The quest for a sustainable energy future hinges significantly on unlocking the potential of hydrogen, a molecule widely hailed as "the fuel of the future." Its promise lies in its ability to decarbonize the global economy through two primary mechanisms: producing storable energy with zero carbon emissions, yielding only water, through combustion or fuel cells, and serving as a crucial alternative to fossil fuels or a chemical feedstock in challenging industrial sectors like steel and cement manufacturing. However, realizing this ambitious vision necessitates overcoming a critical hurdle: the efficient and economical transportation and storage of pure hydrogen.
While significant research efforts are focused on developing cost-effective and low-carbon production methods, a parallel and equally vital challenge lies in moving this nascent fuel from where it’s produced to where it will be consumed. Recognizing this less-discussed but pivotal aspect, a collaborative team led by researchers at the MIT Energy Initiative (MITEI), in partnership with ExxonMobil Technology and Engineering Co., has developed an innovative tool designed to untangle the complexities of hydrogen logistics.
The inherent properties of hydrogen – its status as the lightest gas and its low energy density per volume – present unique transportation challenges. Delivering a substantial amount of energy necessitates a large volume of hydrogen, demanding exceptionally tight seals in containment vessels to prevent its escape. This contrasts sharply with the relative ease of transporting liquid fuels like gasoline. Without robust and efficient means of storage and transport, hydrogen’s potential as a global clean fuel remains constrained.
A Novel Approach to a Complex Challenge
In 2024, supported by funding from ExxonMobil Technology and Engineering Co. through MITEI, the MITEI research team, alongside their Exxon colleagues, embarked on an in-depth examination of various hydrogen transportation strategies. Their extensive analysis concluded that a one-size-fits-all solution is unattainable. The cost and carbon footprint associated with any given transportation method are highly site-specific, varying significantly based on geographical location, available infrastructure, and local energy prices.
Instead of attempting to identify a single "best" method, the researchers opted to create a sophisticated, adaptive tool. This tool empowers users to navigate the diverse array of options, analyze their specific contexts, and select the most suitable pathway for their particular use case. The findings of this comprehensive study, along with the developed analytical tool, are detailed in a recent publication in the esteemed journal Fuel.
The study was spearheaded by former MITEI postdoctoral researchers Gasim Ibrahim, who has since joined Honeywell as an R&D engineer/scientist, and Guiyan Zang, a former MITEI group lead now serving as an associate professor at Washington State University. The research team also included other distinguished MIT co-authors, namely former postdocs Bosong Lin, Jacqueline Garrido, Woojae Shin, and Haoxiang Lai.
Understanding Hydrogen "Carriers" for Global Transport
The foundational assumption guiding the researchers’ work was that for hydrogen to achieve its global potential, it must be capable of being transported over extensive distances, including intercontinental voyages and transoceanic shipments. Given hydrogen’s gaseous nature, converting it into a liquid form prior to shipping emerged as the most practical approach.
While established methods exist for this conversion, the critical questions revolve around identifying the most advantageous methods for large-scale shipping, their associated costs, and their impact on the overall carbon intensity of the delivered hydrogen.
"There hasn’t been a lot of attention paid to addressing those questions," commented Gasim Ibrahim. He noted that while some prior studies have touched upon these issues, their conclusions often vary, and significant uncertainties persist. These uncertainties stem from the inherent variability of costs and carbon emissions across different locations, coupled with a lack of extensive data on how large-scale hydrogen transportation will ultimately function.
To address this knowledge gap, the team conceived and developed an "adaptive tool" designed for user-driven assessments. "So we decided the best thing to do was to develop an adaptive tool that would enable users to perform their own assessments – a tool that could be updated very easily," Ibrahim explained. He further emphasized the tool’s open-source nature, allowing for transparency and community-driven updates. "As the industry develops, and as scale becomes more a factor, the assumptions made in [our initial] assessments of the economics and the carbon intensity [of different shipping methods] will need to be updated."
The Hydrogen Carrier Analysis Tool (HyCAT)
To maintain a clear focus on the transportation and storage aspects, the developed model, christened the Hydrogen Carrier Analysis Tool (HyCAT), deliberately excludes the intricacies of hydrogen production and its end-use applications. HyCAT’s core function is to meticulously calculate the costs and greenhouse gas (GHG) emissions incurred throughout the hydrogen supply chain, from its point of origin to its final delivery.
Ibrahim described HyCAT as an intuitive tool. It features a user-friendly interface with drop-down menus for inputting assumptions, and the analytical results are presented through straightforward bar charts, with accompanying links to detailed tables for deeper dives.
