogen Production: Natural Gas Reforming." Office of Energy Efficiency & Renewable Energy. Accessed June 10, 2024. https://www.energy.gov/eere/fuelcells/hydrogen-production-natural-gas-reforming. ↩