1 Introduction
Hydrogen can be produced from a variety of primary energy sources. When energy is obtained through chemical reactions using hydrogen as a fuel, hydrogen emits only water and no environmental emissions such as carbon dioxide (CO2). Therefore, hydrogen has garnered worldwide attention as an energy carrier that contributes to diversifying energy sources away from fossil fuels and achieving a carbon-neutral society.
Given Japan’s limited natural energy resources and its heavy reliance on imports, it has prioritized hydrogen as a secondary energy source from an early stage to enhance energy security and address environmental challenges. This focus began in 1974, shortly after the 1973 Oil Crisis, with the launch of a national initiative named the “Sunshine Project” to tackle the energy crisis and environmental pollution [
1]. By utilizing inexhaustible and clean energy, including hydrogen, the project sought to alleviate the energy crisis that resulted from the exhaustion of petroleum resources. The hydrogen project focused on research and development of hydrogen-related technologies, including production, mass transportation and storage, utilization and safety technologies, and continued until 1992. Its successor, the World Energy NETwork Project (1993–2002) [
2], aimed to produce hydrogen from abundant renewable energy sources existing worldwide and utilize it as the main energy source as an international clean energy system supply chain. In conjunction with these national projects, Japanese private companies have led the global advancement of hydrogen-related technologies, including the market launch of hydrogen utilization technologies such as residential fuel cells in 2009 and fuel cell vehicles in 2014.
In December 2017, Japan unveiled the Basic Hydrogen Strategy [
3], the world’s first national hydrogen strategy, which outlined a vision for realizing a hydrogen-based society. This strategy emphasized the establishment of an international hydrogen supply chain to meet future domestic demand by utilizing low-cost hydrogen sources that are abundant overseas and utilizing liquid hydrogen carriers. Then, in December 2020, Japan formulated the “Green Growth Strategy for Carbon Neutrality in 2050” (revised in June 2021) [
4] as an industrial policy to create a “virtuous circle between the economy and the environment” to achieve the “Declaration of Carbon Neutrality in 2050” announced by the Japanese government in October 2020. This strategy outlined an action plan for 14 priority sectors expected to grow toward carbon neutrality, with hydrogen being one of the key areas. The “Green Innovation Fund” [
5], valued at two-trillion Japanese Yen, was established among these priority areas. This Fund aimed to provide continuous governmental support for research and development and social implementation in areas where policy effects are significant and long-term continued support is necessary for effective implementation. The Fund is currently implementing hydrogen-related projects, such as the “establishment of large-scale hydrogen supply chains,” “hydrogen production through water electrolysis using power from renewables,” and “fuel NH
3 supply chain establishment.”
In June 2023, the Japanese government released a revised version of its “Basic Hydrogen Strategy” [
6], reflecting two significant milestones: the “Declaration of Carbon Neutrality in 2050” and the drastic change in the global energy supply-demand structure since February 2022. Considering these domestic and international circumstances, the revised Strategy outlines a necessary vision for the public and private sectors to achieve carbon neutrality by 2050. It clearly states the challenges and policies to address and expresses Japan’s commitment to realizing a hydrogen society at an accelerated pace. In line with this revised strategy, the government plans to generate 15 trillion Japanese Yen in public and private hydrogen investments. Additionally, in May 2024, Japan’s parliament enacted the “Hydrogen Society Promotion Act” designed to promote the supply and utilization of low-carbon hydrogen, aiming to transition to a decarbonized economic structure. The law encourages the supply and use of low-carbon hydrogen, which emits less CO
2 during production than conventional methods. The government certifies the business plan of companies that produce and import hydrogen and subsidizes the price difference with conventional fuels.
Generally, hydrogen is produced as a gas from various primary energy sources and then converted into a more transportable medium for storage and transport. Thus, establishing a hydrogen supply chain is essential. While hydrogen itself does not emit CO2 when used as a fuel, environmental emissions are inevitably generated at various stages of the hydrogen supply chain due to the energy and other resources required in each process. Therefore, to contribute to global carbon neutrality, it is imperative that its entire supply chain maintains low carbon emissions from a life cycle assessment (LCA) perspective.
