Production and carbon intensity — route and electricity over color names
Build the habit of evaluating hydrogen by carbon intensity — including route, electricity source, and capture rate — rather than by the color names grey / blue / green.
Hydrogen takes its character from the "route", not the "source"
Hydrogen itself is the same H₂, but its meaning changes greatly depending on how it is produced. Representative entry points are natural-gas reforming, coal or biomass gasification, water electrolysis, and, looking further ahead, thermochemical methods. Thermochemical methods are an umbrella term for processes that combine high-temperature heat (from nuclear reactors or concentrated solar, for example) with multiple chemical reactions to split water; the iodine-sulfur (IS) process is a representative example. In real-world discussions, the key points are which feedstock you start from, which electricity you use, and how CO₂ is handled.
This is where color names like grey, blue, and green are commonly used. But color names are just shorthand. What you really want to look at is carbon intensity — the CO₂-equivalent emitted to make and deliver 1 kg of hydrogen, in units of kg-CO₂e/kg-H₂ — and evaluation needs to include electricity source, capture rate, methane leakage, and the extra energy used for compression and liquefaction.
Electrolysis is not automatically low-carbon
Water electrolysis makes H₂ from water, so it intuitively looks "low-carbon", but the actual carbon intensity depends on the electricity used. The decarbonization benefit is most evident when paired with surplus renewables or low-carbon power sources; running it directly on a grid with high carbon intensity can fail to deliver the expected effect.
Similarly, fossil-derived routes can potentially lower their carbon intensity when combined with CCUS (Carbon Capture, Utilization and Storage), but that does not automatically put them on equal footing. The capture rate (for example, what percentage of CO₂ from a fossil-based step is actually captured), leakage from storage sites, and methane slip all remain as residual issues. That is why, even when a conversation starts with "colors", practical work always comes back to the "route" and the "numbers".
Replacing existing demand is the main focus today
Global hydrogen demand is still dominated by refining, chemical, and industrial uses, and low-emission hydrogen remains a minority. So in the short term it is important to decarbonize hydrogen that is already in use, not only to pursue new applications.
Practice 1–3 — Representative routes and handling color names
Check representative production routes and how to handle color names.
Q1. Which is closest to the most common hydrogen production method today?
Recall where most of today's commercial hydrogen comes from.
The representative common hydrogen production method today is natural-gas reforming. This course uses it as a baseline for comparing other routes.
Q2. Which most directly determines the carbon intensity of hydrogen from water electrolysis?
Electrolysis is a device that converts electricity into H₂.
The carbon intensity of water electrolysis depends mainly on the source of electricity used. Whether it is renewable or a low-carbon source matters.
Q3. Which is the most appropriate way to handle color names such as green / blue / grey?
Colors are labels; evaluation is in numbers.
Color names are useful shorthand, but in practice you need to confirm with carbon intensity, which includes production route, electricity source, and capture rate.
Practice 4–5 — Blue hydrogen and hydrogen as an energy carrier
Check blue hydrogen and how hydrogen looks as an energy carrier.
Q4. Which is closest to a generally accepted description of 'blue hydrogen'?
'Blue' is not a color — it is shorthand for the origin.
Blue hydrogen generally refers to fossil-derived production combined with CO₂ capture and storage.
Q5. Which is closest to hydrogen's role when viewed as a way to convert fluctuating renewable output into hydrogen first and use it later?
Think about which role in 'make → store → transport → use' this matches.
Hydrogen serves as an energy carrier that moves other energy, such as electricity, across time and place for use.
Chapter 3 summary
- Even for the same H₂, carbon intensity changes with the route and the electricity source.
- Grey / blue / green are conversational shorthand; evaluation ultimately comes back to the numbers.
- Electrolysis depends on the electricity; fossil-derived routes depend on the capture rate. Color names alone do not decide.
- In the short term, decarbonizing existing demand (refining, chemicals, industry) is also an important issue.