A rainbow of colors dominates almost every conversation on the transition to a low-carbon economy: green, grey, blue, turquoise, pink, yellow, orange – an ever-increasing palette to describe the same colorless, odorless, and highly combustible molecule, hydrogen. The only difference is the chemical process used to produce it.
Hydrogen is the most abundant element in the solar system, but on Earth it naturally occurs only in its compound form. Therefore, it must be produced from molecules that contain it, such as water or hydrocarbons, through specific processes, including thermo-chemical conversion, biochemical conversion, or water electrolysis.
The colors of hydrogen are crucial for the energy transition because each production pathway generates different amounts of greenhouse gas emissions and results in different production costs. Today, renewable (or green) hydrogen is two to three times more expensive than hydrogen produced from fossil fuels. However, thanks to innovation, economies of scale, and carbon pricing policies, these costs are expected to decrease over time.
Furthermore, the world’s dependence on grey hydrogen has a high carbon cost. A shift to blue hydrogen would reduce carbon emissions by half. Although fossil fuel plants utilizing Carbon Capture and Storage (CCS) are well-suited to mitigate emissions, only the adoption of green hydrogen at scale, with its zero-carbon impact, would fully address emissions concerns associated with the production and consumption of hydrogen.
The Colors of Hydrogen
Black or brown hydrogen refers to hydrogen produced by coal gasification. The black and brown colors sometimes indicate the coal type: bituminous (black) and lignite (brown). This process generates significant CO2 emissions (19 tCO2/tH2).
Blue hydrogen refers to hydrogen produced mainly from natural gas by steam gas reforming, paired with carbon capture and sequestration (CCS). Blue hydrogen has a much lower carbon intensity than grey hydrogen, with estimates ranging from 1 to 4 tCO2/tH2. Although the use of CCS increases costs, blue hydrogen currently remains the cheapest “clean” alternative to grey hydrogen.
Green or renewable hydrogen refers to hydrogen produced from renewable energy sources like wind and solar through a process known as water electrolysis, where an electrolyzer splits water molecules into oxygen and hydrogen. There are no CO2 emissions generated during the production process. Today, green hydrogen costs are significantly more than those of grey hydrogen. It accounts for less than 0.1% of the world’s hydrogen production.
Grey hydrogen refers to hydrogen produced from fossil fuels mainly by steam gas reforming or coal gasification. It generates significant CO2 emissions, between 10-19 tons of CO2 per ton of H2 (tCO2/tH2). Currently, over 95% of the world’s hydrogen consumption is grey hydrogen.
Orange hydrogen refers to emerging processes to produce hydrogen using plastic waste as a feedstock. Orange hydrogen may offer a solution to both the clean energy problem and issues surrounding disposal of plastic waste. Currently, orange hydrogen remains in the early development stage, with various technologies and production processes, including pyrolysis, microwave catalysis, and photo-reforming under evaluation.
Pink hydrogen refers to hydrogen produced by water electrolysis powered using nuclear power, a clean, but non-renewable source of energy. It does not generate CO2 emissions.
Purple hydrogen refers to hydrogen produced by water electrolysis using both nuclear power and heat.
Red hydrogen refers to hydrogen produced by high-temperature catalytic splitting of water using the heat and steam generated from nuclear plants. The process requires much less electricity than traditional electrolysis.
Turquoise hydrogen refers to hydrogen produced from natural gas under a process known as methane pyrolysis, in which natural gas is decomposed into hydrogen and solid carbon at high temperatures. Currently, turquoise hydrogen remains in the early development stage.
Yellow hydrogen refers to green hydrogen produced from solar energy. It does not generate CO2 emissions. Estimates suggest that yellow hydrogen may become the cheapest form of renewable hydrogen in the medium term.