A Guide to the Hydrogen Rainbow
One molecule, two molecules, “green” hydrogen and “blue.” Why is colorless and odorless hydrogen referred to with distinct colors, and what does it all mean? This article is a guide to each identified color of hydrogen, providing insights into production methods, applications, and environmental impacts.
Hydrogen plays a pivotal role in the shift away from fossil fuels, given its capacity to function as an adaptable energy carrier with the potential to be emissions-free. However, hydrogen must exist in pure form to be utilized as an energy source.
Hydrogen production. Image used courtesy of Adobe Stock
Hydrogen is not generally found on Earth in its pure form, so it must be produced using other materials like water, biomass, or fossil fuels. These diverse hydrogen production or extraction approaches are recognized and categorized using color-based identification.
Gray hydrogen is the most commonly used and greatest supplier of hydrogen. Gray hydrogen is primarily produced through a process called steam methane reforming (SMR). This method involves reacting natural gas with steam to produce hydrogen and carbon dioxide. Although SMR is a cost-effective way to produce hydrogen, it results in significant carbon emissions.
Steam methane reforming method of producing hydrogen. Image used courtesy of the Department of Energy
Gray hydrogen is widely used as a feedstock in industrial processes. It is a key component in producing chemicals like ammonia, and it is also used as a refining tool in the petrochemical industry.
Gray hydrogen is considered the least environmentally friendly among the various hydrogen production methods. Given its environmental impact, there is a growing emphasis on transitioning away from gray hydrogen in favor of alternatives, such as green and blue hydrogen.
Green hydrogen is created by electrolysis. This process involves sending an electric current through water to split molecules into hydrogen and oxygen. While various sources can power electrolyzers, the intention is to generate green hydrogen exclusively through surplus electricity from solar or wind, ensuring a production process free from direct greenhouse gas emissions.
The applications for green hydrogen include fuel cells to power larger vehicles, including trains. It could provide a clean and efficient alternative to traditional gas-powered transportation with longer ranges and shorter refueling times.
Because it is generated from renewable sources, green hydrogen can be used for energy storage. When renewable energy production is high, surplus electricity can be diverted to produce hydrogen, which can be converted back to electricity to meet demand.
Green hydrogen has the lowest carbon footprint. Image used courtesy of ResearchGate
While green hydrogen is considered environmentally friendly, challenges include the cost of electrolysis technology and the need for further advancements to make it economically competitive with other forms of hydrogen. However, governments are backing its use, and associated costs could fall.
Blue hydrogen is produced through a process similar to gray hydrogen (SMR of natural gas). However, in the case of blue hydrogen, carbon capture and storage (CCS) technologies can capture and store the carbon emissions SMR produces. This prevents releasing carbon dioxide into the atmosphere.
Blue hydrogen can replace gray hydrogen as a cleaner feedstock option. In general, it is an alternative in sectors where hydrogen is a crucial component, but use for direct electrification may not be practical.
Like green hydrogen, it can also be used in fuel cells for transportation.
While blue hydrogen is considered a cleaner option than gray hydrogen, it is often seen as a transitionary solution on the path to fully green hydrogen. It's important to note that the environmental benefits of blue hydrogen depend on the effectiveness of CCS technologies and the overall carbon footprint of the natural gas used in the production process.
Gold (or White) Hydrogen
The latest addition to the hydrogen color wheel is gold hydrogen (sometimes referred to as white hydrogen). Gold hydrogen is a naturally occurring gas found in subsurface pockets, similar to oil or natural gas.
Initially, many assumed naturally occurring hydrogen, generated through underground chemical reactions, would be in limited and hard-to-locate deposits. However, geologists are currently identifying substantial pockets emerging worldwide. In October, a notably extensive reservoir of natural hydrogen was unearthed in northeastern France.
The consideration here is whether the deposits, found globally, will prove economically viable for extraction. As geologists learn more about gold hydrogen, it could be a low-cost and low-impact hydrogen source for applications similar to green and blue hydrogen.
Pink hydrogen is similar to green hydrogen as it is produced through electrolysis, but this reaction is fueled by nuclear energy. It is sometimes alternatively labeled as purple or red hydrogen.
Using nuclear power offers notable benefits in producing pink hydrogen, both in cost and emissions reductions. Like green and blue hydrogen, pink hydrogen can be a significant alternative to fossil fuels across various industries. Its versatility allows it to serve as a feedstock and energy source in applications such as the cement and steel industries.
Black and Brown Hydrogen
Last on the list of colorful hydrogen are black and brown hydrogen, produced via gasification using black and brown coal, making them the most environmentally damaging option.
This well-established method employed in various industries transforms carbon-rich materials into hydrogen and carbon dioxide. Consequently, the byproducts of gasification are released into the atmosphere.
Color identifiers for types of hydrogen serve as a valuable tool in monitoring production options. With an energy content of about 2.4 times natural gas per kilogram, hydrogen has potential as a clean energy source. However, realizing hydrogen’s full potential in the shift from fossil fuels requires ongoing research, technological progress, and the implementation of supportive policies.