Answer
Hydrogen is classified by color codes based purely on its production pathway, energy source, and resulting carbon footprint:
1. Grey Hydrogen
Grey hydrogen is hydrogen produced from fossil fuel, mainly natural gas/methane, without capturing the CO₂ produced.
Main method: Steam Methane Reforming, SMR CH4+H2O→CO+3H2
Then water-gas shift reaction: CO+H2O→CO2+H2
So final products are: H2+CO2
Key point: CO₂ is released to atmosphere.
Emission: High carbon emission, roughly 9–12 kg CO₂ per kg H₂, depending on process and fuel source.
2. Blue Hydrogen
Blue hydrogen is similar to grey hydrogen, but the CO₂ produced during the process is captured and stored or used.
Production method: Natural gas → SMR/ATR → Hydrogen + CO₂
CO₂ → captured by CCS/CCUS
CCS = Carbon Capture and Storage
CCUS = Carbon Capture, Utilisation and Storage
Key point: It is fossil-fuel based, but with reduced CO₂ emissions.
Emission: Lower than grey hydrogen, but not zero because:
CO₂ capture is not 100%
methane leakage may occur during natural gas extraction and transport
energy is required for carbon capture
3. Green Hydrogen
Green hydrogen is produced by electrolysis of water using renewable electricity, such as solar, wind, hydro, or renewable offshore power.
Reaction: 2H2O→2H2+O2
Electricity source: renewable energy.
Key point: No CO₂ is produced during hydrogen production.
Emission: Very low or near zero, depending on the electricity source and lifecycle emissions.
4. White Hydrogen
White hydrogen means naturally occurring hydrogen found underground in the Earth’s crust.
It is not manufactured like grey, blue, or green hydrogen.
Sources may include:
natural geological reactions
serpentinisation of rocks
radiolysis of water
deep-earth natural hydrogen seepage
Key point: It is natural hydrogen extracted from underground reservoirs.
Emission: Potentially very low, but commercial production is still developing.