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| Fig. 1: Comparison of CO2 emissions estimates for shipping based different sources of Hydrogen. [1,6,7] (Image Source: N. Mishra) |
In 2015, the shipping industry accounted for about 932 million tonnes of CO2 of emissions. This makes the shipping industry's contribution to about 2.6% of the total global emissions of CO2. [1] This has led to a massive pressure on maritime organisations to decarbonise their operations for the world to be able meet its emission goals. Emergent technologies that could potentially solve this problem for the maritime industry include biofuels, battery-electric propulsion, or wind-assisted propulsion. [2] The focus of this article, however, is hydrogen as a fuel. In particular, this article takes a look at the potential emissions from the maritime industry, if it were to transition to hydrogen made from todays technology. There are many ways to produce hydrogen, including using renewable energy, but the most common way is to make hydrogen via Steam Methane Reforming or Autothermal Reforming (ATR), which is an extremely carbon intensive process as both the energy required to make the steam, and the source of the hydrogen itself are very carbon intensive. [3] When burnt, hydrogen has minimal emissions, with water being the only bi-product of complete combustion. Hydrogen is a promising fuel and is especially beneficial for short-sea and coastal shipping, where storage limitations pose less of a challenge. Although promising, substantial investments in infrastructure and technology are needed to overcome challenges in storage, bunkering, and total ownership costs. In this article, however, we do not consider the cost of this transition, or the cost of operating the entire industry on hydrogen, but only the emissions that this fuel would result in.
A significant volume of hydrogen will need to be produced if the global maritime industry was to transition to hydrogen as a source of energy for running its engines. It is estimated that the shipping industry consumed about 215 million tonnes of Residual Fuel Oil, about 77.5 million tonnes of Distillate Oil and about 6 million tonnes of LNG. [1] The energy content of these fuels is tabulated in Table 1.
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| Table 1: Energy Content of the most commonly used shipping fuels. |
Using Table 1 we can estimate the total energy consumed by the shipping industry industry in Joules/year.
| Energy | = | 4.16 × 1010 J tonne-1 × 215 × 106 tonnes y-1 + 4.19 × 1010 J tonne-1 × 77.5 × 106 tonnes y-1 |
| + 4.89 × 1010 × 6 × 106 tonnes y-1 | ||
| = | 1.25 × 1019 J y-1 |
Hydrogen has an energy density of 1.42 × 1011J tonne-1. [9] Using this, we can determine the total amount of hydrogen that would be required to provide 1.25 × 1019 J y-1 worth of energy for the shipping industry.
| Hydrogen Fuel Required | = | 1.25 × 1019 J y-1 1.42 × 1011 J tonne-1 |
= | 8.80 × 107 tonnes y-1 |
This means that to meet the shipping industry's energy demands about 88 million tonnes of hydrogen will be needed. Lets look at the potential emissions if this amount of hydrogen was produced using today's technologies scaled up to meet this demand.
We will consider hydrogen production by three primary methods and estimate emissions from each method.
Coal-Gasification: Hydrogen production from synthesis gas or syngas produced from coal gasification currently results in 18-20 kg of CO2 emissions per kilogram of Hydrogen produced. [4] If used for all of the hydrogen for global shipping, this would amount to approximately 1.76 billion tonnes of CO2 annually, making it significantly more harmful for the environment. In 2018, the total CO2 emissions from the shipping industry stood at approximately 932 million tonnes, meaning this transition would effectively double the total emissions. [1]
Methane Pyrolysis Using Natural Gas and Steam (Steam Methane Reforming): this is a much less carbon-intensive method. Methane pyrolysis uses natural gas as a heat source to produce steam, which is then used to break the bonds of methane yielding in hydrogen and carbon dioxide. This process produces approximately 9.0 kg CO2 per kg of hydrogen produced. [6] At the scale of 88 million tonnes of hydrogen, this approach would still result in 792 million tonnes of CO2 emissions annually, which will not make the smallest dent on the shipping industry's emissions.
Solar-Powered Electrolysis: Hydrogen produced via electrolysis, powered entirely by solar energy, represents the most sustainable option, with nearly zero CO2 emissions if clean energy is consistently supplied. However, this pathway does have lifetime emissions including the emissions from making solar panels etc. These amount to about 3.08 kg of CO2 per kilogram of hydrogen produced. [7] This would mean the total emissions for 95 million tonnes of hydrogen would stand at 271.04 million tonnes of CO2. This does not represent a net-zero scenario but substantially reduces the industry's contribution to global emissions. However, achieving this outcome would require considerable investments in renewable energy infrastructure, particularly in solar panels, to meet the maritime sectors demand at the necessary scale.
These results are summarized in Fig. 1.
Hydrogen presents a promising path toward a decarbonised maritime industry. However, it seems like even the most advanced and cleanest hydrogen production technology will not be able to bring the shipping industry to net-zero emissions. While burning hydrogen would be clean, producing the hydrogen itself would be extremely carbon intensive. There seems to be no existing pathway for decarbonising the maritime industry entirely using hydrogen with the present day's technology. This article does not focus on other challenges of hydrogen such as storage and cost which would add additional hurdles in implementing the hydrogen-powered future of the shipping industry.
© Naman Mishra. 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] N. Olmer et al., "Greenhouse Gas Emissions from Global Shipping, 2013 - 2015," International Council on Clean Transportation, 2017.
[2] "A Pathway to Decarbonise the Shipping Sector by 2050," International Renewable Energy Agency, 2021.
[3] Y. Bicer and A. Dincer, "Life Cycle Assessment of Nuclear-Based Hydrogen and Ammonia Production Pptions: A Comparative Evaluation," Int. J. Hydrogen Energy 42, 21559 (2017).
[4] T. K. Blankand and P. Molly, "Hydrogen's Decarbonization Impact for Industry," Rocky Mountain Institute, January 2020.
[5] "BP Statistical Review of World Energy 2022," British Petroleum, June 2022.
[6] Sun et al., "Criteria Air Pollutants and Greenhouse Gas Emissions From Hydrogen Production in U.S. Steam Methane Reforming Facilities," Environ. Sci. Technol. 53, 7103 (2019).
[7] S. Sadeghi, S. Ghandehariun, and M. A. Rosen, "Comparative Economic and Life Cycle Assessment of Solar-Based Hydrogen Production For Oil and Gas Industries," Energy 208, 118347 (2020).
[8] "Assessment of Fuel Oil Availability," CE Delft, July 2016.
[9] J. O. Abe et al., "Hydrogen Energy, Economy and Storage: Review and Recommendation," Int. J. Hydrog. Energy 44, 15072 (2019).