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Grey hydrogen

Grey hydrogen hydrogen colours natural gas Steam reforming CO2 emissions Industry

Grey hydrogen is hydrogen produced from fossil energy sources. The carbon dioxide released in the process is neither captured nor permanently stored. It usually refers to hydrogen from natural gas. The most important process is steam reforming. In it, methane reacts with steam to form hydrogen and carbon monoxide. In a downstream shift reaction, the carbon monoxide reacts with further steam to form additional hydrogen and CO₂. The gas itself is chemically the same substance as with any other production route. The colour therefore does not describe a property of the molecule, but how it was produced and what climate impact this has. It belongs to the classification system of hydrogen colours.

Emissions per kilogram of hydrogen

What usually matters is the emissions intensity, that is, the amount of greenhouse gases per kilogram of hydrogen. This depends on the system boundary. According to the International Energy Agency (IEA), steam reforming without capture emits around 9 kg CO₂ per kg H₂ directly at the plant site. If the upstream chain of the natural gas is included, i.e. extraction, processing and transport including methane losses, the IEA arrives at around 10 to 12 kg CO₂-equivalent per kg H₂. In an earlier analysis, it gave a range of 10 to 14 kg CO₂-equivalent for hydrogen from natural gas. Depending on the origin of the gas, 1 to 5 kg of this stems from the upstream chain. The widely cited figure of "9 to 12 kg CO₂" therefore mixes two different system boundaries: the lower value applies to direct on-site emissions, the upper value includes the upstream chain.

A simple calculation shows where the emissions come from. Stoichiometrically, one molecule of methane with two molecules of water yields four molecules of hydrogen and one molecule of CO₂. The feedstock alone thus produces about 5.5 kg CO₂ per kg of hydrogen. In addition, there is the CO₂ from the natural gas that serves as fuel and supplies the heat for the strongly endothermic reaction. In this process, natural gas is therefore both feedstock and energy source for the process heat. Methane losses in the upstream chain weigh heavily, because methane is a much more potent greenhouse gas than CO₂. The problem of methane slip therefore also shapes the assessment of grey hydrogen. Looking only at the plant site does not capture the full climate impact.

Production and distinction from other hydrogen colours

Besides natural gas, coal can also be used as a feedstock. According to the IEA, hydrogen from coal gasification without capture causes around 22 to 26 kg CO₂-equivalent per kg H₂, i.e. about twice as much as the natural-gas-based route. Depending on the source, it is called grey, brown or black. In German usage, "grey" usually means hydrogen from natural gas without CO₂ capture. Worldwide in 2023, just under two thirds of production came from natural gas and about 20 percent from coal, in each case without capture. A further share arises as a by-product of other processes, for example in refineries. It cannot be clearly assigned to a colour.

Green hydrogen is produced by electrolysis using electricity from renewable energy. Here the footprint depends above all on the origin of the electricity. Blue hydrogen also uses natural gas, but captures part of the CO₂ and stores it permanently. How climate-friendly it is depends on the capture rate, storage integrity, additional energy demand and the methane emissions of the upstream chain. Turquoise hydrogen is produced by methane pyrolysis: methane is split into hydrogen and solid carbon. Here too, the actual process and energy balance decides, not the colour name. For applications, the H₂ molecule is always the same. For climate policy, support schemes and procurement, however, the production route is what counts.

Significance in industry and the energy system

Grey hydrogen is today the industrial norm. According to the IEA's Global Hydrogen Review 2025, global demand rose to almost 100 million tonnes in 2024, about 2 percent more than in the previous year. The increase was driven by the traditional users, i.e. refineries and industry. Refineries need hydrogen, among other things, for desulphurising fuels and for cracking heavy oil fractions. In the chemical industry, it is a basic material for ammonia, methanol and other products. Low-emission hydrogen has so far covered only a very small part of demand. According to the IEA, global hydrogen production caused around 920 million tonnes of CO₂ in 2023.

