Gray hydrogen is hydrogen gas produced from natural gas through a high-temperature industrial process called steam methane reforming (SMR), with no capture of the carbon dioxide released along the way. It accounts for the vast majority of the world’s hydrogen supply, with roughly 83% of all hydrogen still coming from fossil fuels, primarily through this route.1ScienceDirect. “Colors” of hydrogen: Definitions and carbon intensity The “gray” label exists mainly to distinguish it from cleaner alternatives on the growing hydrogen color spectrum, and it persists because it is cheap, proven, and deeply embedded in global industry.
How Steam Methane Reforming Works
The core of gray hydrogen production is a reaction between methane (the main component of natural gas) and steam at high temperatures, typically between 700 and 1,000 °C. Inside a large catalytic reactor called a reformer, methane molecules break apart in the presence of steam to produce a mixture of hydrogen and carbon monoxide. This reaction requires a significant amount of heat to get going and to sustain, making it what chemists call endothermic.2Renewable and Sustainable Energy Reviews. Evolution paths from gray to turquoise hydrogen via catalytic steam methane reforming: Current challenges and future developments In practice, that heat usually comes from burning additional natural gas, which adds to the process’s overall carbon footprint.
The mixture leaving the reformer is called syngas, short for synthesis gas. It contains hydrogen, carbon monoxide, leftover steam, and some unreacted methane. To squeeze more hydrogen out of this mixture and deal with the carbon monoxide, the syngas passes through a second stage known as the water-gas shift reaction. Here, carbon monoxide reacts with more steam over a different catalyst to produce additional hydrogen and carbon dioxide. In well-designed plants, methane conversion at the exit of the reformer catalyst bed can reach around 87%, and the water-gas shift step drives the remaining carbon monoxide concentration down to very low levels.3ScienceDirect. Investigation of the characteristics of a compact steam reformer integrated with a water-gas shift reactor
After the shift reaction, the gas stream is a mix of hydrogen, carbon dioxide, residual methane, and traces of other gases. The final step is purification, almost always done through a technique called pressure swing adsorption (PSA). Gas is pushed through beds of adsorbent material at high pressure, and the unwanted gases stick to the material while hydrogen passes through. By cycling the pressure up and down across multiple beds, the system can produce hydrogen at purities above 99.99% with recovery rates exceeding 90%.4International Journal of Hydrogen Energy. Review of Polybed pressure swing adsorption for hydrogen purification The carbon dioxide and other rejected gases are typically vented to the atmosphere or burned as fuel in the reformer’s furnace. That venting is what makes the hydrogen “gray.”
How Much Carbon Dioxide Does It Release?
Producing one kilogram of gray hydrogen releases about 29 kg of COâ‚‚ equivalent greenhouse gases.5International Journal of Hydrogen Energy. Comparative techno-environmental analysis of grey, blue, green/yellow and pale-blue hydrogen production That number surprises a lot of people. A kilogram of hydrogen contains roughly the same energy as a gallon of gasoline, so in carbon terms, making hydrogen this way has a substantial footprint before the hydrogen is even used.
About 94% of those emissions come directly from the SMR and shift reactions themselves, where the carbon in methane ends up as COâ‚‚. The remaining fraction comes from upstream activities: extracting and processing the natural gas, running compressors, and transporting the fuel to the plant. During natural gas extraction and processing, methane leakage during drilling and flaring adds emissions ranging from 0.07 to 0.08 kg COâ‚‚ equivalent per kilogram of natural gas handled.6Clean Energy and Sustainability. Supply Chain of Grey-Blue Hydrogen from Natural Gas: A Study on Energy Efficiency and Emissions of Processes Methane is a far more potent greenhouse gas than COâ‚‚ over short time horizons, so even small leaks in the supply chain matter.
The total energy demand to produce a kilogram of gray hydrogen sits around 91 kWh, with roughly 59% of that energy going toward processing the natural gas feedstock itself.5International Journal of Hydrogen Energy. Comparative techno-environmental analysis of grey, blue, green/yellow and pale-blue hydrogen production That is a lot of energy input for one kilogram of output, and it underscores the fundamental tension: hydrogen burns cleanly, but the way most of it is manufactured is anything but clean.
