In the field of hydrogen energy, there is a widely circulated saying: 'Grey hydrogen is undesirable, blue hydrogen can be used, waste hydrogen can be recycled, green hydrogen is the direction.' The terms 'grey hydrogen, blue hydrogen, green hydrogen' mentioned here obviously do not refer to hydrogen having these colors, but rather represent three categories based on the carbon emission intensity during the production process:
1. Grey Hydrogen
Hydrogen produced through the combustion of fossil fuels such as petroleum, natural gas, and coal is known as grey hydrogen. Grey hydrogen accounts for approximately 95% of global hydrogen production. This production method has clear cost advantages compared to blue and green hydrogen, with the lowest cost, mature operational methods, fewer required equipment, and a smaller footprint. However, this method of hydrogen production has significant drawbacks, including the highest carbon emissions and the consumption of non-renewable resources.
2. Blue Hydrogen
Blue hydrogen is also produced from fossil fuels, primarily natural gas. It involves heating, steam, and pressure to convert methane into H₂ and CO₂. During this process, some CO₂ is captured, compressed to over 100 atm, and injected into porous rock formations more than 1000 meters underground, where it can be stored for tens of thousands to millions of years. This process is known as Carbon Capture and Storage (CCS).
While blue hydrogen production does result in some carbon emissions, it generally meets emission restrictions in most countries and emits less carbon than grey hydrogen. Compared to green hydrogen, blue hydrogen has two distinct advantages: lower electricity requirements and lower costs.
However, the capital and operational costs for producing blue hydrogen are still high, and a reliable supply of natural gas is required. Additionally, CCS technology requires relatively rare geological conditions, making blue hydrogen more challenging to obtain on a global scale than green hydrogen.
3. Green Hydrogen
Green hydrogen is produced using renewable energy sources and is characterized by its sustainability. It should support the expansion of the 'green hydrogen' market.
Currently, due to the rapid development of new energy sources such as wind and solar power, the electrolysis of water using renewable energy has become a focal point. This means producing 'green hydrogen' using 'green electricity.' Green electricity is generated from renewable sources such as wind, solar, hydropower, and geothermal energy. It is then used to split water and produce H₂, resulting in extremely low carbon emissions.
The main methods for producing 'green hydrogen' include electrolysis of water using renewable energy, photocatalytic hydrogen production, thermochemical hydrogen production, and biological hydrogen production.
The electrolysis of water for hydrogen production is relatively mature, but it has low production capacity, and there is significant room for improvement in the durability and reliability of electrolysis cells. In terms of cost, producing one kilogram of hydrogen requires about 60 kWh of electricity. The price of 'green hydrogen' is mainly related to the cost of renewable electricity, which must be reduced to below 0.2 yuan per kWh to be cost-competitive with 'grey hydrogen.'
In fact, green hydrogen technology is not limited to these methods.
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