Steam reforming is the most important industrial process for producing hydrogen from carbon-containing energy sources. The hydrogen produced in this way is referred to as gray hydrogen. This method (also known as steam reforming) currently accounts for about 95% of the hydrogen produced worldwide. Today’s plants achieve capacities of up to 100,000 cubic meters per hour.
Procedure
The process is based on the reaction of a carbon-containing energy source with steam. The main feedstocks are natural gas (methane), but methanol, light gasoline, liquefied petroleum gas (LPG), and naphtha are also used. The steam required can be supplied externally or derived from the feedstock itself. Carbon-containing solids, such as bituminous coal or lignite, can also serve as the basis for steam reforming. Their use results in “black” or “brown” hydrogen.
The endothermic reaction requires a continuous supply of heat, which is often provided by burning a portion of the feedstock or through catalytic processes. The process achieves an efficiency of 60 to 70 percent when converting natural gas to hydrogen. By comparison, the efficiency of PEM electrolysis is approximately 70 to 80 percent.
Process Diagram (© Mahler AGS GmbH)
Reaction Mechanism
- Reactor step: In the first step, natural gas (mainly methane) is heated with steam at high temperatures (700–1,000 °C) in a catalytic reactor. This process produces hydrogen and carbon monoxide.
CH₄ + H₂O → CO + 3H₂ CH₄ + H₂O → CO + 3H₂
2. Water-gas shift reaction: In the second step, the carbon monoxide reacts with additional water vapor to form carbon dioxide and more hydrogen.
CO + H₂O → CO₂ + H₂CO + H₂O → CO₂ + H₂
3. Gas purification: In the final step, the gas mixture is purified to remove carbon dioxide and other impurities.
Environmental Impact Assessment and Conclusion
Steam reforming generates significant CO₂ emissions: For every kilogram of hydrogen produced, approximately 9.24 kilograms of CO₂ are emitted. The “gray” hydrogen produced in this way is therefore facing increasing criticism. Various approaches are being pursued to improve its carbon footprint:
- Combining this with carbon capture and storage (CCS) results in “blue” hydrogen
- The gasification of biomass or biogas produces “orange” hydrogen with a better carbon footprint
- Alternative methods such as electrolysis produce “green” hydrogen and are therefore becoming increasingly important
Despite its negative impact on the climate, steam reforming remains the dominant method of industrial hydrogen production for the time being due to its efficiency and cost-effectiveness. In the long term, however, a growing shift toward more climate-friendly production methods is expected.
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