The significance of hydrogen’s lifecycle CO
2 emissions (LCCO
2) has been acknowledged globally. For example, Cho et al. [
7] reviewed 32 LCA publications on hydrogen produced from renewable sources in different countries and regions and provided the LCCO
2 ranges for each renewable source. Oni et al. [
8] conducted a techno-economic and life cycle analysis of three hydrogen production pathways from natural gas (NG) in Canada—steam methane reforming (SMR) with carbon dioxide capture and storage (CCS), autothermal reforming with CCS, and NG decomposition. Verma and Kumar [
9] calculated the lifecycle greenhouse gas (GHG) emissions of hydrogen from underground coal gasification combined with CCS. These studies focus on the well-to-production gate of hydrogen, which covers the extraction of hydrogen feedstock for hydrogen production.
Other studies have evaluated LCCO
2 emissions across entire hydrogen supply chains. For instance, Chen et al. [
10] conducted an LCA of automotive liquid hydrogen fuels in China, considering both renewables and fossil fuels for hydrogen sources and liquid hydrogen (LH) for transport. Wulf and Zapp [
11] analyzed LCA of the hydrogen supply chain using liquid organic hydrogen carriers (LOHC) in Germany, focusing on alkaline water electrolysis using wind power and toluene (TOL)— methylcyclohexane (MCH) and dibenzyl toluene—perhydro dibenzyl toluene systems as the target LOHC. Akhtar et al. [
12] calculated LCCO
2 emissions for hydrogen produced via wind-powered alkaline water electrolysis using different delivery pathways in Western Australia, including compressed hydrogen via pipeline, compressed gas via tube trailer, LH, LOHC (dibenzyl toluene) and NH
3. The authors also evaluated the environmental benefits of using imported renewable hydrogen, transported via liquid hydrogen carriers, as fuel for vehicles [
13] and power generation [
14], compared to conventional technologies in Japan.
In addition to these national and regional LCA studies, the International Partnership for Hydrogen and Fuels Cells in the Economy (IPHE) published a revised methodology for calculating the GHG emission intensity of hydrogen supply chains in November 2022 [
15]. This methodology focuses on well-to-gate emissions, covering the operational emissions from upstream (hydrogen feedstock, extraction of feedstock, and delivery), hydrogen production, and hydrogen distribution and storage. To ensure fair comparability with other energy, the emissions associated with capital goods necessary for the supply chain operation are set out of the boundary of the IPHE methodology.
Based on the IPHE methodology, the International Energy Agency (IEA) also examined the use of hydrogen emission intensity in regulatory and certification schemes in 2023 [
16]. The revised Basic Hydrogen Strategy in Japan [
6] stipulates a low-carbon target based on the IPHE methodology and promotes the use of hydrogen that complies with this target. Considering the current technological level, a low-carbon target of 3.4 kg-CO
2-equivalent (CO
2e) or less per kilogram of hydrogen at the well-to-production gate has been set.
However, it should be noted that this low-carbon target only considers the well-to-production gate of hydrogen. Since Japan aims to establish an international hydrogen supply chain, it is preferable that gaseous hydrogen is converted to liquid form for easier storage and transport. Hence, it is interesting to see how much the LCCO2 of the imported hydrogen will be if the system boundary is expanded to cover the entire hydrogen supply chain from the well-to-production gate.
This paper explores the LCCO
2 emissions of imported hydrogen pathways envisaged in Japan using the IPHE methodology, focusing on the well-to-delivery gate emissions of the pathways. Data used for this analysis were collected in a previous project that investigated the international supply chain of hydrogen produced, including both water electrolysis using renewable electricity and fossil fuel-based hydrogen production with CCS [
17].
2 Method and assumptions
2.1 Overviews
Fig.1 shows the processes involved in the assumed imported hydrogen supply chain to Japan, as defined within the well-to-delivery gate system boundary of this paper. Low-cost, low-carbon hydrogen produced overseas from primary energy sources will be conditioned or converted into liquid hydrogen carriers and stored at loading ports for maritime transport. Upon arrival at Japanese unloading ports, the liquid hydrogen carriers are stored, conditioned, or converted back into gaseous hydrogen before delivery. This paper considers three hydrogen production options and their respective countries: water electrolysis using renewable power in the United Arab Emirates (UAE), steam methane reforming (SMR) combined with CCS in the UAE, and coal gasification combined with CCS in Australia.
