Ion Agirre and Laura Barrio from the SherLOHCk project discuss the challenges of storing and transporting hydrogen
Hydrogen is emerging as the preferred energy source in the battle against greenhouse gas emissions, with a notable emphasis on obtaining green hydrogen. Its integration with renewable sources is pivotal to smoothing out production fluctuations and facilitating energy transport. However, the challenge lies in storing and transporting hydrogen from renewable production sites to application points. The existing technologies for hydrogen storage primarily focuses on boosting its volumetric energy density, yet faces hurdles due to high pressures and low temperatures, slowing down the evolution of the hydrogen economy.
In response to these challenges, there’s a growing interest in liquid organic hydrogen carriers (LOHCs). This innovative technology involves reversible hydrogenation/dehydrogenation reactions utilising cyclic hydrocarbons. What makes LOHCs stand out is their potential to transport hydrogen seamlessly through current liquid fuel infrastructures, offering a loss-free solution for long-term hydrogen storage.
What are the LOHCs?
Under the term LOHCs are an important list of molecules which are able to store and release an average of 6wt-% of hydrogen. The storage consists of a catalytic hydrogenation of a hydrogen- lean compound (LOHC-) yielding a hydrogen-rich compound (LOHC+). Various LOHC molecules are being considered, each with distinct advantages and disadvantages. For instance, methylcyclohexane (MCH) is a cost- effective option using toluene as the carrier molecule, which boasts significant production capabilities estimated at around 22 Mt. However, toluene’s toxicity requires careful handling.
An alternative non-toxic LOHC is Diben- zyltoluene (DBT)/Benzyltoluene (BT) and its mixtures. Although currently more expensive than toluene, scaling up could make it a compelling long-term option, especially due to its non-toxic nature. These systems allow for 6.2 wt% H2 storage, but they require catalytic dehydrogenation, performed at high temperatures (200- 350°C). Three primary commercial hydrogen storage systems dominate the market: compressed gaseous hydrogen (CGH2), liquid hydrogen (LH2), and metal hydrides, with CGH2 claiming over 90% of the market share.
While liquid hydrogen is favoured for extensive storage and transport, it proves technologically impractical for integration with variable renewable energies. Metal hydrides find commercial use primarily in submarines and lack relevance for hydrogen logistics applications. Although compressed H2 appears more competitive for short-distance transportation (<150- 300 km), LOHC systems possess a substantial competitive edge, leveraging low-cost oil infrastructure and offering an easily scalable solution for large-scale hydrogen storage and transport.
An alternative for H2 storage and transportation is using ammonia as a carrier. Ammonia’s advantage lies in not needing to return to the hydrogenation plant. Additionally, it allows more hydrogen to be shipped for the same capacity, reducing transportation costs per kg of H2 due to its higher H2 content. However, converting hydrogen to ammonia requires up to 20% of the energy contained in hydrogen, depending on system size and location. A similar energy loss occurs if ammonia needs reconversion to high-purity hydrogen at its destination. Despite these considerations, ammonia liquefies at -33°C, significantly higher than hydrogen, and contains 1.7 times more hydrogen per m3 than liquefied hydrogen, making it more cost-effective to transport. Ammonia also boasts a well-established international transmission network but poses challenges due to its toxicity, limiting its use in certain sectors. There is also a risk of non-combusted ammonia escaping, leading to particulate matter formation and acidification.
The Role of LOHCs
LOHCs are much easier to transport than hydrogen, making this method a promising alternative. Once the renewable H2 has been generated and adsorbed, the release comes in the final destination. This step is needed to liberate the hydrogen before final consumption and energy is required, which is counterbalanced by their lower transport costs. Therefore, the upcoming efforts must be related to get a more energy efficient dehydrogenation process.
For this purpose, the SherLOHCk project is addressing the following challenges. First: catalyst requirements for the hydrogenation/dehydrogenation steps. Depending on how we’re using LOHCs, the catalysts for hydrogenation and dehydrogenation can be either the same or different. Regardless, what matters most is not just only how active the catalyst is but how selective it is in the reaction. Since the organic molecule is essentially a hydrogen carrier, SherLOHCk aims for numerous cycles of hydrogenation and dehydrogenation without having to swap out the liquid carrier.
In the realm of standard LOHC systems like Aromatics/Cycloalkanes, supported noble metals take the lead as catalysts, prized for their activity and selectivity. However, to reach sustainable processes, the replacement of at least a part of platinoid metals is a priority challenge. The second challenge is the enhancement of thermal properties in the hydrogenation/dehydrogenation process. As mentioned before, both hydrogenation and dehydrogenation steps are energy demanding, especially the latter due to it being a highly endothermic step.
In order to overcome this drawback a catalytic system architecture with excellent thermal properties to reduce energy intensity during loading/unloading processes must be developed. For this purpose, SherLOHCk is working on the development of innovative catalytic system architectures characterised by excellent thermal properties e.g. in the fabrication and structural optimisation of heat conductive cellular structures, through integration of thermo-fluidic models to develop suitable reactor designs and the development of catalytically activated metallic structures.
Getting Ready for Market
After overcoming these challenges, SherLOHCk will assess the economic and environmental feasibility of LOHC technology. To address the economic aspect, a demo scale (>10 kW) will be constructed which will test in continuous operation over at least 200 h on-stream, followed by a scale-up calculations. For environmental viability, a Life Cycle Assessment (LCA) will be employed to analyse not only the LOHC process but also the environmental footprint of the catalyst production.
In summary, LOHC technology holds significant promise in revolutionising hydrogen storage and transportation. Despite the challenges associated with catalyst development and energy- intensive processes, ongoing initiatives, like the SherLOHCk project, are actively addressing these concerns. The inherent advantages of LOHCs, including enhanced transport capacity, reduced operational costs, and improved safety, position them as key contributors to the future of hydrogen logistics. As the industry persists in refining LOHC systems, their potential to drive sustainable advancements in the hydrogen economy presents a huge potential.
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This article was written with the Bilbao School of Engineering, University of the Basque, Spain, in partner with the SherLOHCk project. SherLOHCk project is supported by the Fuel Cells and Hydrogen 2 Joint Undertaking This joint undertaking receives support from the European Union’s Horizon 2020 research and Innovation programme and Hydrogen Europe research.
