Hydrogen can be produced in different ways. The most common method currently used is steam reforming, by which methane is broken down
to produce carbon dioxide and hydrogen. If the carbon dioxide is ultimately released into the atmosphere as a greenhouse gas emission, the hydrogen is termed “grey”. But if the carbon dioxide is stored
or reprocessed, the hydrogen can be referred to as “blue”. Grey and blue hydrogen are both used in industrial processes and for power production.
Blue hydrogen should be replaced by “green” hydrogen over the long term. This is produced by the process of electrolysis, which means that an electrical current is used to split water into its molecular components, hydrogen and oxygen. If the electricity used comes from renewable energy sources, the process is climate-neutral and the hydrogen produced is termed “green”.
Another option for making climate-neutral hydrogen is methane pyrolysis using energy from renewable sources. In this method, methane is split into hydrogen and solid carbon. Solid carbon is a granulate that can be safely stored in old mine shafts, for example, and used again later. The climate-neutral hydrogen produced by methane pyrolysis is called “turquoise” hydrogen.
fig. 8.4 > The four methods for producing hydrogen are distinguished by their source material, the energy source, the necessary production steps, and ultimately by the balance of emissions.