Why Gray Hydrogen Still Dominates
The global hydrogen market is enormous. Dedicated annual production reached about 70 million tons, with a market value around $115 billion, and hydrogen production consumes roughly 2% of global primary energy demand.7Resources for the Future (RFF). Decarbonized Hydrogen in the US Power and Industrial Sectors: Identifying and Incentivizing Opportunities to Lower Emissions The overwhelming majority of that production is gray. The reason is straightforward: cost.
Gray hydrogen currently costs between about $1.50 and $2.50 per kilogram to produce, making it the cheapest option on the market. Blue hydrogen, which uses the same SMR process but adds carbon capture and storage equipment, runs $2.00 to $3.50 per kilogram. Green hydrogen, made by splitting water with renewable electricity, costs $3.50 to $6.00 per kilogram.8International Journal of Hydrogen Energy. Techno-economic analysis of hydrogen production: Costs, policies, and scalability in the transition to net-zero In industries with thin margins that consume massive volumes of hydrogen, like oil refining and ammonia manufacturing, that cost gap is hard to ignore. SMR is also a mature technology with decades of operational experience. Industrial steam reforming has been in use since 1936, and the equipment, catalysts, and engineering practices have been refined continuously since then.9Johnson Matthey Technology Review. Eighty Years of Steam Reforming
In countries like China, which is both the world’s largest hydrogen producer and consumer, gray hydrogen’s dominance is reinforced by cheap domestic natural gas and coal feedstocks, along with a massive existing infrastructure. Analyses confirm that gray hydrogen remains the dominant source in China in the short term due to its mature technology and low price.10International Journal of Hydrogen Energy. Economic analysis of different hydrogen production routes under a CO2 pricing mechanism – A levelized cost of hydrogen based study
Where Gray Hydrogen Gets Used
Most people hear “hydrogen” and think of fuel cells or rocket engines, but the vast majority of gray hydrogen never powers a vehicle or generates electricity. It goes into chemical manufacturing. The single largest use is ammonia production, which feeds the global fertilizer industry. The Haber-Bosch process combines hydrogen with nitrogen from the air to make ammonia, and it is one of the most energy-intensive industrial processes on Earth. Without cheap gray hydrogen, synthetic fertilizer production at its current scale would be economically unthinkable.
Oil refineries are the second-largest consumer. Hydrogen is essential for hydrocracking and hydrotreating, processes that break heavy crude oil fractions into lighter, more valuable fuels and remove sulfur to meet clean-fuel regulations. Methanol production is another major sink, with methanol serving as both a chemical feedstock and a potential fuel. Smaller but growing uses include direct reduction of iron ore in steelmaking and various specialty chemical processes.
The industrial nature of these applications matters for the transition debate. These are not consumer choices where a buyer switches brands. They are capital-intensive, continuous processes where hydrogen quality, pressure, and supply reliability are tightly integrated into plant design. Switching from gray to green or blue hydrogen in an ammonia plant is less like swapping fuels and more like redesigning part of the facility.
The Difference Between Gray, Blue, and Other Fossil Hydrogen
The hydrogen color code has become the standard shorthand in energy discussions, and the distinction between gray and blue hydrogen is the one that matters most for policy. Both start with the same SMR process and the same natural gas feedstock. The only difference is what happens to the COâ‚‚. In a gray plant, it goes into the air. In a blue plant, carbon capture technology intercepts most of the COâ‚‚ before it escapes and routes it to underground geological storage.
That capture step makes a real difference in emissions, but not as much as the label might suggest. With 56% capture efficiency, blue hydrogen still emits about 8.1 kg COâ‚‚ equivalent per kilogram of hydrogen. At 90% capture efficiency, that drops to roughly 3.5 kg COâ‚‚ equivalent.5International Journal of Hydrogen Energy. Comparative techno-environmental analysis of grey, blue, green/yellow and pale-blue hydrogen production Neither figure is zero, and the energy consumption remains nearly the same as gray production because the underlying SMR process is unchanged. Adding capture equipment at 90% efficiency actually raises the energy demand slightly, to around 100 kWh per kilogram versus 91 kWh for gray.5International Journal of Hydrogen Energy. Comparative techno-environmental analysis of grey, blue, green/yellow and pale-blue hydrogen production Retrofitting existing gray plants for carbon capture also introduces engineering challenges, including the energy cost of running vacuum pumps and the fact that some hydrogen can be lost alongside the captured COâ‚‚ stream, reducing overall hydrogen output.11Journal of Environmental Chemical Engineering. Techno-economic analysis of on-site blue hydrogen production based on vacuum pressure adsorption: Practical application to real-world hydrogen refueling stations
Beyond SMR, gray hydrogen can also be produced through other fossil-fuel routes. Autothermal reforming (ATR) combines partial oxidation with steam reforming in a single reactor, using oxygen to provide some of the heat internally rather than relying entirely on external furnaces. This can offer operational flexibility, though ATR has a significantly higher cooling water requirement than conventional SMR.12Journal of Environmental Chemical Engineering. Water intensity for hydrogen production with and without carbon capture and sequestration Coal gasification is another pathway, particularly in regions where coal is cheaper than natural gas. The carbon intensity of coal-based hydrogen is generally higher than SMR-based hydrogen, though advanced gasification designs with carbon capture have demonstrated emissions as low as 0.18 kg COâ‚‚ per kilogram of hydrogen in simulation studies.13International Journal of Hydrogen Energy. Low-emission hydrogen production from gasification of Australian coals – Process simulation and technoeconomic assessment Without capture, coal gasification produces what is sometimes called “brown” or “black” hydrogen, the dirtiest option on the spectrum.
