Hydrogen is the most abundant element in the universe and has the ability to form more chemical compounds than any other element. Although it was considered discovered in 1766 by Henry Cavendish, there are several reports prior to this period regarding a “flammable gas,” which, according to Andrew Szydlo, makes sense because mineral acids (sulfuric and hydrochloric) have been known since the Middle Ages, and metals like iron, zinc, and tin were known to the Ancients.
Basically, each of these metals reacts with acids to produce hydrogen as the main gaseous product. Thus, the first account of the formation of a “foul form of hydrogen (made from iron filings and oil of vitriol) is attributed to the physician and iatrochemist born in Geneva, Turquet de Mayerne (1573–1655).”
Despite being considered the most abundant element in the universe, hydrogen is also the lightest and most ubiquitous, meaning it is present everywhere on Earth (in water, fossil fuels, and all living beings). As Jeremy Rifkin highlights, “when used as an energy form, it becomes the ‘eternal fuel.’ It never runs out and, because it contains no carbon atoms, it does not emit carbon dioxide.” However, hydrogen “rarely exists freely in nature” and must be extracted from natural sources.
For better understanding: “Hydrogen is not a source of energy on its own.” That is, “it is not a primary energy like natural gas or crude oil, which exist freely in nature. It is an energy carrier, a secondary form of energy that needs to be produced, just like electricity.” And, bringing the concept of hydrogen as an element, it is important to understand the current terminology used to define hydrogen as an energy source, based on the degree of carbon emission generated by its production, the process used, and the respective primary energy source.
Although not a universal classification, various regulations, public policies, and hydrogen production projects as an energy source have used the following terminology: i) green hydrogen; ii) blue hydrogen; iii) gray hydrogen; and iv) brown hydrogen. Additionally, v) turquoise hydrogen and vi) purple or pink hydrogen are also highlighted. Indeed, it is quite a varied palette.
The first category – green hydrogen – emerged to describe hydrogen produced through water electrolysis processes (process), where electricity generated from renewable energy sources (such as solar and wind) is used to separate water into hydrogen and oxygen. The green color designation refers to the sustainability and low carbon footprint associated with hydrogen production. It is worth noting that the term “yellow hydrogen” has been used to describe hydrogen produced from solar sources.
Gray hydrogen can be classified as a category that includes the subtypes blue hydrogen and brown hydrogen. The gray color refers to the fact that the respective hydrogen production is made from non-renewable hydrogen sources, such as natural gas or coal. According to the International Renewable Energy Agency (IRENA), gray hydrogen is “produced from methane using steam methane reforming (SMR) or coal gasification.” During this process, the carbon in the fossil fuel is released as carbon dioxide (CO2) – therefore, gray hydrogen has a significant carbon footprint.
The subclass blue hydrogen, although indicating that its production occurs through non-renewable sources, represents the fact that carbon dioxide emissions are captured and permanently stored, usually in underground geological formations. Basically, “natural gas is used in steam methane reforming, followed by carbon capture and storage to produce hydrogen.” Brown hydrogen, in turn, refers to the use of coal “in a gasification process to produce hydrogen,” where no capture occurs.
In economic and market terms, gray hydrogen “is mainly used in the petrochemical industry and ammonia production.” According to A. Ajanovic, M. Sayer, and R. Haas, in an article published in 2022, the largest amount of hydrogen produced is gray hydrogen. In general, “about 6% of the natural gas extracted worldwide and 2% of coal are used for gray hydrogen production each year” – the processing of gray hydrogen through natural gas steam reforming “is a well-established process, resulting in low hydrogen costs.”
Specifically regarding blue hydrogen, Marcus Newborough and Graham Cooley emphasize that, for hydrogen to be categorized as blue, it only requires the installation of a carbon capture and storage device (CCUS) to be considered as such, with no definition of the specific amount of carbon that needs to be captured. “Currently, blue hydrogen is considered an intermediate technology before a complete transition to green hydrogen.”
Regarding turquoise hydrogen, its processing is carried out through methane pyrolysis, and its byproduct, which can be sold in the market, is solid carbon (filamentous carbon or carbon nanotubes). Pink or purple hydrogen refers to hydrogen production through electrolysis, where electricity is “generated from nuclear reactors.”
It is important to note that the International Energy Agency (IEA), for example, does not use colors to refer to the different forms of processing or generation of green hydrogen. It generally refers to low-carbon hydrogen, such as that generated from renewable (solar or wind) or nuclear electricity, and hydrogen produced from fossil fuels that includes carbon capture, utilization, and storage (CCUS) technology.
Regarding actual hydrogen production compared to each color range, according to A. Ajanovic, M. Sayer, and R. Haas, gray hydrogen is currently the main production method, with gigawatt capacity, while green hydrogen is still being deployed on a smaller scale. The explanation for this, somewhat obvious, is still due to the investment costs for production and the price of electricity, which, for green hydrogen generation, still “represents about 90% of the total operational costs.”
However, the expectation is that by 2030, the production of green hydrogen will become cheaper than blue and gray hydrogen. On the other hand, an economic and financial report conducted by ING in 2021 pointed out that green hydrogen was costing twice as much as gray and blue hydrogen: “The costs of hydrogen are approximately €3.80 to €4.80 with energy prices of around €40/MWh, which is the long-term annual average in the Northwest European energy market (Germany and the Netherlands).” Thus, “green hydrogen costs about twice as much to produce compared to gray and blue hydrogen (approximately €2/kg).”
Despite its various “colors,” all the spotlight currently points to green hydrogen, either because of its potential to drive the energy transition or the current European demand – with the expectation of a reduction in production costs. It is expected that the varied hydrogen color palette will become increasingly greener and less gray. And, it is reiterated: although the terminology and classification of hydrogen in colors is not mandatory or standardized, it is noticeable that Brazilian bills tend to adopt this identification (notably PL 1808/2022 and PL 2308/2023).
Reflecting on the hydrogen color palette and its symbolism in the energy transition, hydrogen is considered more than just a chemical element; it emerges as a symbol of society’s quest for harmony with the environment. Although the color-coded nomenclature for its variants is not standardized, it paints a vivid picture of the potential and challenges of hydrogen as an energy carrier. Green hydrogen, representing sustainability and innovation, is the shade that captures environmental aspirations, but still carries the weight of prohibitive costs and emerging infrastructure.
