Energy carriers from different power-to-gas processes could solve many problems with regard to the infrastructure to be built. However, there are significant technical challenges. A comparison.
A great many politicians and business leaders now understand that hydrogen will play a central role in our future energy system, in order for us to be able to store and use renewable energies anytime and anywhere. Storage media, so-called derivates such as ammonia, methanol and LOHC, are a crucial gap in the expansion of the hydrogen economy, because there are hardly any pipelines and it is difficult to transport hydrogen in gaseous form in bulk in tanks, due to its low energy density. However, this will be necessary in the form of seaborne imports, in order to meet the high demand from transport and industry. The industry’s main focus is currently on ammonia, as the infrastructure (facilities, ships) has been established for this and companies have decades of experience in handling this substance. In addition, relatively little energy is required for liquefaction and it can be directly used in the fertiliser and other chemical industries. However, the process chain with its various conversion steps is not very efficient – especially if a cracker extracts the hydrogen again – meaning that much of the primary energy is lost. It is also extremely toxic.
Benefits of e-methane
Instead of allowing hydrogen (H2) to react with nitrogen (N) to produce ammonia (NH3), it is also technically possible to add carbon dioxide (CO2) using the power-to-gas process in order to make climate-neutral methane (CH4). The benefits appear even more impressive if the CO2 is directly extracted from the atmosphere via Direct Air Capture (DAC), for example, and re-released when the e-methane combusts, resulting in zero emissions. If it is converted back into hydrogen (via steam reforming), the CO2 can even be permanently removed from the carbon cycle using Carbon Capture and Storage (CCS). The direct use of the e-methane for heating and industrial process heat is also possible, as the existing natural gas infrastructure can largely be used for this. There would also be no need for current customers to convert their facilities. In addition to existing pipelines and distribution networks, the new LNG import terminals can also continue to operate.
Disadvantages of e-methane
In May 2023, the DVGW Research Centre at the Engler-Bunte-Institut at the Karlsruhe Institute of Technology KIT (DVGW-EBI) published a very clear short study on hydrogen transport options, which also examined the methanation of hydrogen. CO2 is currently only available from point sources such as cement factories, biogas plants or via DAC – overall, far too little for its large-scale use for power-to-gas. DAC is in the very early stages of development, with only a few pilot projects worldwide, and we are still years away from industrial commercial facilities. In addition to this, the process requires a lot of energy.
It’s not just efficiency that matters
Besides the existing infrastructure, the economic efficiency of an energy source also crucially depends on how much of the green electricity originally input into the process reaches the consumer. The above-mentioned study calculated that in the case of ammonia, the utilisation rate of the process chain is around 72 percent, and only 60% with cracking. The energy footprint of green LNG is worse, with the utilisation rate for direct use around 56 percent and with steam reforming to generate hydrogen 47 percent. Overall, several factors must be considered when evaluating the options, as the following diagram shows.
No clear trend towards a specific transfer medium
The study concludes that all transport options include process steps that are currently not commercially available. In the case of liquid hydrogen, these are maritime transport and liquefaction, in the case of green LNG, CO2 transport/sources, in the case of ammonia, cracking and in the case of LOHC, hydrogenation and dehydrogenation plants. The conditions for green LNG are currently the most advanced, but the process chain with the highest future utilisation rate is liquid hydrogen. This suggests that various transport routes will be operated in parallel in future. In addition, technical advances will potentially make transport and conversion processes more efficient and these utilisation rates may change.