As the planet confronts global warming, hydrogen will become an increasingly important energy source, affecting many sectors in which Broadleaf works. Hydrogen is important because it can be environmentally friendly:
- It can be produced from renewables
- Energy can be extracted from hydrogen in an environmentally friendly manner: whether used for combustion or in fuel cells, its primary by-product is water, rather than the carbon dioxide that is generated by burning hydrocarbons such as natural gas, coal, crude oil, other hydrocarbon liquids or biomass.
Hydrogen is described in different ways, according to the energy sources and feedstocks used to produce it and the kinds of by-products that are generated. To assist those interested in this matter of growing importance, this tutorial summarises the colour descriptions attributed to specific forms of hydrogen production and outlines some of the associated technologies.
The primary colours
Table 1 summarises the three main ‘colours’ of hydrogen. Blue hydrogen is similar to grey hydrogen, with the addition of carbon capture and storage (CCS).
Green hydrogen does not involve any greenhouse gases, either in the energy used for producing it or as a by-product. It uses renewable energy to electrolyse water (H2O) into hydrogen (H2) and oxygen (O2).
Renewable energy sources for producing green hydrogen may include:
- Wave and tidal
Hydrogen produced using electrolysis powered by solar energy is sometimes called yellow hydrogen, although this term may also be used to describe hydrogen generated from any energy delivered by an electricity network.
An electrolyser consists of electrodes (an anode and a cathode), separated by an electrolyte that conducts electric current. Electrolysers use different kinds of electrodes, electrolytes, membranes (to separate the gases that are produced, oxygen and hydrogen, while allowing ions to pass through and providing electrical insulation of the electrodes) and catalysts. These determine the efficiency of the electrolysis, the range of current densities and operating temperatures that can be tolerated, and the purity of the gases that are produced.
The diagrams (Figure 1, Figure 2 and Figure 3) outline three kinds of electrolyser, using different forms of technology. They each generate hydrogen and oxygen, with slightly different reactions at the anodes and cathodes:
- In a proton exchange membrane (PEM) electrolyser, the electrolyte is a solid polymer (Figure 1)
- In an alkaline electrolyser, the electrolyte is a liquid alkaline solution, usually of sodium hydroxide (NaOH) or potassium hydroxide (KOH), with a thin diaphragm (Figure 2)
- In a solid oxide electrolysis cell (SOEC), the electrolyte is a porous ceramic, and the water is usually in the form of steam (Figure 3).
PEM and alkaline electrolysers are used commercially. SOEC electrolysis is in pilot development. Another process, anion exchange membrane (AEM) electrolysis, is in laboratory development.
Blue and grey hydrogen
Grey hydrogen is produced from hydrocarbons, using several different feedstocks and processes. The processes all generate carbon dioxide as a by-product; this is commonly released to the atmosphere.
Blue hydrogen is generated by the same processes as grey hydrogen, but the carbon dioxide by-product is captured and either used or stored.
Grey hydrogen is sometimes called black hydrogen.
Reforming and partial oxidation
In steam reforming, steam reacts with a hydrocarbon feedstock to produce hydrogen, carbon dioxide and carbon monoxide. The most widely used process for generating grey hydrogen is steam-methane reforming (SMR). Methane (CH4) is the principal component of natural gas. Figure 4 outlines the process.
- Natural gas is cleaned, to remove impurities and sulphur compounds, leaving a pure methane stream
- Methane and steam enter the reformer. The initial steam reforming reaction, assisted by a catalyst, produces a mix of hydrogen and carbon monoxide, often called syngas: CH4 + H2O –> CO + 3H2
- Reforming is followed by a water gas shift reaction that, again assisted by a catalyst, produces more hydrogen from the syngas and steam: CO + H2O –> CO2 + H2
- Hydrogen and carbon dioxide are separated. Critically for blue hydrogen, the carbon dioxide is captured and either used or stored; it is not released to the atmosphere.
Partial oxidation uses a similar reforming process, with oxygen from air as the oxidising agent (Figure 5). Compared with steam reforming, partial oxidation produces less hydrogen per unit of methane.
Gasification is a high-temperature process that converts organic feedstock into a mixture of hydrogen and carbon monoxide (syngas) and solid by-products. The organic feedstock may be coal, oil or biomass, which reacts in the gasifier with air, oxygen or steam. The syngas is further processed in a water gas shift reaction. As an example, Figure 6 outlines the process for coal gasification.
CCS and CCUS
Carbon capture and storage (CCS) and carbon capture, utilisation and storage (CCUS) is a set of processes that prevent the release of carbon dioxide to the atmosphere.
