While industrial energy requirements are currently met by fossil fuels, they need to be replaced with regenerative electricity or regenerative fuels. “An alternative to fossil fuels is power-to-X fuels, which are synthesised from electrolysis-based hydrogen. The most important power-to-X fuel is currently hydrogen itself. However, before hydrogen can be utilised as an energy source on a widespread basis, there are still some considerable hurdles that need to be overcome in terms of its transportation and storage,” says Gunther Kolb, head of the Fraunhofer IMM energy division, and deputy institute director.
“These include either high space requirements for its storage or other energetically unfavourable conditions. Ammonia (NH3) could become an alternative solution to allow the required hydrogen to be stored and transported with ease.”
To date, ammonia has primarily been known as a source material for agricultural fertilisers. However, it is also a high-quality energy carrier, particularly as a storage medium for hydrogen. “Because ammonia can be liquefied at a moderate temperature of -33°C, its volumetric hydrogen content is significantly higher than that of compressed hydrogen at 700bar. In comparison to hydrogen, liquefied ammonia makes it easier to transport large volumes to wherever it is needed. Because hydrogen produced from ammonia does not contain any carbon oxides or methane, it is also free from greenhouse gases,” explains Kolb.
The conversion of hydrogen into ammonia is a viable option for using it as an energy source both on-site for industrial processes on a large scale as well as on a decentralised basis in the logistics sector. However, ammonia is not very suitable for generating energy through combustion, as it is virtually incombustible in the air.
“In a cracking reactor, ammonia can be split into nitrogen and hydrogen when suitable catalysts are applied. A mixture of ammonia, hydrogen and nitrogen is suited for homogeneous combustion and can be used as an energy source, known as ‘Spaltgas’,” says Kolb. As part of the publicly-funded Spaltgas project, researchers at Fraunhofer IMM and project partners are developing a combustion technology for this gas mixture that will be used in brick firing.
Ammonia can also be used for supplying hydrogen to land vehicles. It can be converted into hydrogen right at the filling station using decentralised plants. This eliminates the need to transport compressed and liquefied hydrogen – a costly and complex process. To this end, Fraunhofer IMM is developing a cracking reactor (‘Ammonpaktor’, pictured). In this reactor, pure hydrogen is produced from ammonia through cracking and subsequent purification, which is then injected into PEM fuel cells. Hydrogen for use in fuel cell vehicles can therefore be produced sustainably from ammonia right at the filling station.
“By utilising the off-gas of the pressure swing adsorption (PSA, used for hydrogen purification) as energy source for the cracking process, we are able to achieve an efficiency of 90%, in comparison to 70% which are achieved [by] conventional technologies.”
The second-gen reactor, currently being made, has a throughput of 25kg/hour of ammonia and produces 70kg of purified hydrogen per day.