The LCCO2 emission E can be calculated using Eq. (1).
where
xi,j is the amount of input
i to process
j, and
ei is the CO
2 emission intensity of input
i. For CO
2 emission intensity, the Japanese Inventory Database for Inventory Analysis (IDEA) [
18] version 2.2, developed by the National Institute of Advanced Industrial Science and Technology and released in December 2017, was used. IDEA version 2.2 provides environmental emission intensity data for approximately 3800 processes, covering nearly all economic activities across Japanese industries. Since IDEA is geographically limited to Japan and CO
2 intensity data for overseas processes in the hydrogen supply chain is scarce, the following assumptions were made for the term
ei in Eq. (1) to calculate emissions related to overseas processes: (a) IDEA data were used for non-energy inputs, assuming that overseas countries have the same emission intensities as Japan; (b) IEA statistics on CO
2 emissions from electricity [
19] were used for electricity inputs (Tab.1); and (c) direct emission factors, which exclude upstream emissions, were used for fuel inputs.
2.2 Hydrogen production
2.2.1 Electrolysis
An alkaline water electrolysis plant with an annual hydrogen production capacity of 2.5 billion N∙m3 and a load factor of 20% is assumed to produce hydrogen using renewable electricity in the UAE. The electricity and pure water intensities for this process are estimated to be 4 kWh/(N∙m3) of hydrogen and 0.9 kg/(N∙m3) of hydrogen, respectively.
2.2.2 Steam methane reforming of natural gas
An SMR plant with an annual hydrogen production capacity of 2.5 billion N∙m3 and a CCS facility is assumed to produce hydrogen from natural gas (NG) in the UAE. The assumed intensities for the plant are 11.3 MJ/(N∙m3) of hydrogen for feedstock NG, 2.27 MJ/(N∙m3) of hydrogen for fuel NG, 0.11 kWh/(N∙m3) of hydrogen for electricity, 1.2×10−3 m3/(N∙m3) of hydrogen for pure water, and 5.00×10−3 m3/(N∙m3) of hydrogen for cooling water. CO2 capture is achieved through the chemical adsorption of CO2 by monoethanolamine solution with a 97% recovery rate. The captured CO2 will be transported 100 km to the CCS site via pipeline and stored in a reservoir at a depth of 1700 m.
2.2.3 Coal gasification
In Australia, a coal gasification plant with an annual hydrogen production capacity of 2.5 billion N∙m3 of hydrogen and a CCS facility are assumed to produce hydrogen from coal. The intensities for feedstock coal, electricity, and cooling water are considered to be 13.6 MJ/(N∙m3) of hydrogen, 0.68 kWh/(N∙m3) of hydrogen and 9.84×10−3 m3/(N∙m3) of hydrogen, respectively. CO2 capture is achieved through adsorption using physical solvents with a recovery rate of 96%. The captured CO2 will be transported 100 km to the CCS site via pipeline and stored in a reservoir at a depth of 1700 m.
2.3 Liquid hydrogen carriers
Gaseous hydrogen produced overseas from renewable sources, NG, and coal is conditioned or converted into liquid hydrogen carriers, such as liquid hydrogen (LH), MCH, and NH
3, for maritime transport and delivery to Japan. The maritime distances from the UAE and Australia to Japan are set at 12000 and 8000 km, respectively. Tab.2 presents the key features of the liquid hydrogen carriers considered in this paper. While this paper primarily focuses on using NH
3 as a hydrogen energy carrier, it is important to note that NH
3 is also a combustible substance and can be directly used as a fuel for gas turbines, boilers, and industrial furnaces [
20].
2.3.1 Liquid hydrogen
Fig.2 illustrates a hydrogen supply chain using LH. Hydrogen will be liquefied using liquefiers with a capacity of 100 t/d and an electricity intensity of 0.55 kWh/(N∙m3) of hydrogen. The liquefied hydrogen will be stored at the loading/unloading ports in spherical vacuum double-shell tanks with a total volume of 50000 m3. The boil-off rate will be set at 0.1%/d. LH tankers with a capacity of 160000 m3 and a boil-off rate of 0.4%/d, will transport the LH. The boiled-off hydrogen will be used as fuel for the tankers.
