Hydrogen is also the most abundant element in the universe. Stars such as the sun consist mostly of hydrogen. The sun is essentially a giant ball of hydrogen and helium gases. Hydrogen occurs naturally on earth only in compound form with other elements in liquids, gases, or solids.
Hydrogen, like electricity, is an energy carrier that must be produced from another substance. Hydrogen can be produced—separated—from a variety of sources including water, fossil fuels, or biomass and used as a source of energy or fuel. Hydrogen has the highest energy content of any common fuel by weight (about three times more than gasoline), but it has the lowest energy content by volume (about four times less than gasoline).
While the use of hydrogen as a fuel dates back more than 150 years, recent announcements have brought hydrogen back into the spotlight as leaders around the world seek to secure a sustainable energy future.
But not all hydrogen is made equal. Although hydrogen is actually a colorless gas, it is commonly referred to by a color to denote how clean it is: black, gray and brown being least clean, a cleaner blue and a true zero-emission green hydrogen.
Here is an explanation of the hydrogen “colors”:
Green Hydrogen. Generating green hydrogen nearly eliminates all carbon emissions because it uses clean, renewable sources. With this method, hydrogen is produced through the electrolysis of water, where electricity generated from solar panels or wind turbines is sent through water, releasing oxygen and (green) hydrogen.
Yellow Hydrogen is a relatively new phrase for hydrogen made through electrolysis using zero-emission solar panels.
Pink Hydrogen is generated through electrolysis powered by nuclear energy.
Purple Hydrogen is made though using nuclear power and heat through combined chemo thermal electrolysis splitting of water.
Red Hydrogen is produced through the high-temperature catalytic splitting of water using nuclear power thermal as an energy source.
White Hydrogen is a naturally-occurring geological hydrogen found in underground deposits and created through fracking. There are no strategies to exploit this hydrogen at present.
Blue Hydrogen is fossil fuel-based (see brown, black, or gray hydrogen), but the carbon emissions from the hydrogen generation process are greatly reduced by using carbon capture and storage. So while blue hydrogen reduces CO2 emissions (by as much as half), it does not go far enough to meet international guidelines for reducing greenhouse gas emissions.
Turquoise Hydrogen can be extracted by using the thermal splitting of methane via methane pyrolysis. The process, though at the experimental stage, remove the carbon in a solid form instead of CO2 gas.
Brown Hydrogen. Roughly 95% of hydrogen is fossil-based. Brown Hydrogen is made via steam-methane reforming, in which steam is used to produce hydrogen from a methane source like natural gas.
Coal gasification is the process of producing syngas—a mixture consisting primarily of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), methane (CH4), and water vapour (H2O)—from coal and water, air and/or oxygen.
Black Hydrogen is produced from the gasification process using coal, which was formed when dead plant matter decays into peat and is converted into coal by the heat and pressure of deep burial over millions of years.
Gray Hydrogen is produced from the gasification process using Lignite, a yellow to dark brown or rarely black coal that formed from peat at shallow depths and temperatures lower than 100 °C.
So while the steam methane reforming and gasification processes produce clean, zero-emission hydrogen fuel, they produce significant amounts of Earth-warming CO2 in the process, making them less-than-ideal choices for a truly clean hydrogen future.
Here is a quick graphic summary produced by NACFE: