‘Green’ versus ‘blue’ hydrogen, and the futility of ‘colours’

There’s a battle of colours underway – and it’s all to do with hydrogen.

“Green” and “blue” hydrogen advocates chastise each other in the media, and government policies, even investor and company decisions, are staked on these colours. The new German government has disavowed subsidies for “blue” hydrogen.

But where do these hydrogen colours come from, and why have they occupied centre-stage in the debate? Are they even useful?

As emphasised at COP26, we’re facing a dire future unless we rapidly decarbonise all of our energy-intensive activities. Substituting fossil-based electricity with renewably-generated electricity is the first step on this path.

But not all energy is provided with electricity – industrial and residential heating, transport fuels and cooking are still reliant on fossil fuels.

Some of these activities can be transitioned to an electrical equivalent. Some activities are difficult or very expensive to electrify, such as ships, aircraft, heavy-duty trucking, industrial heating and chemical manufacturing – for these cases, energy is better provided with molecules. They’re more practical and easier to transport and use than electrons.

The hurdles of hydrogen

Enter hydrogen.

Hydrogen is the lightest and most abundant element in the universe. It’s increasingly seen as useful for its clean energy credentials in fuel-cell buses, trains, aircraft, ships, or burnt as a fuel for domestic and industrial heat. On a mass basis, it provides almost three times as much energy as natural gas or diesel, and produces only water as the product of combustion.

However, while it’s the most abundant element in the universe, there’s almost no natural supply of hydrogen on Earth. It’s always bound up with other elements, such as carbon (in the form of “hydrocarbons”), oxygen (in water), or nitrogen (in ammonia). Nature has already “used” it and locked it away in chemical compounds.

To extract this hydrogen means finding and purifying these hydrogen-containing compounds, and using heat or electricity to break the chemical bonds and liberate the hydrogen.

We must be mindful of the full consequences of its production and supply before we can be sure of its clean credentials.

The many shades of hydrogen

Hydrogen is, therefore, just a molecular energy carrier. Because of this unique nature, we must be mindful of the full consequences of its production and supply before we can be sure of its clean credentials.

The colours assigned to hydrogen are intended to reflect the carbon intensity of the final hydrogen product. There are about nine colour codes used to identify hydrogen production methods.

• “Green” hydrogen is made using entirely renewable energy sources – the exemplar is the electrolytic splitting of water to produce hydrogen and oxygen using 100% renewable electricity.
• “Brown” hydrogen refers to hydrogen extracted from brown coal (and “black” hydrogen from black coal), while “grey” hydrogen uses natural gas as the source. When using these fossil fuels, hydrogen is stripped from the hydrocarbons using steam or oxygen (in a process known as reforming), resulting in a mix of carbon dioxide and hydrogen – the hydrogen is separated and the CO₂ emitted to the environment.
• “Blue” hydrogen arises when some of the emitted CO₂ is captured and sequestered (anything from 0 to 100%), preventing its release to the environment.
• “Turquoise” hydrogen is made when fossil fuels are converted to hydrogen without production of CO₂. By catalytically “cracking” the fossil fuel, we produce a solid carbon product and hydrogen with little to no CO₂ emissions.
• “Pink” hydrogen refers to the use of nuclear energy to produce electricity for hydrogen production via electrolysis (also free of emissions), and “red” hydrogen is produced through the high-temperature catalytic splitting of water using nuclear power.
• “Purple” hydrogen refers to processes in which biomass is used as the fossil fuel source. The CO₂ released in this process is returned to the air from which it came.

Read more: Fuel for thought: Charting a course towards the ammonia economy

While using colours to distinguish production methods was useful in the past when there were only a few alternatives, the proliferation of new, innovative technologies for hydrogen production and supply has led to the question: What colours should we assign these new modes of production? We’re running out of colours.

Further, there’s a wide range of carbon emission-reduction options that can be used even within a single production method – for example, various extents of carbon capture, or varying the fraction of renewable electricity used versus grid electricity to drive the production method.

Fossil fuel-based processes can be supplemented with biogas, further decarbonising them. The “colour” of hydrogen may even evolve over time.