2.3.2 MCH
Fig.3 shows the hydrogen supply chain using MCH. MCH is produced through the chemical reaction of hydrogen and toluene (TOL) at a plant with an annual production capacity of 1.1 Mt/a. Any unreacted hydrogen is captured and used in the plant for heating purposes. The MCH and TOL will be stored at the loading/unloading ports in two tanks of 67200 m3 and one tank of 98100 m3, respectively. They will be transported by sea to Japan in a chemical tanker with a capacity of 81000 m3. Upon arrival, TOL and hydrogen will be obtained by dehydrogenating MCH using the heat from the combustion of city gas, at a processing capacity of 2.15 Mt/a of MCH. The TOL will be transported to hydrogen-producing countries to produce MCH. Notably, the hydrogen obtained from the dehydrogenation process will be unpurified and may contain MCH and TOL vapors. While unpurified hydrogen can be used for direct combustion, a purification process is necessary if hydrogen is to be used in fuel cells.
2.3.3 NH3
Fig.4 shows a hydrogen supply chain using NH3. NH3 is produced from nitrogen and hydrogen at atmospheric pressure in a Haber-Bosch plant with a capacity of 584000 t/a of NH3. The NH3 will be stored at loading/unloading ports in tanks with capacities ranging from 57000 to 81000 m3. It will then be transported by sea to an NH3 tanker with a capacity of 780000 m3, using liquefied natural gas (LNG) as fuel. Upon arrival at the unloading port, the NH3 stored at the unloading port will be transferred to a dehydrogenation (NH3 cracking) plant with a capacity of 2400 t/d of NH3 to produce hydrogen-nitrogen mixed gas using the heat generated from the combustion of city gas. Notably, this calculation does not consider the hydrogen purification process after NH3 cracking.
3 Results and discussion
3.1 Hydrogen production stage emissions
Fig.5 shows the CO
2 emissions during the hydrogen production stage in the assumed hydrogen-producing countries. Two cases are configured for hydrogen from NG and coal, with the electricity inputs for this stage being either grid electricity or renewable electricity. Comparing the calculated emissions in Fig.5 with Japan’s current low-carbon target of 3.4 kg of CO
2e per 1 kg of hydrogen at the well-to-production gate [
6], it is evident that the hydrogen produced through coal gasification exceeds this target due to emissions from grid electricity. However, if the grid electricity input in the Australian process is substituted with renewable electricity, the emissions can be drastically reduced, bringing them below the low-carbon target.
3.2 Supply chain emissions
This subsection shows the LCCO2 emissions using different liquid hydrogen carriers to highlight the key features of each carrier. Two cases are considered for each supply chain as low-carbon options. In Case A, all electricity inputs to the supply chain, except for hydrogen production from renewable electricity, are sourced from the grid in both the hydrogen-producing countries and Japan. In contrast, Case B assumes that all electricity inputs are renewable sources.
3.2.1 Supply chain using liquid hydrogen
Fig.6 shows the LCCO2 emissions from the hydrogen supply chain using LH. In addition to the differences in emissions from the hydrogen production pathways shown in Fig.5, the CO2 emissions from the carrier production stage are also significant. These emissions are primarily due to the large amount of electricity required to cool hydrogen to −253 °C for liquefaction, as well as the high CO2 emission factor of grid electricity in the UAE and Australia, as detailed in Tab.1. In Case B, where all power inputs are sourced from renewable electricity, emissions from the carrier production stage are reduced to zero, reinforcing the earlier point about the benefits of renewable electricity.
3.2.2 Supply chain using MCH
Fig.7 shows the LCCO2 emissions from the hydrogen supply chain using MCH. The emissions from the dehydrogenation process have a significant impact on the total emissions of the MCH supply chain. Even when Japanese domestic grid electricity (Case A) is replaced with renewable electricity (Case B), significant emissions remain in this process. This is primarily attributed to the high emissions from city gas, which is crucial for the endothermic dehydrogenation reaction (MCH→TOL+3H2, ∆H = −204.8 kJ/mol). Therefore, carbonizing the heat supply for the dehydrogenation process is critical to establishing a low-carbon hydrogen supply chain using MCH.
3.2.3 Supply chain using NH3
Fig.8 shows the LCCO2 emissions from the hydrogen supply chain using NH3. The LCCO2 emissions from the NH3 supply chain follow a similar trend to those of the MCH supply chain (Fig.7), though there are differences in the emissions from the carrier production and dehydrogenation stage. In our calculation, it is assumed that NG supplies heat to the Haber-Bosch plant. Therefore, substituting the grid electricity input in Case A with renewable electricity in Case B does not significantly reduce CO2 emissions from this process. Moreover, the NH3 dehydrogenation stage requires less heat (2NH3→N2+3H2, ∆H = −92.2 kJ/mol) compared to the MCH dehydrogenation, resulting in lower CO2 emissions from city gas combustion in this stage.