What Carbon Pricing Does to Gray Hydrogen Economics
Gray hydrogen’s cost advantage is entirely dependent on COâ‚‚ being free to emit. The moment governments put a price on carbon, the economics start to shift. In the European Union, where carbon prices under the Emissions Trading System have climbed substantially in recent years, the cost of emitting that roughly 29 kg of COâ‚‚ per kilogram of hydrogen adds a meaningful surcharge. The EU’s high carbon pricing mechanism increases gray hydrogen costs, though on its own it does not directly make alternatives cheaper.14Academia Green Energy. Policy-driven pathways to cost-competitive green hydrogen: an LCOH-based simulation
The modeling on this is sobering. One study projects that if current policies remain unchanged, China alone could produce 21.6 million tons of gray hydrogen annually by 2060, generating cumulative emissions equivalent to half of 2024 global emissions.15Energy. Policy-driven pathways to decouple China’s hydrogen expansion from carbon lock-in Carbon pricing and targeted subsidies for green hydrogen production emerge in these models as the most effective tools for steering the sector toward net-zero, with green hydrogen subsidies offering the lowest cost per unit of emission reduction.15Energy. Policy-driven pathways to decouple China’s hydrogen expansion from carbon lock-in In the United States, tax credits under the Inflation Reduction Act of up to $3.00 per kilogram for clean hydrogen production are designed to close the gap from the other direction, making green hydrogen competitive rather than making gray hydrogen more expensive.8International Journal of Hydrogen Energy. Techno-economic analysis of hydrogen production: Costs, policies, and scalability in the transition to net-zero
The interesting question is whether carbon pricing alone can kill gray hydrogen or whether direct subsidies for alternatives are necessary. The evidence so far suggests both levers matter, and that relying on just one produces slower, more uneven results. A carbon price punishes the incumbent but does not solve the infrastructure and scaling problems that keep green hydrogen expensive. Subsidies accelerate alternative deployment but leave gray hydrogen economically viable in jurisdictions without carbon costs. Most analysts expect both tools will need to work in tandem for the transition to move fast enough to matter for climate targets.
Process Details That Affect Real-World Performance
The textbook description of SMR is clean and simple, but actual plant performance depends on a web of operational parameters. The ratio of steam to methane fed into the reformer is one of the most important. Higher steam ratios push the reaction further toward hydrogen production and reduce the risk of carbon deposits forming on the catalyst, but they also increase the energy needed to generate all that steam. Optimal performance tends to occur at a steam-to-methane ratio around 2, with pressures in the range of 5 to 10 bar and temperatures between roughly 630 and 830 °C.16Energy. First law energy analysis of thermochemical waste-heat recuperation by steam methane reforming
Catalyst design is another area where gray hydrogen plants have evolved considerably. The nickel-based catalysts used in most reformers are susceptible to poisoning by sulfur compounds in the natural gas feed, so upstream desulfurization is critical. Catalyst deactivation from carbon deposition (coking) is a constant operational concern, particularly when operating at lower steam ratios to save energy. Modern catalyst formulations and reactor designs manage these trade-offs better than older plants, but catalyst management remains one of the key factors separating an efficient gray hydrogen facility from a wasteful one.
