|
|
| Fig. 1: Comparison of cost of "grey", "blue", and "green" H2 production. [1] (Image source: Y. Zhu, after Laughlin and Freund. [1]) |
Steam Methane Reforming (SMR) with Carbon Capture and Storage (CCS) and Methane Pyrolysis are two distinct methods for producing hydrogen with low carbon intensity, each with their unique approaches and environmental impacts. This paper compares the two in terms of their costs in order to understand which method has greater potential as a means of low-carbon hydrogen production.
SMR is the most common method for producing hydrogen today. In SMR, methane (from natural gas) reacts with steam under high pressure and temperature in the presence of a catalyst to produce hydrogen and carbon monoxide. Further reaction of carbon monoxide and steam produces more hydrogen and carbon dioxide. The key to reducing the carbon footprint of this process is CCS. Carbon Capture, Utilization, and Storage involves capturing the CO2 produced during SMR and either reusing it in other industrial processes or storing it underground to prevent its release into the atmosphere. This mitigates the otherwise substantial carbon emissions from SMR, making it a more environmentally friendly option.
Methane Pyrolysis is a relatively newer technology compared to SMR. Methane Pyrolysis decomposes methane into hydrogen and solid carbon under high temperatures, without the presence of oxygen. This process results in hydrogen and solid carbon, rather than CO2. The solid carbon can be used or stored more easily than CO2, potentially making methane pyrolysis a carbon-neutral process.
Methane Pyrolysis has a significant advantage over traditional SMR in terms of carbon emissions. It produces solid carbon instead of carbon dioxide, which is easier to capture and store. SMR, even with CCS, still has a carbon footprint associated with the energy required for capturing and storing CO2. However, if the carbon captured in SMR with CCS is utilized effectively, it can also become a lower-emission process.
SMR with CCS is currently more established and hence more cost-effective due to existing infrastructure and technological maturity. Methane Pyrolysis, being in the developmental stage, tends to be more expensive due to the costs associated with new technology development and scale-up. However, as the technology matures and scales, the cost is expected to decrease.
The current market rate for hydrogen, excluding any carbon tax, is around $1.00 per kilogram in the United States. Estimating the price is challenging since the majority of hydrogen is produced for internal consumption at specific plants, a process known as "captive" production. Furthermore, the prices in the markets where the remaining hydrogen is sold are not publicly disclosed, as these markets are proprietary. [1] Essentially all the Hydrogen produced industrially at the present time is "grey".
Laughlin and Freund therefore estimated the theoretical cost of H2 production using SMR with CCS, termed as "blue" hydrogen, and compared it with cost of "grey" hydrogen (produced by just SMR without CCS) and green hydrogen (produced by water electrolysis). [1] The results are shown in Fig. 1. Their theoretical cost of blue hydrogen in the US is around $1.5/kg. The cost associated with CCS is around $0.47/kg of hydrogen. [2]
H2 produced through methane pyrolysis is termed "turquoise" hydrogen. As mentioned, the technology behind its production is so new that cost predictions are challenging. The chemical process for creating "turquoise" hydrogen is also less energy-efficient compared to the steam reforming method currently employed for "grey" hydrogen production. As a result, its per-kilogram cost is anticipated to fall between the costs of "grey" and "blue" hydrogen. [1]
Machhammer et al. estimated the cost of "turquoise" hydrogen and grey hydrogen to be €3/kg and €2/kg, respectively, based on European natural gas prices. [3] The "grey" hydrogen cost estimate is similar to that from Laughlin and Freund's estimate for Europe. The estimated "turquoise" hydrogen cost is similar to that of "blue" hydrogen cost from Laughlin and Freund. [1] It should be noted that in Machhammer's estimate, the pyrolysis carbon produced is given a €200 per ton of carbon price. This is approximately the price of petroleum coke of moderate purity that is used in metallurgy industry, which is a reasonable approximation.
Parkinson et al. also compared the cost of H2 production using different methods. [4] The cost of H2 produced is around $1.2/kg by SMR, $1.9/kg by SMR + CCS, and $1.3/kg by methane pyrolysis. In this analysis, they compared costs of each method for producing 100kta of hydrogen. It is estimated that the total capital investment would be around $250 million for SMR without CCS and $350 million for methane pyrolysis. The operating cost for SMR is around $95 million and around $120 million for methane pyrolysis. It should be noted that this analysis used the assumption that the carbon by-product would have a value of $150/t. They also conducted a sensitivity analysis to show that the cost of hydrogen would drop to below $0.5/kg if the carbon product has a value of $500/t. Overall, the cost of methane pyrolysis for hydrogen production is higher than that of "grey" hydrogen but lower than "blue" hydrogen, which is in line with the analysis by Laughlin and Freund. [1] Both analyses from Machhammer and Parkinson show that the cost of carbon product has a significant influence on the final cost of hydrogen. [3,4]
The research by Parkinson's group highlighted the largest markets for carbon products, noting that the annual demand for metallurgical coke is approximately 0.6 Gt and for carbon black, used primarily in tire production, is about 10 Mt.[4] This raises the issue of market size and price adequacy for carbon yielded through methane pyrolysis. If methane pyrolysis were to replace SMR for hydrogen production, it would generate an estimated 0.15 Gt of solid carbon annually. [4] While the demand for metallurgical coke exceeds this amount, utilizing the carbon by-product from methane pyrolysis in steel production would emit carbon dioxide, thus negating the benefits of a low-carbon hydrogen production process. Furthermore, the demand for carbon black is significantly less than 0.15 Gt. Therefore, if the carbon from pyrolysis is not used in metallurgical coke, it could oversaturate the remaining carbon market and drastically reduce the carbon price. However, the methane pyrolysis development is still early and having additional carbon by-products that can generate value in the carbon black market could help accelerate the commercialization of this pyrolysis technology.
The analyses above have shown that methane pyrolysis has slightly lower or similar cost to SMR + CCS, and is largely influenced by the value of the carbon by-product. As methane pyrolysis is still an emerging technology, its cost could be potentially lower in the future and more research and investment should be dedicated to improving its technology readiness level (TRL). While SMR + CCS might have slightly higher cost than methane pyrolysis in the long run, its TRL is much higher and is already deployed at commercial scale in various plants. [1] These existing plants should be kept on running to capture CO2 while we further develop methane pyrolysis and water electrolysis technologies.
© Yutong Zhu. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. B. Laughlin and S. W. Freund, "Economics of Hydrogen Fuel," in Machinery and Energy Systems for the Hydrogen Economy, ed. by K. Brun and T. Allison, Elsevier (2022).
[2] "The Future of Hydrogen," International Energy Agency, June 2019.
[3] O. Machhammer, A. Bode, and W. Hormuth, "Financial and Ecological Evaluation of Hydrogen Production Processes on Large Scale," Chem. Eng. Technol. 39, 1185 (2016).
[4] B. Parkinson et al., "Hydrogen Production Using Methane: Techno-Economics of Decarbonizing Fuels and Chemicals," International Journal of Hydrogen Energy 43, 2540 (2018).