What is green hydrogen production and why is it important?

Green hydrogen is a fuel typically generated by electrolysing water with renewable energy sources like wind, water or solar power. The term "green" denotes its eco-friendly nature, identifying it as a clean fuel alternative compared to other colours of hydrogen produced in other ways, including from fossil fuels.

Our modern lifestyles have led to increased consumption of resources like food, housing, and energy, pushing these systems to their limits. Experts from around the globe have repeatedly highlighted that we are facing an unprecedented challenge: climate change.

Green hydrogen is a solution that could help countries around the globe to decarbonise by 2050. 

Producing green hydrogen 

Green hydrogen, the focus of this article, is just one of the colours of hydrogen. The colour categories of hydrogen are intended to reflect the degree of sustainability of the production method. 

Green hydrogen is typically defined as hydrogen produced using electrolysis of water that is powered by renewable electricity. However, this is not the only method to create green hydrogen, it can also be produced using thermal chemical processes (using heat and chemical reactions to release hydrogen from organic materials like biomass), photolysis (solar energy) and biological processes (bacteria and algae). Many of these technologies are still at early stages of development and need further research and investment. 

A growing area of interest is white or gold hydrogen, which is naturally occurring hydrogen found in underground wells. This natural hydrogen appears to be renewable as it is continuously generated deep in the Earth. Energy companies are now exploring geological formations specifically for white hydrogen, which could become a net zero game changer if significant reserves are found, expanding the hydrogen economy.

Other important hydrogen generation techniques include: methane/natural gas reformation (ATR and SMR); biomass gasification; coal gasification; H2S methane reformation; naphtha reformation; methane/natural gas pyrolysis; steam-iron process; steam reforming of waste oil and partial oxidation of heavy oil and coal.

_{Source: Image sourced from Luque-Urrutia, Jesús & Ortiz-García, Thalía & Solà, Miquel & Poater, Albert. (2023) in Green Energy by Hydrogen Production from Water Splitting, Water Oxidation Catalysis and Acceptorless Dehydrogenative Coupling, published under a CC BY 4.0 license.}

Energy reports from 2022 show that only 4% of hydrogen was produced via electrolysis, compared with 49% from steam reforming, 29% from hydrocarbon oxidation and 18% from coal gasification. To be considered a low carbon energy source, hydrogen must meet the UK’s Low Carbon Hydrogen Standard, regardless of the production method or colour classification. This standard helps shift the focus from colour labels to carbon impact. Blue hydrogen, produced through natural gas reforming with carbon capture and storage (CCS), is expected to play a critical role in scaling up the hydrogen economy.

Electrolysis 

Electrolysis is the process of separating hydrogen and  oxygen molecules found in water, using electricity. 

The main barrier currently is the high cost of renewable energy. The cost of electricity, especially from renewable sources like solar or wind, significantly influences the economic viability of hydrogen produced through electrolysis. As the price of renewable energy decreases, the overall cost of green hydrogen production is expected to become more competitive. The cost of underutilised assets from intermittent sources of renewable electricity is also a consideration on the journey to a clean power grid – either underutilised electrolysers or underutilised electricity generation capacity.

While water is needed for electrolysis, its availability is generally not a major barrier, particularly with access to seawater and desalination technologies.

Thermolysis 

Thermochemical water splitting is a hydrogen production process that uses high temperatures (above 1500 ◦C) from solar power or from the waste heat of nuclear power reactions to produce hydrogen.

The process is similar to water electrolysis, but instead of using electricity to split the water, heat is used instead. This technology dates back to 1966, but has struggled to launch into mainstream hydrogen production due to the high temperatures needed for the process which leads to poor efficiencies. 

The concept of thermochemical water splitting cycles (TWSCs) is straightforward: employ a closed-loop system involving two or more chemical reactions to facilitate the breakdown of water and generate hydrogen. 

Photolysis 

In photolysis, sunlight is used with the aid of a catalyst to split water molecules into hydrogen and oxygen.

Using sunlight to split water into hydrogen and oxygen presents promising solutions for green hydrogen production, but still encounters barriers such as efficiency and potential to scale. 

Photoelectrochemical water splitting (PEC) is a promising green hydrogen production method, offering the potential for production efficiency at low operating temperatures. 

To read more about this, take a look at this information from the US Department of Energy’s Office of Energy Efficiency and Renewable Energy. 

Biomass gasification

Biomass gasification uses a controlled process involving heat, steam, and oxygen to convert biomass (renewable organic resources, including agriculture crop residues, forest residues, organic waste, and animal wastes) to hydrogen and other products.

Iron and copper-based catalysts are often used in biomass gasification. 

Biomass is burnt in a small quantity of air to generate hydrogen, methane, carbon dioxide, nitrogen and more. If gasification is coupled with carbon capture, the net carbon emissions of this method can be low.

Biomass gasification is often considered a promising low-carbon energy solution when coupled with carbon capture. However, the environmental impact of this method can vary based on the type of feedstock used and its geographic source. The full lifecycle emissions of biomass are debated, with factors such as transportation, processing, and land-use changes influencing the net carbon footprint

A recent report projects that with anticipated improvements in agricultural practices and plant breeding, up to 1 billion dry tons of biomass could be available for energy use annually. 

Steam reforming

Steam reformation is where natural gas is split into hydrogen and carbon dioxide using steam. The reaction requires heat which is generated by combustion of tail gas and natural gas. The hydrogen is then obtained by adsorptive separation.  

In the case of blue hydrogen, 85%-95% of the produced CO2 during the process is captured and stored underground using industrial carbon capture and storage techniques (CCS). Even greater capture rates are anticipated from the autothermal reforming (ATR) process. Without CCS, the hydrogen is classed as grey hydrogen, which is currently the most utilised hydrogen in the UK, and one of the most damaging to the environment. 

Pyrolysis (turquoise hydrogen) 

Methane pyrolysis is the conversion of natural gas at high temperatures to create hydrogen and solid carbon. This technology has gained interest because it requires less energy than steam reforming and water electrolysis, and the carbon produced is solid which makes it easier to store. It is also understood to perform better environmentally than grey, blue and green hydrogen (using RNG). However, this method of producing hydrogen still relies on fossil fuels. 

It is worth noting that there is a lack of academic information regarding the environmental advantages associated with hydrogen produced through thermal plasma pyrolysis of methane.

Challenges and future outlook of green hydrogen production 

On the journey to net zero, green hydrogen emerges as a promising energy solution however, the widespread adoption is not without its hurdles. 

• Costs: The production of green hydrogen is more expensive compared to grey or blue hydrogen produced from fossil fuels. The main hindrance in the production of green hydrogen is the high electricity price.
• Energy intensity and availability: Electrolysis, one of the primary methods of producing green hydrogen, is an energy-intensive process. The availability of low-cost, renewable energy sources and the variability in weather conditions, such as cloudy days or low wind speeds can impact the production of green hydrogen. However, using excess power produced from the grid that would otherwise be wasted, can be beneficial by cutting the peaks and storing excess energy to stabilise the system.
• Scale-up challenges: the scale-up of green hydrogen production is needed to achieve economies of scale and widespread deployment. It will require overcoming various technical, economic, and regulatory challenges.

Once produced at scale and competitive cost, green hydrogen can also be further converted into other energy carriers, such as ammonia, methanol, methane and liquid hydrocarbons.