Many of these processes have developed over decades around cheap fossil hydrogen, natural gas infrastructure and continuous operation. There, hydrogen serves predominantly as a chemical feedstock, not as an energy carrier. If grey hydrogen is to be replaced by electrolytic hydrogen, additional electricity demand arises. This cannot be assessed merely as an annual quantity. Electrolysers can operate flexibly if storage, offtake profiles and market rules allow it. They can then absorb electricity from periods of high wind and solar generation and contribute to the system's flexibility. Without this flexibility, new electrolysis increases load, may make grid expansion necessary and may demand electricity precisely when renewable generation is scarce.

From an energy-economics perspective, replacing existing grey volumes must be assessed differently from new demand in applications that have so far needed no hydrogen at all. If one kilogram of grey hydrogen is replaced in ammonia production, emissions fall immediately by the roughly 10 to 12 kg CO₂-equivalent mentioned above. For new applications, the reduction depends on what is displaced there.

Economic assessment

Grey hydrogen is usually cheaper than low-emission alternatives. The plants are often fully depreciated, the natural gas supply chains are well established, and the climate costs are not fully reflected in the price. The costs depend mainly on the natural gas price, because natural gas is both feedstock and fuel. Plant efficiency and CO₂ costs also play a role. In the EU, the emissions trading system covers hydrogen production in larger installations. Rising CO₂ prices thus worsen the economics of grey hydrogen. How much depends on free allocation and on international competition.

Anyone replacing grey hydrogen often changes more than just an energy source. Contracts, plant operation, security of supply, certification and product costs also change. For basic materials such as ammonia or steel, even small differences in energy costs can strongly influence competitiveness and location decisions.

Legal classification in the EU

Colour names are not legal terms. EU law works with emission thresholds. Renewable fuels of non-biological origin, the RFNBOs, and low-carbon fuels must cause at least 70 percent lower greenhouse gas emissions over their entire life cycle than a fossil comparator of 94 g CO₂-equivalent per megajoule. The methodology for low-carbon fuels is laid down in Delegated Regulation (EU) 2025/2359 of 8 July 2025. It supplements the Gas Directive (EU) 2024/1788 and was published in the Official Journal on 21 November 2025. For the fossil comparator, it expressly refers to the RFNBO rules of Delegated Regulation (EU) 2023/1185.

By way of illustration: based on the lower heating value of about 120 MJ per kg, 94 g per MJ corresponds to around 11.3 kg CO₂-equivalent per kg of hydrogen. This lies exactly in the range of grey hydrogen. The 70 percent threshold of 28.2 g per MJ corresponds to about 3.4 kg CO₂-equivalent per kg. Grey hydrogen does not meet it. Blue hydrogen meets it only if the capture rate and upstream emissions are right. Grey hydrogen is not prohibited in the EU. However, it counts neither as renewable nor as low-carbon and therefore does not count towards the corresponding quotas and proofs.

Common misconceptions

  • "Hydrogen is automatically climate-friendly." No CO₂ is produced when it is used, but it is produced, to a considerable extent, in the production of grey hydrogen. What counts for the climate impact is the entire chain.

  • "Grey hydrogen is only a transitional issue." The current stock is large and continues to grow. New projects reduce emissions effectively only once they displace existing fossil volumes.

  • "The colour replaces the footprint." Blue hydrogen is not automatically climate-neutral, and green hydrogen does not automatically serve the electricity system. Only emission factors with a clearly defined system boundary are reliable.

  • "Hydrogen consumption is energy consumption." A large part of today's grey volumes is chemically bound, for example in ammonia or methanol. It cannot simply be equated with electricity or fuel consumption.

Grey hydrogen is therefore not a marginal category, but the fossil starting point of many hydrogen debates. The term shows how many emissions are embedded in existing industrial value chains and which changes are necessary if hydrogen is to contribute to decarbonisation.

Sources

Related glossary entries

Edited by eCMS on 12.11.2026 05:05

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