Burning petrol and other forms of fossil fuel to power transport currently accounts for around 20% of global carbon dioxide (CO2) emissions. This has to change if we are to achieve zero greenhouse gas emissions. And even if we only want to reach net zero (that is, we still emit greenhouse gas but capture and store some of it later) then a reduction in transport emissions will still be necessary.
In addition, we burn natural gas for many other purposes, such as heating our homes and creating heat for industrial purposes.
Hydrogen is believed to be a clean alternative. When hydrogen is burnt it combines with oxygen to produce only water (H2O), so no “greenhouse gas” is produced. However, we should note that water vapour is also a greenhouse gas, but its activity is complicated. We will deal with this is a separate article.
In order to replace fossil fuels with hydrogen, we will need to either find or generate vast quantities of hydrogen.
Unfortunately, although hydrogen is the most common substance in the universe, on earth it is almost always found combined with other atoms. Water is a typical example.
“Colours” of hydrogen
So far people have found several different ways of obtaining pure hydrogen. The resulting hydrogen all consists of the same thing: H2 molecules forming a colourless gas. However, it is given different names depending how it was formed.
Some is referred to as green, some as grey or blue, as brown, black, pink, or turquoise, yellow or white. What do these different names mean?
Here is a brief guide.
Naturally occurring hydrogen
White hydrogen is a naturally occurring gas consisting mostly or exclusively of hydrogen molecules. It is the common substance in the universe, making up about 73% of all visible matter. Stars, for example, consist mostly of white hydrogen.
People used to think that it was extremely rare on earth, but recently quantities have been found in underground deposits. It was formed deep within the earth’s mantle and carried up to the surface by geological movements. Some people are beginning to search for and exploit these deposits.
Hydrogen from fossil fuels
Grey: hydrogen from natural gas with release of CO2. Most common form of hydrogen
This is generated from “natural gas”. Like petroleum and other forms of crude oil, natural gas was formed by the rotting of ancient plant in swamps hundreds of millions of years ago. It consists of a mixture of methane (CH4) and other small molecules consisting of carbon and hydrogen.
Heat is used to split these molecules into hydrogen (H2) and carbon dioxide (CO2). The hydrogen is captured and the CO2 is released into the atmosphere, so adding to global warming. The heat is generated by burning more natural gas and so producing even more CO2 in the process.
Currently about 95% of hydrogen production uses this process.
Blue: grey hydrogen with storage of CO2 underground
This, like grey hydrogen, is made by splitting natural gas but the CO2 is captured and stored rather than being released into the atmosphere.
Turquoise: grey hydrogen with CO2 converted to solid
This is the result of splitting natural gas by the process of “pyrolysis”.
Brown: hydrogen from “brown coal”
Brown hydrogen is made by heating “brown coal” in the absence of oxygen. Brown coal is a sedimentary rock formed from peat. Heating it breaks brown coal down to form H2 and many other molecules. The gasification. It’s an established process used in many industries that converts carbon-rich materials into hydrogen and carbon dioxide as well as a number of other gases.
Unwanted by-products are normally released into the atmosphere. However, if those emissions can be stored then the hydrogen can sometimes be called blue.
Black: hydrogen from “black coal”
Black hydrogen is the same as brown hydrogen except the source is black coal rather than the brown variety. Black coal is coal that contains bitumen or asphalt. It is also called bituminous coal.
Hydrogen from Electrolysis
We saw above that when you burn hydrogen it combines with oxygen to produce water (H2O). In this process, energy is released.
The process is reversible. If we add energy we can break apart the water into its constituent parts: oxygen and hydrogen. There are various possible sources for this energy and so the hydrogen that is produced is given different names.
The easiest way to add that energy is by using electricity. The process is called “electrolysis” which means “splitting by electricity”.
Note that electricity is expensive, and this is the major problem holding back the use of hydrogen as a fuel.
They “colour” of the hydrogen produced by electrolysis depends on the source of the electricity.
Green hydrogen: energy from renewable sources
This is made by using green electricity to split water (H2O) into hydrogen and oxygen molecules. Green electricity is generated without adding to global warming. Currently only about 1% of hydrogen production uses this process. 99% is therefore not green. This is because green hydrogen is much more expensive to produce than all the other “colours”.
Pink: energy from nuclear reactors
Some people might consider nuclear reactors as a green source of energy, but nevertheless hydrogen produced by nuclear-generated electricity is called pink rather than green.
Yellow hydrogen: energy from a mix of sources
This hydrogen is produced by electrolysis but the electricity can come from anywhere.
Commonwealth Scientific and Industrial Research Organisation: https://blog.csiro.au/green-blue-brown-hydrogen-explained/