Hydrogen is commonly referred to when talking about low carbon energy sources. The reason for this? Unlike fossil fuels, when hydrogen is burned it solely emits water. While exploring methods to harness this fuel safely, a series of hydrogen production methods have been created. Each production method has been given an associated color.
There are nine main production methods with twelve associated colors: Green, Gray, Blue, Black/Brown, Turquoise, Pink/Purple/Red, Yellow, Orange, and White.
Green hydrogen is both the most popular and most effective low-carbon production method. The World Economic Forum says it is the only climate neutral production method. Gray, Black, and Brown are the most carbon intensive processes. All the other colors emit far less but suffer from significant scaling issues. Below we provide a quick explanation of each.
Green Hydrogen
Green hydrogen uses a process called electrolization to split water molecules into oxygen and hydrogen. Assuming the energy used to complete electrolization is green, this production method releases very little carbon. Green hydrogen could offer an instrumental tool in decarbonizing but is currently less than 1% of hydrogen production. That is because of the prohibitively high cost of electrolyzers and the inconsistency of power from solar and wind sources.
Gray Hydrogen
Ninety five percent of hydrogen produced in the US is gray hydrogen which is a significantly cheaper than green but far more carbon intensive production process. Gray hydrogen is produced through “steam methane reforming” where natural gas/methane is mixed with high-temperature steam to produce hydrogen. For every ton of hydrogen created the process emits 10 tons of CO2.
Blue Hydrogen
Blue hydrogen uses the exact same production process as gray hydrogen but the CO2 emitted is captured and stored underground. It has the exact same emissions intensity, just is offset by carbon capture and storage (the viability of this technology is still unknown).
Black & Brown Hydrogen
Black and brown hydrogen are the most carbon intensive forms of hydrogen production. This process of gasification is the same as gray hydrogen but instead of using gas, it uses coal. Only 4% of US hydrogen is produced using coal gasification but represents 22% of global hydrogen production.
Turquoise Hydrogen
Turquoise hydrogen is the newest form of hydrogen. It uses a process called ‘methane pyrolysis’ which is a similar to the gray hydrogen production process. But, instead the steam production is done through electricity. Additionally, CO2 output is in a solid form which can then be recycled. This process is cheaper than producing green energy but creates dependence on the natural gas used in the production process.
Pink, Purple, and Red Hydrogen
Pink, purple, and red hydrogen utilize the same hydrolysis process of green hydrogen but use different parts of nuclear energy production to produce hydrogen. Pink uses the electricity generated by nuclear power production to split the water, red uses thermolysis instead of hydrolysis (essentially red uses the heat produced from the plant to separate water molecules), and purple uses a mixture of both of these processes. These terms appear to be used interchangeably.
Yellow Hydrogen
Yellow hydrogen has differing definitions. It was initially defined as hydrogen produced using energy from solar power. Since the expansion of hydrogen colors, it has been redefined as hydrogen produced through energy from the electric grid. Thus, the power generation source for the electric grid determines the carbon-intensity of the yellow hydrogen.
Orange Hydrogen
Orange hydrogen is made by injecting water into a targeted geological formation, it both produces hydrogen and also sequesters carbon. The water injections are enriched with carbon dioxide and produces solid carbonates instead of carbon emissions. It has been simulated at the French National Center of Scientific Research but has not been scaled up.
White Hydrogen
The only natural form of hydrogen is white hydrogen. It occurs naturally under the earth’s surface.