Development of a low carbon hydrogen-based economy is seen as a major part of achieving net-zero emissions across the world. Hydrogen is a flexible and potentially non-polluting fuel because it only produces water when it is used in fuel cells or burnt. However, the production of hydrogen can have a high carbon footprint depending on what methods of technology are used. In this article, from our On Hydrogen publication, Dr Amanda Lea-Langton explores the different types of hydrogen production and how its categorisation could be simplified to ensure low carbon delivery.

• The method of manufacturing hydrogen is a fundamental indicator of the hydrogen fuel’s carbon footprint.
• Scaling up the UK’s hydrogen economy can only be achieved if green hydrogen is affordable and abundantly available.
• Policymakers should consider refreshing the Low Carbon Hydrogen Standard and ramp up investment and incentives for renewable electricity to support the production of low carbon green hydrogen.

The energy in hydrogen can be released for use across a vast range of applications from transport to industry and heat. However, hydrogen is considered an energy storage system rather than an energy source itself. The production of hydrogen can have a high carbon footprint depending on what methods of technology are used. There are many ways that hydrogen can be produced from chemical reactions, electro-chemical methods, and nuclear production.

Using a colour system for identifying hydrogen according to the production process was initially a simple way to distinguish between highly polluting production, for example from natural gas (grey hydrogen) and cleaner, low carbon methods from renewable energy sources (green hydrogen). Over time, the spectrum of colours has become increasingly more complex according to more specific definitions of the production processes, with new definitions and colours, alongside new production options. For instance, green hydrogen is now generally only used for production of hydrogen using electrolysers powered by renewable energy, not for all low carbon technology options. Unfortunately, there is no global agreement on the definitions that are used, so specific production routes are sometimes assigned different colours, leading to confusion.

Colours and production – how hydrogen can be defined

Hydrogen gas is colourless, regardless of how it is made. A typical colour scheme to describe the various feedstocks and production processes used in hydrogen manufacture is shown below, although there are variations used in attribution of these colours.

Black or Brown

Established method of production from either black coal or brown coal (lignite) using gasification. Large amounts of CO₂ are released. This is the highest climate impact method.

Grey

Derived from natural gas (methane) using steam methane reforming. This is the most common production type used, with over 90% of hydrogen produced this way. Grey hydrogen has a high climate impact due to CO₂ release.

Blue

Typically derived from natural gas (methane) using steam methane reforming, with the addition of carbon capture and storage technologies (CCS) to store the CO₂ that is produced. This method still uses fossil fuels and is significantly more expensive than grey hydrogen.

Green

Green hydrogen is split from water using electrolysis, with the energy for the reaction coming from renewable sources such as solar or wind. It is considered to have minimal climate impact as no CO₂ is produced from electrolysis.

Purple or pink

This hydrogen is split from water using electrolysis, with the energy for the reaction coming from nuclear power. It produces minimal CO_2, but issues of nuclear waste storage and disposal need to be considered_.

Turquoise

Produced from the pyrolysis of natural gas or methane. The main by-product is solid carbon rather than gaseous CO₂. This method is so far unproven at larger scales.

White

White hydrogen is naturally occurring due to geological processes. Its abundance is unknown but deep geological reservoirs are believed to exist, and some mining projects are in operation, however the potential to extract this type without substantial losses is unproven.

The low carbon colour challenge – problems with colour terminology

Hydrogen in its elemental form is extremely rare in the environment, and so energy is required to produce it from other hydrogen-containing materials such as water or methane. More energy is required to produce and store hydrogen than can be recovered by its end use.

The method of manufacturing hydrogen is a fundamental indicator of the hydrogen fuel’s carbon footprint. Using a colour categorisation seems a simple way to understand the climate impacts, however, even within each colour category, the emissions of damaging climate change gases can be very variable.

Most of the hydrogen produced world-wide is in the form of grey hydrogen. This is a high energy and carbon intensive process, so the hydrogen produced in this way cannot be considered a low carbon source. Indeed, research has shown grey hydrogen can have a higher carbon footprint than using the feedstock natural gas directly. There is pressure from conventional energy providers to move towards blue hydrogen, which is the same production process with the addition of carbon capture and storage to mitigate the carbon footprint. Blue hydrogen as an intermediate solution facilitates a continued reliance on fossil fuel feedstock, which comes at the expense of long-term investment in more sustainable alternatives.

Green hydrogen, defined as using electrolysis of water with renewable electricity, offers a genuinely low carbon source. However, sufficient renewable electricity needs to be available at a competitive cost to ramp up its availability. Green hydrogen is extremely expensive compared to the fossil derived options. Electrolysis can also be powered by nuclear electricity production, which is also a low carbon option, although concerns about nuclear waste handling remain a factor.

Some scientists consider mining accumulations of naturally occurring hydrogen (typically ‘white’ hydrogen) as another low carbon source. However, deposits are likely to be deep within geological structures and extraction without substantial losses and environmental damage could be difficult.

It’s got to be green

To catalyse decarbonisation, there needs to be a transition towards low carbon methods over those with high climate impacts. A move towards green hydrogen over grey hydrogen will lower emissions, green over blue can lower costs associated with storage. Though there are challenges in terms of production, storage and expense, a straightforward first step would be a set classification of colour terminology which is informed by data on emissions intensity.

Bringing together evidence from research, energy stakeholders and government – including findings from recent hydrogen trials – can ensure colour terminology accounts for specific techniques, the relevant carbon footprints from production, and the threshold of emissions to define each colour, irrespective of the technological methods to simplify hydrogen classification.

In the UK, the Department for Energy Security and Net Zero’s Low Carbon Hydrogen Standard (LCHS) demonstrates an important tool in moving towards consensus on low carbon hydrogen. A further refresh of this standard, to include agreement around the intensity of emissions that demarks a particular colour classification or a move solely to an emissions intensity classification, is crucial to ensure low carbon delivery. However, to deliver the most impact, these classifications will need to be harmonised not only nationally but internationally too. The previous government acknowledged that a next step to introducing the LCHS would be to work on a pathway to international alignment for standards and certification. This might include working with stakeholders like the International Energy Agency and the International Organization for Standardization. The Department for Energy Security and Net Zero should convene an international working group to progress this important pathway to alignment and demonstrate leadership.

The previous also government made a start on moving away from high carbon impact hydrogen with the announcement of the largest number of commercial scale green hydrogen production projects. Policymakers must now build on this, ramping up investment and incentives for renewable electricity to support the production of low carbon green hydrogen over fossil fuel driven and CCUS-enabled blue hydrogen. Scaling up the UK’s hydrogen economy can only be achieved if green hydrogen is affordable and abundantly available.