The most common approach to CCS is to capture carbon dioxide and store it permanently in geological formations deep underground. This is often done by injecting carbon dioxide into a producing oil or gas field to enhance the recovery of hydrocarbons. For enhanced oil recovery (EOR), the carbon dioxide mixes with the oil to form a low viscosity, low surface tension fluid, enabling it to flow more freely into production wells. This is usually a closed-loop process, in which any carbon dioxide that returns to the surface in the oil stream is separated and reinjected.
Carbon dioxide may also be captured and used as an industrial feedstock, for example to produce chemicals and fertilisers.
Other colours of hydrogen use variants of the green, blue and grey production processes outlined above.
Pink hydrogen is generated by electrolysis powered by nuclear energy.
With nuclear energy, heat from the nuclear reactor may be used to increase the efficiency of the electrolysis, and to produce steam for solid oxide electrolysis (Figure 3). The steam can also be used for steam methane reforming (Figure 4) and for water gas shift conversion of syngas. Hydrogen produced using nuclear power is sometimes called purple or red.
Nuclear power does not generate any carbon dioxide during its operation, but it does produce nuclear waste.
Turquoise hydrogen is generated from methane using pyrolysis.
Pyrolysis is a high-temperature process that decomposes organic feedstock into syngas, volatile hydrocarbon compounds and solid carbon-rich residue, in an atmosphere that contains little or no oxygen. The volatile compounds may be processed in a reformer, and the syngas processed in a water gas shift reaction.
While pyrolysis can use many kinds of feedstock, for turquoise hydrogen production the feedstock is usually natural gas.
Brown and black hydrogen
Brown hydrogen is generated from brown coal (lignite) or biomass using gasification (Figure 6). The carbon dioxide is not captured.
Black coal can be used in gasification, but it does not contain as much hydrogen as lignite. The hydrogen produced in this way is sometimes called black hydrogen. Confusingly the term black hydrogen is also sometimes used to describe hydrogen from steam reforming of methane, called grey hydrogen in the earlier discussion.
Summary of the spectrum
Figure 7 provides a simplified overview of the hydrogen spectrum.
In terms of their environmental footprints:
- Green, yellow and pink hydrogen are the only colours that do not produce greenhouse gases
- Using natural gas as the feed stock for blue, turquoise or grey hydrogen only makes sense if the hydrogen is needed as a feed in a chemical reaction to make high value products
- If methane-based blue, turquoise or grey hydrogen is used as a fuel, in many circumstances it would have been far more efficient to use the methane directly.
However, the economics of hydrogen production are changing rapidly. Significant research and development effort, on renewable energy generation and on electrolyser technology, shows the promise of reduced capital and operating costs and increased production efficiency for green hydrogen. It is forecast that this will make green hydrogen cost-competitive with blue hydrogen from SMR, currently the cheapest form.
Further information is available from the references below.
There are many books and articles that are relevant to the topics discussed in this note. Here we cite a small number of references, focusing on sources that provide more comprehensive overviews.
AT Kearney (2014) Hydrogen-based energy conversion. AT Kearney Energy Transition Institute. (Available here, accessed 25 June 2021.)
Debarre, R, P Gahlot, C Grillet and M Plaisant (2021) Carbon capture utilization and storage: Towards net-zero. Kearney Energy Transition Institute. (Available here, accessed 25 June 2021.)
Decourt, B, R Debarre, S Alias and P Gahlot (2018) Electricity storage gaining momentum AT Kearney Energy Transition Institute. (Available here, accessed 25 June 2021.)
IEA (2019) The future of hydrogen: Seizing today’s opportunities. Technology report prepared by the IEA for the G20, Japan. International Energy Agency, Paris. (Available here, accessed 25 June 2021.)
IEA (2020) Hydrogen. Tracking report. International Energy Agency, Paris (Available here, accessed 25 June 2021.)
IRENA (2020) Green hydrogen cost reduction: Scaling up electrolysers to meet the 1.5°C climate goal. International Renewable Energy Agency, Abu Dhabi. (Available here, accessed 25 June 2021.)
IRENA (2020) Green hydrogen: A guide to policy making. International Renewable Energy Agency, Abu Dhabi. (Available here, accessed 25 June 2021.)
IRENA (2021) Renewable power generation costs in 2020. International Renewable Energy Agency, Abu Dhabi. (Available here, accessed 25 June 2021.)
IRENA (2021) Green hydrogen supply: A guide to policy making. International Renewable Energy Agency, Abu Dhabi. (Available here, accessed 25 June 2021.)
L’Huby, T, P Gahlot and R Debarre (2020) Hydrogen applications and business models: Going blue and green? Kearney Energy Transition Institute. (Available here, accessed 25 June 2021.)