3.3 Comparison of supply chain emissions
All the results shown in Fig.6–Fig.8 are integrated into Fig.9 to compare the LCCO2 emissions across different liquid hydrogen carriers. The current Japanese low-carbon target of 3.4 kg of CO2e per 1 kg of hydrogen applies only to the well-to-production gate boundary. Fig.9 indicates that the supply chain emissions could exceed this target unless the appropriate low-carbon options are selected. The LH supply chain is particularly electricity-intensive, and Fig.9 confirms that substituting the grid electricity use with renewable electricity can significantly reduce supply chain emissions. In contrast, reducing emissions at the dehydrogenation stage is crucial for establishing low-carbon MCH and NH3 supply chains.
4 Conclusions
This paper presents the LCCO2 emission estimates from hydrogen imported to Japan. Gaseous hydrogen, produced through water electrolysis using renewable electricity in the UAE, NG SMR in the UAE, and coal gasification in Australia, have been transformed into liquid hydrogen carriers (LH, MCH, and NH3) for maritime transport to Japan. Upon arrival in Japan, these hydrogen carriers were transformed back into gaseous hydrogen. The key findings of the LCCO2 emissions estimates can be summarized as follows:
• The CO2 emissions attributed to electricity inputs for hydrogen production from fossil fuels combined with CCS can be significant, particularly if grid electricity has a high CO2 emission factor (Fig.5). Using low-carbon renewable electricity at this stage can significantly reduce CO2 emissions from hydrogen production.
• Regarding the emissions from the LH supply chain (Fig.6), since the electricity input for transforming gaseous hydrogen into LH is significant, the CO2 emissions from this carrier production stage are dominated by the high CO2 emission factor of grid electricity. However, substituting the grid electricity with renewable electricity can dramatically reduce emissions, reducing the LCCO2 of the LH supply chain.
• The CO2 emissions during the dehydrogenation stage are dominant in the LCCO2 emissions of the MCH supply chain (Fig.7). Low-carbon heating sources for the dehydrogenation process are essential for reducing emissions.
• The emission profile of the NH3 supply chain resembles that of the MCH (Fig.8), with the dehydrogenation stage being a primary emission source. Additionally, adopting low-carbon measures for utility inputs in the Haber-Bosch process is essential for improving NH3 supply chain emissions.
The selection of liquid hydrogen carriers is a significant issue in shaping Japan’s hydrogen economy. However, each carrier faces unique challenges, and it is currently impossible to determine which will dominate in the long-term. In addition, their applications are expected to be tailored according to their chemical properties and the availability of infrastructures. Therefore, the Japanese government envisions the future potential of all three carriers and supports research and development to overcome technological barriers without prioritizing any one option.
As noted earlier, Japan has set a low-carbon well-to-production target of 3.4 kg of CO2e per 1 kg of hydrogen. Since the transformation of gaseous hydrogen into liquid carriers is necessary for the international hydrogen supply chain Japan aims to establish, the results of this paper indicate that setting a well-to-delivery gate emissions target could provide another useful benchmark in selecting the most promising liquid hydrogen carrier to achieve Japan’s carbon neutrality goals.
The calculations in this paper follow the IPHE methodology, focusing only on operational well-to-gate emissions. However, a previous calculation by the authors on the LCA of fuel NH
3 [
21] revealed that emissions from capital goods, particularly those related to renewable power generation and energy storage, can be significant when NH
3 is produced from renewable energy. For GHG emissions in the hydrogen supply chain, the ISO/TS 19870 [
22] provides international guidelines for GHG accounting, using the IPHE methodology as its foundation. According to this standard, the quantification of the capital goods emissions “shall” be provided for information separately using relevant values taken from literature evaluated by calculations following the relevant ISO documents. Our future work will include these capital goods emissions in the hydrogen supply chain.
LCCO2 estimates will undoubtedly be a critical criterion for selecting the environmentally preferable carriers. However, it is equally important to consider the economic profiles of different carriers, as their costs will influence their roles in the hydrogen economy.
It should be noted that the LCCO2 estimates in this paper were calculated under specific conditions, and the results for each supply chain may vary significantly, depending on the data used and the assumptions made.
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