While HyCAT operates with a defined boundary – "incoming hydrogen to outgoing hydrogen" – Ibrahim clarified that users are expected to incorporate specific local factors into their analyses. This includes the carbon intensity and cost of producing the incoming hydrogen. "So that will inform the final values that come out of a HyCAT analysis," he stated, underscoring why the tool’s output is inherently variable depending on the user’s inputs and geographical context.
Based on user-defined assumptions, HyCAT calculates the cost and GHG emissions across five key stages of the hydrogen supply chain:
- Conversion to Carrier: The process of transforming gaseous hydrogen into a transportable liquid or chemical compound.
- Storage at Origin: Holding the converted hydrogen before shipment.
- Maritime Transport: The voyage across oceans or large bodies of water.
- Storage at Destination: Holding the hydrogen upon arrival.
- Conversion Back to Hydrogen: Releasing the pure hydrogen from its carrier.
Exploring the Landscape of Liquefaction and Chemical Carriers
A pivotal decision in any hydrogen transport plan involves how to convert the gaseous hydrogen into a transportable liquid or compound and, subsequently, how to recover the pure hydrogen at the destination.
One primary method is direct liquefaction, which involves super-cooling hydrogen gas to extremely low temperatures. However, this process is energy-intensive. "you would need to consume about a third of the energy content of the hydrogen to make the gaseous hydrogen cold enough to liquify," Ibrahim pointed out. Furthermore, maintaining the cryogenic state during storage and transit requires highly efficient insulation to prevent re-gasification and loss. The principal advantage of direct liquefaction is that it does not involve chemical reactions.
Alternatively, hydrogen can be transported using chemical "carriers." These are liquid chemical compounds that have the ability to absorb hydrogen atoms under specific conditions and release them under others. This approach effectively converts the volatile hydrogen gas into a more manageable liquid form. The process necessitates two chemical reactions: one to bind the hydrogen to the carrier and another to release it at the destination.
In their demonstration runs, the researchers evaluated three promising hydrogen carrier compounds, each with its own set of advantages and disadvantages:
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Toluene-Based Carrier: This method involves adding hydrogen to toluene, a hydrocarbon. While the specific chemical reaction has not been extensively studied, a significant drawback is the typical origin of toluene from the oil and gas industry, leading to a relatively high initial carbon intensity. Additionally, some toluene is lost over time, requiring periodic replenishment.
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Synthetic Methane: This carrier is produced by reacting hydrogen with carbon dioxide. Ibrahim highlighted that this reaction has the benefit of consuming carbon dioxide, which can be captured from the atmosphere. However, a byproduct of this reaction is water, leading to a loss of some hydrogen with each reaction cycle.
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Ammonia: Formed by reacting hydrogen with nitrogen from the air, ammonia production is a well-established commercial process. "We’ve been producing ammonia for a long time," Ibrahim stated, adding that existing infrastructure for its transport and storage is robust. While Ibrahim considers ammonia a "most promising option," he noted that the reaction required to release the hydrogen from ammonia has not yet received extensive research attention.
Varied Conclusions and Future Directions
The initial analyses conducted using HyCAT revealed a compelling insight: the optimal hydrogen transportation strategy is not universal. "As we developed the tool, we saw that the ‘best’ carrier was very specific to the supply chain at hand," Ibrahim observed. He elaborated that the selection is contingent upon numerous factors, including the shipping distance, prevailing energy and shipping costs in both exporting and importing nations, the capital investment required for necessary facilities at both ends of the supply chain, and many other variables.
Ibrahim and his team are now planning a follow-up study where they intend to leverage HyCAT to scrutinize specific supply chains under a defined set of conditions. This will involve identifying assumptions with high degrees of uncertainty and exploring the range of potential outcomes. "Then we’ll be able to say, ‘under these conditions, this carrier is better than that one,’ or ‘this carrier is better at cost, but worse at carbon intensity,’" Ibrahim articulated.
For the present, the primary takeaway from the study, according to Ibrahim, is the absence of a singular, definitive answer. He cautioned decision-makers against generalizing or extrapolating findings from existing literature to their unique circumstances. Instead, he advocates for the widespread adoption of HyCAT as a tool for informed decision-making. By utilizing the tool, organizations can gain a comprehensive understanding of their available options, align these with their strategic objectives and values, optimize their supply chains, and ultimately contribute to making clean-burning hydrogen a tangible reality. The implications for global energy markets and climate change mitigation efforts are profound, suggesting a future where localized solutions, informed by robust analytical frameworks, will pave the way for hydrogen’s widespread integration.
