New Energies, new possibilities

Researchers at the Korea Institute of Science and Technology (KIST) have developed a steam-carrier-adopted composite membrane reactor system to produce pure H2 (>99.99%) from ammonia with high productivity (>0.35 mol-H2 gcat−1 h−1) and ammonia conversion (>99%) at a significantly reduced operating temperature (<723 K). A paper on their work is published in the Journal Of Membrane Science.

Membrane reactor for production of H2 from NH3. Credit: KIST

Coupling of a custom developed palladium/tantalum composite metallic membrane and ruthenium on lanthanum-doped alumina catalysts allowed stable operation of the membrane system with significant mass transfer enhancement. Various reactor assemblies involving as-fabricated membranes and catalysts are experimentally compared to suggest the optimal configuration and operating conditions for future applications. Steam is adopted as a sweep gas, presenting efficient H2 recovery (>91%) while replacing conventionally utilized noble carrier gases that require additional gas separation processes. The steam carrier presents similar membrane reactor performance to that of noble gases, and the water reservoir used for steam generation acts as an ammonia buffer via scrubbing effects.

—Park et al.

Although the need to build a global clean energy supply network has been noted worldwide, there are constraints when it comes to transporting renewable energy in the form of electricity across long distances. This has resulted in a growing demand for a technology that can convert surplus renewable energy into hydrogen and transport the hydrogen to the target destination for utilization.

Hydrogen gas, however, cannot be transported in large amounts due to the limitations in the amount that can be stored per unit volume. A strategy suggested to overcome this issue is the use of chemicals in liquid form as hydrogen carriers, similar to the current method of transporting fossil fuels in a liquid form.

Liquid ammonia (hydrogen storage density per volume: 108kg-H2/m3) is capable of storing around 1.5 times more hydrogen than liquefied hydrogen under the same volume. Unlike the conventional hydrogen production method of natural gas steam reforming in which large amounts of carbon dioxide is emitted in the production process, the hydrogen production method using ammonia only leads to the generation of hydrogen and nitrogen.

Despite the many advantages presented by ammonia, there has been relatively little research on producing high-purity hydrogen from ammonia and generating electricity in conjunction with fuel cells.

The research team at KIST developed a low-cost membrane material and a catalyst for decomposition of ammonia into hydrogen and nitrogen. By combining the catalyst and membrane, the research team created an extraction device that is capable of decomposing ammonia and separating pure hydrogen at the same time. With the developed technology, it is possible to continuously produce high-purity hydrogen. The system can even be applied to small power generation devices by directly connecting it with fuel cells without any additional hydrogen purification processes.

The research team substantially reduced the ammonia decomposition temperature from 550 ˚C to 450 ˚C, thereby lowering energy consumption and doubling the hydrogen production speed compared to the conventional technology. Also, using the low-cost metal membrane, it was able to produce at least 99.99% pure hydrogen without any high-cost isolation process such as pressure swing adsorption (PSA).

Currently, storage- and transportation-related infrastructure for ammonia has been commercialized and used worldwide for intercontinental transportation. If the newly developed technology from KIST is applied to such infrastructure, it will help Korea take a step closer to the hydrogen economy.

“We’re planning a follow-up study to develop a compact hydrogen power pack that does not emit any carbon dioxide, based on the recently developed technology, and apply it to urban aerial mobilities (e.g. drone taxis), unmanned aerial vehicles, ships, and other modes of transportation.

—Dr. Jo Young Suk from KIST

This study was carried out, with a grant from the Ministry of Science and ICT (MSIT), as Institutional R&D Program of KIST and the Core Renewable Energy Technology Development Project of the Korea Energy Technology Evaluation and Planning.

Resources

• Yongha Park, Junyoung Cha, Hyun-Taek Oh, Taeho Lee, Sung Hun Lee, Myung Gon Park, Hyangsoo Jeong, Yongmin Kim, Hyuntae Sohn, Suk Woo Nam, Jonghee Han, Chang Won Yoon, Young Suk Jo (2020) “A catalytic composite membrane reactor system for hydrogen production from ammonia using steam as a sweep gas,” Journal of Membrane Science, Volume 614, 118483 doi: 10.1016/j.memsci.2020.118483.

発電や船舶燃料などで注目。ただ、コスト低減が必要など実用化のハードルは高い。

東亜石油の製油所に運び込まれたメチルシクロヘキサン。水素を分離し発電用燃料に

脱炭素エネルギーの切り札として洋上風力とともに、期待が高いのが水素である。

千代田化工建設、三菱商事、三井物産、日本郵船の4社は、世界でも例のない形で水素を長距離輸送する実証試験を始めた。

昨年12月、ブルネイで天然ガス由来の水素を利用して製造された「メチルシクロヘキサン」（MCH）を積み込んだ船が、川崎港に初めて到着した。MCHは東亜石油の製油所内の設備で水素とトルエンに分離。その水素を発電用燃料として同じ製油所内の火力発電所で燃やす試験が今年5月に始まった。分離されたトルエンは船に積まれ、ブルネイに戻る。そこで再び天然ガスから改質された水素と合成され、MCHとして利用される。

水素は燃焼させても二酸化炭素（CO2）が発生しないことから、脱炭素エネルギーとして期待を集めている。しかし、常温では気体で存在し、体積当たりのエネルギー密度が天然ガスの3分の1程度と低い。液体にするにはマイナス253度まで冷却しなければならず、輸送方法の確立が課題だった。

そこで千代田化工は、水素をトルエンと化学合成してMCHにし、体積を500分の1とすることで長距離輸送を実現した。ガソリンなどと同じ常温の液体として輸送でき、既存の石油インフラを活用できるのも強みだ。実証試験は11月まで行われ、安全かつ低コストで輸送できるかを確認する。

「再エネ水素」の挑戦

日本は水素の研究開発において、世界でもトップを走ってきた。政府は2017年12月に「水素基本戦略」を策定。カーボンフリー（脱炭素）のエネルギーの新たな選択肢として、水素を明示した。

しかし近年、ドイツなど欧州諸国との競争が熾烈化。日本が優位性を維持するには、実証試験を経て実用化を急ぐ必要がある。そのカギを握るプロジェクトが福島県浪江町で7月にスタートした。

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