A complex calculation

The total accounting of the greenhouse gas emissions associated with each kilogram of hydrogen delivered to the end user is a complex task, specific to each project, location and time, and, in fact, even for a given project varies operationally – day to day and seasonally.

Such an analysis, called a life cycle analysis, or LCA, is complex, but essential for each project if we’re to provide accurate and reliable guidance for decision-makers.

The entire supply train must be considered – extraction and fabrication of the energy system, extraction and production of the chemical compounds used to supply the hydrogen, the conversion process of the compounds to hydrogen, the storage, transport, and disposition of the hydrogen to the end user.

It’s only when this comprehensive accounting is carried out that we can answer the question: “What are the greenhouse gas emissions reductions possible when using hydrogen for my application, compared with the alternatives?”

The dominant benchmark method of hydrogen production is steam methane reforming (SMR). A modern SMR plant produces between nine and 11kg CO₂e per kg hydrogen; this is “grey” hydrogen. Adding conventional carbon capture and storage to SMR (to produce “blue hydrogen”) can reduce the carbon intensity to between 1.5 and 5kg CO₂e per kg hydrogen, depending on how much carbon capture is undertaken.

Extra methane is needed to power the carbon capture plant (unless renewable electricity is used), and the additional fugitive methane emissions must be accounted for if an honest assessment of carbon intensity is to be made.

Some studies suggest emissions may even be higher than “grey” hydrogen if these aren’t accounted for. The assumptions concerning leakage rates into the atmosphere and global warming potentials (for methane, carbon dioxide and hydrogen) associated with the production and any implicit distribution of hydrogen must be clearly and honestly accounted for in the life cycle analysis.

Advocates of “green” hydrogen must likewise ensure their total process from manufacture of the hydrogen to delivery is lower than this benchmark emissions intensity, and is at least competitive on pricing, in the absence of a carbon tax, if they want a market-driven uptake.

A recent analysis undertaken by Monash researchers considered a large-scale solar-electrolysis plant producing hydrogen in northwest Australia. An overriding factor is intermittency of renewable electricity and consequent electrolyser turndown.

To avoid excessive turndown, either energy storage is needed, or grid buffering. Accounting for all the factors associated with production of the solar panels and battery backup, water desalination, hydrogen compression and storage, electrical transmission systems, and electrolyser manufacture, it showed a greenhouse gas intensity of about 2-3kg CO₂e per kg.

This is significantly lower than the benchmark case, but not zero. If buffering with predominantly fossil fuel-based grid power is used, even to a modest extent, carbon intensity rises rapidly to equal or surpass that of SMR.

Our most urgent need now is to decarbonise in the quickest, most cost-effective way. The use of hydrogen at large scale will need considerable infrastructure to be built for distribution and supply.

The consensus from most techno-economic models is that producing hydrogen purely from renewables will not be available at low cost and at scale until the 2030s. If we need large-scale, low-cost hydrogen now, we need to use available low-emitting processes to produce this hydrogen and demonstrate through a full life-cycle analysis, for each project, a positive reduction in CO₂e emissions. The technology used to deliver lower carbon-intensive hydrogen is irrelevant.

We’re still at the early stages of this transition from colours to numbers. The European CertifHy project that operated from 2014 to 2016 developed a common European-wide definition of green hydrogen, recognising certification of “low carbon hydrogen production” (less than 4.4kgCO₂/kg H₂) routes as equivalent to “green” hydrogen.

The recent National Hydrogen Strategy lays out the need for Australia to play a leading role internationally, developing methods with our trading partners to determine emissions associated with hydrogen production and delivery.

Trials are underway by the Clean Energy Regulator collaboratively with the Department of Industry, Science, Energy and Resources, and co-designed with industry and key stakeholders. The final outcome should be a rigorous, universally agreed-upon methodology to attach numbers (not colours) to every kilogram of hydrogen produced to reflect its true carbon intensity.

We should welcome all options that enable low-carbon hydrogen to play a role in decarbonising our energy systems, and stop focusing on colours.