Green hydrogen is made by using clean electricity from renewable energy sources, such as solar or wind power, to electrolyze water. Electrolysers use an electrochemical reaction to split water into its components of hydrogen and oxygen, emitting zero-carbon dioxide in the process.
Green hydrogen currently makes up a small percentage of the overall hydrogen, because production is very expensive. Just as energy from wind power has reduced in price, green hydrogen will come down in price as it becomes more common.
Blue hydrogen is produced mainly from natural gas, using a process called steam reforming, which brings together natural gas and heated water in the form of steam. The output is hydrogen – but also carbon dioxide as a by-product. That means carbon capture and storage (CCS) is essential to trap and store this carbon.
Blue hydrogen is sometimes described as ‘low-carbon hydrogen’ as the steam reforming process doesn’t actually avoid the creation of greenhouse gases.
Grey hydrogen is created from natural gas, or methane, using steam methane reformation but without capturing the greenhouse energy made in the process.
Currently, this is the most common form of hydrogen production.
There is also a gasification process that uses coal as a feedstock, creating brown hydrogen, which also releases carbon dioxide and can be put in the same category as grey. The process used to create hydrogen from natural gas is called steam methane reforming (SMR), where high-temperature steam (700°C–1,000°C) is used to produce hydrogen from a methane source, such as natural gas.
Brown and Black Hydrogen:
Brown hydrogen (made from brown coal) and black hydrogen (made from black coal) are produced through gasification.
These black and brown hydrogen are the absolute opposite of green hydrogen in the hydrogen spectrum and the most environmentally damaging. Any hydrogen made from fossil fuels through the process of ‘gasification’ is called black or brown hydrogen.
It’s a process used in many industries that converts carbon-rich materials into hydrogen and carbon dioxide. As a result, gasification releases those by-products into the atmosphere.
White hydrogen is a naturally occurring geological hydrogen found in underground deposits and created through fracking. Fracking is the process of drilling down into the earth before a high-pressure water mixture is directed at the rock to release the gas inside. Water, sand, and chemicals are injected into the rock at high pressure which allows the gas to flow out to the head of the well.
It is mostly found in gaseous form.
A new contender is turquoise hydrogen, this is called because the method of synthesizing it is regarded as sitting somewhere between green and blue hydrogen production.
This is a new entry in the hydrogen colour charts and production has yet to be proven at scale.
Turquoise hydrogen is made using a process called methane pyrolysis to produce hydrogen and solid carbon. Methane splitting is not a new concept, and a few commercial-scale plants already exist in North America.
The process uses natural gas purely as a feedstock, with all energy for heating and splitting methane coming from electricity. So if the electricity is sourced from renewable energy, the whole process is basically carbon neutral.