be sent for permanent storage (CCS) or utilisation (CCU). • Turquoise hydrogen is produced by the pyrolysis of methane, producing hydrogen and elemental carbon, thus avoiding CO 2 emissions. This process is in the early stages of commercialisation. The carbon intensity of turquoise hydrogen is determined by the source of methane and the supply of heat to drive the pyrolysis process, and can range from 1.5 to less than 0.6 kgCO₂e/H₂ (Singhania, et al., 2023), (Schubak, 2022). • Green hydrogen is generated by the electrolysis of water using electricity. The carbon intensity for hydrogen from electrolysis ranges from 1 to 0.3, depending on the share of renewable energy in the electricity mix. Electrolysis is the only process that uses water in place of hydrocarbons as the hydrogen source. Table 1 illustrates the range of carbon intensities for the different colours of hydrogen. There is a progressive fall in carbon intensity values, moving from black or brown to grey, then blue to turquoise or green hydrogen. Carbon intensities may approach zero for turquoise or green hydrogen compared with as much as 18-20 KgCO₂/KgH₂ for coal. While using colours to differentiate routes to produce hydrogen has been valuable, a consensus is emerging that carbon intensity is more appropriate (IEA, 2023). Given that there is a range of intensities for any given production route, this will drive the necessary shift towards production methods with the lowest carbon intensity, for example, ensuring that renewable electricity is used for electrolysis. This is reflected in the increasing level of investments in hydrogen electrolysis projects worldwide. Alkaline and polymer electrolyte membrane (PEM) electrolysers are both commercially available technologies. Worldwide, the three biggest manufacturers in 2022 were all Chinese companies, with a combined manufacturing capacity of 4.1 GW (Klevstrand, 2022). Global investments amounted to $1.2 billion, representing a doubling in commissioned capacity to 1.2 GW, with 25% of this capacity installed in China (Klevstrand, 2023). Advanced electrolyser technologies such as solid oxide and anion exchange membranes are nearing commercial deployment.
100%
300
Brown and grey Blue Green Share of Green H
90%
250
80%
70%
200
60%
150
50%
40%
100
30%
20%
50
10%
0%
0
2019
2030
2040
2050
of carbon dioxide (CO 2 ) and, as a result, has the highest ‘carbon intensities’, greater than 18kg CO₂/kgH₂ (see Figure 1 ). • Grey hydrogen is produced from methane or natural gas via the steam methane reforming process (SMR) with carbon intensities ranging from 11 to 9kgCO₂/kgH₂. • Blue hydrogen uses an additional process to capture the CO 2 emissions from gasification or reforming to reduce the carbon intensity of the process ranging from 10 to 0.6 kgCO₂e/kgH₂, determined by the effectiveness of the carbon capture process. The captured carbon must then Figure 1 Global hydrogen production outlook Source: (NITI Aayog, 2022)
Gasi cation & reforming Reforming (SMR) Gasi cation & reforming plus CCS Reforming with CCS Production process
Carbon intensity (kg CO /kg H )
Colour Feedstock
Brown Coal or lignite
20 to 18
Methane (natural gas)
Grey
11 to 9
9.2 to 3.5
Coal or lignite
Blue
Methane Biomethane Methane (natural gas)
3.9 to 0.8
1.5 to <0.6 1 to ≈0
Pyrolysis
Turquoise
Biomass
Gasi cation
1.7 to 1.5
Green
Solar, wind electricity plus electrolysis
1 to ≈0
Water
Table 1 The different colours of hydrogen based on feedstock and production process Modified from (Nelson, 2021)
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