Hydrogen Colors

Hydrogen is portrayed as being zero-emission rather than a zero-tailpipe emission solution. While it’s true that hydrogen only results in water vapor when used within a vehicle, that’s not telling of the whole story. To properly determine hydrogen’s lifecycle carbon impact, you must factor in those emissions that occur during production.

Hydrogen is both an energy carrier and an energy source ("natural hydrogen", also called "white" or sometimes "gold" hydrogen). With the exception of naturally occurring hydrogen, it must be produced from a primary energy source such as renewables, nuclear, or fossil fuels. Depending on how it is produced, hydrogen is assigned a color – and every color, a carbon impact spectrum. Is your supply of hydrogen carbon-intensive, low carbon, carbon-neutral, or our favorite: carbon-negative?

The use of colors to designate the carbon intensity of hydrogen is not ideal and has resulted in a growing range of colors, which can create more confusion than clarity. Ultimately, the goal is to understand the mass of hydrogen produced for a given mass of green house gas emitted to atmosphere.

Hydrogen colors

Grey hydrogen

This is the most common method of hydrogen production, which involves natural gas steam methane reforming (SMR). The process generates large amounts of carbon dioxide emissions, which contributes to climate change. Steam methane reformation accounts for 95% of current hydrogen production around the world. During the reforming process, methane (CH4) that’s sourced from natural gas reacts with steam (H2O) – resulting in hydrogen, carbon monoxide, and small amounts of carbon dioxide. What differentiates grey hydrogen from other forms that rely on SMR is that there is no carbon capture and sequestration (CCS) involved. This means that the resulting CO₂ and CO are released directly into the atmosphere. Yet, grey hydrogen is only considered to have a moderate greenhouse gas (GHG) impact – producing slightly fewer emissions than both black and brown hydrogen.

Blue hydrogen

Blue hydrogen is sourced from fossil fuel. This method of hydrogen production is similar to grey hydrogen, but it captures and stores the carbon dioxide emissions. The process is called carbon capture and storage (CCS), and it reduces greenhouse gas emissions significantly. When it comes to blue hydrogen, at the most basic level, it goes one step beyond grey, black, and brown hydrogen to lower its carbon impact. Not only does it use natural gas (or coal) in steam methane reformation to produce hydrogen, but also incorporates carbon capture and sequestration (CCS). What does this mean? The resulting carbon dioxide and carbon monoxide that is produced from SMR is captured before it can be released into the atmosphere and then stored underground. 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 avoid the creation of greenhouse gases.

Green hydrogen

Also known as renewable hydrogen – is produced from water, using a process called electrolysis. This process generates zero direct carbon emissions. This is arguably because there is also some sort of GHG emissions scope 2 and/or 3</ref>and is considered the most sustainable method of hydrogen production

This involves a strong electrical current – generated from renewable sources like solar or wind energy – being passed through purified water. The apparatus used to do this is called an electrolyser. The electrochemical reaction splits the water into its constituent elements, hydrogen and oxygen.

Because the electricity that powers the electrolyser comes from renewable sources, and the only by-product of the process is oxygen, the production of green hydrogen releases no greenhouse gas emissions. It’s a sustainable and environmentally friendly means of hydrogen production.

Hydrogen can be stored for long periods of time. This means that green hydrogen production can use the excess solar and wind energy generated on sunny and windy days, and then convert the energy produced by these intermittent resources back into electricity when the market needs it, helping to firm the electricity grid.

Green hydrogen is relatively expensive to produce, which is why it currently makes up a small percentage of overall hydrogen production. But as renewable energy prices fall and electrolysers become cheaper and more efficient, it’s expected to become increasingly common.

Yellow hydrogen

Yellow hydrogen uses mixed-origin grid energy as its feedstock. Depending on what is available, it can be supplied by a variety of sources including solar, alternative renewables, and fossil fuels. To produce yellow hydrogen, these sources of grid electricity undergo electrolysis – a process that uses an electric current to split water molecules into hydrogen and oxygen. The average carbon impact of yellow hydrogen is moderate, though it can fluctuate based on the energy used to make it. When electricity sourced from coal or conventional natural gas is used, the resulting hydrogen’s carbon intensity (CI) score will be higher. Conversely, when electricity from renewable sources is used, the CI score will decrease accordingly.

Turquoise hydrogen

Turquoise hydrogen is produced from a process called pyrolysis. Pyrolysis occurs when methane (CH4) from natural gas is decomposed at very high temperatures to generate hydrogen (2H2) and solid carbon (C). Unlike prior selections, turquoise hydrogen does not produce carbon dioxide or carbon monoxide – giving it a low-carbon classification. However, this means of producing hydrogen is still in its experimentation phase and has not been proven at scale.

Turquoise hydrogen may be cheaper than electrolysis, but it still uses natural gas as a feedstock, which many consider problematic given how much methane is leaking during its production. But the gas industry will likely be pumping for this and we will be hearing more about it.

Brown hydrogen

This method of hydrogen production involves Coal gasification of Lignite, often referred to as brown coal, is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. This process generates large amounts of carbon dioxide emissions and other pollutants. It is not a sustainable method of hydrogen production and is not widely used.

Black hydrogen:

Black hydrogen is referred to hydrogen produced from Bituminous coal, or black coal gasification. Gasification relies on high temperatures, those exceeding 700°C or 1,292°F, to break down fossil-based or organic carbonic materials – all without combustion. Instead, using a controlled flow of oxygen and/or steam, black coal is broken down into hydrogen (H₂), carbon dioxide (CO₂), and carbon monoxide (CO). Although, when it comes to separating hydrogen from the other chemical compounds, the resulting CO₂ and CO are released into the atmosphere. For this reason, black hydrogen is associated with having a high carbon impact.

Pink hydrogen

Pink hydrogen is generated through electrolysis of water by using electricity from a nuclear power plant. Regardless of what color you use to describe it, this hydrogen is made the same way. Steam is extracted from nuclear power plants and passed through an electrolyzer, producing two outputs: pure hydrogen and oxygen. Since there are no direct carbon emissions that result from this process, the GHG impact of producing pink/purple/red hydrogen is minimal.

Purple hydrogen

Purple hydrogen is made though using nuclear power and heat through combined chemo thermal electrolysis splitting of water. This method of hydrogen production uses biomethane or biogas from organic waste to produce hydrogen through SMR or electrolysis with water. This process generates fewer carbon emissions than grey hydrogen and can help reduce waste.

Red hydrogen

Red hydrogen is produced through the high-temperature catalytic splitting of water, also called Thermochemical water splitting using energy from concentrated solar power or from the waste heat of nuclear power reaction as energy source. The chemical reactions produce hydrogen and oxygen from water. This is a long-term technology pathway, with potentially low or no greenhouse gas emissions.

White hydrogen

White hydrogen is naturally produced gas. Naturally occurring hydrogen is a form of geological chemical element found in the air and in underground deposits. White hydrogen is the natural form of H2 that is found in underground deposits released via fracking, and in the air.