Heat integration is where plant economics really get decided. The reformer operates at very high temperatures, and recovering waste heat from the hot syngas exiting the reactor, the flue gases from the furnace, and the shift reactor all contribute to overall efficiency. A well-integrated plant recycles much of this heat to raise the steam needed for the reaction, preheat the feed gas, or generate electricity for onsite use. Poor heat integration means burning more natural gas for the same hydrogen output, which drives up both cost and emissions.
Water Use and Other Overlooked Environmental Costs
Carbon dioxide gets most of the attention, but gray hydrogen production also consumes substantial amounts of water. Steam is a reactant, not just a heat carrier, so every molecule of hydrogen produced requires water as a chemical input. On top of that, cooling systems throughout the plant use large volumes of water. Autothermal reforming variants require about 72% more cooling water than conventional SMR, largely because of the air separation unit needed to supply pure oxygen.12Journal of Environmental Chemical Engineering. Water intensity for hydrogen production with and without carbon capture and sequestration
In water-stressed regions, this can be a genuine constraint on where gray hydrogen plants can operate. It is an issue that gets amplified rather than solved by the transition to blue hydrogen, since adding carbon capture equipment typically increases both the energy and water demands of the plant. Green hydrogen from electrolysis also requires water, but the volumes and the nature of the demand differ. For regions planning large-scale hydrogen economies, water availability is an infrastructure question that sits alongside pipeline capacity and storage.
Pipeline Infrastructure and the Blending Question
Gray hydrogen plants are typically located near their customers, either onsite at refineries and chemical plants or connected by dedicated hydrogen pipelines. These pipelines are made of specialized steels because hydrogen molecules are small enough to diffuse into the metal lattice and cause a phenomenon called hydrogen embrittlement, where the steel becomes brittle and prone to cracking.
As hydrogen production scales up and policymakers explore blending hydrogen into existing natural gas pipelines as a short-term decarbonization measure, the embrittlement question becomes critical. Research shows that in pipelines carrying hydrogen-natural gas blends, corrosion and hydrogen embrittlement act together in ways that are not yet fully understood. The risk increases with higher hydrogen blending ratios and with higher-strength pipeline steels.17Natural Gas Industry B. Research progress on corrosion and hydrogen embrittlement in hydrogen–natural gas pipeline transportation Impurities like hydrogen sulfide and CO₂ in the gas stream compound the problem. The failure mechanisms when corrosion and embrittlement coexist remain an active area of research, which means that proposals to repurpose natural gas infrastructure for hydrogen transport carry genuine engineering uncertainty. This matters for gray hydrogen producers considering future transitions, since the same pipelines that carry their natural gas feedstock today might need to carry hydrogen or blended gas tomorrow.
Alternative Reforming Approaches
Standard SMR is not the only way to reform methane. Autothermal reforming, mentioned earlier for its higher water demands, has one practical advantage: by combining partial oxidation with steam reforming in a single vessel, it generates some of its own heat internally. This makes the reactor easier to control and potentially easier to pair with carbon capture, since the CO₂ stream can be more concentrated. A two-stage approach, where partial oxidation of methane is followed by a separate water-gas shift reaction with individually controlled temperatures, can achieve higher hydrogen yields than single-stage autothermal reforming. When methane, oxygen, and steam are in their ideal stoichiometric ratio, the two-stage process achieves a hydrogen yield of 2.89 moles per mole of methane at a shift temperature of 200 °C, compared to 2.25 for the single-stage approach at 800 °C.18International Journal of Hydrogen Energy. Thermodynamic analysis of hydrogen production from methane via autothermal reforming and partial oxidation followed by water gas shift reaction
These alternatives are not exotic laboratory curiosities. ATR is used commercially, particularly in large-scale plants designed from scratch for hydrogen or methanol production. The choice between conventional SMR and ATR often comes down to the specific feedstock composition, the desired scale, and whether carbon capture is part of the initial design. For existing gray hydrogen plants, though, the installed base is overwhelmingly conventional SMR, and that will not change quickly given the capital costs involved.
There is also growing interest in “turquoise” hydrogen, produced by methane pyrolysis rather than reforming. Instead of reacting methane with steam, pyrolysis cracks methane directly into hydrogen and solid carbon using high temperatures, producing no COâ‚‚ if the process heat comes from a clean source. The solid carbon can potentially be sold or sequestered far more easily than gaseous COâ‚‚. This technology is still early-stage compared to the nearly nine decades of operational history behind steam reforming, but it represents one possible pathway for fossil-fuel-based hydrogen production without the emissions that define the “gray” label.