By Afriyie Ankamah
Hydrogen is derived from many sources and known by many colours: brown, black, purple, pink, red, turquoise, blue, green and even white and is the most abundant element in the universe. It is essential for life and its predominant use is in the refining and ammonia industries. It is also used in the transport industry as a fuel and this sector is projected to be the fastest growing usage sector within the next few decades. In its pure state it is very scarce to find as it is mostly combined with other substances such as water.
Here, we’re discussing the different methods for producing hydrogen – and what the different colours mean.
How is Hydrogen Produced?
Hydrogen production can broadly be classified into fossil, renewable and nuclear sources (see Figure 1).
Figure 1. Sources of hydrogen production and their colours. Source: newenergy.slb.com
Hydrogen Production from Fossil Sources
Production using coal and natural gas constitute the fossil sources and these are achieved by reforming, pyrolysis and gasification processes.
- The reforming of the fossil-based sources—mainly natural gas—to form hydrogen is currently achieved by three main processes:
- steam methane reforming (SMR)
- partial oxidation
- autothermal reforming (ATR).
- Coal gasification process involves the use of crushed coal through a process of partial oxidation to produce hydrogen.
- Pyrolysis is also a means to produce hydrogen when methane is left to decompose producing solid carbon and no carbon dioxide.
Producing Hydrogen from Renewable Sources
With regards to the renewable sources, electricity from wind, solar, hydro and geothermal sources are used to electrolyse (split) water into hydrogen and oxygen.
Using Nuclear to Produce Hydrogen
Electricity from nuclear sources—mainly uranium—is also used to produce hydrogen through the electrolysis process.
To produce hydrogen from renewable or nuclear sources, there are four different types of electrolysis that can be used:
- alkaline water electrolysis (AWE)
- solid oxide electrolysis (SOE)
- polymer electrolyte membrane (PEM)
- Microbial electrolysis cell (MEC).
In addition to electrolysis, there is also gasification of renewable sources. The main renewable that is used in this process is biomass and this gasification is achieved through thermochemical processes.
The Colours of Hydrogen
You’ve seen how hydrogen is produced and from their various sources, but what’s behind the colours? Let’s take them one by one:
- Green hydrogen. This is hydrogen produced from wind or solar energy
- Yellow hydrogen. Hydrogen only from solar sources.
- Purple, pink or red hydrogen is produced from nuclear energy.
- Grey hydrogen is produced from natural gas without capturing the carbon dioxide emitted in the production process.
- Blue hydrogen is produced from natural gas when the emissions are captured and stored permanently.
- Black or brown hydrogen is produced from coal, depending on the coal used.
- Turquoise hydrogen is produced from pyrolysis, and this produces a solid carbon aside from the hydrogen.
- White hydrogen is produced using electricity from the grid (which is a mixture of electricity sources) in the production process.
These various hydrogen sources have significant differences in their carbon footprint. Fossil sources generate tons of carbon dioxide while renewable and nuclear sources have zero carbon emissions. According to the IEA, hydrogen production generates around 830 million tonnes of carbon dioxide per year which when placed in perspective is equivalent to the carbon emissions of the UK and Indonesia combined. Currently, less than 0.4% of hydrogen production uses electrolysis with the electricity from renewable sources and about 98% of hydrogen production emit significant amount of carbon dioxide.
The cost of production also varies with the different production methods. Currently, the fossil-based sources are the cheapest compared to the renewable options, however, there are projections by IRENA that the cost of green hydrogen will be competitive by the next few decades due to projections in lowering renewable electricity and electrolyser costs.
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